Symmetric spectra isotopic perturbation

The l3C NMR spectrum of the C4H7+ cation in superacid solution shows a single peak for the three methylene carbon atoms (72) This equivalence can be explained by a nonclassical single symmetric (three-fold) structure. However, studies on the solvolysis of labeled cyclopropylcarbinyl derivatives suggest a degenerate equilibrium among carbocations with lower symmetry, instead of the three-fold symmetrical species (13). A small temperature dependence of the l3C chemical shifts indicated the presence of two carbocations, one of them in small amounts but still in equilibrium with the major species (13). This conclusion was supported by isotope perturbation experiments performed by Saunders and Siehl (14). The classical cyclopropylcarbinyl cation and the nonclassical bicyclobutonium cation were considered as the most likely species participating in this equilibrium. 

Prakash et al. (1985) used the deuterium isotope effect on the l3C NMR spectrum of [47] to provide further evidence for the symmetrical, homoaromatic nature of this ion. They prepared the specifically deuterated trishomocyclopropenyl cation [57] by superacid treatment of the corresponding alcohol [58]. The 13C NMR spectrum of [57] displayed a triplet for the deuterated methine only 0.2 ppm to higher field than the undeuterated methine, indicating only an isotopic perturbation of resonance and not a rapidly equilibrating classical ion system (see Siehl, 1987). 

The key feature of the NMR spectra of 91 is its simplicity. Thus the 13C NMR spectrum consists of only two resonances at 4.9 and 17.6 ppm, indicating either a symmetrical trishomocyclopropenium cation, 93, or rapid equilibration between three equivalent structures (Scheme 37). The positions of the 13C NMR resonances of the cation strongly suggested the formulation of its structure as the trishomocyclopropenium ion, 93210. This conclusion was reinforced by the preparation of the deuterated cation and examination of the isotopic perturbation of its 13C chemical shifts208 211, and measurement of the 13C H coupling constants209. 

Saunders and Kates (1980) have applied the isotopic perturbation technique to ion [31] (see Section 4). The perturbation by the deuterium in [422] of the C-nmr spectrum of [31] was smaller than 0.1 ppm, confirming the proposed static three-fold symmetric structure for the ion. 

This is true even if a slow 6,2-hydride shift in secondary cations converts part of [103] (i.e. 3,3-D2-[100a,bj) into 5,5-D2-[101]. Cation 5,5-D2-[101] would lack an equilibrium isotope effect because of the symmetry of an assumed Wagner-Meerwein rearrangement between D2-[101a] and D2-[101b], This does not vitiate the validity of the isotopic perturbation test, which shows that a double energy minimum of secondary cations [103] is not present. A static symmetrical hypercoordinated structure [102] is in accord with the observed results. If the 6,2,1-hydride shift in [102] was not frozen out on the timescale of the experiment, the C spectrum of [102] measured below — 150 C is still conclusive, although it results from an equal mixture of isomers of [102] labelled at C-3, C-5 and C-7. 

A r-bridged structure, rather than rapid equilibrating cr-bonded unsym-metrical structures for allyl-lithium were indicated from the C n.m.r. spectrum of allyl-lithium-ld in THF using the Saunders isotopic perturbation method. " However, in one study a symmetric Ji-bridged structure was proposed in contrast to a non-symmetric i-bridged structure in another. 2-R-allyl-(R=Pr or Bu ), crotyl-, or prenyl-potassium, symmetric -complexes result in EtjO or THF. The H n.m.r. and u.v. spectra of allyl-,M+ (M=Na, K, Rb, or Cs) were reported. 

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