Chemical shift tensors calculations

Table n. Principal values of the chemical shift tensor calculated with uncorrected and corrected hybrids ... 

This paper presents quantum mechanical studies of the, 5N and, 3C chemical shifts in both the N7-H and N9-H tautomeric forms of purine. Quantum mechanical calculations of the chemical shifts were used to assign the NMR resonances and the spatial orientation of the principal axes of the chemical shift tensors. Calculations in purine and in a pyridine-methanol complex model provide insights on the importance of the intermolecular interactions on the chemical shifts of the nucleic acid bases. 

Tab. 6.2 Calculated principle values nucleic acid bases and base pairs and orientations of chemical shift tensors of the imino nitrogens in...

Tab. 6.3 Calculated carbon chemical shift tensors in nucleosides.

The DD-CSA cross-correlated relaxation, namely that between 13C-1H dipole and 31P-CSA, can also be used to determine backbone a and C angles in RNA [65]. The experiment requires oligonucleotides that are 13C-labeled in the sugar moiety. First, 1H-coupled, / - DQ//Q-II CP spectra are measured. DQ and ZQ spectra are obtained by linear combinations of four subspectra recorded for each q-increment. Then, the cross-relaxation rates are calculated from the peak intensity ratios of the doublets in the DQ and ZQ spectra. The observed cross-correlation rates depend on the relative orientations of CH dipoles with respect to the components of the 31P chemical shift tensor. As the components of the 31P chemical shift tensor in RNA are not known, the barium salt of diethyl phosphate was used as a model compound with the principal components values of -76 ppm, -16 ppm and 103 ppm, respectively [106]. Since the measured cross-correlation rates are a function of the angles / and e as well, these angles need to be determined independently using 3/(H, P) and 3/(C, P) coupling constants. 

NMR Chemical Shift Tensor Components eor Mes3Si /HCBiiMe5Br6 and Calculated Tensor Components eor MessSi", 1. " The Orientation oe the Eigenvectors Relative to the Molecular Frame is Given in Fig. 1... 

The electronic stractures of 2-4 were probed by their He(I) and He(II) photoelectron spectra. Calculations reported on isodesmic hydrogenation reactions of model compounds were consistent with significant jt-delocalisation in 2 and 3 support for that view came inter alia from chemical shift tensors for 2—4, determined from Si H CPM AS-NMR spectra. 

Fig. 3. Chemical shift tensor orientations for the isopropyl cation, propionylium ion, benzoylium ion, and benzenium ion obtained from MP2 chemical shift calculations.

We calculated the chemical shift tensor of the isolated cation using the GIAO-MP2 implementation of Gauss (65), with tzp (C) and dz (H) basis... 

Figure 56)155. Compounds investigated included three forms of tetramesityldisilene the solvent-free form 69156, the toluene adduct 69-C7Hs157 and the tetrahydrofuran solvate 69 THF158. Also studied were a second tetraaryldisilene (70)159 dialkyldiaryl-substituted disilene, 71160, and the only tetraalkyldisilene known to be stable as a solid, 72161. Three silyl-substituted disilenes, 73, 74 and 75, were also investigated162,163. To assist in the interpretation of the experimental results, ab initio molecular orbital calculations of the 29Si chemical shift tensors were carried out for model disilene molecules. 

As mentioned above, the principal values of chemical shift tensor give information about three dimensional electronic state of a molecule. However, in order to understand behavior of the principal values, one should obtain information about the orientation of the principal axis system of a chemical shift tensor with respect to the molecular fixed frame. The orientations of the principal axis systems of the chemical shift tensors of L-alanine Cp -carbons in some peptides were calculated, whose L-alanine moieties have different main-chain dihedral-angles, (( >,v /H-57.40,-47.50)[aR-helix], (-138.8°,134.7°)[ pA-sheet], (-66.3°,-... 

For our final example of theoretical NMR in catalysis, we again turn to carbenium ion chemistry. Here we study the formation of the isopropyl cation on frozen SbF5, a strong Lewis acid (27). In contrast to the studies presented earlier, with this system we were able to experimentally measure the chemical shift tensor. Because the full tensor is naturally obtained from NMR calculations, a comparison can readily be made. In addition, for the isopropyl cation we also studied the effect the medium (in this case, the charge balancing anion) had on the chemical shift tensor. 

Extremely Fast Calculation of UC Chemical Shift Tensors Using the Bond Polarization Theory... 

The semi-empirical bond polarization theory provides a fast method for the calculation of 13C chemical shift tensors with an accuracy level comparable to ab initio methods for most molecules. There are no restrictions in the size of the molecular system, so that the method can be applied to large molecules or crystal lattices. Chemical shift calculations were carried out for several small molecules and compared to ab initio results. Additionally the calculated chemical shift tensor for the fullerence Qo is presented. 

The authors of this work proposed a semi-empirical scheme for the calculation of 13C chemical shift tensors based on the bond polarization theory (5). This method can reproduce 13C chemical shift tensors with deviations from experiment comparable to the errors of the ab initio methods. One major advantage is that the calculations can be performed for large molecular systems with hundreds of atoms even on a PC computer. In contrast to the ab initio method a set of empirical parameters is needed for the calculations. In the case of the bond polarization theory these parameters can be estimated directly from experimental chemical shifts solving a set of linear equations. 

Within the framework of the bond polarization the shift components of the unpolarized bond and the parameter Aai giving the polarization influence on the chemical shift are determined empirically from solving a set of linear equations 1 for a number of substances where both the chemical shift tensor and the molecular structure are known. The bond polarization energies Vai are calculated as effect of surrounding net atomic charges qx on atomic hybrids %. With the bond polarity parameter the polarization energy can be calculated. 

The origin of equations 1 and 2 was explained in more detail in (6). The calibration of the bond polarization parameters for, 3C chemical shift tensors for C(sp3/sp2)-C, C(sp3/sp2)-H, C(sp3/sp2)-0, and C(sp3)-N o-bonds as well as C(sp2)-C and C(sp2)-0 71-bonds was described in detail in (5). Structural data and tensorial single crystal, 3C NMR data of 20 crystalline substances including sugars, polycyclic aromatic compounds, and amino acids were used in this calculation. 

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