The absorption spectrum of nitrosyl chloride

Abstract

Although the photodecomposition of nitrosyl chloride has often been quoted as a typical photochemical reaction, there is considerable disagree ­ ment in the literature with regard to its exact mechanism. Early workers (Kiss 1923; Bowen and Sharpe 1925) found a quantum efficiency, γ , of 0.5 for the visual region, while Kistiakowsky (1930) found a value of 2. Quanta of wave-lengths shorter than 7600 A have sufficient energy to dissociate NOCl. Kistiakowsky, however, found fine structure in the spectrum as far down as 4100 A and suggested therefore that the reaction occurred as a result of a collision between a photochemically activated molecule and a normal molecule. On the other hand, Allmand (1931) pointed out that the fact that γ was not reduced (Kistiakowsky 1930) either by sevenfold dilution with nitrogen or reduction in pressure by a factor of nine, indicated a primary dissociation mechanism. He suggested the possibility of this occurring without the disappearance of rotational fine structure. Recently, Winn (1939) has found photochemical decomposition of NOCl to occur with a quantum efficiency of 2 down to extremely low pressures. The existence of rotational fine structure associated with primary dissociation still awaits clarification. The authors have therefore reinvestigated the absorption spectrum of nitrosyl chloride in the visible and ultraviolet regions, and have measured the extinction coefficients over a wide range. Manganini (1889) was the first to study this spectrum but made only qualitative measurements. Leermakers and Ramsperger (1932), incidental to another problem, measured the extinction coefficient of NOCl over a small part of the visible region, but made no mention of fine structure. The molecular structure of NOCl has been investigated recently by both infra-red and electron diffraction methods (Bailey and Cassie 1934; Ketelaar and Palmer 1937; Fox and Martin 1938). The molecule is apparently triangular, the only non-linear completely asymmetrical triatomic molecule reported in the literature.* The other nitrosyl halides are presumably similar. The characteristics of the vibrations of such a molecule still await complete solution.