J . Phys. Chem. 1989, 93, 3639-3643
3639
If this picture is correct, then the system can cross from the
5Sto the 5Dand then to the 7S ground state. The SDcan access
I sG$J Figure 6. Qualitative potential energy surface for Mn+-CH4 system.
to the empty C-H antibonding orbital. The 5Sstate has spinpaired d electrons to donate to the antibonding orbital, but the 4s orbital has an electron preventing donation of the C-H electrons to the 4s orbital. Hence the sS state will tend to act like an odd electron radical and abstract H or CH3. The 5Sstate can correlate with either quartet or sextet MnH+ or MnCH3+ product. The 7S ground state should only react by H-atom abstraction to give a sextet MnH+ product. This is generally consistent with the reactivity of these states in endothermic reactions with H2 as described by A r m e n t r ~ u t . ~ - "The energy of H-M-CH3 was estimated by taking the second bond to be equal to the bond formed by Cr+ since Cr+ and MnL+ (L = H, CH3) have five unpaired d electrons and are in a sense i s o e l e c t r ~ n i c . ~ ~
the ground state directly. Note that the system can readily get caught in the SDpotential well (H-Mn-CH3+) and move rc peatedly past the 7S crossing point until relaxation to the ground state occurs. An argument essentially similar to this has been postulated by Armentrout and ceworkers to rationalize the collisional relaxation of excited Cr+ by CH4.16 We note also that the repulsive nature of the 7S Mn+-CH4 potential surface is evident from recent examination of clustering between the atomic metal ion and CH4 by Weisshaar and co-workers.I7 The failure of N2 to deexcite Mn+ supports the proposed mechanism. N 2 simply does not have a C-H bond to which the metal can oxidatively add. The attractive surface that couples ground- and excited-state manifolds in the methane case is absent in the N 2 case. Similarly the presence of C-H bonds in dimethylamine suggests that energy transfer eliminates the possibility of charge transfer in its interactions with Mn'. Efficient energy transfer may also be the reason larger alkanes fail to react with excited Mn+. Registry No. Mn', 14127-69-6; Mn2(CO),,,, 10170-69-1; CH4, 7482-8. (15) The energy of (H-Mn+-CHJ was estimated by D(H-MnC-CH3) = (D(Mn+-H) + D(Cr+-CH,) + D(Mn+-CH,)
+ D(Cr+-H)) / 2
D(M+-H) and D(M+-CHJ were taken from: Armentrout, P. B. Metal Ligand Bond Energies. In Structure, Reactiuity, and Thermochemistry of Ions; Ausloos, P., Lias, S. G., Eds.; Reidel: Boston, 1986; pp 182-184. (16) Georgiadis, R.; Armentrout, P. B. J . Phys. Chem. 1988, 92, 7067. (17) Tonkyn, R.; Ronan, M.; Weisshaar, J. C. J . Phys. Chem. 1988, 92, 92-102.
Multireference Configuration Interaction Study of the Reaction H,
+ BO
-
H
+ HBO
Michael Page Laboratory for Computational Physics and Fluid Dynamics, Naval Research Laboratory, Washington, DC 20375 (Received: September 26, 1988)
-
The reactants, transition state, and products for the collinear abstraction reaction H2 + BO H + HBO have been studied by using ab initio MCSCF and multireference CI techniques with basis sets up to triple
