THEORETICAL STUDY ON ISOTROPIC HYPERFINE COUPLING CONSTANTS OF SECOND-ROW ELEMENTS IN PI-ELECTRON RADICALS

The Isotropie electron-nuclear spin interactions of the second-row elements in the Periodic Table are interpreted semiquantitatively on the basis of the molecular orbital theory. For an atom Y that is bonded to m atoms, X i(i= ,2,⋯,m), the hyperfine constant, OY, has the form aY=OTYY0+ ∑ mQxiYpii0+∑ i-1m RYXiYpyi0 where pYY0, pii0, and p Yi0 are elements of the π-electron spin density matrix, and QBA and RABA are the σ-π parameters for the nucleus A and resulting from the π AO spin density matrix elements, pBB0 and pAB 0. The spin polarization through the σ-π interaction is shown to be divided into three terms according to their mechanisms; (i) the spin polarization in the AB σ bond, which induces opposite spin densities on atomic orbitals of atoms A and B, (ii) the spin polarization which appears as the difference in the hybrid ratios of the hybrid σ-atomic orbitals for different spins, and (iii) the spin polarization of the inner-shell electrons. An empirical analysis correlating the observed hyperfine coupling constants with the calculated π-atomic-orbital spin density matrix elements by the above equation gives the following results: for a coplanar CHC2 fragment QCC, QC′C, and R CC′C are found to be 46.0±0.1, -17.3±0.1, and - 1.95±0.25 G, respectively; for a NC2 fragment QNN=29.0±1.6, QC N=-4.3±0.8, and RNCN=0.9±1.5 G and for 17O in a carbonyl fragment QOO=-22. 0±3.0, QCO75.4±7.6, and ROC O±9.6 G.