Exploring chemical reaction mechanisms via high accuracy ab initio quantum chemistry
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The mechanisms and properties of SN2 halide identity exchange, radical-radical abstraction reactions and hydroxycarbene tunneling reactions are investigated via high accuracy quantum mechanical computations. The origin of the SN2 intrinsic reaction rate enhancement has been addressed in this study. A detailed analysis is performed of R–CH2X + X– (RCH2 = propyl, ethyl, methyl, allyl, benzyl, acetonitrile; X = F, Cl) SN2 identity exchange reactions. In the traditional view, fast SN2 rates for substrates with a multiple bond at Cβ (carbon adjacent to the reacting Cα center) are primarily due to π-conjugative stabilization in the SN2 transition state. However, our results give a definite answer that electrostatic interactions among Cα, Cβ and the attacking halide anion are the main driving force of the SN2 rate acceleration. The quantum tunneling computations demonstrate that several members of the hydroxycarbene family (R–COH, R = H, methyl, phenyl) are able to penetrate through ~30 kcal mol–1 activation barriers to isomerize to aldehyde derivatives (R–CHO) with half-lives of 1–2 hrs. The intrinsic reaction path is mapped out and the Wentzel–Kramers–Brillouin approximation is used to compute tunneling probabilities. In addition, semi-classical transition state theory shows that multidimensional vibrational coupling increases the half-life by a factor of 3-5 relative to the one-dimensional reaction coordinate analysis. Finally, rigorous one-dimensional reaction profiles are constructed for several radical-radical abstraction reactions using Mukherjee state-specific multireference coupled cluster theory (Mk-MRCC). We demonstrate that the performance of Mk-MRCCSD(T) is superior to other popular multireference methods and its root-mean-square deviation from full CI is only 0.24 kcal mol–1 for binding energies. The energy profile for each level of theory is extrapolated to the complete basis set limit to facilitate future kinetic studies.