Matrix-Isolation Study of the Infrared and Ultraviolet Spectra of the Free Radical HCO. The Hydrocarbon Flame Bands

Abstract

The photoproduction of H or D atoms from a variety of sources in a carbon monoxide matrix or in an argon matrix to which a small concentration of carbon monoxide has been added leads to the appearance of prominent ultraviolet absorptions between 2100 and 2600 Å, all of which may be assigned to HCO or DCO. Both the CO-stretching and the bending vibrational modes are appreciably excited in the transition. Evidence is presented indicating that the transition observed in the matrix experiments is the same one responsible for the hydrocarbon flame bands. Using the frequencies observed in the matrix experiments, a tentative assignment for the hydrocarbon flame bands has been proposed which is in reasonable agreement with the observed band structure of the emission system. In the upper state, the carbon and oxygen atoms of HCO are approximately singly bonded. Observation of the infrared absorption frequencies of isotopically substituted HCO in an argon matrix has prompted reconsideration of the valence force field appropriate to ground-state HCO. Interaction between the CH-stretching and the CO-stretching modes has been found to play an important role. Factors leading to the stabilization of HCO in an argon matrix in the present experiments, in contrast to the results of previous studies, are discussed.