Isotropic shielding constants

Isotropic shielding constants for the heavy elements are listed in Table 1 and show a consistent picture. In aU cases we find that relativity reinforces the shielding. 

Table 1. Isotropic shielding constants (ppm) for F, I and Xe calculated with different response formalisms (see text)...

Table VII. Prediction of 170 NMR chemical shifts assuming AE = T for the decavanate ion [V10O28]6 Experimental values and attributions are from ref (74). Atomic parametrization was r t(V) = 135 pm and ex°(V) = 3.633 eV leading to e2t = 3.9 eV. The computed diamagnetic isotropic shielding constant was found to be ad = 418.1 0.1 ppm for all oxygen sites.

The literature describing theoretical studies on monocyclic thietanes and thietes until 1995 has been fully covered in CHEC-II(1996). However, the described papers were confined to semi-empirical or molecular orbital(MO) calculations. During the last decade, computational abilities have increased dramatically, and several papers have appeared dealing with ab initio calculations of thietane structures. Extensive ab initio calculations have been carried out in order to establish the isotropic shielding constant a and chemical shifts 8 of 14 variously substituted thietanes 1 <2000MRC468>. These values have been computed using the HF/6-31++G and geometries for this purpose... 

Table 3.1 The statistically averaged 15N NMR isotropic shielding constants of pyridine in aqueous solution, aN, (in ppm) and corresponding solvent shifts, AaN. Experimental data is taken from Ref. [46]. Results from Ref. [44]...

Table 6 Isotropic shielding constant (in ppm) for X being oxygen or carbon and the shift compared to the gas-phase value. QM denotes the system that is treated quantum mechanically, and the water part describes the approach for treating the solvent. For the latter, PCMl and PCM2 denotes polarizable continuum models and TIP3P and SPC two different force helds. - marks gas-phase calculations. All results are from ref. 33... 

Table 7 Lowest electronic excitation energy (in eV) and isotropic shielding constant (in ppm) <7. n gives the number of water molecules that are explicitly treated quantum theoretically. PCM denotes the standard polarized continuum method, whereas in PCM the radii spheres surrounding each atom in the construction of the cavity have been scaled by 91.7%. Finally, MM marks results obtained with an explicit description of the solvent. All results are from ref 35... 

Table 8 Change in lowest electronic excitation energy (in eV) and in isotropic shielding constant (in ppm) due to solvation for the molecules of Fig. 1. PCM denotes results with the...

Absolute isotropic shielding constants (a, ppm) computed at various theoretical levels for C, N and O. Density functional values are computed using the 6-311+G(2d,p) basis set and 6-31 lG(d,p) geometries. Nuclei are labelled from left to right. 

In Table 11 we present their calculated isotropic shielding constants for different nuclei of various systems using different computational schemes. It is obvious from the Table that the agreement between the calculated and the measured values is far from perfect, independent of the calculational method. It shall, however, be stressed that, being the second-order derivatives of the total energy, these quantities are very sensitive to any numerical inaccuracies. 

Tests regarding the ability of current computational approaches to reproduce NMR properties were provided by Bjomsson et al. [745, 746]. They predicted a small gas-to-liquid shift for the isotropic shielding constants. This again indicates that environmental effects on NMR shifts are well captured by cluster approaches. Gester et al. [747] investigated the gas-to-liquid shift for liquid ammonia. 

Table 5 Calculiited Isotropic Shielding Constants a, in ppm) of Small Molecules...

Calculated isotropic shielding constants of C, N, O, and F nuclei for a set of small molecules are summarized in Table 5. DFT predicted shielding constants compare reasonably well with experimental and MP2 results. The deviations are larger in molecules (Nz and CO) that have unsaturated bonds, where shielding constants are either very small, or are negative. Although the DFT results are not as accurate as those obtained from MP2, DFT approaches represent a reasonable trade-off between accuracy and cost, as DFT is more cost-effective than MP2. 

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