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Clusters of tetracene molecules with different numbers of attached (Ar)N, (Ne)N and (H2)N particles (N = 1-2000) are assembled inside superfluid He nanodroplets and studied via laser-induced fluorescence. The frequency shift of the fluorescence spectrum of the tetracene molecules is studied as a function of cluster size and pickup order of tetracene and cluster species. For (Ar)N and (Ne)N clusters, our results indicate that the tetracene molecules reside inside the clusters when tetracene is captured by the He nanodroplet before the cluster species; conversely, the tetracene molecules stay on the surface of the clusters when tetracene is captured after the cluster species. In the case of (H2)N clusters, however, tetracene molecules reside inside the (H2)N clusters irrespective of the pickup order. We conclude that (Ar)N and (Ne)N clusters are rigid at T = 0.38 K, while (H2)N clusters of up to N = 2000 remain fluxional at the same temperature. The results may also indicate the occurrence of heterogeneous nucleation of the (H2)N clusters, which is induced by the interaction with tetracene chromophore molecules. [ABSTRACT FROM AUTHOR]

Computer simulations and experiments have been performed to quantify the effects of nozzle shape and skimmer placement on high-density supersonic jets. It is shown that the on axis beam intensity achieved is much higher than intensity achieved using standard sonic nozzles. Changes in skimmer design and positioning are required to allow this intense jet to propagate in a typical supersonic beam setup. [ABSTRACT FROM AUTHOR]

We present an ab initio study of the effect of spin-orbit coupling on the dynamical properties of the Tl-(0001 ) surface as well as on the electron-phonon interaction at the surface. The calculations based on density-functional theory were carried out using a linear response approach and a mixed-basis pseudopotentia] method. It is shown that the spin-orbit effects on the phonon spectrum and the electron-phonon interaction at the Fermi level of the surface are weak but conspire to a reduction in the electron-phonon coupling. [ABSTRACT FROM AUTHOR]

A simple expression for the asymptotic exchange energy of heteronuclear dimers is derived from the surface integral method. A five-dimensional hypersurface, consisting of all spherical surfaces centered at the nucleus of the atom with higher ionization energy, more appropriate for the case where the two atoms have different ionization energies, is used in the surface integral. All integrals are carried out analytically. Compared with the exchange energy of Smirnov and Chibisov, which is also obtained from the surface integral method with a hypersurface consisting of all infinite planes perpendicular to the internuclear axis, the present result is much simpler. The exchange energies of alkali hydrides are computed as an illustration. It is shown that the present method and the method of Smirnov and Chibisov are complementary to each other. [ABSTRACT FROM AUTHOR]

The utility of a continuous beam of He droplets for the assembly and surface deposition of AgN clusters, (N) ~ 300-6000, is studied with transmission electron microscopy. Images of the clusters on amorphous carbon substrates obtained at short deposition times have provided for a measure of the size distribution of the metal clusters. The average sizes of the deposited clusters are in good agreement with an energy balance based estimate of AgN cluster growth in He droplets. Measurements of the deposition rate indicate that upon impact with the surface the He-embedded cluster is attached with high probability. The stability of the deposited clusters on the substrate is discussed. [ABSTRACT FROM AUTHOR]

F1-ATPase is the catalytic domain of F1Fo-ATP synthase and consists of a hexameric arrangement of three noncatalytic α and three catalytic ß subunits. We have used unbiased molecular dynamics simulations with a total simulation time of 900 ns to investigate the dynamic relaxation properties of isolated β-subunits as a step toward explaining the function of the integral F1 unit To this end, we simulated the open (βE) and the closed (βTP) conformations under unbiased conditions for up to 120 ns each using several samples. The simulations confirm that nucleotide-free ßE retains its open configuration over the course of the simulations. The same is true when the neighboring α subunits are included. The nucleotide-depleted as well as the nucleotide-bound isolated ßTP subunits show a significant trend toward the open conformation during our simulations, with one trajectory per case opening completely. Hence, our simulations suggest that the equilibrium conformation of a nudeotide-free β-subunit is the open conformation and that the transition from the closed to the open conformation can occur on a time scale of a few tens of nanoseconds. [ABSTRACT FROM AUTHOR]

Recent experiments (Wang et al., 2010) have found evidence of phase transitions of gases adsorbed on a single carbon nanotube. In order to understand the observations, we have carried out classical grand canonical Monte Carlo simulations of this system, for the cases of Ar and Kr on zigzag and armchair nanotubes with radius R > 0.7 nm. The calculated behavior resembles the experimental results in the case of Ar. However, the prominent, ordered phase found for Kr in both simulations and (classical) energy minimization calculations differs from that deduced from the experimental data. A tentative explanation of the apparent discrepancy is that the experiments involve a nanotube of rather large radius (>1.5 nm). [ABSTRACT FROM AUTHOR]

A stochastic theory is formulated for the sticking probability of a projectile scattered from a surface. The theory is then explored by applying it to a generalized Langevin equation model of the scattering dynamics. The theory succeeds in describing the known features of trapping on surfaces. At low energies sticking will occur only if there is an attractive interaction between the projectile and the surface. The probability of sticking at low energies is greater the lower the temperature and the deeper the attractive well of the particle as it approaches the surface. The sticking probability in the absence of horizontal friction tends to be lower as the stillness of the surface increases. However, in the presence of horizontal friction, increased stiffness may lead to an increase in the sticking coefficient. A cos2(θi) scaling is found only in the absence of corrugation and horizontal friction. The theory is then applied successfully to describe experimentally measured sticking probabilities for the scattering of Xe on a Pt(111) surface. [ABSTRACT FROM AUTHOR]

The method of local increments is used in connection with an embedded cluster approach and wave function based quantum chemical ab initio methods to describe the adsorption of a single CO molecule on the MgO(001) surface. The first step in this approach is a conventional Hartree-Fock calculation. The occupied orbitals are then localized by means of the Foster-Boys localization procedure, and the full system is decomposed into several "subunits" that consist of the orbitals localized at the CO molecule and at the Mg and O atoms of the MgO cluster. The correlation energy is expanded into a series of local n-body increments that are evaluated separately and independently. In this way, big savings in computer time can be achieved because (a) the treatment of a large system is replaced with a series of much faster calculations for small subsystems and (b) the big basis sets necessary for describing dispersion effects are only needed for the atoms in the respective subsystem while all other atoms can be treated by medium size Hartree-Fock type basis sets. The coupled electron pair approach, CEPA, an approximate coupled cluster method, is used to calculate the correlation energies of the various subsystems. For the vertical adsorption of CO on top a Mg atom of the MgO(001 ) surface with the C atom toward Mg, the individual one- and two-body increments are calculated as functions of the CO-MgO separation and a full potential energy curve is constructed from them. A very shallow minimum with an adsorption energy of 0.016 eV at a Mg-C distance of 3.04 Å is found at the Hartree -Fock level, while inclusion of correlation (dispersion) effects shortens the Mg-C distance to 2.59 Å and yields a much larger adsorption energy of 0.124 eV. This is in very good agreement with the best experimental value of 0.14 eV. The basis set superposition error, BSSE, was fully corrected for by the counterpoise method and the bonding mechanism was analyzed at the Hartree-Fock level by means of the constrained space orbital variation, CSOV, analysis. [ABSTRACT FROM AUTHOR]

We develop a model, based on pairwise additive He-Mg and He-O interactions, for the potential energy of He adsorbates above a rigid MgO(100) surface. The attractive long-range He-Mg and He-O interactions are assumed to have the form C6/r6 with the C6 coefficients determined from atomic data within the context of the Slater-Kirkwood approximation. The repulsive short-range He-Mg and He-O interactions are assumed to have the form Cp/rp, with the exponent p and the Cp coefficients taken as adjustable parameters. We find that for p = 9, the Cp coefficients can be chosen so that the laterally averaged He-MgO(100) pairwise additive interaction supports low-lying selective adsorption states, some of whose energies agree very well with the states' apparent energies inferred from experimental measurements. However, for realistic values of the adjustable parameters that define our model, the lateral corrugation of the model pairwise additive He-MgO(100) potential energy surface far exceeds the corrugation that has been inferred both from experimental measurements and from density functional calculations of the short-range He-MgO(100) interaction. [ABSTRACT FROM AUTHOR]

There have been a few experimental reports of self-assembled adlayers of bis-fullerene molecules on solid substrates. Most of these studies suggest the adsorbate molecules are lying down on the surface, with the fullerene moieties almost close packed. However, very little theoretical work has been carried out on such systems, and little is known about the roles played by different parts of the potential energy in driving the self-assembly. We carry out a Temperature Replica Exchange Monte Carlo study here of two representative bis-fullerene molecules on a metal substrate. We use a coarse-grained model potential energy function, in which certain parameters can be varied within the range of their experimental uncertainty. The molecules investigated consist of two fullerene moieties bonded by a rigid bridging group. In particular, the effect of the strength of the fullerene interaction EFG with the substrate (nominally Au(111)) has been investigated in detail. To ensure efficient sampling of the rugged potential energy surfaces encountered in the simulations, we utilize replica exchange techniques. These enable us to construct free energy landscapes for the system. We find that for relatively low values of EFG the molecules form standing-up adlayers. By contrast, for higher values of EFG lying-down adlayers dominate. For one molecule, two different crystalline adlayer morphologies have been identified. The detailed structure of the lying-down layer is a function of the temperature and of the group used to bridge the fullerene moieties. [ABSTRACT FROM AUTHOR]

The use of damping functions to correct the multipole expansion at short-range is explored. Damping functions for the terms in the multipole expansion can be determined ab initio as a linear combination of analytic functions of the separation between sites, but there are additional short-range terms that have different angular dependence. The approach provides a detailed ab initio description of the penetration energy correction to the multipole expansion in an easily comprehensible form. [ABSTRACT FROM AUTHOR]

We present potential energy curves calculated at the CCSD(T) level of theory for Ga+-RG and In+ -RG complexes (RG = He-Rn). Spectroscopic parameters have been derived from these potentials and compared to previously calculated parameters for the Al+-RG and Tl+-RG complexes. Additionally, for some cases, we compare these parameters with those obtained from electronic spectroscopic studies on excited states of the neutral species, arising from atomic-based d ← p excitations. The Ga+-RG and In+-RG potentials have also been used to calculate the transport coefficients for M+ traveling through a bath of RG atoms. [ABSTRACT FROM AUTHOR]

It is shown that a commensurate square monolayer solid of Kr/NaCl(001) can be stabilized with a model incorporating a rather large energy corrugation amplitude. Then a square bilayer is formed under a compression and preempts the formation of an incommensurate triangular monolayer lattice. The lattice dynamics of the commensurate monolayer may be complex, because the modeling admits the possibility that it has a (2 x 2) unit cell with four Kr atoms. [ABSTRACT FROM AUTHOR]

Infrared laser spectroscopy was used to probe the unique rotational dynamics of the HCN−M (M Na, K, Rb, Cs) complexes formed on the surface of helium droplets. The 1CH stretch ro-vibrational spectra were measured revealing what appears to be the Pand Rcontours of a nearly rigid linear rotor. To simulate the linear molecule spectra, given a rotational temperature of 0.37 K, effective moments of inertia, IB, were required to be 104−105amu·Å2larger than the ab initio predicted values. The large moments of inertia were found to be strongly dependent on both the mass of the complex and the size of the helium droplet, consistent with a model where the dopant is located in a dimple site on the surface of the droplet. In this model, the moment of inertia is representative of the rotational motion of the dopant on the surface about an inertial axis through the center of the droplet. [ABSTRACT FROM AUTHOR]

Infrared−infrared double resonance spectroscopy is used to probe the vibrational dynamics of molecular complexes solvated in helium nanodroplets. We report results for the acetylene−HCN and cyanoacetylene−HCN binary complexes, each having two stable isomers. We find that vibrational excitation of an acetylene−HCN complex results in a population transfer to the other isomer. Photoinduced isomerization is found to be dependent on both the initially excited vibrational mode and the identity of the acetylene−HCN isomer. However, population transfer is not observed for the cyanoacetylene−HCN complexes. The results are rationalized in terms of the ab initio intermolecular potential energy surfaces for the two systems with particular emphasis on the long-range barriers to rearrangement. [ABSTRACT FROM AUTHOR]

Rovibronic energy levels and properties of the Br(2P)−HCN complex were obtained from three-dimensional calculations, with HCN kept linear and the CN bond frozen. All diabatic states that correlate to the 2P32and 2P12states of the Br atom were included and spin−orbit coupling was taken into account. The 3 × 3 matrix of diabatic potential surfaces was taken from the preceding paper (paper 1). In agreement with experiment, we found two linear isomers, Br−NCH and Br−HCN. The calculated binding energies are very similar:  D0352.4 cm-1and D0349.1 cm-1, respectively. We established, also in agreement with experiment, that the ground electronic state of Br−NCH has (1/2) and that Br−HCN has a ground state with (3/2), where the quantum number, , is the projection of the total angular momentum, J, of the complex on the intermolecular axis R. This picture can be understood as being caused by the electrostatic interaction between the quadrupole of the Br(2P) atom and the dipole of HCN, combined with the very strong spin−orbit coupling in Br. We predicted the frequencies of the van der Waals modes of both isomers and found a direct Renner−Teller splitting of the bend mode in Br−HCN and a smaller, indirect, splitting in Br−NCH. The red shift of the CH stretch frequency in the complex, relative to free HCN, was calculated to be 1.98 cm-1for Br−NCH and 23.11 cm-1for Br−HCN, in good agreement with the values measured in helium nanodroplets. Finally, with the use of the same potential surfaces, we modeled the Cl(2P)−HCN complex and found that the experimentally observed linear Cl−NCH isomer is considerably more stable than the (not observed) Cl−HCN isomer. This was explained mainly as an effect of the substantially smaller spin−orbit coupling in Cl, relative to Br. [ABSTRACT FROM AUTHOR]

The three adiabatic potential surfaces of the Br(2P)−HCN complex that correlate to the 2P ground state of the Br atom were calculated ab initio. With the aid of a geometry-dependent diabatic mixing angle, also calculated ab initio, these adiabatic potential surfaces were transformed into a set of four diabatic potential surfaces required to define the full 3 × 3 matrix of diabatic potentials. Each of these diabatic potential surfaces was expanded in terms of the appropriate spherical harmonics in the atom-linear molecule Jacobi angle . The dependence of the expansion coefficients on the distance Rbetween Br and the HCN center of mass and on the CH bond length was fit to an analytic form. For HCN in its equilibrium geometry, the global minimum with De800.4 cm-1and Re6.908a0corresponds to a linear Br−NCH geometry, with an electronic ground state of symmetry. A local minimum with De415.1 cm-1, Re8.730a0, and a twofold degenerate ground state is found for the linear Br−HCN geometry. The binding energy, De, depends strongly on the CH bond length for the Br−HCN complex and much less strongly for the Br−NCH complex, with a longer CH bond giving stronger binding for both complexes. Spin−orbit coupling was included and diabatic states were constructed that correlate to the ground 2P32and excited 2P12spin−orbit states of the Br atom. For the ground spin−orbit state with electronic angular momentum j(3/2) the minimum in the potential for projection quantum number ±(3/2) coincides with the local minimum for linear Br−HCN of the spin-free case. The minimum in the potential for projection quantum number ±(1/2) occurs for linear Br−NCH but is considerably less deep than the global minimum of the spin-free case. According to the lowest spin−orbit coupling included adiabatic potential the two linear isomers, Br−NCH and Br−HCN, are about equally stable. In the subsequent paper, we use these potentials in calculations of the rovibronic states of the Br−HCN complex. [ABSTRACT FROM AUTHOR]

We report accurate quantum three-dimensional calculations of highly excited intermolecular vibrational states of the van der Waals (vdW) complexes tetracene·He and pentacene·He in the S1excited electronic state. The aromatic molecules were taken to be rigid and the intermolecular potential energy surfaces (IPESs) were modeled as a sum of atom−atom Lennard-Jones pair potentials. The IPESs are corrugated in the direction of the long (x) axis of the aromatic molecules, due to the presence of the symmetrically equivalent global double minimum for tetracene·He, and a triple minimum (central global minimum and two equivalent local minima) for pentacene·He, on each side of the aromatic plane. Both IPESs have two additional minor equivalent local minima further away from the center of the molecule. The vdW vibrational states analyzed in this work cover about 80% of the well depths of the IPESs. The mode coupling is generally weak for those states whose out-of-plane (z) mode is unexcited. However, the z-mode fundamental is strongly coupled to the short-axis (y) in-plane mode, so that the pure z-mode excitation could not be identified. The He atom exhibits large in-plane spatial delocalizaton already in the ground vdW vibrational state, which increases rapidly upon the excitation of the in-plane xand ymodes, with little hindrance by the corrugation of the aromatic microsurfaces. For the vdW vibrational energies considered, the He atom spatial delocalization reaches xand yvalues of ∼5 and 4 Å, respectively, and is limited only by the finite size of the aromatic substrates. Side-crossing delocalization of the wave functions on both sides of the molecular plane is found at excitation energies >30 cm-1, giving rise to the energy splittings of the pairs of states symmetric/antisymmetric with respect to the aromatic plane; the splittings show strong vdW vibrational mode specificity. [ABSTRACT FROM AUTHOR]

Electronic absorption spectra of bare and methanol-solvated radical anions of benzophenone ((C6H5)2CO) and acetophenone ((C6H5)CH3CO) were measured by monitoring the photodetachment efficiency in the gas phase. Strong absorption bands due to autodetachment after transitions to bound excited states were observed. Stepwise spectral shifts approaching the limit of the condensed phase spectra were found to occur as the cluster size increases. In the case of benzophenone radical anion, the solvation of two methanol molecules exhibits the near convergence to the limit, representing the full coordination with the solvent molecules around the carbonyl group. For the acetophenone case, the coordination number was not apparently determined because of their relatively small shifts. Relationships between hydrogen bonding and electronic structure are analyzed for the spectral shifts with the aid of calculations based on density functional theory. The calculational results show that the coordination angle of the solvent molecule is affected mostly by steric hindrance around the carbonyl group, and that there is no evidence for reorientation due to specific hydrogen bonding interaction with the singly occupied orbital, which has been formerly persisted for an interpretation of the transient absorption following pulse radiolysis in alcoholic solutions. An alternative possibility involving deformation with respect to intramolecular coordinates is discussed. [ABSTRACT FROM AUTHOR]

The rotational excitation spectrum, including the vibrational shift of the rotational band, of several CO isotopomers solvated in He clusters has been calculated. Reptation quantum Monte Carlo simulations are used in conjunction with an accurate He−CO potential energy surface, which quantitatively describes the rovibrational spectrum of the binary complex. Our simulations, when compared with number-selective infrared spectra taken for different isotopomers, help discriminate among the alternative assignments proposed for cluster sizes around 15 He atoms. The origin of the vibrational band has a red shift that is nearly linear with the cluster size within the first solvation shell and is almost constant up to the largest cluster studied, well beyond completion of the second solvation shell. A blue upturn at even larger sizes would be needed to attain the nanodroplet limit, as recently estimated from the isotopic dependence of the measured R(0) transitions. [ABSTRACT FROM AUTHOR]

We studied proposed steps for the enzymatic formation of -carboxyglutamic acid by density functional theory (DFT) quantum chemistry. Our results for one potentially feasible mechanism show that a vitamin K alkoxide intermediate can abstract a proton from glutamic acid at the -carbon to form a carbanion and vitamin K epoxide. The hydrated carbanion can then react with CO2to form -carboxyglutamic acid. Computations at the B3LYP/6-311G level were used to determine the intermediates and transition states for the overall process. The activation free energy for the gas-phase path is 22 kcal/mol, with the rate-limiting step for the reaction being the attack of the carbanion on CO2. Additional solvation studies, however, indicate that the formation of the carbanion step can be competitive with the CO2attack step in high-dielectric systems. We relate these computations to the entire vitamin K cycle in the blood coagulation cascade, which is essential for viability of vertebrates. [ABSTRACT FROM AUTHOR]

Relative integrated cross sections are measured for spin−orbit-conserving, rotationally inelastic scattering of NO (212), hexapole-selected in the upper -doublet level of the ground rotational state (j0.5), in collisions with D2at a nominal energy of 551 cm-1. The final state of the NO molecule is detected by laser-induced fluorescence (LIF). The state-selected NO molecule is oriented with either the N end or the O end toward the incoming D2molecule by application of a static electric field Ein the scattering region. This field is directed parallel or antiparallel to the relative velocity vector v. Comparison of signals taken for the different applied field directions gives the experimental steric asymmetry SA, defined by SA (v↑↓E− v↑↑E)/(v↑↓Ev↑↑E), which is equal to within a factor of −1 to the molecular steric effect, Si→f≡ (D2→NO− D2→ON)/(D2→NOD2→ON). The dependence of the integral inelastic cross section on the incoming -doublet component is also measured as a function of the final rotational (jfinal) and -doublet (final) state. The measured steric asymmetries are similar to those previously observed for NO−He scattering. Spin−orbit manifold-conserving collisions exhibit a larger propensity for parity conservation than their NO−He counterparts. The results are interpreted in the context of the recently developed quasi-quantum treatment (QQT) of rotationally inelastic scattering Gijsbertsen, A.; Linnartz, H.; Taatjes, C. A.; Stolte, S. J. Am. Chem. Soc.2006, 128, 8777. The QQT predictions can be inverted to obtain a fitted hard-shell potential that reproduces the experimental steric asymmetry; this fitted potential gives an empirical estimate of the anisotropy of the repulsive interaction between NO and D2. QQT computation of the differential cross section using this simple model potential shows reasonable agreement with the measured differential cross sections. [ABSTRACT FROM AUTHOR]

We have recorded the S1← S0electronic spectra of Biphenylene and its Ar and O2van der Waals complexes inside helium nanodroplets using beam depletion detection. In general, the spectrum is similar to the previously reported high-resolution REMPI spectrum. The zero phonon lines, however, are split similar to the previously reported tetracene case. The calculated potential energy surface predicts that helium atoms can simultaneously occupy all equivalent global minima positions. Therefore, it appears that the splitting cannot be explained either by different isomers or by tunneling. Furthermore, surprisingly the splitting is retained for the Ar van der Waals complexes (and possibly for the O2complex as well). This case suggests that the current models of the origin of zero phonon line splitting and the helium solvation are incomplete. [ABSTRACT FROM AUTHOR]

The hole-burning (HB) spectra of phenol−Arn(PhOH−Arn) clusters with n1 and 2 have been measured in a molecular beam to clarify the possible existence of isomers. Two species were identified to give rise to signals in the S1−S0spectrum recorded for the n1 cluster; however, one of the species was found to originate from dissociation of an n2 cluster. Similarly, three species were observed in the spectrum of the n2 cluster, and two of them were assigned to n3 and larger clusters. The spectral contamination from larger size clusters was quantitatively explained by the dissociation after photoexcitation. The analysis of the spectra demonstrates that only a single isomer exists in the molecular beam for both the n1 and the n2 clusters. In addition to two previously detected intermolecular modes, a third low-frequency mode, assigned to an intermolecular bending vibration, is observed for the first time in the HB spectrum of the n2 cluster. The assignments of the intermolecular vibrations were confirmed by ab initio MO calculations. The observation of the third intermolecular vibration suggests that the geometry of the n2 cluster has Csor lower symmetry. [ABSTRACT FROM AUTHOR]

In support of mass-selected infrared photodissociation (IRPD) spectroscopy experiments, coupled-cluster methods including all single and double excitations (CCSD) and a perturbative contribution from connected triple excitations CCSD(T) have been used to study the V(H2O) and ArV(H2O) complexes. Equilibrium geometries, harmonic vibrational frequencies, and dissociation energies were computed for the four lowest-lying quintet states (5A1, 5A2, 5B1, and 5B2), all of which appear within a 6 kcal mol-1energy range. Moreover, anharmonic vibrational analyses with complete quartic force fields were executed for the 5A1states of V(H2O) and ArV(H2O). Two different basis sets were used:  a Wachtersf V8s6p4d1f basis with triple- plus polarization (TZP) for O, H, and Ar; and an Ahlrichs QZVPP V11s6p5d3f2g and Ar9s6p4d2f1g basis with aug-cc-pVQZ for O and H. The ground state is predicted to be 5A1for V(H2O), but argon tagging changes the lowest-lying state to 5B1for ArV(H2O). Our computations show an opening of 2°−3° in the equilibrium bond angle of H2O due to its interaction with the metal ion. Zero-point vibrational averaging increases the effectivebond angle further by 2.0°−2.5°, mostly because of off-axis motion of the heavy vanadium atom rather than changes in the water bending potential. The total theoretical shift in the bond angle of about 4° is significantly less than the widening near 9° deduced from IRPD experiments. The binding energies (D0) for the successive addition of H2O and Ar to the vanadium cation are 36.2 and 9.4 kcal mol-1, respectively. [ABSTRACT FROM AUTHOR]

The state-to-state vibrational predissociation (VP) dynamics of the hydrogen-bonded ammonia−acetylene dimer were studied following excitation in the asymmetric CH stretch. Velocity map imaging (VMI) and resonance-enhanced multiphoton ionization (REMPI) were used to determine pair-correlated product energy distributions. Following vibrational excitation of the asymmetric CH stretch fundamental, ammonia fragments were detected by 2 1 REMPI via the 1E‘ ‘ ← 1A1‘ and ‘1A1‘ ← 1A1‘ transitions. The fragments'' center-of-mass (c.m.) translational energy distributions were determined from images of selected rotational levels of ammonia with one or two quanta in the symmetric bend (2umbrella mode) and were converted to rotational-state distributions of the acetylene co-fragment. The latter is always generated with one or two quanta of bending excitation. All the distributions could be fit well when using a dimer dissociation energy of D0900 ± 10 cm-1. Only channels with maximum translational energy <150 cm-1are observed. The rotational excitation in the ammonia fragments is modest and can be fit by temperatures of 150 ± 50 and 50 ± 20 K for 12and 22, respectively. The rotational distributions in the acetylene co-fragment pair-correlated with specific rovibrational states of ammonia appear statistical as well. The vibrational-state distributions, however, show distinct state specificity among channels with low translational energy release. The predominant channel is NH3(12) C2H2(24or 1415), where 4and 5are the trans- and cis-bend vibrations of acetylene, respectively. A second observed channel, with much lower population, is NH3(22) C2H2(14). No products are generated in which the ammonia is in the vibrational ground state or the asymmetric bend (14) state, nor is acetylene ever generated in the ground vibrational state or with CC stretch excitation. The angular momentum (AM) model of McCaffery and Marsh is used to estimate impact parameters in the internal collisions that give rise to the observed rotational distributions. These calculations show that dissociation takes place from bent geometries, which can also explain the propensity to excite fragment bending levels. The low recoil velocities associated with the observed channels facilitate energy exchange in the exit channel, which results in statistical-like fragment rotational distributions. [ABSTRACT FROM AUTHOR]

The McyclohexaneAr (M Li, Na, and K) cluster ions were investigated using infrared photodissociation spectroscopy in the C−H stretching region. The alkali metal cation binds to the cyclohexane ring above the ring on the S6axis via 3coordination. The C−H stretching modes are perturbed due to binding of the metal cation and display a significant spread in frequency. The shifts are greatest for the Liand decrease for Naand Kwith increasing ionic radius. It has been observed that cyclohexane displays greater selectivity for Liover Nathan the cyclic ether, 12-crown-4. The charge transfer interaction between Liand cyclohexane is believed to be responsible for the selectivity of Liover other alkali metal ions. [ABSTRACT FROM AUTHOR]

Van der Waals clusters of phthalocyanine with 1−4 argon atoms formed inside superfluid helium nanodroplets have been investigated by recording fluorescence excitation spectra as well as emission spectra. The excitation spectra feature a multitude of sharp lines when recorded in superfluid helium droplets in contrast to the respective spectra measured in a seeded supersonic beam (Cho et al. Chem. Phys. Lett.2000, 326, 65). The pickup technique used for doping of the phthalocyanine and the argon into the droplets allows for nondestructive analysis of the cluster sizes. Alternation of the pickup sequence gives information on the binding site of the argon atoms. The investigation of dispersed emission spectra in helium droplets can be used as a special tool for the identification of %@mt;sys@%0%@sx@%0%@be@%0%@sxx@%%@mx@%transitions within the variety of sharp lines seen in the excitation spectra. Thus, different isomers of the clusters can be distinguished. Moreover, the emission spectra reveal information on dynamic processes such as vibrational predissociation of the van der Waals complexes and interconversion among isomeric species. The binding energy of the phthalocyanine−argon1complex in helium droplets was estimated to be at most 113 cm-1. [ABSTRACT FROM AUTHOR]

Nonrelativistic clamped-nuclei pair interaction energy for ground-state helium atoms has been computed for 12 interatomic separations ranging from 3.0 to 9.0 bohr. The calculations applied the supermolecular approach. The major part of the interaction energy was obtained using the Gaussian geminal implementation of the coupled-cluster theory with double excitations (CCD). Relatively small contributions from single, triple, and quadruple excitations were subsequently included employing the conventional orbital coupled-cluster method with single, double, and noniterative triple excitations CCSD(T) and the full configuration interaction (FCI) method. For three distances, the single-excitation contribution was taken from literature Gaussian-geminal calculations at the CCSD level. The orbital CCSD(T) and FCI calculations used very large basis sets, up to doubly augmented septuple- and sextuple-zeta size, respectively, and were followed by extrapolations to the complete basis set limits. The accuracy of the total interaction energies has been estimated to be about 3 mK or 0.03% at the minimum of the potential well. For the attractive part of the well, the relative errors remain consistently smaller than 0.03%. In the repulsive part, the accuracy is even better, except, of course, for the region where the potential goes through zero. For interatomic separations smaller than 4.0 bohr, the relative errors do not exceed 0.01%. Such uncertainties are significantly smaller than the expected values of the relativistic and diagonal Born−Oppenheimer contributions to the potential. [ABSTRACT FROM AUTHOR]

In this work, we report on a photoionization study of the microhydration of the four DNA bases. Gas-phase clusters of water with DNA bases guanine (G), cytosine (C), adenine (A), and thymine (T) are generated via thermal vaporization of the bases and expansion of the resultant vapor in a continuous supersonic jet expansion of water seeded in Ar. The resulting clusters are investigated by single-photon ionization with tunable vacuum-ultraviolet synchrotron radiation and mass analyzed using reflectron mass spectrometry. Photoionization efficiency (PIE) curves are recorded for the DNA bases and the following water (W) clusters:  G, GWn(n1−3); C, CWn(n1−3); A, AWn(n1,2); and T, TWn(n1−3). Appearance energies (AE) are derived from the onset of these PIE curves (all energies in eV):  G (8.1 ± 0.1), GW (8.0 ± 0.1), GW2(8.0 ± 0.1), and GW3(8.0); C (8.65 ± 0.05), CW (8.45 ± 0.05), CW2(8.4 ± 0.1), and CW3(8.3 ± 0.1); A (8.30 ± 0.05), AW (8.20 ± 0.05), and AW2(8.1 ± 0.1); T (8.90 ± 0.05); and TW (8.75 ± 0.05), TW2(8.6 ± 0.1), and TW3(8.6 ± 0.1). The AEs of the DNA bases decrease slightly with the addition of water molecules (up to three) but do not converge to values found for photoinduced electron removal from DNA bases in solution. [ABSTRACT FROM AUTHOR]

We present the mid-IR (400−1800 cm-1) spectra of 9-ethyl guanine, guanosine, and 2-deoxyguanosine measured by IR−UV double-resonance spectroscopy. We compare the recorded mid-IR spectra with the spectra of the most stable structures obtained from RI-MP2 and RI-DFT-D calculations. The results confirm the enol form for all structures and demonstrate the efficacy of a new approach to DFT calculations that includes dispersion interactions. [ABSTRACT FROM AUTHOR]

The anions of CsC2and CsC4produced by sputtering a graphite surface with Cswere mass-selected and trapped in neon matrixes at 6 K. The electronic absorption spectra of CsC2and CsC4, obtained by photodetachment of electrons from the anions, were measured subsequently and reveal strong absorptions in the visible range, which resemble the known band systems of C2-and C4-, respectively. The origin band of CsC2(500.4 nm) and CsC4(442.6 nm) is shifted by ∼1100 or by ∼700 cm-1to the blue from the position of the %@mt;sys@%0%@sx@%0%@be@%0%@sxx@%%@mx@%band of C2-or C4-. The observed system of CsC2is assigned to the 2B2−X 2A1electronic transition of the T-shaped form. The CsC4spectrum is consistent with a C4-chain slightly perturbed by the Cs atom. The oscillator strength of the observed electronic transition of CsC2and CsC4is an order of magnitude larger than for the respective carbon anions. CsC2-has a weak electronic transition, assigned to 1B2−X 1A1in the C2vform, with origin band at 516.5 nm. [ABSTRACT FROM AUTHOR]

A new organometallic complex, Co(CO)2NOtBuNC,was synthesized and investigated by photoelectron spectroscopy (PES) and threshold photoelectron photoion coincidence (TPEPICO) spectrometry in order to determine its ionization energy as well as the bond energies in the ionic forms. The assignment of the nine peaks in the PES was based on Kohn−Sham molecular orbital energies, and an adiabatic ionization energy of 7.30 ± 0.05 eV was determined. In the TPEPICO experiment, the following 0 K onsets were determined for the various fragment ions:  CoCONOtBuNC(8.17 ± 0.05 eV); CoNOtBuNC(9.01 ± 0.05 eV); and CotBuNC(10.42 ± 0.05 eV). Because the photon source did not extend above 14 eV, we could not observe the bare Coion in the experiment. The heat of formation of the CotBuNCion was estimated by ab initio and DFT calculations of the CoLtBuNC → CotBuNCL (L CO, NO, NH3, H2O, PMe3) substitution enthalpies. [ABSTRACT FROM AUTHOR]

The dynamics of vibrational wave packets excited in Na2dimers in the triplet ground and excited states is investigated by means of helium nanodroplet isolation (HENDI) combined with femtosecond pump−probe spectroscopy. Different pathways in the employed resonant multiphoton ionization scheme are identified. Within the precision of the method, the wave packet dynamics appears to be unperturbed by the helium droplet environment. [ABSTRACT FROM AUTHOR]

Negatively charged sodium auride clusters, NanAun-(n1−3), have been investigated experimentally using photoelectron spectroscopy and ab initiocalculations. Well-resolved electronic transitions were observed in the photoelectron spectra of NanAun-(n1−3) at several photon energies. Very large band gaps were observed in the photoelectron spectra of the anion clusters, indicating that the corresponding neutral clusters are stable closed-shell species. Calculations show that the global minimum of Na2Au2-is a quasi-linear species with Cssymmetry. A planar isomer of D2hsymmetry is found to be 0.137 eV higher in energy. The two lowest energy isomers of Na3Au3-consist of three-dimensional structures of Cssymmetry. The global minimum of Na3Au3-has a bent-flake structure lying 0.077 eV below a more compact structure. The global minima of the sodium auride clusters are confirmed by the good agreement between the calculated electron detachment energies of the anions and the measured photoelectron spectra. The global minima of neutral Na2Au2and Na3Au3are found to possess higher symmetries with a planar four-membered ring (D2h) and a six-membered ring (D3h) structure, respectively. The chemical bonding in the sodium auride clusters is found to be highly ionic with Au acting as the electron acceptor. [ABSTRACT FROM AUTHOR]

We report the vibrationally mediated photodissociation dynamics of C2H4excited through the 2Agstate. Vibrational state-selected ions were prepared by two-photon resonant, three-photon ionization of ethylene via (, 3s) and (, 3p) Rydberg intermediate states in the wavelength range 298−349 nm. Absorption of a fourth photon led to dissociation of the cation, and images of the product ions C2H3and C2H2were simultaneously recorded using reflectron multimass velocity map imaging. Analysis of the multimass images yielded, with high precision, both the total translational energy distributions for the two dissociation channels and the branching between them as a function of excitation energy. The dissociation of ions that were initially prepared with torsional excitation exceeding the barrier to planarity in the cation ground state consistently gave enhanced branching to the H elimination channel. The results are discussed in terms of the influence of the initial state preparation on the competition between the internal conversion to the ground state and to the first excited state. [ABSTRACT FROM AUTHOR]

The excited state dynamics of silver atoms embedded in helium nanodroplets have been investigated by a variety of spectroscopic techniques. The experiments reveal that 5p 2P12← 5s 2S12excitation of embedded silver atoms results almost exclusively in the ejection of silver atoms populating the 2P12state. In contrast, excitation to the 5p 2P32state leads to the ejection of not only silver atoms in the 2P12, 2P32, and 2D52excited states but also of AgHe and AgHe2. These AgHe exciplexes are mainly formed in the A212electronic state. In addition, it is found that a considerable fraction of the 2P32excited silver atoms become solvated within the helium droplets, most probably as AgHe2. The observations can be accounted for by a model in which the metastable 2D52state of silver acts as a doorway state in the relaxation of 2P3/2excited silver atoms. [ABSTRACT FROM AUTHOR]

Small clusters of group 1 metal complexes with hydrogen fluoride molecules M(HF)n, M Li, Na, and K, are studied with the ab initio molecular orbital method. The trimer M(HF)3forms a C3vcluster, in which the metal atom is ionized and the ejected electron is trapped on the top of three equivalent HF molecules. The optimized geometric structure of Li(HF)3is almost identical with that of the ion pair Li(HF)3Cl-by replacing a Cl-anion with an ejected electron e-; thus Li(HF)3can be described as Li(HF)3e-. The entity e-is trapped under the electrostatic field created by three HF bond dipoles; and at the same time, the HF bonds are polarized and weakened. A triplet anion e-(HF)3Li(HF)3e-is stable and is a possible anion unit of electrides. [ABSTRACT FROM AUTHOR]

We present results of path integral Monte Carlo simulations of LiH solvated in superfluid 4He clusters of size up to N100. Despite the light mass of LiH and the strongly anisotropic LiH−He potential with a large repulsion at the hydrogen end, LiH is solvated inside the cluster for sufficiently large N. Using path integral correlation function analysis, we have determined the dipole (J1) rotational excitations of the cluster and a corresponding effective rotational constant Beffof the solvated LiH. We predict that Beffis greatly reduced with respect to the gas-phase rotational constant B, to a value of only about 6% of B. This exceptionally large reduction of the rotational constant is due to the highly anisotropic 4He solvation structure around LiH. It does not follow the previously established trend of a relatively small Breduction for light molecules, showing the strongest reduction of all molecules in 4He to date. Comparison of the calculated rotational spectra of LiH in helium obeying Bose and Boltzmann statistics, respectively, demonstrates that the Bose statistics of helium is an essential requirement for obtaining well-defined molecule rotational spectra in helium-4. [ABSTRACT FROM AUTHOR]

Rovibrational spectra are measured for the HCCCN·HCN and HCN·HCCCN binary complexes in helium droplets at low temperature. Though no Q-branch is observed in the infrared spectrum of the linear HCN·HCCCN dimer, which is consistent with previous experimental results obtained for other linear molecules, a prominent Q-branch is found in the corresponding infrared spectrum of the HCCCN·HCN complex. This Q-branch, which is reminiscent of the spectrum of a parallel band of a prolate symmetric top, implies that some component of the total angular momentum is parallel to the molecular axis. The appearance of this particular spectroscopic feature is analyzed here in terms of a nonsuperfluid helium density induced by the molecular interactions. Finite temperature path integral Monte Carlo simulations are performed using potential energy surfaces calculated with second-order Möller−Plesset perturbation theory, to investigate the structural and superfluid properties of both HCCCN·HCN(4He)Nand HCN·HCCCN(4He)Nclusters with N≤ 200. Explicit calculation of local and global nonsuperfluid densities demonstrates that this difference in the rovibrational spectra of the HCCCN·HCN and HCN·HCCCN binary complexes in helium can be accounted for by local differences in the superfluid response to rotations about the molecular axis, i.e., different parallel nonsuperfluid densities. The parallel and perpendicular nonsuperfluid densities are found to be correlated with the locations and strengths of extrema in the dimer interaction potentials with helium, differences between which derive from the variable extent of polarization of the CN bond in cyanoacetylene and the hydrogen-bonded CH unit in the two isomers. Calculation of the corresponding helium moments of inertia and effective rotational constants of the binary complexes yields overall good agreement with the experimental values. [ABSTRACT FROM AUTHOR]

Making use of the coupled cluster variant CCSD(T) in conjunction with Dunning's cc-pVTZ basis set, equilibrium structures and complete harmonic force fields have been calculated for the linear carbon chains of type C2n1with n5−9. With the exception of C3, which is a well-known “floppy” molecule with an extremely shallow bending potential, all members of the C2n1series up to C19appear to behave like fairly normal semirigid molecules. The IR active bending vibrations of lowest wavenumber for C17and C19, which may be of interest to forthcoming far-infrared astronomy, are predicted to occur at 13.1 and 11.1 cm-1, respectively, with corresponding absolute IR intensities of 6.6 and 5.9 km mol-1. Huge IR intensities are calculated for one antisymmetric stretching vibration per chain (19 948, 29 632, and 30 040 km mol-1for C15, C17, and C19, respectively). A quantitative description of these vibrations may require the explicit consideration of anharmonicity effects and electronic structure calculations going beyond CCSD(T). [ABSTRACT FROM AUTHOR]

Energy transfer dynamics at the gas−liquid interface have been probed with a supersonic molecular beam of CO2and a clean perfluorinated-liquid surface in vacuum. High-resolution infrared spectroscopy measures both the rovibrational state populations and the translational distributions for the scattered CO2flux. The present study investigates collision dynamics as a function of incident angle (inc0°, 30°, 45°, and 60°), where column-integrated quantum state populations are detected along the specular-scattering direction (i.e., scatinc). Internal state rovibrational and Doppler translational distributions in the scattered CO2yield clear evidence for nonstatisticalbehavior, providing quantum-state-resolved support for microscopic branching of the gas−liquid collision dynamics into multiple channels. Specifically, the data are remarkably well described by a two-temperature model, which can be associated with both a trapping desorption (TD) component emerging at the surface temperature (TrotTS) and an impulsive scattering (IS) component appearing at hyperthermal energies (Trot> TS). The branching ratio between the TD and IS channels is found to depend strongly on inc, with the IS component growing dramatically with increasingly steeper angle of incidence. [ABSTRACT FROM AUTHOR]

A study has been made of the ion chemistry of a series of small molecules that have been embedded in helium nanodroplets. In most instances, the molecules H2O, SO2, CO2, CH3OH, C2H5OH, C3H7OH, CH3F, and CH3Cl have been allowed to form clusters, and reactivity within these has been initiated through electron impact ionization. For two of the molecules studied, CF2Cl2and CF3I, reactivity is believed to originate from single molecules embedded in the droplets. Electron impact on the droplets is thought to first create a helium ion, and formation of molecular ions is then assumed to proceed via a charge hopping mechanism that propagates though the droplet and terminates with charge-transfer to a molecule or cluster. The chemistry exhibited by many of the cluster ions and at least one of the single molecular ions is very different from that observed for the same species in isolation. In most cases, reactivity appears to be dominated by high-energy bond breaking processes as opposed to, in the case of the clusters, ion−molecule reactions. Overall, charge-transfer from Hedoes not appear to be a “soft” ionization mechanism. [ABSTRACT FROM AUTHOR]

A linear AC trap for polar molecules in high-field seeking states has been devised and implemented, and its characteristics have been investigated both experimentally and theoretically. The trap is loaded with slow 15ND3molecules in their ground state (para-ammonia) from a Stark decelerator. The trap's geometry offers optimal access as well as improved loading. We present measurements of the dependence of the trap's performance on the switching frequency, which exhibit a characteristic structure due to nonlinear resonance effects. The molecules are found to oscillate in the trap under the influence of the trapping forces, which were analyzed using 3D numerical simulations. On the basis of expansion measurements, molecules with a velocity and a position spread of 2.1 m/s and 0.4 mm, respectively, are still accepted by the trap. This corresponds to a temperature of 2.0 mK. From numerical simulations, we find the phase−space volume that can be confined by the trap (the acceptance) to be 50 mm3(m/s)3. [ABSTRACT FROM AUTHOR]

Infrared spectra of the NH stretching vibrations of (NH3)nclusters (n2−4) have been obtained using the helium droplet isolation technique and first principles electronic structure anharmonic calculations. The measured spectra exhibit well-resolved bands, which have been assigned to the 1, 3, and 24modes of the ammonia fragments in the clusters. The formation of a hydrogen bond in ammonia dimers leads to an increase of the infrared intensity by about a factor of 4. In the larger clusters the infrared intensity per hydrogen bond is close to that found in dimers and approaches the value in the NH3crystal. The intensity of the 24overtone band in the trimer and tetramer increases by a factor of 10 relative to that in the monomer and dimer, and is comparable to the intensity of the 1and 3fundamental bands in larger clusters. This indicates the onset of the strong anharmonic coupling of the 24and 1modes in larger clusters. The experimental assignments are compared to the ones obtained from first principles electronic structure anharmonic calculations for the dimer and trimer clusters. The anharmonic calculations were performed at the Møller−Plesset (MP2) level of electronic structure theory and were based on a second-order perturbative evaluation of rovibrational parameters and their effects on the vibrational spectra and average structures. In general, there is excellent (<20 cm-1) agreement between the experimentally measured band origins for the N−H stretching frequencies and the calculated anharmonic vibrational frequencies. However, the calculations were found to overestimate the infrared intensities in clusters by about a factor of 4. [ABSTRACT FROM AUTHOR]

The photoionization and photoelectron spectroscopy of pure He droplets were investigated at photon energies between 24.6 eV (the ionization energy of He) and 28.0 eV. Time-of-flight mass spectra and photoelectron images were obtained at a series of molecular beam source temperatures and pressures to assess the effect of droplet size on the photoionization dynamics. At source temperatures below 16 K, where there is significant production of clusters with more than 104atoms, the photoelectron images are dominated by fast electrons produced via direct ionization, with a small contribution from very slow electrons with kinetic energies below 1 meV arising from an indirect mechanism. The fast photoelectrons from the droplets have as much as 0.5 eV more kinetic energy than those from atomic He at the same photon energy. This result is interpreted and simulated within the context of a “dimer model”, in which one assumes vertical ionization from two nearest-neighbor He atoms to the attractive region of the He2potential energy curve. Possible mechanisms for the slow electrons, which were also seen at energies below IE(He), are discussed, including vibrational autoionizaton of Rydberg states comprising an electron weakly bound to the surface of a large HeNcore. [ABSTRACT FROM AUTHOR]

n-Alkanols can occur in a multitude of energetically competitive conformational states. Using the OH stretching vibration as an infrared and Raman spectroscopic sensor in supersonic jet expansions, the torsional preferences around the C−O and C−Cbonds are probed for n-propanol through n-hexanol. Raman detection is more powerful for isolated monomers, whereas IR spectroscopy is more sensitive for molecular complexes. The subtle IR vibrational shift induced by the nanocoating of n-alcohols with Ar atoms is shown to alternate with chain length. A large number of alcohol dimer absorptions is observed and subjected to collisional relaxation and nanocoating conditions. Essential features of the dimer spectra are modeled successfully by a simple force field approach. Exploratory quantum chemical calculations up to the MP2/aug-cc-pvqz level encourage a rigorous theoretical study of the subtle conformational aspects in monomers and possibly also in dimers of linear alcohols. [ABSTRACT FROM AUTHOR]

In this article, a new technique we call Beam Action Spectroscopy via Inelastic Scattering (BASIS) is demonstrated. BASIS takes advantage of the sensitivity of rotational state distributions in a supersonic molecular beam to inelastic scattering within the beam. We exploit BASIS to achieve increased sensitivity in two very different types of experiments. In the first, the UV photodissociation spectrum of OClO is recovered by monitoring intensity changes in the pure rotational transition of a spectator molecule (OCS) downstream from the nozzle, revealing a new vibrational structure in the region between 30 000 and 36 000 cm-1. In the second, the mid-IR vibrational spectrum of acetylene is recorded simply by monitoring a single pure rotational transition of OCS co-expanded with acetylene. The technique may prove particularly fruitful when an excitation process produces product dark states that are not easily probed by conventional spectroscopy. [ABSTRACT FROM AUTHOR]

The observed V3torsional barriers measured by microwave spectroscopy for nine methyl groups attached to peptide bond linkages in five gas-phase biomimetics have been found to differ considerably from one molecule to the next and even depend on the position of substitution, being sensitive to structural changes at the other end of the peptide bond. In the search for an explanation for these results, ab initio calculations have been performed at the HF/6-311G(d,p) level of theory and interpreted in terms of the natural bond orbitals and resonance structures of the peptide bond. These calculations reveal that resonance delocalization in peptide bonds is influenced by methyl conformation through the coupling of vicinal to orbital interactions with the nto . Thus, CN double-bond character increases (and CO double-bond character decreases) as the methyl group is rotated from the syn to the anti position. A quasilinear correlation exists between the barriers to internal rotation of attached methyl groups and the relative importance of the two principal resonance structures that contribute to the peptide bond. [ABSTRACT FROM AUTHOR]