Ionic versus apolar behavior of the tetramethylammonium ion in water

Abstract

Neutron diffraction involving seven isotope substitution experiments is used to extract the methyl hydrogen to the water hydrogen partial structure factor in a 2.0 m aqueous solution of tetramethylammonium chloride, in order to investigate the orientation of water molecules around the TMA ion. The correlation function of the TMA centers with water molecule centers is obtained and shows that the average distance is about 4.7 Å, in agreement with the result found previously using a nitrogen isotope substitution. The technique of spherical harmonic reconstruction is used to obtain orientational correlation functions between TMA ions and water molecules. The results show that the TMA ion has a predominantly ‘‘apolar’’ character in water but that a range of water molecule orientations around the ion are possible.