Comparison of the CH...N and CH...O Interactions Involving Substituted Alkanes
Journal of Molecular Structure
The CH⋯N interaction between FnH3−nCH (proton donor) and NH3, CH3NH2, and CH2NH (acceptor) is examined by ab initio calculations and compared with the comparable CH⋯O systems. The interaction is strengthened by 1.2 kcal/mol with each F added to the donor, slightly larger than the 1.0 kcal/mol increment for CH⋯O. Like CH⋯O, the CH⋯N interaction behaves like a conventional H-bond in terms of electron density shifts and the magnitudes of the various components of the interaction energy. The CH⋯N interactions are also like CH⋯O in that they both die off more slowly as the H-bonds are stretched, in comparison to corresponding conventional H-bonds. Contrary to the behavior of the OH donor, the CH bond contracts and undergoes a blue shift upon forming a complex with either N or O acceptor, although these changes are somewhat smaller for CH⋯N and not as systematic. The differing trends are attributed to the level of cancellation between opposing forces, and not due to any fundamental difference between the two types of interaction.
Comparison of the CH...N and CH...O Interactions Involving Substituted Alkanes Y. Gu, T. Kar, S. Scheiner J. Mol. Struct. 2000 552 17-31