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POLAR (ACYCLIC) ISOMER OF FORMIC ACID DIMER: RAMAN SPECTROSCOPY STUDY.

ROMAN M. BALABIN, Department of Chemistry and Applied Biosciences, ETH Zurich, 8093 Zurich, Switzerland.

Formic (methanoic) acid spectral range of 575--1150 cm^-1 has been studied by gas-phase Raman spectroscopy method. A weak Raman-active vibration of polar (acyclic) HCOOH dimer has been found at 864\pm2.1 cm^-1 and assigned using quantum chemistry data. The temperature-dependence of intensity ratios of Raman lines was used to evaluate the thermodynamic parameters of polar dimer. Its experimental dimerization enthalpy (\bigtriangleupH) was found to be -8.6\pm0.2 kcal mol^-1. Entropy of dimerization has been evaluated using theoretical (MP2) Raman scattering activities. Its value (\bigtriangleupS) is estimated as -36\pm2 cal mol^-1 K^-1. The results are compared with the published experimental data and calculations. The presented results can be used for molecular dynamics simulations, hydrogen bond energy estimation, and analysis of CH₂O₂ vapor density measurements.