Ab initio calculations of hydrogen-bonded carboxylic acid cluster systems : Dimer evaporations

Ab initio calculations were performed on proton bound formic acid and acetic acid clusters, (RCOOH)_nH⁺ at various levels of theory. Three types of competitive structures were deduced: Open-chain structures are favored energetically for lower members (n ≤ 5) of the cluster series, chains terminated with a cyclic dimer unit at one end for intermediate n and chains terminated by cyclic dimer units at both ends are favored for high n (n ≥ 6). The computational results corroborate previously reported experimental results from this laboratory, according to which there is a changeover from monomer evaporation from clusters of low n to dimer evaporation at n ≥ 6 for R = H. Fairly good agreement was obtained between calculated and experimental binding energies. © 1996 American Chemical Society.