[printer friendly version]


"Molecular Thermodynamics Visualized"

Dudley Herschbach
Texas A\&M University


Classical thermodynamics, developed in the 19th century, dealt solely with macroscopic properties of matter. Quantum statistical mechanics triumphantly provided means to evaluate all thermodynamic quantities from molecular properties, via the partition function. This route to thermodynamics has proved extremely useful, especially for gas phase processes. As usually formulated, however, the partition function is not easily visualized. Moreover, applications often involve a ratio of the partition functions for two molecules having several atoms or groups in common. The contributions of many of the degrees of freedom associated with the common groups then tend to cancel, but this does not find natural expression in the usual formulation. We present an unorthodox form of the partition function for a polyatomic molecule that deserves to be better known. This form has, for each atom, a factor that corresponds to free translational motion of the atom within an effective volume defined by the average vibrational amplitudes of the atoms and the geometrical configuration of the neighboring atoms. When treating chemical reactions, in many cases the only volume elements that do not cancel to a practical approximation are those for the atoms in the vicinity of the reaction site. Thus the important factors, in particular the large volume changes that occur in transforming vibrational degrees of freedom into rotational and translational ones, can be readily visualized.

Tuesday, October 4, 2011
IQSE 578, 11:30 a.m.
Mitchell Physics Building

(Pizza, salad, and soda to be served at 12:30 p.m. outside IQSE 578)