Activation strain model

Figure 10.36 Activation strain model analysis of the CH, dissociation on the Rh(lll) surface. Interaction energy as a function of reaction coordinate (black line). of the CH,...

We observe the initial bond order of the CH bond that is cleaved to be close to one. It decreases to 0.29 in the transition state. This implies a more substantial weakening in contrast to the assumption of the activation strain model. The M-H and M-C bond orders are, respectively, 0.38 and 0.43, to be compared with the final state values 0.42 and 0.54, respectively. Compared to the initial CH bond interaction in the transition state, the CH bond interaction has been substantially weakened. One also finds that the M-C and M-H interactions are already close to those of the final state. The bonding interactions are distributed in a close to even distribution over the three bonds of H-C, C-Rh, and H-Rh. In the final state, the dissociating H atom moves to a twofold adsorption site. The closeness of the bondorders of the C-Rh bond and one of H-Rh bonds to those in the final state illustrates the late transition state nature of CH bond activation. 

Keywords Activation strain model Bond activation Bond theory Catalysis Density functional calculations Halogenated phosphine ligands Ligands Transition metal complexes... 

The activation strain model of chemical reactivity [16-19] is a fragment-based approach to tmderstand (trends in) chemical reactivity, in terms of the intrinsic properties of reference fragments. Due to its fragment-based nature, the model is most often applied to bimolecular processes, such as oxidative additions [24] as in this work, but also SN2-reactions [16, 25, 26], pericyclic reactions [27], and even barrier-free bond formations, such as hydrogen or halogen bonds [28]. However, also unimolecular processes are successfully studied using the activation strain... 

Diefenbach A, De Jong GT, Bickelhaupt FM (2005) Fragment-oriented design of catalysts based on the activation strain model. Mol Phys 103 995... 

De Jong GT, Bickelhaupt FM (2007) Transition-state energy and position along the reaction coordinate in an extended activation strain model. ChemPhysChem 8 1170... 

Van Zeist W-J, Bickelhaupt FM (2010) The activation strain model of chemical reactivity. Org... 

This and subsequent papers by Gorelsky [37] employed the activation strain model [38] to analyze trends in the CMD activation barriers (see Figure 1.14 for C-H activation of benzene). In this approach, the activation energy, for... 

Figure 1.14 The activation strain model illustrated for C-H activation of benzene at Pd(Ph)(K -OAc)(PMej) with component energies indicated in kcal mol [37a].

Ess, Periana, Goddard, and coworkers subsequently compared four-membered and six-membered C-H activation transition states in a B3LYP study on the C-H activation of benzene at Ir(acac)2(X) species (X = OH, OAc) [66]. An activation strain model was used to compare the performance of hydroxide and acetate, where the latter could access both four-membered and six-membered transition states, depending on whether the proton transfers onto the Ir-bound or pendant oxygen, respectively. C-H activation via a six-membered process is favored and is associated with a lower term compared to the more constrained four-membered transition states. The acetate and hydroxide four-membered transition states are very similar. 

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