MECHANISMS OF ELECTROPHILIC SUBSTITUTIONS OF ALIPHATIC-HYDROCARBONS - CH4+NO+

The substitution reaction of methane with the nitrosonium cation, a model electrophile, was investigated computationally at the Hartree-Fock and correlated MP2, MP4SDTQ, and CISD levels of theory, using standard basis sets (6-31G(d), 6-31G(dp), and 6-31+G(dp) for geometry optimizations and TZ2P for energy single points on the most critical structures). The energetically favored reaction course leads to N-protonated nitrosomethane, H3CHNO+ (6). The initial complex of CH4 and NO+ in C(s) symmetry is bound by -3.7 kcal mol-1 (MP4SDTQ/6-31+G(dp)/MP2/6-31+G(dp)+ZPVE//MP2/6-31+G(dp)). In the critical step, the electrophile NO+ attacks carbon directly, rather than a C-H bond, to yield a pentacoordinate intermediate (3) with a hydrogen unit attached to a H2CNO+ cation moiety [DELTAH0(CISD+Q/TZ2P//MP2/6-31G()dp)+ZPVE//MP2/6-31G(dp))=57.3 kcal mol-1]. This unusual mode of attack, proceeding through a transition structure which also has three-center two-electron (3C-2e) CHH bonding, can be visualized in two ways. During the reaction, tetrahedral methane distorts to lower symmetry (C(s)) and binding between the electrophile and the developing lone pair occurs. The energy required for the methane distortion is partly recovered from the new bonding interaction to the electrophile. An alternative pathway involving the insertion of NO+ into a CH bond is less favorable by 14.4 kcal mol-1 (MP4SDTQ/6-31G(dp)//MP2/6-31G(dp) + ZPVE//MP2/ 6-31G(dp)). The reaction proceeds exothermically through hydrogen rearrangements to yield N-protonated nitrosomethane, with an overall reaction enthalpy of -9.1 kcal mol-I (MP4SDTQ/6-31G(dp)//MP2/6-31G(dp) + ZPVE//MP2/6-31G(dp)). The global minimum on the CH4NO+ potential hypersurface is H2NCHOH+, protonated formamide.