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

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 similar to 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 pi-stacked structure is most stable for (pyridine)(2), while (pyridine)(3)prefers the structures stabilized by dipole-dipole and C-HMIDLINE HORIZONTAL ELLIPSIS pi 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 (nu(3)), the symmetric N-H stretching (nu(1)) band, and the first overtone bands of the N-H bending vibrations (2 nu(4)). The anharmonic calculations including the Fermi-resonance between nu(1)and 2 nu(4)well reproduced the observed spectra.