THEORETICAL COMPARISON OF TETRAHEDRANE AND CYCLOBUTADIENE BY AB INITIO TECHNIQUES

The relative stability of the two C4H4 isomers tetrahedrane and cyclobutadiene is investigated by means of ab initio SCF MO and CI calculations. It is pointed out that the single determinantal wave function of the SCF method gives an intrinsically better representation of the tetrahedrane ground state, a classical closed-shell species, than that of cyclobutadiene, with its partially filled valence shell. Configuration interaction (CI) is introduced to increase the flexibility of the molecular wave functions employed, thereby removing the greater part of the bias toward tetrahedrane exhibited by the simple SCF treatment. The CI calculations thus find tetrahedrane to be considerably less stable (by 70 kcal/mole) than cyclobutadiene in its rectangular ground state; a stable minimum within the tetrahedral arrangement is indicated, however, for CC distances slightly smaller than the normal single-bond value. Tetrahedrane is found to have no such potential minima in any of its excited states and to be unstable with respect to dissociation into two acetylene molecules by from 70 to 100 kcal/mole. © 1969, American Chemical Society. All rights reserved.