THERMODYNAMIC FUNCTIONS OF PENDULAR MOLECULES

External electric or magnetic fields can hybridize rotational states o f individual dipolar molecules and thus create pendular states whose f ield-dependent eigenproperties differ qualitatively from those of a ro tor or an oscillator. The pendular eigenfunctions are directional, so the molecular axis is oriented. Here we use quantum statistical mechan ics to evaluate ensemble properties of the pendular states. For linear molecules, the partition function and the averages that determine the thermodynamic functions can be specified by two reduced variables inv olving the dipole moment, field strength, rotational constant, and tem perature. We examine a simple approximation due to Pitzer that employs the classical partition function with quantum corrections. This provi des explicit analytic formulas which permit thermodynamic properties t o be evaluated to good accuracy without computing energy levels. As ap plications we evaluate the high-field average orientation of the molec ular dipoles and field-induced shifts of chemical equilibria.