Chemical shielding 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. 

Fig.3 The dependences on the dihedral angles(< >,i /), of the isotropic chemical shielding constant for the L-alanine residue Cp- (a)and Ca-(b) carbons in peptides. Chemical shielding calculations were carried out using the GIAO-CHF method with 4-31G ab initio MO basis set. The 4-31G optimized geometries for the model molecules, N-acetyl-N -methyl-L-alanineamide, were employed.

P. B. Karadakov and K. Morokuma, ONIOM as an efficient tool for calculating nmr chemical shielding constants in large molecules, Chem. Phys. Lett., 317 (2000) 589-596. 

Chemical shifts, in terms of the chemical shielding constants, are given as ... 

It could be mentioned that in this work, a new improved nuclear factor was obtained. This may be the very first time MD simulations have contributed in the determination of a natural constant. Absolute shielding constants are important quantities, besides in NMR spectroscopy, as the experimental values for theoreticians calculating shielding constants quantum mechanically. An interesting theoretical application of including the short-time-scale thermal fluctuations from MD trajectories to quantum chemical calculations of chemical shielding constants is reported by Chesnut and Rusiloski [78]. and by Woolf et al. [79]. 

The first DFT calculations of Co chemical shielding constants were done using the SOS-DFPT-IGLO approach. The preliminary results indicated a significant improvement over data previously calculated by INDO and FPT-CNDO/2. Despite this improvement, the calculated Co chemical shifts are... 

Fig. 19. Correlation between solid-state Co isotropic chemical shifts ( ) and the isotropic chemical shielding constants (criso) calculated at 6-311 -t- G /B3PW91 level. All the calculations were performed on the bare complexes using experimental geometries. Data were taken from refs 53 and 167, and Table 1.

Table 21.4 C magnetic shielding constant (ppm) of PE and PVC Functional Experimental Calculated Chemical shielding constants...

Functional Experimental Calculated Chemical shielding constants ... 

ONIOM as an efficient tool for calculating NMR chemical shielding constants in large molecules ... 

Karadakov PB, Morokitma K (2000) ONIOM as an efficient tool for calculating NMR chemical shielding constants in large molecttles. Chem Phys Lett 317 589-596 Kanpp M, Malkina OL, Malkin VG (1997) The calculation of 0 chemical shielding in transition metal 0X0 complexes. 1. Comparison of DFT and ab initio approaches, and mechanism of relativity-induced shielding. J ChemPhys 106 9201-9212... 

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