INTERNAL CONROTATION AND DISROTATION IN H2BCH2BH2 AND DIBORYLMETHANE 1,3 H EXCHANGE

The paths of correlated internal disrotation (barrier less than 0.4 kc al/mol) and conrotation (barrier around 1.9 kcal/mol) of the two BH2 g roups in H2BCH2BH2 have been computed employing nb initio [MP2(full)/6 -31G*] and density functional theory (Becke3LYP/6-311+G**) methods. T wo B-C ... B(p) hyperconjugative interactions stabilize the C-5 symmet ric H2BCH2BH2 isomer (a), The B-C ... B(p) hyperconjugative stabilizat ion, evaluated by homodesmotic reactions and using the orbital deletio n procedure (which ''deactivates'' the ''vacant'' born p orbital), is less than 6 kcal/mol in diborylmethane. The B-C ... B( p) stabilizatio n is shown to be remarkably large in C4B6H10 (Td). At MP2(fu)/6-31G*, disproportionation into I and methane is only 5.6 kcal/mol exothermic . The 1,3 H exchange in diborylmethane is an asynchronous process and proceeds sia a doubly bridged cyclic intermediate with 9.3 kcal/mol ba rrier. Structures with ''planar tetracoordinate'' carbon are stabilize d considerably by BH2 substituents, but they are still high in energy. (C) 1997 John Wiley & Sons, Inc.