Is pyridinium hydrochloride a simple hydrogen-bonded complex C5H5N•••HCl or an ion pair C5H5NH+•••Cl- in the gas phase?: An answer from its rotational spectrum

The ground-state rotational spectra of the three isotopomers (C5H5N)-N-14 ... (HCl)-Cl-35, (C5H5N)-N-14 ... (HCl)-Cl-37 and (C5H5N)-N-14 ... (DCl)-Cl-35 of a complex formed by pyridine with hydrogen chloride have been observed by using a fast-mixing nozzle in combination with a pulsed-nozzle, FT microwave spectrometer Rotational constants A(0), B-0, C-0, centrifugal distortion constants, Delta(J), Delta(JK), delta(J), delta(K) and nuclear quadrupole coupling constants chi(aa)(A) and chi(bb)(A) - chi(cc)(A) (where A = N-14 or Cl) were determined in each case. A detailed interpretation of the spectroscopic constants led to the conclusion that the observed complex has a planar, C-2v geometry, with the HCl subunit forming a hydrogen bond to N and lying along the C-2 axis of pyridine. The nitrogen to chlorine distance was determined to be 2.999(2) Angstrom. The magnitudes of the nuclear quadrupole coupling constants chi(aa)(A) and the intermolecular stretching force constants k(sigma) in comparison with those expected in the hydrogen-bond C5H5N ... HCl and ion-pair C5H5NH+... Cl- limits show that the extent of proton transfer from HCl to pyridine is small.