Correlated second-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 electronic con elation and Fermi contact operators as perturbation terms, has been performed for the evaluation of the indirect couplings of protons and carbon-13 nuclei of even- and odd-membered polymethine chains (conjugated polyenes and cyanines), in the frame of the CS-INDO molecular orbital method. The effect of electronic correlation on the values computed 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 leading to negative coupling constants, but the nucleus s-density parameter s(N)(2)(0) of hydrogen or carbon- 13 cannot be reduced to a unique value, valid everywhere. (C) 1998 Published by Elsevier Science B.V. All rights reserved.