Matrix-isolation photolysis of SO2, O3 and H2O: evidence for the H2O:SO3 complex

Weakly Bound Complex Formation between HCN and CH3Cl: A Matrix-Isolation and Computational Study

The matrix-isolated infrared spectrum of a hydrogen cyanide-methyl chloride complex was investigated in a solid argon matrix. HCN and CH₃Cl were co-condensed onto a substrate held at 10 K with an excess of argon gas, and the infrared spectrum was measured using Fourier-transform infrared spectroscopy. Quantum chemical geometry optimization, harmonic frequency, and natural bonding orbital calculations indicate stabilized hydrogen- and halogen-bonded structures. The two resulting weakly bound complexes are both composed of one CH₃Cl molecule bound to a (HCN)₃ subunit, where the three HCN molecules are bound head-to-tail in a ring formation. Our study suggests that─in the presence of CH₃Cl─the formation of (HCN)₃ is promoted through complexation. Since HCN aggregates are an important precursor to prebiotic monomers (amino acids and nucleobases) and other life-bearing polymers, this study has astrophysical implications toward the search for life in space.

The Structures, Molecular Orbital Properties and Vibrational Spectra of the Homo- and Heterodimers of Sulphur Dioxide and Ozone. An Ab Initio Study

The structures of a number of dimers of sulphur dioxide and ozone were optimized by means of a series of ab initio calculations. The dimer species were classified as either genuine energy minima or transition states of first or higher order, and the most probable structures consistent with the experimental data were confirmed. The molecular orbitals engaged in the interactions resulting in adduct formation were identified and relations between the orbitals of the dimers of the valence isoelectronic monomer species were examined. The vibrational spectra of the most probable structures were computed and compared with those reported in the literature, particularly with spectra observed in cryogenic matrices. The calculations were extended to predict the properties of a number of possible heterodimers formed between sulphur dioxide and ozone.

Competitive reaction pathways in vibrationally induced photodissociation of H2SO4

Vibrationally induced photodissociation of sulfuric acid into H2O + SO3 is investigated based on reactive molecular dynamics (MD) simulations. Multisurface adiabatic reactive MD simulations allow us to follow both, H-transfer and water elimination after excitation of the ν9 OH-stretching mode. Analysis of several thousand trajectories finds that the H2O and SO3 fragments have distinct final state distributions with respect to translational, rotational, and vibrational degrees of freedom. Rotational distributions peak at quantum numbers j ≤ 5 for water and j ≈ 60 for SO3. The final state distributions should be useful in identifying products in forthcoming experiments. Based on the MD trajectories, a kinetic scheme has been developed which is able to explain most of the trajectory data and suggests that IVR is very rapid. Typical lifetimes of the excited complex range from several 10 picoseconds to hundreds of nanoseconds, depending on the excitation level. Including temperature and pressure profiles characteristic for the stratosphere in the kinetic model shows that excitations higher than ν9 = 4 can significantly contribute to the photolysis rate. This extends and specifies earlier work in that multi-level modeling is required to understand the significance of vibrationally induced decomposition pathways of sulfuric acid in the middle atmosphere.

Prediction of tetraoxygen reaction mechanism with sulfur atom on the singlet potential energy surface

The mechanism of S+O₄ (D(₂h)) reaction has been investigated at the B3LYP/6-311+G(3df) and CCSD levels on the singlet potential energy surface. One stable complex has been found for the S+O₄ (D(₂h)) reaction, IN1, on the singlet potential energy surface. For the title reaction, we obtained four kinds of products at the B3LYP level, which have enough thermodynamic stability. The results reveal that the product P3 is spontaneous and exothermic with -188.042 and -179.147 kcal/mol in Gibbs free energy and enthalpy of reaction, respectively. Because P1 adduct is produced after passing two low energy level transition states, kinetically, it is the most favorable adduct in the ¹S+¹O₄ (D(₂h)) atmospheric reactions.

Theoretical Studies of the Reaction Channels on the SO2/OH/NO Singlet Potential Energy Surface

Ab initio MP2/6-311++G(2d,2p) investigation of the SO2/OH/NO singlet potential energy surface (PES) has been performed with the aim to localize and describe the existing minima and transition states linking them. The systematic studies have revealed seven minima, with the trans-HONO-SO2 complex (1t) being the global minimum. Eight transition states between minima or between minima and the relevant reactant species have been described. Several available izomerization and dissociation routes have been identified and discussed. The most favorable association of HOSO2 and NO was found to be a barrierless process forming nitrososulfonic acids. Isomerizations between trans-, cis-, and gauche- nitrososulfonic acids (2t, 2c, and 2g) are possible with low-energy barriers. The HOSO2 and NO species can also react via another channels involving high-energy transition states to produce the HOSO-NO2 (3) and HNO-SO3 (4) complexes.

Counterpoise corrected geometries of hydrated complexes

We have calculated the equilibrium geometries of the hydrated complexes, H2O.CO2, H2O.CS2,H2O.OCS, H2O.SO2, and H2O.SO3, in the electronic ground state. We have used the coupled cluster with singles, doubles, and perturbative triples ab initio method with a correlation consistent augmented triple-zeta basis set. We find that a counterpoise corrected optimization scheme is important for an accurate description of the geometries. These high level ab initio calculated geometries are of comparable quality to those obtained experimentally.

A Gas- and Condensed-Phase Density Functional Study of Donor−Acceptor Complexes of Sulfur Trioxide

A series of donor-acceptor complexes containing sulfur trioxide have been studied in the gas and condensed phases using density functional theory. The condensed phase is represented using the polarizable continuum model. The systems investigated include complexes of nitrogen-containing donor molecules, (CH(3))(n)H(3-n)N (n = 0-3), with SO(3) and complexes of oxygen-containing donor molecules, (CH(3))(m)H(2-m)O (m = 0-2), with SO(3). Significant differences are observed between the gas- and condensed-phase properties of the complexes as a result of the ability of the condensed-phase medium to support higher charge separation between the donor and acceptor. The gas/condensed-phase behavior of two nitrogen-containing complexes, (CH(3))H(2)N-SO(3) and (CH(3))(2)HN-SO(3), has been investigated for the first time. These complexes exhibit properties intermediate to the previously observed H(3)N-SO(3) and (CH(3))(3)N-SO(3) complexes. Systematic trends in the gas- and condensed-phase structure and properties have been observed as methyl groups are added to the donor molecule. In addition, two oxygen-containing complexes, CH(3)OH-SO(3) and (CH(3))(2)O-SO(3), have been characterized for the first time. The differences between the gas- and condensed-phase properties of the oxygen-containing complexes are, in many cases, larger than those of the nitrogen-containing complexes, and therefore they represent an intriguing new class of complexes for potential experimental observation. Finally, a strong correlation between the charge transfer and binding energy has been obtained for both the nitrogen- and oxygen-containing complexes of sulfur trioxide.

About the stability of sulfurous acid (H2SO3) and its dimer

The characterization and isolation of sulfurous acid (H2SO3) have never been accomplished and thus still remain one of the greatest open challenges of inorganic chemistry. It is known that H2SO3 is thermodynamically unstable. In this study, however, we show that a Ci-symmetric dimer of sulfurous acid (H2SO3)2 is 3.5 kcal mol-1 more stable than its dissociation products SO2 and H2O at 77 K. Additionally, we have investigated the kinetic stability of the sulfurous acid monomer with respect to dissociation into SO2 and H2O and the kinetic isotope effect (KIE) on this reaction by transition-state theory. At 77 K, the half-life of H2SO3 is 15 x 10(9) years, but for the deuterated molecule (D2SO3) it increases to 7.9 x 10(26) years. At room temperature, the half-life of sulfurous acid is only 24 hours; however, a KIE of 3.2 x 10(4) increases it to a remarkable 90 years. Water is an efficient catalyst for the dissociation reaction since it reduces the reaction barrier tremendously. With the aid of two water molecules, one can observe a change in the reaction mechanism for sulfurous acid decomposition with increasing temperature. The most likely mechanism below 170 K is via an eight-membered transition-state ring; yet, above 170 K, a mechanism with a six-membered transition state ring becomes the predominant one. For deuterated sulfurous acid, this change in reaction mechanism can be observed at 120 K. Consequently, between 120 and 170 K, different predominant reaction mechanisms occur for the decomposition of normal and deuterated sulfurous acid when assisted by two water molecules. However, the much longer half-life of deuterated sulfurous acid and the stability of the sulfurous acid dimer at 77 K are encouraging for future synthesis and characterization under laboratory conditions.

Toward elimination of discrepancies between theory and experiment: the rate constant of the atmospheric conversion of SO3 to H2SO4

The hydration rate constant of sulfur trioxide to sulfuric acid is shown to depend sensitively on water vapor pressure. In the 1:1 SO3-H2O complex, the rate is predicted to be slower by about 25 orders of magnitude compared with laboratory results [Lovejoy, E. R., Hanson, D. R. & Huey, L. G. (1996) J. Phys. Chem. 100, 19911-19916; Jayne, J. T., Poschl, U., Chen, Y.-m., Dai, D., Molina, L. T., Worsnop, D. R., Kolb, C. E. & Molina, M. J. (1997) J. Phys. Chem. A 101, 10000-10011]. This discrepancy is removed mostly by allowing a second and third water molecule to participate. An asynchronous water-mediated double proton transfer concerted with the nucleophilic attack and a double proton transfer accompanied by a transient H3O+ rotation are predicted to be the fastest reaction mechanisms. Comparison of the predicted negative apparent "activation" energies with the experimental finding indicates that in our atmosphere, different reaction paths involving two and three water molecules are taken in the process of forming sulfate aerosols and consequently acid rain.