Cyclopropylcarbinyl cations calculations

Distinct evidence for the equilibration of bicyclobutonium with a minor isomer, bisected cyclopropylcarbinyl cation, comes from the ultra-low temperature CPMAS studies of Myhre, Webb and Yannoni25. They have observed a major isomer, the bicyclobutonium ion, with a l3C chemical shift of 15 ppm for the pentacoordinated carbon, and a minor bisected cyclopropylcarbinyl cation, whose cationic center s chemical shift was found to be at 235 ppm. The NMR chemical shifts of the cation are also comparable with those calculated by the IGLO method at that temperature26 27. The energies of these cations were shown to be nearly the same (AAH° = 0.05 kcalmol1). 

The planar cyclobutyl cation and the perpendicular cyclopropylcarbinyl cation are of nearly equal energy and much less stable than the bisected cyclopropylcarbinyl or bicyclobutonium ion (ca 36 kcalmol"1) while the latter two structures have very similar energies. Inclusion of correlation at the MP4SDQ/6-31G //MP2-6-31G levels showed that bicyclobutonium ion is more stable than the bisected cyclopropylcarbinyl cation only by 0.7 kcalmol"1. Selected structural parameters for the bisected and perpendicular conformers (6 and 7) of the cyclopropylcarbinyl cation as well as the bicyclobutonium ion 2, as calculated at the MP2/6-31G level, are as shown26. 

At the lowest temperatures studied ( —140°C), 13C NMR spectroscopy indicates that 519 is still an equilibrating mixture of bisected o-delocalized cyclopropylcarbinyl cations 521 and bicyclobutonium ion 522.171 172 From the comparison of calculated NMR shifts, the low-lying species is considered to be the bicyclobutonium ion.171,172... 

The mechanism for bromination of semibullvalene has been proposed, based on quantum chemical calculations. The reaction pathway involves concerted bromine addition and cyclopropane ring opening to form an allylic cation, without the intermediacy of a bromonium or a cyclopropylcarbinyl cation.18... 

The ab /w/nWIGLO/NMR method has been used to determine the relative distribution and stability difference of the cyclopropylcarbinyl cation and cyclobutyl cation in solu-tion. Agreement between IGLO chemical shifts and experimental shifts could only be obtained when assuming a rapid equilibrium between the two cations. Over the range of temperatures considered (-61 to -132° C), a cyclobutyl cation structure with an axial H atom and short 1,3-distances of 1.65 A (bicyclobutonium ion structure) was found to be more stable by 0.5 kcalntoT For the gas phase, however, the cyclopropylcarbinyl cation was calculated to be 0.26 kcalmoT more stable [MP4/6-31G(d)//MP2/6-31G(d) calculations including vibrational corrections]. ... 

The rotational barrier for the secondary I -methyl cation 30 could not be measured experimentally, but the barrier for the tertiary r,T-dimethyl cation 31 could be estimated from the observed line broadening at + 80 °C as 18 kcalmoT Theoretical calculations at the STO-3G level gave a value of 17.7 kcalmoT for the latter cation. The secondary cationic system is a mixture of two isomers (cw-30a and trans-3 h) due to the restricted rotation across the Cl—CT bond. In the tertiary dimethyl cation 31, the methyls expectedly show distinct NMR absorptions <5 H 3.14 (cw-Me), 2.60 trans-Mo) <5 C 30.2 (cw-Me), 33.7 trans-MQ). The cationic center s absorptions are also interesting in that they are highly shielded compared with other related cations, including cyclopropylcarbinyl cations. The tertiary cationic center has <5 C of 249.6, the secondary cationic center <5 C of 220.8 and the primary cationic center <5 C of 191.4. Calculations at MNDO and STO-3G suggested that these cations have enhanced vinyl-bridging character compared to the simple cyclopropylcarbinyl model". ... 

Cyclobutyl- or cyclopropylcarbinyl chloride or the corresponding alcohols react with SbFj to give stable ion solutions with identical H- and C-nmr spectra. These spectra indicate either an ion with three-fold symmetry, the tricyclobutonium ion [32], or a set of rapidly equilibrating, less symmetrical ions with the same effective averaged symmetry. These include cyclopropylcarbinyl cation [33], bicyclobutonium ion [34] or bent cyclobutyl cations [35]. Theoretical calculations predict similar energies for [33]-[35], but indicate that [32] is less likely (Hehre and Hiberty, 1974 Hehre, 1975). Isotopic perturbation studies by Saunders and Siehl (1980) indicate that the... 

Mechanism (82) would, however, produce unusual C-satelIites, and both explanations (83) and (84) were disregarded after comparison with the average chemical shift for the six proton group (6.1 ppm) calculated from 4-methyl-cyclopropylcarbinyl cation [104], a thermodynamically favoured rearrangement product of [113] observed at higher temperatures. Average chemical... 

Molecular orbital calculations using 4-3IG and 6-3IG basis sets led to the conclusion that the minimum energy structure for C4Ht was the bisected cyclopropylcarbinyl cation 61 (Table 12) with a plane of symmetry defined by the formally cationic carbonA second structure (64) was also found as an energy minimum that was subtly different from 61, but possessed an unsymmetrical cyclopropyl ring with one corner bent toward the... 

L. R. Schmitz and T. S. Sorensen, J. Am. Chem. Soc., 104,2605 (1982). This paper is concerned mostly with substituent effects on the classical cyclopropylcarbinyl cation structure, but footnote 2 in this reference provides a useful listing of all earlier calculations on the system. 

The dominant contributor to the reactivity of vinylcyclopropanes in any radical reaction is the form (4a), the cyclopropylcarbinyl radical system. The opening of a cyclopropylcarbinyl radical to a butenyl radical is among the fastest radical processes known, with a rate constant of 1.3 x 10 sec . The various stereoelectronic effects of this rearrangement have been reviewed. - The structure of (4a), deduced from its ESR spectrum - and in agreement with calculations (STO-36 basis set), is in the bisected conformation shown, predicted to be 1.4 kcal mol more stable than its perpendicularly oriented counterpart. Above -100 C only the butenyl radical (4b) can be detected. Substituent effects do not seem to operate here when the substituents are on the cyclopropane (i.e. product stabilization). The cyclopropylcarbinyl cation and anion have structures similar to (4a), bisected conformations (5) and (6), respectively. A concise summary of solvolytic and mechanistic data for system (5) has recently appeared. Reviews of cyclopropylcarbinyl anions and carbenes are also available. ... 

An example of the application of the decay technique to a structural problem is provided by the demonstration of the existence of cyclic C4Ht isomers, namely, the cyclobutyl and the cyclopropylcarbinyl cations, as stable species in the dilute gas state (12-15). Theoretical approaches led to somewhat conflicting conclusions, reliable STO-3C calculations shoving that the cyclopropylcarbinyl ion in the bisected configuration (I)... 

By the ab initio method Hehre and Hiberty have calculated different conformations of the homoallylic ions C H. and the isomeric bisected cyclopropylcarbinyl cation to show all the homoallylic ions I-IV being energetically less stable than ion V by 20-30 kcal/mole. The solvolysis products whose composition points to the intermediate formation of homoallylic rations can 1% attribute either to the fast trapping of unstable ions by counterions or to the change of the stability ratio of the isomeric ions 4 by solvation effects. 

A comparison of the C NMR spectra between 7-norbornenyl ions and typical a-cyclopropylcarbinyl cations indicates that in the former the C cation centre signals are shifted 100-200 ppm to the high field. In choosing between the non-classical structure 19 and the rapid equilibrium of a-cyclopropylcarbinyl ions one should take into account that the calculated averaged shift for the signal of C should be 1/2 (250 - - 80) = 165 ppm which differs by 40 ppm from the experimental... 

In conclusion, most of the evidence and molecular orbital calculations (see p. 27) support the view that cyclopropylcarbinyl cations have a bisected conformation that is stabilized because of the increased p-character of the bent C—C cyclopropane bonds which increases greatly their C—C hyperconjugative ability. The most important evidence that has been interpreted as arguing against this formulation is the NMR spectra of the cyclopropylmethyl cation itself, and hence there must be certain reservations about the < bove conclusion concerning this ion. It must also be... 

According to ab initio HF calculations (3-21G basis set), the C-C bond of cyclopropane is 1.513 A long, in good agreement with experiment. A comparable calculation on cyclopropylcarbinyl cation gives the bond lengths shown. Rationalize the differences. 

Transient Three-membered-Ring Compounds.—A review on protonated cyclopropanes as reactive intermediates has appeared. The reaction of methylcyclo-propane with proton is predicted, by EH-MO calculations, to involve two dominant paths (i) removal of hydride ion to generate cyclopropylcarbinyl cation, and (ii)... 

---