The interaction of fluorosilanes with hydrogen fluoride clusters: strongly blue-shifted hydrogen bonds

Abstract

The equilibrium structures, binding energies, and vibrational spectra of the complexes formed between hydrogen fluoride clusters (HF) n (1≤ n≤4) and the fluorosilanes SiHF 3, SiH 2F 2, and SiH 3F are investigated within the second-order Møller–Plesset perturbation theory method applying extended basis sets. It is shown that Si–F⋯H–F halogen–hydrogen bonds are formed in the most stable open dimers, SiHF 3–HF, SiH 2F 2⋯HF, and SiH 3F⋯HF. No Si–H⋯F–H hydrogen bonds occur in these dimers. Nevertheless, blue shifts of Si–H stretching frequencies are calculated. All three trimers, fluorosilane–(HF) 2, all three tetramers, fluorosilane–(HF) 3, and two of the pentamers, fluorosilane–(HF) 4, form cyclic structures with strong Si–F⋯H–F halogen–hydrogen bonds and weak Si–H⋯F–H contacts, the latter displaying, nevertheless, strongly blue-shifted Si–H stretching frequencies. These blue shifts are comparable in size to those of the corresponding fluoromethane–(HF) n complexes and are with about +50 cm −1 for the case n=3 among the largest ever calculated and definitely the largest for Si–H bonds. In the title complexes, the formation of the Si–F⋯H–F halogen–hydrogen bonds induces a substantial stretching of this Si–F bond, which in turn leads to a significant contraction of the fluorosilane Si–H bond in the Si–H⋯F–H hydrogen bond. This disposition of the fluorosilane monomers is demonstrated with the aid of suitable potential energy surface scans and appears to be a prerequisite for the formation of strongly blue-shifted hydrogen bonds.