Principal Investigator Harold W Schranz Project s10

Research School of Chemistry Machine VP


Efficient Calculation of Statistical and Dynamical Reaction Rates for Large Dimensional Molecular Systems

Knowledge of how quickly chemical reactions occur is an essential ingredient in the rigorous modelling of combustion, industrial and atmospheric reaction systems. This project focuses on the development of new methods for the accurate prediction of rate constants for chemical reaction. Crucial to this development is a more complete understanding of how and on what timescale energy moves about a molecule.

The current dominant theories of unimolecular reaction are statistical. A fundamental assumption is that the timescale on which energy moves about a reactant molecule is much shorter than the timescale for reaction. It is assumed that intramolecular vibrational energy redistribution (IVR) is globally rapid throughout the molecular phase space (Fig. 1). Further assumptions are made in the application of such statistical theories. Traditionally, reactant molecules are modelled as a set of separable harmonic oscillators and rigid rotors. Angular momentum conservation is often neglected or approximated.






Figure 1. Statistical evolution of trajectories through molecular phase space for a unimolecular reaction

It has been widely thought that the assumption of rapid IVR referred to above is valid for sufficiently large polyatomics. Much of the supporting evidence for this view comes from indirect experimental studies of IVR and comparisons of statistical and dynamical calculations. However, in recent studies, we have shown that even in the presence of fast IVR rates between some modes the reaction dynamics can be extremely nonstatistical. Secondly, most comparisons of statistical and dynamical calculations have made simplifying assumptions which render the comparisons ambiguous. Frequently, the potential energy surface used for the dynamical calculations is approximated by a normal mode analysis for the statistical calculations. In addition, commonly used initial state selection procedures used for the dynamical calculations often cause artifacts such as short time transients which need to be deconvoluted from the true dynamical behaviour.

It is apparent that in order to clearly identify the presence or absence of statistical behaviour in a chemical reaction it is necessary to compare statistical and dynamical calculations performed for exactly the same model under the same conditions.


What are the basic questions addressed?

How to calculate a rate constant for reaction?

When are statistical theories valid?

The role of statistical and dynamical behaviour in chemical reactions?

How to take account of non-statistical effects in a general theory?

Can we take advantage of non-statistical effects e.g. mode specific excitations of reactants causing product rate and yield enhancement?

How and on what timescale does energy move around within a molecule?

What are the results to date and future of the work?

Significant progress has been made on a number of related research topics: firstly, how and how fast energy moves around an isolated molecule; secondly, how efficiently energy moves between molecules; and thirdly, how the process of energy transfer influences the subsequent dynamics of chemical reaction. A further topic involves the efficient calculation of certain statistical mechanical quantities used in current statistical rate theories.

How energy moves around a molecule

An initial classical study considered the nonlinear resonant interaction resulting from kinematic coupling between the torsion mode and other modes in sequentially bonded ABBA type tetra-atomic molecules. It was found that the nonlinear resonant interactions were most likely to involve the symmetric bending mode. This finding stimulated our later detailed quantum and classical dynamical studies which were facilitated by employing a reduced dimensional model (Fig. 2). Furthermore, the observed rate of torsional isomerisation is compared to the predictions of Transition State Theory. The importance of statistical or dynamical behaviour is thus related to the observed extent of intramolecular vibrational energy redistribution (IVR).

Figure 2. Wavepacket time evolution an initial excited (20 quanta) torsion state into an excited symmetric bend state for a quantum two-mode model. (a) t=0.0 ps; (b) t=0.05 ps; (c) t=0.1 ps; (d) t=0.3 ps.

These studies are being extended to larger molecular systems. A potential energy surface for benzene which incorporates the dominant potential couplings was constructed on the basis of ab initio data. The surface was employed for a full dimensional classical mechanical molecular dynamics study of IVR. Comparisons were made with recent experimental observations regarding the extent and timescale of IVR involving the ring modes. The linewidths found experimentally were instrument limited at 1 cm-1 for a range of excited ring modes for excitations of between 1200 and 8200 cm-1 yielding an upper limit on the IVR rate of 0.094 ps-1. This result is consistent with preliminary trajectory calculations which reveal an initially rapid decay followed by slow IVR at longer times.

How energy moves between molecules

In gas-phase unimolecular reactions, intramolecular energy transfer in isolated molecules competes with intermolecular energy transfer between molecules. The extent to which either energy transfer process can be described by statistical theories is dependent on the extent of intra- and intermolecular coupling present. The role of such couplings in determining the nature and extent of energy transfer was examined in a model study of arrays of one dimensional monatomic chains under tensile stress. It was noted that chain arrays (where chains are only weakly coupled to each other) show greatly increased nonstatistical effects.

Intermolecular (collisional) energy transfer was reviewed in the context of unimolecular reaction rate theory. The concept of a collision is found to differ between statistical theories and reality or simulations; some care is required in comparisons. Results obtained in detailed classical trajectory simulations of small molecule collisions were surveyed and discussed with an emphasis on nonergodic effects (e.g. angular momentum conservation) and the dependence of energy transfer on the hardness and capture strength of the intermolecular potential.

How energy transfer influences the subsequent reaction dynamics

A classic prototype reaction in the study of unimolecular reactions is the isomerisation of methyl isocyanide. Most experimental studies have given rate constants consistent with statistical behaviour even though trajectory studies have predicted non-statistical and mode specific behaviour. Our more detailed theoretical study has indicated that the nature of the potential energy surface and the type of initial excitation can affect the extent of IVR and hence the subsequent reaction dynamics (Fig. 3).

Fig. 3. Statistical (TST) and dynamical (trajectory) calculations of the rate of isomerisation of CH3NC compared with experimental results.

In summary, some detailed insights are being gained into how the process of (intramolecular and intermolecular) energy transfer depends on the nature of the molecular system and how this behaviour can influence the (statistical versus dynamical) nature of the chemical reaction.

New method for calculating anharmonic densities of states

A key element in the application of statistical theories of chemical reaction is the calculation of statistical quantities such as the density of states for a reactant molecule. In order to faciliate the application of statistical theories, we developed a simulation method for the estimation of anharmonic densities of states of classical molecular models. The method is based on the equilibrium energy distribution established in an uncoupled dimer of the anharmonic molecule and a reference molecule whose density of states is known analytically as a function of energy. Applications to one-dimensional chain molecules and small clusters of atoms joined by Morse bonds indicate that the method is both simple and reliable.

What computational techniques are used and why is a supercomputer required?

For the classical dynamical simulations, large numbers of trajectories need to be generated by integrating the classical equations of motion. In the statistical studies, long Markov walks need to be performed in order to properly sample the high dimensional phase space of the reactant molecule. Both types of calculations are numericallly intensive, the former requiring large amounts of CPU time but vectorizes well. A further type of calculation performed are quantum dynamical simulations of energy transfer. The current method does not vectorize well, due to the highly complex nature of the matrix elements, but use of a supercomputer is essential as memory requirements are larger than available on workstations at the RSC.

Publications

Collisional Energy Transfer in Unimolecular Reactions. Statistical Theory and Classical Simulation, S. Nordholm and H.W. Schranz, in Advances in Chemical Kinetics and Dynamics, (John R. Barker ed.), JAI Press, 1995, Vol. 2A, pp. 245-281.

The Fragmentation of One Dimensional Monatomic Chains under Tension - Simulation and Statistical Theory, K. Bolton, S. Nordholm, and H. W. Schranz, Stuart Rice Festschrift, J. Phys. Chem., 99, 2477-2488 (1995).

Intramolecular Vibrational Energy Redistribution and Torsional Isomerization: A Model Classical and Quantum Study, H. W. Schranz and Michael A. Collins, in Proceedings of the First Electronic Computational Chemistry Conference, edited S. Bachrach, CD-ROM version, (ARInternet , Landover MD, 1995).

On the Estimation of Anharmonic Densities of States of Molecules, L. Ming, S. Nordholm, and H. W. Schranz, Chem. Phys. Lett. submitted