Raman spectral studies of solutions of formic acid and methyl formate

Abstract

Raman spectroscopy was used to study solutions of formic acid in water, acetonitrile, 1,4-dioxane, and dichloromethane and solutions of methyl formate in methyl d-formate and acetonitrile.

A very clear non-coincidence effect, NCE (i.e., Vaniso. -Viso. =/ 0), exists for formic acid. Dilution in water led to a reduction of the NCE. Such behaviour is indicative of resonance energy transfer, which implies significant intermolecular coupling in formic acid and short range, short term order in the liquid. Dilution in acetonitrile and 1,4-dioxane led to the development of peaks at ~1735 cm-1 and ~1765 cm-1. By comparison with methyl formate, the band at ~1735 cm-1 was signed to uncoupled formic acid monomer. The band at ~1765 cm-1 was assigned to a formic acid-solvent complex. Independent evidence (perturbations of the solvent spectra) was found to support the existence of a complex. In the case of acetonitrile, this complex was investigated more deeply. A "Job" plot gave ambiguous results concerning the co-ordination number, n, of the "complex". "Equilibrium constants" were calculated for several possible stoichiometries. At high relative amounts of acetonitrile the "equilibrium constant" increased dramatically (for each of the stoichiometries). At high concentrations of acetonitrile, a mixture of complexes may exist.

A non-coincidence effect also exists in methyl formate but is much smaller than in formic acid. The NCE was studied as a function of concentration and compared with several models in the literature. The behaviour of the NCE for the isotopic dilution (in methyl d-formate) conformed with the model tested. However, the non-isotopic dilution (in acetonitrile) did not conform with the models. The full width at half maximum of the carbonyl band exhibited unusual behaviour, first increasing and then decreasing, indicating a change in the nature of the intermolecular potential with dilution.