Hydrogen bonding. Part. 76. Molecular orbital investigation of the stability of tertiary amine-hydrogen fluoride compounds in aqueous solution

Abstract

N,N-Dimethyl-1-adamantamine-hydrogen fluoride (2MAAHF) does not dissociate in aqueous solution, while 2MAAHCl and 2MAAHBr readily ionize in H2O. Semi-empirical (AM1-SM2) calculations demonstrate that the enthalpic barrier to dissociation of 2MAAHF in aqueous solution is four times as high as that of 2MAAHCl and three times as high as that of 2MAAHBr. Ab initio calculations (3-21G(*) and 3-61G∗) show that 2MAAHF is a tightly bound molecular complex between neutral HF and neutral amine, with a low dipole moment and little negative charge exposed on fluorine. 2MAAHCl and 2MAAHBr are ion-pairs with an electrostatic hydrogen bond between cation and anion, large dipole moments, and exposed halogen atoms with high electrostatic potential; these factors explain the ease of ionization of 2MAAHCl and 2MAAHBr. Parallel studies on other tertiary amines give similar results.