VALENCE ISOMERS OF BENZENE AND THEIR RELATIONSHIP TO THE ISOELECTRONIC ISOMERS OF AS6

We report the results of geometry-optimized ab initio SCF-MO calculati ons for As6 in structural forms corresponding to the five valence isom ers of benzene, C6H6, With which As6 is valence-isoelectronic. We used Scuseria's 752+P polarized basis set for arsenic. The resulting energ y ordering of As6 isomeric structures is very similar to what we found previously for P6 isomers but quite different from that known for the C6H6 valence isomers. For As6, as for P6, prismane and benzvalene str uctures have the lowest energies while the planar hexagon (benzene) ha s the highest. The range of energies of the five structures is small, around 45 kcal/mol, somewhat larger than for the P6 isomers but far sm aller than for C6H6. Calculated bond distances are easily identified w ith experimental examples of AsAs single, double, and aromatic bonds. The energy of 2As6 lies only 30 kcal/mol above that of 3As4. Calculate d energy changes of homodesmotic reactions involving As4 and As6 give estimates of strain energies and resonance energies. The strain energi es range from 2 kcal/mol for Dewar benzene to 33 kcal/mol for prismane . These are uniformly lower than comparable calculated strain energies of P6 isomers and much lower than strain energies of the valence isom ers of C6H6. Our estimate of the resonance energy of planar hexagonal As6 is 17.6 kcal/mol, somewhat larger than that for the same structure for P6 and smaller than the calculated resonance energy of benzene, C 6H6. Using calculated strain and resonance energies for As6 isomers, w e obtain an estimate of 60 kcal/mol for the As=As double bond energy. As with carbon and phosphorus, the homoatomic double bond is weaker th an two homoatomic single bonds, and it is this result that establishes the basic energy ordering of As6 isomers. Calculated charge densities for As6 isomers are uniformly smaller than those for comparable P6 st ructures. Weaker bonds, smaller strain energies, and lower charge dens ities are properties that are consistent with the fact that valence AO 's of larger principal quantum number are more diffuse.