Ab initio SCF and CI [configuration interaction] study of the electrocyclic transformations of cyclopropyl and allyl systems

Abstract

Ab initio SCF and Cl calculations employing a double f Gaussian basis have been carried out to deduce minimum energy surfaces for the electrocyclic transformations of the cyclopropyl cation, radical, and anion respectively to the corresponding allylic species. In each of the symmetry-forbidden interconversions, it is concluded that the ring-opening phase does not occur simultaneously with the actual CH2 rotation. In the allowed disrotatory process of the cation, the methylene rotation begins very shortly after CC stretch is initiated and thereafter the two types of geometrical changes proceed in a synchronous fashion for virtually the entire reaction. Nevertheless, the calculations indicate that in the allowed (conrotatory) anion interconversion CH2 rotation and ring opening again occur in well-separated phases, just as in fact was found before for the allowed thermochemical transformation of cyclobutene and butadiene. The different patterns of behavior in this respect are seen to be related to distinctions in the relative stabilities of the various ring compounds. The importance of Cl in the calculations for the C3H5 systems is much smaller than has been noted in the earlier C4H6 study, with the result that the differences in the activation energies of the respective forbidden and allowed processes for both the cation and anion are calculated to be much greater than the corresponding difference in the cyclobutene-butadiene case. Analysis of a number of energy terms for the dis and con modes of the various C3H5 transformations suggests that the agree-(1) (a) Instituí für Physikalische Chemie; (b) Lehrstuhl für Theoretische Chemie; (c) University of Nebraska.(2) W.

Time-resolved photoelectron spectroscopy of the allyl radical: The lifetimes of the ultraviolet bands

“…͑c͒ A third possibility would be excited state isomerization, leading to the formation of either cyclopropyl or methylvinyl radical. Photocyclization in agreement with the Woodward-Hoffmann rules was suggested by theoretical studies 19 and resonance Raman experiments. 4 At the excitation energies present in our experiment, the cyclopropyl radical would lose a hydrogen atom and form the closed-shell molecule cyclopropene.…”

“…Irradiation at 254 nm, 18 yielded a mixture of allene, propyne, acetylene, methane, and traces of propene, while irradiation at 400 nm supposedly yielded cyclopropyl radicals. 17 Theoretical studies 19 suggested a disrotatory photocyclization upon excitation in the UV bands, a reaction pathway that was supported by the results of a resonance Raman experiment 4,13,14 performed at 224.63 nm, but no direct information on the decay properties of the UV band system is yet available. Thus the results have to be regarded as inconclusive.…”

Kinetics and dynamics in the photodissociation of the allyl radical

“…8 The isomerization to cyclopropyl has also been the subject of several theoretical investigations. 6,9,10 Nevertheless, the evidence for such a reaction is inconclusive.…”

Electron Spin Resonance of Oxiranyl Radicals in Solution: Configurational Stabilities and Rearrangement Reactions

“…In constrast to the cyclopropyl radical where the mode of ring opening is now being extensively discussed [35] we do not know of any theoretical treatment of the oxiranyl rearrangement (2) and therefore again delay quantitative statements. However, qualitatively, the effects of substitution may easily be explained if the ring opening of oxiranyl takes the same stereochemical course as cyclopropyl.…”