THE IR, RAMAN AND NMR SPECTRA AND CONFORMATIONS OF CYCLOHEXYLALLENE

Article abstract of DOI:10.1016/0022-2860(90)80244-E

Three conformers of cyclohexylallene, designated I, II and III, were observed in the IR and Raman spectra.Metastable crystals formed by annealing an amorphous solid from 80 K to 140 K, and crystals formed under a pressure of 30 kbar at ambient temperature, contained conformer II.The stable crystals obtained after heating the metastable crystals to 195 K contained conformer I.The conformer III was observed as a minor component in a nitrogen matrix heated to 900 K before deposition.Negligible intensity variations between the bands of conformer I and II were observedwith nozzle temperatures 300-900 K in the argon and nitrogen matrices, indicating a ΔH⁰(I-II) below 0.4 kJ mol^-1 while ΔH⁰(III-II) was equal to 6.5 kJ mol^-1 and a barrier higher than ca. 11 kJ mol^-1 was observed between the latter.Variable temperature measurements in Raman indicate ΔH⁰(I-II) ca. 0.9 kJ mol^-1 in the liquid.The 13C NMR spectra show that in deuteromethanol solution the equatorial conformer dominates ( = 2percent a at 193 K, ΔG⁰(a-e) ca. 6.4 kJ mol^-1).Analysing the temperature dependence of the vicinal H-H coupling across the cyclohexane - allene carbon bond on the basis of a gauche <-> anti equilibrium it is found 40percent anti and 60percent gauche in methanol solution at ambient temperature.We conclude that the abundant conformers II and I being respectively the anti and the gauche conformer, while III is the anti axial conformer.