Hydration of amines, diamines, polyamines and amides studied by NMR

Abstract

The spin–lattice relaxation times, T₁, of natural abundance H₂¹⁷O have been measured for aqueous solutions of nine amines, five diamines, two polyamines and five amides, as a function of the concentration at 25 °C. The water-accessible surface areas of the solute molecules were calculated. The coordination number, n_h, and the rotational correlation times, τ^h_c, of water molecules around the solute molecules were estimated and compared with that of pure water τ⁰_c. The value of τ^h_c/τ⁰_c= 2.02 for tert-butylamine is the largest and that of τ^h_c/τ⁰_c= 0.97 for urea the smallest obtained. The value of n_h(τ^h_c/τ⁰_c– 1) was defined as the dynamic hydration number (DHN).

The partial molar volumes and the partial molar heat capacities for these homologues at infinite dilution are linearly dependent on their DHN.