MOLECULAR DENSITY-OF-STATES FROM ESTIMATED VAPOR-PHASE HEAT-CAPACITIES

Heat capacity data between 298 and 1500K are used to derive a reduced set of apparent vibrational frequencies that can be used for estimatio n of molecular density of states, rho(E). Estimates for a number of mo lecule and radical species, using a reduced set of three frequencies w ith noninteger degeneracies, are shown to compare favorably to direct count methods, which require specification of the complete frequency s et. Use of the reduced set of three frequencies leads to significant i mprovement in calculations of rho(E)/Q as compared to similar calculat ions which use only a single geometric- or arithmetic-mean frequency a pproximation. Since vapor phase heat capacity data of molecules and ra dicals can be estimated accurately by a group additivity formalism, th is approach provides a method to estimate rho(E) for use in calculatio ns of pressure effects in unimolecular and chemical activation reactio n systems. The accuracy of the rho(E)/Q distributions obtained from he at capacity data makes this a viable method for those cases where the complete frequency distribution is not known. It is especially valuabl e for those cases where contributions to rho(E) from internal rotors o r low frequency vibrations such as inversions are not well known. This approach is useful for quantum RRK or inverse Laplace transform calcu lations of k(E) since no assignment of transition state properties is necessary. The reduced frequency set can also be combined with Delta H -f(298) and S(298) to provide a compact data set to describe thermodyn amic properties at any temperature. (C) 1997 John Wiley & Sons, Inc.