A comprehensive theoretical study on the hydrolysis of carbonyl sulfide in the neutral water.

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.

Distinct pi-bonding capability between phosphinidene and phosphonium ion: a computational study.

The heavy dipnictenes (RE=ER, where E=P, As, Sb, and Bi with the substituent R) have essentially planar geometry and appreciable strength in pi-bonding, unlike related heavier main group 14 analogues of alkenes as concluded recently by Power. This work demonstrated that the protonated pnictenes behave more like the heavy carbene for their weak pi-bonding character from the computational study with the B3LYP/6-311++G** method. For example, although both phosphinidene (HP) and the phosphonium ion (H2P+) are isoelectronic to silylenes, the pi-bonding tendency of the former is rather strong and it forms a planar adduct with both the stable carbene and stable silylene ((HCNH)2E, where E=C and Si). In contrast, the latter forms trans-bent adducts with the two species. These results can be interpreted in terms of the Carter-Goddard-Malrieu-Trinquier (CGMT) model, and the fact that the value of DeltaEST [E(triplet)-E(singlet)] of the HP fragment increases significantly after protonation. All other heavy pnictenes resemble the phosphinidene. In contrast, nitrene (HN) and nitrenium (H2N+) have a ground triplet state, thus both have strong pi-bonding character similar to that of carbene.

Theoretical study on the identity ion pair SN2 reactions of LiX with CH3SX (X=Cl, Br, and I): structure, mechanism, and potential energy surface.

Three archetypal ion pair nucleophilic substitution reactions at the methylsulfenyl sulfur atom LiX+CH3SX-->XSCH3+LiX (X=Cl, Br, and I) are investigated by the modified Gaussian-2 theory. Including lithium cation in the anionic models makes the ion pair reactions proceed along an SN2 mechanism, contrary to the addition-elimination pathway occurring in the corresponding anionic nucleophilic substitution reactions X-+CH3SX-->XSCH3+X-. Two reaction pathways for the ion pair SN2 reactions at sulfur, inversion and retention, are proposed. Results indicate the inversion pathway is favorable for all the halogens. Comparison of the transition structures and energetics for the ion pair SN2 at sulfur with the potential competition ion pair SN2 reactions at carbon LiX+CH3SX-->XCH3+LiXS shows that the SN2 reactions at carbon are not favorable from the viewpoints of kinetics and thermodynamics.

Computational studies on stable triplet states of heteroacetylenes and the effects of halogen substituents.

This paper describes theoretical studies of halogen-substituted heteroacetylenes (XCMY, M = Si and Ge; X, Y = H, Cl and F) performed at the QCISD(T)/6-311G//QCISD/6-31G level of theory. The electronegative halogen substituents destabilize the singlet state such that the triplet state tends to become favorable. The triplet state has the bifunctional electronic structure of a triplet carbene joined to a heavy singlet carbene. We found that the substituents effectively reduce the energy of the donor-acceptor interactions (E(D-A)) between the two in-plane lone pairs of electrons of the singlet state; therefore, the remaining pi bond is less favorable energetically than the triplet state with a sigma bond. A related phenomenon occurs for the homonuclear heavy acetylenes in singlets in which the lead compound RPbPbR switches to a Pb-Pb sigma bond from the pi bonds observed for the lighter acetylenes.

Ene reaction of arynes with alkynes.

Arynes, generated in situ from ortho-silylaryl triflates, undergo ene reaction with alkynes possessing propargylic hydrogen in the presence of KF/18-crown-6 in THF at room temperature to give substituted phenylallenes. Various terminal and internal alkynes as well as different arynes can be used to give the corresponding phenylallenes in good to moderate yields. The reaction of alkyne without propargylic hydrogen gave an acetylenic C-H addition product (a phenylalkyne) and a dehydro Diels-Alder product (a phenanthrene).

Ab initio computational insight into the ion-pair S(N)2 reaction of lithium isothiocyanate and methyl fluoride in the gas phase and in acetone solution.

The ion-pair S(N)2 reaction LiNCS + CH3F with two mechanisms, inversion and retention, was investigated at the MP2(full)/6-311+G**//HF/6-311+G** level in the gas phase and in acetone solution. All HF-optimized structures were confirmed by vibrational frequency analysis. Based on IRC analyses, eight possible reaction pathways in the title reaction are proposed. The inversion mechanism through a six-membered-ring transition-state structure is the most favorable. Methyl thiocyanate should form preferentially in the gas phase and the more stable methyl isothiocyanate will be the main product in CH3COCH3. The retardation of the reaction in CH3COCH3 solution was attributed to the differences in the solvation free energies in the separated reactants and transition structures. All of the theoretical results are consistent with the experiment.

Modified Gaussian-2 level investigation of the identity ion-pair SN2 reactions of lithium halide and methyl halide with inversion and retention mechanisms.

Identity ion-pair S(N)2 reactions LiX + CH(3)X --> XCH(3) + LiX (X = F, Cl, Br, and I) have been investigated in the gas phase and in solution at the level of the modified Gaussian-2 theory. Two possible reaction mechanisms, inversion and retention, are discussed. The reaction barriers relative to the complexes for the inversion mechanism [DeltaH(cent) ( not equal )(inv)] are found to be much higher than the corresponding values for the gas phase anionic S(N)2 reactions, decreasing in the following order: F (263.6 kJ mol(-1)) > Cl (203.3 kJ mol(-1)) > Br (174.7 kJ mol(-1)) > I (150.7 kJ mol(-1)). The barrier gaps between the two mechanisms [DeltaH(cent) ( not equal ) (ret) - DeltaH(cent) ( not equal ) (inv)] increase in the order F (-62.7 kJ mol(-1)) < Cl (4.4 kJ mol(-1)) < Br (24.9 kJ mol(-1)) < I (45.1 kJ mol(-1)). Thus, the retention mechanism is energetically favorable for fluorine and the inversion mechanism is favored for other halogens, in contrast to the anionic S(N)2 reactions at carbon where the inversion reaction channel is much more favorable for all of the halogens. The stabilization energies for the dipole-dipole complexes CH(3)X. LiX (DeltaH(comp)) are found to be similar for the entire set of systems with X = F, Cl, Br, and I, ranging from 53.4 kJ mol(-1) for I up to 58.9 kJ mol(-1) for F. The polarizable continuum model (PCM) has been used to evaluate the direct solvent effects on the energetics of the anionic and ion-pair S(N)2 reactions. The energetic profiles are found to be still double-well shaped for most of the ion-pair S(N)2 reactions in the solution, but the potential profile for reaction LiI + CH(3)I is predicted to be unimodal in the protic solvent. Good correlations between central barriers [DeltaH(cent) ( not equal ) (inv)] with the geometric looseness of the inversion transition state %C-X( not equal ), the dissociation energies of the C-X bond (D(C-X)) and Li-X bond (D(Li-X)) are observed, respectively.

Theoretical Study of Reactions of Arduengo-Type Carbene, Silylene, and Germylene with CH(4).

The potential energy surfaces corresponding to the reaction of cyclic, unsaturated diaminocarbene (DAC), -silylene (DAS), and -germylene (DAG) with methane have been investigated by employing the B3LYP and CCSD(T) levels of theory. Our model calculations demonstrate that the electronic perturbation effect should play a significant role in determining the magnitude of their singlet-triplet splitting. Namely, the singlet-triplet gap of DAC, DAS, and DAG shows the opposite order as the parent compounds (CH(2), SiH(2), and GeH(2)), as well as the compounds with pi-donor substituents (C(NH(2))(2), Si(NH(2))(2), and Ge(NH(2))(2)). Our theoretical investigations suggest that the heavier the X center (X = C, Si, Ge), the larger the insertion barrier, and the less exothermic (or the more endothermic) the insertion reaction. Namely, the chemical reactivity decreases in the order DAC > DAS > DAG. Even so, all the species are predicted to be kinetically stable with respect to insertion reactions with alkanes. Moreover, it is found that a singlet state DAC, DAS, or DAG inserts in a concerted manner, and that the stereochemistry at the X center (X = C, Si, and Ge) is preserved.

Cycloadditions of 16-Electron 1,3-Dipoles with Ethylene. A Density Functional and CCSD(T) Study.

The 1,3-dipolar cycloaddition (DC) reactions of ethylene with nitrile ylide (CNC), nitrile imine (CNN), nitrile oxide (CNO), diazomethane (NNC), azine (NNN), and nitrous oxide (NNO) in the gas phase were examined using the density functional theory and CCSD(T) calculations. All of the structures, including the precursor complexes and the transition structures, were completely optimized at the B3LYP/6-31G level with single-point energies evaluated at CCSD(T)/6-311G. The theoretical results suggest that the activation energies for the DC reactions of nitrile-type molecules (CNC, CNN, and CNO) are small (5.1-11 kcal/mol) and these reactions are very exothermic (-77 to -46 kcal/mol). In contrast, the DC reactions of NNC, NNN, and NNO are less exothermic (-39 to -6.0 kcal/mol) and have larger activation barriers (13-29 kcal/mol). Moreover, this work shows that the configuration mixing (CM) model based on Pross and Shaik's theory can successfully predict the relative ordering of the activation energy and reaction enthalpies of DC reactions. Combining our theoretical calculations and the CM model, the following conclusion emerges: a 16-electron 1,3-dipole reactant with more electropositive substituents at the terminal positions will possess a smaller singlet-triplet splitting. This will facilitate cycloaddition with the dipolarophile and will result in a larger exothermicity.