HYDRATION IN POLYMER STUDIED THROUGH MAGIC-ANGLE-SPINNING NUCLEAR-MAGNETIC-RESONANCE AND HETERONUCLEAR C-13(H-1) OVERHAUSER ENHANCEMENT SPECTROSCOPY - CROSS-RELAXATION AND LOCATION OF WATER IN POLY(ACRYLAMIDE)

A combination of magic angle spinning (MAS) and heteronuclear C-13{H-1 } Overhauser enhancement spectroscopy (HOESY) is shown to be a powerfu l technique for studying hydration in polymers. This is demonstrated i n poly(acrylamide)-water system. The increased spectral resolution due to MAS is shown to resolve polymer-polymer and polymer-water dipolar correlations in the two dimensional HOESY experiment. The 2D experimen t is thus shown to lead to an indirect detection of water interacting with the polymer. The one dimensional transient Overhauser experiment involving selective inversion of water allows the study of cross-relax ation between water protons and carbonyl carbon in the polymer side ch ain. The cross-relaxation rate is rationalized in terms of a direct di pole-dipole interaction between the carbonyl carbon and the hydrated b ound water. Based on temperature dependent O-17 spin-lattice relaxatio n time measurements and a two-step motional model for water, we gather that water molecules close to the observed polymer site reorient anis otropically, typically an order of magnitude slower than in pure water . The correlation time for bound water mobility has been estimated to be 0.58X10(-10) s at 298 K, and, in turn, has been used to locate hydr ated water at a distance of 3.45 Angstrom from amide carbonyl. This is the first time such an estimate has been made for hydrated water in a polymer using HOESY data. (C) 1995 American Institute of Physics.