H Use of Computational Chemistry Software Packages

H.1 Computational Chemical Engineering

As a prologue to the future, our profession is evolving to one of molecular chemical engineering. For chemical reaction engineers, computational chemistry and molecular modeling could well be our future.

Thermodynamic properties of molecular species that are used in reactor design problems can be readily estimated from thermodynamic data tabulated in standard reference sources such as Perry’s Handbook or the JANAF Tables. Thermochemical properties of molecular species not tabulated can usually be estimated using group contribution methods. Estimation of activation energies is, however, much more difficult owing to the lack of reliable information on transition-state structures, and the data required to carry out these calculations is not readily available.

Recent advances in computational chemistry and the advent of powerful, easy-to-use software tools have made it possible to estimate important reaction-rate quantities (such as activation energy) with sufficient accuracy to permit incorporation of these new methods into the reactor design process. Computational chemistry programs are based on theories and equations from quantum mechanics, which until recently, could only be solved for the simplest systems such as the hydrogen atom. With the advent of inexpensive high-speed desktop computers, the use of these programs in both engineering research and industrial practice is increasing rapidly. Molecular properties such as bond length, bond angle, net dipole moment, and electrostatic charge distribution can be calculated. Additionally, reaction energetics can be accurately determined by using quantum chemistry to estimate heats of formation of reactants, products, and also for transition-state structures.

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Examples of commercially available computational chemistry programs include Spartan, developed by Wavefunction, Inc. (www.wavefun.com), and Cerius2 from Molecular Simulations, Inc. (www.accelrys.com). The Web module in Chapter 3 on molecular reaction engineering (see www.umich.edu/~elements/5e/ ) gives an example of what we can expect in the future.

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