ELECTRON CORRELATION-EFFECTS ON THE THEORETICAL CALCULATION OF NUCLEAR-MAGNETIC-RESONANCE SPIN-SPIN COUPLING-CONSTANTS

The equation-of-motion coupled cluster singles and doubles (EOM-CCSD) method for general second-order properties is derived providing a quad ratic, CI-like approximation and its linked form from coupled cluster (CC) energy derivative theory. The effects of the quadratic contributi on, of the atomic basis set employed, and of electron correlation on N MR spin-spin coupling constant calculations using EOM-CCSD methods are investigated for a selected set of difficult molecules, notably CH3F, B2H6, CH3CN, C2H4, and CH3NH2. We find that the quadratic contributio n is insignificant for the couplings in the molecules considered in th is study and in addition the quadratic contribution only slightly depe nds on the basis set used. Therefore it seems well justified to use th e less expensive CI-like approximation or its linked-diagram form to e valuate spin-spin coupling constants. The Fermi-contact contribution s hows the largest variation with the change of basis sets. The diamagne tic spin-orbit (DSO) and the spin-dipole (SD) contribution vary little , seemingly being converged at the DZP level while the paramagnetic sp in-orbit (PSO) term shows moderate variations. Except for very few cas es, the FC contribution is dominant in all the couplings in the select ed set of molecules and it is also most sensitive to the inclusion of electron correlation. The other contributions are less affected by ele ctron correlation. Although of lesser importance, the significance of the noncontact contributions and electron correlation effects on accur ate calculation of coupling constants such as (1)J((CF)-C-13-F-19) in CH3F and (1)J((CN)-C-13-N-15) in CH3CN is clearly demonstrated. (C) 19 96 American Institute of Physics.