The rotational spectrum and nature of the heterodimer in trimethylammonium bromide vapor

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Abstract
The ground‐state rotational spectra of three symmetric‐top isotopomers (CH3)3 14N⋅⋅⋅H 79Br, (CH3)3 14N⋅⋅⋅H 81Br, and (CH3)3 14N⋅⋅⋅D 79Br of the heterodimer of trimethylamine and hydrogen bromide have been detected by pulsed‐nozzle, Fourier‐transform microwave spectroscopy. The spectroscopic constants B 0, D J , D J K , χ(14N), and χ(Br) have been determined for each of the isotopomers and for (CH3)3 14N⋅⋅⋅H 81Br have the values 1161.6294(2) MHz, 0.148(5) kHz, 7.77(2) kHz, −2.883(7) MHz, and 99.645(7) MHz, respectively. A comparison of the 14N– and 81 Br–nuclear‐quadrupole coupling constants χ(14N) and χ(Br) with those expected on the basis of a hydrogen‐bonded model (CH3)3N⋅⋅⋅HBr and an ion‐pair model (CH3)3NH+⋅⋅⋅Br− leads to the conclusion that in the heterodimer trimethylamine‐hydrogen bromide there is a significant extent of proton transfer from HBr to (CH3)3N. The value of the intermolecular stretching force constant k σ=82(3) N m−1 determined from D J is also compared with those expected for the limiting models of the dimer and is found to lie close to that associated with the ion‐pair limit.