THEORETICAL STUDY OF GEOMETRY REACTIVITY AND SPECTRUM OF CYCLOPROPANE

Ab initio SCF-MO and CI calculations have been carried out for the cyclopropane molecule C3H6 for a series of CCC internuclear angles in order to study the geometry of this molecule in its ground and excited states; from this treatment an equilibrium CCC angle of 65° is indicated. A basic similarity between cyclopropane and the series of symmetric AB2 molecules (and others) is pointed out; all possess C2v symmetry throughout their respective bending processes and their angular correlation diagrams show a general agreement between shapes of corresponding orbital energy curves. The great disparity in the geometries of cyclopropane and its isoelectronic AB2 and HAB2 counterparts ozone and formate ion is thereupon related to the fact that these systems possess different ground-state electronic configurations; in turn it is shown that this distinction is caused by the ability of hydrogen AO's to selectively alter the stability of the orbitals of a parent AB2 molecule. Development of this concept also allows a consistent explanation for the unusual reactivity of cyclopropane. CI calculations are employed to study the electronic spectrum of C3H6 and they indicate that the majority of the known ultraviolet absorptions of this system cannot reasonably be assigned to vertical transitions; instead O-O excitations involving upper states with wide-angle equilibrium geometries are suggested.