Intramolecular Vibrational Energy Redistribution and Torsional Isomerization: A Model Classical and Quantum Study

by

Harold W. Schranz

Research School of Chemistry, ANU

Motivation

Current Study

How Do We Get Nonlinear Resonance?

Full Dimensional Classical Study

Reduced Dimensional Quantum and Classical Study

Nonlinear Resonance and Torsional Isomerization

Concluding Remarks

Acknowledgements

• How energy moves amongst the vibrational modes of a molecule, a process known as intramolecular vibrational energy redistribution (IVR), is of great significance.

  
  
  

• IVR is crucial in determining the rates and mode specificity of chemical reactions.

  
  
  

• Rapid (subps) IVR facilitated by the Fermi resonance of X-H stretching modes with bond bending modes has been well studied (experimental/theoretical).

• e.g. torsion modes may have overtone states close in energy to the fundamentals, overtones or combination levels of the remaining vibrational modes (e.g. bends).

  
  

• Nonlinear resonance involving torsion is important in understanding the simplest type of chemical reaction, conformational isomerisation.

  
  

• The very large amplitude of torsional motion and the periodic character of relevant terms in the Hamiltonian suggests there may be some novel dynamical effects.

• Classical and quantum simulations of models of HOOH and CH3OOCH3 (united methyl groups).

• Simplest molecules which allow torsion.

• ABBA molecules have symmetry which restrict the number of interactions.

• Probe the importance of nonlinear resonance interactions between the torsion and higher frequency modes.

• Study 2 : 1 (Fermi resonance), 3 : 1, 4 : 1, and combination resonances.

• The hydrogen and methyl substituents illustrate important mass effects.

There is a close correspondence of the full dimensional classical ABBA system with the classical and quantum reduced dimensional systems for the case of a 2:1 resonance.

The 2:1 resonant interaction produces IVR between the torsion and symmetric bend on a very fast subpicosecond time scale, even with relatively poor frequency matching.

Weaker IVR can occur by 4:1 processes on a much longer timescale.

The facile 2:1 energy transfer pathway involving the torsion and symmetric bend and torsion has been shown to directly influence the dynamics of the isomerization reaction.

The results of this study are relevant to a number of isomerization reactions where the reaction coordinate is essentially a torsion.

For example, the much studied photoisomerizations of cis- and trans-stilbene and their substituted analogs involve an ethylenic torsion as a primary component of the reaction coordinate.

The interaction of this coordinate with other relatively low-frequency coordinates, such as the symmetric bend, symmetric phenyl twist and the out-of-plane phenyl bend, could determine the nature of the reaction dynamics.

The author gratefully acknowledges:

Mick Collins, Research School of Chemistry,
Australian National University

Roger Edberg,
Australian National University Supercomputer Facility