Carbocations spectroscopy

The differentiation of bridged nonclassical from rapidly equilibrating classical carbocations based on NMR spectroscopy was difficult because NMR is a relatively slow physical method. We addressed this question in our work using estimated NMR shifts of the two structurally differing ions in comparison with model systems. Later, this task... 

As mentioned, we also carried out IR studies (a fast vibrational spectroscopy) early in our work on carbocations. In our studies of the norbornyl cation we obtained Raman spectra as well, although at the time it was not possible to theoretically calculate the spectra. Comparison with model compounds (the 2-norbornyl system and nortri-cyclane, respectively) indicated the symmetrical, bridged nature of the ion. In recent years, Sunko and Schleyer were able, using the since-developed Fourier transform-infrared (FT-IR) method, to obtain the spectrum of the norbornyl cation and to compare it with the theoretically calculated one. Again, it was rewarding that their data were in excellent accord with our earlier work. 

NMR spectroscopy is ideal for detecting charged fluorinated intermediates and has been applied to the study of increasingly stable carbocation and carbanion species. Olah [164, 165] has generated stable fluorocarbocations m SbFj/SOjClF at low temperatures The relatively long-lived perfluoro-rerr-butyl anion has been prepared as both the cesium and tris(dimethylamino)sulfonium (TAS) salts by several groups [166, 167, 168], Chemical shifts of fluonnated carbocations and carbanions are listed m Table 23. 

Allylic bromination, 339-340 mechanism of, 339-340 Allylic carbocation, electrostatic potential map of, 377, 489 resonance in, 488-489 SN1 reaction and, 376-377 stability of, 488-489 Allylic halide, S l reaction and. 377 S j2 reaction and, 377-378 Allylic protons, ]H NMR spectroscopy and, 457-458... 

When compound 17-1 is dissolved in FS03H at -78°C, NMR spectroscopy shows that a carbocation is formed. If the solution is then allowed to warm to — 10°C, a different ion forms. The first ion gives compound 17-2 when... 

The kinetics of deuterium isotope exchange between diphenyl phosphine and t-butylthiol have been studied by H n.m.r. spectroscopy.274 A negative temperature coefficient was observed for the reaction of a perf1uoroalky1 phosphite with a fluorinated aldehyde.275 The kinetics for the reaction of alcohols with phosphoryl trichloride bore strong similarities to those of carboxylic acid derivatives.276 An interesting report desribed the solvolysis of ary 1 hydroxymethyl-phosphonates. It was shown that a phosphoryl group does not prevent carbocation formation on an immediately adjacent carbon atom.277... 

The values of ks/kp for partitioning of carbocations are most conveniently determined as the ratio of the yields of products from the competing nucleophile addition and proton transfer reactions (equation 1 derived for Scheme 2). The determination of these product yields has been simplified in recent years by the application of high-pressure liquid chromatography (HPLC). Typically, the product peaks from an HPLC analysis are detected and quantified by UV-vis spectroscopy. In cases where the absorbance of reactants and products is small, substrates may be prepared with a chromophore placed at a sufficient distance so that its effects on the intrinsic reactivity of the carbocationic center are negligible. For example, the aliphatic substrates [1]-Y have proved to be very useful in studies of the reactions of the model tertiary carbocation [1+].21,23... 

Partitioning of carbocations between addition of nucleophiles and deprotonation, 35, 67 Perchloro-organic chemistry structure, spectroscopy and reaction pathways, 25, 267 Permutational isomerization of pentavalent phosphorus compounds, 9, 25 Phase-transfer catalysis by quaternary ammonium salts, 15, 267 Phosphate esters, mechanism and catalysis of nucleophilic substitution in, 25, 99 Phosphorus compounds, pentavalent, turnstile rearrangement and pseudoration in permutational isomerization, 9, 25... 

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

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... 

Carbocations have been generated in zeolites and were characterized by CPMAS 13C NMR spectroscopy and accompanying quantum chemical calculations.110 13C NMR isotropic shifts calculated at the GIAO-MP2/TZP/DZ level for the 1, 3-dimethylcyclopentenyl (98) and 1,2,3-trimethylcyclopentenyl cations (99)111 are in agreement with the observed experimental spectra of the carbocations in the zeolite.112... 

Bicyclic and polycyclic carbocations, NMR spectroscopy, 145-150 1, 2-dimethyl-2-norbornyl cation, 149 bicyclobutonium ions, 145-146, 146/ 3- /<7o-trialkylsilylbicyclobutonium ions, 147-148... 

Heteroatom stabilized carbocations, NMR spectroscopy, 156-158 halomethyl cation, 156... 

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