Tunneling Isomerization of Small Carboxylic Acids and Their Complexes in Solid Matrixes: A Computational Insight

We have studied hydrogen-atom tunneling in the cis-to-trans conformational change of some carboxylic acid monomers and formic acid (FA) complexes and dimers at the MP2­(full) and CCSD­(T) levels of theory within the Wentzel–Kramers–Brillouin approximation. The barrier for the minimum energy path, where the OH bond length and the COH bending angle are optimized, is found to be a good approximation for providing the highest barrier transparency. The matrix effect on the transmission coefficients of <i>cis</i>-FA monomer, <i>trans</i>–<i>cis</i> FA dimer (tc1), and <i>cis</i>-acetic acid monomer are modeled by the polarizable continuum model (PCM) at the MP2­(full) level of theory in different environments. For the <i>cis</i>-FA monomer and <i>trans</i>–<i>cis</i> FA dimer (tc1), the calculated transmission coefficients agree with the experimental lifetimes observed in noble-gas solids. However, this method cannot reproduce the experimental results obtained for <i>cis</i>-acetic acid. Moreover, the long lifetime of <i>cis</i>-FA and <i>cis</i>-acetic acid in the N₂ environment cannot be reproduced either, which is most probably due to specific interactions that are not included in the PCM. The calculation for <i>cis</i>-HCOOD shows a strong decrease of the barrier transparency compared to that for <i>cis</i>-HCOOH, which is consistent with the experiments. In general, good agreement is observed between the calculated barrier transparency (including PCM) and experimental tunneling rate. However, some exceptions are found, which shows that additional factors influence the tunneling rate