Low-temperature carbon-13 magnetic resonance of solids. 1. Alkenes and cycloalkenes

Abstract

The 13C shift tensors for a large variety of olefins including some of the cyclic variety are reported at 20 K. On the basis of existing studies and symmetry arguments the orientation of the tensor elements can in general be assigned to specific molecular axes for the olefmic carbons. With this information conclusions are then drawn, relating trends in the various tensor elements to geometric and electronic effects. The method shows great promise in elucidation of the geometrical origins of chemical shifts. Often the range of shifts encompassed by any individual tensor element is 3 times greater than that of the isotropic shifts in the series. Also described is the apparatus for obtaining 13C cross-polarization spectra at cryogenic temperatures.

Singularities of magnetic‐field induced electron current density: A study of the ethylene molecule

“…Also, the theoretical average carbon shielding matches favorably with the experimental data in the same table. A fairly good qualitative agreement with Zilm et al [31], who reported the experimental carbon chemical shift tensor, has been found.…”

Understanding chemical shielding tensors using group theory, MO analysis, and modern density‐functional theory

“…ADF uses MOs that are generated from linear combinations of fragment orbitals (FOs) (108) to parameterize the electron density, according to Kohn and Sham (109), which is then used to calculate the total electronic energy. Using the calculated energy, the magnetic shielding is determined as described in Appendix B, and the total paramagnetic shielding contribution, s :: p , is arbitrarily decomposed into The contributions to paramagnetic shielding have been analysed according to pair-wise MO mixing several times before, although the analysis was often performed using approximate or empirical methods (110,111), qualitative discussions (40,(112)(113)(114)(115), or schematised representations of the MOs (116)(117)(118). More thorough treatments have tended to account for observed isotropic chemical shift values (39,119); however, relatively few (compared with the total number of accounts of observed/calculated CS) can be found where this discussion is extended to include anisotropic CS (38,41,43,48,120,121).…”

31P solid state nuclear magnetic resonance principal values of the chemical shift tensors of phosphane oxides, phosphane sulphides and phosphane selenides

“…techniques allow the determination of the chemical shift tensor components which provide a much richer source of information than the isotropic shielding values. Studies must be made in two directions, firstly on simple model compounds [33] and secondly, on families of compounds in which the geometry around the nucleus under study and the nature of the substituents are changed. The interpretation of the data which is directly related to the electronic structure and to the geometry remains a difficult problem.…”