B3LYP study of the conformers and rotamers of isostructural N-methylpiperidine betaine hydrochloride and (1-methylcyclohexyl)-acetic acid. The effect of electrostatic attraction on rotation barriers

The structure and rotational barriers of axial and equatorial N-methylpiperidine betaine hydrochlorides, MPBH·Cl, and (1-Methylcyclohexyl)-acetic acid, MCHA, have been carried out at the B3LYP/6-31G(d,p) level of theory. The calculated rotational barrier for the axial and equatorial stereoisomers about the N+–C(8) bond in MPBH·Cl are 8.17 and 8.30 kcal/mol, whereas those for the rotation about the C(1)–C(8) bond in MCHA are 4.11 and 4.91 kcal/mol, respectively. The higher rotation barriers in MPBH·Cl are a result of the electrostatic attractions between the positively charged nitrogen atom with the negatively charged oxygen atoms of the carboxylic group and the chloride anion. Structures of the most stable rotamers of MPBH·Cl and MCHA are analyzed.