An Ab Initio Molecular Dynamics Study of the Hydrolysis Reaction of Sulfur Trioxide Catalyzed by a Formic Acid or Water Molecule.

Ab initio molecular dynamics (AIMD) calculations have been performed to investigate the role of dynamical and steric effects in formic acid (FA) or H2O-catalyzed gas phase hydrolysis of SO3 to form sulfuric acid. This was done by colliding FA or H2O with the SO3-H2O complex and the water dimer with the SO3 molecule and analyzing the outcomes of 230 AIMD trajectories. Our calculations show that, within simulation times used, sulfuric acid is formed in 5% of FA collisions but is not produced when H2O collides with the SO3-H2O complex or when the water dimer collides with the SO3 molecule. We also find that FA collisions have about 2 times higher probability to form the prereactive complex than H2O collisions. Moreover, our simulations show that the SO3-H2O-FA prereactive complex is more stable in time than the SO3-H2O-H2O prereactive complex. These findings indicate that the FA-catalyzed mechanism is favored over the H2O one when looking from the steric and dynamic effect point of view. Additionally, AIMD simulations starting from the optimized structure of the SO3-H2O-FA prereactive complex have been computed to qualitatively estimate the rate of the sulfuric acid formation. Collisional energy has been observed to promote sulfuric acid formation more effectively than thermal excitation.

Broadband photoacoustic spectroscopy of CH414 with a high-power mid-infrared optical frequency comb.

We report a photoacoustic spectroscopy setup with a high-power mid-infrared frequency comb as the light source. The setup is used in broadband spectroscopy of radiocarbon methane. Owing to the high sensitivity of a cantilever-enhanced photoacoustic cell and the high-power light source, we can reach a detection limit below 100 ppb in a broadband measurement with a sample volume of only a few milliliters. The first infrared spectrum of CH414 is reported and given a preliminary assignment. The results lay a foundation for the development of optical detection systems for radiocarbon methane.

Ab Initio Molecular Dynamics Simulations of the Influence of Lithium Bromide on the Structure of the Aqueous Solution-Air Interface.

We present the results of ab initio molecular dynamics simulations of the solution-air interface of aqueous lithium bromide (LiBr). We find that, in agreement with the experimental data and previous simulation results with empirical polarizable force field models, Br- anions prefer to accumulate just below the first molecular water layer near the interface, whereas Li+ cations remain deeply buried several molecular layers from the interface, even at very high concentration. The separation of ions has a profound effect on the average orientation of water molecules in the vicinity of the interface. We also find that the hydration number of Li+ cations in the center of the slab Nc,Li+-H2O ≈ 4.7 ± 0.3, regardless of the salt concentration. This estimate is consistent with the recent experimental neutron scattering data, confirming that results from nonpolarizable empirical models, which consistently predict tetrahedral coordination of Li+ to four solvent molecules, are incorrect. Consequently, disruption of the hydrogen bond network caused by Li+ may be overestimated in nonpolarizable empirical models. Overall, our results suggest that empirical models, in particular nonpolarizable models, may not capture all of the properties of the solution-air interface necessary to fully understand the interfacial chemistry.

Ab initio molecular dynamics studies of formic acid dimer colliding with liquid water.

Ab initio molecular dynamics simulations of formic acid (FA) dimer colliding with liquid water at 300 K have been performed using density functional theory. The two energetically lowest FA dimer isomers were collided with a water slab at thermal and high kinetic energies up to 68kBT. Our simulations agree with recent experimental observations of nearly a complete uptake of gas-phase FA dimer: the calculated average kinetic energy of the dimers immediately after collision is 5 ± 4% of the incoming kinetic energy, which compares well with the experimental value of 10%. Simulations support the experimental observation of no delayed desorption of FA dimers following initial adsorption. Our analysis shows that the FA dimer forms hydrogen bonds with surface water molecules, where the hydrogen bond order depends on the dimer structure, such that the most stable isomer possesses fewer FA-water hydrogen bonds than the higher energy isomer. Nevertheless, even the most stable isomer can attach to the surface through one hydrogen bond despite its reduced hydrophilicity. Our simulations further show that the probability of FA dimer dissociation is increased by high collision energies, the dimer undergoes isomerization from the higher energy to the lowest energy isomer, and concerted double-proton transfer occurs between the FA monomers. Interestingly, proton transfer appears to be driven by the release of energy arising from such isomerization, which stimulates those internal vibrational degrees of freedom that overcome the barrier of a proton transfer.

Effects of Global and Local Anharmonicities on the Thermodynamic Properties of Sulfuric Acid Monohydrate.

We use state-of-the-art electronic structure calculation methods and large basis sets to obtain reliable values for the thermodynamic properties of sulfuric acid monohydrate and study the effects of vibrational anharmonicity on these properties. We distinguish between two forms of vibrational anharmonicity: local anharmonicity, which refers to the anharmonicity of the vibrational modes of a given cluster conformer, and global anharmonicity, which originates from accounting for the presence of different conformers in the first place. In our most accurate approach, we solve the nuclear Schrödinger equation variationally for the intermolecular large-amplitude motions, thus quantum-mechanically accounting for the presence of higher-energy conformers for both reactants and products, while using the standard vibrational perturbational approach for the other vibrational modes. This results in a value of -11.0 kJ/mol for the reaction Gibbs free energy at 298.15 K. When standard vibrational perturbational approaches are employed, the effects of local anharmonicity depend heavily on the choice of the electronic structure calculation basis set. In fact, better results can often be achieved by combining a simple harmonic treatment for the vibrational partition function with a statistical mechanical accounting of global anharmonicity. Thus, we recommend that future studies that intend to include anharmonicity start by accounting for the presence of higher-energy conformers and only then consider whether local anharmonicity calculations are feasible and necessary.

Generalized Intermolecular Interaction Tensor Applied to Long-Range Interactions in Hydrogen and Coinage Metal (Cu, Ag, and Au) Clusters.

We present a novel formulation for the intermolecular interaction tensor, which is used to describe the long-range electrostatic, induction, and dispersion interactions. Our formulation is based on concepts drawn from combinatorial analysis and Clifford calculus and enables us to present the interaction tensor in a form that is simple to use and suitable for both numerical and symbolic analyses. We apply the derived formulas to calculate the long-range interaction coefficients in hydrogen and coinage metal (Cu, Ag, and Au) clusters. The electronic structure calculations are performed at the CCSD(T) level, with triple-ζ and quadruple-ζ basis sets. The multipole moments and dispersion coefficients are obtained as fits to the derived interaction formulas. The most important interaction parameters are obtained accurately and are in good agreement with other results.

The effect of large amplitude motions on the vibrational intensities in hydrogen bonded complexes.

We have developed a model to calculate accurately the intensity of the hydrogen bonded XH-stretching vibrational transition in hydrogen bonded complexes. In the Local Mode Perturbation Theory (LMPT) model, the unperturbed system is described by a local mode (LM) model, which is perturbed by the intermolecular modes of the hydrogen bonded system that couple with the intramolecular vibrations of the donor unit through the potential energy surface. We have applied the model to three complexes containing water as the donor unit and different acceptor units, providing a series of increasing complex binding energy: H2O⋯N2, H2O⋯H2O, and H2O⋯NH3. Results obtained by the LMPT model are presented and compared with calculated results obtained by other vibrational models and with previous results from gas-phase and helium-droplet experiments. We find that the LMPT model reduces the oscillator strengths of the fundamental hydrogen bonded OH-stretching transition relative to the simpler LM model.

Pair-potential approach to accurate dispersion energies between group 12 (Zn, Cd, Hg) clusters.

Dispersion interactions between group 12 (Zn, Cd, Hg) metal clusters are studied at the CCSD(T) level with triple-ζ basis sets. We use atomic orientation dependent C6 dispersion coefficients derived from simple model systems to calculate the intermolecular dispersion energy between larger metal clusters. By using an atomic pair-potential model, we are able to obtain highly accurate results comparable to the CCSD(T) level of theory. For all the systems studied, our method results in a robust description of the dispersion energy at long distances, and it is able to outperform the examined dispersion corrected DFT calculations.

Dispersion interactions in small zinc, cadmium, and mercury clusters.

Dispersion interactions in small clusters of group XII (Zn, Cd, Hg) metal atoms are studied at the CCSD(T) level with triple-ζ basis sets. A pair potential model together with a least-squares fit to the interaction potential energy surface is used to calculate interatomic dispersion coefficients, which are found to be in good agreement with atomistic calculations. The angular dependence of the dispersion interaction, extracted by explicitly accounting for the leading order electrostatic and induction terms, is determined with the aid of a coupled spherical harmonic expansion. The distance dependence of the orientation averaged dispersion energy is modeled by an integral average of our potential model, which is able to robustly account for the average dispersion energy to a remarkable degree.

The effect of large amplitude motions on the transition frequency redshift in hydrogen bonded complexes: a physical picture.

We describe the vibrational transitions of the donor unit in water dimer with an approach that is based on a three-dimensional local mode model. We perform a perturbative treatment of the intermolecular vibrational modes to improve the transition wavenumber of the hydrogen bonded OH-stretching transition. The model accurately predicts the transition wavenumbers of the vibrations in water dimer compared to experimental values and provides a physical picture that explains the redshift of the hydrogen bonded OH-oscillator. We find that it is unnecessary to include all six intermolecular modes in the vibrational model and that their effect can, to a good approximation, be computed using a potential energy surface calculated at a lower level electronic structure method than that used for the unperturbed model.

Characterization of the dispersion interactions and an ab initio study of van der Waals potential energy parameters for coinage metal clusters.

Dispersion interactions and van der Waals C(6) coefficients are studied in small model systems involving copper, silver, and gold atoms. We investigate a novel method where the intermolecular dispersion interactions are characterized by interatomic C(6) coefficients that can be used to formulate the dispersion energy as a separate contribution to the total interaction energy at long distances. We obtain the C(6) coefficients from a least-squares fit to the interaction energy surface. Other significant effects to the long-range interaction energy such as multipole-multipole and induction energy are explicitly taken into account. The electronic structure calculations are performed at the coupled cluster level including single, double, and perturbative triple excitations (CCSD(T) level) with triple-ζ basis sets.

Ab initio structural and vibrational investigation of sulfuric acid monohydrate.

We employ ab initio methods to find stable geometries and to calculate potential energy surfaces and vibrational wavenumbers for sulfuric acid monohydrate. Geometry optimizations are carried out with the explicitly correlated coupled-cluster approach that includes single, double, and perturbative triple excitations (CCSD(T)-F12a) with a valence double-ζ basis set (VDZ-F12). Four different stable geometries are found, and the two lowest are within 0.41 kJ mol(-1) (or 34 cm(-1)) of each other. Vibrational harmonic wavenumbers are calculated at both the density-fitted local spin component scaled second-order Møller-Plesset perturbation theory (DF-SCS-LMP2) with the aug-cc-pV(T+d)Z basis set and the CCSD-F12/VDZ-F12 level. Water O-H stretching vibrations and two highly anharmonic large-amplitude motions connecting the three lowest potential energy minima are considered by limiting the dimensionality of the corresponding potential energy surfaces to small two- or three-dimensional subspaces that contain only strongly coupled vibrational degrees of freedom. In these anharmonic domains, the vibrational problem is solved variationally using potential energy surfaces calculated at the CCSD(T)-F12a/VDZ-F12 level.

An ab initio study of van der Waals potential energy parameters for silver clusters.

We employ ab initio calculations of van der Waals complexes to study the potential energy parameters (C(6) coefficients) of van der Waals interactions for modeling of the adsorption of silver clusters on the graphite surface. Electronic structure calculations of the (Ag(2))(2), Ag(2)-H(2), and Ag(2)-C(6)H(6) complexes are performed using a coupled-cluster approach that includes single, double, and perturbative triple excitations (CCSD(T)), Møller-Plesset second-order perturbation theory (MP2), and spin-component-scaled MP2 (SCS-MP2) methods. Using the atom pair approximation, the C(6) coefficients for silver-silver, silver-hydrogen, and silver-carbon atom systems are obtained after subtracting the energies of quadrupole-quadrupole interactions from the total electronic energy.

Acceptor tunneling motion and O-H stretching vibration overtones of the water dimer.

We have carried out a detailed investigation of the acceptor tunneling in the water dimer. This motion is responsible for the largest splitting of vibrational states in the dimer, the so-called acceptor splitting. Our results confirm that this splitting is due to a coupled 2-fold motion: The internal rotation of the donor with respect to the O-O axis, and the wagging of the acceptor. The minimum potential energy path along the corresponding coordinate was computed using the coupled-cluster ab initio with single, double, and perturbative triple excitations (CCSD(T)) method and the augmented correlation consistent polarized valence quadruple zeta basis set (aug-cc-pVQZ). The pure acceptor tunneling energy levels were obtained by the variational method with a free rotor basis. The acceptor splittings associated with the O-H stretching overtone states of the water dimer were calculated with a simple model which employs adiabatic separation between the tunneling motion and high-frequency vibrations.

Calculation of the O-H stretching vibrational overtone spectrum of the water dimer.

The O-H stretching vibrational overtone spectrum of the water dimer has been calculated with the dimer modeled as two individually vibrating monomer units. Vibrational term values and absorption intensities have been obtained variationally with a computed dipole moment surface and an internal coordinate Hamiltonian, which consists of exact kinetic energy operators within the Born-Oppenheimer approximation of the monomer units. Three-dimensional ab initio potential energy and dipole moment surfaces have been calculated using the internal coordinates of the monomer units using the coupled cluster method including single, double, and perturbative triple excitations [CCSD(T)] with the augmented correlation consistent valence triple zeta basis set (aug-cc-pVTZ). The augmented correlation consistent valence quadruple zeta basis set (aug-cc-pVQZ), counterpoise correction, basis set extrapolation to the complete basis set limit, relativistic corrections, and core and valence electron correlations effects have been included in one-dimensional potential energy surface cuts. The aim is both to investigate the level of ab initio and vibrational calculations necessary to produce accurate results when compared with experiment and to aid the detection of the water dimer under atmospheric conditions.

Simulation of inversion motion and N-H stretching overtone spectra of aniline.

A curvilinear internal coordinate Hamiltonian is used to simulate the N-H stretching overtone spectra and the associated inversion splittings in aniline. A simple local mode type model is applied to the N-H stretching and H-N-H bending modes. Geometric algebra is employed to derive the kinetic energy operator for the large amplitude inversion motion. Electronic structure calculations at the Moller-Plesset second order perturbation theory and correlation consistent aug-cc-pVTZ basis set level are used to obtain model parameters, some of which have been optimized with the least-squares method using experimental vibrational term values as data. The observed N-H stretching overtone vibrational levels and the inversional tunneling splittings are well reproduced with our approach.