Bottom-bound transition states

Figure 7.19 (a) Back donation in the side-bound transition state using chelated catalyst, (b) Back donation in the bottom-bound transition state using chelated catalyst, (c) Back donation in bottom-bound transition state using non-chelated catalyst. Adapted from Ref [75]. 

The side-bound transition states were also found to be stabilized by greater d Jt back donation than that found for the bottom-bound transition states (Figure 7.19) [75]. In the preferred conformation, the chelating adamantyl group locks the orientation of the NHC and positions the NHC % orbital in the same plane as the ruthenium-aikyiidene bond. In the bottom-bound transition state, the aikyiidene was horizontal, and the n orbital of the aikyiidene was located in the same plane as the NHC it orbital. Thus, the same filled metal d orbital was found to be involved in back donations to the two, empty re orbitals. In the side-bound transition states, the aikyiidene re orbital was pointed out-of-plane to interact with another filled d orbital on ruthenium. 

Grubbs and Houk reported the computed energy barriers for the coupling of ethene with 59, which indicated a clear preference for the formation of the side-bound ruthenacyclobutane intermediate (Figure 4.5). The preference for the side-bound mechanism was attributed to a combination of steric and electronic factors. Since the alkylidene adopts a horizontal conformation in the bottom-bound transition states, steric repulsion of the alkylidene and... 

Often the adsorbed species are bound rather strongly and can be considered immobile at the bottom of a vibrational well. The transition state may, however, have several possibilities, being, for example, a precursor that is highly mobile in two... 

Figure 17.21 (part II) shows the 1 1 complex from (DHQD)2-PHAL and 0s04 together with the stereostructure, which is derived from the previous discussion, in the transition state of the asymmetric dihydroxylation. Here, the alkene nestles between the amine-complexed 0s04 on the one side and the methoxyquinoline residue on the other. The enantioselectivity of the dihydroxylation is the result of the alkene s preference to nestle in this niche with the orientation shown here. This orientation is characterized by the fact that no repulsion may occur between the alkene and the bottom of this niche, i.e., the central heterocycle of (DHQD)2-PHAL. This is the case if and only if the. s/r-bound hydrogen atom (as the smallest double bond substituent in the substrate) points in the direction of the central heterocycle. 

The small effective mass of unpaired atoms allows us to cool them to temperatures higher than that corresponding to the bottom of the diatomic band. The price is, however, that most of the atoms are discarded and only a small fraction of cl/periodic potential is a sparse-lattice analogy of the transition from Mott-insulator to a superfluid state in the fully occupied lattice, recently observed in Ref. [Greiner 2002],... 

In these catalysts, olefin approach occurs from the bottom-face of the catalyst (i.e., anti to the NHC) leading to a bottom-bound metallacyclobutane (Figure 14). DFT calculations using allylbenzene as a model substrate indicate that the transition state... 

Cavallo and coworkers [14] subsequently clarified the role of solvent effects in the side- and bottom-bound mechanisms. In the reaction of the second-generation methylidene with ethylene, the bottom-bound olefin complex was found to be more stable in the gas phase (by 3 kcalmol" ), while the side-bound complex was more stable in CH2CI2 (by 3kcalmol ). However, in the subsequent transition state for metallacyclobutane formation, the bottom-bound geometry was favored in both solution and the gas-phase (by 4 and 11 kcal mol , respectively). This was rationalized as being due to steric effects between the alkene and the mesityl group of the ligand, which can be avoided in the olefin complexes, but not in the transition state. 

Figure 7.18 Structures of the transition states in the side- and bottom-bound pathways. Adapted from Ref [75].

To study the effects of incorporating the anharmonic nature of the generalized normal modes transverse to the MEP on the vibrational partition function factor, Q° (T,s), in the generalized transition state partition function, Q (T,s), in eq. (4), we computed at the saddle point of surface 5SP from sets of either harmonic or anharmonic bound vibrational energy levels E /hc (in wave numbers) [176], where Vj,...,V5 are the vibrational quantum numbers and the energy is measured relative to the saddle point (i.e., from the bottom of the vibrational well). That is, we take... 

Fig. 31. Implications about the contractile mechanism in insect flight muscle. Blue is insect flight muscle SI shape in pre-powerstroke state (Al-Khayat et al., 2003), and green is chicken skeletal muscle SI in the rigor state with no nucleotide bound (Rayment et al., 1993a). The actin filament (right) is shown with the Z-band at the bottom and M-band at the top. A transition from the pre-powerstroke/resting SI shape to the rigor/end of post-powerstroke shape would involve an axial swing of the lever arm by 100 A, resulting in the sliding of the actin filaments past the myosin filaments and toward the M-band.

In theory, the lysocline records the sedimentary expression of the saturation horizon, that is the depth-dependent transition from waters oversaturated to waters undersaturated with respect to carbonate solubility (Figure 4). The lysocline thus marks the top of a depth zone, bounded at the bottom by the CCD, over which the bulk of carbonate dissolution in the ocean is expected to occur in response to saturation state-driven chemistry. The thickness of this sublysocline zone, as indicated by the vertical separation between the lysocline and CCD, is variable and is governed by the rate of carbonate supply, the actual dissolution gradient, and... 

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