Weak C-H···O, C-H···F and C-H···p intermolecular hydrogen bonds and internal dynamics of molecular complexes by Fourier transform microwave spectroscopy

In this contribution, we show how rotational spectroscopy, combined with molecular beam techniques, can supply detailed information on the chemical features of Weak Hydrogen Bond (WHB), and provide insight on the internal dynamics of the units constituent the molecular complexes in unrivaled precision. 1) C-H×××O WHB.We report the first rotationally resolved investigation of a complex, cyclobutanone-trifluoromethane, with a C-H×××O interaction involving one carbonylic oxygen. This bridge, together with two C-H×××F weak hydrogen bonds, stabilizes the configuration shown aside. 2) C-H×××F WHB.The internally highly dynamic fluoromethane-trifluoromethane molecular complex is stabilized by three CH×××F weak hydrogen bonds and electrostatic dipole-dipole interaction, thereby balancing its equilibrium structure towards one of two plausible structures. The two subunits, bound by 5.3 kJ/mol, are not rigidly locked but exhibit large amplitude motions by almost freely spinning around their symmetry axes, with V3 barriers of 0.84 (CF3) and 0.36 (CH3) kJ/mol. 3) C-H×××p WHB.The pure rotational spectra of the normal and of the benzene-13C isotopomers of the benzene-trifluoromethane molecular complex have been measured by molecular beam-Fourier transform microwave spectroscopy. The normal species is a symmetric top, with the symmetry axes, C3 of CHF3 and C6 of Benzene lying along the same line. The rotational spectrum of the benzene-13C isotopomer is that of a slightly asymmetric top. Both species are characterized by an almost free rotation of the two subunits with respect to each other.