Shielding calculations

Rauhut, G., Puyear, S., Wolinski, K., Pulay, P., 1996, Comparison of NMR Shielding Calculated from Hartree-Fock and Density Functional Wave Functions Using Gauge-Including Atomic Orbitals , J. Phys. Chem., 100,... 

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. 

How does one do relativistic calculations Although two formalisms had been in the literature outlining relativistic approaches to shielding calculations, no calculations had been carried out for molecules. 

There is a need for shielding calculations including electron correlation at higher levels than have already been reported at the conference (MBPT2, SOPPA/SOLO) to really find out how large the electron correlations effects are in molecules such as N2, HCN, CO. 

While DFT may or may not be more accurate than MP2 for absolute shielding calculations is debatable, the strength of the DFT method in calculations of shieldings is in the ability of DFT to provide a consistent picture over a wide range of chemical systems, since calculations can be done at a very modest computational cost compared to MP2. Among the successes of the method is in ligand chemical shifts in transition metal complexes. For example, 13C, 170,31P and H chemical shifts for oxo (12,14,15), carbonyl (16-19), interstitial carbide (20), phosphine (21,22), hydride (23), and other ligands have been successfully reproduced to within tens of ppm in... 

What is the current state of shielding calculations for small molecules This question can be asked in the context of state of the art calculations of shielding in molecules... 

The developments since 1992 include very accurate calculations of the shielding surfaces in the immediate vicinity of the potential minimum for diatomic molecules. Rovibrational corrections to shielding calculated with these more accurate shielding surfaces (CCSD(T) level calculations) are shown in Table VIII. 

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.

Predicted 235U Absolute Shieldings. Calculated uranium shieldings for selected molecules are collected in Table V. 

The basis sets used in the calculations were taken from the GAUSSIAN94 basis set library. All geometry optimizations employed the 6-31G basis. The most accurate shielding calculations presented in this article involve, in a locally-dense context, 6-311++G(2d,2p) sets of basis functions on the fluorines, while the carbons and hydrogens are described within the 6-31G basis. 

NMR Shielding Calculations. The influence of the combination of basis set, wave-function type and theoretical geometry on 19F shielding in monofluorobenzene were investigated in much greater detail than for the other fluorobenzenes. The results of the calculations are presented in Figures 1-3. 

A Conformational Study of the l-Alanine Residue in Polypeptides by Ab Initio UC NMR Shielding Calculation... 

In this paper, we aim to elucidate the correlation between the chemical shift tensors and the conformation of peptides, by carrying out NMR shielding calculations using the ab initio GIAO-CHF MO with 6-31G basis set, in order to understand the 13C chemical shift and chemical shift tensor behavior of the peptides and polypeptides. 

Figure 1. Structure of N-acetyl-N -methyl-L-alanine amide used in NMR shielding calculations.

The chromophores of rhodopsin and bacteriorhodopsin are 11 -cis- and all-trans-retinal Schiff bases, respectively. Upon binding to the proteins, their unsaturated carbons show anomalous 13C chemical shifts compared with those of corresponding model compounds. This indicates the occurrence of interactions between the chromophore and its surrounding protein matrix. Ab inito shielding calculation reveals that the major part of such anomalous shifts originates in the conformational change of the chromophore. 

Ab inito chemical shielding calculation, combined with solid-state NMR experiment, is a powerful tool to elucidate the structures of active sites in proteins and enzymes. 

The geometries of all the molecules studied were optimized using the same basis sets as in the shielding calculation. Then, the initial geometries of all-trans and 11 -cis retinals were taken from the corresponding crystal structures (19, 20). 

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