The calculation of C-13 chemical shielding tensors in ionic compounds utilizing point charge arrays obtained from Ewald lattice sums

An embedded ion method is proposed for accurately calculating the C-13 chem ical shift tensors in ionic compounds. The method models an ionic crystal b y embedding an ion of interest inside an array of point charges. The potent ial, produced by an infinite ionic lattice, at the location of the ion of i nterest can be simulated accurately utilizing a point charge array obtained by the Ewald summation method. The Ewald summation method, as implemented in the computer program EWALD, in conjunction with the quantum-mechanics pr ogram GAUSSIAN 98 is used to generate a self-consistent point charge array that simulates the Ewald potential in a defined region at the center of the array. Subsequently, the chemical shift tensor calculation is performed us ing GAUSSIAN 98 on the ion of interest positioned in the region inside the point charge array in which the Ewald potential is established. The embedde d ion method was tested on potassium methyl-trithiocarbonate (KS2CSCH3) who se crystal lattice is composed of potassium cations and molecular S2CSCH3- anions. The principal values of the C-13 chemical shift tensors in KS2CSCH3 were measured in a stationary cross polarization nuclear magnetic resonanc e experiment. It is shown that the correlation between experimental and cal culated principal values improves significantly when the C-H bond distances are optimized from their x-ray values. It is further demonstrated that a s ubstantial improvement in the correlation is obtained when the chemical shi elding tensor calculation is performed on an S2CSCH3- anion embedded inside a point charge array obtained by the Ewald summation method. The embedded ion method was completed applying the B3P86/cc-pVTZ, B3LYP/cc-pVTZ, and MP2 /cc-pVDZ quantum-mechanical computations and the various results are compar ed and analyzed. (C) 2001 American Institute of Physics.