Potential surface for a nonconcerted reaction. Tetramethylene

Abstract

We study a potential surface for the tetramethylene diradical, CH2CH2CH2CH2, a likely intermediate in the nonconcerted fragmentation of cyclobutane to two ethylenes. Outside a clearly defined region of a distorted cyclobutane we find two apparent minima, in geometry close to trans and gauche conformations of a model «-butane. These minima appear to be unstable with respect to fragmentation to two ethylenes. In fact, a motion in any degree of freedom away from the distorted cyclobutane region leads to a level crossing and a tetramethylene species whose orbitals resemble those of two interacting ethylenes. If by the term "intermediate" is meant a true minimum in the many dimensional potential surface then our calculations imply there is no such species intervening between cyclobutane and two ethylenes. We suggest that a large energetically flat region of a potential surface will in the context of modern collision theory have the same operational consequences as a true intermediate, and that such a region, termed a "twixtyl," may be a common feature of nonconcerted reactions.

On the Electronic Nature of a Butadienyl Biradical – Experiments and ab initio MO Calculations

“…[22] Therefore, for the values listed in Table 2, we assume that the CASPT2/6-31G* valbeing genuine biradicals. In contrast to the case of butane-1,3-diyl, [20] through bond coupling does not become domi-ues underestimate the experimental figures by 7Ϫ10 kcal/ mol, while the CASSCF values deviate by 25 kcal/mol. The nant in 10a.…”

Roaming: A Phase Space Perspective

“…In such cases, the ends of the chain often become capable of largeamplitude relative motions with little associated change in potential energy. Hoffmann et al (50) were the first to suggest the generality of such phenomena, and, in recognition that the associated PES regions possessed characteristics somewhere between those of minima and saddle points, coined the term twixtyl for the molecular species so generated. The flat PES regions themselves have been likened to geological features: a caldera (51, 52) if access to the plateau is protected by barriers, or a mesa (53) if it is not.…”

The generation and behavior of biradical intermediates in crystalline media

“…First, however, we very briefly review some of the current theories that relate biradical structure and their partitioning between cyclization and cleavage. A mainstay of the interpretation of solution phase 1,4-biradical reactivity over the years has been the idea that, in order for efficient cleavage to occur, the radical-containing porbitals should be parallel to the carbon-carbon sigma bond undergoing scission [3]; Hoffmann has termed this the (90,90) geometry [34]. In their more stable conformations, such species may be either gauche (structure 33, Scheme 15) or anti (structure 34, Scheme 15).…”

“…bHoffmann notation [34] for the extent of overlap between the p-orbital at C(1) with the C(2)--C(3) bond. A value of 0 ° indicates orthogonality, and a value of 90 ° indlcates full overlap.…”

“…The atom numbering refers to the figure in Table IV. eHoffmann notation [34] for the extent of overlap between the p-orbital at C(4) with the C(2)-C(3) bond. The p-orbital at C(4) is assumed to lie normal to the C(3)--C(4)-C(5) plane.…”