Torsional Angular Dependence of (1)J(Se,Se) and Fermi Contact Control of (4)J(Se,Se): Analysis of (n
May 1st, 2008 | by admin |Torsional Angular Dependence of (1)J(Se,Se) and Fermi Contact Control of (4)J(Se,Se): Analysis of (n)J(Se,Se) (n=1-4) Based on Molecular Orbital Theory.
(n)J(Se,Se) (n=1-4) nuclear couplings between Se atoms were analyzed by using molecular orbital (MO) theory as the first step to investigating the nature of bonded and nonbonded (n)J(Se,Se) interactions between Se atoms. The values were calculated by employing Slater-type triple xi basis sets at the DFT level, which were applied to structures optimized with the Gaussian 03 program. The contribution from each occupied MO (psi(i)) and psi(i)–>psi(a) (psi(a)=unoccupied MO) transition was evaluated separately. (1)J(Se,Se) was calculated for the MeSeSeMe model compound, which showed a typical dependence on the torsion angle (phi(C(Me)SeSeC(Me))). This dependence explains the small values (</=64 Hz) of (1)J(obsd)(Se,Se) observed for RSeSeR\’ and large values (330-380 Hz) of (1)J(obsd)(Se,Se) observed for 4-substituted naphtho[1,8-c,d]-1,2-diselenoles, which correspond to synperiplanar diselenides. The HOMO–>LUMO and HOMO-1–>LUMO transitions contribute the most to (1)J(Se,Se) at phi=0 and 180 degrees to give large values of (1)J(Se,Se), whereas various transitions contribute and cancel each other out at phi=90 degrees to give small values of (1)J(Se,Se). Large (4)J(obsd)(Se,Se) values were also observed in the nonbonded SeSe, SeSe==O, and O==SeSe==O interactions at naphthalene 1,8-positions. The Fermi contact (FC) term contributes significantly to (4)J(Se,Se), whereas the paramagnetic spin-orbit (PSO) term contributes significantly to (1)J(Se,Se). (2)J(Se,Se) and (3)J(Se,Se) were analyzed in a similar manner and a torsional angular dependence was confirmed for (3)J(Se,Se). Depending on the structure, the main contribution to (n)J(Se,Se) (n=2, 3) is from the FC term, with a lesser contribution from the PSO term. Analysis of each transition enabled us to identify and clearly visualize the origin and mechanism of the couplings.
Nakanishi W, Hayashi S.
Department of Material Science and Chemistry, Faculty of Systems Engineering, Wakayama University, 930 Sakaedani, Wakayama 640-8510, Japan, Fax: (+81) 73-457-8253.