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

Abstract

Synthesis and 1H NMR spectra in D 2O of 4 betaines and 19 betaine complexes with mineral acids containing 2 or 3 CH 2 groups in the tether, N +–(CH 2) n –COO −, n=2,3, and diverse volume of the positively charged groups are reported. In compounds containing three CH 2 groups in the tether and three substituents at the nitrogen atom or α, α′-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 CH 2 groups in the tether or three CH 2 groups and unsubstituted pyridine ring, the multiplet changes to a triplet and gives apparent A 2X 2 and A 2M 2X 2 spectra, respectively, consistent with no significant conformational preference. Both the number of CH 2 groups in tether and the bulkiness of the charged groups are responsible for the observed differences of N +CH 2 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 trans, but in aqueous solution it is reverse.