Is protonated ammonium fluoride an ion-neutral complex in the gas phase?

Abstract

The two-dimensional rotation of the ammonium ion within NH4+-.FH is compared with hindered internal rotations in ethane. Both have bound levels below their internal rotation barriers as well as higher lying free rotor levels. Rotation in NH4+.-FH is the large-amplitude limit of bending vibrations, whereas free rotations in ethane are the large-amplitude limits of torsional oscillations. The accuracy of ab initio computations is calibrated using another tetrahedral-diatomic complex, CH4-.FH, for which MP4(SDTQ) calculations using the 6-3 1 1G** basis set reproduce experimentally determined spectroscopic properties. The calculations predict that the internal rotation barrier for NH4+-.FH is 13 kJ mol-', much lower than the calculated dissociation energy, DO = 48 kJ mol-'. From the quantization of the two-dimensional rotation, statistical mechanics predicts that NH4+--FH exists as an ion-neutral complex for a significant fraction of the time at temperatures above 300 K. We therefore propose that it represents a paradigm for ionic bonding between polyatomic partners in the gas phase. Conformationally mobile molecules are widely viewed as being highly fluxional. Nearly 120 years ago, Le Bel and van't Hoff proposed that free rotation occurs around single bonds. This description is satisfactory for many purposes, although most chemists are well aware that even the simplest example, ethane, has a 12 kJ mol-' barrier to internal rotation.' An ethane molecule has four bound levels corresponding to torsional oscillations. The energy gap between the zero point and the highest bound torsional level is 9 kJ mol-'. The next level, which is approximately 10 kJ mol-' above the zero point, corresponds to free internal rotation of one methyl with respect to the other. At sufficiently elevated temperatures, ethane behaves as though the methyl groups turn