Isotopic perturbation method

A selective and brief survey of some of the more recent studies follows. Askani et al. (1984), using isotope perturbation methods (Siehl, 1987), showed that both the dimethyl [92] and the tetracarbomethoxy [93] semibullvalenes are Cope systems. In a related study, mild perturbation of the symmetrical semibullvalenes [94] and [95] is achieved by replacing one of the methyl groups with an ethyl group [94a] and [95a]. Again these systems were shown to be rapidly equilibrating tautomers and not neutral homoaromatics (Gompper et al., 1985). 

Saunders, Telkowski and Vogel developed another powerful method for distinguishing nonclassical from classical cations using the deuterium isotopic perturbation method. ... 

The isotopic perturbation method was first developed using H-nmr spectroscopy and deuterium as the perturbing isotope. The isomerization of 3-deuterio-2,3-dimethyl-2-butyl cation (Saunders et al., 1971), 2,3,3-trimethyl-2-butyl cation and cyclopentyl cation (Saunders and Vogel, 1971a) and methyl interchange in 2,3-dimethyl-2-butyl cation [10] (Saunders and Vogel, 1971b) were studied. 

The bridged nonclassical structure [117] has also received support from Schleyer et al. (1980) in a comparison of C-nmr chemical shift sums of carbocations and their respective hydrocarbon precursors . Further evidence for [117] has been given by Servis and Shue (1980) who applied their isotopic perturbation method to ion [113]. 

Uvarov et al. demonstrated the suitability of the Saunders isotopic perturbation method, applied to vicinal C-D couplings, in order to measure the conformational equilibrium of deuterated cycloalkanes. To this end, the secondary isotope effect on "7(C,D) was determined as shown in Eq. (89), where I represents (Di)cyclohexane, and II represents (l,I-D2)cyclohexane. 

Ab initio calculations for a model dication (CH)g also support the nonclassical pyramidal structure Of late Hogeveen has corroborated the nonclassical structure of dication 506 with both Saunders isotopic perturbation method and the comparison — after Schleyer et al. — of the sum of all chemical carbon shifts in the NMR spectrum of the ion with that for the neutral hydrocarbons 507 and 508 the calculated differences were 54.6 and 195.7 ppm per charge unit, respectively, typical for nonclassical ions. 

The isotopic perturbation method is a powerful one for demonstrating the unsymmetrical nature of equilibrating systems however, as indicated above, the nature of the dissymmetry requires care in interpretation. In particular, the evaluation of the magnitude of the averaged coupling... 

The kind of information obtainable through the use of the isotopic perturbation method is manifold and the problems which can be investigated cover a broad range. Fundamental questions connected to the interplay between molecular dynamics and structure like the distinction between time-averaged symmetry of molecules with a multiple minimum energy... 

The isotopic perturbation method is very useful for studying fractional bonding and hypercoordination in coordinatively unsaturated and electron deficient compounds such as transition metal complexes or carbocations. The 2-norbornyl cation and the bicyclobutonium cation are the most prominent examples of carbocations whose structures have led to much controversial discussion (the so-called classical-nonclassical ion controversy). The application of the isotopic perturbation methed is likely to be the most decisive piece of nmr evidence for the hypercoordinated structure of these two cations in solution. 

The bicyclo[3.2.1]octa-3,6-dien-2-yl anion [27] was proposed as the prototype of bishomoaromatic anions. The isotopic perturbation method has been applied to this system to obtain experimental evidence on its nature (Christl et al., 1983). 

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. 

In contrast to cyclobutadiene, tri-tert-butylcyclobu-tadiene is a stable compound. The equilibrium between the two double-bond isomers cannot be frozen out in a NMR experiment at 88 K. This suggests the activation energy for the process is less than 2.5 kcal/mol. A similar conclusion was derived using the isotopic perturbation method of Saunders and examining the NMR spectrum of vicinally C-dilabeled cyclobutadiene. Here, the rate of interconversion exceeds 10 s at 25 K. 

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