Pair Configuration of Maximum Occurrence Probability in Liquid Formic Acid as Determined from Neutron Scattering and Proton Magnetic Relaxation Results

Formic and acetic acid aggregation in the liquid state

The microscopic structure of neat formic and acetic acid have been measured by neutron diffraction with H/D substitution on SANDALS at the ISIS neutron spallation source. These data, together with complementary x-ray data, have been modeled via the empirical potential structure refinement (EPSR) method, which integrates information obtained from the diffraction data in a Monte Carlo simulation in order to provide a three-dimensional model of the system under study compatible with the measured structure factors. Two models have been generated for each acid, in order to test their consistency, with positive results. The final structure obtained is that of two liquids that are very similar to each other, with high connectivity although rather disordered. They present a hierarchy of probability for hydrogen bond formation, where weaker bonds involving the carbonyl hydrogen for formic acid or the methyl hydrogen for acetic acid are more abundant than the stronger bonds involving the hydroxyl hydrogen. Cooperative effects are found to be fundamental for the description of aggregation of formic and acetic acid, but the structure in the liquid presents a greater variety of bonds than in the solid state.

Quantum-chemical study of CHCl3-SO2 association

CHCl(3)-SO(2) association is studied by high-level quantum-chemical calculations of stationary points of the dimer electronic potential-energy hypersurface, including correlated second-order Moller-Plesset and CCSD(T) calculations with basis sets up to 6-311++G(d,p). During geometry optimization, frequency, and energy calculations, a self-written computer code embedding the GAMESS ab initio program suite applies counterpoise correction of the basis set superposition error. A CH...O hydrogen-bonded complex (DeltaE(0)=-8.73 kJmol) with a 2.4 A intermolecular H...O distance and two very weak van der Waals complexes (DeltaE(0)=-3.78 and -2.94 kJmol) are located on the counterpoise-corrected potential-energy surface. The intermolecular interactions are characterized by Kitaura-Morokuma interaction energy decompositions and Mulliken electron population analyses. The unusual hydrogen bond is distinguished by a CH-bond contraction, a pronounced enhancement of the IR intensity and a shift to higher frequency ("blueshift") of the CH-stretching vibration compared to the CHCl(3) monomer. Spectroscopy and association in liquid solution is also discussed; our results provide an alternative explanation for features in the CH-stretching vibration spectrum of chloroform dissolved in liquid sulfur dioxide which have been attributed previously to an intermolecular Fermi resonance.

Car−Parrinello Molecular Dynamics Simulation of Liquid Formic Acid

First-principles molecular dynamics has been used to investigate the structural, vibrational, and energetic properties of formic acid, formic acid-formate anion dimers, and liquid formic acid in a periodically repeated box with 32 formic acid molecules. We found that in liquid formic acid the hydrogen-bonded clusters mainly consist of linear branching chains. From our simulation, we got good agreement with the available structural and dynamical data. We also studied the proton transfer in the cis-formic acid-formate anion dimer, and we showed that this proton transfer does not have any potential barrier. The hydrogen bonding statistics as well as the mean lifetime of the hydrogen bonds are analyzed.