Vibrational spectroscopy of the three isomers of 1,4-difluorobutadiene

Infrared and Raman spectra were recorded for the trans,trans (EE), cis,cis (ZZ), and cis,trans (ZE) isomers of 1,4-difluorobutadiene (DFBD). From these spectra and frequencies predicted from the adiabatic connection method, which is a hybrid of Hartree-Fock and density-functional theories, complete assignments of fundamentals were made for the observable s-trans configurations. The fundamentals for the trans,trans isomer are (in cm(-1)): (a(g)) 3091, 3048, 1681, 1325, 1280, 1151, 1121, 409, 383; (a(u)) 934, 798, 227, 154; (b(g),) 897, 830, 397; and (b(u)) 3086, 3056, 1638, 1299, 1221, 1088, 621, 133. The fundamentals for the cis,cis isomer are (in cm(-1)): (a(g)) 3118, 3088, 1676, 1410, 1248, 1134, 946, 751, 232; (a(u)) 914, 762, 330, 78; (b(g)) 897, 789, 580; and (b(u)) 3109, 3092, 1624, 1340, 1215, 1044, 632, 165. The fundamentals for the cis,trans isomer are (in cm(-1)): (a') 3114, 3082, 3062, 3036, 1690, 1629, 1391, 1313, 1253, 1224, 1138, 1129, 1008, 706, 504, 308, 138; and (a") 929, 887, 824, 758, 526 (calculated), 230, 155. Overall agreement between the assignments and the predicted frequencies is quite good. With allowance for the difference between modes strongly dependent on CF or CCl motions, a very good correlation was found between the fundamentals of the three isomers of DFBD and the corresponding isomers of 1,4-dichlorobutadiene. Both sets of isomers are of special interest because they exhibit the cis effect, in which the cis,cis isomer has the lowest energy and the trans, trans isomer has the highest energy.