An ab initio study of internal rotation in cyclohexanecarboxaldehyde and cyclohexanecarboxylic acid fluoride and chloride

The conformational equilibrium and structure of cyclohexanecarboxaldehyde and cyclohexanecarboxylic acid fluoride and chloride were investigated by ab initio calculations with 3-21G (or 3-21G*) and 6-31G* basis sets. Additionally, a single point second-or third-order Møller-Plesset calculations with the HF/6-31G* optimized structural parameters were performed. The gauche was calculated to be thermodynamically the preferred conformation for the three molecules with a chair cyclohexane ring and an equatorial substituent. The aldehyde and the fluoride were determined to have the trans while the chloride to have the cis for the high energy form. From the potential surface scan and transition states optimization, the gauche to trans barrier was predicted to increase in the order: chloride > fluoride > aldehyde. The inclusion of electron correlation in the calculations made a noticeable change on energy differences and barriers in the aldehyde. For the halides, electron correlation slightly affected the barriers only, particularly in the fluoride compound.