Quantum mechanics computational thermochemistry

Recent advances in computational chemistry have made it possible to calculate enthalpies of formation from quantum mechanical first principles for rather large unsaturated molecules, some of which are outside the practical range of combustion thermochemistry. Quantum mechanical calculations of molecular thermochemical properties are, of necessity, approximate. Composite quantum mechanical procedures may employ approximations at each of several computational steps and may have an empirical factor to correct for the cumulative error. Approximate methods are useful only insofar as the error due to the various approximations is known within narrow limits. Error due to approximation is determined by comparison with a known value, but the question of the accuracy of the known value immediately arises because the uncertainty of the comparison is determined by the combined uncertainty of the approximate quantum mechanical result and the standard to which it is compared. 

It should be recognized, however, that although bond and group additivity rules represent useful tools when applicable, they are restricted to molecules built from standard bonds and groups. Consequently, when new molecules are encountered, for which available bond and group values do not apply, their thermochemistry must be determined either experimentally or via computational quantum mechanics. 

As stated above, the thermochemistry of free radicals can also be estimated by the group additivity method, if group values are available. With the exception of a few cases reported in Benson (1976), however, such information presently does not exist. Therefore, we rely on the model compound approach (for S and Cp) and bond dissociation energy (BDE) considerations and computational quantum mechanics for the determination of the heats of formation of radicals. 

In summary, bond and group additivity rules, as well as the model compound approach, in conjunction with statistical mechanics, represent useful tools for the estimation of thermochemical properties. However, their utility for the determination of thermochemistry of new classes of compounds is limited, especially with regard to the determination of Aiff. For new classes of compounds, we must resort to experiments, as well as to computational quantum mechanical methods. 

In summary, computational quantum mechanics methods represent powerful new tools for the estimation of thermochemistry. However, their routine use clearly must be avoided, as there still are unresolved limitations of these methods. Consequently, we must continue to rely on conventional methods, experiments, and chemical intuition for the estimation of thermochemical properties. 

In addition to experiments, a range of theoretical techniques are available to calculate thermochemical information and reaction rates for homogeneous gas-phase reactions. These techniques include ab initio electronic structure calculations and semi-empirical approximations, transition state theory, RRKM theory, quantum mechanical reactive scattering, and the classical trajectory approach. Although still computationally intensive, such techniques have proved themselves useful in calculating gas-phase reaction energies, pathways, and rates. Some of the same approaches have been applied to surface kinetics and thermochemistry but with necessarily much less rigor. 

Martin, J. M. L. Computational thermochemistry a brief overview of quantum mechanical approaches, Aww. Rep. Comput. Chem. 2005,1, 31-43. 

Contemporary advances in computer hardware and software have brought about a immense increase in our ability to calculate (as distinct from measuring) Ahyd Him. Modem computational thermochemistry includes various empirical, semiempirical, and quantum mechanical methods for calculating AhydH29g of linear and cyclic alkenes, polyalkenes, alkynes and polyalkynes. There is at present no critical survey of computational methods for determining Ahyd- 298 which Chapter 3 supplies. 

The CCCBDB is currently the only computational chemistry or physics database of its kind. This is due to the maturity of quantum mechanics to reliably predict gas-phase thermochemistry for small (20 nonhydrogen atoms or less), primarily organic, molecules, plus the availability of standard-reference-quality experimental data. For gas-phase kinetics, however, only in the past two years have high-quality (<2% precision) rate-constant data become available for H and OH transfer reactions to begin quantifying uncertainty for the quantum mechanical calculation of reaction barriers and tunneling. There is a critical need for comparable quality rate data and theoretical validation for a broader class of gas-phase reactions, as well as solution phase for chemical processing and life science, and surface chemistry. 

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