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33846639687
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The fact that such catalysis leads to an increased barrier in most cases of course implies that in reality the reaction is not likely to utilize the catalyst in this way but would proceed either in an uncatalyzed manner or with the catalyst in almost a spectator role. Nonetheless, the lower barriers observed in some cases demonstrate the subtleties in such catalytic reactions. The purpose of our calculations is to illustrate what happens in a constrained situation so as to indicate the fundamental principles at work and, thus, provide insights into the design of potential catalysts for such reactions
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The fact that such catalysis leads to an increased barrier in most cases of course implies that in reality the reaction is not likely to utilize the catalyst in this way but would proceed either in an uncatalyzed manner or with the "catalyst" in almost a spectator role. Nonetheless, the lower barriers observed in some cases demonstrate the subtleties in such catalytic reactions. The purpose of our calculations is to illustrate what happens in a constrained situation so as to indicate the fundamental principles at work and, thus, provide insights into the design of potential catalysts for such reactions.
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42
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0004194783
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Oxford University Press: Oxford
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(c) Fleming, I. Pericyclic Reactions; Oxford University Press: Oxford, 1999.
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Pericyclic Reactions
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Fleming, I.1
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45
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33846595290
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In a simplified treatment, the 1,2-hydrogenation may be regarded as a [2, 2]-type reaction, which is also symmetry forbidden
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In a simplified treatment, the 1,2-hydrogenation may be regarded as a [2 + 2]-type reaction, which is also symmetry forbidden.
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46
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33846608724
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-1, respectively.
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-1, respectively.
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47
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33846594371
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298 values of benzene, 1,3-cyclohexadiene, and 1,4-cyclohexadiene from the NIST Chemistry Webbook (Linstrom P. J., Mallard W. G., Eds. NIST Chemistry WebBook, NIST Standard Reference Database Number 69; National Institute of Standards and Technology: Gaithersburg, MD 20899, June 2005 (http://webbook.nist.gov)).
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298 values of benzene, 1,3-cyclohexadiene, and 1,4-cyclohexadiene from the NIST Chemistry Webbook (Linstrom P. J., Mallard W. G., Eds. NIST Chemistry WebBook, NIST Standard Reference Database Number 69; National Institute of Standards and Technology: Gaithersburg, MD 20899, June 2005 (http://webbook.nist.gov)).
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49
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33846639238
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We find that this effect is quite general: an additional molecule of H2 substantially lowers the barrier for the uncatalyzed 1,2-hydrogenation of ethene and several other unsaturated substrates
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2 substantially lowers the barrier for the uncatalyzed 1,2-hydrogenation of ethene and several other unsaturated substrates.
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-
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50
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33846647515
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We have carried out additional calculations on structures corresponding to HF complexed to the benzene or H2 moieties of the TS of Figure 8, and the derived barrier is not significantly affected by the presence of HF; i.e, the HF molecule is effectively a spectator in these cases. The results in Figure 7 demonstrate the principles at work when the acid catalysts are more actively involved
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2 moieties of the TS of Figure 8, and the derived barrier is not significantly affected by the presence of HF; i.e., the HF molecule is effectively a spectator in these cases. The results in Figure 7 demonstrate the principles at work when the acid catalysts are more actively involved.
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51
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33846586491
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2 moieties of the uncatalyzed TS of Figure 9 show that the barrier (measured from the reactant complex) is only very slightly lowered from that for the uncatalyzed reaction.
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2 moieties of the uncatalyzed TS of Figure 9 show that the barrier (measured from the reactant complex) is only very slightly lowered from that for the uncatalyzed reaction.
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52
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33846634695
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All proton affinities were computed at the G3(MP2)-RAD level and are taken from ref 8d with the exception of HBr, for which the proton affinity was computed as part of the present study.
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All proton affinities were computed at the G3(MP2)-RAD level and are taken from ref 8d with the exception of HBr, for which the proton affinity was computed as part of the present study.
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