Features of weak C-H···O and C-H···F intermolecular hydrogen bonds by Fourier transform microwave spectroscopy

So much scientific interest has been dedicated during the last years to the Weak Hydrogen Bond (WHB), a non covalent interaction which manifests itself in myriad ways in Structural Chemistry and Biology, that this topic deserved recently the publication of a book [1]. Only a few rotationally resolved spectroscopic investigations, which detail specific or local interactions, are available, however, on WHB. The three C-H×××F-C WHB interactions of the difluoromethane dimer [2] and the three C-H···O interactions of dimethylether dimer [3] have been characterized from their pure rotational spectra. Each of these interactions has been estimated to have an energy of about 2 kJ/mol and a little blue shift of the C-H stretching Here we report the results of the investigations of the rotational spectra of some heterodimers held together through three WHB’s. 1) Oxirane-trifluoromethane. The investigation of the parent and of its two 13C species has been used to establish a Cs geometry, with the two moieties bound by one C-H×××O and two C-H×××F-C hydrogen bonds. An overall bonding energy of about 6.7 kJ/mol has been derived from the centrifugal distortion analysis. The lengths of the C-H×××O and C-H×××F hydrogen bonds, r(O×××H) and r(F×××H), are 2.37 and 2.68 Å, respectively. The C-H···F-C interactions are at the origin of the HCF3 internal rotation motion barrier of 0.55(1) kJ/mol giving rise to the A-E splittings observed in the rotational spectra. 2) 1,4-dioxane-trifluoromethane. The FT-MW spectrum of the Cs conformation with CHF3 axial with respect to 1,3-dioxane has been assigned. It is stabilized by one C-H×××O and two C-H×××F weak hydrogen bonds. 3) cyclobutanone-trifluoromethane. This is the first rotationally resolved investigation of a complex 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. [1] - “The weak hydrogen bond in structural chemistry and biology” IUCr Monographs on crystallography, Vol. IX (G.R.Desiraju and T.Steiner Ed.s) Oxford University Press, (2001). [2] - (a) Caminati, W.; Melandri, S.; Moreschini, P.; Favero, P.G.; Angew. Chem. Int. Ed., 1999, 38, 2924; (b) Blanco, S.; López, J. C.; Lesarri, A.; Alonso, J. L.; J. Mol. Struct. 2002, 612, 255. [3] - Y. Tatamitani, B. Liu, J. Shimada, T. Ogata, P. Ottaviani, A. Maris, W. Caminati and J. L. Alonso, J. Am. Chem. Soc., 124 (2002) 2739.