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

The rotational spectra of two isotopomers, C5H5N...H79Br and C5H5N...H81Br, 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 14N and Br nuclei were determined. The rotational constants indicate that the observed complex has a planar geometry of C2v symmetry, with the HBr subunit lying along the C2 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) Å 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 C5H5NH+...Br- to its valence-bond description. The intermolecular force constant kσ=57.8(16) N m-1 determined from the centrifugal distortion constants ΔJ of the two isotopomers is also consistent with a strongly bound complex involving partial proton transfer. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]