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Potential energy profile , olefin

The potential energy profile is smooth without excessive barriers and too stable intermediates which would break the sequence of steps. The rate-determining step is found to be olefin insertion followed by isomerization, supporting the Halpern mechanism. Isomerization of the ethyl hydride complex is an important part of the rate-determining step. These two reactions, exothermic overall, has an overall barrier height of about 20 kcal/mol. The trans ethyl hydride complex, the product of ethylene insertion, may not be a local minimum (per MP2 calculation) and these two steps may well be a combined single step. [Pg.81]

Figure 1. Potential energy profile of the entire catalytic cycle in the Halpern mechanism for olefin hydrogenation, in kcal/mol at the RHF level, relative to l+C H +H. Numbers in parentheses are the MP2 energy at the RHF optimized geometries, relative to 4. Figure 1. Potential energy profile of the entire catalytic cycle in the Halpern mechanism for olefin hydrogenation, in kcal/mol at the RHF level, relative to l+C H +H. Numbers in parentheses are the MP2 energy at the RHF optimized geometries, relative to 4.
In this work, we have compared the potential energy profiles of the model catalytic cycle of olefin hydrogenation by the Wilkinson catalyst between the Halpern and the Brown mechanisms. The former is a well-accepted mechanism in which all the intermediates have trans phosphines, while in the latter, proposed very recently, phosphines are located cis to each other to reduce the steric repulsion between bulky olefin and phosphines. Our ab initio calculations on a sterically unhindered model catalytic cycle have shown that the profile for the Halpern mechanism is smooth without too stable intermediates and too high activation barrier. On the other hand, the key cis dihydride intermediate in the cis mechanism is electronically unstable and normally the sequence of elementary reactions would be broken. Possible sequences of reactions can be proposed from our calculation, if one assumes that steric effects of bulky olefin substituents prohibits some intermediates or reactions to be realized. [Pg.91]

Zhu J, Fogarty HA, Mollerstedt H, Brink M, Ottosson H (2013) Aromaticity effects on the profiles of the lowest triplet-state potential-energy surfaces for rotation about the C=C bonds of olefins with five-membered ring substituents an example of the Impact of Baird s rule. Chem Eur J 19 10698-10707... [Pg.334]


See other pages where Potential energy profile , olefin is mentioned: [Pg.205]    [Pg.77]    [Pg.81]    [Pg.39]    [Pg.119]    [Pg.21]    [Pg.367]    [Pg.71]    [Pg.72]    [Pg.1126]    [Pg.249]    [Pg.238]    [Pg.110]    [Pg.44]    [Pg.75]    [Pg.283]   


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