Formic acid tetramers: structure isomers in the gas phase

HF, DFT and MP2 calculations are employed to predict the structures and stabilities of formic acid tetramers. Three nearly equally stable structures were identified: a stack of two dimers (“sandwich”, C, symmetry), a cyclic structure (S4 symmetry) and a “butterfly” structure obtained from the nearly planar cycle by folding. The latter was the global minimum when electron correlation was taken into account. DFT and MP2 yield the same energetic ordering but DFT overestimates hydrogen bonding. The stability is discussed in terms of two- and three-body contributions