Deuterium isotope effects on the carbon-13 chemical shifts of carbocations

The nature of carbocations has been studied by the determination of deuterium isotope effects on the carbon-13 chemical shifts for carbocations with different charge-delocalization mechanisms. The deuterium isotope effects on the carbon-13 chemical shifts showed different directions and different magnitudes for different types of carbocations. The nature of classical-nonclassical cations plays an important role in determining the type of isotope shift and appreciable substituent effects which were observed. Deuterium isotope shifts on the cationic carbon centers are positive (downfield shift) for 2-substituted-2-norbornyl cations with deuterium substitution at the C6 position, zero for classical delocalized 3-methyl-3-nortricyclyl cations with deuterium substitution in the methyl group, and negative (upfield shift) for nonclassical trishomocyclopropenyl cations with deuterium substitution at the β-carbon position. Deuterium isotope effects on carbon-13 chemical shifts have also been examined as a function of increasing electron demand in a series of 2-aryl-2-norbornyl cations. The onset of σ-delocalization in the 2-aryl-2-norbornyl cations as a result of substitution of electron withdrawing groups on the phenyl ring give enhanced positive values of the isotope shifts for deuterium substitution at the C6 exo position and reduced positive values of the isotope shifts for deuterium substitution at the C1 position. All the observed isotope shifts are explained by a substantial contribution from the vibrational origin of the isotope effect