Knowledge Bank

In a search for rotational structure of the gauche rotamer of butadiene, high-resolution (0.002 cm$\mathrm{<mrow\; data-mjx-texclass="ORD">-1</mrow>}$) infrared spectra were recorded at a long path length in regions that grew in intensity when the temperature was raised from $0irc$C to $50irc$C. These regions were also ones where Huber-Walchli and Gunthard had observed bands for a second, higher energy rotamer, when hot butadiene was frozen in an argon matrix at 4 K.} \textbf{1981}, $37A$, 285.} From medium-resolution infrared spectra, DeMare and coworkers had concluded that a band at 749 cm$\mathrm{<mrow\; data-mjx-texclass="ORD">-1</mrow>}$ was from the gauche rotamer.} \textbf{1997}, $101$, 3998.} Our high-resolution analysis of this largely A-type band indicates it is, however, probably a difference tone [$\nu 15$($bg$) - $\nu 13$($au$)] of the trans rotamer. Attempts to identify other subband series attributable to the gauche rotamer, which is present to only about 5% at room temperature,} \textbf{ 2001}, $105$, 7569.} failed. Much of the searching occurred in the wings of the 525-cm$\mathrm{<mrow\; data-mjx-texclass="ORD">-1</mrow>}$ band for $\nu 12$({\it a}) of the trans rotamer, which has been analyzed for the first time. In addition to rotational transitions associated with the $\nu 12$ fundamental, the rotational structure in hot bands was also analyzed.