Structure and Spectra of the Oxalyl Halides

Abstract

<p> The ultraviolet absorption spectrum of oxalyl chloride-fluoride has been recorded under high resolution, and has been attributed to a superposition of the spectra of the cis isomer (which appears weakly) and the trans isomer. The ultraviolet spectra of the cis isomers of oxalyl bromide, oxalyl chloride and oxalyl fluoride have also been identified. With the help of theoretical calculations, discrete absorption in the ultraviolet spectrum of trans oxalyl chloride-fluoride has been attributed to the superposition of four systems: (a) the singlet-singlet and singlet-triplet transitions which involve promotion of an electron from the highest energy non-bonding orbital (n₁) to the lowest energy antibonding orbital (π₁*) and (b) the singlet-singlet and singlet-triplet transitions which involve promotion from n₁ to the second lowest energy antibonding π orbital (π₂*). The S-S and S-T, n₁ → π₁* transitions have been analyzed in detail. </p> <p> Theoretical calculations have been carried out which indicate that α,β diketones, promotion of an electron to the lowest energy antibonding skeletal orbital (σ₁*) produces states which have a tendency towards dissociation along the C-C bond. It has been postulated that the diffuseness of the high energy absorption spectrum results from such a molecular dissociation, the Aᵤ(π₁,π₁*) state being strongly predissociated by the Aᵤ(n₁,σ₁*) state while the Aᵤ(n₁,π₁*) state is strongly predissociated by the Aᵤ(π₁,σ₁*) state. </p> <p> It has been postulated that the fluid phases of glyoxal and biacetyl consist of an equilibrium mixture of these molecules in various degrees of aggregation (e.g., monomeric, dimeric, trimeric,...). An ultraviolet band system previously assigned to a second n → π* transitions of these molecules has been reassigned to the first n → π* transition of a polymeric species. Some evidence has been assembled which supports this hypothesis. </p>