Hydration structure around the carboxyl group studied by neutron diffraction with C-12/C-13 and H/D isotopic substitution methods

摘要

Time-of-Flight (TOF) neutron diffraction measurements have been carried out on aqueous 8 mol% sodium acetate solutions in D2O. Scattering cross sections that were observed for sample solutions involving C-12/C-13 and H/D isotopically substituted acetate ions were used to derive the first-order difference functions, Delta(H)(Q) and Delta(C)(Q), and corresponding distribution functions, G(H)(r) and G(C)(r), which describe the environmental structure around the methyl and the carboxyl groups within the acetate ion, respectively. Structural parameters concerning the first hydration shell of the carboxyl group within the acetate ion were obtained through the least squares fit to the observed intermolecular difference function, Delta(C)(inter)(Q). The nearest neighbor C-O ... D-W1 (C-O: carboxyl carbon atom, D-W1: water deuterium atom) distance, r(C-O ... D-W1), and the angle, angleC(O) ... D-W1- O-W (O-W : water oxygen atom), were determined to be 2.63( 1) Angstrom and 120( 1)degrees, respectively. The coordination number, n(C-O ... D-W1), was obtained to be 4.0(1). These results are consistent with the hydration structure in which water molecules in the first hydration shell of the carboxyl group are hydrogen-bonded with oxygen atoms of the carboxyl group.