The i.r. spectroscopy of some highly conjugated systems—IV. Effects of variation in the donor atom. Rules for predicting double bond frequency in heterenones

Abstract

The comparative effect of O, N and S as donor atoms is explored for heterenones, defined as compounds in which donors are joined to carbonyl via a carbon double bond. Bond assignments are proposed and the results are collated with those obtained previously [1–3] to give self-consistent rules, good with certain qualifications to ±5–10 cm −1, for predicting double bond frequency in heterenones that contain one carbonyl group and one or two donor atoms (Table 4). Salient features include separate base-lines for the s-cis and s-trans conformations; constant corrections for N-acylation and for the replacement of COR by CO 2R; and a configurational asymmetry such that donor atoms have two-thirds the effect in the CIS-as in the TRANS-position. There are auxiliary correction factors for hydrogen bonding, coupling to δNH, and some steric effects. It is shown how these rules can throw light on the electronic character of complex structures that incorporate the heterenone moiety. There are systematic deviations among CIS-s-cis enaminones due to induced electron donation, and two sorts of steric anomaly: lack of planarity (ν m rises) and self-compensating changes in νCO and νCC that do not affect ν m, confirmed as the true index of conjugation. Also discussed are the reasons for anomalous νCC values in compounds containing sulphur; diagnostic criteria for distinguishing νCC from aromatic modes; and further extensions to the original coupling hypothesis.