GIAO calculations

Alkorta, I., Elguero, J., 1998, Ab Initio Hybrid DFT-GIAO Calculations of the Shielding Produced by Carbon-Carbon Bonds and Aromatic Rings in H NMR Spectroscopy , New J. Chem., 381. 

It could appear interesting to apply Chestnut s approximate infinite-order perturbation-theory prescription for isotropic NMR chemical shieldings (25) in order to extrapolate our HF//MP2 and MP2//MP2 results to higher orders of perturbation theory. However, due to the feet that reference MP3-GIAO and MP4-GIAO results are not available, even for C6H5F, we did not feel that the use of ratios, derived from MPn-GIAO calculations on smaller moleules, would be sufficiently well justified. 

The results presented in this paper strongly suggest that, at the present moment, the most efficient and sufficiently accurate approach for calculating 19F NMR shielding values in fluorobenzenes and other related molecules is a HF-GIAO calculation on top of a ground-state geometry optimized at the MP2 level of theory. 

Figure 4. Graph of the GIAO-calculated shielding increment (Ag) of a proton of methane (oriented as in Figure 3) vs. distance above the center of ethene.

Figure 7. GIAO-calculated shielding increment (Ao) of "ghost atoms" (triangles) and the McConnell equation-calculated shielding increment (circles) 2.0 A above ethene vs. lateral distance from the center of the carbon-carbon double bond.

Structure GIAO (calculated) Equation 2 (predicted) Experimental (observed)... 

The acid-base properties of 4-nitroso-5-aminopyrazoles 50 have been studied. In particular, protonation sites have been determined where salts 51 are formed [66], The effects produced on II, 13C, and 15N chemical shifts by protonation and by hydrogen-bonding solvents on five azoles (imidazole, 4,5-dimethylimidazole, pyra-zole, 3,5-dimethyl-pyrazole, and 4,5-dihydro-3-methyl-2//-benz[g]indazole) have been determined experimentally. Phase effects on the 13C chemical shifts of the C-4 atom of pyrazole were discussed, based both on empirical models and on GIAO calculations of absolute shieldings in different complexes. The special case of the chemical shifts of pyrazoles in the solid state, where they form multiple N-H N hydrogen bonds, was also studied theoretically [88],... 

By a combination of NMR data in the solid state and GIAO calculations, the existence of a proton transfer in hemi-salt 78 (solid state proton transfer, SSPT) has been established. This is an important result because SSPT remains an infrequent phenomenon (see Sect. 3.4). 

To explore the possible antiaromaticity of 5//-dibcnz fc/ azcpinc (5) we have studied its behavior in sulfuric acid as a solvent using NMR (including GIAO calculations) [89], Instead of the expected cations 79 and 7 we isolated the cations of 9-formylacridine (80) and of 9,90-ethene-1,2-diyl-bis-acridinc (81). A mechanism for this unexpected dismutation was proposed. 

The second equation for 15N includes nitroso derivatives (very far removed from other 15N chemical shifts). The usefulness of 15N chemical shifts (combined with GIAO calculations and some X-ray structure determinations) was demonstrated in the case of C-bromo pyrazoles 152-158 [151],... 

In all cases when there is a bromine atom at position 3(5), 155-158, the tautomer present in the solid state is the 3-bromo one a. In solution, the same tautomer is the major one. DFT calculations justify the predominance of 3-bromo tautomers over 5-bromo ones and provide some useful chemical shifts obtained through GIAO calculations. 

F, and 29Si as appropriate). In selected cases, PCM-GIAO calculations were also performed to investigate the extent of solvent effects on the computed NMR shifts. 

The Determination of Absolute Configuration Using DFT/GIAO Calculations of VCD Spectra... 

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