Conformational preferences of isostructural N-methylpiperidine betaine and (1-methylcyclohexyl) acetic acid studied by PM3 and B3LYP calculations.: The effect of electrostatic interactions on the rotation barrier

PM3 conformational analysis has been carried out on the axial and equatorial N-methylpiperidine betaine (N-carboxymethyl-N-methylpiperidinium inert salt), MPB, and (1-methylcyclohexyl)acetic acid, MCHA. The terms axial and equatorial refer to the positions of CH2COO(H) substituents. The rotation barriers for H3C-N+-CH2-COO- and H3C-C-CH2-COOH units in the axial and equatorial stereoisomers are 9.9 and 6.1 kcal/mol for MPB and 3.9 and 2.4 kcal/mol for MCHA, respectively. An increase of the rotation barriers on going from MCHA to MPB is result of appearance of the Coulombic interactions between the charged groups in the latter compounds. The intramolecular C-H . . .O hydrogen bonding plays a minor role in determining the conformation. Four conformers of MPB have been analyzed by B3LYP level calculation with 6-31G(d,p) and 6-311++G(d,p) basis sets and the results demonstrate that the most stable is the axial-gauche and the least stable is the axial-anti conformer. (C) 2001 Elsevier Science B.V. All rights reserved.