GIAO-DFT

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. 

Over the years, several computational methods have been developed. The variational theory can be used either without using experimental data other than the fundamental constants (i.e., ab initio methods) or by using empirical data to reduce the needed amount of numerical work (i.e., semiempirical data methods). There are various levels of sophistication in both ab initio [HF(IGLO), DFT GIAO-MP2, GIAO-CCSD(T)] and semiempirical methods. In the ab initio methods, various kinds of basic sets can be employed, while in the semiempirical methods, different choices of empirical parameters and parametric functions exist. The reader is referred to reviews of the subject.18,77... 

We refer to the literature for a more detailed discussion of this point (3,7), as well as for the actual implementation within the framework of the DFT-GIAO method for NMR shieldings (6,7,9). 

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

In order to determine the Absolute Configuration (AC) of an enantiomer of a chiral molecule of defined specific rotation, its VCD spectrum is measured and compared with the predicted DFT/GIAO VCD spectra of the two enantiomers. Assuming that the... 

DFT/GIAO predicted spectra are accurate, the predicted spectrum of one enantiomer will be in excellent agreement with the experimental spectrum, while for the other enantiomer the agreement will be very poor. By way of illustration, the B3PW91/TZ2P VCD spectra of the R- and S- enantiomers of PHTP are compared with the experimental VCD spectrum of (+) - PHTP in Figure 2.17, and the calculated and experimental rotational strengths... 

Thus, significant improvements of calculated rotational strengths await the incorporation of anharmonicity and solvent effects and the development of superior functionals. In the meantime, it is clear that the current DFT/GIAO methodology is of very high... 

The principal values, 6n, 622, 633, of the chemical shift tensor were obtained for each form by CP/MAS NMR experiments. The orientation of the principal axes with respect to the molecular frame was investigated by P CP/single-crystal NMR for the complex with propan-2-ol. The principal axis 1 of both chemically equivalent phosphorus atoms is nearly parallel to the P-S bond and the principal axis 3 is very close to the P=S bond. The comparison of experimental P 6 parameters with theoretical data calculated by the DFT GIAO approach provided complementary information about the most sensitive NMR parameters, which best characterize the nature of the C-H -S contacts. 

In theoretical work, the molecular complex was built (Scheme 5) with the methane guest molecule in close proximity to one of the thiono-sulfurs of the model compound and the P1=S-H-C unit was aligned in the plane of the S=P1-S bonds. The distance, d, between the sulfur and the carbon in the P1=S-H-C fragment was varied in the range 2.4-4.1 A. Several DFT GIAO calculations... 

Osmialowski and Gawinecki116 have also calculated 15N NMR chemical shifts for aniline derivatives as catalogued in Table 16. The largest differences between experimental and DFT/GIAO calculated 15N NMR chemical shifts can be greatly diminished when medium effects (in the form of aniline-CHCl3 complexes) are taken into account. These calculations are still in progress. 

Table 18 Calculated Metal Shieldings and Chemical Shifts (ppm) in Transition Metal Carbonyls TM(CO)e (TM = Cr, Mo, W) and in Using DFT-GIAO... 

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