Rotational spectrum of the trimethylamine-hydrogen iodide dimer: An ion pair (CH3)3NH+⋅⋅⋅I− in the gas phase

The ground-state rotational spectra of the two isotopomers [(CH3)3N,HI] and [(CH3)3N,DI] of a dimer formed between trimethylamine and hydrogen iodine have been observed in the gas phase by the pulsed-nozzle, F–T microwave technique. The spectroscopic constants B0, DJ, DJK, χ(14N) and χ(I) have been determined for both isotopomers and have the values 894.0954(2) MHz, 0.010 17(18) kHz, 5.880(10) kHz, −2.451(8) MHz, and −341.204(14) MHz, respectively for [(CH3)3N,HI]. It is concluded, by comparing the iodine nuclear quadrupole coupling constant χ(I) and the intermolecular stretching force constant kσ with those of model hydrogen-bonded (H3P...HI) and ion-pair (Na+...I−) analogs, that (CH3)3NH+...I− is the appropriate description for the gas-phase dimer. An experimentally defined fractional ionic character indicates that the extent of proton transfer increases smoothly along the series [(CH3)3N,HX], where X=F, Cl, Br, and I. Simple energetic considerations are shown to be in agreement with this conclusion.