Knowledge Bank

We are studying dimethylacetylene (2-butyne) in the $1000-1100cm-1$ region the $\nu 11$ and $\nu 11$ and $\nu 15$ (methyl rocking) fundamental bands. The spectrum was recorded at a temperature of -60 C with a pressure of 1.4 Torr, an absorption path of 8 m, and a resolution of $00.0014cm-1$, using our modified double-pass Bomem FT spectrometer and cooled multiple-traversal cell. The spectrum is congested, with thousands of resolved and partly-resolved lines as well as many regions where the rotational structure is unresolved. In spite of this complexity, we have been able to make significant progress in the analysis of the bands. More than 200 p, Q, and R-branch transitions in $Ki=0$ (with $K\prime \prime =0$ and 6) and $Ki=3$ (with $K=3$) subbands have been assigned, where $Ki$ is the torsional quantum number. Analysis of these lines gives a value of $B=0.11190cm-1$ for the rotational constant of the molecule in its ground state, Q-branches of numerous subbands with other values of $Ki$ can also be recognized in the spectrum, and their analysis will be discussed. Dimethylacetylene has long been of theoretical interest as one of the simplest examples of a molecule with a near-zero barrier to internal rotation. However, as far as we know the present study constitutes the first rotationally analyzed spectrum of this fundamental species.