Strong hydrogen bonding in the gas phase: fluoromethane⋯hydronium versus fluoromethane⋯ammonium

Abstract The proton-bound heterodimers MeF⋯HOH 2 + and MeF⋯HNH 3 + are here reported for the first time. Ion structures have been characterized by collisionally-induced dissociation and their enthalpies of association computed ab initio at the MP4(SDQ)/6–311G ∗∗ 6D level as ΔH 450 ° = - 105.9 kJ mol −1 and ΔH 450 ° = − 71.0 ± 0.8 kJ mol −1 , respectively. Complete equilibration between the latter species and its constituents has been demonstrated at a pressure of 0.48 Torr in the ion source: establishment of equilibrium at that pressure takes place on the 0.1 ms timescale at 190°C. Theoretical van't Hoff plots give ab initio entropies of association, which match the experimental value published for H 2 O⋯HNH 3 + . The value for McF⋯HOH 2 + is ΔS 450 ° = − 103.0 J mol −1 K −1 , but the entropy for MeF⋯HNH 3 + depends on how the ammonium rocking motions are considered in the large-amplitude limit. Viewing them as a pair of harmonic vibrations yields ΔS 450 ° = − 68.1 J mol −1 K −1 , while looking at these degrees of freedom as a two-dimensional hindered internal rotor gives ΔS 450 ° = − 17.6 ± 1.1 or ΔS 450 ° = − 52.3 ± 1.1 J mol −1 K −1 , depending on whether internal rotation is treated as completely coupled or completely uncoupled from external rotation. Analysis of calculated vibrational frequencies, electron densities, and rotational barriers leads to the conclusion that the large difference in entropy of association between MeF⋯HOH 2 + and MeF⋯HNH 3 + reflects the low degree of covalency in the ammonium-fluorine proton bridge and the comparative accessibility of purely electrostatic bonding orientations.