Semiautomated Transition State Localization for Organometallic Complexes with Semiempirical Quantum Chemical Methods

We
present an efficient computational protocol for robust transition
state localization that can be routinely applied to complex (organometallic)
reactions. The capabilities of the combination of extended tight-binding
semiempirical methods (GFNn-xTB) with a state-of-the-art
transition state localization algorithm (mGSM) is
demonstrated on a modified version of the MOBH35 benchmark set, consisting
of 29 organometallic reactions and transition states. Furthermore,
for three examples we demonstrate how error-prone the conventional
(manual) approach based on chemical intuition can be and how errors
are avoided by a semiautomated generation of reaction profiles. The
performance of the GFNn-xTB methods is carefully
assessed and compared with that of the widely used PM6-D3H4 and PM7
semiempirical methods. The GFNn-xTB methods show
much higher success rates of 89.7% (GFN1-xTB) and 86.2% (GFN2-xTB)
compared with 72.4% for PM6-D3H4 and 69.0% for PM7. The barrier heights
and reaction energies are computed with much better accuracy at reduced
computational cost for the GFNn-xTB methods compared
with the PMx methods, allowing a semiquantitative
assessment of possible reaction pathways already at a semiempirical
level. The mean error of GFN2-xTB for the barrier heights (8.2 kcal
mol–1) is close to what low-cost density functional
approximations provide and substantially smaller than the corresponding
error of the competitor methods.