Conformational Equilibria of β-Alanine and Related Compounds as Studied by NMR Spectroscopy

Abstract

The vicinal proton−proton couplings of the dipolar form of β-alanine in water, alcohol−water, and dimethyl sulfoxide−water solutions indicate little conformational preference and are consistent with an essentially statistical equilibrium of the gauche or trans conformations. The position of the equilibrium is only slightly affected, over a temperature range of about 130°, by changes in dielectric constants ranging from 30 to 80 or by massive changes in ionic strength. Quantum-mechanical calculations at the HF/6-31G^(**) and LMP2/cc-pVTZ levels were found to give rather good parallels with experiment, although suggesting the gauche conformation to be 2−3 kcal/mol more stable in water or methanol than actually observed. A number of related compounds, such as N,N,N-trimethyl-β-alanine and N,N-diethyl-β-alanine, as well as the conjugate acid and conjugate base of β-alanine, also show no significant conformational preference in water solution. In conformity with these results, the zwitterionic form of piperidine-3-carboxylic acid (nipecotic acid) has about the same preference for equatorial carboxylate as cyclohexanecarboxylic acid itself. Taurine shows no significant conformational preference except in basic solution, where the couplings indicate about 53% of the gauche conformation. In contrast, N,N,N-trimethyltaurine is predominantly trans in acidic or neutral solution. The conformational equilibria of the N,N,N-trimethyltaurine species are most likely governed by steric hindrance, because there are rather large tetrahedral groups at each end of the ethano chains. Yet, even here the energy difference between gauche and trans is only about 1.2 kcal.