Ab initio studies of some hydrogen-bonded complexes of fluoroform – Evidence for blue-shifted behaviour

We have extended a previous study of the complexes of fluoroacetylene with ammonia, water, hydrogen fluoride, phosphine, hydrogen sulphide and hydrogen chloride to the corresponding complexes of fluoroform, as proton donor, with the same series of proton acceptors, in a search for the existence of blue-shifted hydrogen-bonded adducts among these associated species. The complexes with NH3 and PH3 were found to exhibit lengthening of the CH bond of CHF3, but those with H2O, HF, H2S and HCl showed the opposite behaviour. At the harmonic level, with the exception of the NH3 complex, the CH stretching wavenumber of CHF3 was shifted to higher values, while the infrared intensity of this mode decreased uniformly on complexation. At the anharmonic level, on the other hand, all six CH stretching wavenumber shifts were to the blue. In the complexes with NH3, H2O, PH3 and H2S, the CHF3 molecule experienced a net increase of electronic charge but in those with HF and HCl it was found to be the electron donor. A topological charge density analysis showed the presence of bond critical points in the CH⋯X hydrogen-bonded region of all the complexes except CHF3·HF while, in the case of CHF3·HCl, two hydrogen bond critical points and a ring critical point were found. With the exception of the CHF3·NH3 species the remaining complexes successfully meet all the criteria for blue-shifted hydrogen bonding behaviour.