Spectrochimica Acta. Vol. 48A, No. 8, pp. 1115-1126. 1992 0584-8539/92 $5.00+ 0.00 Printed in Great Britain ~) 1992 Pergamon Press Lid A re-examination of the infrared and ultraviolet spectroscopy of trifluoroacetyl fluoride and trifluoroacetyl chloride: an experimental and theoretical study J. S. FRANCISCO Department of Chemistry, Wayne State University, Detroit, MI 48202, U.S.A. and I. H. WILLIAMS School of Chemistry, University of Bath, Bath, BA2 7AY, U.K. (Received 22 November 1991; in final form 16 January 1992; accepted 10 February 1992) Abstract--The fundamental IR vibrational modes of trifluoroacetyl fluoride CF3C(O)F and trifluoroacetyl chloride CF3C(O)CI have been re-examined by ab initio molecular orbital calculations and compared with literature assignments. Several bands of the IR spectrum are reassigned. The Q-branch and integrated absorption cross-sections have been measured for vl, v3, 1,4 and v~l fundamental bands for both pressurized and unpressurized samples on each molecule. The UV absorption spectra of CF3C(O)F and CF3C(O)CI show a structureless continuum with a maximum at 210 nm (Omax = 3.20+0.02 x 10-2°cm 2 molecule -~) and 255 nm (Om~ = 7.66 + 0.26 X 10 -2o cm 2 molecule-l), respectively. The nature of the electronic transition giving rise to the UV absorption spectrum for CF3C(O)F and CF3C(O)CI has been examined by ab initio molecular orbital calculations. It is attributed to the/[ IA"~--~tA' electronic transition. 1. INTRODUCTION STRONG EVIDENCEsuggests that C1 atoms produced by the photolysis of chlorofluorocar- bons (CFCs) are responsible for the depletion of stratospheric ozone [1]. As a result, a new generation of compounds known as hydrochlorofluorocarbons (HCFCs) have been designed to degrade in the troposphere, thereby suppressing the transport of chlorine- containing molecules to the stratosphere. Degradation is likely initiated in the troposphere by reaction with OH or HO2 radicals by abstraction of a hydrogen atom from the parent HCFC. Oxidation of resulting fragments is currently a major experimental and theoretical focus in our group. Preliminary considerations of the oxidation of CF3CX2H (where X -- F or Cl) suggest that trifluoroacetyl halides are likely stable intermediates formed in the photo-oxidation process [2]. Trifluoroacetyl fluoride, CF3C(O)F, is suggested to result from the degrada- tion of HCFC-134 (CFaCFzH) or HCFC-127 (CF3CH2F), while trifluoroacetyl chloride is a possible degradation product of HCFC-124 (CFaCFCIH) or HCFC-123 (CF3CCI3H) [2, 31. Identification and quantitation of these species in the atmosphere is achieved by using characteristic IR absorption bands, assuming that the bands are properly assigned and the corresponding absorption cross-sections are known. In the present work, the vibrational spectra of trifluoroacetyl fluoride and trifluoroacetyl chloride have been re- examined with the aid of ab initio molecular orbital calculations of the normal modes of these compounds, for comparison with the experimentally determined spectra. IR cross- sections are also measured for selected bands that facilitate quantitative measurements of these species in the upper atmosphere. Furthermore, the presence of these com- pounds contributes to the greenhouse effect by adsorbing infrared radiation in the atmospheric window [4]. It is in the region between 1250 and 750 cm -1 that absorption by trace gases reduces heat loss and thus contributes to warming of the lower atmosphere. Measurement of IR cross-sections of bands in the 1240-750 cm-1 region gives a measure of the amount of radiation that can be absorbed by these compounds. There are no reports of the absorption cross-section for the UV spectra of CF3C(O)F 1115