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Articles you may be interested in NMR shielding tensors for density fitted local second-order Møller-Plesset perturbation theory using gauge including atomic orbitals A converse approach to the calculation of NMR shielding tensors Analyzing NMR shielding tensors calculated with two-component relativistic methods using spin-free localized molecular orbitals Natural chemical shielding analysis of nuclear magnetic resonance shielding tensors from gauge-including atomic orbital calculations Erratum: Localized orbital/local origin method for calculation and analysis of NMR shieldings. Applications to 1 3C shielding tensors [J.A theory ofNMR shielding tensors is derived from Ramsey's expressions, using the framework of the random phase approximation (RP A) and localized molecular orbitals. By expanding angular momentum terms relative to a local origin for each orbital and using properties of the RP A solutions, we arrive at shielding expressions that contain no reference to an overall gauge origin and that lead to appropriate damping of basis set errors in contributions from distant groups. The expressions allow an analysis of the shielding into intrinsic bond and bond-bond coupling contributions. The resulting method is a variant ofthe coupled-Hartree-Fock approach. Ab initio results are presented for 13C isotropic shieldings and shielding tensors in a number of organic molecules ranging in size up to benzene. These results agree very well with experiment, for both isotropic shieldings and principal tensor components, even in double-zeta basis sets. For some low symmetry molecules we study the asymmetry of the shielding tensor and show that the antisymmetry can be quite large even if the anisotropy is moderate. The bond analyses shed light on the effects of molecular structural features on the shiel dings and their anisotropies, and emphasize in particular the conformational effects in bond-bond coupling contributions.

In a previous study (Chemical Physics Letters 2005, 401 , 385) we computed the optical rotatory dispersion of (S)-propylene oxide in gas phase and solution using the hierarchy of coupled cluster models CCS, CC2, CCSD, and CC3. Even for the highly correlated CC3 model combined with a flexible basis set, the theoretical gas-phase specific rotation at 355 nm was found to be negative in contrast to the experimental result. We argued that vibrational contributions could be crucial for obtaining a complete understanding of the experimental result. Here, we show that this indeed is the case by using coupled cluster models and density functional theory methods to calculate the vibrational contributions to the gas-phase specific rotation at 355, 589.3, and 633 nm. While density functional theory (B3LYP and SAOP functionals) overestimates the specific rotation at 355 nm by approximately 1 order of magnitude and yields an incorrect sign at 589.3 and 633 nm, the coupled cluster results are in excellent agreement with the experimentally measured optical rotations. We find that all vibrational modes contribute significantly to the optical rotation and that temperature effects must be taken into account.

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The components of the full nuclear shielding tensor are analyzed with particular regard to the origins of the antisymmetric component of the shielding of nuclei at low-symmetry molecular sites. As an aid we propose the study and display of the shielding response vector, i.e., the nuclear shielding field per unit applied magnetic field. The analysis is based on ab initio calculations in the localized orbital-local origin method for cyclopropane, cyclopropene, ethylene oxide, ethylene imine, and diazirine, and also includes a discussion of the mechanism for the large antisymmetric component predicted for the unsaturated nuclei in cyclopropene and diazirine, both in terms of bond and lone pair contributions and in terms of a decomposition of the dominant paramagnetic contributions into molecular point group species. Display of the component of the shielding vector parallel to the applied field is shown to be a valuable alternative to the common ellipsoid representations. Display of the components perpendicular to the applied field provides a picture of the relation of the antisymmetry to molecular structure and suggests its direct observation through monitoring the perpendicular response. Finally, we show that, contrary to a recent claim, the eigensolutions of the full tensor in the presence of antisymmetry play no role in the observed anisotropies.

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