The vibrational spectrum of pyrrole (C4H5N) and furan (C4H4O) in the gas phase

We have recorded the absorption spectrum of pyrrole (C5H5N) and furan (C4H4O) on a broad spectral range in the infrared region and under various experimental conditions, using a Fourier transform interferometer. The resolution was good enough to reliably identify band shapes. We have performed the vibrational assignment of the vibration-rotation bands observed between 500 and 12,000 cm(-1) for pyrrole, and 500 and 9000 cm(-1) for furan. The assignment of the fundamental bands is supported by ab initio calculations at the MP2 level of approximation, considering both band positions and absolute absorption intensities. These calculations also allowed the normal mode coordinates to be drawn for the two species and their geometrical structure to be calculated, Vibrational mode numbering and symmetries are updated to latest conventions in the literature. In the combination and overtone spectral ranges, the band shape information guided the assignment, which could be achieved for most of the numerous observed bands. An extensive set of vibration constants was obtained for each species. The results from the literature available on each species are considered at each step of the investigation. (C) 2001 Elsevier Science B.V. All rights reserved.