Electronic effects on 1H-azepines valance tautomerization: an ab initio comparative study

Ab initio HF/6-31G*, B3LYP/6-31G* and MP2/6-31G*//HF/6-31G are employed for calculations of equilibrium constants (Keq), rate constants (kr), activation electronic energies (ΔE#), enthalpies (ΔH#), and Gibbs free energies (ΔG#) for conversions of 3(X)-1H-azepine, 1 (where X=H, NH2, OCH3, CH3, CF3, F, Cl, Br, NO2 and CN) to their corresponding 2(X)-benzene imines, 2 (Series 1). The same calculations are carried out for conversions of 4(X)-1H-azepines, 1′, to analogous 3(X)-benzene imines, 2′ (Series 2). Equilibriums related to Series 1 with different (X) substitutions occur with Keqs=1.4×107 (H), 4.8×104 (NH2), 5.6×103 (OCH3), 9.7×105 (CH3), 1.1×107 (CF3), 3.1×105 (F), 7.6×105 (Cl), 1.2×106 (Br), 2.4×108 (NO2) and 2.4×106 (CN). Similarly, for Series 2, the Keqs are: 1.4×107 (H), 7.1×107 (NH2), 9.7×106 (OCH3), 9.4×106 (CH3), 3.0×107 (CF3), 9.6×107 (F), 7.1×107 (Cl), 9.3×107 (Br), 1.0×109 (NO2) and 1.2×108 (CN). A Hammet ρ value of 1.70 is obtained for Series 2. Due to steric effects, meaningful ρ is not fond for Series 1.