The anharmonic vibrational spectra of alpha-D-glucose, beta-D-glucose, and sucrose are computed by the vibrational self-consistent field (VSCF) method, using potential energy surfaces from electronic structure theory, for the lowest energy conformers that correspond to the gas phase and to the crystalline phase, respectively. The results are compared with ultraviolet-infrared (UV-IR) spectra of phenyl beta-D-glucopyranoside in a molecular beam, with literature results for sugars in matrices and with new experimental data for the crystalline state. Car-Parrinello dynamics simulations are also used to study temperature effects on the spectra of alpha-D-glucose and beta-D-glucose and to predict their vibrational spectra at 50, 150, and 300 K The effects of temperature on the spectral features are analyzed and compared with results of the VSCF calculations conducted at OK. The main results include: (i) new potential surfaces, constructed from Hartree-Fock, adjusted to fit harmonic frequencies from Moller-Plesset (MP2) calculations, that give very good agreement with gas phase, matrix, and solid state spectra; (ii) computed infrared spectra of the crystalline solid of alpha-glucose, which are substantially improved by including mimic groups that represent the effect of the solid environment on the sugar; and (iii) identification of a small number of combination-mode transitions, which are predicted to be strong enough for experimental observation. The results are used to assess the role of anharmonic effects in the spectra of the sugars in isolation and in the solid state and to discuss the spectroscopic accuracy of potentials from different electronic structure methods.

Vibrational energy flow and conformational transitions following excitation of the OH stretching mode of the most stable conformer of glycine are studied by classical trajectories. ‘‘On the fly’’ simulations with the PM3 semiempirical electronic structure method for the potential surface are used. Initial conditions are selected to correspond to the v = 1 excitation of the OH stretch. The main findings are: (1) An an equilibrium-like ratio is established between the populations of the 3 lowest-lying conformers after about 10 picoseconds. (2) There is a high probability throughout the 150 ps of the simulations for finding the molecule in geometries far from the equilibrium structures of the lowest-energy conformers. (3) Energy from the initial excited OH (v = 1) stretch flows preferentially to 5 other vibrational modes, including the bending motion of the H atom. (4) RRK theory yields conformational transition rates that deviate substantially from the classical trajectory results. Possible implication of these results for vibrational energy flow and conformational transitions in small biological molecules are discussed.

First-principles anharmonic vibrational calculations are carried out for the Raman spectrum of the C-H stretching bands in dodecane, and for the C-D bands in the deuterated molecule. The calculations use the Vibrational Self-Consistent Field (VSCF) algorithm. The results are compared with liquid-state experiments, after smoothing the isolated-molecule sharp-line computed spectra. Very good agreement between the computed and experimental results is found for the two systems. The combined theoretical and experimental results provide insights into the spectrum, elucidating the roles of symmetric and asymmetric CH(3) and CH(2) hydrogenic stretches. This is expected to be very useful for the interpretation of spectra of long-chain hydrocarbons. The results show that anharmonic effects on the spectrum are large. On the other hand, vibrational degeneracy effects seem to be rather modest at the resolution of the experiments. The degeneracy effects may have more pronounced manifestations in higher-resolution experiments. The results show that first-principles anharmonic vibrational calculations for hydrocarbons are feasible, in good agreement with experiment, opening the way for applications to many similar systems. The results may be useful for the analysis of CARS imaging of lipids, for which dodecane is a representative molecule. It is suggested that first-principles vibrational calculations may be useful also for CARS imaging of other systems.

Noble-gas hydrides such as HXeCCH are prepared in cryogenic noble-gas matrices where they are stable. Molecular dynamics simulations reported here predict that HXeCCH is chemically stable in clusters of acetylene, and that stability prevails for temperatures of at least 150 K, at which the clusters are liquid-like. The HXeCCH(C(2)H(2))(n) clusters are studied for sizes up to n = 7. Ab Initio Molecular Dynamics trajectories of 10 ps duration are computed using BLYP-D DFT potential. The liquid-like nature of the system at 150 K is reflected in large amplitude motion of intermolecular distances and orientations. In addition, structures, energetics, NBO charges and bonding analysis at equilibrium are also reported. Complexation is found to be energetically favorable, and to increase the stability of the HXeCCH molecule. The significance of the existence of stable liquid-like complexes of noble-gas hydrides is discussed.

High level ab initio calculations of clusters comprised of water, HCl, and ON-ONO2 are used to study nitrosyl chloride (ClNO) formation in gas phase water clusters, which are also mimics for thin water films present at environmental interfaces. Two pathways are considered, direct formation from the reaction of gaseous HCl with ON-ONO2 and an indirect pathway involving the hydrolysis of ON-ONO2 to form HONO, followed by the reaction of HONO with HCl to form ClNO. Surprisingly, direct formation of ClNO is found to be the dominant channel in the presence of water despite the possibility of a competing hydrolysis of ON-ONO2 to form HONO. A single water molecule effectively catalyzes the ON-ONO2 + HCl reaction, and in the presence of two or more water molecules the reaction to form ClNO becomes spontaneous. Direct formation of ClNO is fast at room and ice temperatures, indicating the possible significance of this pathway for chlorine activation chemistry in both the polar and midlatitude troposphere, in volcanic plumes and indoors. The reaction enthalpies, activation energies, and rate constants for all studied reactions are reported. The results are discussed in light of recent experiments.

Several complementary experimental and theoretical methodologies were used to explore water uptake on sodium chloride (NaCl) particles containing varying amounts of sodium dodecyl sulfate (SDS) to elucidate the interaction of water with well-defined, environmentally relevant surfaces. Experiments probed the hygroscopic growth of mixed SDS/NaCl nanoparticles that were generated by electrospraying aqueous 2 g/L solutions containing SDS and NaCl with relative NaCl/SDS weight fractions of 0, 5, 11, 23, or 50 wt/wt %. Particles with mobility-equivalent diameters of 14.0(+/- 0.2) nm were size selected and their hygroscopic growth was monitored by a tandem nano-differential mobility analyzer as a function of relative humidity (RH). Nanoparticles generated from 0 and 5 wt/wt % Solutions deliquesced abruptly at 79.1 (+/- 1.0)% RH. Both of these nanoparticle compositions had 3.1(+/- 0.5) monolayers of adsorbed surface water prior to deliquescing and showed good agreement with the Brunauer-Emmett-Teller and the Frenkel-Halsey-Hill isotherms. Above the deliquescence point, the growth curves could be qualitatively described by Kohler theory after appropriately accounting for the effect of the particle shape on mobility. The SDS/NaCl nanoparticles with larger SDS fractions displayed gradual deliquescence at a RH that was significantly lower than 79.1%. All compositions of SDS/NaCl nanoparticles had monotonically Suppressed mobility growth factors (GF(m)) with increasing fractions of SDS in the electrosprayed solutions. The Zdanovskii-Stokes-Robinson model was used to estimate the actual fractions of SDS and NaCl in the nanoparticles; it suggested the nanoparticles were enhanced in SDS relative to their electrospray solution concentrations. X-ray photoelectron spectroscopy (XPS), FTIR, and AFM were consistent with SDS forming first a monolayer and then a crystalline phase around the NaCl core. Molecular dynamics simulations of water vapor interacting with SDS/NaCl slabs showed that SDS kinetically hinders the initial water uptake. Large binding energies of sodium methyl sulfate (SMS)-(NaCl)(4), H(2)O-(NaCl)(4), and SMS-H(2)O-(NaCl)(4) calculated at the MP2/cc-pVDZ level suggested that placing H(2)O in between NaCl and surfactant headgroup is energetically favorable. These results provide a comprehensive description of SDS/NaCl nanoparticles and their properties.

The separable VSCF approximation and the VSCF-PT2 method are extensively used for anharmonic vibrational spectroscopy calculations of large molecules. VSCF-PT2 uses second-order perturbation theory to correct the VSCF level, and is thus more accurate. It is shown by test calculations for a series of amino acids and peptides that the mean deviation of VSCF from VSCF-PT2 frequencies decreases for an increasing number of modes N, the decrease scaling roughly with log N. It is conjectured that the result is a manifestation of improved mean accuracy of VSCF as a mean-field approximation, a consequence of increased averaging with increasing numbers of modes. There is no systematic increase in VSCF accuracy with N for individual vibrational transitions. Increased accuracy of VSCF with N is found for certain groups of transitions, e.g., N-H stretches. The results are expected to be useful in choosing methods for spectroscopy calculations of extended systems such as large peptides, proteins, and nucleic acids.

It is found that HRnCCH and HRnOH are metastable, chemically bound compounds of radon. These molecules are studied by multi-reference ab initio methods. Equilibrium geometry, NBO partial charges and bond orders, harmonic frequencies, are calculated. Intrinsic life-times are obtained by calculating the dissociation barriers and related partition functions, and by applying transition state theory. HRnCCH and HRnOH are found to be protected by an energy barrier of 2.1 and 0.79 eV, respectively. Using transition state theory, HRnOH is predicted to have a half-life of 1 h at about 230 K. HRnCCH is found to be kinetically stable at room temperature with its lifetime limited by the lifetime of the radioactive Rn atom. The significance of compound formation of radon with acetylene and water is discussed.

Molecular dynamics (MD) simulations are carried out for the complex of glucose with KNO3 and for complexes of the type glucose-KNO3-(H2O)(n), for n = 3 water molecules added to the system, charge separation into K+ and NO3- ions takes place; (4) for the sugar-water system with n = 11 water molecules all hydroxyl groups are hydrated with the glucose adopting a surface position, indicative of a surfactant property of the sugar; and (5) comparison of DL_POLY with MP2/TZP structure predictions indicates that the empirical force field predicts global and local minimum structures reasonably well, but errs in giving the energy rankings of the different minima. The implications of the results on the effects of salts on saccharides are discussed.

Simulations show that photodissociation of methyl hydroperoxide, CH(3)OOH, on water clusters produces a surprisingly wide range of products on a subpicosecond time scale, pointing to the possibility of complex photodegradation pathways for organic peroxides on aerosols and water droplets. Dynamics are computed at several excitation energies at 50 K using a semiempirical PM3 potential surface. CH(3)OOH is found to prefer the exterior of the cluster, with the CH(3)O group sticking out and the OH group immersed within the cluster. At atmospherically relevant photodissociation wavelengths the OH and CH(3)O photofragments remain at the surface of the cluster or embedded within it. However, none of the 25 completed trajectories carried out at the atmospherically relevant photodissociation energies led to recombination of OH and CH(3)O to form CH(3)OOH. Within the limited statistics of the available trajectories the predicted yield for the recombination is zero. Instead, various reactions involving the initial fragments and water promptly form a wide range of stable molecular products such as CH(2)O, H(2)O, H(2), CO, CH(3)OH, and H(2)O(2).

The vibrational self-consistent field (VSCF) method assumes separability in normal modes in its usual version. However, the method fails in cases such as soft torsional modes which are better treated by angular variables. We develop VSCF equations based on the assumption of wave function separability in internal coordinates. To test the method, simple illustrative applications to small systems are provided: trans-HONO, cis-HONO, H2S2, and H2O2. The code directly uses points from ab initio calculations, and the method proves to be accurate for all types of transitions. For typical torsional transitions, the error in the computed frequency is smaller than that of VSCF in normal coordinates. The wave functions for the torsional mode are compared with the corresponding normal mode wave functions. The differences are substantial. The results are encouraging for extension of the model for large polyatomic systems. Work along these lines is in progress. (C) 2010 Elsevier B. V. All rights reserved.

The kinetic stability of the HXeOH and HXeOXeH molecules, chemically bound compounds made of Xe atoms and water, is studied by multi-reference ab initio methods. The decomposition paths, the transition states and the rates of dissociation as a function of temperature, are calculated. HXeOH and HXeOXeH are found to be protected by an energy barriers of 0.59 and 0.4 eV, respectively. Applying transition state theory, HXeOH and HXeOXeH have intrinsic half-lives of 1 h at 170 and 120 K, respectively. Implications of the results for the kinetic stability of these species are discussed. (C) 2009 Elsevier B. V. All rights reserved.

The HXeCCH center dot center dot center dot H(2)C(2) complex has been characterized by IR spectroscopy in a xenon matrix and ab initio calculations. This species exhibits a blue shift of the H-Xe stretching mode (19-28 cm(-1)) in comparison with the HXeCCH monomer, which indicates stabilization of the H-Xe bond upon complexation. The complex results from annealing-induced diffusion of acetylene molecules above 50 K to isolated HXeCCH formed at lower temperature. In addition, a weak absorption with a blue shift of +51 cm(-1) was tentatively assigned to the HXeCCH complex with two acetylene molecules. These experimental observations are supported by ab initio calculations. (C) 2009 Elsevier B.V. All rights reserved.

Structural properties of large NO(3)(-)center dot(H(2)O)(n) (n = 15-500) clusters are studied by Monte Carlo simulations using effective fragment potentials (EFPs) and by classical molecular dynamics simulations using a polarizable empirical force field. The simulation results are analyzed with a focus on the description of hydrogen bonding and solvation in the clusters. In addition, a comparison between the electronic structure based EFP and the classical force field description of the 32 water cluster system is presented. The EFP simulations, which focused on the cases of n = 15 and 32, show an internal, fully solvated structure and a ‘‘surface adsorbed’’ structure for the 32 water cluster at 300 K, with the latter configuration being more probable. The internal solvated structure and the ‘‘surface adsorbed’’ structure differ considerably in their hydrogen bonding coordination numbers. The force field based simulations agree qualitatively with these results, and the local geometry of NO(3)(-) and solvation at the surface-adsorbed site in the force field simulations are similar to those predicted using EFPs. Differences and similarities between the description of hydrogen bonding of the anion in the two approaches are discussed. Extensive classical force field based simulations at 250 K predict that long time scale stability of ‘‘internal’’ NO(3)(-), which is characteristic of extended bulk aqueous interfaces, emerges only for n > 300. Ab initio Moller-Plesset perturbation theory is used to test the geometries of selected surface and interior anions for n = 32, and the results are compared to the EFP and MD simulations. Qualitatively, all approaches agree that surface structures are preferred over the interior structures for clusters of this size. The relatively large aqueous clusters of NO(3)(-) studied here are of comparable size to clusters that lead to new particle formation in air. Nitrate ions on the Surface of such clusters may have significantly different photochemistry than the internal species. The possible implications of surface-adsorbed nitrate ions for atmospheric chemistry are discussed.