15min:

NORMAN C. CRAIG, Department of Chemistry, Oberlin College, Oberlin, OH 44074; ROBERT L. SAMS, ESML, Pacific Northwest National Laboratory, Richland, WA 99352.

In a search for rotational structure of the gauche rotamer of butadiene, high-resolution (0.002 cm^-1) infrared spectra were recorded at a long path length in regions that grew in intensity when the temperature was raised from 0^\circC to 50^\circC. These regions were also ones where Huber-Wälchli and Günthard had observed bands for a second, higher energy rotamer, when hot butadiene was frozen in an argon matrix at 4 K.\footnoteP.~Huber-Wälchli, Hs.~H.~Günthard, Spectrochim. Acta \textbf1981, 37A , 285. From medium-resolution infrared spectra, DeMaré and coworkers had concluded that a band at 749 cm^-1 was from the gauche rotamer.\footnoteG.~R.~DeMaré, Yu.~N.~Panchenko, J.~ Vander~Auwera, J. Phys. Chem. A \textbf1997, 101 , 3998. Our high-resolution analysis of this largely A-type band indicates it is, however, probably a difference tone [ ₁₅( b_g) - ₁₃(\textit a_u )] 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,\footnoteJ.~Saltiel, D.~F.~Sears, Jr., A.~M.~Turek, J. Phys. Chem. A \textbf 2001, 105 , 7569. failed. Much of the searching occurred in the wings of the 525-cm^-1 band for ₁₂( a_u ) of the trans rotamer, which has been analyzed for the first time. In addition to rotational transitions associated with the ₁₂ fundamental, the rotational structure in hot bands was also analyzed.