The nature of the complex formed between pyridine and hydrogen bromide in the gas phase: An experimental approach using rotational spectroscopy

Abstract

The rotational spectra of two isotopomers, C(5)H(5)N...H(79)Br and C(5)H(5)N...H(81)Br, of a complex formed by pyridine and hydrogen bromide were observed by using a pulsed-jet, Fourier-transform microwave spectrometer that incorporated a mixing nozzle. Rotational constants, centrifugal distortion constants, and nuclear quadrupole coupling constants for the (14)N and Br nuclei were determined. The rotational constants indicate that the observed complex has a planar geometry of C(2v) symmetry, with the HBr subunit lying along the C(2) axis of pyridine. These conclusions are in good agreement with those obtained earlier from infrared spectroscopy in cryogenic matrices and ab initio calculations. The distance r(N...H)=1.137(2) A was obtained by fitting the rotational constants under the assumption of an unchanged pyridine geometry and with r(H-Br) fixed at the value obtained through an ab initio calculation. The nuclear quadrupole coupling constants were interpreted, with the aid of ab initio calculations for the complex, as well as hydrogen-bonded and ion-pair limiting models for the interaction of pyridine and HBr, to establish that the complex may be visualized in terms of a substantial contribution (>60%) of the ionic structure C(5)H(5)NH(+)...Br(-) to its valence-bond description. The intermolecular force constant k(sigma)=57.8(16) N m(-1) determined from the centrifugal distortion constants Delta(J) of the two isotopomers is also consistent with a strongly bound complex involving partial proton transfer.