Conformation of cis,trans-1,3,5-trimethylcyclohexane-1,3,5-tricarboxylic acid derivatives: crystal structures and semiempirical molecular orbital calculations

Abstract

The conformation of three derivatives of the cis,trans epimer of Kemp’s triacid with cis-carboxy groups, namely the 4-tert-butylbenzyl ester (1), the phenethyl ester (2) and the phenethyl amide (3) was investigated. The determination of crystal structures showed that, while 1 and 2 are in the boat conformation, previously unobserved in this family of compounds, 3 is in the chair conformation, all of them with axial carboxy groups. In the first two cases, dimers are formed by hydrogen bonding between carboxy groups of adjacent molecules whereas in the third case, the presence of other hydrogen bonds involving the amide groups gives rise to the formation of layers. The conformations observed were compared with that assumed by 1 in its uranyl complex previously reported. Semiempirical molecular orbital calculations have shown that the chair and boat forms have intrinsically similar conformational stabilities. We also investigated by X-ray diffraction and differential scanning calorimetry the phase transition presented by 1.