14N nuclear quadrupole resonance of the molecular complexes of urea.

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The 14N nuclear quadrupole resonance (NQR) was studied in four kinds of molecular complexes of urea, such as urea-nitric acid, urea-phosphoric acid, urea-oxalic acid, and urea-ammonium chloride. The 14N quadrupole coupling constants and asymmetry parameters at liquid nitrogen temperature are as follows: |e2Qq⁄h|=3.157 MHz and η=0.475 for the HNO3 complex; |e2Qq⁄h|=3.277 MHz and η=0.414 for the H3PO4 complex; |e2Qq⁄h|=3.456 MHz and η=0.363 for the (COOH)2 complex; and |e2Qq⁄h|=3.544 MHz and η=0.298 for the NH4Cl complex. From the temperature dependence of the resonance lines in these complexes, the strength of hydrogen bonds are discussed. The differences of the electron densities at the nitrogen atoms between urea and its complexes were derived from the corresponding NQR parameters, and the bond characters in these complexes are discussed.
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