The detailed hydration mechanism of carbonyl sulfide (COS) in the presence of up to five water molecules has been investigated at the level of HF and MP2 with the basis set of 6-311++G(d, p). The nucleophilic addition of water molecule occurs in a concerted way across the C==S bond of COS rather than across the C==O bond. This preferential reaction mechanism could be rationalized in terms of Fukui functions for the both nucleophilic and electrophilic attacks. The activation barriers, DeltaH( not equal) (298), for the rate-determining steps of one up to five-water hydrolyses of COS across the C==S bond are 199.4, 144.4, 123.0, 115.5, and 107.9 kJ/mol in the gas phase, respectively. The most favorable hydrolysis path of COS involves a sort of eight-membered ring transition structure and other two water molecules near to the nonreactive oxygen atom but not involved in the proton transfer, suggesting that the hydrolysis of COS can be significantly mediated by the water molecule(s) and the cooperative effects of the water molecule(s) in the nonreactive region. The catalytic effect of water molecule(s) due to the alleviation of ring strain in the proton transfer process may result from the synergistic effects of rehybridization and charge reorganization from the precoordination complex to the rate-determining transition state structure induced by water molecule. The studies on the effect of temperature on the hydrolysis of COS show that the higher temperature is unfavorable for the hydrolysis of COS. PCM solvation models almost do not modify the calculated energy barriers in a significant way.

Computational investigations on the gas-phase nucleophilic substitution reactions of p-substituted phenoxides (p-Y-C6H4O-, Y = OH, CH3O, CH3, H, F, Cl, CF3) with halomethanes (CH3X, X = F, Cl, Br, and I) were performed by the B3LYP and MP2 methods with the 6-311+G(d,p) basis set. Calculated results indicate that the reactions are more endothermic only when the substrate is a lighter halide. The complexation enthalpies, the key parameters in the transition state (TS), the central barriers, overall barriers, overall reaction enthalpies, and the charge of the O4 atom in the TSs all present good correlations with the Hammett constants sigma of substituents in the nucleophile. Leffler-Grunwald rate equilibrium relationships predict the degree of bond formation in the transition state suggesting that the reactions have progressed 31%, 24%, 24%, and 21% in the TS for halomethanes (X = F, Cl, Br, and I), respectively. The TS structure with substituents in the nucleophile is not kinetically but thermodynamically controlled, similar to the earlier results. Furthermore, the excellent relationship between the central barrier heights and the looseness of the transition state structure indicates that the stretching of the cleaving bond is one of the major factors determining the central barrier heights. The nucleophilicity of the nucleophile decreases with the increase of the electron-withdrawing power of substituent Y in the nucleophile, while the leaving-group ability of the halogen atom increases with the decrease of its Mulliken electronegativity.

In a kinetic experiment on the SN2 reaction of sodium p-nitrophenoxide with iodomethane in acetone-water mixed solvent, Humeres et al. (J. Org. Chem. 2001, 66, 1163) found that the reaction depends strongly on the medium, and the fastest rate constant was observed in pure acetone. The present work tries to explore why acetone can enhance the reactivity of the title reactions. Accordingly, we make a mechanistic study on the reactions of sodium p-nitrophenoxide with halomethanes (CH3X, X = Cl, Br, I) in acetone by using a supramolecular/continuum model at the PCM-MP2/6-311+G(d,p)//B3LYP/6-311+G(d,p) level, in which the ion pair nucleophile is microsolvated by one to three acetone molecules. We compared the reactivity of the microsolvated ion pair nucleophiles with solvent-free ion pair and anionic ones. Our results clearly reveal that the microsolvated ion pair nucleophile is favorable for the SN2 reactions; meanwhile, the origin of the enhanced reactivity induced by microsolvation of the nucleophile is discussed in terms of the geometries of transition state (TS) structures and activation strain model, suggesting that lower deformation energies and stronger interaction energies between the deformed reactants in the TS lead to the lower overall reaction barriers for the SN2 reaction of microsolvated sodium p-nitrophenoxide toward halomethanes in acetone.

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ABSTRACT:Computational modeling by an ab initio molecular orbital method (HF and MP2) with the 6-311ϩG(d,p) and 6-311ϩϩG(2df,2pd) basis sets on the tautomerism of three monochalcogenocarboxylic acids CH 3 C(AO)XH (X ϭ S, Se, and Te) in the polar and aprotic solution tetrahydrofuran (THF) was undertaken using the combined supramolecular/continuum method, in which CH 3 OCH 3 was considered as a realistic model for THF in the supermolecule. Our calculations showed that the tautomerism involving CH 3 OCH 3 not only gives the preference of the enol form CH 3 C(AX)OH (X ϭ S, Se, and Te) in the tautomeric equilibrium, but also significantly lowers the tautomeric barriers by Ͼ34 kJ/mol in THF solution. These results support the experiments of Kato and coworkers, in contrast with earlier Delaere et al.'s theoretical prediction for the tautomeric barrier of thioformic acid HC(AO)SH . . . O(CH 3 ) 2 , in which the barrier is increased by ϳ6 kJ/mol relative to reactants complex because of the constraint in locating the tautomeric transition structure.

ABSTRACT:Computational investigations by an ab initio molecular orbital method (HF and MP2) with the 6-311ϩG(d,p) and 6-311ϩϩG(2df, 2pd) basis sets on the tautomerism of three monochalcogenosilanoic acids CH 3 Si(AO)XH (X ϭ S, Se, and Te) in the gas phase and a polar and aprotic solution tetrahydrofuran (THF) was undertaken. Calculated results show that the silanol forms CH 3 Si(AX)OH are much more stable than the silanone forms CH 3 Si(AO)XH in the gas-phase, which is different from the monochalcogenocarboxylic acids, where the keto forms CH 3 C(AO)XH are dominant. This situation may be attributed to the fact that the SiOO and OOH single bonds in the silanol forms are stronger than the SiOX and XOH single bonds in the silanone forms, respectively, even though the SiAX (X ϭ S, Se, and Te) double bonds are much weaker than the SiAO double bond. These results indicate that the stability of the monochalcogenosilanoic acid tautomers is not determined by the double bond energies, contrary to the earlier explanation based on the incorrect assumption that the SiAS double bond is stronger than the SAO double bond for the tautomeric equilibrium of RSi(AO)SH (R ϭ H, F, Cl, CH 3 , OH, NH 2 ) to shift towards the thione forms [RSi(AS)OH]. The binding with CH 3 OCH 3 enhances the preference of the silanol form in the tautomeric equilibrium, and meanwhile significantly lowers the tautomeric barriers by more than 34 kJ/mol in THF solution.

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Computational investigations on the gas-phase homodimerization and the keto-to-enol tautomerism of monochalcogenocarboxylic acids, CH3C(=O)XH (X = S, Se, Te) , were performed using ab initio molecular orbital methods (HF and MP2) with the 6-311+G(d, p) and 6-311+G(2df, 2df) basis sets. Calculated results indicate that the dimerization enthalpy values, ΔH, for the enol-dimers, [CH3C(=X)OH]2 (X = S, Se, Te) , are notably higher than those for the corresponding keto-dimers, [CH3C(=O)XH]2 (X = S, Se, Te) , while the ΔH values decrease as the electronegativity of chalcogen atom is lowered, for both the keto- and enol-dimers. It is found that the homodimerization of monochalcogenocarboxylic acids is thermodynamically unfavorable because the releasing dimerization heat cannot overcome the loss of entropy. This is contrasted with the case in the carboxylic acid, where the much higher dimerization enthalpy is responsible for the favorable dimerization in the gas phase. Our results also suggest that the tautomeric reactions of the monochalcogenocarboxylic acids in the gas phase may proceed by an eight-membered ring TS with intermolecular double proton transfer and lower tautomeric barrier, instead of by direct intramolecular proton transfer in monomer with a highly strained four-membered TS. The geometrical and energetic characteristics of the homodimers and tautomeric TSs are also further elucidated by NBO analysis.