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 re ported. In compounds containing three CH2 groups in the tether and three su bstituents at the nitrogen atom or alpha, alpha'-disubstituted pyridine rin g, a characteristic multiplet for an AA'MM'X-2 spin system is observed. Thi s is consistent with preference for trans conformation (68-85%). In the spe ctra 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 n o significant conformational preference. Both the number of CH: groups in t ether and the bulkiness of the charged groups are responsible for the obser ved 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.