Shielding calculation methods

Mauri et al have provided the only ab initio shielding calculation method for extended periodic networks, which they have applied to crystals and also to small molecules containing atoms in the first row of the Periodic Table. In an effort to extend the ab initio calculations of shielding in extended networks using periodic boundary conditions to involve other than light atoms, Mauri et... 

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... 

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.

Computational Details. Restricted Hartree-Fock (RHF) calculations were carried out using Gaussian 94 (45) and ACES II (46) on an IBM RISC/6000 computer. The gauge independent atomic orbitals (GIAO) method was used for the shielding calculations (47). All second-order many-body perturbation theory (MBPT2, also referred to as MP2) calculations were performed with ACES II (46). 

In order to improve on the RHF values, electron correlation is included in the ab initio method by using MP2. Comparing the results for the 6-31 lG(d,p) basis set for all the diatomics, electron correlation results in a sizable correction to the RHF results differences of 8.7 ppm for A1H, 8.0 ppm for A1F, 3.7 ppm for A1C1 and 7.4 ppm for A1NC are noted. In the case of A1H, the isotropic value of the aluminum shielding calculated at the MP2 level is greater than the result from RHF theory however, for the fluoride, chloride and isocyanide, the opposite trend holds. Gauss et al. (42) also observed this trend for the hydride, fluoride and chloride. 

For fused metals anomalous results for the association factor x below 1 are found, whilst fused salts give peculiar results, sometimes indicating an association factor of 10. (In such cases, equation (1) must be used, since the critical temperatures are unknown and must be very high.) These are, however, extreme cases of application. The abnormalities in general seem to be connected with the value of the critical temperature, At present, it is fairly generally agreed that, whilst low values of k for liquids about room temperature point to association, the calculation of an association factor x by Ramsay and Shields s method is unjustified. ... 

---