The relationship of ε_L and ε_R 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 ε_L and ε_R 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 ε_L and ε_R 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.