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, Δ_J, Δ_JK, δ_J, δ_K and nuclear quadrupole coupling constants χ_aa(A) and χ_bb(A) − χ_cc(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, C_2v 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 χ_aa(A) and the intermolecular stretching force constants k_σ 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.