Potential energy surfaces and thermodynamic properties of weakly bound molecular complexes H2O-O3 and H2O-SO2

Новый метод для теоретического исследования поверхностей потенциальной энергии (ППЭ) и оценки термодинамических параметров структурно-нежестких молекулярных комплексов при явном учете ровибрационной динамики мономеров в составе комплекса применен для оценки термодинамических свойств водородно-связанных комплексов H2OO3 и H2OSO2. Новый подход включает классическое рассмотрение межмолекулярных движений мономеров, прямой неэмпирический расчет межмолекулярного потенциала и вычисление соответствующих статсумм методом Монте-Карло (интегрирование с простой случайной выборкой, позволяющее оценивать энтропийные вклады). Полные шестимерные межмолекулярные ППЭ комплексов, включающие 14016 (H2OO3) and 9672 (H2OSO2) структурно-уникальных точек, были рассчитаны неэмпирическим методом (MP2/6-311++G(2d,2p)). Локальные минимумы рассчитанной ППЭ были использованы как стартовые точки для дальнейшей оптимизации геометрии на уровнях QCISD/aug-ccpVTZ и MP2/aug-ccpVQZ. Было найдено, что структура глобального минимума комплекса H2OSO2 согласуется с экспериментальными данными, в то время как глобальный минимум H2OO3 соответствует искаженной асимметричной водородно-связанной структуре (C1), отличной от структуры, предложенной на основе интерпретации микроволновых спектров и предшествующих квантовохимических расчетов более низкого уровня. Использование нового метода позволило впервые оценить термодинамические функции и константы комплексообразования H2OO3 и H2OSO2 при явном учете их ровибрационной динамики. Наилучшие оценки стандартной константы равновесия K0(298), полученные новым методом, составляют 1.0510-2 (H2OO3) и 3.1510-2 (H2OSO2). С учетом эмпирической поправки, выведенной из анализа констант равновесия (H2O)2 и (D2O)2, эти значения составляют 8.710-3 и 2.510-2. Полученные результаты могут оказать существенное влияние на результаты существующих моделей атмосферных процессов

Potential energy surfaces of hydrogen-bonded complexes H2O...O3 and H2O...SO2 and their thermodynamic properties with explicit accounting of rovibrational contributions

A new approach for the theoretical exploration of the potential energy surface and thermodynamic properties evaluation of structurally-nonrigid molecular complexes explicitly accounting the rovibrational dynamics of monomers within the complex has been developed and applied to the estimation of thermodynamic properties of the hydrogen bonded complexes H2OO3 and H2O-SO2. The new approach consists of the classical consideration of intermolecular motions of monomers, direct ab initio calculation of intermolecular potential, and the Monte Carlo integration during the partition function evaluation. The full six-dimensional intermolecular potential energy surfaces (PES) of these complexes required for the thermodynamic calculations and consisted of 14016 (H2O-O3) and 9672 (H2O-SO2) unique points were calculated by the ab initio (MP2/6-311++G(2d,2p)) method. The local minima found at the calculated PESﾒs were used as starting points for the farther high-level geometry optimization (up to QCISD/aug-ccpVTZ and MP2/aug-ccpVQZ). It was found that the structure of the global minimum of H2O-SO2 complex is in the agreement with the available experimental data whereas the global minimum of H2O-O3 corresponds to the twisted asymmetric hydrogen-bonded structure (C1) different from that one proposed on the basis of microwave spectra and previous quantum chemical calculations. By using the new approach, the thermodynamic properties and the equilibrium constants of H2O-O3, and H2O-SO2 were obtained for the first time with explicit accounting their rovibrational dynamics. The best estimates for the K0(298) of the complexes H2O-O3, and H2O-SO2 are 1.05*10-2 and 3.15*10-2, respectively (empirically corrected values are 8.7*10-3 and 2.5*10-2). The corresponding uncertainty of these values is estimated as 25-35% (based on the known values of (H2O)2 and (D2O)2 equilibrium constants). The results obtained can significantly affect the conclusions made on the basis of many modern atmospheric models. The work was supported by the Russian Foundation for Basic Research (project No. 07-03-00390

Quantum Chemical Study Of The Initial Step Of Ozone Addition To The Double Bond Of Ethylene

The mechanisms of the initial step in chemical reaction between ozone and ethylene were studied by multireference perturbation theory methods (MRMP2, CASPT2, NEVPT2, and CIPT2) and density functional theory (OPW91, OPBE, and OTPSS functionals). Two possible reaction channels were considered: concerted addition through the symmetric transition state (Criegee mechanism) and stepwise addition by the biradical mechanism (DeMore mechanism). Predicted structures of intermediates and transition states, the energies of elementary steps, and activation barriers were reported. For the rate-determining steps of both mechanisms, the full geometry optimization of stationary points was performed at the CASPT2/cc-pVDZ theory level, and the potential energy surface profiles were constructed at the MRMP2/cc-pVTZ, NEVPT2/cc-pVDZ, and CIPT2/cc-pVDZ theory levels. The rate constants and their ratio for reaction channels calculated for both mechanisms demonstrate that the Criegee mechanism is predominant for this reaction. These results are also in agreement with the experimental data and previous computational results. The structure of DeMore prereactive complex is reported here for the first time at the CCSD(T)/cc-pVTZ and CASPT2/cc-pVDZ levels. Relative stability of the complexes and activation energies were refined by single-point energy calculations at the CCSD(T)-F12/VTZ-F12 level. The IR shifts of ozone bands due to formation of complexes are presented and discussed. © 2012 American Chemical Society

Quantum Chemical Study of the Initial Step of Ozone Addition to the Double Bond of Ethylene

The mechanisms of the initial step in chemical reaction between ozone and ethylene were studied by multireference perturbation theory methods (MRMP2, CASPT2, NEVPT2, and CIPT2) and density functional theory (OPW91, OPBE, and OTPSS functionals). Two possible reaction channels were considered: concerted addition through the symmetric transition state (Criegee mechanism) and stepwise addition by the biradical mechanism (DeMore mechanism). Predicted structures of intermediates and transition states, the energies of elementary steps, and activation barriers were reported. For the rate-determining steps of both mechanisms, the full geometry optimization of stationary points was performed at the CASPT2/cc-pVDZ theory level, and the potential energy surface profiles were constructed at the MRMP2/cc-pVTZ, NEVPT2/cc-pVDZ, and CIPT2/cc-pVDZ theory levels. The rate constants and their ratio for reaction channels calculated for both mechanisms demonstrate that the Criegee mechanism is predominant for this reaction. These results are also in agreement with the experimental data and previous computational results. The structure of DeMore prereactive complex is reported here for the first time at the CCSD­(T)/cc-pVTZ and CASPT2/cc-pVDZ levels. Relative stability of the complexes and activation energies were refined by single-point energy calculations at the CCSD­(T)-F12/VTZ-F12 level. The IR shifts of ozone bands due to formation of complexes are presented and discussed