THE NATURE OF THE CARBON-CARBON BONDS IN CYCLOPROPANE AND CYCLOBUTANE - A COMPARISON BASED ON SPIN-COUPLED THEORY

The electronic structure of the smallest cycloalkanes: cyclopropane (C3H6) and cyclobutane (C4H8) is studied by means of spin-coupled (SC) theory-the most general single-configuration approach. The comparison between the main characteristic features of the corresponding SC wave functions: orbital overlaps, orbital shapes, and correlation energies per electron pair indicates that the bonding along the carbon-carbon sigma-bond frameworks in cyclopropane and cyclobutane is rather similar, which represents a convincing theoretical explanation for their surprisingly close conventional ring strain energies. These conclusions are further confirmed by the results of full-valence SC VB (valence bond) calculations and are juxtaposed with SC and SC VB results for an n-alkane of a similar size, propane (C3H8). Although the sigma-electron sextet involved in the carbon-carbon bonds in cyclopropane appears to be isocongugate to the pi-electron system of benzene, its SC description is found to involve no significant resonance, and the optimal mode of coupling the spins of the six valence electrons is decidedly different from that in benzene. This undermines attempts to explain the properties of C3H6 by invoking the idea of a-aromaticity.