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In more than 60 years of research on molecular excitons, there has been extensive theoretical work but few experimental investigations have rigorously tested the predictions of exciton coupling theories. In centrosymmetric doubly H-bonded molecular dimers with identical chromophores, the S0-->S1 electronic transition dipole moments of the monomers combine in a parallel and antiparallel fashion, giving the S0-S1 and S0-->S2 transitions of the dimer. One of these is strictly symmetry-forbidden and the other fully allowed. Minimal perturbations such as 12C/13C or H/D isotopic substitution lift the symmetry restrictions sufficiently to render both transitions allowed. The excitonic (Davydov) splitting can then be measured as the energy difference between the respective vibrationless O0(0) bands. We have measured the mass-specific vibronic spectra of the centrosymmetric H-bonded dimers (2-pyridone)2 and (2-aminopyridine)2 that are supersonically cooled to a few K and isolated in molecular beams, using two-color resonant two-photon ionization spectroscopy. Comparison of the all-12C- and 13C- isotopomer spectra yield excitonic splittings of delta(exp) = 43.5 and 10.5 cm(-1), respectively. The corresponding splittings calculated by high-level ab initio methods (RI-CC2/aug-cc-pVTZ) are 20 to 50 times larger. These purely electronic ab initio exciton splittings need to be reduced ('quenched') by vibronic coupling to the optically active vibrational modes. Only after quenching are the experimentally observed exciton splittings correctly reproduced.

Among the weak intermolecular interactions found in proteins, the amide N--H...pi interaction has been widely observed but remains poorly characterized as an individual interaction. We have investigated the isolated supersonic-jet-cooled dimer of the cis-amide and nucleobase analogue 2-pyridone (2PY) with benzene and benzene-d6. Both MP2 and SCS-MP2 geometry optimizations yield a T-shaped structure with a N--H...pi hydrogen bond to the benzene ring and the C=O group above, but far from the C--H bonds of benzene. The CCSD(T) calculated binding energy at the optimum geometry is De = 25.2 kJ/mol (dissociation energy D0 = 21.6 kJ/mol), corresponding to the H-bond strength of the water dimer or of N--H...O hydrogen bonds. The T-shaped geometry is supported by the infrared-ultraviolet depletion spectra of 2PY x benzene: The N--H stretch vibrational frequency is lowered by 56 cm(-1), and the C=O stretch vibration is lowered by 10 cm(-1), relative to those of bare 2PY, indicating a strong N--H...pi interaction and a weak interaction of the C=O group. The benzene C--H infrared stretches exhibit very small shifts (approximately 2 cm(-1)) relative to benzene, signaling the absence of interactions with the benzene C--H groups. The infrared spectral shifts are consistent with a strong nonconventional pi hydrogen bond and a T-shaped structure for 2PY x benzene. Symmetry-adapted perturbation theory calculations show that the N--H...pi interaction is by far the dominant stabilization factor.

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]

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]

Spectral tuning effects on visible chromophores by hydrogen bonds are central to the chemistry of vision and of photosynthesis. A model for large spectral tuning effects by hydrogen bond switching is provided by the 7-hydroxyquinoline·HCOOH complex, which forms two isomers, CTN1 and CTN2, both with an HCOOH...N hydrogen bond but with different (quinoline)C–H...O==C hydrogen bonds. A 180° rotation of the HCOOH moiety around the O–H...N hydrogen bond exchanges the C–H...O hydrogen bonds, rotates the dipole moment of HCOOH, and leads to an ∼850 cm-1 shift of the electronic spectrum. Mass-selected S1←S0 resonant two-photon ionization, UV-UV holeburning, S1→S0 fluorescence spectra, and photoionization efficiency curves of the two 7-hydroxyquinoline·HCOOH isomers were measured in supersonic expansions. Comparison to ab initio calculations allow us to determine the H-bond connectivity and structure of the two isomers and to assign their inter- and intramolecular vibrations. The Franck-Condon factors of the intermolecular shear vibration χ in the S1←S0 spectra indicate that the weak C–H...O hydrogen bond contracts markedly in the CTN1 isomer but expands in the CTN2 isomer. These changes of H-bond lengths agree with the spectral shifts. In contrast, the strong O–H...N hydrogen bond undergoes little change upon S1←S0 excitation. [ABSTRACT FROM AUTHOR]

The 2-aminopyridine·2-pyridone (2AP·2PY) dimer is linked by N–H...O==C and N–H...N hydrogen bonds, providing a model for the Watson–Crick hydrogen bond configuration of the adenine·thymine and adenine·uracil nucleobase pairs. Mass-specific infrared spectra of 2AP·2PY and its seven N–H deuterated isotopomers have been measured between 2550 and 3650 cm-1 by IR laser depletion combined with UV two-color resonant two-photon ionization. The 2PY amide N–H stretch is a very intense band spread over the range 2700–3000 cm-1 due to large anharmonic couplings. It is shifted to lower frequency by 710 cm-1 or ≈20% upon H bonding to 2AP. On the 2AP moiety, the “bound” amino N–H stretch gives rise to a sharp band at 3140 cm-1, which is downshifted by 354 cm-1 or ≈10% upon H bonding to 2PY. The amino group “free” N–H stretch and the H–N–H bend overtone are sharp bands at ≈3530 cm-1 and 3320 cm-1. Ab initio structures and harmonic vibrations were calculated at the Hartree–Fock level and with the PW91 and B3LYP density functionals. The PW91/6-311++G(d,p) method provides excellent predictions for the frequencies and IR intensities of all the isotopomers.© 2004 American Institute of Physics. [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]

Hydrogen-bonded complexes of the photoacid 1-naphthol with NH[sub 3] and ND[sub 3] were investigated by resonant two-photon ionization, spectral hole burning, and fluorescence spectroscopies. Although the intermolecular vibrations are weak in both absorption and emission, with typical Franck–Condon factors <2% relative to the electronic origin, all six intermolecular modes were identified, namely the hydrogen bond stretch σ, the ammonia torsion τ, two in-plane wags β[sub 1] and β[sub 2], and two out-of-plane rocking motions ρ[sub 1] and ρ[sub 2]. Several ammonia torsional excitations were observed, with spacings in good agreement with the S[sub 0]- and S[sub 1] state effective torsional barriers derived by Humphrey and Pratt [J. Chem. Phys. 104, 8332 (1996)]. The β[sub 1], β[sub 2], and ρ[sub 2] vibrational excitations exhibit large (2–8 cm[sup -1]) torsional splittings, which indicate strong anharmonic coupling with the ammonia internal rotation. The observed Franck–Condon factors of the intermolecular stretching vibration imply a contraction of the O–H···N hydrogen bond by approx. 0.01 Å upon S[sub 1]←S[sub 0] excitation. © 2001 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]

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]

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]

We review experimental and theoretical investigations of excited-state hydrogen atom transfer (ESHAT) reactions along unidirectionally hydrogen bonded solvent ‘wire’ clusters. The solvent wire is attached to the aromatic ‘scaffold’ molecule 7-hydroxyquinoline (7HQ), which offers an O–H and an N hydrogen bonding site, spaced far enough apart to form two- to four-membered wires. S 1 &ThickSpace;←&ThickSpace; S 0 photoexcitation renders the O–H group more acidic and the quinolinic N more basic. This provides a driving force for the enol → keto tautomerization, probed by the characteristic fluorescence of the 7-ketoquinoline in the molecular beam experiments. For 7-hydroxyquinoline·(NH 3 ) 3 , excitation of ammonia-wire vibrations induces the tautomerization at &nvsim;200&ThickSpace;cm -1 . Different reaction pathways have been explored by excited-state ab initio calculations. These show that the reaction proceeds by H-atom transfer along the wire as a series of Grotthus-type translocation steps. There is no competition with a mechanism involving successive proton translocations. The rate-controlling S 1 state barriers arise from crossings of a π π* with a Rydberg-type πσ* state and the proton and electron movements along the wire are closely coupled. The excited state reactant, H-transferred intermediates and product structures are characterized. The reaction proceeds by tunnelling, as shown by deuteration of the solvent molecules (ND 3 ) in the wire. The first step of the reaction exhibits intra/intermolecular vibrational mode selectivity. Substitution of NH 3 by one, two or three H 2 O molecules in the wire leads to increasing threshold with each additional H 2 O molecule, up to >2000&ThickSpace;cm -1 for the 7-hydroxyquinoline·(H 2 O) 3 water-wire cluster. No 7-ketoquinoline fluorescence is observed upon insertion of even a single H 2 O molecule. The calculations show that insertion of each H 2 O molecule into the solvent wire introduces a high barrier, which blocks any further H-atom transfer. [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]

We report an accurate ab initio study of the effects of chirality on the intermolecular interactions between two small chiral molecules bound by a single hydrogen bond. The methods used are second-order Møller-Plesset theory (MP2), as well as density functional theory with the B3LYP functional. The differential interaction energy between two homochiral molecules, e.g., R···R[sup ′] and the analogous heterochiral molecules R···S[sup ′] measures the degree of chiral discrimination, termed the chirodiastaltic energy, ΔE[sub chir]. Formation of the O-H···O hydrogen bond between the chiral H-bond donor HOOH and the chiral H acceptor 2-methyl oxirane leads to four diastereomeric complexes. There are two distinct contributions to the chirodiastaltic energies, the diastereofacial contribution which controls the face or side of the acceptor to which the H bond is formed, and the diastereomeric contribution, which is the energy difference between two complexes formed by (M)- and (P)-HOOH to the same face. The largest chirodiastaltic energy is ΔE[sub chir]=0.46 kcal/mol (6% of the binding energy) between the syn-(M)- and syn-(P)-HOOH·2-methyl oxirane complexes. The chiral 2,3-dimethyloxirane acceptor is C[sub 2] symmetric and hence offers two identical faces. Here the chirodiastaltic energy is identical to the diastereomeric energy, and is calculated to be ΔE[sub chir]=0.36 kcal/mol or 4.5% of the binding energy. © 2000 American Institute of Physics. [ABSTRACT FROM AUTHOR]

The 2-pyridone dimer, (2PY)[sub 2], which is linked by two antiparallel N-H···O hydrogen bonds, is a model for hydrogen bonded nucleotide base pairs, e.g., the uracil dimer. Mass- and isomer-selected S[sub 2]←S[sub 0] vibronic spectra of supersonically cooled (2PY)[sub 2] were measured by laser two-color resonant two-photon ionization and UV/UV-holeburning techniques. The latter allows the identification of the spectrum of the 2-pyridone·2-hydroxypyridine mixed dimer, present at approx. 5% relative concentration. S[sub 2]→S[sub 0] fluorescence emission spectra show dominantly the hydrogen bond shearing vibration ν[sub 4][sup ″] at 98.5 cm[sup -1] and the stretching vibration ν[sub 6][sup ″] at 163.5 cm[sup -1]. The hydrogen bond stretching vibration force constant was determined to be 75.4 N/m, or 37.7 N/m per hydrogen bond, a very high value. The ν[sub 2][sup ″] (a[sub u]) torsional and the ν[sub 3][sup ″] (b[sub u]) slanting vibrations were also identified. Ground state structures, rotational constants, harmonic intermolecular and intramolecular vibrational frequencies, interaction, and dissociation energies were calculated using Hartree-Fock and density functional (B3LYP) methods. The B3LYP/6-311++(2d,2p) results are in excellent agreement with all experimental observations. © 2000 American Institute of Physics. [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 excitonic S1/S2state splitting and thelocalization/delocalization of the S1and S2electronic states are investigated in the benzonitrile dimer (BN)2and its 13C and d5isotopomers by mass-resolved two-color resonant two-photon ionizationspectroscopy in a supersonic jet, complemented by calculations. Thedoubly hydrogen-bonded (BN-h5)2and (BN-d5)2dimers are C2hsymmetric with equivalentBN moieties. Only the S0→ S2electronicorigin is observed, while the S0→ S1excitonic component is electric-dipole forbidden. A single 12C/13C or 5-fold h5/d5isotopic substitution reduce thedimer symmetry to Cs,so that the heteroisotopic dimers (BN)2-(h5– h513C), (BN)2-(h5– d5), and (BN)2-(h5– h513C) exhibitboth S0→ S1and S0→S2origins. Isotope-dependent contributions Δisoto the excitonic splittings arise from the changes of theBN monomer zero-point vibrational energies; these range from Δiso(12C/13C) = 3.3 cm–1to Δiso(h5/d5) = 155.6 cm–1. The analysisof the experimental S1/S2splittings of sixdifferent isotopomeric dimers yields the S1/S2exciton splitting Δexc= 2.1 ± 0.1 cm–1. Since Δiso(h5/d5) ≫ Δexcand Δiso(12C/13C) > Δexc, complete and near-complete exciton localization occursupon 12C/13C and h5/d5substitutions, respectively, as diagnosedby the relative S0→ S1and S0→ S2origin band intensities. The S1/S2electronic energy gap of (BN)2calculatedby the spin-component scaled approximate second-order coupled-cluster(SCS-CC2) method is Δelcalc= 10 cm–1. This electronicsplitting is reduced by the vibronic quenching factor Γ. Thevibronically quenched exciton splitting Δelcalc·Γ = Δvibroncalc= 2.13cm–1is in excellent agreement with the observedsplitting Δexc= 2.1 cm–1. Theexcitonic splittings can be converted to semiclassical exciton hoppingtimes; the shortest hopping time is 8 ps for the homodimer (BN-h5)2, the longest is 600 ps for the(BN)2(h5– d5) heterodimer. [ABSTRACT FROM AUTHOR]

Studies the hydrogen bond vibrations of 2-aminopyridine-2-pyridone complex, a Watson-Crick analogue of adenine-uracil. Ab initio results; Resonant two-photon ionization spectra; Geometry and force constant change upon electronic excitation.

NH···πhydrogen bonds occur frequently between the amino acid side groupsin proteins and peptides. Data-mining studies of protein crystalsfind that ∼80% of the T-shaped histidine···aromaticcontacts are CH···π, and only ∼20% areNH···π interactions. We investigated the infrared(IR) and ultraviolet (UV) spectra of the supersonic-jet-cooled imidazole·benzene(Im·Bz) complex as a model for the NH···πinteraction between histidine and phenylalanine. Ground- and excited-statedispersion-corrected density functional calculations and correlatedmethods (SCS-MP2 and SCS-CC2) predict that Im·Bz has a Cs-symmetric T-shaped minimum-energystructure with an NH···π hydrogen bond to theBz ring; the NH bond is tilted 12° away from the Bz C6axis. IR depletion spectra support the T-shaped geometry:The NH stretch vibrational fundamental is red shifted by −73cm–1relative to that of bare imidazole at 3518cm–1, indicating a moderately strong NH···πinteraction. While the S0(A1g) → S1(B2u) origin of benzeneat 38 086 cm–1is forbidden in the gas phase,Im·Bz exhibits a moderately intense S0→ S1origin, which appears viathe D6h→ Cssymmetry lowering of Bz byits interaction with imidazole. The NH···π ground-statehydrogen bond is strong, De=22.7 kJ/mol (1899 cm–1). The combinationof gas-phase UV and IR spectra confirms the theoretical predictionsthat the optimum Im·Bz geometry is T shaped and NH···πhydrogen bonded. We find no experimental evidence for a CH···πhydrogen-bonded ground-state isomer of Im·Bz. The optimum NH···πgeometry of the Im·Bz complex is very different from the majorityof the histidine·aromatic contact geometries found in proteindatabase analyses, implying that the CH···π contactsobserved in these searches do not arise from favorable binding interactionsbut merely from protein side-chain folding and crystal-packing constraints.The UV and IR spectra of the imidazole·(benzene)2clusterare observed via fragmentation into the Im·Bz+masschannel. The spectra of Im·Bz and Im·Bz2arecleanly separable by IR hole burning. The UV spectrum of Im·Bz2exhibits two 000bands corresponding to the S0→ S1excitations of the two inequivalentbenzenes, which are symmetrically shifted by −86/+88 cm–1relative to the 000band of benzene. [ABSTRACT FROM AUTHOR]

Mass-and isomer-selected infrared spectra of 2-pyridone·(NH3)nclusters with n=1–3 were measured in the NH and CH stretch fundamental region(2400–3700 cm–1) using infrared (IR) laserdepletion spectroscopy combined with resonant two-photon ionizationUV laser detection. The IR depletion spectra reveal three differentH-bonding topologies of these clusters: The n= 1and 2 clusters form ammonia bridges stretching from the N–Hto the CO group of the cis-amide functionof 2-pyridone (2PY), giving rise to intense and strongly red-shifted(2PY)NH and ammonia NH stretch bands. For n= 3,two isomers (3X and 3Y) are observed in the IR spectra: The spectrumof 3X is compatible with an ammonia-bridge structure like n= 2, with the third NH3accepting an H-bondfrom C6–H of 2PY. The IR spectrum of 3Y exhibitsa broad IR band in the 2500–3000 cm–1rangeand is characteristic of a bifurcated double-bridged structure inwhich the first NH3 accepts an H-bond from the (2PY)NH and donatestwo H-bonds to the other two ammonias, both of which donate to theCO group of 2PY. This double-donor/double-bridge H-bondingpattern increases the acceptor strength of the first ammonia and dramaticallylowers the (2PY)NH stretching frequency to ∼2700 cm–1. For all clusters the ammonia 2ν4HNH bend overtonesin the 3180–3320 cm–1region gain intensityby anharmonic coupling (Fermi resonance) to the hydrogen-bonded ammoniaNH stretches, which are red-shifted into the 3250–3350 cm–1region. The experimental results are supported byoptimized structures, vibrational frequencies, and IR intensitiescalculated using density-functional theory with the B3LYP and PW91functionals, as well as with the more recent functionals B97-D andM06-2X, which are designed to include long-range dispersive interactions. [ABSTRACT FROM AUTHOR]