ACCURATE CALCULATION OF NMR CHEMICAL-SHIFTS

An overview over currently available methods for the calculation of NM R chemical shifts is given. After a brief discussion of the basis aspe cts, 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 quant um chemical approaches will be discussed. The importance of electron c orrelation effects for the accurate determination of absolute shieldin g constants and relative shifts is demonstrated and the accuracy of va rious approaches assessed by a careful comparison with experiment, bef ore a few representative applications will be presented. In case of th e carboranes C2B3H5 and C2B3H7, electron-correlated calculations were essential to resolve discrepancies between SCF calculations and experi ment, while for the vinyl-substituted vinyl cations CH2 = C+ - CH = CH CH3 correlated NMR chemical shift calculations - but not the SCF calcu lations - allow the unequivocal assignment of the measured C-13 signal s to the Z and E isomers formed in superacid solutions. Finally, an ou tlook with respect to pending problems in the area of NMR chemical shi ft calculations is given.