Molecular force measurement in liquids and solids using vibrational spectroscopy

Shifts in molecular vibrational frequencies are used to measure intermolecular forces in liquids and solids as a function of external pressure. The force along a particular bond within a molecule is derived from 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 performed 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 three methylsiloxanes. Comparison of these and previous gas-to-liquid (or solid) and high-pressure vibrational frequency shift results reveal a large variation in the force on different types of bonds, while families of similar bonds experience a similar force at a given external pressure, with only a weak dependence on the location of the bond within the solute or the molecular structure of the surrounding solvent. Physical interpretations of the results using both continuum and perturbed hard sphere fluid models are suggested.