A theoretical study of the neutral, protonated, and deprotonated trimers of HF and HCl

Abstract

The structures of the neutral, protonated, and deprotonated trimers of HF and HC1 have been optimized at the MP2/6-31 + G (d,p) level of theory. The neutral trimers are cyclic with C3h symmetry. The protonated HF and HCl trimers have C 2v, and C 2 symmetry, respectively, while the deprotonated trimers have C 2v symmetry. Correlated trimer binding energies have been evaluated with the 6-31 + G(2d,2p) basis set using Møller-Plesset perturbation theory at second (MP2), third (MP3), and fourth (MP4) order; the coupled clusters doubles method (CCD), and CCD with non-iterative inclusion of triples (CCD (T)); quadratic configuration interaction (QCISD), and QCISD with non-iterative inclusion of triples (QCISD (T)); the averaged coupled-pair functional (ACPF); the linearized coupled clusters method (LCCM); and configuration interaction with all single and double excitations (CISD), all relative to a single-reference Hartree-Fock function. The size-consistency error in the CISD energies was corrected using the Davidson and Pople corrections, and by use of the supermolecule approach. The size-consistency error in the ACPF binding energies is less than 0.1 kcal mol −1. All size-consistent methods predict similar binding energies for the HF trimers. There is more variation among these methods in the binding energies of the HC1 trimers, with MP2 predicting the highest binding energies. (HF) 3 and (HCl) 3 are predicted to be about three times more stable than the corresponding dimers. The charged complexes F 3H 4 +, F 3H 2 −, Cl 3H 4 +, and Cl 3H 2 − have hydrogen bonds which are weaker than the hydrogen bonds in the corresponding dimers.