Electron-correlated calculations, nuclear chemical shifts

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. 

J. Gauss, Effects of electron correlation in the calculation of nuclear magnetic resonance chemical shifts, J. Chem. Phys. 99 (1993) 3629. 

As suggested by Mo and Schleyer [7a], a reference molecule with the same number of diene conjugations is a better choice than a molecule with the same number of ji-electrons, because the ASE values (computed using the block-localized wave function, BLW, method) exhibit a better correlation with the nuclear-independent chemical shift (NICS) values. We followed their suggestion and adopted the former approach for the calculation of the ASEs using the AE values derived from the EDA. 

Second-Order Many-Body Perturbation Theory Using Gauge-Including Atomic Orbitals. J. Gauss,/. Chem. Phys, 99, 3629 (1993). Effects of Electron Correlation in the Calculation of Nuclear Magnetic Resonance Chemical Shifts. 

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