The relationship of εL and εR from consideration of bisignate C.D. curves of α,β-unsaturated ketones

Abstract

From reconsideration of experimental evidence of ∆ε and ε curves and extrapolation of comments by Beecham and Collins1 it is concluded that the occurrence of bisignate n- π* c.d. curves for ketones and α,β-unsaturated ketones does not require hypothesizing the existence of two spectral bands of opposite sign and therefore indicates that the relationship between εL and εR for a single transition does not necessarily involve a simple constant amplitude factor as the conventional interpretation holds. An alternative interpretation of bisignate curves is proposed, based on one spectral band, the relationship between εL and εR involving differential half-widths of the vibronic components of the L series relative to the R series and a variable relative-amplitude scale along the vibronic progression. If appropriate scale and differential half-width parameters are used, the characteristics of monosignate and bisignate curves can be demonstrated by means of synthetic curves.