Synchrotron-Based High Resolution Far-Infrared Spectroscopy of -Butadiene.

The high resolution far-infrared spectrum of -butadiene has been reinvestigated by Fourier-transform spectroscopy at two synchrotron radiation facilities, SOLEIL and the Canadian Light Source, at temperatures ranging from 50 to 340 K. Beyond the well-studied bands, two new fundamental bands lying below 1100 cm, ν and ν, have been assigned using a combination of cross-correlation (ASAP software) and Loomis-Wood type (LWWa software) diagrams. While the ν analysis was rather straightforward, ν exhibits obvious signs of a strong perturbation, presumably owing to interaction with the dark ν + ν state. Effective rotational constants have been derived for both the = 1 and = 1 states. Since only one weak, infrared active fundamental band (ν) of -butadiene remains to be observed at high resolution in the far-infrared, searches for the elusive conformer can now be undertaken with considerably greater confidence in the dense ro-vibrational spectrum of the form.