RELATIVE ENERGIES AND STRAIN ENERGIES OF PROPOSED STRUCTURES FOR P-8

Phosphorus is known to form a wide range of homoatomic clusters. Known examples that contain the Ps nucleus are the substituted and anionic clusters P8R4, P8R6, and P8(6-). Although the neutral P8 naked cluster has never been prepared, there has been theoretical interest in this molecule in recent years. Neutral Ps would be isoelectronic with (CH)8 hydrocarbons for which at least 13 structural isomers have been prepared. We report the results of geometry optimized ab initio SCF MO calculations with 4-31G* and 6-31G* basis sets for P8 in the cubane, octabisvalene, and cuneane isomeric forms. These structures are connected by single bonds only and are therefore likely to be the lowest energy forms of P8. Indeed, calculated bond distances are all consistent with single P-P bonds. At the 4-3 IG* level all three structures correspond to real minima on the energy surface. The cuneane structure has the lowest energy for both basis sets. At the 6-31G* level the cuneane form of P8 is only a few kcal/mol higher in energy than 2P4. Energy changes for homodesmotic reactions involving Pg give estimates of strain energies for the three P8 Structures. These range from 45 kcal/mol for cuneane to 76 kcal/mol for cubane, much smaller than the estimated strain energies for (CH)8 in cuneane and cubane forms: 137 and 166 kcal/mol, respectively. Using the strain energy additivity rule, we can decompose the strain energies of the larger cage and polycyclic structures to obtain estimates of strain energies of component monocycles. Applying the additivity rule again, we can estimate strain energies of Pg structures for which ab initio SCF MO calculations have yet to be done.