Theoretical study of O2-H2O: potential energy surface, molecular vibrations, and equilibrium constant at atmospheric temperatures

Composition of the Water Dimer and the Heterodimers of Water with N2 and O2 in Earth's Atmosphere

The mole fractions χ and number concentrations n of the water dimer and the heterodimers H2O-N2 and H2O-O2 in Earth's atmosphere are reported up to 20 km. The water dimer data is obtained from published values of the equilibrium constant based on the water equation of state. The mixed equilibrium constants for the heterodimers are obtained from the respective second virial coefficients using an approach introduced by Stogryn and Hirschfelder that extracts the components pertaining to pairwise interactions producing bound and metastable dimers. From these calculations, χ and n for the water dimer and the (H2O)(N2) and (H2O)(O2) heterodimers at standard sea level are 1.79(6) × 10-5, 4.77(12) × 10-5 and 9.90(5) × 10-6 and 4.55(15) × 1014, 1.23(3) × 1016 and 2.56(1) × 1015, respectively. Analytical expressions are provided for these quantities for altitudes between 0-20 km and temperatures from 200-300 K. Sea level values of χ and n are given for two specific locations.

Explicitly correlated ab initio potential energy surface and predicted rovibrational spectra for H2O-N2 and D2O-N2 complexes

An ab initio intermolecular potential energy surface (PES) for the van der Waals complex of H₂O-N₂ that explicitly incorporates the intramolecular Q₂ bending normal mode of the H₂O monomer is presented. The electronic structure computations have been carried out at the explicitly correlated coupled cluster theory [CCSD(T)-F12] with an augmented correlation-consistent triple zeta basis set and an additional bond function. Analytic five-dimensional intermolecular PESs for ν₂(H₂O) = 0 and 1 are obtained by fitting to the multi-dimensional Morse/long-range potential function form. These fits to 40 890 points have the root-mean-square (rms) discrepancy of 0.88 cm^-1 for interaction energies less than 2000.0 cm^-1. The resulting vibrationally averaged PESs provide good representations of the experimental microwave and infrared data: for microwave transitions of H₂O-N₂, the rms discrepancy is only 0.0003 cm^-1, and for infrared transitions of the A₁ symmetry of the H₂O(ν₂ = 1 ← 0)-N₂, the rms discrepancy is 0.001 cm^-1. The calculated infrared band origin shifts associated with the ν₂ bending vibration of water are 2.210 cm^-1 and 1.323 cm^-1 for H₂O-N₂ and D₂O-N₂, respectively, in good agreement with the experimental values of 2.254 cm^-1 and 1.266 cm^-1. The benchmark tests and comparisons of the predicted spectral properties are carried out between CCSD(T)-F12a and CCSD(T)-F12b approaches.

High-resolution crystal structure of copper amine oxidase fromArthrobacter globiformis: assignment of bound diatomic molecules as O2

The crystal structure of a copper amine oxidase fromArthrobacter globiformiswas determined at 1.08 Å resolution with the use of low-molecular-weight polyethylene glycol (LMW PEG; average molecular weight ∼200) as a cryoprotectant. The final crystallographicRfactor andRfreewere 13.0 and 15.0%, respectively. Several molecules of LMW PEG were found to occupy cavities in the protein interior, including the active site, which resulted in a marked reduction in the overallBfactor and consequently led to a subatomic resolution structure for a relatively large protein with a monomer molecular weight of ∼70 000. About 40% of the presumed H atoms were observed as clear electron densities in theFo−Fcdifference map. Multiple minor conformers were also identified for many residues. Anisotropic displacement fluctuations were evaluated in the active site, which contains a post-translationally derived quinone cofactor and a Cu atom. Furthermore, diatomic molecules, most likely to be molecular oxygen, are bound to the protein, one of which is located in a region that had previously been proposed as an entry route for the dioxygen substrate from the central cavity of the dimer interface to the active site.

Molecular orbital model of the influence of interaction between O2 and aluminosilicate sites on the triplet-singlet energy gap and reactivity

The behavior of O(2) molecule in models of acid aluminosilicate sites on any kind of material was investigated using reliable QM ab initio calculations. The triplet-singlet energy gap of isolated O(2) was calculated at confident levels of theory with different basis sets as a reference. Models of aluminosilicate active sites interacting with oxygen in their singlet and triplet electronic states were considered for two kinds of O(2) arrangements. Geometry optimizations were performed on both non-corrected and corrected BSSE potential energy surfaces, realizing that good modeling of heavy atom-hydrogen interactions is sensitive to BSSE corrections during these processes. Energies were further evaluated at higher level of theory to test tendencies. Singlet oxygen appears more attractive to active aluminosilicate sites than those calculated with triplet oxygen, indicating a source of oxidative efficiency for designed nanostructures containing such molecular residues. It was clearly seen that aluminosilicate groups, appearing ubiquitously in several materials, could reduce the O(2) triplet-singlets energy gap by at least 10 kJ/mol. Some elegant features of oxygen interactions with such sites were further analyzed by means of the atoms in molecules (AIM) theory.

The water-oxygen dimer: First-principles calculation of an extrapolated potential energy surface and second virial coefficients

The systematic intermolecular potential extrapolation routine (SIMPER) is applied to the water-oxygen complex to obtain a five-dimensional potential energy surface. This is the first application of SIMPER to open-shell molecules, and it is the first use, in this context, of asymptotic dispersion energy coefficients calculated using the unrestricted time-dependent coupled-cluster method. The potential energy surface is extrapolated to the complete basis set limit, fitted as a function of intermolecular geometry, and used to calculate (mixed) second virial coefficients, which significantly extend the range of the available experimental data.

Infrared Spectroscopy of Ozone−Water Complex in a Neon Matrix

Matrix isolation infrared spectroscopy has been applied to study an ozone-water complex of atmospheric interest. The complex was identified in the spectral region of three normal modes of ozone and water. Ab initio calculation at MP4(SDQ), QCISD, and CCSD(T) levels indicates the existence of only one stable conformer, which accords with the present experimental result. This conformer belongs to the Cs symmetry group where two molecular planes of ozone and water are perpendicular to the Cs symmetry plane. The binding energy was calculated to be 1.89 kcal/mol at the CCSD(T)/6-311++G(3df,3pd)//CCSD(T)/6-311++G(d,p) level of theory. The formation constant and atmospheric abundance of the ozone-water complex are estimated using the thermodynamic and spectroscopic data obtained.