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

Abstract

The 31P and 77Se magic angle spinning (MAS) nuclear magnetic resonance (NMR) 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 values 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-splitting-ratio method of Eichele and Wasylischen. For the C(S) molecule 1, delta J was obtained to be +640 +/- 260 Hz from the 31P spectrum and +550 +/- 140 Hz from the 77Se spectrum. Density functional theory (DFT) calculations give a delta J value of about +705 Hz. The value of delta J could not be determined unambiguously by analysis of the 31P spectra for the C1 molecules 2; nevertheless, an estimation of delta J was possible. The principal axis 3 of the phosphorus shielding tensor was determined to be nearly parallel to the PSe bond in 1 and 2. For the selenium shielding of 1, the same orientation was found, whereas in 2, the principal axis 2 of the selenium shielding was found to be oriented nearly along the PSe bond. The experimentally determined phosphorus nuclear magnetic shielding tensors agree well with those calculated by the IGLO method. For those two principal values of the selenium-shielding tensors corresponding to directions nearly perpendicular to the SeP bond, the agreement between calculated and experimental values is satisfactory. For the third one, corresponding to the principal axis close to the SeP bond, the calculated deshielding contributions are distinctly too small for both compounds investigated. Trends observed for the calculated molecular orbital (MO) contributions to the shielding as well as possible reasons for the underestimation of the deshielding contributions along the SeP bond are discussed.