Ab initio shielding calculations

Recently, ab initio shielding calculations based on well-established theories, IGLO (individual gauge for localized orbitals)9, GIAO (gauge including atomic orbital)10 and LORG... 

By applying polarization functions, ab initio shielding calculations for some polyenals and their Schiff bases reproduce the experimental values well even on the carbonyl and the imine carbons using the LORG theory without including correlation effects. In addition, there is a trend that the calculation with polarization functions yields smaller anisotropies of chemical shieldings than those without polarization functions. 

To describe the numerical results of the ab initio shielding calculations, a choice of functional form is required to represent the shielding surface in analytic form. For the rare gas pair, it was found that the inverse R dependence of a (R) has a behavior c lose to R 6 7, R 6 8, and R 7 4. Therefore, a function of the form... 

Mauri et al have provided the only ab initio shielding calculation method for extended periodic networks, which they have applied to crystals and also to small molecules containing atoms in the first row of the Periodic Table. In an effort to extend the ab initio calculations of shielding in extended networks using periodic boundary conditions to involve other than light atoms, Mauri et... 

An additional source of chemical shielding anisotropies is that of ab initio theoretical calculations.20 25 There has been considerable progress in this area of molecular quantum mechanics, particularly with the use of gauge-invariant atomic orbitals within the framework of self-consistent-field (SCF) perturbation theory.26 In many cases the theoretical quantities have been extremely accurate and have served not only as a corroboration of experimental quantities but also as a reliable source of new data for molecules of second-row atoms (i.e., Li through F). 

Because of the proton s chemical importance and its favorable characteristics for NMR detection, the overwhelming bulk of experimental investigation and correlation of chemical shifts has been centered on the proton. However, the situation regarding the determination of proton shielding anisotropies has been unsatisfactory in many respects. Similarly, the field of ab initio theoretical calculations of proton shielding tensors has, until very recently, enjoyed little success. Both these factors are related to the fact that the proton chemical shifts are comparatively small and hence influenced strongly by secondary factors such as neighboring-atom electron distribution and medium effects. 

Recent ab initio type calculations of nuclear shielding of carbon, nitrogen and oxygen in small molecules by Ditchfield et al, [171], essentially show that the diamagnetic contribution to the total shielding for second row nuclei does not vary appreciably from one molecule to another. 

The thermotropic liquid crystal, 4,4 -diheptylazoxybenzene (HAS), exhibiting isotropic, nematic and smectic phases, has been studied through e NMR. The temperature dependence of e chemical shifts and spin-lattice relaxation times of the Xe gas dissolved in HAS showed clear signatures of the phase transitions. Theoretical models have been used to understand the influence of the different phases on the isotropic and anisotropic parts of the chemical shielding. From the studies it is also inferred that in the smectic phase, Xe atoms preferentially occupy interlayer spacings rather than the interiors. Bent-core or banana-shaped molecules display an array of novel chiral liquid crystalline phases. NMR studies on two of the banana core moieties have been analyzed using ab initio structure calculations and the steric inertial frame model. ... 

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