GIAO method , chemical shift calculation

To compare with the experimental NMR data, the 13C isotropic chemical shifts of all of the acetylide and vinylidene complexes were calculated. These systems were too large for us to be able to do chemical shift calculations at the MP2 level. We thus used the GIAO method at the RHF level. Fortunately, the RHF level of theory appears to be adequate for these complexes. We used a mixed basis set for the GIAO, with tzplarge+ on C, tzplarge on H, tzp on the Mg and O associated with the adsorbed species, and dzp on the remaining Mg and O (26). 

In their study of structures, energies, and C NMR chemical shifts of C4H7 and C5H/ ions, Olah et al. have recently found - that the ko delocalized bisected cyclopropylmethyl cation 110 and the nonclassical bicyclobutonium ion 111 (both of G symmetry) are minima for C4H7 (MP2/cc-pVTZ level). At the MP4(SDTQ)/cc-pVTZ//MP2/cc-pVTZ + ZPE level, structure 111 is more stable by 0.4 kcal mol than structure 110. On the basis of C NMR chemical shift calculations [GIAO-CCSD(T) method] and relative energies, the equilibrium of the two structures in superacid solutions most likely account for the experimentally observed NMR chemical shifts. 

Wolinski K, Haacke R, Hinton JF, Pulay P (1997) Methods for parallel computation of SCF NMR chemical shifts by GIAO method Efficient integral calculation, multi-Fock algorithm, and pseudo-diagonalization. J Comp Chem 18 816-825... 

For strained or unsaturated cations such as the cyclobutenyl cation (6), or the cyclohexadienyl cation (7) (i.e., protonated benzene), no good agreement between IGLO (or GIAO)-SCF and experimental chemical shifts could initially be obtained. In these cases, electron correlation effects need to be included in the chemical shift calculations, in addition to the geometry optimizations which have to be performed at correlated levels as well. This necessity could be demonstrated with the advent of electron-correlated methods such as GIAO-MP2. 

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