Method IGAIM

The individual gauges for atoms in molecules (IGAIM) method is based on Bader s atoms in molecules analysis scheme. This method yields results of comparable accuracy to those of the other methods. However, this technique is seldom used due to large CPU time demands. 

There are several alternative computational methods for obtaining NMR data such as the GIAO method (Wolinski et al., 1990), the LORG method (Bouman and Hansen, 1989), and IGAIM (Keith and Bader, 1992). 

The H and chemical shifts of some unsaturated species, their carbanions and the corresponding lithium complexes were calculated by the IGAIM method and compared with experimental values in THF solution. Examples are the lithium salt of the allyl carbanion (102), the dimeric form of the latter (103) and the lithium salt of the bicyclo[3.2.1]octa-3,6-dienyl carbanion (104) . 

Calculations using GIAO and IGAIM methods have enabled the chemical shifts of methyl cation and anion to be determined by gauging the shielding of the nucleus by neighbouring electrons the experimental data for cyclopentadienide ion, benzene, tropylium ion, and cyclooctatetraene dianion have also been reproduced and the data have been correlated by a simple model that also accounts for the large downfield shifts found for carbenes.14... 

Wiberg and Zilm used the Atoms in Molecules based IGAIM method to calculate the F NMR shielding for the alkyl fluorides from methyl fluoride to i-butyl fluoride and to separate it into the contribution from each of the MOs. 

Table 3 Comparison of absolute C shieldings in (ppm) calculated by the IGAIM method and the CHF procedure, using the same 6-31 G (2d, 2p) basis set, with experimental values taken as thermal averages at 300 K in the limit of zero gas density...

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