RECENT PUBLICATIONS

(AVAILABLE FOR SNAIL MAIL DISTRIBUTION - Email: Harold.Schranz@anu.edu.au)

Harold W. Schranz and Michael A. Collins, Nonlinear resonance and torsional dynamics: Model simulations of HOOH and CH3OOCH3, J. Chem. Phys. 98, 1132-1148 (1993). PAPER (POSTSCRIPT)

Simple models of the vibrational dynamics of HOOH and CH3OOCH3 are investigated by classical trajectory methods. Nonlinear resonances due to kinematic coupling between the torsional motion and symmetric bond bending are found to have significant dynamical effects in some cases. The timescales and magnitudes of these energy transfer processes are examined.

Michael A. Collins and Harold W. Schranz, Quantum simulations of nonlinear resonance and torsional dynamics, J. Chem. Phys. 100, 2089-2103 (1994). PAPER (POSTSCRIPT)

A simple model of the vibrational dynamics of ABBA type sequentially bonded tetra-atomic molecules is investigated by quantum mechanical methods. The model Hamiltonian excludes bond stretching and asymmetric bending but includes the kinematic coupling between the torsional motion and symmetric bond bending which results in nonlinear resonances. The effect of this coupling on energy levels and the timescale of intramolecular energy transfer is evaluated and discussed in terms of both resonant and nonresonant effects.

Harold W. Schranz and Michael A. Collins, A model classical study of nonlinear resonance and torsional isomerisation, J. Chem. Phys. 101, 307-321 (1994). PAPER (POSTSCRIPT)

A simple two degree of freedom classical model of the nonlinear resonance interaction between symmetric bond bending and torsional motion in linearly bonded tetra-atomic molecules is investigated. The rate and mechanism of intramolecular vibrational energy redistribution at different relative vibrational frequencies is evaluated, and comparison is made with quantum calculations. The rate of torsional isomerisation is compared to the prediction of transition state theory, and related to the observed intramolecular vibrational energy redistribution.

Kim Bolton, Sture Nordholm, and Harold W. Schranz, The Fragmentation of One Dimensional Monatomic Chains under Tension - Simulation and Statistical Theory, Stuart Rice Festschrift, J. Phys. Chem. 99, 2477-2488 (1995). PAPER (POSTSCRIPT) | FIGURES (POSTSCRIPT) | PAPER (HTML)

A one dimensional monatomic chain under tensile stress has been studied by molecular dynamics. The aim is to determine the lifetime to chain breakage and to ascertain whether this event can be described by simple statistical reaction rate theory. Chains of two to twenty atoms have been simulated. A simple transition state theory, equivalent to a nucleation theory for one dimensional fluids, gives the main features of the decay rate coefficient as a function of the applied stress. For finite chain lengths an anharmonic RRKM theory provides a more accurate rate coefficient but chain healing (reversible decay) in the simulated motion causes a significant deviation particularly at high chain energy. The simulation is extended to chain arrays which show greatly increased nonstatistical effects.

Sture Nordholm and Harold W. Schranz, Collisional Energy Transfer in Unimolecular Reactions. Statistical Theory and Classical Simulation, in Advances in Chemical Kinetics and Dynamics, edited by J. R. Barker (JAI Press, Conn., 1995) Volume 2A, pp 245-281. PAPER & FIGURES (BINHEX) | PAPER(POSTSCRIPT)

A review is presented of our recent work on collisional energy transfer in the context of unimolecular reaction rate theory. The main aim is to find a collision frequency w and a conditional energy transfer probability density P(E',E) describing the transfer from energy E before to energy E' after the collision. The traditional strong collision assumption and the more recent ergodic collision theory are considered. Extensions obtained by introducing an assumption of impulsive collisions are noted. The collision frequency concept is found to differ in the statistical theories (hit or miss) and reality or simulations (continuous scale between hit and miss) requiring attention in comparisons of results. Results obtained in detailed classical trajectory simulations of small molecule collisions ( Ar + Br2, Br2 + Br2, Ar + SO2) are surveyed and discussed with an emphasis on angular momentum conservation and other nonergodic effects. The dependence of energy transfer efficiency on the hardness and capture strength of the intermolecular potential is also discussed.

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

An initial 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.

In order to facilitate a quantum study of the nonlinear resonance between the symmetric bend and torsion modes a reduced dimensional model was employed. The low dimensionality of the system also makes it amenable to the methods used commonly in the study of ergodic properties of nonlinear classical dynamical systems e.g. surfaces of section, phase space plots. The rate of torsional isomerization is compared to the prediction of Transition State Theory, and related to the observed intramolecular vibrational energy redistribution (IVR).

The dependence of the nonlinear resonance on the relevant kinematic terms in the Hamiltonian is clearly demonstrated in both the quantum and classical studies. Whereas the mechanisms for the nonlinear resonance is essentially the same, the exact frequency matching required, and strength and timescales of the resulting energy transfer can be significantly different. The extent to which classical studies of IVR can be used to make quantitative predictions will be discussed.

Harold W. Schranz and Michael A. Collins, Intramolecular Vibrational Energy Redistribution and Torsional Isomerization: A Model Classical and Quantum Study, in Ultrafast Chemical and Physical Processes in Molecular Systems, Proceedings of Femtochemistry: The Lausanne Conference, (M. Chergui ed.) World Scientific, Singapore, 1996, pp 206-209. PAPER (POSTSCRIPT)

The nonlinear resonant interaction resulting from kinematic coupling between the torsion mode and other modes in sequentially bonded ABBA type tetra-atomic molecules is considered and found to most likely involve the symmetric bending mode. The rate of torsional isomerization is related to the observed extent of intramolecular vibrational energy redistribution (IVR).

Harold W. Schranz, Thomas D. Sewell and Sture Nordholm, Statistical and Dynamical Behaviour in the Isomerisation of Methyl Isocyanide, in Ultrafast Chemical and Physical Processes in Molecular Systems, Proceedings of Femtochemistry: The Lausanne Conference, (M. Chergui ed.) World Scientific, Singapore, 1996, pp 202-205. PAPER (POSTSCRIPT)

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 given strong indications of non-statistical and mode specific behaviour. The aim of our more detailed theoretical study is to determine the role of statistical and nonstatistical behaviour in the subsequent reaction dynamics of locally and microcanonically excited CH3NC molecules.

A simulation method for the estimation of anharmonic densities of states of classical molecular models is described. 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.

Harold W. Schranz and Thomas D. Sewell, Statistical and Dynamical Behaviour in the Unimolecular Reaction Dynamics of Polyatomic Molecules, THEOCHEM, 368 (1996) 125-131.
PAPER (POSTSCRIPT) | FIGS. (POSTSCRIPT) | PAPER (PDF) | Elsevier WWW Site (HTML)

The 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.

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, the presence of a fast IVR rate, as derived from some experiments, does not automatically ensure the reaction dynamics will be statistical. In fact, 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.

In the present paper, we investigate results of recent statistical and dynamical calculations performed on identical potential energy surfaces for a range of polyatomic molecules. Our ultimate goal is to determine how the extent and timescale of IVR plays a role in determining the statistical or nonstatistical behaviour in the subsequent unimolecular reaction dynamics of locally and microcanonically excited polyatomic molecules.

Harold W. Schranz, Mode to Mode Energy Flow Amongst the Ring Modes of Benzene, THEOCHEM, 368 (1996) 119-124.
PAPER (POSTSCRIPT) | FIGS. (POSTSCRIPT) | PAPER (PDF) | Elsevier WWW Site (HTML)

How energy moves around the vibrational modes of a polyatomic molecule, such as benzene, is of considerable interest, both spectroscopically and theoretically. Much effort has been devoted to observing and understanding how energy is transferred out of initially excited overtone states involving the CH stretching modes. Significantly less study has been devoted to an understanding of how and on what timescale energy is transferred amongst the ring modes. Such low frequency modes dominate the vibrational state density and also play a role in the irreversible nature of the decay out of C-H modes.

The motivation for the present classical trajectory study, is to model the results of 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 the results of our trajectory calculations which reveal an initially rapid decay followed by slow IVR at longer times.

Terry J. Frankcombe, Rob Stranger and Harold W. Schranz, The intermolecular potential energy surface of CO₂-Ar and its effect on collisional energy transfer Internet Journal of Chemistry, 1 (1998) 12. PAPER (POSTSCRIPT) | ABSTRACT (PDF) | PAPER (PDF) | Paper (HTML) Paper (Internet J. Chem.)

Classical molecular dynamics (MD) is a useful way of studying intra- and inter-molecular energy transfer in gas-phase collisional processes. However to perform these MD simulations a global analytic potential energy surface (PES), which is not directly experimentally observable, is required to determine the forces on the constituent atoms of the colliding species. Often a functional form of the PES is assumed and the parameters adjusted until the surface reproduces a set of experimental observables.

We have used both conventional ab initio methods and density functional theory methods to generate points on the PES for the carbon dioxide-argon colliding system at different levels of theory. To the intermolecular part of these potential profiles we have fit a variety of simple functional forms. We used MD simulations to look at the effect of the subtle differences in shape of the PES on the average energy transfered per collision (<E>) and higher energy transfer moments (such as <E²>). There were found to be significant quantitative differences between the energy transfer moments generated using the different functional forms, even when these forms were fit reasonably well to the same data.

M.A. Bennett, M. Bown, D.C.R. Hockless, J.E. McGrady, H.W. Schranz, R. Stranger, and A.C. Willis, Dissociative and non-dissociative pathways in the endo to exo isomerization of tetramethyl-o-xylylene complexes of ruthenium and osmium, ML₃{h⁴-o-C₆Me₄(CH₂)₂} {M=Ru, L= PMe₃; M=Os, L=PMe₃, PMe₂Ph}. Formation of Hexamethylbenzene-1,2-diyl complexes by ligand addition to the exo- osmium complex, Organometallic 17 (1998) 3784-3797. Abstract | HTML | PDF | Supporting Information

Treatment of the (eta(6)-hexamethylbenzene)ruthenium(II) and -osmium(II) salts [M(O-2-CCF3)L-2(eta(6)-C6Me6)]PF6 (M = Ru, L = PMe3; M = Os, L = PMe3, PMe2Ph) in the presence of L with KO-t-Bu gives exclusively the endo- (tetramethyl-o-xylylene)metal(0) complexes ML3{eta(4)-endo-o-C6Me4(CH2)(2)}, endo-1, -2, and -3, respectively, in high yield; these-are protonated by an excess of triflic acid (CF3SO3H, TfOH) to give the (eta(6)-hexamethylbenzene)metal(II) salts [ML3(eta(6)-C6Me6)](OTf)(2) [M = Ru, L = PMe3 (4); M = Os, L = PMe3 (5); M = Os, L = PMe2Ph (6)). Complexes 4-6 revert to endo-1-3 on treatment with. KO-t-Bu, whereas for M=Ru, L=PMe2Ph the complexes [ML3(eta(6)-C6Me6)](2+) and [M(O2CCF3)L-2(eta(6)-C6-Me6)](+)/L react with KO-t-Bu to give exclusively the exo isomer, Ru(PMe2Ph)(3){eta(4)-exo-o-(CH2)(2)C6Me4} (exo-7). The endo complexes 1-3 are converted quantitatively into the corresponding exo isomers in toluene in the temperature range 65-106 degrees C, the process being first order in endo complex. Kinetics studies in the presence of PMe3 (for 1 and 2) or PMe2-Ph (for 3) indicate that two pathways are available: one depends on initial dissociation of L and proceeds through a bis(ligand) intermediate or intermediates, e.g., ML2{endo-o-C6-Me4(CH2)(2)} and ML2{exo-o-(CH2)(2)C6Me4}, and the other does not. The dissociative mechanism is predominant for M = Ru, L = PMe3, whereas the nondissociative or direct mechanism plays the dominant, possibly exclusive, role for M = Os, L = PMe3. The osmium(0) compound exo-2 adds PMe3 irreversibly to give the sigma-bonded (hexamethylbenzene-1,2-diyl)osmium(II) complex Os(PMe3)(4){kappa(2)-o-(CH2)(2)C6Me4} (8), whereas the corresponding PMe2Ph derivative 9 is in equilibrium with exo-3 and PMe2Ph and cannot be isolated; the ruthenium(0) compound exo-1 is inert toward PMe3. Density functional calculations on the model compounds ML3-{eta(4)-exo-o-(CH2)(2)C6H4} and ML4{kappa(2)-o-(CH2)(2)C6H4}(M = RU, Os; L = PH3) correctly reflect the observed stability order Os > Ru for the diyl complex but predict the latter to be more stable than the eta(4) complex far both elements. In this case, the usual computational simplification of replacing a tertiary phosphine by PH3 is probably unjustified, The molecular structures of the eta(4) complexes endo-3, exo-3, and exo-1 and of the diyl complex 8 have been determined by X-ray crystallography. The endo- to exo-o-xylylene isomerizations are compared with the intramolecular migrations that occur in Fe(CO)(3)(eta(4)-polyene) and Cr(CO)(3)(eta(6)-substituted-naphthalene) complexes.

H.W. Schranz, Chemistry and the World Wide Web, Chemistry in Australia, December 1998, pp. 9-11. PAPER (PDF) | Paper (HTML)

A brief overview of a few of the many current and potential uses of the WWW in Chemistry. The focus is on selected examples of useful chemical WWW sites, the emergence of electronic journals and electronic conferences. Some useful paper-based references and WWW links are given.

Harold W. Schranz, Harold W. Schranz, Sean C. Smith, Alexander M. Mebel and Sheng H. Lin, Prediction of absolute rate coefficients and product branching ratios for the C(3P) + allene reaction system, J. Chem. Phys. 117(2002) 7055-7067.
PAPER PREPRINT (PDF) | PAPER PROOF (PDF) | J. Chem. Phys. 117(2002) | PAPER (JCP)

Complex chemical reactions in the gas phase can be decomposed into a network of elementary e.g., unimolecular and bimolecular! steps which may involve multiple reactant channels, multiple intermediates, and multiple products. The modeling of such reactions involves describing the molecular species and their transformation by reaction at a detailed level. Here we focus on a detailed modeling of the C(3P)+allene (C3H4) reaction, for which molecular beam experiments and theoretical calculations have previously been performed. In our previous calculations, product branching ratios for a nonrotating isomerizing unimolecular system were predicted. We extend the previous calculations to predict absolute unimolecular rate coefficients and branching ratios using microcanonical variational transition state theory (m-VTST) with full energy and angular momentum resolution. Our calculation of the initial capture rate is facilitated by systematic ab initio potential energy surface calculations that describe the interaction potential between carbon and allene as a function of the angle of attack. Furthermore, the chemical kinetic scheme is enhanced to explicitly treat the entrance channels in terms of a predicted overall input flux and also to allow for the possibility of redissociation via the entrance channels. Thus, the computation of total bimolecular reaction rates and partial capture rates is now possible.

(AVAILABLE FOR SNAIL MAIL DISTRIBUTION - Email: Harold.Schranz@anu.edu.au)

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Last revised Fri Oct 11 11:20:49 EST 2002 - Harold W. Schranz Email: Harold.Schranz@anu.edu.au