VIBRATIONAL-SPECTRA AND ASSIGNMENTS FOR 1-FLUOROCYELOBUTENE AND 1-CHLOROCYCLOBUTENE - REVISED ASSIGNMENT FOR CYCLOBUTENE

Gas-phase infrared spectra and liquid-phase Raman spectra are presented for 1-chlorocyclobutene and 1-fluorocyclobutene. Complete assignments of vibrational fundamentals are proposed for these molecules of C(s) symmetry. For 1-chlorocyclobutene (cm-1): (a') 3090, 2947, 2940, 1595, 1452, 1434, 1250, 1197, 1165, 1120, 916, 891, 869, 493, 299; (a") 2980, 2966, 1140, 1070, 1016, 854, 751, 395, 211. For 1-fluorocyclobutene (cm-1): (a') 3111, 2954, 2948, 1667, 1464, 1437, 1306, 1214, 1189, 1154, 972, 911, 866, 651, 412; (a") 2982, 2969, 1142, 1076, 1020, 855, 764, 436, 264. Frequencies of CH-rich modes correlate closely for these isotopomer-like molecules. These results along with a Raman spectrum of liquid cyclobutene near -100-degrees-C have provided evidence for assigning five unsettled vibrational fundamentals of cyclobutene. These modes in cm-1 are (a2) nu-9 = 2944, nu-11 = 1011, and nu-12 = 846 and (b1) nu-19 = 903 and nu-20 = 888. Ring-puckering frequencies for a variety of halogen-substituted cyclobutenes are compared and found to follow an understandable pattern.