GIAO-CHF method

NMR shifts ( H, 13C, and 1SN) of 1-alkyl-, 2-alkyl-, and 3-aryltetrazolo[l,5- ]pyridinium salts have also been measured <1999JST119>. The data are compiled in Table 3. The 1SN shifts of these salts seemed of particular importance as they revealed quite big shielding changes for the nitrogen nuclei. These chemical shifts were also calculated by the ab initio GIAO-CHF method, and the result was found to be in fairly good agreement with the experimental values. 

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.

Table 2 Calculated l3C chemical shielding for the CP carbon of the L-alanine residue in peptides and polypeptides by the 4-31G GIAO-CHF method...

Figure 8. Plots of the calculated oxygen chemical shielding using the GIAO-CHF method against the hydrogen bond length.

The NMR shieldings are calculated by ab initio GIAO-CHF method with the 6-31G basis set. N-acetyl-N -methyl-L-alanine amide (Ac-L-Ala-NH-Me)... 

Most recently, the calculated chemical shift maps of the C and Cp carbons of the t-Ala residue were obtained using the GIAO-CHF method with an ab initio 4-31G basis set as shown in Fig. 1.3 in Chapter 1 [4], of... 

The 13C chemical shift contour map for the Cp carbon of the L-alanine residue in peptides and polypeptides was made as a function of the dihedral angles(, W) by using the experimental data. Also, the corresponding calculated map was made by using the ab initio coupled Hartree-Fock method with the gauge included atomic orbitals(GIAO-CHF). From these results, it was found that the calculated map explains the chemical shift behavior of the a-helix and p-sheet forms in poly(L-alanine) and some proteins. This suggests that the calculated map is applicable to the structural analysis of proteins with complicated structure. 

Most recently, ab initio calculations for the NMR chemical shifts have become available for medium-size molecules because of the remarkable power capabilities of modernworkstations, personal computers and supercomputers [32]. This leads to a quantitative discussion on the chemical shift behaviors. For example, the ab initio MO calculation with the 4-31G basis set using the GIAO-CHF (gauge independent atomic orbital-coupled Har-tree-Fock) method on A-acetyl-A -methyl-L-alanineamide which is the same model molecule as the case of the above FPT INDO calculation will be... 

Since about 1990, powerful post-Hartree-Fock approaches for the inclusion of electron correlation in chemical shift calculations have been developed and applied in main group chemistry (see NMR Chemical Shift Computation Ab Initio and NMR Chemical Shift Computation Structural Applications). Unfortunately, these correlated methods are computationally too demanding at present to be applied to transition metal complexes and clusters of chemically relevant size. In particular, the least expensive post-CHF method available, the MP2-GIAO approach, is expected to fail for systems with significant nondynamical correlation effects. 

All methods mentioned so far such as IGLO, LORG, GIAO, and CHF with a common gauge origin are of the CHF type, i.e., the many-electron wave function is approximated as a single Slater determinant. Methods for the calculation of NMR chemical shifts that go beyond this approximation, i.e., that include correlation effects, have been developed recently and are discussed in Section 5. One further neglects relativistic effects. These effects can be important if heavy atoms are present in a molecule and are discussed in Section 6 of this article. 

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