Far-infrared study of heteroconjugated and homoconjugated hydrogen bonds between trimethylamine N-oxide and various carboxylic acids as a function of the pKa of the acid

Abstract

Six trimethylamine N-oxide–carboxylic acid systems have been studied in the FIR region with regard to the hydrogen-bond vibration ν_σ. These systems were previously investigated in acetonitrile using MIR spectroscopy. In the FIR these systems could be studied only in chloroform solutions. Therefore, the MIR studies were repeated in chloroform solutions. Owing the CH acidity of chloroform (with the more acidic systems) homoconjugated NO⁺H⋯ON ⇌ NO⋯H⁺ON bonds are favoured compared with the heteroconjugated ones. In chloroform the other results remained essentially the same. In the FIR only one hydrogen bond vibration of the heteroconjugated CO₂H⋯ON ⇌ CO^–₂⋯H⁺ON complexes is found. In the less acidic systems, where broad single-minimum proton potentials with at best a small barrier are present, the ν_σ band is found at ca. 210 cm^–1 whereas it is found at ca. 170 cm^–1 with the double-minimum systems, indicating that these hydrogen bonds are loosened due to the interaction with the environment. In the case of the homoconjugated species this vibration is found at 240 cm^–1. Changing the acid–base ratio these bands can easily be distinguished. The intensity of the homoconjugated bands increases with increasing acidity of the acids.