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The benzoic acid dimer, (BZA)2, is a paradigmatic symmetric hydrogen bonded dimer with two strong antiparallel hydrogen bonds. The excitonic S1/S2 state splitting and coherent electronic energy transfer within supersonically cooled (BZA)2 and its 13C-, d1 -, d2 -, and 13C/d1 - isotopomers have been investigated by mass-resolved two-color resonant two-photon ionization spectroscopy. The (BZA)2-(h - h) and (BZA)2-(d - d) dimers are C2h symmetric, hence only the S2 ← S0 transition can be observed, the S1 ← S0 transition being strictly electric-dipole forbidden. A single 12C/13C or H/D isotopic substitution reduces the symmetry of the dimer to Cs, so that the isotopic heterodimers (BZA)2 - 13C, (BZA)2 -(h - d), (BZA)2 -(h13C-d), and (BZA)2 -(h - d13C) show both S1 ← S0 and S2 ← S0 bands. The S1/S2 exciton splitting inferred is Δexc = 0.94 ± 0.1 cm-1. This is the smallest splitting observed so far for any H-bonded gas-phase dimer. Additional isotope-dependent contributions to the splittings, Δiso, arise from the change of the zero-point vibrational energy upon electronic excitation and range from Δiso = 3.3 cm-1 upon 12C/13C substitution to 14.8 cm-1 for carboxy H/D substitution. The degree of excitonic localization/delocalization can be sensitively measured via the relative intensities of the S1 ← S0 and S2 ← S0 origin bands; near-complete localization is observed even for a single 12C/13C substitution. The S1/ S2 energy gap of (BZA)2 is Δcalcexc=11 cm-1 when calculated by the approximate second-order perturbation theory (CC2) method. Upon correction for vibronic quenching, this decreases to Δvibronexc=2.1 cm-1 [P. Ottiger et al., J. Chem. Phys. 136, 174308 (2012)], in good agreement with the observed Δexc = 0.94 cm-1. The observed excitonic splittings can be converted to exciton hopping times τexc. For the (BZA)2-(h - h) homodimer τexc = 18 ps, which is nearly 40 times shorter than the double proton transfer time of (BZA)2 in its excited state [Kalkman et al., ChemPhysChem 9, 1788 (2008)]. Thus, the electronic energy transfer is much faster than the proton-transfer in (BZA)2*. [ABSTRACT FROM AUTHOR]

We analyze the vibronic band structure of the excitonically coupled S1←S0/S2←S0 excitations of the 2-aminopyridine (2AP) self-dimer (2AP)2, using a linear vibronic coupling model [R. Fulton and M. Gouterman, J. Chem. Phys. 41, 2280 (1964)]. The vibronic spectra of supersonically cooled (2AP)2 and its 13C-isotopomer were measured by two-color resonant two-photon ionization and UV/UV-depletion spectroscopies. In the C2-symmetric form of (2AP)2, the S1←S0 (1A←1A) transition is very weak, while the close-lying S2←S0 (1B←1A) transition is fully allowed. A single 12C/13C isotopic substitution breaks the symmetry of the dimer so that the (2AP)2-13C isotopologue exhibits both S1 and S2 electronic origins, which are split by 11 cm-1. In Fulton–Gouterman-type treatments, the linear vibronic coupling is mediated by intramolecular vibrational modes and couplings to intermolecular vibrations are not considered. For (2AP)2, a major vibronic coupling contribution arises from the intramolecular 6a′ vibration. However, the low-energy part of the spectrum is dominated by intermolecular shear (χ′) and stretching (σ′) vibrational excitations that also exhibit excitonic splittings; we apply a linear vibronic coupling analysis for these also. The respective excitation transfer integrals VAB are 50%–80% of that of the intramolecular 6a′ vibration, highlighting the role of intermolecular vibrations in mediating electronic energy exchange. The S1/S2 electronic energy gap calculated by the approximate second-order coupled-cluster method is ∼340 cm-1. This purely electronic exciton splitting is quenched by a factor of 40 by the vibronic couplings to the Franck–Condon active intramolecular vibrations. [ABSTRACT FROM AUTHOR]

In the Cage[1] isomer of the water hexamer, the free O-H bonds of the two end-cap water molecules can flip between "up" (u) and "down" (d) orientations, giving four conformers denoted uu, ud, du, and dd. Using the Møller-Plesset second order perturbation method and large basis sets, we calculate fully relaxed potential energy curves as a function of both u↔d torsional angles, denoted φ[SUB1], φ[SUB2]. These predict du as the lowest conformer, with uu nearly degenerate and ud and dd at 30-40 and 50- 70 cm[SUP-1] higher energy, respectively. Along φ[SUB1] the torsional barriers are about 200 cm[SUP-1], along φSUB1]. The torsional zero-point energies are high, the φ[SUB2] between 80 and 110 cm vibrational ground states are strongly delocalized and averaging of the cluster properties is important along both φ[SUB1] and φ[SUB2]. The dipole moment components vary strongly along both φ[SUB1] and φ[SUB2]: μ[SUBa] changes from +0.8 to +2.2 D, μ[SUBb] from +0.5 to +1.2 D, and μ[SUBc] from +1.4 to -0.9 D. The φ[SUB2] torsional fundamental of H[SUB2]O[SUB6] is predicted in the range 65- 72 cm[SUP1] with an intensity of ≈0.5 D[SUP2],theφ[SUP1] fundamental is in the 22- 32 cm[SUP-1] range, with an intensity of ≈0.3 D[SUP2]. Both excitations are b/c hybrid perpendicular bands with a dominant b component. The torsional overtones should be very weak. [ABSTRACT FROM AUTHOR]

We present an ab initio theoretical study of five low-energy isomers of the water hexamer {Chair, Cage(du)[1], Book, Prism, and Boat}, their intramolecular vibrations, binding energies De and dissociation energies D[sub 0]. Møller-Plesset second order perturbation calculations using the aug-cc-pVTZ basis set at aug-cc-pVDZ optimized geometries including vibrational zero point energy corrections predict Chair to be the most stable isomer, followed closely by Cage(du)[1] (+0.02 kcal/mol) and Book (+0.05 kcal/mol), while Prism is 0.15 kcal/mol higher. The Boat conformer is least stable at both the De and DO levels. The main focus is on the intramolecular normal modes of the five isomers. The calculated O-H stretching frequencies and intensities are compared to recent infrared spectra of water hexamer in supersonic jets, liquid-helium droplets and solid para-hydrogen matrices. The IR spectra indicate that Book and Chair are major species in the latter two environments and may also exist in supersonic jets. The (H[sub 2]O)[sub 6] gas phase interconversion equilibria are calculated and predict that the most abundant isomer is Chair below 8 K, Cage between 8-26 K, and Book above 26 K. Several of the low-frequency vibrational modes are identified as low-amplitude precursors of the Chair+-+Book ↔ Cage isomerization pathways. [ABSTRACT FROM AUTHOR]

Investigates the coupling of a Jahn-Teller pseudorotation in a 9-hydroxytriptycene molecule. Interpretation of the triptycene spectra; Quantification of potential energy surfaces of vibrational modes; Interaction between Jahn-Teller and torsional potentials.

We report a combined experimental and theoretical study of the van der Waals isomers and intermolecular vibrations of the 2,3-dimethylnaphthalene·Ne complex in the S1 electronic state. The two-color resonant two-photon ionization spectrum exhibits eight bands within ≊40 cm-1 of the electronic origin. Theoretical considerations in combination with hole-burning spectroscopic measurements show that the transition closest to the electronic origin (at 000+5 cm-1) arises from an isomer which is different from that responsible for the other seven bands in the spectrum. The latter involve excitations of the intermolecular vibrations of the main isomer of 2,3-dimethylnaphthalene·Ne. Accurate three-dimensional quantum calculations of the van der Waals vibrational levels of the complex were performed using a discrete variable representation method. Combination of theory and experiment led to a complete assignment as well as to a quantitative theoretical reproduction of the experimental intermolecular vibrational level structure, and a parametrization of the intermolecular potential energy surface, modeled as sum of atom–atom Lennard-Jones pair potentials. This potential surface exhibits a global minimum above (and below) the aromatic ring plane of 2,3-dimethylnaphthalene and a shallower local minimum at C2v geometry, on the C2 axis of the molecule, adjacent to the two methyl groups. The main and minor isomers identified experimentally are associated with the global and the local minimum, respectively. The quantum calculations were extended to ≊1000 van der Waals vibrational states, i.e., to energies up to 78% of D0. These include levels localized either in the global or local minima, as well as highly excited vibrational states delocalized over all three potential minima, providing comprehensive insight into the quantum dynamics of the high-lying van der Waals states of an atom–large aromatic molecule complex. © 1995 American... [ABSTRACT FROM AUTHOR]

We report a detailed experimental and theoretical study of the intermolecular vibrational levels of the 1,2-dimethylnaphthalene·Ar van der Waals complex in the S1 electronic state. Due to the low symmetry of the aromatic molecular substrate, excitations in all three van der Waals modes (two in-plane, one out-of-plane) are allowed in the electronic spectrum, leading to the most complete data set of intermolecular vibrational transitions measured so far for an atom-large molecule complex. Two-color resonant two-photon ionization spectra reveal twelve bands within ≊95 cm-1 of the electronic origin. Accurate quantum three-dimensional calculations of the van der Waals vibrational levels were performed, using a method based on the 3D discrete variable representation. Combination of theory and experiment allowed a complete and quantitative interpretation of the level structure, as well as an accurate parametrization of the intermolecular potential energy surface (PES), modeled as sum of atom–atom Lennard-Jones pair potentials. The parametrization was partly transferable from other aromatic molecules, but methyl group parameters had to be adapted. Quantum number assignments of the vibrational states were possible for the low-lying levels, but difficult or impossible for states at higher energies, due to strong mode coupling, mainly between the two in-plane modes. © 1995 American Institute of Physics. [ABSTRACT FROM AUTHOR]

A spectroscopic study of supersonic jet-cooled catechol (1,2-dihydroxybenzene) and its d1- and d2-isotopomers, deuterated at the hydroxy groups, was performed by resonant two-photon ionization (R2PI) and fluorescence emission techniques, and supplemented by molecular-beam hole-burning experiments. The latter prove that one single rotamer of catechol is dominant under molecular beam conditions. The complicated vibrational structure in the S0→S1 spectrum from the 000 band to 400 cm-1 above is not due to three different rotamers, as previously thought, but is due to the excitation of a vibrational progression associated mainly with the torsion of the hydroxy groups. The torsional bands are very prominent in the R2PI spectra, but are weak in the emission spectra. Detailed analysis of the torsional bands was based on a fit to the S1 and S0 state frequencies and the Franck–Condon factors in absorption and emission, using a double-minimum potential for the S1 state and a harmonic potential for the S0 state. In the S1 state one of the two –O–H torsional mode frequencies is lowered from τ2≊250 to ≊50 cm-1, and the molecule is only quasiplanar with respect to the –O–H torsional coordinates. [ABSTRACT FROM AUTHOR]

A combined experimental and theoretical study of the intermolecular vibrations of the o-xylene·Ar van der Waals complex is reported for both the S0 and S1 electronic states. Two-color resonant two-photon ionization and fluorescence emission spectra of the vdW mode region of supersonic jet-cooled o-xylene·Ar exhibit five bands within 70 cm-1 of the electronic origin, which arise from low-frequency large-amplitude intermolecular vibrations. Accurate quantum 3D calculations of vdW vibrational levels were performed, based on the 3D discrete variable representation. Apart from the restriction to the J=0 state the calculated eigenstates are exact for the intermolecular potential energy surface (PES) employed. The PES is represented as a sum of Lennard-Jones (LJ) pair potentials, and the direct comparison between theory and experiment enabled calibration of the LJ parameters. Very good agreement was achieved for both the S0 and S1 states of o-xylene·Ar. The quantum 3D calculations provide a quantitative description of the vdW level structure up to ≊70 cm-1 above the vdW ground state. The low-energy eigenfunctions have nodal patterns analogous to the 2,3-dimethylnaphthalene·Ar complex. However, in the energy range 40–60 cm-1 the vdW mode eigenfunctions change over to 2D radial-oscillator-type wave functions, similar to those of benzene·Ar, but switch back to Cartesian type above 60 cm-1. The S1 state vdW levels of 2,3-dimethylnaphthalene [M. Mandziuk, Z. Bacˇic, T. Droz, and S. Leutwyler, J. Chem. Phys. 100, 52 (1994)] were recalculated with the present parameters, and the agreement between experimental and calculated frequencies is improved. [ABSTRACT FROM AUTHOR]

The effect of rare gas complexation on the electronically excited S1(E’) state of triptycene (T), which is Jahn–Teller distorted, was investigated by two-color resonant two-photon ionization (2C-R2PI) spectroscopy of the supersonically cooled van der Waals complexes triptycene·Nen, n=1–3. These complexes afford unique possibilities to study the effects of weak intermolecular interactions on the intramolecular Jahn–Teller coupling. Since the atoms are adsorbed at high-symmetry positions, the system symmetry is lowered from D3h(n=0) to C2v for n=1 and 2, but reverts to D3h for n=3. A Jahn–Teller (A1+E)xe coupling model including a uniaxial external strain component was applied successfully to calculate the S1 state levels and S1←S0 electronic spectra of all three complexes. The spectrum of T·Ne3 was fully interpreted without inclusion of strain, implying a highly symmetric D3h structure in which each of the three V-shaped compartments of triptycene is occupied by a single Ne atom. In contrast, the vibronic spectra of T·Ne and T·Ne2 were fitted with a considerable uniaxial strain of -19.73 and 19.07 cm-1, respectively, confirming both the predicted C2v geometry of the complexes as well as the equal magnitude, but opposite sign of the intramolecular distortion induced by one or two Ne atoms. These distortions correspond to a small change of the angle of the V-shaped compartments by ∼0.8°. The spectra of T·Ne and T·Ne2 are much more complex compared to triptycene and T·Ne3 due to the splitting of the E vibronic levels in C2v symmetry, and the appearance of additional transitions to levels that are weak or symmetry forbidden in bare triptycene. The calculated Jahn–Teller potential energy surfaces and vibronic wave functions are discussed and classified in terms of their symmetry and localization/delocalization properties. [ABSTRACT FROM AUTHOR]

The vibronic structure in the S1(E)←S0(A1) resonant two-photon ionization (R2PI) spectrum of supersonically cooled 9-fluorotriptycene is assigned using three different Jahn–Teller (JT) model Hamiltonians for the excited 1E state—Exe, (A+E)xe, and (A+E)x(a2+e). The basic Exe interpretation is satisfactory. However, the fitted vibronic band frequencies and intensities are improved by including coupling to a second excited state 1A1 in an exciton model. Some further observed absorption bands are only assignable by invoking a molecular Barnett effect (momentum coupling to an a2 vibration). The measured fluorescence emission spectra from different S1 vibronic levels are quantitatively reproduced within all three coupling schemes by the parameters fitted to the R2PI spectrum. Results are compared to previous calculations on unsubstituted triptycene. The JT stabilization energy is decreased by ∼10% upon fluoro bridgehead substitution, which is rationalized by the electron-withdrawing effect of the F atom. For the same reason, the exciton splitting between the S1 and the S2 states, as calculated in the (A+E)xe model, is reduced relative to triptycene. The ground state vibrational frequencies in the range 0–700 cm-1 are calculated using the semiempirical MOPAC 6.0/AM1 method and compared with the measured S0 frequencies, as well as those of triptycene. A 273 cm-1 degenerate C–F bending mode predicted by the AM1 calculation may explain several unassigned features in the higher-energy (200–360 cm-1) part of the R2PI spectrum, and may represent an example of e+e multimode coupling within a degenerate electronic state. [ABSTRACT FROM AUTHOR]

Molecular dynamics simulations were performed for van der Waals clusters naphthalene·Arn, n=1 to 4. For all isomers and conformers of these clusters, dynamical quantities such as velocity autocorrelation functions, vibrational power spectra, and semiclassical electronic absorption spectra were calculated over a wide energy range, and averaged over a canonical distribution at temperatures in the range T=5 to 30 K. Electronic absorption spectra were calculated for the origin bands according to the semiclassical method [L. E. Fried and S. Mukamel, J. Chem. Phys. 96, 116 (1992)] and are compared with the corresponding experimental naphthalene·Arn R2PI spectra [T. Troxler and S. Leutwyler, J. Chem. Phys. 95, 4010 (1991)]. The appearance of distinct absorption bands due to specific isomers for a given cluster size, as observed experimentally, is well reproduced by the simulations. Comparison of calculated electronic shifts for different isomers allows clear assignments in the experimental spectra. Increasing the simulation temperature to T=15–25 K is accompanied by band broadening and the appearance of sidebands towards the blue. Especially strong sidebands appear for naph·Ar2 and all clusters containing the Ar2 subunit, due to large-amplitude surface rotation/translation of the argon dimer on the naphthalene surface, in agreement with experiment. For clusters containing the n=3 and n=4 subunits the spectral broadening is smaller. For the n=4 (4+0)-isomer, the calculated band shape increases less than for the other n=4 isomers, mainly due to the motional narrowing effect of cluster fluxionality. Above 25 K, isomerization between different possible topological structures also occurs by side-crossing motion of one or several argon atoms. [ABSTRACT FROM AUTHOR]

Van der Waals (vdW) clusters of naphthalene with argon were synthesized in supersonic beam expansions. Mass-selective absorption spectroscopy was carried out by using fragmentation-free two-color resonant two-photon ionization with mass-spectrometric detection. Electronic spectra of naph·Arn (n≤6) were recorded at the vicinity of the naphthalene S1←S0 electronic origin (32 020 cm-1 ), and corresponding spectra for the 810 transition up to n=30. For n=3, the spectra due to two different isomers could be separated. Clear evidence for the existence of different cluster isomers was also found for n=6, 8, 9, 12–14, and 25–28. In comparison with most other solvent clusters M·Arn (M is the aromatic molecule), the spectral red shifts of 000 and vibronic bands of the naph·Arn clusters are very small. Analysis of the size dependence of the electronic spectral shifts indicates that the stepwise solvation of the naphthalene molecule proceeds predominantly on one side of the molecule. Two semicyclic trends in the spectral shifts are interpreted as successive wetting-nonwetting transitions which occur with increasing solvent cluster size. In the proposed mechanism for these transitions, part of the innermost solvent layer ‘‘unwets’’ the substrate, and moves to the second or third layer, thereby forming a nonwetting dropletlike cluster on one side of the naphthalene substrate. [ABSTRACT FROM AUTHOR]

Excited-state proton transfer from 1-naphthol to water was studied as a function of solvent system size, from supersonically cooled neutral clusters, 1-naphthol·(H2O)n, n=1–50, to bulk ice and water. Occurrence or nonoccurrence of proton transfer was detected and studied using cluster-size-specific laser-spectroscopic techniques: resonant two-photon ionization (R2PI) and laser-induced fluorescence emission. Depending on cluster size or solution phase, three qualitatively different types of excited-state behavior were observed: (1) For small clusters, n≤7, both the R2PI and fluorescence spectra of the clusters were similar in nature to the spectra of bare 1-naphthol; (2) the medium-size clusters (n=8–20) show incremental spectral shifts which indicate successive stages of molecular solvation, and the spectra approach that of 1-naphthol in bulk ice at n≊20; (3) the fluorescence spectra for large clusters, n≥20, show increasing emission intensity below 25 000 cm-1, characteristic of the emission of the excited-state 1-naphtholate anion. Excited-state proton transfer appears to occur in the largest observed clusters (n≥30), yet the fluorescence spectra do not converge fully to that of 1-naphtholate anion in bulk water. These three behaviors are discussed in terms of a model based on three distinct excited states connected by two electronic and geometric rearrangement processes. The model accounts in a unified way for the complete range of aqueous solvation behavior observed here as well as in many other solvent systems studied previously. The extent of proton transfer reaction is largely solvent controlled, the major determinants being the proton affinity of the solvent or solvent cluster, and its ability to resolvate the nascent ion pair on a subnanosecond time scale. In bulk ice, the slow solvent relaxation results in complete absence of excited-state proton transfer from both 1- and 2-naphthol. [ABSTRACT FROM AUTHOR]

Efficient excited-state proton transfer in neutral acid–base clusters α-naphthol·Bn has been detected and studied by a combination of laser spectroscopic techniques (resonant two-photon ionization, fluorescence excitation, and emission spectroscopy). S1 state proton transfer was observed for B=NH3 and n≥4, as evidenced by several criteria: (a) large red shift and substantial broadening of the R2PI spectra of the n≥4 clusters relative to those of the bare α-naphthol and smaller clusters; (b) very large Stokes shift (∼8000 cm-1) of the emission spectra of the n≥4 clusters; (c) complete broadening of the fluorescence emission band for the n≥4 clusters; and (d) a striking similarity of the emission band position and width of the latter spectra to the emission spectrum of the α-naphtholate anion in basic aqueous solution. No proton-transfer reaction was observed for small solvent clusters with B=NH3 and n≤3, nor for any of the pair complexes studied, which involve a single base partner [B=triethylamine, 3-dimethylamino-1-aminopropane, 1,4-bis(dimethylamino)butane] which we have studied so far. This behavior illustrates the difficulty of achieving charge separation in neutral gas-phase complexes or clusters. A critical gas-phase proton affinity PAcrit =248±3 kcal/mol was determined for proton transfer to take place in the α-naphthol·Bn (or base B) system. [ABSTRACT FROM AUTHOR]

The structures, binding energies, and vibrational frequencies of fully optimized water clusters (H2O)n, n=1– 4, were computed with ab initio molecular orbital theory at the SCF level using different basis sets. The SCF procedure allows satisfactory predictions for these properties compared with experimental results. For quantitative predictions of binding energies and geometrical parameters, a basis set including polarization functions is needed. With respect to intramolecular vibrational frequencies and frequency shifts, however, the split valence basis set 4-31G leads in all cases to the best rationalization of the available experimental data. Analysis of intramolecular force constants, frequencies, and eigenvectors for n=2 to 4 shows that (i) a transition from highly localized (n=2) to highly delocalized (n=4) vibrational modes takes place; (ii) the delocalized O–H vibrations of cyclic (H2O)n clusters (n≥3) can be described as longitudinal/transverse optical phonons; (iii) internal force constants for the hydrogen bonding O–H stretch, k(O–Hb), decrease strongly with size, leading to a decrease in the νbridge O–H frequencies; a similar decrease is found for the intramolecular bending force constant k(Θ); (iv) force constants for the free O–H stretching motions k(O–Hf) stay nearly constant, as do the corresponding vibrational frequencies; (v) intermolecular stretch–stretch couplings increase substantially with n and dominate over intramolecular stretch–stretch couplings for n≥3; similarly, intermolecular bend–libration couplings are very important. [ABSTRACT FROM AUTHOR]

Presents the calculation of a four-dimensional torsional potential for the cyclic water tetramer using high-level ab initio methods. Use of the Gaussian basis set; Calculations of ab initio in the asymmetric unit of the torsional space; Representation of the symmetry relationships between sets of degenerate stationary points and interconversion pathways.

Studies the intermolecular vibrations and van der Waals isomerism of the 2,3-dimethylnaphthalene...He van der Waals complex. Potential energy surface; Quantum calculations of the van der Waals vibrational levels; Reproduction of observed bands as intermolecular vibration excitations; Delocalization of van der Waals over the potential surface.

Presents information on a study which obtained diamond powders by carbon dioxide-laser-induced decomposition of carbon hydrogen at low pressures and temperatures. Methodology of the study; Results; Conclusions.

The optical absorption spectra of the B 2Π3/2 ← X 2Π3/2 transitions of chloro-, bromo-, and iodocyanide cations in neon matrices at 4.5 K have been obtained and vibrationally analyzed. The origin band of the A 2Σ+ ← X 2Π3/2 transition of ICN+ is also observed as well as the absorption bands of the CN radical in the spectra of ClCN+ and BrCN+. These features and the vibrational frequency data are discussed. [ABSTRACT FROM AUTHOR]

The stimulated-emission-pumping/resonant 2-photon ionization (SEP-R2PI) method was used to determine the intermolecular dissociation energies D0 of jet-cooled 1-naphthol(1NpOH)·S complexes, where S is a linear molecule (N2, CO, CO2, OCS, N2O, and ethyne) or symmetric-top molecule (2-butyne) that contains double or triple bonds. The dissociation energies D0(S0) are bracketed as follows: 6.68 ± 0.08 kJ/mol for S=N2, 7.7 ± 0.8 kJ/mol for CO, 12.07 ± 0.10 kJ/mol for CO2, 13.03 ± 0.01 kJ/mol for N2O, 14.34 ± 0.08 kJ/mol for ethyne, 15.0 ± 1.35 kJ/mol for OCS, and 29.6 ± 2.4 kJ/mol for 2-butyne. The minimum-energy structures, vibrational wavenumbers, and zero-point vibrational energies were calculated using the dispersion-corrected density functional theory methods such as B97-D3 and B3LYP-D3 with the def2-QZVPP basis set. These predict that N2 and CO are dispersively bound Face complexes (S bound to a naphthalene Face), while CO2, N2O, and OCS adsorb into the "Notch" between the naphthyl and OH groups; these are denoted as Notch complexes. Ethyne and 2-butyne form Edge complexes involving H-bonds from the —OH group of 1NpOH to the center of the molecule. The presence of a double or triple bond or an aromatic C=C bond within S does not lead to a specific calculated geometry (Face, Notch or Edge). However, a correlation exists between the structure and the sign of the quadrupole moment component Θzz of S: negative Θzz correlates with Face or Notch, while positive Θzz correlates with Edge geometries. [ABSTRACT FROM AUTHOR]

The S1/S2 state exciton splittings of symmetric doubly hydrogen-bonded gas-phase dimers provide spectroscopic benchmarks for the excited-state electronic couplings between UV chromophores. These have important implications for electronic energy transfer in multichromophoric systems ranging from photosynthetic light-harvesting antennae to photosynthetic reaction centers, conjugated polymers, molecular crystals, and nucleic acids. We provide laser spectroscopic data on the S1/S2 excitonic splitting Δexp of the doubly H-bonded o-cyanophenol (oCP) dimer and compare to the splittings of the dimers of (2-aminopyridine)2, [(2AP)2], (2-pyridone)2, [(2PY)2], (benzoic acid)2, [(BZA)2], and (benzonitrile)2, [(BN)2]. The experimental S1/S2 excitonic splittings are Δexp = 16.4 cm-1 for (oCP)2, 11.5 cm-1 for (2AP)2, 43.5 cm-1 for (2PY)2, and <1 cm-1 for (BZA)2. In contrast, the vertical S1/S2 energy gaps Δcalc calculated by the approximate second-order coupled cluster (CC2) method for the same dimers are 10-40 times larger than the Δexp values. The qualitative failure of this and other ab initio methods to reproduce the exciton splitting Δexp arises from the Born-Oppenheimer (BO) approximation, which implicitly assumes the strong-coupling case and cannot be employed to evaluate excitonic splittings of systems that are in the weak-coupling limit. Given typical H-bond distances and oscillator strengths, the majority of H-bonded dimers lie in the weak-coupling limit. In this case, the monomer electronic-vibrational coupling upon electronic excitation must be accounted for; the excitonic splittings arise between the vibronic (and not the electronic) transitions. The discrepancy between the BO-based splittings Δcalc and the much smaller experimental Δexp values is resolved by taking into account the quenching of the BO splitting by the intramolecular vibronic coupling in the monomer S1 ← S0 excitation. The vibrational quenching factors Γ for the five dimers (oCP)2, (2AP)2, (2AP)2, (BN)2, and (BZA)2 lie in the range Γ = 0.03-0.2. The quenched excitonic splittings Γ·Δcalc are found to be in very good agreement with the observed splittings Δexp. The vibrational quenching approach predicts reliable Δexp values for the investigated dimers, confirms the importance of vibrational quenching of the electronic Davydov splittings, and provides a sound basis for predicting realistic exciton splittings in multichromophoric systems. [ABSTRACT FROM AUTHOR]

Mass- and isomer-selected S[sub 1]←S[sub 0] resonant two-photon ionization and S[sub 1]→S[sub 0] fluorescence spectra were obtained for the supersonically cooled 7-hydroxyquinoline·(NH[sub 3])[sub n] clusters with n=2-16. For n=2 and 3, the absorption and emission spectra exhibit discrete and narrow bands, characteristic of nonreactive hydrogen bonded ammonia-chain clusters. For n>=4, the S[sub 1]←S[sub 0] R2PI spectra are completely broadened, with an onset at approx. 355 nm. For n=4-7, a weak fluorescence emission is observed, Stokes-shifted by approx. 185 nm, with a maximum at approx. 540 nm, which shows discrete structure on a broad background. From comparison to fluorescence emission observed in bulk solution, we conclude that S[sub 1] state enol→keto tautomerization occurs. For the n>=7 or 8 clusters, the fluorescence emission spectra become completely unstructured and shift to the blue, peaking at 435-450 nm. This emission indicates the occurrence of either S[sub 1]-state proton transfer to the ammonia solvent cluster and formation of the 7-HQ anion, or of the ground-state intracluster enol→keto tautomerization reaction. © 2000 American Institute of Physics. [ABSTRACT FROM AUTHOR]

The S2 ↏ S1 vibronic spectrum of the ortho-cyanophenol dimer (oCP)2 is analyzed in a joint experimental and theoretical investigation. Vibronic excitation energies up to 750 cm-1 are covered, which extends our previous analysis of the quenching of the excitonic splitting in this and related species [Kopec et al., J. Chem. Phys. 137, 184312 (2012)]. As we demonstrate, this necessitates an extension of the coupling model. Accordingly, we compute the potential energy surfaces of the orthocyanophenol dimer (oCP)2 along all relevant normal modes using the approximate second-order coupled cluster method RI-CC2 and extract the corresponding coupling constants using the linear and quadratic vibronic coupling scheme. These serve as the basis to calculate the vibronic spectrum. The theoretical results are found to be in good agreement with the experimental highly resolved resonant two-photon ionization spectrum. This allows to interpret key features of the excitonic and vibronic interactions in terms of nodal patterns of the underlying vibronic wave functions. [ABSTRACT FROM AUTHOR]

The gas-phase rotational motion of hexafluorobenzene has been measured in real time using femtosecond (fs) time-resolved rotational Raman coherence spectroscopy (RR-RCS) at T = 100 and 295 K. This four-wave mixing method allows to probe the rotation of non-polar gasphase molecules with fs time resolution over times up to ~5 ns. The ground state rotational constant of hexafluorobenzene is determined as B0 = 1029.740(28) MHz (2σ uncertainty) from RR-RCS transients measured in a pulsed seeded supersonic jet, where essentially only the v = 0 state is populated. Using this B0 value, RR-RCS measurements in a room temperature gas cell give the rotational constants Bv of the five lowest-lying thermally populated vibrationally excited states v7/8, v9, v11/12, v13, and v14/15. Their Bv constants differ from B0 by between -1.02 MHz and +2.23 MHz. Combining the B0 with the results of all-electron coupled-cluster CCSD(T) calculations of Demaison et al. [Mol. Phys. 111, 1539 (2013)] and of our own allow to determine the C-C and C-F semi-experimental equilibrium bond lengths re(C-C) = 1.3866(3) Å and re(C-F) = 1.3244(4) Å. These agree with the CCSD(T)/wCVQZ re bond lengths calculated by Demaison et al. within ±0.0005 Å. We also calculate the semi-experimental thermally averaged bond lengths rg(C-C)=1.3907(3) Å and rg(C-F)=1.3250(4) Å. These are at least ten times more accurate than two sets of experimental gas-phase electron diffraction rg bond lengths measured in the 1960s. [ABSTRACT FROM AUTHOR]

The quenching of the excitonic splitting in hydrogen-bonded molecular dimers has been explained recently in terms of exciton coupling theory, involving Förster's degenerate perturbation theoretical approach [P. Ottiger, S. Leutwyler, and H. Köppel, J. Chem. Phys. 136, 174308 (2012)]. Here we provide an alternative explanation based on the properties of the adiabatic potential energy surfaces. In the proper limit, the lower of these surfaces exhibits a double-minimum shape, with an asymmetric distortion that destroys the geometric equivalence of the excitonically coupled monomers. An effective mode is introduced that exactly reproduces the energy gain and amount of distortion that occurs in a multi-dimensional normal coordinate space. This allows to describe the quenched exciton splitting as the energy difference of the two (S1 and S2) vibronic band origins in a one-dimensional (rather than multi-dimensional) vibronic calculation. The agreement with the earlier result (based on Förster theory) is excellent for all five relevant cases studied. A simple rationale for the quenched exciton splitting as nonadiabatic tunneling splitting on the lower double-minimum potential energy surface is given. [ABSTRACT FROM AUTHOR]

The gas-phase ground-state dissociation energy D0(So) of an isolated and cold bimolecular complex is a fundamental measure of the intermolecular interaction strength between its constituents. Accurate D0 values are important for the understanding of intermolecular bonding, for benchmarking high-level theoretical calculations, and for the parameterization of dispersion-corrected density functionals or force-field models that are used in fields ranging from crystallography to biochemistry. We review experimental measurements of the gas-phase D0(S0) and Do(Si) values of 55 different M-S complexes, where M is a (hetero)aromatic molecule and S is a closed-shell solvent atom or molecule. The experiments employ the triply resonant SEP-R2PI laser method, which involves M-centered (S0 -> S1) electronic excitation, followed by S1 -> S0 stimulated emission spanning a range of So state vibrational levels. At sufficiently high vibrational energy, vibrational predissociation of the M-S complex occurs. A total of 49 dissociation energies were bracketed to within ≤1.0 kj/mol, providing a large experimental database of accurate noncovalent interactions. [ABSTRACT FROM AUTHOR]

The gas-phase ground-state dissociation energy D0(S0) of an isolated and cold bimolecular complex is a fundamental measure of the intermolecular interaction strength between its constituents. Accurate D0 values are important for the understanding of intermolecular bonding, for benchmarking high-level theoretical calculations, and for the parameterization of dispersion-corrected density functionals or force-field models that are used in fields ranging from crystallography to biochemistry. We review experimental measurements of the gas-phase D0(S0) and D0(S1) values of 55 different M⋅S complexes, where M is a (hetero)aromatic molecule and S is a closed-shell solvent atom or molecule. The experiments employ the triply resonant SEP-R2PI laser method, which involves M-centered (S0 → S1) electronic excitation, followed by S1 → S0 stimulated emission spanning a range of S0 state vibrational levels. At sufficiently high vibrational energy, vibrational predissociation of the M⋅S complex occurs. A total of 49 dissociation energies were bracketed to within ≤1.0 kJ/mol, providing a large experimental database of accurate noncovalent interactions. [ABSTRACT FROM AUTHOR]

Femtosecond degenerate four-wave mixing (fs-DFWM) rotational coherence spectroscopy (RCS) has been used to determine the rotational and centrifugal distortion constants of the 00 00 ground and 01 10 vibrationally excited states of gas-phase CS2. RCS transients were recorded over the 0–3300 ps optical delay range, allowing the observation of 87 recurrences. The fits yield rotational constants B00 00=3.271 549 2(18) GHz for 12C32S2 and B00 00=3.175 06(21) GHz for the 12C32S34S isotopomer. The rotational constants of the degenerate 01 10 bending level of 12C32S2 are B01 10=3.276 72(40) and 3.279 03(40) GHz for the e and f substrates, respectively. These fs-DFWM rotational constants are ten times more accurate than those obtained by CO2 laser/microwave heterodyne measurements and are comparable to those obtained by high-resolution Fourier transform infrared spectroscopy. Ab initio calculations were performed at two levels, second-order Mo\ller-Plesset theory and coupled-cluster singles, doubles, and iterative triples [CCSD(T)]. The equilibrium and vibrationally averaged C==S distances were calculated using large Dunning basis sets. An extrapolation procedure combining the ab initio rotational constants with the experiment yields an equilibrium C==S bond length of 155.448 pm to an accuracy of ±20 fm. The theoretical C==S bond length obtained by a complete basis set extrapolation at the CCSD(T) level is re(C==S)=155.579 pm, or 0.13 pm longer than that in the experiment. [ABSTRACT FROM AUTHOR]

The excited-state hydrogen-atom transfer (ESHAT) reaction of the 7-hydroxyquinoline·(NH3)3 cluster involves a crossing from the initially excited 1ππ* to a 1πσ* state. The nonadiabatic coupling between these states induces homolytic dissociation of the O–H bond and H-atom transfer to the closest NH3 molecule, forming a biradical structure denoted HT1, followed by two more Grotthus-type translocation steps along the ammonia wire. We investigate this reaction at the configuration interaction singles level, using a basis set with diffuse orbitals. Intrinsic reaction coordinate calculations of the enol→HT1 step predict that the H-atom transfer is preceded and followed by extensive twisting and bending of the ammonia wire, as well as large O–H...NH3 hydrogen bond contraction and expansion. The calculations also predict an excited-state proton transfer path involving synchronous proton motions; however, it lies 20–25 kcal/mol above the ESHAT path. Higher singlet and triplet potential curves are calculated along the ESHAT reaction coordinate: Two singlet-triplet curve crossings occur within the HT1 product well and intersystem crossing to these Tn states branches the reaction back to the enol reactant side, decreasing the ESHAT yield. In fact, a product yield of ≈40% 7-ketoquinoline·(NH3)3 is experimentally observed. The vibrational mode selectivity of the enol→HT1 reaction step [C. Manca, C. Tanner, S. Coussan, A. Bach, and S. Leutwyler, J. Chem. Phys. 121, 2578 (2004)] is shown to be due to the large sensitivity of the diffuse πσ* state to vibrational displacements along the intermolecular coordinates. [ABSTRACT FROM AUTHOR]

The structure of the 1-naphthol (NH[SUB3])[SUB2] cluster was investigated by rotational coherence spectroscopy (RCS), mass selective one- and two-color resonant two-photon ionization (R2PI) experiments and ab initio calculations. RCS measurements yielded rotational constants of 1-naphthol (NH[SUB3])[SUB2] as A = 1197, B = 500, and C = 413 MHz, as well as those for several isotopomers. The counterpoise-corrected second-order Mφller-Plesset perturbation theory (MP2) method predicts two isomers A and B. Both structures have hydrogen bonded naphthol-OH… NH[SUB3]… (NH[SUB3] chains, with the second NH[SUB3] (bent above the proximal aromatic ring and pointing towards the p-electron system and have nearly the same binding energy. The experimental rotational constants agree better with those calculated for structure B. The B3LYP and PW91 density functional methods also predict two isomers A, B with the rotational constants of B in acceptable agreement with experiment. Based on two-color R2PI experiments using low ionization frequency to suppress cluster fragmentation, the S[SUB1]←S[SUB0] electronic origin region of the 1-naphthol (NH[SUB3])] [SUB2 - 4] cluster series as reassigned, in agreement with the work of Dedonder-Lardeux et al. [Phys. Chem. Chem. Phys. 3, 4316 (2001)]. In one-color experiments, the 1-naphthol (NH[SUB3])[SUB3] cluster fragments with nearly 100% efficiency into the 1-naphthol (NH[SUB3])[SUB2SUP+] mass channel. [ABSTRACT FROM AUTHOR]

The S[sub 1]↔S[sub 0] and S[sub 2]↔S[sub 0] vibronic spectra of the supersonically cooled 2-pyridone dimer (2PY)[sub 2] and its [sup 13]C-, d[sub 1]-, and d[sub 2]-isotopomers were investigated by two-color resonant two-photon ionization and fluorescence spectroscopies. For the C[sub 2h] symmetric (2PY)[sub 2]–h[sub 2] and (2PY)[sub 2]–d[sub 2] complexes, the 2PY moieties are equivalent and the S[sub 1]↔S[sub 0] ([sup 1]A[sub g]↔[sup 1]A[sub g]) transition is forbidden. A single H/D or [sup 12]C/[sup 13]C isotopic substitution reduces the symmetry to C[sub s], so that (2PY)[sub 2]–[sup 13]C and (2PY)[sub 2]–d[sub 1] both exhibit S[sub 1]↔S[sub 0] and S[sub 2]↔S[sub 0] transitions. The S[sub 1]/S[sub 2] state exciton splittings are 43.6 cm[sup -1] and 52.4 cm[sup -1], respectively. These are analyzed in terms of a Frenkel model and compared to calculated splittings based on ab initio monomer transition dipole moments. For (2PY)[sub 2]–d[sub 1], whose 2PY subunits are different, an excitation transfer time of 318 fs is calculated from the exciton splitting. The S[sub 1]↔S[sub 0] and S[sub 2]↔S[sub 0] spectra are analyzed and assigned. Several b[sub u] intermolecular vibrations of S[sub 1] appear via vibronic coupling to the S[sub 2](B[sub u]) state. Combination of the fluorescence data from excitation of the S[sub 1] and S[sub 2] origins and vibrational excitations of (2PY)[sub 2]–h[sub 2], (2PY)[sub 2]–d[sub 1], and (2PY)[sub 2]–d[sub 2] allows the determination of the six S[sub 0] state intermolecular vibrational frequencies.© 2002 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Accurate O–H...O hydrogen-bond dissociation energies were measured for the supersonic-jet-cooled complexes 1-naphthol·S with S=D[sub 2]O, ethanol, oxirane, and oxetane. A mass-selective pump–dump–probe method was used, combining stimulated emission pumping with resonant two-photon ionization and ion-dip techniques. The ground-state dissociation energies D[sub 0](S[sub 0]) are 5.83±0.13 kcal/mol for d[sub 1]-1-naphthol·D[sub 2]O, 7.94±0.02 kcal/mol for 1-naphthol·ethanol, 7.71±0.14 kcal/mol for 1-naphthol·oxirane and >8.17 kcal/mol for 1-naphthol·oxetane. The D[sub 0]’s increase by 5%–7% upon excitation of 1-naphthol to the S[sub 1] state. These dissociation energies are compared to those of the analogous complexes with S=H[sub 2]O, methanol, NH[sub 3], and ND[sub 3] [Chem. Phys. Lett. 246, 291 (1996)]. The trends in D[sub 0] are compared to the electric dipole moments μ, molecular polarizabilities α¯, and gas-phase proton affinities of the H bond acceptor molecules. For the O-containing acceptors, the D[sub 0]’s correlate well with α¯, but the only good overall correlation for both O- and N-containing acceptors was found between the dissociation energies and proton affinities. © 2002 American Institute of Physics. [ABSTRACT FROM AUTHOR]

The intermolecular potential energy surface and the torsional-puckering (TP) motion of the cyclic homodromic water pentamer were theoretically investigated at the level of second-order MoilerPlesset perturbation theory using double- and triple-zeta correlation-consistent basis sets including diffuse functions. Extensive searches yielded three cyclic homodromic stationary points: (i) the puckered global minimum structure GS, (ii) a torsional-puckering saddle point, 62 cm[SUP-1] higher, denoted TS1, and (iii) a planar C[SUB5h] symmetric structure (Hessian index 7), 800 cm[SUP-1] higher in energy. Harmonic vibrational frequencies and normal modes were calculated for all stationary points. Based on the energies and second derivatives of the GS and TS 1 structures a cyclic minimum energy path for the large-amplitude TP motion with ten permutationally equivalent minima was constructed. The ground and all excited states are delocalized by TP pseudorotation, giving vibrationally averaged C[SUB5h] symmetry. The first pseudorotational TP level lies at ≈1 cm[SUP-1] (≈0.6 cm[SUP-1]) and the ten lowest TP states within a band of ≈20 cm[SUP-1] (8 cm[SUP-1]) for (H[SUP2]O)[SUB5] [(D[SUB2]O)[SUB5]]. The corresponding TP dipole moment functions were also calculated, and intensities for the far-infrared torsional-puckering transitions of (H[SUB2]O)[SUB5] and (D[SUB2]O)[SUB5] evaluated up to 300 cm[SUP-1]. [ABSTRACT FROM AUTHOR]

Mass-selective ground-state vibronic spectra of molecular van der Waals complexes carbazole·S, S=N2, CO, and CH4, were measured by stimulated emission pumping followed by resonant two-photon ionization of the vibrationally hot complexes. S0-state vibrational modes were accessed from ≊200 cm-1 up to the ground-state dissociation limit D0(S0) of the van der Waals bond. Above D0, efficient vibrational predissociation of the complexes occurs, allowing accurate determination of the van der Waals dissociation energies as 627.2±7.9 cm-1 for N2, 716.5±29.8 cm-1 for CO, and 668.6±15.1 cm-1 for CH4. In the S1 excited state, the van der Waals binding energies increase to 678.5±8.0, 879.2±29.9, and 753.8±15.2 cm-1, respectively. The relative increases upon electronic excitation are about 8% and 13% for N2 and CH4, similar to the analogous rare gases Ar and Kr. For CO, the relative increase of van der Waals binding energy is 23%. The differences are primarily due to electrostatic interactions. © 1995 American Institute of Physics. [ABSTRACT FROM AUTHOR]

We report a combined spectroscopic and theoretical investigation of the intermolecular vibrations of supersonic jet-cooled phenol·(H2O)3 and d1-phenol·(D2O)3 in the S0 and S1 electronic states. Two-color resonant two-photon ionization combined with time-of-flight mass spectrometry and dispersed fluorescence emission spectroscopy provided mass-selective vibronic spectra of both isotopomers in both electronic states. In the S0 state, eleven low-frequency intermolecular modes were observed for phenol·(H2O)3, and seven for the D isotopomer. For the S1 state, several intermolecular vibrational excitations were observed in addition to those previously reported. Ab initio calculations of the cyclic homodromic isomer of phenol·(H2O)3 were performed at the Hartree–Fock level. Calculations for the eight possible conformers differing in the position of the ‘‘free’’ O–H bonds with respect to the almost planar H-bonded ring predict that the ‘‘up–down–up–down’’ conformer is differentially most stable. The calculated structure, rotational constants, normal-mode eigenvectors, and harmonic frequencies are reported. Combination of theory and experiment allowed an analysis and interpretation of the experimental S0 state vibrational frequencies and isotope shifts. © 1995 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Mass-selective ground-state vibrational spectra of jet-cooled carbazole·R (R=Ne, Ar, Kr, and Xe) van der Waals complexes were obtained by populating ground-state intra- and intermolecular levels via stimulated emission pumping, followed by time delayed resonant two-photon ionization of the vibrationally hot complex. By tuning the dump laser frequency, S0 state vibrational modes were accessed from ≊200 cm-1 up to the dissociation energy D0. Upon dumping to ground-state levels above D0, efficient vibrational predissociation of the complexes occurred, allowing us to determine the S0 state van der Waals binding energies very accurately. The D0(S0) values are <214.5±0.5 cm-1 (R=Ne), 530.4±1.5 cm-1 (R=Ar), 687.9±4.0 cm-1 (R=Kr), and 890.8±1.6 cm-1 (R=Xe). In the S1 state, the corresponding binding energies are larger by 9% to 12%, being <222.9±1.0 cm-1, 576.3±1.6 cm-1, 756.4±4.5 cm-1, and 995.8±2.5 cm-1, respectively. © 1995 American Institute of Physics. [ABSTRACT FROM AUTHOR]

An intermolecular potential energy surface was derived for the hydrogen-bonded water trimer as a function of the three torsional angles ω1, ω2, ω3, for energies up to 1300 cm-1 (3.7 kcal/mol) above the global minimum. The O...O distances and the intramolecular geometry of the H2O molecules are held fixed. This surface is based on the ab initio calculations presented in a companion paper [W. Klopper et al., J. Chem. Phys. 103, 1085 (1995)], which involve very large basis sets and the most extensive treatment of correlation energy for calculations of (H2O)3 so far. The 70 ab initio interaction energies, multiplied by six due to the S6 symmetry of the surface, were fitted using an analytical potential function, with an average error of ≊11 cm-1. This potential provides a rapidly computable analytical expression for use in calculations of torsional eigenfunctions and -values and other properties of this cluster. Also given is a classification of the low-lying torsional wave functions according to nodal properties. © 1995 American Institute of Physics. [ABSTRACT FROM AUTHOR]

The minimum energy structure of the cyclic water trimer, its stationary points, and rearrangement processes at energies <1 kcal/mol above the global minimum are examined by ab initio molecular orbital theory. Structures corresponding to stationary points are fully optimized at the Hartree–Fock and second-order Mo\ller–Plesset levels, using the 6-311++G(d,p) basis; each stationary point is characterized by harmonic vibrational analyses. The lowest energy conformation has two free O–H bonds on one and the third O–H bond on the other side of an approximately equilateral hydrogen-bonded O...O...O (O3) triangle. The lowest energy rearrangement pathway corresponds to the flipping of one of the two free O–H bonds which are on the same side of the plane across this plane via a transition structure with this O–H bond almost within the O3 plane. Six distinguishable, but isometric transition structures of this type connect six isometric minimum energy structures along a cyclic vibrational-tunneling path; neighboring minima correspond to enantiomers. The potential energy along this path has C6 symmetry and a very low barrier V6=0.1±0.1 kcal/mol. This implies nearly free pseudorotational interconversion of the six equilibrium structures. The corresponding anharmonic level structure was modeled using an internal rotation Hamiltonian. Two further low-energy saddle points on the surface are of second and third order; they correspond to crown-type and planar geometries with C3 and C3h symmetries, respectively. Interconversion tunneling vibrations via these stationary points are also important for the water trimer dynamics. A unified and symmetry-adapted description of the intermolecular potential energy surface is given in terms of the three flipping coordinates of the O–H bonds. Implications of these results for the interpretation of spectroscopic data are discussed. [ABSTRACT FROM AUTHOR]

A combined experimental and theoretical study of the 2-naphthol·H2O/D2O system was performed. Two different rotamers of 2-naphthol (2-hydroxynaphthalene, 2HN) exist with the O–H bond in cis- and trans-position relative to the naphthalene frame. Using Hartree–Fock (HF) calculations with the 6-31G(d,p) basis set, fully energy-minimized geometries were computed for both cis- and trans-2HN·H2O of (a) the equilibrium structures with trans-linear H-bond arrangement and Cs symmetry and (b) the lowest-energy transition states for H atom exchange on the H2O subunit, which have a nonplanar C1 symmetry. Both equilibrium and transition state structures are similar to the corresponding phenol·H2O geometries. The H-bond stabilization energies with zero point energy corrections included are ≊5.7 kcal/mol for both rotamers, ≊2.3 kcal/mol stronger than for the water dimer, and correspond closely to the binding energy calculated for phenol·H2O at the same level of theory. Extension of the aromatic π-system therefore hardly affects the H-bonding conditions. The barrier height to internal rotation around the H-bond only amounts to 0.5 kcal/mol. Harmonic vibrational analysis was carried out at these stationary points on the HF/6-31G(d,p) potential energy surface with focus on the six intermolecular modes. The potential energy distributions and M-matrices reflect considerable mode scrambling for the deuterated isotopomers. For the a’ intermolecular modes anharmonic corrections to the harmonic frequencies were evaluated. The β2 wag mode shows the largest anharmonic contributions. For the torsional mode τ (H2O H-atom exchange coordinate) the vibrational level structure in an appropriate periodic potential was calculated. On the experimental side resonant-two-photon ionization and dispersed fluorescence emission spectra of 2HN·H2O and d-2HN·D2O were measured. A detailed assignment of the bands in... [ABSTRACT FROM AUTHOR]

Extensive ab initio calculations of the phenol·H2O complex were performed at the Hartree–Fock level, using the 6-31G(d,p) and 6-311++G(d,p) basis sets. Fully energy-minimized geometries were obtained for (a) the equilibrium structure, which has a translinear H bond and the H2O plane orthogonal to the phenol plane, similar to (H2O)2; (b) the lowest-energy transition state structure, which is nonplanar (C1 symmetry) and has the H2O moiety rotated by ±90°. The calculated MP2/6-311G++(d,p) binding energy including basis set superposition error corrections is 6.08 kcal/mol; the barrier for internal rotation around the H bond is only 0.4 kcal/mol. Intra- and intermolecular harmonic vibrational frequencies were calculated for a number of different isotopomers of phenol·H2O. Anharmonic intermolecular vibrational frequencies were computed for several intermolecular vibrations; anharmonic corrections are very large for the β2 intermolecular wag. Furthermore, the H2O torsion τ around the H-bond axis, and the β2 mode are strongly anharmonically coupled, and a two-dimensional τ/β2 potential energy surface was explored. The role of tunneling splitting due to the torsional mode is discussed and tunnel splittings are estimated for the calculated range of barriers. The theoretical studies were complemented by a detailed spectroscopic study of h-phenol·H2O and d-phenol·D2O employing two-color resonance-two-photon ionization and dispersed fluorescence emission techniques, which extends earlier spectroscopic studies of this system. The β1 and β2 wags of both isotopomers in the S0 and S1 electronic states are newly assigned, as well as several other weaker transitions. Tunneling splittings due to the torsional mode may be important in the S0 state in conjunction with the excitation of the intermolecular σ and β2 modes. [ABSTRACT FROM AUTHOR]

The irregular vibronic structure resolved in the S1←S0 resonant two-photon ionization (R2PI) spectrum of supersonically cooled triptycene (9,10-dihydro-9,10[1’2’]benzenoanthracene) is assigned in terms of a single-mode E’xe’ Jahn–Teller vibronic Hamiltonian for the excited state, with linear and quadratic coupling terms. The Jahn–Teller active vibrational mode is a benzene wagging framework mode. To fit to the observed vibronic levels yields a very low frequency νe’ =47.83 cm-1 and linear and quadratic terms are k=1.65 and g=0.426. This fit accounts for ≊98% of the observed absorption band intensities over the observable range 0–350 cm-1. The quadratic term is unusually large, leading to localization of the lowest vibronic levels in the three symmetry-equivalent minima. Emission spectra from 13 vibronic levels in the excited E’ state show extended vibrational progressions with up to 25 members in the analogous e’ ground state vibration, which is highly harmonic in the electronic ground state. The Franck–Condon factors of the fluorescence emission spectra calculated with the E’ state Jahn–Teller parameters fitted to the absorption spectrum also yield a quantitative fit to observed emission intensities. The eigenvectors of the E’ state vibronic levels are hence determined to great precision; the lowest five can be classified as radial oscillator and/or hindered rotor states, while higher levels have mixed character. Several eigenvectors are strongly localized in the upper sheet of the adiabatic Jahn–Teller surface, corresponding to ‘‘cone’’ states. [ABSTRACT FROM AUTHOR]

Structures and order–disorder transitions of the rare-gas solvent clusters carbazole·Arn were studied for n=1–7 by molecular dynamics and Monte Carlo simulation methods. This study was motivated by the investigation of these clusters by size-specific electronic spectroscopy. Cluster structures corresponding to absolute and local energy minima were obtained by molecular-dynamics slow cooling and annealing. For n=1–3, single minimum-energy structures of Cs symmetry were found, with all n atoms on the same side of the molecular substrate. For n=4–7, the number of energetically low-lying monolayer isomers rises very rapidly with n. Orientational ordering of the adcluster by the substrate is unimportant for n≥4. The isomers for n≥4 differ by (i) rotational/translational displacements of the cluster relative to the substrate, (ii) promotion of atoms to the second solvent layer, and (iii) adsorption of atoms on the second side of the substrate. Order–disorder transitions of the monolayer solvent clusters were studied by MC methods as a function of temperature. Four different quasi-two-dimensional order–disorder transitions were found: the racemization and surface-decoupling transitions reflect the loss of cluster–substrate orientational correlation, while the cluster rigid–fluxional and melting transitions derive from the loss of intracluster bond-orientational order, and increase in bond-length fluctuations. Analogies to order–disorder phase transitions in two-dimensional rare-gas films are discussed. [ABSTRACT FROM AUTHOR]

Excited-state proton transfer was studied in supersonically cooled neutral acid-base clusters of 2-naphthol·(NH3)n with n=1-10, using size-selective two-color resonant two-photon ionization (R2PI) and fluorescence emission techniques. The smaller clusters (n≤3) show vibrationally resolved spectra and do not exhibit excited-state proton transfer. Two rotamers (cis and trans) were observed for each cluster size; these exhibit very different electronic-vibrational couplings to the intermolecular modes, reflecting the orientation of the (NH3)n cluster relative to the electronic transition dipole moment of the aromatic chromophore. Excited-state proton transfer occurs for n≥4, as evidenced by (a) a large spectral red shift (≊8000 cm-1) of the fluorescence emission band; (b) large increase in the emission bandwidth; (c) similarity of the fluorescence band position and width to that of 2-naphtholate anion in neutral or basic aqueous solution. In absorption, the S1←S0 bands are substantially broadened for clusters with n≥4, but are not shifted to the red, as is the case for the analogous 1-naphthol·(NH3)n clusters. The cluster-size dependence of the proton transfer reaction is mainly controlled by the cluster proton affinity. [ABSTRACT FROM AUTHOR]

Carbazole·B complexes (B=H2O, D2O, NH3) were synthesized and cooled in pulsed supersonic nozzle beams. The intermolecular hydrogen-bond vibrations and dissociation energies were studied by several laser-spectroscopic techniques (fluorescence excitation and emission, resonance-two-photonionization with mass-specific detection). The following results were obtained for both S0 and S1 electronic states: (1) determination of the structural symmetry of the complexes, (2) measurement of the intermolecular stretching (νσ) and bending (νβ) frequencies, (3) determination of stretching force constants, (4) hydrogen-bond dissociation energies D0 for carbazole·H2O/D2O, and (5) electronic spectral shifts δν relative to the bare carbazole molecule. The latter are large (500–710 cm-1) and reflect an increase of the hydrogen-bond energy by approx. 40% upon electronic excitation. Fermi resonance couplings between the intermolecular νσ and an intramolecular b1 vibration of carbazole are observed and partially analyzed. To complement the experimental work, extensive ab initio quantum chemical calculations of the same complexes were performed at the Hartree–Fock level. The calculated complex structures are consistent with the experimental information. Intermolecular potential-energy curves for the stretching vibrational coordinate were calculated for a number of increasingly flexible basis sets (STO-3G, 4-31G, 6-31G, 4-31G*); anharmonic vibrational frequencies were then obtained numerically. Excellent agreement with the experimental intermolecular stretching frequencies was found for all three complexes using the 4-31G* basis set. Good agreement with experiment was also found for the calculated 4-31G* hydrogen-bond dissociation energy. [ABSTRACT FROM AUTHOR]

Presents discussions on molecular dynamics. Significance of the two-electron bond in predicting molecular structure; Chemical concepts of hydrogen bonding and proton transfer; Details on acid dissociation and dissolution.

The ground-state intermolecular dissociation energies D0(S0) of supersonic-jet cooled intermolecular complexes of 1-naphthol (1NpOH) with the bi- and tricycloalkanes trans-decalin, cis-decalin, and adamantane were measured using the stimulated-emission-pumping/resonant two-photon ionization (SEP-R2PI) method. Using UV/UV holeburning, we identified two isomers (A and B) of the adamantane and trans-decalin complexes and four isomers (A–D) of the cis-decalin complex. For 1NpOH·adamantane A and B, the D0(S0) values are 21.6 ± 0.15 kJ/mol and 21.2 ± 0.32 kJ/mol, those of 1NpOH·trans-decalin A and B are 28.7 ± 0.3 kJ/mol and 28.1 ± 0.9 kJ/mol, and those of 1NpOH·cis-decalin A and B are 28.9 ± 0.15 kJ/mol and 28.7 ± 0.3 kJ/mol. Upon S0 → S1 electronic excitation of the 1NpOH moiety, the dissociation energies of adamantane, trans-decalin, and the cis-decalin isomer C change by <1% and those of cis-decalin isomers A, B, and D increase only slightly (1%–3%). This implies that the hydrocarbons are dispersively adsorbed to a naphthalene "face." Calculations using the dispersion-corrected density functional theory methods B97-D3 and B3LYP-D3 indeed predict that the stable structures have face geometries. The B97-D3 calculated D0(S0) values are within 1 kJ/mol of the experiment, while B3LYP-D3 predicts D0 values that are 1.4–3.3 kJ/mol larger. Although adamantane has been recommended as a "dispersion-energy donor," the binding energies of the trans- and cis-decalin adducts to 1NpOH are 30% larger than that of adamantane. In fact, the D0 value of 1NpOH·adamantane is close to that of 1NpOH·cyclohexane, reflecting the nearly identical contact layer between the two molecules. [ABSTRACT FROM AUTHOR]

Reports coherent anti-Stokes Raman (CARS) spectra of water dimers and higher clusters in seeded supersonic jets. Determination of the gas-phase structure of water clusters from microwave/radio frequency spectra; spectral position of the CARS bands; Results of ab initio calculations.

The S1 excited-state lifetime of jet-cooled 1-ethylcytosine (1ECyt) is ∼1 ns, one of the longest lifetimes for cytosine derivatives to date. Here, we analyze its S0 → S1 vibronic spectrum using two-color resonant two-photon ionization and UV/UV holeburning spectroscopy. Compared to cytosine and 1-methylcytosine, the S0 → S1 spectrum of 1ECyt shows a progression in the out-of-plane "butterfly" mode ν 1 ′ , identified by spin-component scaled-second-order coupled-cluster method ab initio calculations. We also report time-resolved S1 state nonradiative dynamics at ∼20 ps resolution by the pump/delayed ionization technique. The S1 lifetime increases with the number of ν 1 ′ quanta from τ = 930 ps at v 1 ′ = 0 to 1030 ps at v 1 ′ = 2 , decreasing to 14 ps at 710 cm−1 vibrational energy. We measured the rate constants for S1 ⇝ S0 internal conversion and S1 ⇝ T1 intersystem crossing (ISC): At the v′ = 0 level, kIC is 8 × 108 s−1 or three times smaller than 1-methylcytosine. The ISC rate constant from v′ = 0 to the T1(3ππ*) state is kISC = 2.4 × 108 s−1, 10 times smaller than the ISC rate constants of cytosine, but similar to that of 1-methylcytosine. Based on the calculated S1(1ππ*) state radiative lifetime τrad = 12 ns, the fluorescence quantum yield of 1ECyt is Φfl ∼ 7% and the intersystem crossing yield is ΦISC ∼ 20%. We measured the adiabatic ionization energy of 1-ethylcytosine via excitation of the S1 state as 8.353 ± 0.008 eV, which is 0.38 eV lower than that of amino-keto cytosine. Measurement of the ionization energy of the long-lived T1(ππ*) state formed via ISC reveals that it lies 3.2–3.4 eV above the S0 ground state. [ABSTRACT FROM AUTHOR]

We have measured the intermolecular dissociation energies D0 of supersonically cooled 1-naphthol (1NpOH) complexes with solvents S = furan, thiophene, 2,5-dimethylfuran, and tetrahydrofuran. The naphthol OH forms non-classical H-bonds with the aromatic π-electrons of furan, thiophene, and 2,5-dimethylfuran and a classical H-bond with the tetrahydrofuran O atom. Using the stimulated-emission pumping resonant two-photon ionization method, the ground-state D0(S0) values were bracketed as 21.8 ± 0.3 kJ/mol for furan, 26.6 ± 0.6 kJ/mol for thiophene, 36.5 ± 2.3 kJ/mol for 2,5-dimethylfuran, and 37.6 ± 1.3 kJ/mol for tetrahydrofuran. The dispersion-corrected density functional theory methods B97-D3, B3LYP-D3 (using the def2-TZVPP basis set), and ωB97X-D [using the 6-311++G(d,p) basis set] predict that the H-bonded (edge) isomers are more stable than the face isomers bound by dispersion; experimentally, we only observe edge isomers. We compare the calculated and experimental D0 values and extend the comparison to the previously measured 1NpOH complexes with cyclopropane, benzene, water, alcohols, and cyclic ethers. The dissociation energies of the nonclassically H-bonded complexes increase roughly linearly with the average polarizability of the solvent, α ¯ (S). By contrast, the D0 values of the classically H-bonded complexes are larger, increase more rapidly at low α ¯ (S), but saturate for large α ¯ (S). The calculated D0(S0) values for the cyclopropane, benzene, furan, and tetrahydrofuran complexes agree with experiment to within 1 kJ/mol and those of thiophene and 2,5-dimethylfuran are ∼3 kJ/mol smaller than experiment. The B3LYP-D3 calculated D0 values exhibit the lowest mean absolute deviation (MAD) relative to experiment (MAD = 1.7 kJ/mol), and the B97-D3 and ωB97X-D MADs are 2.2 and 2.6 kJ/mol, respectively. [ABSTRACT FROM AUTHOR]