MOLECULAR-FORCE MEASUREMENT IN LIQUIDS AND SOLIDS USING VIBRATIONAL SPECTROSCOPY

Shifts in molecular vibrational frequencies are used to measure interm olecular forces in liquids and solids as a function of external pressu re. The force along a particular bond within a molecule is derived fro m its measured vibrational frequency shift using an expression for the perturbation of a quantum anharmonic oscillator in a classical bath. New pressure induced frequency shift and force measurements are perfor med on the C=C bond in 1-octene, trans-2-octene and trans-4-octene (in both pure liquids and methanol solutions), and on the Si-O bond in th ree methylsiloxanes. Comparison of these and previous gas-to-liquid (o r solid) and high-pressure vibrational frequency shift results reveal a large variation in the force on different types of bonds, while fami lies of similar bonds experience a similar force at a given external p ressure, with only a weak dependence on the location of the bond withi n the solute or the molecular structure of the surrounding solvent. Ph ysical interpretations of the results using both continuum and perturb ed hard sphere fluid models are suggested.