Nuclear quadrupole interactions in cadmium complexes: Semiempirical and abinitio calculations

Semiempirical calculations, based on the so-called angular overlap model, h ave been compared with ab initio methods (MP2) for the calculation of nucle ar quadrupole interactions (NQI's) in cadmium complexes with biologically r elevant ligands (H2O, OH-, cysteinate, carboxylate, and imidazole). The ass umptions on which the semiempirical model is based have been tested and the comparison indicates that: 1) A change in the Cd-ligand bond length by 0.1 Angstrom may change the electric field gradient (EFG) by about 0.2 a.u.. A simple scheme to incorporate such effects in the semiempirical method is s uggested. 2) The effect of ligand-ligand interactions is up to about 0.2 a. u. for the largest diagonal: element of the EFG tensor for the tested compl exes, and such effects can significantly influence the so-called asymmetry parameter. 3) The position of non-coordinating atoms on the ligands can in some cases (e.g. the hydrogen atoms of water) significantly influence the E FG. The combined effect of non-coordinating atoms and ligand-ligand interac tions may cause deviations of up to 0.35 a.u. between ab initio and the sem iempirical calculations. 4) In the semiempirical model each ligand is chara cterised by one parameter, the so-called partial nuclear quadrupole interac tion. This parameter has been evaluated by ab initio calculations, and agre ement was found within about 0.2 a.u. (approximate to 40 Mrad/s) for all li gands except imidazole. 5) A change in the coordination number from 2 to 6 may change the partial NQI by about 0.3 a.u.