Ab initio studies on hydrogen-bonded chains. IV. Structure and stability of formic acid chains

Abstract

Abstract Equilibrium geometries and relative stabilities of several possible catemeric motifs of formic acid have been investigated with the aid of the ab initio crystal orbital method applying basis sets from minimal to double-zeta quality. Extensive geometry optimization has been carried out on these infinite chains of formic acid molecules. For the purpose of comparison syn- and anti-monomers and the cyclic dimer of formic acid have been treated at the same methodical level. Results on equilibrium geometries, hydrogen-bond energies, dipole moments and a limited number of harmonic force constants and vibrational frequencies are compared with experimental data available. Moreover a full in-plane force field for the cyclic dimer is presented. The chain structure corresponding to the experimentally observed β-conformation of solid formic acid turned out to be the most stable structure exhibiting a larger stabilization energy per hydrogen bond than the cyclic dimer. The vapor-phase energy difference between syn- and anti-monomer is considerably reduced in the corresponding polymers resulting in a second, energetically low-lying structure which might be of importance for the vibrational dynamics of solid formic acid.