Hydrogen Bonds in the 1:1 and 2:1 Complexes of Trigonelline with Mineral Acids studied by FTIR, 1H and 13 C NMR Spectroscopies

Abstract

The 1:1 and 2:1 complexes of trigonelline (TRG) with HBr, HI, HNO 3 , HClO 4 and HBF 4 have been synthesized and their FTIR, 1 H and C NMR spectra were analyzed. The 1:1 complexes are divided into two groups. The first group includes complexes with HBr, HI and HNO 3 , in which a proton is transferred from the acid to TRG and the anions are bonded with the carboxylic group, COOH...X - (1). The second comprises complexes with HClO 4 and HBF 4 , with two molecules of protonated TRG forming dimer, like benzoic acid, and the anions interacting exclusively with the positively charged nitrogen atoms (2). All 2:1 complexes crystallized with one water molecule and their FTIR spectra are of HadŽi type iii, characterized by an intense broad (continuum) absorption below 1600 cm - 1 typical of a short-strong hydrogen bond (SSHB) with a delocalized proton and a single vC=O band. In these complexes the water molecule interacts electrostatically with one positively charged nitrogen atom and the anions with the second one (3). The water molecule additionally forms a hydrogen bond with Br, I and NO 3 ions. The proton and carbon chemical shifts of the complexes in D 2 O have been determined and discussed.