FAR-INFRARED STUDY OF HETEROCONJUGATED AND HOMOCONJUGATED HYDROGEN-BONDS BETWEEN TRIMETHYLAMINE N-OXIDE AND VARIOUS CARBOXYLIC-ACIDS AS A FUNCTION OF THE PK(A) OF THE ACID

Six trimethylamine N-oxide-carboxylic acid systems have been studied i n the FIR region with regard to the hydrogen-bond vibration nu(sigma). These systems were previously investigated in acetonitrile using MIR spectroscopy. In the FIR these systems could be studied only in chloro form solutions. Therefore, the MIR studies were repeated in chloroform solutions. Owing the CH acidity of chloroform (with the more acidic s ystems) homoconjugated NO+H ... ON half arrow right over half arrow le ft CO2- ... 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 CO 2H ... ON half arrow right over half arrow left CO2- ... H+ON complexe s is found. In the less acidic systems, where broad single-minimum pro ton potentials with at best a small barrier are present, the nu(sigma) band is found at ca. 210 cm-1 whereas it is found at ca. 170 cm- 1 wi th the double-minimum systems, indicating that these hydrogen bonds ar e loosened due to the interaction with the environment. In the case of the homoconjugated species this vibration is found at 240 cm-1. Chang ing the acid-base ratio these bands can easily be distinguished. The i ntensity of the homoconjugated bands increases with increasing acidity of the acids.