An investigation of the trimethylammonium chloride molecule in the vapor phase by pulsed-nozzle, Fourier-transform microwave spectroscopy

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Abstract
The ground-state rotational spectra of the symmetric-top isotopomers [(CH3)314N, H35Cl], [(CH3)314N, D35Cl], [(CH3)315N, H35Cl], and [(CH3)315N, H37Cl] of a dimer formed between trimethylamine and hydrogen chloride have been observed in the vapor above heated samples of solid trimethylammonium chloride by pulsed-nozzle, Fourier-transform, microwave spectroscopy. Spectroscopic constants have been determined in each case and for [(CH3)314N, H35Cl] the values are B0=1800.4605(2) MHz, DJ=0.320(10) kHz, DJK=13.59(11) kHz, χ(35Cl)=−21.625(5) MHz, and χ(14N)=−3.504(5) MHz. The observed rotational constants B0 indicate that the nuclei N, H, and Cl lie along the C3 symmetry axis of the molecule in the order N⋅⋅⋅H⋅⋅⋅Cl with the distance r(N⋅⋅⋅Cl)=2.8164(3) Å. A comparison of the values of the nuclear quadrupole coupling constants χ(35Cl) and χ(14N) with those in related dimers and molecules leads to the evocation of some ionic character (CH3)3N +H C̄l arising from a significant extent of proton transfer from Cl to N. This conclusion is reinforced when the values of χ(35Cl) and χ(14N) predicted on the basis of a recent electrostatic hydrogen-bonded model (CH3)3N⋅⋅⋅HCl of the dimer are considered. The intermolecular stretching force constants kσ=84(3) N m−1 determined from DJ has a magnitude closer to that expected in the ion-pair limit than in the hydrogen-bond limit.