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Microwave spectra for seven isotopomers of cis-1,2-difluoroethylene have been reinvestigated and extended with the pulsed-molecular-beam Fourier transform method. Rotational Hamiltonians that include a full set of quartic centrifugal distortion constants have been fit for each isotopomer. Ground-state rotational constants for the trans isomer were available from an investigation of the high-resolution infrared spectra of several isotopomers. Equilibrium rotational constants have been extracted for the cis and trans isomers from the experimental ground-state constants and the vibration-rotation constants computed at the MP2/6-31G(d) level. Equilibrium structures have been fit to the equilibrium principal moments of inertia for both isomers, and isomeric differences in the bond lengths and bond angles have been interpreted in terms of the strong influence of fluorine substitution. For cis,trans-1,4-difluorobutadiene a similar derivation of the equilibrium structure has been achieved through the combination of published microwave data and quantum chemical calculations. This substance provides an opportunity to examine the influence of cis and trans fluorine substitution within a single molecule. Evidence is weighed for the adequacy of vibration-rotation constants computed with the MP2/6-31G(d) model.

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

3-Fluorocyclobutene (1) and 3-chlorocyclobutene (2) have been obtained by simpler syntheses and in higher yield than previously reported. One of the synthetic steps involves a three-phase-transfer-catalysis technique that could be of wider utility. New examples are cited of applying AgF2, an underutilized reagent, as a fluorinating agent near room temperature. Compounds 1 and 2 have been characterized by mass, NMR and vibrational spectroscopies. The NMR spectra have been fully analyzed. Useful group frequencies in the vibrational spectra of halocyclobutenes have been identified. The five monochloro- monofluorocyclobutane precursors of fluorocyclobutenes have also been separated and characterized. From NMR coupling constants, conformer mixtures have been identified for three of the isomers.

The microwave spectra of four isotopic species of 3,3‐difluorocyclopropene have been investigated in the region from 18–40 GHz. Structural parameters obtained are r (C1C2) =1.321±0.001, r (C1C3) =1.438±0.007, r (CH) =1.075±0.001 , r (CF) =1.365±0.005 A, &FCF=105.5±0.5°, &C1C3C2=54.6±0.4°, &HC1C2=148.4±0.1°. From the Stark effect the dipole moment has been determined to be 2.98±0.02 D.

Attribution complete des fondamentales de vibration du compose du titre a partir des spectres IR et Raman, et des calculs en coordonnees normales d'ordre zero

$^{1}$N. C. Craig, J. Pranata, J. R. Sprague, and P. S. Stevens, J. Am. Chem. Soc., 106, 7637 (1984) $^{2}$P. Pulay, G. Fogarasi, and J. E. Boggs, J. Chem. Phys., 74, 3399 (1981)

Infrared spectra under a variety of experimental conditions and Raman spectra for the liquid phase have been recorded for the cis and trans isomers of 1,2-difluorocyclopropane and the 1,2-d$sub 2$, 3,3-d$sub 2$, and d$sub 4$ deuterated modifications. Almost all of the 168 fundamentals of this set of eight molecules have been observed and assigned convincingly. For cis-1,2- difluorocyclopropane-d$sub 0$ the fundamentals are (a') 3105, 3063, 3023, 1450, 1365, 1224, 1135, 1047, 862, 784, 468, 209 cm$sup -1$; (a'') 3055, 1346, 1150, 1089, 1060, 993, 739, 621, 319 cm$sup -1$. For trans-1,2-difluorocyclopropane- d$sub 0$ the fundamentals are (a) 3070, 3021, 1457, 1380, 1203, 1132, 1068, 961, 842, 415, 279 cm$sup -1$; (b) 3110, 3070, 1304, 1161, 1072, 1005, 937, 783, 452, 303 cm$sup -1$. Only the 1089-cm$sup -1$ mode in the cis and the 937-cm$sup -1$ mode in the trans are in doubt. Group frequency descriptions are discussed qualitatively, and attention is drawn to the fundamentals near 1450 cm$sup -1$ which are characteristic of fluorocyclopropanes and appear to be largely due to ring stretching. From previously published equilibrium data for the cis-to-trans isomerization and the fundamental frequencies of the two isomers, an electronic energy difference of -2800 +- 200 cal/mol hasmore » been calculated. In contrast, the trans isomer of 1,2-difluoroethylene has an energy 1100 cal/mol greater than the cis, and the trans rotamer of 1,2-difluoroethane has an energy about 600 cal/mol greater than the gauche. (auth)« less