Dimerization of formic acid - An example of a "noncovalent" reaction mechanism

The pulse deposition technique allows selectively the isolation of monomeri c or dimeric formic acid in argon matrices at 7K. Warning of matrices conta ining the monomer M from 7 K to 40 K results in the decrease of M and forma tion of a dimer B. This dimer is also labile, and further warming finally p roduces a second dimer A. By comparison with density functional theory (DFT ) calculations and gas phase IR spectra taken from the literature, the latt er dimer A was identified as the C-2h- symmetrical cyclic dimer. The unstab le dimer B was identified as the acyclic C-S-symmetrical dimer. An activati on energy of 2.3 kcal mol(-1) was calculated for the B --> A rearrangement at the B3LYP/6-311 + + G(d,p) level of theory, which is in qualitative agre ement with the experimental finding of a slow thermal reaction under the co nditions of matrix isolation.