Molecular density of states from estimated vapor phase heat capacities

Document Type

Article

Publication Date

1-1-1997

Abstract

Heat capacity data between 298 and 1500K are used to derive a reduced set of apparent vibrational frequencies that can be used for estimation of molecular density of states, p(E). Estimates for a number of molecule and radical species, using a reduced set of three frequencies with noninteger degeneracies, are shown to compare favorably to direct count methods, which require specification of the complete frequency set. Use of the reduced set of three frequencies leads to significant improvement in calculations of p(E)/Q as compared to similar calculations which use only a single geometric- or arithmetic-mean frequency approximation. Since vapor phase heat capacity data of molecules and radicals can be estimated accurately by a group additivity formalism, this approach provides a method to estimate p(E) for use in calculations of pressure effects in unimolecular and chemical activation reaction systems. The accuracy of the p(E)/Q distributions obtained from heat capacity data makes this a viable method for those cases where the complete frequency distribution is not known. It is especially valuable for those cases where contributions to p(E) from internal rotors or low frequency vibrations such as inversions are not well known. This approach is useful for quantum RRK or inverse Laplace transform calculations of fe(E) since no assignment of transition state properties is necessary. The reduced frequency set can also be combined with ΔHf(298) and S(298) to provide a compact data set to describe thermodynamic properties at any temperature. © 1997 John Wiley & Sons, Inc.

Identifier

0031104192 (Scopus)

Publication Title

International Journal of Chemical Kinetics

External Full Text Location

https://doi.org/10.1002/(SICI)1097-4601(1997)29:3<161::AID-KIN2>3.0.CO;2-S

ISSN

05388066

First Page

161

Last Page

170

Issue

3

Volume

29

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