Intramolecular energy transfer and the driving mechanismsfor large-amplitude collective motions of clusters

摘要

This paper uncovers novel and specific dynamical mechanisms that initiate large-amplitudecollective motions in polyatomic molecules. These mechanisms are understood in terms ofintramolecular energy transfer between modes and driving forces. Structural transition dynamics ofa six-atom cluster between a symmetric and an elongated isomer is highlighted as an illustrativeexample of what is a general message. First, we introduce a general method of hyperspherical modeanalysis to analyze the energy transfer among internal modes of polyatomic molecules. In thismethod, the (3n – 6) internal modes of an n-atom molecule are classified generally into three coarselevel gyration-radius modes, three fine level twisting modes, and (3n –12) fine level shearingmodes. We show that a large amount of kinetic energy flows into the gyration-radius modes whenthe cluster undergoes structural transitions by changing its mass distribution. Based on this fact, weconstruct a reactive mode as a linear combination of the three gyration-radius modes. It is shownthat before the reactive mode acquires a large amount of kinetic energy, activation or inactivation ofthe twisting modes, depending on the geometry of the isomer, plays crucial roles for the onset of astructural transition. Specifically, in a symmetric isomer with a spherical mass distribution,activation of specific twisting modes drives the structural transition into an elongated isomer byinducing a strong internal centrifugal force, which has the effect of elongating the mass distributionof the system. On the other hand, in an elongated isomer, inactivation of specific twisting modesinitiates the structural transition into a symmetric isomer with lower potential energy by suppressingthe elongation effect of the internal centrifugal force and making the effects of the potential forcedominant. This driving mechanism for reactions as well as the present method of hypersphericalmode analysis should be widely applicable to molecular reactions in which a system changes itsoverall mass distribution in a significant way.