Temperature and length scale dependence of tetraalkylammonium ion-amide interaction.
We have studied the temperature and length scale dependence of the energetics of the pair interaction of well-established hydrophobic solutes tetraalkylammonium bromides with hydrophilic formamide (FA) and hydrophobic hexamethylphosphoric triamide (HMPT). Our results do indicate the anomalous length scale dependence of the tetraalkylammonium cation-amide interaction in water. As the cation size is increased, the unfavorable enthalpy of interaction is increased rather linearly until the maximum is reached, after which there appears to be a reversal of the trend. We believe that this phenomenon arises from the impossibility of water to maintain its H-bond network near large tetraalkylammonium cations that leads to the formation of a somewhat disordered solute hydration shell. The energetic cost for overlapping this shell with the amide hydration shell in water is noticeably smaller than that for tetraalkylammonium cations of a moderated size.
Kustov
Andrey V
AV
Institute of Solution Chemistry of Russian Academy of Sciences, Ivanovo State University of Chemistry and Technology, Ivanovo 153045, Russia. kustov@isuct.ru
Korolev
Valeriy P
VP