RELATIVE IMPORTANCE OF FERMI CONTACT, ORBITAL DIPOLE AND SPIN DIPOLE TERMS FOR P-C COUPLINGS IN PHOSPHA-AROMATIC COMPOUNDS

The three contributions (Fermi-contact, orbital-dipole, and spin-dipole) to the P-C couplings of phosphorin and two of its methylated derivatives, benzo(b)phosphorin and benzo(c)phosphorin, are examined by SCF-INDO-FPT calculations. An important overall result is that the orbital-dipole and spin-dipole terms are quite important and make decisive contributions compared to the Fermi-contact term. In particular, the orbital term provides the principal contribution to 1J (P-C), white 2J and 3J (P-C) are controlled by the spin-dipole interactions and so behave also the long-range couplings over more than three bonds. Furthermore, theory predicts 1J, 2J (P-C, syn) and 4J negative while 2J (P-C, anti), 3J and 5J positive. The absolute values and internal trend of couplings in phosphorin are accounted for correctly and there is some evidence of the correctness of the predicted signs. Analogies and difference between N-C and P-C couplings of aza- and phospha-aromatic congener systems are examined. © 1979.