THE RING-OPENING OF CYCLOPROPYLIDENE TO ALLENE - GLOBAL FEATURES OF THE REACTION SURFACE

The global features of the groundstate ring opening of cyclopropylidene to allene are studied by means of ab-initio FORS MCSCF calculations based on a minimal AO basis set. The energy surface is completely mapped out in terms of three reaction coordinates, namely the CCC ring-opening angle and two angles describing the rotations of the CH2 groups. For each choice of these three variables, the twelve remaining internal coordinates are optimized by energy minimization. In the initial phase of the reaction, as the CCC angle opens, the CH2 groups rotate in a disrotatory manner, maintaining C(s) symmetry. This uphill reaction path leads to a transition region which occurs early, for a CCC angle of about 84-degrees. In this transition region the reaction path branches into two pathways which are each others' mirror images. The system exhibits thus a bifuracting transition region. Passed this region, the two pathways are overall conrotatory in character. However, these downhill reaction paths to the products are poorly defined because, from a CCC opening angle of about 90-degrees on, the CH2 groups can rotate freely and isoenergetically in a synchronized, cogwheel-like manner and this disrotatory motion can mix unpredictably with the conrotatory downhill motion. There is no preference for any one of the two reaction pathways yielding the two stereoisomers of allene and the reaction is therefore nonstereospecific with respect to the numbered hydrogen atoms. The global surface is documented by means of contour maps representing slices corresponding to constant CCC angles. The bifurcating transition region is mapped in detail.