Nuclear Magnetic Resonance Spectra in Solution

One of the most powerful tools in the study of carbocations is nuclear magnetic resonance (NMR) spectroscopy. This method yields direct information—through chemical shifts, coupling constants, and the temperature dependence of band shapes— about the structure and dynamics of carbocations. 

An empirical criterion based on additivity of 13C NMR chemical shifts for distinguishing classical trivalent and higher coordinate carbocations has been developed by 

Schleyer, Olah, and co-workers.55 In this method, the sums of all 13C chemical shifts of carbocations with their respective hydrocarbon precursors are compared. A trivalent carbocation has a sum of chemical shifts of at least 350 ppm higher than the sum for the corresponding neutral hydrocarbon. This difference can be rationalized by partly attributing it to the hybridization change to sp2 and to the deshielding influence of an unit positive charge in the trivalent carbocation. Since higher coordinate carbocations (nonclassical ions) have penta- and hexa-coordinate centers, the sum of their chemical shifts relative to their neutral hydrocarbons is much smaller, often by less than 200 ppm. 

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