IR–VUV spectroscopy of pyridine dimers, trimers and pyridine–ammonia complexes in a supersonic jet

Abstract

The infrared spectra of the C–H stretching vibrations of (pyridine)_m, m = 1–3, and the N–H stretching vibrations of (pyridine)_m–(NH₃)_n, m = 1, 2; n = 1–4, complexes were investigated by infrared (IR)–vacuum ultraviolet (VUV) spectroscopy under jet-cooled conditions. The ionization potential (IP₀) of the pyridine monomer was determined to be 74 546 cm⁻¹ (9.242 eV), while its complexes showed only smooth curves of the ionization thresholds at ∼9 eV, indicating large structural changes in the ionic form. The pyridine monomer exhibits five main features with several satellite bands in the C–H stretching region at 3000–3200 cm⁻¹. Anharmonic calculations including Fermi-resonance were carried out to analyze the candidates of the overtone and combination bands which can couple to the C–H stretching fundamentals. For (pyridine)₂ and (pyridine)₃, most C–H bands are blue-shifted by 3–5 cm⁻¹ from those of the monomer. The structures revealed by random searching algorithms with density functional methods indicate that the π-stacked structure is most stable for (pyridine)₂, while (pyridine)₃ prefers the structures stabilized by dipole–dipole and C–H⋯π interactions. For the (pyridine)_m–(NH₃)_n complexes, the mass spectrum exhibited a wide range distribution of the complexes. The observed IR spectra in the N–H stretching vibrations of the complexes showed four main bands in the 3200–3450 cm⁻¹ region. These features are very similar to those of (NH₃)_n complexes, and the bands are assigned to the anti-symmetric N–H stretching band (ν₃), the symmetric N–H stretching (ν₁) band, and the first overtone bands of the N–H bending vibrations (2ν₄). The anharmonic calculations including the Fermi-resonance between ν₁ and 2ν₄ well reproduced the observed spectra.