Covalent-Bond Asymmetries from Zeeman-Split NQR

An experimental technique is developed for the measurement of asymmetry parameters in electric quadrupole coupling of nuclei with spin $I\frac{3}{2}$ in covalent bonds in organic single crystals. The theoretical details of Zeeman‐split nuclear quadrupole resonance spectra, upon which the technique is based, are examined and found to yield a particularly simple approximate expression for the asymmetry parameter of the electric field gradient tensor, which has been previously shown to be proportional to the double‐bond character of the covalent bond. The technique is applicable to atoms bound to a single neighbor in the organic molecule. The limitations of the approximations involved in obtaining the simple formula are examined in detail, and it is found that an order‐of‐magnitude smaller probable error may be obtained than has been possible with previous geometrical techniques. Correction formulas are presented which may yield still another order‐of‐magnitude better precision if desired. Experimental results on the Cl³⁵ resonances in para‐dichlorobenzene, para‐chloroaniline, and 1245 tetrachlorobenzene demonstrate the method, along with verifying the second‐ and third‐order perturbation theoretic results which derive the maximum magnetic field—about 2400 Oe—in which the simple formulas may be used. Other experimental measurements of the asymmetry parameters of C–Cl bonds are tabulated.