Knowledge Bank

Alkanes appear capable of forming hydrogen bonds when sufficiently activated by neighboring electronegative substituents. In this context, the $C-H\cdots O$ interaction between the substituted alkanes trifluoromethane and difluoromethane with oxirane has been analyzed by molecular beam Fourier transform microwave spectroscopy. The ground state rotational spectra of $C2H4O\cdots HCF3$, $\mathrm{<mrow\; data-mjx-texclass="ORD">13</mrow>}CCH4O\cdots HCF3$, $C2H4O\cdots H13CF3$, $C2H4O\cdots H2CF2$, $C2H4O\cdots H213CF2$ and $\mathrm{<mrow\; data-mjx-texclass="ORD">13</mrow>}CCH4O\cdots H2CF2$ isotopomers have been studied, in their natural abundances, in the frequency range $6-18$ GHz. A $Cs$ symmetry has been established for the oxirane$\cdots$trifluoromethane complex with the C-H bond of trifluoromethane pointing to the domain of the nonbonding electron pairs of the O atom. The cooperative effect of two $C-F\cdots H-C$ interactions increases the stability of the complex. The barrier to internal rotation of the $CF3$ group has been determined from the observed A-E splittings. In the oxirane$\cdots$difluoromethane complex the determined structural data reveals the existence of two bifurcated hydrogen bonds: one between the $CH2$ group of difluoromethane with the O atom of oxirane and the other between the methylenic C-H groups of oxirane with the closest fluorine atom of difluoromethane. To our knowledge this is the first experimental evidence of this behaviour. The $C-H\cdots S$ interaction has been also analyzed in the trifluoromethane$\cdots$tiirane complex. The rotational spectra of $C2H4S\cdots HCF3,C2H434S\cdots HCF3,\mathrm{<mrow\; data-mjx-texclass="ORD">13</mrow>}CCH4S\cdots HCF3$ and $C2H4S\cdots H13CF3$ isotopic species have been studied in their natural abundance.