Structure and Ultraviolet Spectra of Ethylene, Butadiene, and Their Alkyl Derivatives

A critical survey is made of existing knowledge on the absorption spectra near $\lambda 2000$ of ethylene, butadiene, and their alkyl derivatives. Particular emphasis is given to the nature of the molecular structures and electronic states involved. The results of some preliminary quantum-theoretical computations of C. A. Rieke and the writer on electronic term values and on red shifts due to hyperconjugation with alkyl groups are reported. It is concluded that decreases in ionization potential on alkyl substitution are due mainly to charge transfer as suggested by Price, but that red shifts in spectra result partly from this and partly from hyperconjugation. Various points raised in papers of Price and collaborators, and in the paper of Carr, Pickett, and Stücklen (CPS) in this issue, are discussed. The spectroscopic and refractivity data, combined with the computations, favor the $s$-trans form as the stable form in butadiene and its open-chain derivatives, but there is some evidence pointing to the presence of admixture of the $s$-cis form; further study is needed. (The nomenclature $s$-cis and $s$-trans is here proposed for the arrangement of groups around a single bond.) The stability of the planar arrangement in various electronic states of the molecule and its positive ion is discussed; the evidence indicates that the planar form is barely stable in the ion. Attention is called to the need of further study of the phenomenon of photo-chemical cis↔trans conversion by light near $\lambda 2000$.