MODEL PREDICTIONS OF MEAN LIBRATIONAL FREQUENCIES FOR RIGID MOLECULES IN DILUTE DECALIN SOLUTION

The product of the reduced inertia Jr of a dipolar molecule and the square of the far infrared absorption maximum wave-number νmax that it displays in dilute solution is shown, using a three-parameter Mori/itinerant oscillator model, to be related to the mean-square action on the molecule and hence by a simple argument to the solute volume of rotation V. Estimates of V have been made for seventeen rigid molecules (mainly substituted phenyls) and their νmax values have been measured in decalin solution at 293 and 110 K. Provided allowance is made for translation-rotation coupling in four cases, the derived relation Irν2max = (8aa2c2kT)-1V2P(0) is found to be approximately obeyed at both temperatures with P(0) (the solute-independent mean-square torque acting on a molecule of unit V) having a value of 4.0 × 1017 (N m-2)2 at both temperatures. It should now be possible to predict μmax for other solutes in decalin if their structures are known. © 1979.