A comparison between entropies of aromatic compounds from quantum mechanical calculations and experiment

In this work, we perform a set of quantum mechanical and statistical mechanical calculations to generate the entropy of five simple, aromatic compounds-benzene, toluene, p-xylene, m-xylene and o-xylene-in the ideal gas state. We systematically examine how the choice of quantum mechanical level of theory and size of basis set impact the agreement between theory and experiment. Regardless of level of theory and basis set, all calculations require an empirical scaling factor to correct the vibrational contribution to the entropy. Once this scaling factor is applied, there is at most nominal advantage in more sophisticated levels of theory or increased basis set size, while a heavy computational penalty is paid for the more advanced theory. We find that the variation in scaling factor across these aromatic compounds is on average 0.3%. Across a range from 250 to 540 K, the difference in the entropies obtained from all quantum mechanical calculations and from experiment is less than half a percent. (c) 2006 Elsevier B.V. All rights reserved.