Anisotropy of chemical shift and J coupling for P-31 and Se-77 in trimethyl and triphenyl phosphine selenides

The P-31 and Se-77 magic angle spinning (MAS) nuclear magnetic resonance (N MR) experiments for selenium-77 enriched (70%) trimethylphosphine selenide 1 and triphenylphosphine selenide 2 were carried out in order to determine the nuclear magnetic shielding tensors of both nuclei and to establish valu es of the phosphorus-selenium indirect spin-spin coupling anisotropy Delta J. The m = +1/2 and m = - 1/2 subspectra were analysed by the dipolar-split ting-ratio method of Eichele and Wasylischen. For the C-s molecule 1, Delta J was obtained to be +640 +/- 260 Hz from the P-31 spectrum and + 550 +/- 140 Hz from the Se-77 spectrum. Density functional theory (DFT) calculation s give a Delta J value of about + 705 Hz. The value of Delta J could not be determined unambiguously by analysis of the P-31 spectra for the C-1 molec ules 2; nevertheless, an estimation of Delta J was possible. The principal axis 3 of the phosphorus shielding tensor was determined to be nearly paral lel to the PSe bond in 1 and 2. For the selenium shielding of 1, the same o rientation was found, whereas in 2, the principal axis 2 of the selenium sh ielding was found to be oriented nearly along the PSe bond. The experimenta lly determined phosphorus nuclear magnetic shielding tensors agree well wit h those calculated by the IGLO method. For those two principal values of th e selenium-shielding tensors corresponding to directions nearly perpendicul ar to the SeP bend, the agreement between calculated and experimental value s is satisfactory. For the third one, corresponding to the principal axis c lose to the SeP bond, the calculated deshielding contributions are distinct ly too small for both compounds investigated. Trends observed for the calcu lated molecular orbital (MO) contributions to the shielding as well as poss ible reasons for the underestimation of the deshielding contributions along the SeP bond are discussed. (C) 1998 Elsevier Science B.V. All rights rese rved.