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Neutron diffraction measurements with 14N/15N and H/D isotopic substitution methods have been carried out on aqueous 15 mol% urea solutions, in order to obtain information on the hydration structure of the amino group within the urea molecule in concentrated aqueous solutions. The observed first-order difference function, 0HΔN(Q), and the N–H partial structure factor, aNH(Q), were analyzed by the least squares fitting procedure. The nearest neighbor N···O and N···H distances were obtained to be 2.92(1) Å and 3.42(1) Å, respectively. The average hydration number per one amino group was determined to be 2.0(1). The tilt angle between the N–H···O axis and the molecular plane of the hydrogen bonded water molecule is estimated to be 51(5)°.

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