Does the vapour of trimethylphosphonium bromide contain ion pairs (CH3)3PH+⋯ Br– ? An answer from the rotational spectrum

Abstract

The rotational spectra of three symmetric-top isotopomers of the heterodimer in trimethylphosphonium bromide vapour have been detected and analysed to give the ground-state spectroscopic B₀, D_J, D_JK, χ(Br), C_N(Br) and C_K(Br) in each case. Comparison of the bromine nuclear quadrupole coupling constants χ(Br) with those of various model hydrogen-bonded dimers B⋯HBr and that of the ion-pair model Na⁺⋯Br^– establishes that the heterodimer should be described as (CH₃)₃P⋯HBr rather than (CH₃)₃PH⁺⋯Br^–. The conclusion is reinforced by the corresponding comparison of the intermolecular stretching force constants k_σ(as determined from D_J values). This finding is in sharp contrast to the situation in trimethylammonium bromide vapour, in which the heterodimer species is (CH₃)₃NH⁺⋯Br^–. The reason for the difference in behaviour of the N and P species is attributed to the greater effective ionic radius of the trimethylphosphonium ion.