High-resolution infrared spectra of the two nonpolar isomers of 1,4-difluorobutadiene

High-resolution (0.0013 cm−1) infrared spectra have been recorded for trans,trans-1,4-difluorobutadiene (ttDFBD) and cis,cis-1,4-difluorobutadiene (ccDFBD). The rotational structure in two C-type bands (ν10 and ν12) and one A-type band (ν22) for ttDFBD and in two C-type bands (ν11 and ν12) for ccDFBD has been analyzed. Ground state and upper state rotational constants, except for ν10 of ttDFBD, have been fitted. Band centers are 934.1 cm−1 (ν10), 227.985 cm−1 (ν12), and 1087.919 cm−1 (ν22) for ttDFBD. Band centers are 762.891 cm−1 (ν11) and 327.497 cm−1 (ν12) for ccDFBD. The small inertial defects in the ground state confirm that both isomers are planar. Obtaining the ground state rotational constants for the two isomers of DFBD is a first step toward determining their semi-experimental equilibrium structures.