Weak hydrogen bonds unveiled by Fourier Transform Microwave spectroscopy

With Fourier Transform Microwave Spectroscopy we have observed and assigned the rotational spectra of several molecular adducts namely: (CH2FCl)2, CH2FCl∙∙∙H2O and (CH3)2O∙∙∙ CHF3 obtaining straightforward information on their structural arrangement and some information on their dynamics. The structural information is extracted from the spectroscopic constants and compared to high level ab initio calculations which help to justify and understand the observations. All dimers show unambiguously the presence of weak hydrogen bonds of the type C-H∙∙∙Cl, C-H∙∙∙F, C-H∙∙O and they are stabilized by multiple weak bonds. We also observed fine structures associated with the rotational transitions and those are due to nuclear quadrupole spin interaction or to the presence of large amplitude motions occurring in the dimer. In the case of nuclear quadrupole spin interaction, the determined molecular constants can be used as additional structural information, since the effect depends on the orientation of the electric field gradient at the nucleus involved (in our case the chlorine nucleus). The internal motions effects were observed for CH2FCl∙∙∙H2O and (CH3)2O∙∙∙ CHF3 for which the spectra showed a splitting of the rotational transitions. In both cases the fine effect was used to extract information on the potential energy surface associated with the motion which in the case of weak non covalent bonds is usually shallow and allows tunneling across low barriers. The case of CH2FCl∙∙∙H2O is special since there is the possibility of forming two different adducts: the HOH ∙∙∙Cl one and the HOH ∙∙∙F one. The rotational spectrum shows the presence of only the dimer bound to the chorine atom. So, in this case it is the water molecule that chooses, and we show experimentally that it prefers chorine!