Use of microclusters to simulate cage, trapping, and chaperon effects in association reactions

Xiche Hu, William L. Hase

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Association reactions of HArn (n = 4, 12, 13) microclusters with CH3 to form CH4 are used to study microscopic solvation dynamics. Potential energy surfaces for the CH3 + HArn systems are constructed from a previously fitted H + CH3 ab initio potential and Ar-Ar, Ar-C, and Ar-H Lennard-Jones interactions. Classical trajectory and reaction path calculations for the CH3 + HArn systems illustrate the relationship between the structure of a microcluster and its chemical reactivity. Solvating the H atom with Arn is found to have the following three important effects on the association dynamics: (1) caging, a steric effect responsible for attenuating the association reactivity; (2) trapping, the Arn shell creates a van der Waals well in which low relative translational energy collisions are trapped for long times; and (3) chaperon, a vibrationally/rotationally hot CH4 is formed as H associates with CH3 and Arn acts as an energy sink to stabilize the excited CH4. For CH3 reaction with a Ar6(H)Ar6 cluster, which has a H atom in the interior of an icosahedral Ar12 shell, a van der Waals well exists on the reaction path in which CH3 is physisorbed on the surface of the Ar12 shell. Both reaction path and classical trajectory calculations show that relaxation of this solvation shell structure is necessary for CH4 formation to occur. Trapping in the van der Waals well enhances CH4 formation by providing a sufficiently long interaction time for relaxation of the Ar12 shell structure. If the H atom is sitting on the surface of the Arn moiety this relaxation is not necessary, since reaction can occur by both direct and hopping mechanisms.

Original languageEnglish
Pages (from-to)7535-7546
Number of pages12
JournalJournal of physical chemistry
Issue number19
StatePublished - 1992


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