Hydration and dissociation of hydrogen fluoric acid (HF)

The hydration and dissociation phenomena of HF(H₂O)_n (n ≤ 10) clusters have been studied by using both the density functional theory with the 6-311++G*[sp] basis set and the Møller-Plesset second-order perturbation theory with the aug-cc-pVDZ+(2s2p/2s) basis set. The structures for n ≥ 8 are first reported here. The dissociated form of the hydrogen-fluoric acid in HF(H₂O)_n clusters is found to be less stable at 0 K than the undissociated form until n = 10. HF may not be dissociated at 0 K solely by water molecules because the HF H bond is stronger than the OH H bond, against the expectation that the dissociated HF(H ₂O)_n would be more stable than the undissociated one in the presence of a number of water molecules. The dissociation would be possible for only a fraction of a number of hydrated HF clusters by the Boltzmann distribution at finite temperatures. This is in sharp contrast to other hydrogen halide acids (HCl, HBr, HI) showing the dissociation phenomena at 0 K for n ≥ 4. The IR spectra of dissociated and undissociated structures of HF(H ₂O)₂ are compared. The structures and binding energies of HF(H₂O)_n are found to be similar to those of (H ₂O)_n+1. It is interesting that HF(H₂) _n=5,6,10 are slightly less stable compared with other sizes of clusters, just like the fact that (H₂O)_n=6,7,11 are slightly less stable. The present study would be useful for the experimental/spectroscopic investigation of not only the dissociation phenomena of HF but also the similarity of the HF-water clusters to the water clusters.