Organic anions. Part 2. Spectroscopic studies of 1,3-diarylpropenide ions

Abstract

Electronic absorption, ¹H n.m.r. and ¹³C n.m.r. spectra have been determined for eight sodium (and in some cases lithium and potassium) 1,3-diarylpropenides (I) in liquid ammonia. These show that, in this medium, contact ion pairs are not formed and that not only the trans,trans-anions (I) but also small amounts of the cis,trans-anions (II) are present. The rate of interconversion of the trans,trans- and cis,trans-forms is too low to be measured by n.m.r. spectroscopy but from the proton-decoupled ¹³C n.m.r. spectra of the unsubstituted diphenylpropenide (IIIa) it is possible to measure the rate of rotation of the phenyl group. The barrier to this rotation, ΔG^‡_rot= 10.9 kcal mol^–1, is essentially independent of the nature of the counter-ion (Li⁺, Na⁺, or K⁺).