Quantitative infrared and near-infrared gas-phase spectra for pyridine: Absolute intensities and vibrational assignments

Abstract

Using vetted methods for generating quantitative absorption reference data, broadband infrared and near-infrared spectra (total range 11,000–600 cm−1) of pyridine vapor were recorded at 0.1 cm−1 spectral resolution, with the analyte thermostatted at 298 K and pressure-broadened to 1 atmosphere using N2 ballast gas. The quantitative spectrum is reported for the first time, and we have re-assigned some of the 27 fundamental modes. Fundamental assignments were confirmed by IR vapor phase band shapes, FT-Raman measurements and comparison with previous analyses. For the 760-Torr vapor-phase IR data several bands show resolved peaks (Q-branches). We have also assigned for the first time many combination and overtone bands in the mid- and near-IR. Assignments were made via comparison to theoretically calculated frequencies and intensities: The frequencies were computed with Gaussian03 with the anharmonic option, using MP2 and the ccpvtz basis set. The intensities were taken from a VSCF calculation in GAMESS using Hartree–Fock (for overtones and combination bands) or from the harmonic MP2 for fundamentals. Overtone and combination band harmonic and anharmonic frequencies, as well as intensities were also calculated using the CFOUR program. It is seen in the NIR spectrum near 6000 cm−1 that the very strong bands arise from the C-H first overtones, whereas only much weaker bands are observed for combination bands of C-H stretching modes. Certain features are discussed for their potential utility for atmospheric monitoring.