Nitrogen quadrupole coupling in paraelectric ammonium sulfate

The quadrupole coupling parameters for ^pre14N in the two chemically inequivalent sites in paraelectric ammonium sulfate have been measured by proton-nitrogen double resonance for the interval 225 ≤T≤365 K. Transition frequencies are represented to within ca. 0.2 kHz in this interval by v₊ (I) = 242.43 - 0.3380T; v₋(I) = 144.03 - 0.1855 T; v(II)=147.08 - 0.1294 T; v₋ (II) = 73.39 - 0.0281 T kHz, where T is the temperature in degrees Kelvin. These data are discussed in the context of neutron diffraction thermal parameters and an STO-3G calculation of the field gradients at the two sites. Although the temperature dependence is qualitatively in agreement with the structure data, the discrepancy between the calculated and observed asymmetry parameters, particularly for site I, suggests that hydrogen bonding to sulfate is an important feature in determining the ammonium ion charge distribution and hence the field gradient tensor at nitrogen. At 296.1 K the coupling parameters are e²gQ/h (I) = 154.53 kHz, η(I) = 0.684; e²gQ/h (II) = 115.71 kHz, η1(II) = 0.749.