CH4-NXN - A COMPARISON BETWEEN THE STABILIZED X=F SERIES AND THE DESTABILIZED X=CN SERIES

Reasons for the striking difference in DELTA-E values obtained from the isodesmic equation CX4 + 3CH4 --> 4CH3X for CF4 (+ 58 kcal/mol) and C(CN)4 (-54 kcal/mol) are examined by natural population analysis starting from the Hartree-Fock 6-31G** wavefunction. Localized strictly orthogonal orbitals are generated and the energy gain from nonbonding orbital interactions is estimated by deletion of the Fock matrix elements between the corresponding orbitals. The total energy is thus divided into a part arising from a hypothetical Lewis structure and a part due to hyperconjugation. Despite the large apparent destabilization in C(CN)4, the absolute energy contribution from hyperconjugation is larger than in CF4. The hyperconjugative stabilization per substituent, however, increases along the series from CH3F to CF4, while it decreases slightly in the cyano series. Since isodesmic equations measure the stabilization relative to CH3X, a large stabilization in the F series and destabilization for X = CN result. The contributions from the Lewis structures are destabilizing in both cases.