FINITE TD SYMMETRY MODELS FOR DIAMOND - FROM ADAMANTANE TO SUPERADAMANTANE (C35H36)

Seven clusters based on the diamond lattice, ranging in size from cyclohexane (C6H12, D3d) to the ''adamantane-of-adamantanes'' (or superadamantane-5, C35H36, T(d)), have been computed by molecular mechanics (MM2 and MM3), semiempirical self-consistent-field (SCF) (MINDO/3, MNDO, AM1, and PM3) methods, and ab initio SCF (with the minimal STO-3G, DZ, and DZP basis sets) theory. Molecular geometries are fully optimized under appropriate symmetry constraints in the MM2, MM3, and ab initio SCF methods, and by using Cartesian coordinates using semiempirical methods. Particular attention has been paid to the carbon-carbon bond lengths, total energies, and heats of formation. The largest cluster studied here is still far from the infinite diamond lattice. Comparisons among the computational methods reveal the merits and shortcomings of each.