CORRELATED 2ND-ORDER PERTURBATION STUDIES OF SPIN-SPIN NUCLEAR COUPLING-CONSTANTS FROM CS-INDO MOLECULAR-ORBITALS - POLYMETHINE COMPOUNDS

A second-order double perturbation treatment, using both the electroni c con elation and Fermi contact operators as perturbation terms, has b een performed for the evaluation of the indirect couplings of protons and carbon-13 nuclei of even- and odd-membered polymethine chains (con jugated polyenes and cyanines), in the frame of the CS-INDO molecular orbital method. The effect of electronic correlation on the values com puted for the contact terms produces a significant increase of the (1) J and (3)J couplings and a decrease of the (2)J couplings, sometimes l eading to negative coupling constants, but the nucleus s-density param eter s(N)(2)(0) of hydrogen or carbon- 13 cannot be reduced to a uniqu e value, valid everywhere. (C) 1998 Published by Elsevier Science B.V. All rights reserved.