We have studied the unimolecular dissociation reaction of CH 3 SCH 3 (DMS) and CH 3 SCH 2 radical theoretically. The structures of reactants, activated complexes, and products have been optimized at the MP2/6-311G(D,P) level. Energies have been derived from single point MP4SDTQ/6-311G(D,P) calculations at the MP2 geometries. The barrier height corrected for zero point energy for the unimolecular dissociation of DMS to CH 3 and SCH 3 in MP4SDTQ and CAS(2,2)MP2 calculations was found equal to 295.3 and 310.0 kJ mol -1 , respectively. The barrier height corrected for zero point energy for the dissociation reaction of CH 3 SCH 2 radical to CH 3 and SCH 2 was calculated to be 135.5 kJ mol -1 at the MP4SDTQ level of theory. DMS is a C 2V molecule with two C 3V tops. The potential constants and barrier height for the torsional motion of methyl groups in DMS were also calculated. At the MP4SDTQ level of theory, the torsional barrier height for a methyl group was found to be 8.21 kJ mol -1 . Generalized transition state theory and RRKM method were employed to calculate the rate constants for the two reactions in the title in a temperature range of 300-3000 K. According to generalized transition state theory, we have found the Arrhenius parameters for the unimolecular dissociation reactions of DMS and CH 3 SCH 2 , k 1 ) 5.3 × 10 15 exp(-318.8 kJ mol -1 /RT) s -1 and k 3 ) 9.2 × 10 13 exp(-138.4 kJ mol -1 /RT) s -1 , respectively. According to RRKM method, we have found the highpressure limiting rate constant values: k 1 ) 6.1 × 10 15 exp(-317.2 kJ mol -1 /RT) s -1 and k 3 ) 4.4 × 10 13 exp(-138.0 kJ mol -1 /RT) s -1 .
The nature of weak van der Waals interactions in different complexes of some atmospheric molecules such as CO 2, N 2 O , and N 2 was examined. Ab initio calculation was carried out at MP2 level of theory using Dunning's aug-cc-pVTZ basis set. Bader's theory of atoms in molecules (AIM) was employed to analyze electron density and to characterize the nature and properties of van der Waals interactions. A set of criteria, having been proposed in the context of AIM theory, was examined for these complexes. In spite of the parameter kinetics energy, per electron density is expected to be greater than unity for closed-shell interactions; we obtained values less than unity for many of these polyatomic systems. A set of limitations has also been outlined for the values of two AIM quantities: total energy density, H(r), and Laplacian of electron density, ∇2ρ, which correspond to different bond natures.
The kinetic energy pressure (KEP) is quantified in terms of displaced charges and their orbital representations. Two deformation density matrices are introduced to separate reorganization of the electron density due to Pauli antisymmetrization from that of orbital relaxation. The formalism is applied to interaction of carbon nanotubes with hydrogen molecule and the results confirmed that KEP has the main contribution to such interaction. Also, it was found that the contribution of KEP can be easily traced in the complex formation.
ABSTRACT:In this research, deformation density matrix has been introduced as matrix representation of the density difference between the complex and fragments. The deformation density matrix is then diagonalized to obtain the magnitude of displaced charges as eigenvalues. Correspondingly, the eigenvectors reveal the spaces responsible for reorganization of the electrons because of the complex formation. The formalism has been applied on some CO 2 planar clusters, and the results showed that how the deformation density can be successfully separated into in-plane and out-of-plane contributions.
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