Analysis of Rotational Structure in the High-Resolution Infrared Spectrum and Assignment of Vibrational Fundamentals of Butadiene-2,3-13C2

Abstract The 2,3- 13 C 2 isotopomer of butadiene was synthesized, and its fundamental vibrational fundamentals were assigned from a study of its infrared and Raman spectra aided with quantum chemical predictions of frequencies, intensities, and Raman depolarization ratios. For two C-type bands in the high-resolution (0.002 cm −1 ) infrared spectrum, the rotational structure was analyzed. These bands are for ν 11  ( a u ) at 907.17 cm −1 and for ν 12  ( a u ) at 523.37 cm −1 . Ground state and upper state rotational constants were fitted to Watson-type Hamiltonians with a full quartic set of centrifugal distortion constants and two sextic ones. For the ground state, A 0  = 1.3545088(7) cm −1 , B 0  = 0.1469404(1) cm −1 , and C 0  = 0.1325838(2)  cm −1 . The small inertial defects of butadiene and two 13 C 2 isotopomers, as well as for five deuterium isotopomers as previously reported, confirm the planarity of the s-trans rotamer of butadiene.