The Interplay between experiment and theory computational NMR spectroscopy of carbocations

Institute of Organic Chemistry I, Ulm University, Albert-Einstein-Allee II, D-89069, Ulm, Germany 

The IGLO method, an important breakthrough for the calculation of magnetic properties, was developed in the mid-1980s by Kutzelnigg et al.2 After some 

ADVANCES IN PHYSICAL ORGANIC CHEMISTRY VOLUME 42 ISSN 0065-3160 DOI 10.1016/S0065-3160(07)42003-2 

The CH cation 1, protonated methane, is the parent of hypercoordinated carbocations containing a five coordinated carbon atom. It is elusive in solution and has not been observed by NMR spectroscopy but gas-phase infrared investigations have shown its fluxional structure which has been proven by ab initio molecular dynamic simulation.18 

For the CH with a Cs symmetrical structure 1 a 13C NMR chemical shift of 11.5 ppm is calculated. This is 7.1 ppm more shielded than that calculated for the hydrocarbon CH4 at the GIAO-MP2 level.19 The shielding effect in 1 is as expected for a hypercoordinated carbocation carbon.20 The calculated minimum energy structure for diprotonated methane (CHi+) 2 has C2v symmetry with two 3c-2e 

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The interplay between experiment and theory computational NMR spectroscopy of carbocations... 

Tetrahedral intermediates, derived from carboxylic acids, spectroscopic detection and the investigation of their properties, 21, 37 The interplay between experiment and theory computational NMR spectroscopy of carbocations, 42, 125... 

A. Sharma, A.G. Namdeo and K.R. Mahadik, Molecular Markers New Prospects in Plant Genome Analysis , Pharmacognosy Rev., 2008, 2, 23, [online computer file]. Avail. URL http //www.phcog.net/reviews/issue3/4.pdf H.-U. Siehl, The Interplay between Experiment and Theory Computational NMR Spectroscopy of Carbocations , Adv. Phys. Org. Chem., 2008, 42, 125. 

The generation of carbocations in strongly acidic media, and the characterization of their structure by NMR in the 1950s was a breathtaking accomplishment that led to the award of the Nobel Prize in Chemistry to George Olah. Over the past 50 years NMR spectroscopy has evolved as the most important experimental method for the direct study of structure and dynamics of carbocations in solution and in the solid state. Hans-Ullrich Siehl provides an excellent review of computational studies to model experimental NMR spectra for carbocations. This chapter provides an example of how the fruitful interplay between theory and experiment has led to a better understanding of an important class of reactive intermediates. 

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