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

Abstract

The ground-state rotational spectra of the three isotopomers C5H514N···H35Cl, C5H514N···H37Cl and C5H514N···D35Cl 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 A0, B0, C0, centrifugal distortion constants, ΔJ, ΔJK, δJ, δK and nuclear quadrupole coupling constants χaa(A) and χbb(A) − χcc(A) (where A = 14N 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, C2v geometry, with the HCl subunit forming a hydrogen bond to N and lying along the C2 axis of pyridine. The nitrogen to chlorine distance was determined to be 2.999(2) Å. The magnitudes of the nuclear quadrupole coupling constants χaa(A) and the intermolecular stretching force constants kσ 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.