Polar (Acyclic) Isomer of Formic Acid Dimer: Gas-Phase Raman Spectroscopy Study and Thermodynamic Parameters

The formic (methanoic) acid spectral range of 575-1150 cm(-1) has been studied by the gas-phase Raman spectroscopy method in a temperature region between 25 and 45 degrees C. A weak Raman-active vibration of polar (acyclic) HCOOH dimer (a-FAD) has been found at 864 +/- 2.1 cm(-1) and assigned using quantum chemistry data. The contours of the formic acid monomer (FAM) line at 1104 cm(-1) and the a-FAD line were deconvoluted using ab initio data to obtain precise total integral intensities. The temperature dependence of the intensity ratios was used to evaluate the thermodynamic parameters of the polar dimer. Its experimental dimerization enthalpy (Delta H(a-FAD)) was found to be -8.6 +/- 0.2 kcal mol(-1). The entropy of dimerization has been evaluated using theoretical (MP2) Raman scattering activities. Its value (Delta S(a-FAD)) is estimated as -36 +/- 2 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 CH(2)O(2) vapor density measurements.