How to predict activation barriers - Conformational transformations of compounds CH3C(CH2PPh2)(3-n)[CH2P(oTol)(2)](n)Mo(CO)(3) (n=1-3): Force field calculations versus NMR data

Tripod metal entities tripodM are sterically congested systems. The conform ations adopted by compounds CH3C(CH2PPh2)(3-n)[CH2P(oTol)(2)](n)Mo(CO)(3) ( n =1: 1, n = 2: 2, n = 3: 3) will thus be largely determined by the repulsi ve forces acting in these molecules. The steric demand of the o-tolyl group s impedes their free rotation and enantiomerization processes referring to the compounds as a whole are sufficiently slow to permit their analysis by NMR techniques. Through a combination of line-shape analysis, EXSY methods, and coalescence experiments, the Delta G double dagger values for these co nformational enantiomerization processes have been determined as Delta G do uble dagger(298K) = 54.3, 57.9, 65.5 kJ.mol(-1) for compounds 1, 2, and 3, respectively. By an exhaustive search on a force field generated hypersurfa ce, activation energies of 53, 57 and 69 kJ.mol(-1) have been calculated. T hus, the force field approach correctly reproduces the dependence of the ac tivation energy on the degree of o-tolyl substitution. Moreover, the force field simulation also gives an insight into the individual microsteps of th e enantiomerization pathways.