Unrestricted second and fourth order Møller−Plesset perturbation theory (MP2 and MP4), density functional theory (B3LYP and BHandHLYP), coupled cluster (CCSD(T)), and quadratic configuration interaction (QCI) calculations have been performed using both the 6-311++G(d,p) and 6-311++G(2d,2p) basis sets, to study the OH hydrogen abstraction reaction from formic acid. A complex mechanism involving the formation of a very stable prereactive complex is proposed, and the rate coefficients are calculated over the temperature range 296−445 K, using classical transition state theory. The following expressions, in L mol -1 s -1, are obtained for the acidic, for the formyl, and for the overall temperature-dependent rate constants: k I = (1.37 ± 0.40) × 10 7 exp[(786 ± 87)/ T ], k II = (5.93 ± 1.39) × 10 8 exp[(−1036 ± 72)/ T ], and k = (5.28 ± 2.35) × 10 7 exp[(404 ± 125)/ T ], respectively. An extremely large tunneling factor results for the acidic path, as a consequence of the presence of a high and narrow effective activation barrier. The contribution of the formyl path to the overall rate coefficient, as well as the magnitude of the tunneling effect, explain the observed non-Arrhenius behavior.
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Galano et al. (2002) studied this question.