Raman spectral study of 'neat' formic acid and aqueous and organic solutions of formic acid

The Raman spectra of ‘neat’ formic acid and formic acid in several solvents (water, acetonitrile, 1,4‐dioxane and dichloromethane) were measured. In aqueous solution a very clear non‐coincidence effect (NCE) indicative of resonance energy transfer was observed. The size of the NCE (ca 64 cm−1) indicates that strong intermolecular forces exist between the formic acid molecules, no doubt augmented by hydrogen bonding. In acetonitrile and 1,4‐dioxane, two bands (at 1735 and 1765 cm−1) develop with dilution. The 1735 cm−1 band was assigned to ‘free’ formic acid monomer and the band at 1765 cm−1 to formic acid ‘complexed’ to the solvent. Liquid formic acid is unlike acetic acid in that it does not arrange itself into discrete, long‐lived dimers or polymers. It is viewed as a collection of monomeric units which interact through hydrogen bonding, thereby imposing local, short‐term order in the liquid. This allows the resonance energy transfer to occur and results in a large non‐coincidence effect. Copyright © 1999 John Wiley & Sons, Ltd.