Low barrier methyl rotation in 3-Pentyn-1-ol as observed by microwave spectroscopy

It is known that the barrier to internal rotation of the methyl groups in ethane (\textbf{1}) is about 1000 \wn.\footnote{R. M. Pitzer, \emph{Acc. Chem. Res.}, \textbf{1983}, \emph{16}, 207–210.} If a C-C-triple bond is inserted between the methyl groups as a spacer (\textbf{2}), the torsional barrier is assumed to be dramatically lower, which is a common feature of ethinyl groups in general. \newline \indent To study this effect of almost free internal rotation, we measured the rotational spectrum of 3-pentyn-1-ol (\textbf{3}) by pulsed jet Fourier transform microwave spectroscopy in the frequency range from 2 to 26.5 GHz. Quantum chemical calculations at the MP2/6-311++G(d,p) level of theory yielded five stable conformers on the potential energy surface. The most stable conformer, which possesses C$_{1}$ symmetry, was assigned and fitted using two theoretical approaches treating internal rotations, the rho axis method (\emph{BELGI-C$_{1}$}) and the combined axis method (\emph{XIAM}). The molecular parameters as well as the internal rotation parameters were determined. A very low barrier to internal rotation of the methyl group of only 9.4545(95) \wn \ was observed. \begin{wrapfigure}{r}{0pt} \includegraphics[scale=0.55]{3Py1.eps} \end{wrapfigure