Nuclear Quadrupole Resonance (NQR):

            Atoms which have nuclear spins greater than ½ behave as if the distribution of charge within the nucleus is non-spherical. The nucleus does not behave as a dipole because the nuclear charge distribution remains centrosymmetric. It does, however, possess an electrical quadrupole moment (C0₂, which is centrosymmetric and linear, is an example of a molecule which has an electric quadrupole moment).

                      

   

        

           

           

Species	Ionic character of the M-Cl bond %
[PtCI6]2-	44
[PdC16] 2-	43
[lrCI6] 2-	47
[0sCI6] 2-	46
[ReCl6] 2-	45
[WCl6] 2-	43
[SnCI6] 2-	66
[TeCl6] 2-	68
[SeCI6] 2-	59

             In an applied non-uniform electrostatic field, the non-uniformly charged nucleus can  take up at least two  orientations,  one  of which  is  more stable  than the  others  (the  number  of orientations depends on the  magnitude of the  nuclear quadrupole moment). It is possible to excite the nucleus from a lower to an upper state by application of suitable radio-frequency radiation. This is a classical description of the phenomenon, but the essentials are carried over into a quantum mechanical treatment.  In practice, the non-uniform electrostatic field is generated by the charge distribution around, but very close to, the nucleus. Clearly, this is  a  phenomenon  which  is only applicable to atoms in a low-symmetry environment (but, note carefully, this does not  automatically  mean  a low-symmetry  complex).  The most-studied of the nuclei which exhibit quadrupole resonance spectra are ³⁵Cl and ³⁷Cl. The method is inherently insensitive and the high concentrations of these isotopes in samples such as solid K₂[PtC₁₆] is  a  great advantage. Only solids can be studied, anyhow, because the molecular tumbling in a liquid or gas averages the effect to zero. The sensitivity of the method has increased in recent years by the advent of pulse (such as those used in NMR) and also, in suitable cases, by not looking at the NQR nucleus itself but, rather, at one which is energetically coupled to it and which is NMR-active.

             

          It is likely that because of these developments the future will see a wider use of NQR spectroscopy. It is also likely that the interpretation of the data will become more sophisticated.  Traditionally, the experimental data have been interpreted to give the percentage ionic character of a bond.  This is because, for example, in the Cl^- ion all of the p orbitals are equally occupied whilst in Cl₂ the σ bond, if composed of p orbitals only, corresponds to one electron in the p_σ orbital of each chlorine atom, and so Cl^- and Cl₂ differ in their resonant frequencies. Interpolation allows a value for the ionic character of a Cl-M bond to be determined from the chlorine resonance frequencies in Cl^- and Cl₂. Some correction may be applied to allow for the fact that a pure chlorine p orbital may not be involved in the M-Cl bond. When there are two non-equivalent NQR nuclei in the unit cell of a solid these give rise to separate resonances which may be resolvable.  In this way NQR spectroscopy gives structural information. Both bromine, ⁷⁹Br and ⁸¹Br, and iodine, ¹²⁷I, but not fluorine, give NQR spectra, as too may ¹⁴N, ⁵⁵Mn, ⁵⁹Co, ⁶³Cu, ⁶⁵Cu, ⁷⁵As, ¹²¹Sb, ¹²³Sb, ²⁰¹ Hg, and ^2o9Bi.