Nuclear Quadrupole Resonance and Bonding in Crystalline Ammonia

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Abstract
Nuclear quadrupole coupling constants (eQq) of 14N have been determined as a function of temperature for each of the isotopic species NH3, NH2D, NHD2, and ND3 in the crystalline state. The temperature dependences are explained, following a theory proposed by Bayer, in terms of an averaging of the electric field gradient (q) by torsional lattice motions. Frequencies and moments of inertia known from other spectroscopic studies of NH3 and ND3 were employed in the calculations, which also led to values for the quadrupole coupling constants without zero‐point vibrations. These values turned out to be the same (−3.47 Mc/sec) within 1% for NH3 and ND3, showing that the different field gradients for the various isotopic species in the crystalline solid are well explained in terms of lattice motions. Comparison of the vibrationless, or ``quasistatic'' values, with the coupling constants of the gaseous molecules (−4.08 Mc/sec) then gave the shift produced by the crystalline field, here called the quasistatic shift. This is large relative to the changes produced by isotopic substitution in the gaseous molecule. Calculations of the electric field and the field gradient produced at a given 14N nucleus by surrounding molecules in the known crystal structure were made to explain the quasistatic shift. Molecules containing the nearest hydrogens were treated as a distribution of four point charges, and the others, to a total of about 100, were taken as point dipoles. Both models were consistent with a crystalline dipole moment evaluated from the gaseous value and the known molecular polarizability, taking into account the reaction field of the lattice. The quasistatic shift was orders of magnitude too large to be accounted for by the direct field gradient of surrounding neighbors; it is better described in terms of a redistribution of electrons in the nitrogen p orbitals by polarization of the molecule in the crystal field.