Hydrogen bonding. Part 77. Molecular orbital study of C–H⋯F hydrogen bonds in tetramethylammonium fluoride; potential effects on solid state structure

Abstract

The infrared (IR) spectrum of tetramethylammonium fluoride suggests that it contains the strongest C–H⋯F− hydrogen bonds yet observed. Ab initio 3-21G(∗) calculations were used to examine potential solid state arrangements of cation about anion. The favored state is one in which four cations surround each F− in a D2d arrangement and four F− surround each cation. Each F− acts as acceptor of four hydrogen bonds of −10.8kcalmol−1, one from each cation. This arrangement, similar to that of tetramethylammon chloride, is consonant with the IR spectrum of the cation in solid tetramethylammonium fluoride. In the preferred form of the monomeric gas phase ion-pair F− lies against one triangular face of the Td cation with three CHF hydrogen bonds of −11.5kcalmol−1 each. Constraint of F− in the gas phase ion-pair to interaction with a single cation hydrogen results in a tightly bound molecular complex between HF and trimethylammonium methylide with an interaction energy of −27kcalmol−1; however, this structure is not seen elsewhere and apparently does not play a role in the solid salt.