Stella M. Resende

Atmospheric reaction between the HS radical and chlorine

Abstract The mechanism and the kinetics of the reaction between HS and Cl2 leading to HSCl and Cl were studied theoretically. The effect of the complete basis set limit (CBS) was included by extrapolating the results obtained with cc-pVDZ, cc-pVTZ and cc-pVQZ correlation consistent basis sets functions. At the highest level of calculation, CCSD(T), the calculated rate constant is 1.0×10−12 cm3 molecule−1 s−1. The activation Gibbs free energy was calculated to be 7.4 kcal/mol, which is only 0.2 kcal/mol above the value determined experimentally. The enthalpy and Gibbs free energies of the reaction are −6.9 and −6.5 kcal/mol, respectively.

Radiative and predissociative lifetimes of the A 2Σ+ state (v′=0,1) of SH and SD: A highly correlated theoretical investigation

Doublet and quartet states of the HS radical correlating with H(2S)+S(3P,1D,1S) were investigated by ab initio calculations, at the CASSCF-MRCI/aug-cc-pV5Z level of theory. Molecular parameters and spectroscopic constants obtained for both the ground (X 2Π) and the first excited (A 2Σ+) states represent the best overall theoretical description of this system to date. Transition moments, transition probabilities, and radiative and predissociative lifetimes were also determined for the X 2Π–A 2Σ+ system. The values calculated for the radiative lifetimes of the A state show that previous results were too large. Theoretical predissociative lifetimes, although quite sensitive to the region of crossing of the potential energy curves, reproduce the experimental trends.

What is so mysterious about the electronic states of SCI

The 15 lowest lying doublet electronic states of the molecule SCI have been investigated theoretically at a high level of correlation treatment (MRCI). For the ground state (X 2II), spectroscopic constants were obtained from a set of eight vibrational intervals. This result extends our knowledge about this state beyond the experimentally known data that presently were derived from only two bands. Spin-orbit constants, transition probabilities and radiative lifetimes complement its spectroscopic characterization. For the excited electronic states, a global view of the doublet states is presented that can help us understand the scarcity of experimental data on electronic transitions for this system and also the difficulty of assigning the only two transitions so far recorded. Most of these states are repulsive, and for the few high lying bound ones, of Rydberg character, avoided crossings restrict the number of accessible vibrational states. Crossing by repulsive states and predissociation is also a factor that can prevent further emissions. Two new bound excited states, 2δ and 2Σ, predicted in this study, are of direct relevance to an interpretation of the limited experimental data available on electronic transitions.

A high level theoretical investigation of the N2O4→2 NO2 dissociation reaction: Is there a transition state?

The N2O4→2 NO2 dissociation reaction was investigated at a high level of theory using the couple cluster with all single and double excitations and connected triples [CCSD(T)] and complete active space self-consistent field approaches, and the cc-pVDZ, aug-cc-pVDZ, and cc-pVTZ basis sets. Only at the coupled cluster level a first-order saddle point was found connecting reactant and products. Collectively, structural, vibrational, and thermodynamic data for the three stationary points represent the best theoretical description of this reaction system to date, and are in good agreement with available experimental results. Unimolecular transition state theory rate constants (k∞) were also evaluated at 250, 298.15, and 350 K. At the CCSD(T)/cc-pVTZ level of calculation these results are 0.62×101, 1.90×103, and 1.66×105 s−1, respectively. Known experimental results at 298 K vary from 1.7×105 to 1.0×106 s−1. Including an estimate for basis set superposition error, we predict ΔH2980 for the dissociation reaction...

Thermochemistry of atmospheric sulfur compounds

Abstract The heats of formation at 298.15 K of important atmospheric sulfur species were calculated using a high level of electronic correlation, CCSD(T), extrapolated to the complete basis set limit. Our results for SH, CH 3 SH, CH 3 S, CH 3 SO and CH 3 SO 2 , are, respectively, 34.4, −5.3, 29.7, −16.8 and −53.1 kcal mol −1 . By carefully choosing the chemical reactions, this investigation reports the most accurate Δ H 0 f values available to date for these species.

Enthalpy of Formation of CH3SO and CH3SO2: A Difficult Case in Quantum Chemistry

The enthalpies of formation of CH3SO and CH3SO2 at 298.15 K were determined at a high level of electronic correlation, CCSD(T), and with the use of Woon and Dunnings procedure to reach the complete basis set limit. This study led to values of -16.7 kcal mol-1 and -53.1 kcal mol-1, respectively, for CH3SO and CH3SO2, which are expected to be the most accurate ones to date. Analysis of existing calculations at various levels of theory clearly shows the need of large basis sets and a high level of electronic correlation treatment to produce reliable and accurate results.

The reaction of SH with O2: A theoretical high level investigation

The atmospheric interaction of SH with O2 was investigated using single and multireference theoretical techniques. Several possibilities for the products were tested, the most important being the formation of HSOO. The rate constant for this reaction was calculated as being 4.5 × 10−22 cm3 molecule−1 s−1. It is a very slow reaction and the product is unfavorable thermodynamically by 6.07 kcal mol−1 at 298 K. This reaction is not expected to be a significant one in the atmosphere. Our calculations also show that the species SOOH is not bound, and that the decomposition of HSOO will lead to SO + OH. The rate constant of this unimolecular reaction was predicted to be 1.2 × 10−14 s−1.

Does SH really react with O3 in the ground state

Abstract The minimum energy path for the reaction of SH with O 3 was investigated at a highly correlated level of calculation using the multireference MP2 and configuration interaction (CI) methods, with basis sets ranging from DZP to aug-cc-pVTZ. Starting from ground state reactants and following the minimum energy path, ground state products and a significant activation energy barrier are obtained. This energy barrier led to a kinetic constant of 2.3×10 −17 cm 3 / molecule s for atmospheric conditions of 1 atm and 298 K. These results contrast with experimental findings of the formation of an excited HSO molecule and a kinetic constant several orders of magnitude greater. Since no electronic crossing was also verified in our calculations, these results are suggestive that the experimental kinetic determination might be involving reactants in their excited states, probably O 3 .

Theoretical study of the photodissociation cross section of HSCl

The photoabsorption spectra of HSCl are predicted on the basis of a single one-variable model. Potential curves for the six lowest-lying electronic states and transition moment functions were evaluated at a high level of correlation treatment (CASSCF/MRCI/cc-pVQZ). Similarly to HOCl, it is predicted that the presence of HSCl in the stratosphere, besides coupling the chemistry of sulfur and chlorine, might also have an important role in the ozone depletion cycle.

Thermodynamical analysis of the atmospheric fate of the CH3SCH2O2 radical

The thermodynamic properties of nine possible reactions for the atmospheric fate of the CH3SCH2O2 radical are analysed in this work. The reactant species considered were NO, NO2, HO2, CH3O2, CH3S, CH3SO, CH3SO2, O and O2. The calculations were performed at the UCCSD(T)/cc-pVTZ//MP2/6-31G(d) level of theory. Reactions of the CH3SCH2O2 radical with NO, NO2, HO2, CH3S, CH3SO, CH3SO2 and O are exothermic and spontaneous. In the reaction with NO2, the equilibrium will be highly dislocated towards the reactants. The reaction with CH3O2 radical is endothermic by a small quantity, but spontaneous. The estimated rate constant indicates that it will not be important. Reaction with O2 leading to the CH3SCH2O and O3 products is thermodynamically unfavourable, having a positive reaction enthalpy. The thermodynamical features of the reactions with O and HO2 radicals indicate that these reactions should be important in this process, and kinetic studies of these reactions are strongly suggested. The heats of formation of the CH3O2, CH3SO, CH3SO2, CH3SO3, CH3SCH2O, CH3SCH2O2H, CH3SCH2O2NO, CH3SCH2O2NO2, CH3SCH2O3 and CH3SCH2O4 species were determined.