Interconversion of diborane (4) isomers

Highly correlated electronic structure calculations using many-body perturbation theory (MBPT) and coupled-cluster (CC) gradient techniques are applied to a study of a reaction pathway which links the two forms [C _2v and D_2d] of diborane(4) [B₂H₄]. A reaction coordinate which preserves C₂ symmetry is studied, as this mechanism is allowed by orbital symmetry rules. However, calculations show that the minimum energy path does not conform to this idealized mechanism. Rather, the reaction coordinate bifurcates, and the transition state contains no nontrivial elements of symmetry. At the level of partial fourth-order many-body perturbation theory [SDQ-MBPT(4)] with a large triple-zeta plus double polarization basis set, differences in distances between the hydrogen atoms corresponding to the bridge atoms in the C_2v form and the two boron atoms [δr(B-H) = |r(B₁-H) - r(B₂-H)-] are 0.14 and 0.81 Å, reflecting the pronounced asymmetry of the transition state structure. We find that the C_2v isomer should be the thermodynamically favored form of B₂H₄. At the coupled-cluster singles and doubles level with a noniterative treatment of triple excitation effects, the barrier to isomerization is found to be 6.3 kcal/mole using a large generally contracted basis set. This result, along with statistical arguments concerning the rate of reaction, suggest that equilibrium may be established relatively rapidly, and that rigid molecule treatments of B₂H₄ spectra may not be appropriate.