Electron-correlated calculations, nuclear density functional theory

Also in 2-substituted ethanesulphonates,35 the 33S chemical shift has a reverse substituent effect and correlates with both Taft substituent constants and the chemical shift of the carboxylic carbon in related carboxylic acids. It seems that the substituent effect does not depend on conformation, but prevailingly on intramolecular electronic effects. Density functional theory (DFT) calculations of 33S nuclear shielding constants and natural bond orbital (NBO) analysis made it possible to conclude that substituents cause a variation in the polarization of the S-C and S-O bonds and of the oxygen lone pairs of the C — S03 moiety. This affects the electron density in the surroundings of the sulphur nucleus and consequently the expansion or contraction of 3p sulphur orbitals. 

The methods that will be discussed in the following are all of the ab initio type. Given the molecular field free Hamiltonian in Eq. (2.9), with the nuclear coordinates and charges and the electronic mass given as parameters, in these methods all integrals over this Hamiltonian or parts of it are evaluated ab initio, i.e. by strict application of the appropriate mathematical rules and without using further data from experiment or otherwise. The emphasis will be in particular on the SCF and on so-called correlated methods. Semi-empirical or density functional theory (DFT) methods on the other hand will not be mentioned explicitly. However, most of what will be said about SCF-based methods for the calculation of properties will also apply to semi-empirical or DFT methods, because one can consider to a certain extent the semi-empirical and DFT methods as variants of SCF, just with a slightly different Hamiltonian. 

Density functional theory (DFT) is an alternative to HF methods for describing molecular electronic structure. Electron correlation effects are explicitly included in DFT calculations. Coupled DFT (CDFT), together with the IGLO method, has been used in some nuclear shielding calculations and some results for F and are shown in Table 7. 

Ab initio molecular orbital calculations. This method uses the Hartree-Fock self-consistent field (SCF) theory with one-electron molecular orbitals. It is based on the variation theorem to seek the nuclear geometry of the hydrogen bonded complex with lowest energy, and it makes use of no experimental data. Additional elaborations (electron correlation) or related approaches (density functional theory) are possible. 

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