Daniel H. Ess, PhD, Brigham Young University
Theory of Divalent Main Group H2 Activation
The structure, barrier heights, thermodynamics, electronic properties, and dynamics of H2 activation by singlet divalent main group compounds (ER2; E = C, Si, Ge) were studied using density functional theory (DFT), absolutely localized molecular orbitals (ALMO), and quasiclassical trajectories (QCTs). ALMO energy and charge decomposition calculations revealed that in the transition state carbene-type compounds act as ambiphiles toward H2 while heavier analogs (Si and Ge) act as nucleophiles. We also found that classic frontier molecular orbital (FMO) energy gaps do not provide a reasonable estimate of energy stabilization gained in the transition state or an accurate description of electronic character of the reaction. Examination of barrier heights and reaction energies showed a clear kinetic-thermodynamic relationship for ER2 activation of H2. QCTs show that dihydrogen approach and reaction with CR2 may involve geometries that significantly deviate from those expected based on a static transition-state structure. In contrast, SiR2 trajectories involve geometries close to the side-on approach that would be predicted by the static transition state. Trajectories also demonstrated that addition of H2 to CR2 and SiR2 is dynamically concerted.