Direct experimental evidence that the X-H∙∙∙Cl bond is stronger than X-H∙∙∙F bond (X=O, C)

We have observed and assigned the rotational spectra of the molecular adducts CH2FCl∙∙∙H2O and (CH2FCl)2 with Fourier Transform Microwave Spectroscopy, obtaining straightforward information on their structural arrangement and internal dynamics. a) O-H∙∙∙Cl vs O-H∙∙∙F From the spectroscopic constants of various isotopomers of the water moiety we have extracted the structural information on the CH2FCl∙∙∙H2O complex and the structural data is compared to high level ab initio calculations. In the case of CH2FCl∙∙∙H2O there is the possibility of forming two different adducts: the O-H∙∙∙Cl one and the O-H∙∙∙F one. The ab initio energies for the two conformations are very similar with the O-H∙∙∙F bound complex slightly more stable (ca. 20 cm-1). The rotational spectrum shows only the presence of the dimer which is hydrogen bound to the chorine atom although both conformations should have sizeable dipole moment components and should thus be observable. So it seems that the chlorine bound complex is more stable than the fluorine bound one. In this case it is the water molecule that chooses, and we show experimentally that it prefers chorine. The internal motions effects observed for CH2FCl∙∙∙H2O were 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. b) C-H∙∙∙Cl vs C-H∙∙∙F The dimer (CH2FCl)2 shows unambiguously the presence of weak hydrogen bonds of the type C-H∙∙∙Cl, C-H∙∙∙F and is stabilized by multiple bonds. Between all possible conformations, the observed one is the one bearing the highest number of C-H∙∙∙Cl interactions. 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 in this case the observed geometrical arrangement corresponds to the lowest energy conformational minimum. In both complexes we observed fine structures of the rotational transitions due to nuclear quadrupole spin interaction or to the presence of large amplitude motions occurring in the dimer. The nuclear quadrupole spin interaction constants are 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).