Donor-acceptor complexes formed by perfluoro-organo bromides and iodides with nitrogenous and other bases. II. Band shapes and widths in the absorption spectrum of gaseous CF₃I-N(CH₃)₃

Abstract

The absorption band centred at c. 77 cm^-1 in gaseous mixtures of CF₃I and N(CH₃)₃ previously reported and attributed to the N-I stretching mode of the complex CF₃I-N(CH₃)₃ has been carefully re-examined. This band is of interest as an example of a low frequency ?dissociative type? vibrational mode of a weak molecular complex. The band is asymmetric and apparently structureless with a half intensity width at room temperature of 28-30 cm^-1. The width of the band may be accounted for as arising from transitions v_i + v_i+1 where v_i is the vibrational quantum number of the N-I stretching mode with v_i up to c. 10 making appreciable contribution to the intensity on the low wave-number side. Centrifugal distortion in the complex is considered. Centrifugal stretching and consequent weakening of the bond may shift the band envelope 2-3 cm^-1 to lower wave numbers. Assessment of these and other factors affecting the band shape suggest that the fundamental frequency is probably c. 90 cm^-1. The band shape of the vibrational mode of the complex at c. 272 cm^-1 is briefly discussed. Many of the considerations presented in this paper should apply to vibration-rotation band shapes in other weak molecular complexes. Some general consequences of anharmonicity for the interpretation of the spectra of weak molecular complexes are discussed.