TY - JOUR
T1 - Use of microclusters to simulate cage, trapping, and chaperon effects in association reactions
AU - Hu, Xiche
AU - Hase, William L.
PY - 1992
Y1 - 1992
N2 - 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.
AB - 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.
UR - http://www.scopus.com/inward/record.url?scp=0008894123&partnerID=8YFLogxK
U2 - 10.1021/j100198a012
DO - 10.1021/j100198a012
M3 - Article
AN - SCOPUS:0008894123
SN - 0022-3654
VL - 96
SP - 7535
EP - 7546
JO - Journal of physical chemistry
JF - Journal of physical chemistry
IS - 19
ER -