Knowledge Bank

The low-temperature environment of a free jet expansion provides an excellent laboratory for probing the conformational behaviour of hydrogen bonding. The first observation of axial and equatorial hydrogen-bonded complexes has been reported for tetrahydropyran and pentamethylene sulphide complexes with HCl and $HF.a$ Both six-membered rings present two nonequivalent lone pairs at the O or S atoms and the formation of hydrogen bonds gives rise to two different axial and equatorial conformers. Recently, we have also reported axial and equatorial hydrogen bonds for trimethylene sulphide$\cdots HCl.b$ For isolated trimethylene sulphide the ring-puckering tunnellig motion makes equivalent the nonbonding electron pairs at the S atom. This equivalence is broken by effect of complexation, giving rise to axial and equatorial conformers. In this contribution we present the results on the related pentamethylene and trimethylene sulphide$\cdots$ HF complexes. An unsubstituted alkane such as methane might act as a proton donor only in certain extraordinary circunstances. However 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 alkane trifluoromethane and oxirane has been analyzed by MB-FTMW spectroscopy for the parent and $\mathrm{<mrow\; data-mjx-texclass="ORD">13</mrow>}C$ isotopomers. A $r0$-like structure has been derived and the barrier to internal rotation of the $CF3$ group has been determined from the observed A-E splittings. A MB-FTMW Spectrometer incorporating a laser ablation source has been constructed in our lab which avoids the use of high temperature techniques commonly used for vaporising low-volatility compounds. As a preliminary test, the rotational spectrum of 1,3,5-trithiane (m.p. $216\circ C$) has been observed. The sensitivity of the instrument proved to be high enough to observe $\mathrm{<mrow\; data-mjx-texclass="ORD">33</mrow>}S$ isotopic species in natural abundance.