Pyrolysis jet spectroscopy: The S1–S band system of formyl cyanide, HCOCN, and DCOCN

Abstract

The à 1A″–X̃ 1A′ band systems of HCOCN and DCOCN have been studied in depth using the technique of pyrolysis jet spectroscopy. Ab initio predictions of molecular structures and vibrational frequencies for the ground and excited state were made, to assist in the assignment of the vibronic structure in the laser-induced fluorescence (LIF) spectra. A resolved emission spectrum obtained by laser population of the 81 level of the excited state was analyzed to establish the ground state vibrational frequencies. Both the LIF and resolved emission spectra are consistent with a substantial elongation of the C=O bond on excitation, and smaller changes in the other geometric parameters. The excited state inertial defect, the pattern of bands involving the antisymmetric vibrations, and the ab initio predictions all suggest that the excited state structure is only slightly nonplanar. A rotational analysis of the 910 band of the DCOCN LIF spectrum has provided the first experimental rotational constants for the ground state of formyl cyanide-d1, which should be useful in attempts to observe the microwave spectrum. The uv spectrum of formyl cyanide is found to be remarkably similar to that of propynal.