Hydrogen bonding Part 36. NMR study of 14N to CH coupling as a measure of hydration of tetramethylammonium and tetrapropylammonium halides

Abstract

Correlation of development of 14N to CH NMR coupling with cation mobility has been used to study hydration behaviour of tetramethylammonium and tetrapropylammonium halides in the range between saturated solutions and the point where full ion mobility is attained. The tetramethylammonium ion in solutions of the chloride forms an initial tightly bound clathrate with 4 moles H 2O per mole of cation (H 2O/salt=4), and a larger, looser hydration shell with an additional 3 moles H 2O. The tetramethylammonium ion presumably also forms an initial clathrate with H 2O/salt=4 in solutions of the fluoride; however, the salt binds water so tightly that ion mobility does not commence until H 2O/salt = 7.5, and full mobility is not reached until H 2O/salt≈10. Tetramethylammonium bromide shows full ion mobility in the saturated solution with H 2O/salt= 8.75. The larger tetrapropylammonium ion forms an initial clathrate with H 2O/salt=9; both the chloride and bromide show development of a larger hydration shell. Comparison with previously studied tetraalkylammonium halides demonstrates that the formation or lack of formation of a larger hydration shell is not a function of hydration type (normal type A or hypobarogenic type B) of the tetraalkylammonium halide; rather, it is a combined function of cation size and anion electronegativity.