THE ROTATIONAL SPECTRUM AND NATURE OF THE HETERODIMER IN TRIMETHYLAMMONIUM BROMIDE VAPOR

The ground-state rotational spectra of three symmetric-top isotopomers (CH3)3 14N⋯H 79Br, (CH 3)3 14N⋯H 81Br, and (CH 3)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 B0, Dj, DJK, X( 14N), and X(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 81Br-nuclear-quadrupole coupling constants X(14N) and X(Br) with those expected on the basis of a hydrogen-bonded model (CH3)3N⋯HBr and an ion-pair model (CH3)3 N+H⋯B -r leads to the conclusion that in the heterodimer trimethylamine-hydrogen bromide there is a significant extent of proton transfer from HBr to (CH3)3 N. The value of the intermolecular stretching force constant kσ = 82 (3) N m-1 determined from DJ 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. © 1990 American Institute of Physics.