The conformations and 1H nuclear magnetic resonance parameters of dibenzylideneacetone

Abstract

Dibenzylideneacetone, (C₆H₅CHCH)₂CO, with two degrees of rotational freedom imparted by the two single bonds between the olefin and the carbonyl group was studied to determine its stable conformers by ¹H n.m.r. spectroscopy and by an INDO molecular orbital method. A solvent effect induced by benzene on the trans-olefinic protons and a least squares analysis of the temperature dependence of the internal chemical shift (δ_AB) between the olefinic protons have revealed support of the theoretical calculations of δ_AB that the major conformation is s-cis,cis and the minor one is s-cis,trans. An INDO calculation of the conformers concerned confirmed this deduction together with the absence of the s-trans,trans form. δ_AB is a characteristic parameter of the conformation with the values 0.5 and 1.5 p.p.m. for the s-cis and s-trans forms, respectively, in the olefinic portions of dibenzylideneaċetone.