Six trans-trans and five trans-cis dimeric structures of formic acid (HCOOH) are revealed by ab initio calculations. Four trans-trans and two trans-cis dimers are identified in the IR absorption spectra in argon matrices. The trans-cis dimers are obtained by narrow-band IR excitation of the vibrational transitions of the trans-trans dimers. Two trans-trans (tt3 and tt6) and one trans-cis (tc4) dimer are characterized experimentally for the first time. The tunneling decay rates of two trans-cis dimers (tc1 and tc4) are evaluated at different temperatures. A greater lifetime of the trans-cis dimers at elevated temperatures compared to the cis-monomer suggests that the high-energy conformers can be stabilized upon hydrogen bonding.

The higher energy conformer (cis) of HCOOD is prepared by vibrational excitation of the trans form. The cis conformer decays back to the conformational ground state (trans) via tunneling of deuterium. The tunneling process in HCOOD in rare gas matrices is extremely slow (in scale of weeks). We present new measurements of the tunneling rate constants, which characterize the efficiency of the cis-to-trans conversion process in Ne, Ar, Kr, and Xe matrices. The tunneling rates of HCOOD follow the trend k(Xe) approximately = k(Kr)>k(Ar) approximately = k(Ne), which is anomalous with respect to the reaction barrier of the solvated molecule. We propose a semiempirical energetic scheme of solid state solvation, which is consistent with all experimental observation. The temperature dependence of the tunneling constants rates of HCOOD is very weak compared to HCOOH in all matrices. The fundamental vibrational frequencies of the cis and trans conformers of HCOOD in various matrices are reported.

We prepared the first cis-trans dimer of formic acid and measured its vibrational spectrum in a low-temperature Ar matrix. This preparation was done by selective vibrational excitation of the trans-trans noncyclic dimer. It was found that the stability of the cis-trans dimer against proton tunneling is strongly improved compared to the monomer, especially at elevated temperatures (>30 K). This surprising phenomenon was explained by differences in dynamical, energetic, and vibrational properties of the dimer and monomer. The obtained results show that the proton tunneling reactions can be strongly modified in the hydrogen-bonded solid network compared to the monomeric species.

We study the conformational reorganization of formic acid (FA) in solid neon and report the higher-energy cis-FA monomer and one form of the trans-cis FA dimers. They were prepared by selective vibrational excitation of the trans-FA monomer and trans-trans dimer. The proton tunneling decay of cis-FA monomer is surprisingly very fast in solid neon, two orders of magnitude faster than in solid argon. It was also found that the stability of the trans-cis dimer against proton tunneling is enormously enhanced in solid neon compared to the monomer (by a factor of approximately 300). These results are discussed in terms of matrix solvation and hydrogen bonding.