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We have demonstrated the application of many-body expansions to calculations of the anharmonic, local-mode, OH-stretching vibrational frequencies of water clusters. We focused on five low-lying isomers of the water hexamer and the DD*(20,1) isomer of (H2O)21. Our approach provides accurate OH-stretching vibrational frequencies when treating one- and two-body interactions with the CCSD(T)-F12 level of theory and the three- and four-body interactions with the DF-MP2-F12 level. Additionally, we have investigated the physical origin of the large contribution that two- and three-body interactions make to the shifts of vibrational frequencies using symmetry-adapted perturbation theory (SAPT). We conclude that while two-body vibrational frequency shifts can be correlated linearly with electrostatic energies, all strongly shifted three-body interactions can be correlated to the induction energy with a single regression coefficient of approximately 70 cm–1 (kcal·mol–1)−1.

Metal-organic frameworks (MOFs) show considerable promise as materials for gas storage and separation. Many MOF structures have open metal sites, which allow for coordination of gas molecules to the metal centers. In this work, we use coupled-cluster and symmetry-adapted perturbation theory to probe the interaction between hydrogen gas and unsaturated metal sites in mimic structures based on the MOF HKUST-1. The interactions are of a mixed electrostatic/dispersive nature, with the relative magnitudes of these components dependent on the metal center. The strongest binding was found for magnesium- and zinc-containing MOFs, with an overall interaction energy of -4.5 kcal mol(-1).

It's the Network Numerous reactions of small molecules and ions in the atmosphere take place in the confines of watery aerosols. Relph et al. (p. 308 ; see the Perspective by Siefermann and Abel ) explored the specific influence of a water cluster's geometry on the transformation of solvated nitrosonium (NO + ) to nitrous acid (HONO). The reaction involves (O)N–O(H) bond formation with one water molecule, concomitant with proton transfer to additional, surrounding water molecules. Vibrational spectroscopy and theoretical simulations suggest that certain arrangements of the surrounding water network are much more effective than others in accommodating this charge transfer, and thus facilitating the reaction.

A polarizable force field that explicitly includes contributions from exchange repulsion, dispersion, charge penetration, and multipole electrostatics was developed to describe the interaction between bromine and water. This force field was combined with a polarizable force field for water and used in molecular dynamics simulations to calculate the relative energetics of three bromine clathrate hydrates. The simulations predict the tetragonal structure (Allen, K. W.; Jeffrey, G. A. J. Chem. Phys. 1963, 38, 2304) to be the most stable, with the CS-I and CS-II cubic structures being less stable. Although the CS-II species is not the most stable energetically, we argue that it could be formed under conditions of low bromine concentration, in agreement with recent measurements (Goldschleger, I. U.; Kerenskaya, G.; Janda, K. C.; Apkarian, V. A. J. Phys. Chem. A 2008, 112, 787) that provide evidence for three different bromine hydrate crystal types.

Si2H4 and Si7H8 cluster models are constructed for studying the role of high-order electron correlation effects on the buckling of SiSi dimers on the Si(100) surface. CASPT3, multi-reference coupled-pair functional, and the diffusion Monte Carlo methods are used to examine whether correlation effects not recovered in CASPT2 calculations are important for the buckling of the dimers. The calculations show that such high-order correlation effects are indeed important for determining the relative stability of the buckled and unbuckled structures.

In this work we examine three water models, including one developed by us, with distributed polarizable sites. These models are assessed by comparison with the results of MP2 calculations. The use of distributed polarizable sites is found to be especially important for describing the 3-body interaction energies of water clusters. It is also shown that all-atom repulsion terms are necessary to accurately describe cluster geometries.

As a step toward a first principles characterization of the optical properties of chlorine hydrate, we have calculated the electronic absorption spectrum of a chlorine molecule trapped in dodecahedral (H2O)20 and hexakaidodecahedral (H2O)24 cages. For comparison, spectra were also calculated for an isolated Cl2 molecule as well as for selected Cl2(H2O)n, n < or =8, clusters cut out of the Cl2(H2O)20 cluster, allowing us to follow the evolution of the low-lying excited states with increasing number of surrounding water molecules. Although encapsulation of a chlorine molecule within the water cages has relatively little effect on its low-lying valence transitions, it does result in a large number of solvent-to-solute charge-transfer transitions at energies starting near 48,000 cm(-1).

Theoretical studies of the H2O.O2 complex have been carried out over the past decade, but the complex has not previously been experimentally identified. We have assigned IR vibrations from an H2O.O2 complex in an inert rare gas matrix. This identification is based upon theoretical calculations and concentration dependent behavior of absorption bands observed upon co-deposition of H2O and O2 in argon matrixes at 11.5 +/- 0.5 K. To aid assignment, we have used a harmonically coupled anharmonic oscillator local mode model with an ab initio calculated dipole moment function to calculate the OH-stretching and HOH-bending frequencies and intensities in the complex. The high abundance of H2O and O2 makes the H2O.O2 complex likely to be significant in atmospheric and astrophysical chemistry.

We show that a simple two-dimensional vibrational model can explain most of the features observed in the first and second OH-stretching overtone region of the room temperature cis,cis-HOONO spectrum. The model uses ab initio calculated parameters and includes the OH-stretching mode coupled to the internal torsion of the OH group.

We have recorded vapor-phase CH-stretching overtone spectra of 3-hexyne in the ΔvCH = 2−8 regions and of 3-hexyne-d10 in the ΔvCD = 2−7 regions. We have recorded CH-stretching overtone spectra of butane in the ΔvCH = 2−7 regions and of propane in the ΔvCH = 7 and 8 regions to facilitate comparison with the 3-hexyne spectra. The spectra were recorded at room temperature with the use of conventional and intracavity laser photoacoustic spectroscopy. The spectra can be explained in terms of a harmonically coupled local-mode model (HCAO) with one oscillator for each of the nonequivalent CH bonds. Spectral assignments are made and correlated with ab initio geometry optimized structures. The intensities of the CH stretching transitions are calculated using the HCAO local-mode model and ab initio dipole moment functions. Parameters for the calculations come from a local-mode analysis of the spectra and from ab initio calculations. Our simple calculations are in good agreement with the observed absolute and relati...

The vibrational spectroscopy of the nitrate-water isotopologues is studied in the O-H and O-D stretching regions using temperature-dependent infrared multiple photon dissociation spectroscopy combined with calculations of the anharmonic spectra. At a temperature of 15 K a series of discrete peaks is observed in the IRMPD spectra of NO3(-)·H2O, NO3(-)·HDO, and NO3(-)·D2O. This structure is considerably more complex than predicted by harmonic calculations. A signal is only observed in the hydrogen-bonded O-H (O-D) stretching region, characteristic of a double hydrogen-bond donor binding motif. With increasing temperature, the peaks broaden, leading to a quasi-continuous absorption from 3150 to 3600 cm(-1) (2300-2700 cm(-1)) for NO3(-)·H2O (NO3(-)·D2O) and, above 100 K, an additional band in the free O-H (O-D) stretching region, suggesting the population of complexes containing only a single hydrogen bond at higher internal energies. Vibrational configuration interaction calculations confirm that much of the structure observed in the IRMPD spectra derives from progressions in the water rocking mode resulting from strong cubic coupling between the O-H (O-D) stretch and water rock degrees of freedom. The spectra of both NO3(-)·H2O and NO3(-)·D2O display a strong peak that does not derive from the water rock progression but results instead from a Fermi resonance between the O-H (O-D) stretch and H-O-H (D-O-D) bend overtone. Additional insight into the nature of the O-H (O-D) stretch and water rocking coupling in these complexes is provided by an effective Hamiltonian that allows for the cubic coupling between the O-H stretch and water rock degrees of freedom.

We have recorded the CH-, OH-, and OD-stretching fundamental and overtone spectra of catechol (1,2-dihydroxybenzene, pyrocatechol) and selectively deuterated catechol. Conventional and intracavity photoacoustic spectroscopy were used to record room temperature spectra of catechol in solution and in the vapor phase, whereas nonresonant ionization detected spectroscopy was used to study catechol in a supersonic jet. The spectra can be explained in terms of a local mode model with one oscillator for each of the nonequivalent CH, OH, or OD bonds. Intensities of the CH-, OH-, and OD-stretching transitions were calculated with an anharmonic oscillator local mode model and ab initio determined dipole moment functions. Our simple calculations are in good agreement with the observed intensities. Line widths in the jet-cooled spectra are discussed in terms of intramolecular vibrational redistribution. The spectroscopic and theoretical results are in agreement with a relatively weak intramolecular hydrogen bond.

Localized orbital coupled cluster calculations have been carried out for eight isomers of the biomolecule mimic 3-(4-hydroxyphenyl)-N-benzylpropionamide (HNBPA). A number of choices for generation of orbital domains, and treatment of electron pair interactions have been tested. The results of these calculations are compared to those obtained from localized orbital MP2 and SCS-MP2 calculations. It is found that the SCS-MP2 method provides relative conformational energies very similar to those obtained from the higher-level coupled cluster calculations.

We have explored the room temperature response of metal nanoparticle decorated single-walled carbon nanotubes (NP-SWNTs) using a combination of electrical transport, optical spectroscopy, and electronic structure calculations. We have found that upon the electrochemical growth of Au NPs on SWNTs, there is a transfer of electron density from the SWNT to the NP species, and that adsorption of CO molecules on the NP surface is accompanied by transfer of electronic density back into the SWNT. Moreover, the electronic structure calculations indicate dramatic variations in the charge density at the NP-SWNT interface, which supports our previous observation that interfacial potential barriers dominate the electrical behavior of NP-SWNT systems.

We describe theoretical methods for the calculation of vibrational and electronic transitions in sulfuric acid, from which absorption cross sections can be obtained in the infrared through to the vacuum ultraviolet region. In the absence of experimental cross sections these calculations provide invaluable input for the assessment of the atmospheric photolysis of sulfuric acid. The vibrational model is based on a local mode model that includes the OH-stretching and SOH-bending vibrations, while the electronic transitions are calculated with coupled cluster response theory. These approaches are sufficient to describe the dominant vibrational transitions in the near infrared and visible regions, the lowest lying electronic transitions in the ultraviolet region and the higher energy electronic transitions in the region of Lyman- α radiation. We highlight the influence quantum mechanical calculations have had in the recent discussion of the atmospheric photolysis of sulfuric acid, and show that theoretical calculations can provide absorption cross sections of an accuracy that is useful in atmospheric science.

We have calculated the frequencies and intensities of the hydrogen-bonded OH-stretching transitions in the water dimer complex. The potential-energy curve and dipole-moment function are calculated ab initio at the coupled cluster with singles, doubles, and perturbative triples level of theory with correlation-consistent Dunning basis sets. The vibrational frequencies and wavefunctions are found from a numerical solution to a one-dimensional Schrödinger equation. The corresponding transition intensities are found from numerical integration of these vibrational wavefunctions with the ab initio calculated dipole moment function. We investigate the effect of counterpoise correcting both the potential-energy surface and dipole-moment function. We find that the effect of using a numeric potential is significant for higher overtones and that inclusion of a counterpoise correction for basis set superposition error is important.

We have recorded the vibrational absorption spectrum of 1,1,1,2-tetrafluoroethane (HFC-134a) in the fundamental and first five CH-stretching overtone regions with the use of Fourier transform infrared, dispersive long-path, intracavity laser photoacoustic, and cavity ringdown spectroscopies. We compare our measured total oscillator strengths in each region with intensities calculated using an anharmonic oscillator local mode model. We calculate intensities with 1D, 2D, and 3D Hamiltonians, including one or two CH stretches and two CH stretches with the HCH bending mode, respectively. The dipole moment function is calculated ab initio with self-consistent-field Hartree-Fock and density functional theories combined with double- and triple-zeta-quality basis sets. We find that the basis set choice affects the total intensity more than the choice of the Hamiltonian. We achieve agreement between the calculated and measured total intensities of approximately a factor of 2 or better for the fundamental and first five overtones.

We have simulated the HOONO vibrational overtone spectrum with use of a local mode Hamiltonian that includes the OH-stretching, OOH-bending, and NOOH-torsional modes and coupling between all three modes. The local mode parameters and the dipole moment function are calculated with coupled-cluster ab initio theory and an augmented Dunning-type triple-zeta basis set. We investigate the accuracy of the local mode parameters obtained from two different potential-energy fitting routines, as well as the sensitivity of these parameters to the basis set employed. We compare our simulated spectra to previously published action spectra in the first and second OH-stretching overtone regions. In addition we have recorded the spectrum in the OH-stretch and OOH-bend combination region around 7700 cm-1 and we also compare to this. Our simulated spectrum is in qualitative agreement with experiment in the first and second OH-stretching overtone and in the stretch-bend regions.

[1] We present the first microphysical calculations confirming that photolysis of sulfuric acid vapor by visible light is responsible for the formation of the springtime “CN layer” observed in the polar stratosphere. Our calculations show that the recently proposed photolysis mechanism is also sufficient to explain observations of vertically increasing SO2 mixing ratios in the upper stratosphere. Such photolysis, however, does not sufficiently explain the limited observations above 40 km of vertically decreasing H2SO4 and SO3 vapor, suggesting an additional loss mechanism there. We rule out previous speculation regarding reaction of H2SO4 with O(1D) or OH as inconsistent with observations of SO2. Rather, the loss is consistent with a permanent sink for sulfur, such as H2SO4 neutralization by metals on meteoritic dust. In light of such removal, H2SO4 photolysis to SO2 gains added importance in preserving gaseous sulfur in the upper stratosphere. We also present new cross sections derived from ab initio calculations for H2SO4 absorption at visible and Lyman-α wavelengths and evaluate their atmospheric implications. Our atmospheric model reveals that photolysis of H2SO4 by Lyman-α radiation is responsible for up to one third of the SO2 in the mesosphere and 10% of the particles in the polar stratospheric CN layer.

The lowest-energy electronic transitions in the hydroxyl radical and the hydrogen bound complex H(2)O.HO have been studied using ab initio methods. We have used the complete active-space self-consistent field and multireference configuration interaction (MRCI) methods to calculate vertical excitation energies and oscillator strengths. At the MRCI level the lowest-lying (2)Sigma(+)--(2)Pi electronic transition is redshifted by about 2500 cm(-1) upon formation of the H(2)O.HO complex. We propose that this transition could be used to identify the complex in the gas phase, which in turn could be used to examine the role of H(2)O.HO in atmospheric reactions.

[1] Estimates of absorption optical depths for the bound complexes H2O-H2O and the sum of H2O-N2, H2O-O2, and H2O-Ar at visible and near-infrared wavelengths are compared to the same quantities calculated from a frequently used water continuum parameterization (MT_CKD) and from a theoretical far wing water vapor lineshape theory. The temperature dependences of some of these optical depths are also compared. The comparisons suggest qualitatively that water complexes may contribute to the continuum at these wavelengths, and show that the temperature dependence of the continuum might provide insight into the role of the complexes in the atmosphere. Because of the dearth of laboratory measurements of the continuum at these wavelengths, and because the current estimates for the equilibrium constants of these water vapor complexes remain highly uncertain, more observations are needed before the importance of water complexes can be accurately quantified.

We have measured the infrared spectrum of H2O.HO in argon matrices at 11.5 +/- 0.5 K. We have also calculated the vibrational frequencies and intensities of the H2O.HO complex. As a result of these measurements and calculations, we have assigned a previously unassigned absorption band at 3442.1 cm-1 to the OH stretch in the radical complexed to the water molecule. This absorption originates from a complex that is situated in a different site within the argon matrix to those absorptions already assigned to this vibration at 3452.2 and 3428.0 cm-1. We observe a decrease in intensity of the OH radical stretching vibration of the H2O.HO complex upon isotopic substitution of D for H that agrees well with our calculations.

Resonant two-photon ionization (R2PI), resonant ion-dip infrared (RIDIR), and UV-UV hole-burning spectroscopies have been employed to obtain conformation-specific infrared and ultraviolet spectra under supersonic expansion conditions for O-(2-acetamidoethyl)-N-acetyltyramine (OANAT), a doubly substituted aromatic in which amide-containing alkyl and alkoxy side chains are located in para positions on a phenyl ring. For comparison, three single-chain analogs were also studied: (i) N-phenethyl-acetamide (NPEA), (ii) N-(p-methoxyphenethyl-acetamide) (NMPEA), and (iii) N-(2-phenoxyethyl)-acetamide (NPOEA). Six conformations of OANAT have been resolved, with S(0)-S(1) origins ranging from 34,536 to 35,711 cm(-1), denoted A-F, respectively. RIDIR spectra show that conformers A-C each possess an intense, broadened amide NH stretch fundamental shifted below 3400 cm(-1), indicative of the presence of an interchain H bond, while conformers D-F have both amide NH stretch fundamentals in the 3480-3495 cm(-1) region, consistent with independent-chain structures with two free NH groups. NPEA has a single conformer with S(0)-S(1) origin at 37,618 cm(-1). NMPEA has three conformers, two that dominate the R2P1 spectrum, with origin transitions between 35,580 and 35,632 cm(-1). Four conformations, one dominate and three minor, of NPOEA have been resolved with origins between 35,654 and 36,423 cm(-1). To aid the making of conformational assignments, the geometries of low-lying structures of all four molecules have been optimized and the associated harmonic vibrational frequencies calculated using density functional theory (DFT) and RIMP2 methods. The S(0)-S(1) adiabatic excitation energies have been calculated using the RICC2 method and vertical excitation energies using single-point time-dependent DFT. The sensitivity of the S(0)-S(1) energy separation in OANAT and NPOEA primarily arises from different orientations of the chain attached to the phenoxy group. Using the results of the single-chain analogs, tentative assignments have been made for the observed conformers of OANAT. The RIMP2 calculations predict that interchain H-bonded conformers of OANAT are 25-30 kJ/mol more stable than the extended-chain structures. However, the free energies of the interchain H-bonded and extended structures calculated at the preexpansion temperature (450 K) differ by less than 10 kJ/mol, and the number of extended structures far outweighs the number of H-bonded conformers. This entropy-driven effect explains the presence of the independent-chain conformers in the expansion, and cautions future studies that rely solely on relative energies of conformers in considering possible assignments.

Vertical and adiabatic excitation energies of the lowest (2)A(') excited state in the water-hydroxyl complex have been determined using coupled cluster, multireference configuration interaction, multireference perturbation theory, and density-functional methods. A significant redshift of about 0.4 eV in the vertical excitation energy of the complex compared to that of the hydroxyl radical monomer is found with the coupled cluster calculations validating previous results. Electronic excitation leads to a structure with near-equal sharing of the hydroxyl hydrogen by both oxygen atoms and a concomitantly large redshift of the adiabatic excitation energy of approximately 1 eV relative to the vertical excitation energy. The combination of redshifts ensures that the electronic transition in the complex lies well outside the equivalent excitation in the hydroxyl radical monomer. The complex is approximately five times more strongly bound in the excited state than in the ground state.