Differences in proton-proton coupling constants of N+-CH2-CH2 protons of some betaines, N+-(CH2)2)3-COO-, and their complexes in aqueous solution

Synthesis and H-1 NMR spectra in D2O of 4 betaines and 19 betaine complexes with mineral acids containing 2 or 3 CH2 groups in the tether, N+-(CH2)(n)-COO-, n = 2, 3, and diverse volume of the positively charged groups are reported. In compounds containing three CH2 groups in the tether and three substituents at the nitrogen atom or alpha, alpha'-disubstituted pyridine ring, a characteristic multiplet for an AA'MM'X-2 spin system is observed. This is consistent with preference for trans conformation (68-85%). In the spectra of compounds with two CH2 groups in the tether or three CH2 groups and unsubstituted pyridine ring, the multiplet changes to a triplet and gives apparent A(2)X(2) and A(2)M(2)X(2) spectra, respectively, consistent with no significant conformational preference. Both the number of CH: groups in tether and the bulkiness of the charged groups are responsible for the observed differences of N+CH2 multiplicity and reflect changes in conformational preferences. According to the PM3 calculations, in the gas phase a gauche-like conformer is more stable than the irans, but in aqueous solution it is reverse. (C) 2001 Elsevier Science B.V. All rights reserved.