Fig. 15.2.  Mossbauer spectra of Fe(CN)₆^{4 -}, (a), Fe[(CN)₅NO]^2-,   (b) and FeSO₄ (s) (c).  For quadrupole moment to influence ΔE, there has to be an electric field gradient at the nucleus which is present in (b) and (c) above because they possess sufficient asymmetry to possess electric field gradients at the nucleus. Spherical symmetry or perfect octahedral symmetry does not produce electric field gradients.