The ions [CH3S]+, [C2H5S]+ and [CH3O]+ formed by electron-impact

THEORETICAL STUDIES ON BOND DISSOCIATION ENERGIES FOR SOME THIOL COMPOUNDS BY DENSITRY FUNCTIONAL THEORY AND CBS-Q METHOD

Quantum chemical calculations are used to estimate the bond dissociation energies (BDEs) for 15 thiol compounds. These compounds are studied by employing the hybrid density functional theory (B3LYP, B3PW91, B3P86, PBE0) methods and the complete basis set (CBS-Q) method together with 6-311G** basis set. It is demonstrated that B3P86 and CBS-Q methods are accurate for computing the reliable BDEs for thiol compounds. In order to test whether the non-local BLYP method suggested by Fu et al.19 is general for our study and whether B3P86 method has a low basis set sensitivity, the BDEs for seven thiol compounds are also calculated using BLYP/6-31+G* and B3P86 method with 6-31+G*, 6-31+G**, and 6-311+G** basis sets for comparison. The obtained results are compared with the available experimental results. It is noted that B3P86 method is not sensitive to the basis set. Considering the inevitable computational cost of CBS-Q method and the reliability of the B3P86 calculations, B3P86 method with a moderate or a larger basis set may be more suitable to calculate the BDEs of the C–SH bond for thiol compounds.

THEORETICAL STUDIES ON HEATS OF FORMATION FOR SOME THIOL COMPOUNDS BY DENSITY FUNCTIONAL THEORY AND CBS-Q METHOD

The heats of formation (HOFs) for 15 thiol compounds are calculated by employing the hybrid density functional theory (B3LYP, B3PW91, B3P86) methods with 6-311G** basis set and the complete basis set (CBS-Q) method. It is demonstrated that the B3P86 and CBS-Q methods are accurate to compute the reliable HOFs for thiol compounds. In order to test whether the B3P86 method has a low basis set sensitivity, the HOFs for six thiol compounds are also calculated by using the B3P86 method with 6-31+G*, 6-31+G**, and 6-311+G** basis sets for comparison. We also extend our study by employing the nonlocal BLYP method together with 6-31+G* basis set to calculate the HOFs for thiol compounds. The obtained results are compared with the experimental results. It is noted that the B3P86 method is not sensitive to the basis sets. Considering the inevitably computational cost of CBS-Q method and the reliability of the B3P86 calculation, the B3P86 method with a moderate or a larger basis set such as 6-311G** and 6-311+G** may be more suitable to calculate the HOFs of thiol compounds. In addition, we believe that the maximum error associated with the calculated HOFs is less than 6 kcal/mol for the B3P86/6-311G** method and it is expected that the error bar is more likely 1–5 kcal/mol for the HOFs of thiol compounds.

Quasiclassical Trajectory Study of the Collision-Induced Dissociation Dynamics of Ar + CH3SH+ Using an Ab Initio Interpolated Potential Energy Surface

Classical trajectory calculations have been performed to investigate the collision-induced dissociation (CID) of the CH(3)SH(+) cation with Ar atoms. A new intramolecular potential energy surface for the CH(3)SH(+) cation is evaluated by interpolation of 3000 ab initio data points calculated at the MP2/6-311G(d,p) level of theory. The new potential energy surface includes seven accessible dissociation channels of the cation. The present QCT calculations show that migration of hydrogen atoms, leading to the rearrangement CH(3)SH(+) <--> CH(2)SH(2)(+), is significant at the collision energies considered (6.5-34.7 eV) and that the formation of CH(3)(+), CH(3)S(+), and CH(2)(+) cations takes place primarily by a "shattering" mechanism in which the products are formed just after the collision. The theoretical product abundances are found to be in qualitative agreement with the experimental data. However, at high collision energies, the calculated total cross sections for the formation of CH(3)(+) and CH(2)SH(+) cations are noticeably larger than the experimental determinations. Several features of the dynamics of the CID processes are discussed.

Quasiclassical trajectory study of the collision-induced dissociation of CH[sub 3]SH[sup +]+Ar

Quasiclassical trajectory calculations were carried out to study the dynamics of energy transfer and collision-induced dissociation (CID) of CH(3)SH(+) + Ar at collision energies ranging from 4.34 to 34.7 eV. The relative abundances calculated for the most relevant product ions are found to be in good agreement with experiment, except for the lowest energies investigated. In general, the dissociation to form CH(3)(+) + SH is the dominant channel, even though it is not among the energetically favored reaction pathways. The results corroborate that this selective dissociation observed upon collisional activation arises from a more efficient translational to vibrational energy transfer for the low-frequency C-S stretching mode than for the high-frequency C-H stretching modes, together with weak couplings between the low- and high-frequency modes of vibration. The calculations suggest that CID takes place preferentially by a direct CH(3)(+) + SH detachment, and more efficiently when the Ar atom collides with the methyl group-side of CH(3)SH(+).

Ion-molecule reactions of [C, H(3), S](+) ions and evidence for long-lived triplet CH(3)S(+)

Gas-phase [C, H(3), S](+) ions obtained by electron impact from (CH(3))(2)S at 14 eV undergo two distinct low-pressure ion-molecule reactions with the parent neutral: proton transfer and charge exchange. The kinetics of these reactions studied by Fourier transform ion cyclotron resonance (FT-ICR) techniques clearly suggests the [C, H(3), S](+) species to be a mixture of isomeric ions. While proton transfer is consistent with reagent ions displaying the CH(2)SH(+) connectivity, the observed charge exchange strongly argues for the presence of thiomethoxy cations, CH(3)S(+), predicted to be stable only in the triplet state. Charge exchange reactions are also observed in the reaction of these same [C, H(3), S](+) ions with benzene, toluene and phenetole. For these substrates, the CH(2)SH(+) ions can promote proton transfer and electrophilic methylene insertion in the aromatic ring with elimination of H(2)S. The results obtained for the different substrates suggest that the fraction of long-lived fraction of thiomethoxy cations obtained at 14 eV by electron ionization of dimethyl sulfide amounts to ~(22 -/+ 4)% of the [C, H(3), S](+) fragments.