Hydrogen bonding. Part 81. Infrared and molecular orbital study of the hydrates of N,N-dimethyl-1-adamantamine-hydrogen fluoride

Abstract N , N -dimethyl-1-adamantamine-hydrogen fluoride is a molecular species which does not dissociate in aqueous solution, and which contains a covalent three-center NHF bond. This compound forms three hydrates, a stoichiometric liquid pentahydrate, a reasonably stable crystalline trihydrate, and an ephemeral crystalline monohydrate. Infrared spectra and ab initio geometry optimization demonstrate that the monohydrate forms a dimeric cluster of C i symmetry with a C 2 h planar (H 2 O·F) 2 unit sandwiched between bound amine units. This structure contains 4HOHF, 2CHF, 4CHOH 2 , and 2NHF hydrogen bonds. The association energies of these bonds are estimated by molecular orbital methods, and the possible reasons for the extraordinary instability of the (H 2 O·F) 2 unit relative to similar units in quaternary ammonium fluoride monohydrates are discussed.