Constant shielding

Z is tlie atomic number and cr is a shielding constant, determined as below, n is an effective principal quantum number, which takes the same value as the true principal quantum number for u = 1, 2 or 3, but for u = 4, 5, 6 has the values 3.7, 4.0, 4.2, respectively. The shielding constant is obtained as follows ... 

File shielding constant for the valence electrons of silicon is obtained using SlateZs rules as followrs. The electronic configuration of Si is (ls )(2s 2p )(3s 3p ). We therefore count... 

Shielding constants reported in experimental studies are usually shifts relative to a standard compound, often tetramethylsilane (TMS). In order to compare predicted values to experimental results, we also need to compute the absolute shielding value for TMS, using exactly the same model chemistry. Here is the relevant output for TMS ... 

The shielding constant s is calculated, for any of the sets of orbitals, from... 

Frenking s group showed that the Group 11 isocyanides M—NC (M = Cu, Ag and Au) are less well bound compared with the corresponding cyanides M—CN [276]. They also studied CO coordination on Cu, Ag+ and Au with Au(CO)2 being the most stable of all Group 11 dicarbonyl complexes [281]. Vaara et al. demonstrated the importance of relativistic effects in the 13-C NMR nuclear shielding constant in... 

Gauss, J., Stanton, J. F., 1996, Perturbative Treatment of Triple Excitations in Coupled Cluster Calculations of Nuclear Magnetic Shielding Constants , J. Chem. Phys., 104, 2574. 

Lee, A. M., Handy, N. C., Colwell, S. M., 1995, The Density Functional Calculation of Nuclear Shielding Constants using London Atomic Orbitals , J. Chem. Phys., 103, 10095. 

Malkin, V. G., Malkina, O. L., Salahub, D. R., 1993, Calculations of NMR Shielding Constants by Uncoupled Density Functional Theory , Chem. Phys. Lett., 204, 80. 

This shielding constant, a, is made up of three terms (eq. 2.2)... 

In the first, quantum mechanics can be used to calculate the NMR coupling constant between two nuclear spins in the molecular environment or the NMR shielding constant. Jensen [8] in Chapter 10 of the reference provides a comprehensive introduction to the methods for calculating these molecular properties. The NMR coupling constant, KAB, is related to the second derivative of the energy with respect to the internal magnetic moments, /u, arising from the nuclear spin of the two atoms, A and B. 

The shielding constant, crA, is related to a mixed second derivative of the energy with respect to an internal magnetic nuclear moment due to spin, and external magnetic field, Bext. 

Table 1.29Si NMR shielding constants a and chemical shifts 6 in ppm and their localized contributions for SiX4 (I) and XSifNH (II)...

Case58 investigated the effect of ring currents on NMR shielding constants by means of the DFT calculations. The studied rings included the ones commonly found in proteins and nucleic acids. The shielding constants were calculated for methane molecule placed in several positions relative to the ring. The calculations provided data needed to derive structural parameters from measured chemical shifts in proteins and nucleic acids. 

It is important to note the orientation dependence of the shielding constant, cr, and the fact that shielding is proportional to the applied field, whence the need for chemical shift reference materials such as tetramethylsilane. 

The concept of magic-angle spinning arises from the understanding of the shielding constant, cr (Eq. 4). This constant is a tensor quantity and, thus, can be related to three principal axes ... 

H and 119Sn NMR. Deuterium isotope 132 effects on shielding constants and spin-spin coupling constants. 

Figure 1.4 shows a significant deviation between the isolated cluster calculations and the full calculation. The situation is, however, considerably improved by the presence of the classical point charges in the QM/MM calculation. Here the whole bandwidth of chemical shielding constants is present, and correlation with the reference values is excellent. 

This shows that the second and further solvation shells still have a non-negligible effect on NMR chemical shielding constants through the long-range electrical field they create. The approximation of an isolated molecular cluster in vacuo is valid for large clusters only this eventually makes determination of the shieldings of all protons computationally much more expensive than the fully periodic ab initio calculation. 

The second kind of NMR isotope effect has its origin in the mass differences between isotopes. In NMR mass effects show up as isotopic differences in NMR shielding constants and coupling constants. Their theoretical rationalization is... 

Table 12.4c Contributions of different vibrational modes to the zero point IE on 13C shielding constants of 13CH4 and 13CD4, - [ — ] (in ppm)...

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