2-Norbornyl cation calculations

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. 

Similarly to the 2-norbornyl cation, comparison of calculated (IGLO/DZ//MP2/ 6-31G(d)) and experimental 13C NMR chemical shifts allowed to differentiate between the hypercoordinated 70 and the trivalent form 71 of the bicyclo[2.1.1.]hexyl cation.85 The experimental (157.8 ppm) and calculated (158.5 ppm) values for Cl and C2 (averaged signal) are reported to be nearly identical for the symmetrically bridged... 

Intramolecular interaction energy calculated for the element norbornyl cations, see Ref. 40. 

Theoretical quantum mechanical calculations903-908 have also been performed on the 2-norbornyl cation at various levels. These calculations reveal a significant preference for the o-delocalized nonclassical structure. An extensive calculation by Schaefer and co-workers906 using full geometry optimization for symmetrically and... 

TABLE 40. MP2/6-31G(d) calculated energies (kcal mol-1) of substituted norbornyl cation K+ (Figure 61) and KHa... 

The problems associated with theoretical calculations of the energies of protonated cyclopropane are readily apparent and the norbornyl skeleton increases the complexity of computation. Early quantum mechanical calculations of the MINDO/3, STO-3G (MINDO/3 geometry)and ab initio STO-3G type show the corner-protonated norbornyl cation less stable (- 9143, i44 (g i3)i45. 5 9144 4.9 kcal mol" ... 

Diese discussed results, together with theoretical calculations where the classical form of 2-norbomyl cation is not even an energy minimum, clearly proves the symmetrically (or very close to symmetrical) bridged structure of the 2-norbornyl cation (126) involving hypercoordinate carbons. 

In further theoretical studies, Schleyer and coworkers found" that the nonclassical 2-norbornyl cation (126) is more stable by 13.6kcalmoE than the 127 classical structure, and 126 is the only stable form in the gas phase and in solution, and the classical form (127) is unlikely to be involved in solvolysis reactions [MP4(fc)SDQ/6-31G /MP2(full)/6-31G -p ZPVE level]. They also performed a rigorous ab initio calculation and defined the classical 127 structure as an artifact and found that it is a transition state in the rearrangement of 126 into the bridged 2-norpinyl cation. The computed MP2-GIAO 0 chemical shifts for 126 are close to the experimental values, whereas those calculated by Schindler (IGLO) for 127 deviate considerably. The comparison of the chemical shifts for the two structures with the experimental data proves once again the symmetrical nonclassical structure. 

Recently, ab initio calculations (MP2/6-311G ) have been used to study nine nonclassical carbocations derived from cyclic hydrocarbons, including the 2-norbornyl cation. Electron density and chemical shifts, geometries, energetics, and orbital interactions within the NBO formalism were computed and analyzed. In sharp contrast to the findings by Werstiuk s group, 3c-2e bonding... 

Unequivocal evidence for the existence of the 7-norbornyl cation, however, was subsequently provided by Sunko and coworkers by using the cryogenic matrix isolation technique. They codeposited 7-chloronorbornane 146 and a large excess of SbFj at -263°C then slowly warmed it to -173°C. The IR spectrum was in very good agreement with the calculated frequencies [MP4(sdq,fc)/6-31G //MP2(full)6-31G + ZPE level] for the 7-norbornyl cation 80 (Scheme 5.11). Further warming gave the 2-norbornyl cation 126 at -73°C. Tlie possibility of the formation of a complex of 146 with Sbf. was ruled out when similar experiments with both 146 and 147 resulted in identical spectra. Recent ab initio calculations support the nonclassical bent structure of the cation. 

The IR spectrum published by Koch et al. (1990) was recorded using Vancik and Sunko s technique (1989) in which 2-chloronorbornane and SbF are deposited simultaneously from the gas phase on a Csl window at 77 K. During a slow temperature increase to 150 K, formation of the carbocation could be observed by the change in the IR spectrum. Koch et al. (1990) compared the frequencies and intensities of 21 absorption bands observed in the range 878-3110 cm with results that were calculated on the basis of a structure of the nonclassical 2-norbornyl cation obtained by Koch et al. (1989) using the 6-31G basis set and dynamic electron correlation on the MP2 level. The calculated IR bands correlated very well with the experimental frequencies and intensities. The calculated structure of this cation was. 

In the context of the historical development of our knowledge of the structure of the 2-norbornyl cation since Winstein and Trifan s postulate in 1949, this comparison with the 1,2-dimethyl derivative is very important for aspects provided by the theory of scientific discoveries. As discussed in our book on aromatic diazo compounds (Zollinger, 1994, Chap. 9), verifications are never definitive, in science proofs are not possible (the term proof should only be used in mathematics) falsifications, however, can be definitive. It was shown that molecular orbital calculations and their application to experimental data on the IR and NMR spectra for the 2-norbornyl cation are consistent with a symmetrical, nonclassical cationic intermediate, but not with a rapid equilibrium between two classical intermediates. The... 

The ab initio calculations taking into account electron correlations have led to the conclusion that the nonclassical 2-norbomyl ion is more stable than the classical one by 8-13 kcal/mole ( , cf. similar conclusions have been drawn by the authors of In the edge-protonated 2-norbornyl cation was shown to be comparable in stability with the comer-protonated one. 

Table 9. Calculated and Experimental Chemical Shifts and Values for 2-Norbornyl Cations...

It is interesting to compare the 2-norbornyl cation with its lower homologue — the 2-bicyclo[2,l,l]hexyl cation. Calculations with the MINDO/3 method show that the steric strain in the bicyclo[2,l,l]hexyl ion makes it a particularly favourable model for exhibitingCT-participation... 

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