How ammonia participates in hydrogen bonding

High resolution spectral data on molecular complexes observed in free jet expansions and in particular rotationally resolved spectra, provide a wealth of information on the structure and the detailed nature of non-covalent interactions. While many studies can be found on hydrogen bonded complexes with water, less are the ones on ammonia complexes, and the question if ammonia can act as a proton donor was arisen.1 We report the results of Fourier Transform Microwave Spectroscopy (FTMW) and Free Jet Absorption Millimeterwave (FJAMMW) spectroscopy assisted by high level ab initio and electrostatic model calculations of molecular adducts of organic molecules with ammonia: namely CF3H•••NH3, t-butanol•••NH3 and glycidol•••NH3. Our results confirm the idea that ammonia does not easily form hydrogen bonds and in all three cases ammonia acts as the proton acceptor. This is true also in the case of the weak proton donor CF3H. In the first two cases: CF3H•••NH3 and t-butanol•••NH3, ammonia undergoes a completely free rotation about the hydrogen bond, although the data at millimeter wave frequencies, show that the previous model used for the spectral analysis2 compels further development. Glycidol (oxiranemethanol) is a chiral molecule for which many studies are reported in the literature and it consists of an oxirane ring to which a methanol group and is attached. Due to the presence of two hydrogen bond contact sites, the alcoholic hydrogen and the etheric oxygen, the isolated molecule of glycidol presents an intramolecular hydrogen bond in two different low energy conformations 1 and G. We observe two different geometries of the adduct where the ammonia molecule inserts in the intramolecular hydrogen bond forming a hydrogen bond with the alcoholic hydrogen and a secondary interaction with the etheric oxygen (see figure below). Besides the presence of multiple hydrogen bonds, the ammonia moiety still undergoes an internal rotation motion about the principal hydrogen bond and the potential energy barrier determined for this motion is substantially an estimation of the dissociation energy of the secondary interaction. Relative intensity measurements, from which the populations related to different conformation can be inferred, show that the population fraction of the lowest energy molecular adduct (G2•••NH3) is higher than expected from the calculated relative energies. This accounts for a kinetic effect in the formation of the weakly bound adduct in the supersonic expansion. 1 D.D. Nelson, G.T. Fraser, W. Klamperer, Science 238 1670 (1987) 2 G.T. Fraser, F.J. Lovas, R.D. Suenram, D.D. Nelson, W. Klemperer J. Chem. Phys. 84 5983 (1986)