ACCURATE CALCULATION OF NMR CHEMICAL-SHIFTS

An overview over currently available methods for the calculation of NMR chemical shifts is given. After a brief discussion of the basis aspects, i.e. definition of the shielding tenser and gauge-origin problem in the calculation of magnetic properties, recently developed methods for calculating NMR chemical shifts based on electron-correlated quantum chemical approaches will be discussed. The importance of electron correlation effects for the accurate determination of absolute shielding constants and relative shifts is demonstrated and the accuracy of various approaches assessed by a careful comparison with experiment, before a few representative applications will be presented. In case of the carboranes C2B3H5 and C2B3H7, electron-correlated calculations were essential to resolve discrepancies between SCF calculations and experiment, while for the vinyl-substituted vinyl cations CH2 = C+ - CH = CHCH3 correlated NMR chemical shift calculations - but not the SCF calculations - allow the unequivocal assignment of the measured C-13 signals to the Z and E isomers formed in superacid solutions. Finally, an outlook with respect to pending problems in the area of NMR chemical shift calculations is given.