DENSITY-OF-STATES OF ONE-DIMENSIONAL HINDERED INTERNAL ROTORS AND SEPARABILITY OF ROTATIONAL DEGREES OF FREEDOM

Analytical formulas for the density-of-states and sum-of-states functi ons of internal one-dimensional hindered rotors are developed and anal yzed. Quantum effects are taken into account via the inverse Laplace t ransform of the classical rotational partition function corrected for quantum effects by the method of Fitter and Gwinn. Density-of-states a nd partition functions of i-C3H7, n-C4H10, n-C5H12, and sec-C4H9 are c alculated under a separability assumption with internal hindered rotor s treated either classically or classically with corrections for quant um effects. Results are compared with those obtained in a full treatme nt with explicit accounting for interaction between internal and exter nal (overall) rotations. It is demonstrated that separability of the r otational degrees of freedom can be assumed in most cases with reasona ble accuracy, the main source of disagreement between the two approach es being not the separability assumption but the approximation of a co mplex torsional potential with a simpler sinusoidal function.