STRUCTURES AND DISSOCIATION-ENERGIES OF THE COMPLEXES CHMFN-CENTER-DOT-CENTER-DOT-CENTER-DOT-NH4-3) - AN AB-INITIO MOLECULAR-ORBITAL STUDY((N+M=4, N=1)

Intermolecular hydrogen bonding interactions between C-F groups and po sitively charged N-H groups have been investigated. Ab initio molecula r orbital calculations were used to determine the structures and disso ciation energies of the complexes CHmFn...NH4+ (n + m = 4, n = 1-3) at the MP2/6-31 ++G*//MP2/6-31++G** computational level. The value of D elta H-298(o) for the dissociation of the complex CH3F NH4+, in which the orientation of the grouping C-F...H-N is nearly linear, is found t o be 13.5 kcal/mol. For CH2F2...N-4(+) and CHF3 NH4+, however, the dis sociation energy per H...F interaction drops significantly as the C-F. ..H-N+ geometry becomes progressively less linear. Thus, near-linear i nteractions (C-F...H-N) between C-F groups and positively charged N-H groups are sufficiently strong to play a role in the alignment of mole cules in crystals and complexes, particularly if there are several suc h interactions per molecule and other stronger interactions are absent .