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Direct chemical dynamics simulations: coupling of classical and quasiclassical trajectories with electronic structure theory

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In classical and quasiclassical trajectory chemical dynamics simulations, the atomistic dynamics of collisions, chemical reactions, and energy transfer are studied by solving the classical equations of motion. These equations require the potential energy and its gradient for the chemical system under study, and they may be obtained directly from an electronic structure theory. This article reviews such direct dynamics simulations. The accuracy of classical chemical dynamics is considered, with simulations highlighted for the F + CH3OOH reaction and of energy transfer in collisions of CO2 with a perfluorinated self‐assembled monolayer (F‐SAM) surface. Procedures for interfacing chemical dynamics and electronic structure theory computer codes are discussed. A Hessian‐based predictor–corrector algorithm and high‐accuracy Hessian updating algorithm, for enhancing the efficiency of direct dynamics simulations, are described. In these simulations, an ensemble of trajectories is calculated which represents the experimental and chemical system under study. Algorithms are described for selecting the appropriate initial conditions for bimolecular and unimolecular reactions, gas‐surface collisions, and initializing trajectories at transition states and conical intersections. Illustrative direct dynamics simulations are presented for the Cl + CH3I SN2 reaction, unimolecular decomposition of the epoxy resin constituent CH3NHCHCHCH3 versus temperature, collisions and reactions of N‐protonated diglycine with a F‐SAM surface that has a reactive head group, and the product energy partitioning for the post‐transition state dynamics of C2H5F → HF + C2H4 dissociation. © 2012 John Wiley & Sons, Ltd.

This article is categorized under:

  • Theoretical and Physical Chemistry > Reaction Dynamics and Kinetics
Figure 1.

Classical and quantum harmonic RRKM rate constants for the unimolecular dissociation of C2H5 → H + C2H4. Solid and dashed lines show classical and quantum state countings, respectively. The rate constants are in unit of s−1 and energy is in kcal/mol. (Reproduced with permission from Ref 29. Copyright 1982, John Wiley & Sons, Ltd.)

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Figure 2.

Comparison of quantum survival probability (solid line) with classical result (dashed line) for overtone excitation of benzene. (Reproduced with permission from Ref 31. Copyright 1992, American Institute of Physics.)

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Figure 3.

The reaction coordinate potential energy curve for the F + CH3OOH reaction. s is the distance along the intrinsic reaction coordinate. (Reproduced with permission from Ref 37. Copyright 2007, American Chemical Society.)

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Figure 4.

Distributions, P(J), of the CO2 rotational quantum number resulting from collisions of 15 K CO2 with a 300 K perfluorinated hydrocarbon surface. (Reproduced with permission from Ref 38. Copyright 2007, American Chemical Society.)

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Figure 5.

An illustration of the (k − 1)th step and the kth step of the Hessian‐based predictor–corrector integrator, where the dashed curves indicate predicted trajectories and the solid curves are corrected trajectories.

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Figure 6.

Total energies of the Cl + CH3I chemical reaction system for simulations of 140 fs. Time step = 1.0 fs and K = 12. The units for the horizontal and vertical axes, respectively, are fs and kcal/mol. In the figure, pCFD–Bofill denotes the perturbed compact finite difference (CFD)–Bofill scheme and qN‐Bofill denotes the Bofill scheme based on the first‐order Taylor expansion. (Reproduced with permission from Ref 51. Copyright 2010, American Institute of Physics.)

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Figure 7.

Total energies of the Cl + CH3I chemical reaction system for simulations of 140 fs. Time step = 1.0 fs and K = 12. The units for horizontal and vertical axes, respectively, are fs and kcal/mol. In the figure, pCFD–PSB denotes the perturbed compact finite difference (CFD)–Bofill scheme and qN‐PSB denotes the power symmetric Broyden scheme based on the first‐order Taylor expansion. (Reproduced with permission from Ref 51. Copyright 2010, American Institute of Physics.)

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Figure 8.

View of a typical trajectory that shows the indirect roundabout mechanism observed for the SN2 reaction Cl + CH3I. (Reproduced with permission from Ref 112. Copyright 2008, The American Association for the Advancement of Science.)

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Figure 9.

Plot of ln(kd) versus 1/T for CH3NHCHCHCH3 → ·CH3 + ·NHCHCHCH3 dissociation. kd is in unit of s−1 and T is in K. (Reproduced with permission from Ref 88. Copyright 2011, American Chemical Society.)

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Figure 10.

Probability of peptide fragmentation, surface fragmentation and reactivity, as a function of collision energy, for collisions of gly2 − H+ with COCl‐SAM and CHO‐SAM surfaces. These are nonexclusive events and the possibility for all three to occur in a single trajectory is clear. (Reproduced with permission from Ref 117. Copyright 2011, American Institute of Physics.)

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Figure 11.

Populations of the HF vibrational states for C2H5F → HF + C2H4 dissociation, with different amounts of excitation energy in the C2H5F transition state □, results of the MP2/6‐311++G** simulations •, results of the MP2/6‐31G* simulations ▴, experimental results. (Reproduced with permission from Ref 118. Copyright 2006, American Chemical Society Created using data from Refs 35 and 120.).

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