Vibrational Assignments and a Potential Function ... - ACS Publications

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The Journal of Physical Chemistry, Vol. 82, No. 9, 1978

nitrogen and an interchain space large enough to accommodate the water molecules. Likewise, it can be added that in the micellar solutions studied here, there appears to be no water molecules in a hydrocarbon-like environment. Finally, it was noted that, a t 25 "C, the appearance of the 950-nm absorption in solution of R4NBr seems related to the minimum in the concentration dependence of their apparent molal volumes (4").However, from the 4"data presently available25a t 60 " C , this correlation does not hold.

N. C. Craig, R. A. MacPhail, and D. A. Spiegel

Canada and of the France-Qu6bec scientific exchange program. References and Notes J. Paquette and C. Jolicoeur, J . Solution Chem., 6, 403 (1977). P. Philip and C. Jolicoeur, J. Phys. Chem., 77, 3071 (1973). C. Jolicoeur, N. D. The, and A. Cabana, Can. J. Chem., 49, 2008 (1971). C. Jolicoeur and P. R. Philip, J . Solution Chem., 4, 3 (1975). R. W. Gurney, "Ionic Processes in Solution", McGraw-Hill, New York, N.Y., 1954, Chapter 16. H. S. Frank, J. Phys. Chem., 67, 1554 (1963). W. Y. Wen, J . Solution Chem., 2, 253 (1973). J. E. Desnoyers and C. Jolicoeur, "Modern Aspects of Electrochemistry", J. O'M. Bockris and 6. E. Conway, Ed., Plenum Press, New York, N.Y., No. 5, 1969, Chapter 1. P. S. Ramanathan, C. V. Krishnan, and H. L. Friedman, J . Solution Chem., I,237 (1972). W. H. Streng and W. Y. Wen, J . Solution Chem., 3, 865 (1974). J. Davies, S. Ormondroyd, and M. C. R. Symons, J. Chem. SOC., Faraday Trans. 2, 4, 686 (1972). M. M. Marciacq-Rousselot, A. de Trobriand, and M. Lucas, J . Phys. Chem., 76, 1455 (1972). H. G. Hertz, Wafer, Compr. Treat., 3, Chapter 16 (1973). C. Jolicoeur, P. Bernier, E. Firkins, and J. K. Saunders, J. Phys. Chem., 80, 1908 (1976). P. Picker, E. Tremblay, and C. Jolicoeur, J . Solution Chem., 3, 377 (1974). M. Lucas and A. de Trobriand, C.R. Acad. Sci. (Paris),274, 1361 (1972). J. Lang, C. Tondre, R. Zana, R. Bauer, H. Hoffman, and W. Ulbricht, J . Phys. Chem., 79, 276 (1975). P. A. Leduc and J. E. Desnoyers, Can. J . Chem., 51, 2993 (1973). S. Ablett, M. D. Barratt, F. Franks, M. D. Pedley, and D. S. Reid, "L'eau et les syst6mes biologiques", Colloque International du CNRS, No. 246, ed. CNRS, Paris, 1976, p 105. S. Lindenbaum, J . Phys. Chem., 75, 3733 (1971). M. Lucas, A. de Trobirand, and M. Ceccaldi, J. Phys. Chem., 79, 913 (1975). 0. D. Bonner and Y. S. Choi, J. Phys. Chem., 78, 1723 (1974). A. Bruneau and J. Corset, Can. J . Chem., 52, 915 (1974). G. R. Choppin and N. J. Hornug, Spectrochim. Acta, Part A , 30, 1615 (1974). A. LoSurdo and H. E. Wirth, J . Phys. Chem., 76, 1333 (1972).

5. Conclusion The results of the present investigation first provide confirming evidence to previous assignment of near-IR hydration spectra of the R4N+ ions. The differential spectra obtained with various salts of Bu4Nt show that, in the presence of small hydrophylic anions, mutual disturbance of the hydration cospheres of the various ions is within 10% of the total hydration effect a t 0.5 m. However, when both ions contain large hydrophobic groups (e.g., tetrabutylammonium hexanoate), the spectra exhibit an important concentration dependence which has been assigned to partial disruption of the structured hydration cospheres. Based on this interpretation, the spectral data are capable of predicting a major part of the heat of dilution of these salts. Finally, it is shown that concentrated solutions of the larger R4NBr salts exhibit a new absorption band ca. 950 nm. From the spectra of water diluted in various solvents, this component can be assigned to water molecules retained between alkyl chains of pairs, or higher clusters, of the hydrophobic ions. Acknowledgment. The authors gratefully acknowledge the financial support of the National Research Council of

Vibrational Assignments and a Potential Function for 3,3-Difluorocyclopropene-do, - d l , and - d z Norman C. Craig," Richard A. MacPhail, and David A. Splegel Department of Chemistry, Oberiin College, Oberlin, Ohio 44074 (Received November 2, 1977) Publication costs assisted by the Petroleum Research Fund

Complete assignments of the vibrational fundamentals of 3,3-difluorocyclopropeneand its dl and dz isotopic modifications are derived from infrared and Raman spectra. The 15 fundamentals for the undeuterated molecule are as follows, in cm-l: (al) 3150, 1598, 1343, 946, 769, 500; (az)883, 393; (b,) 3128, 1131, 968, 522; (b,) 1094, 680,416. An 18-parameter potential function is fit to the full set of frequencies for the three isotopic species. Contributions of internal coordinates to normal modes are analyzed in terms of potential energy distributions, and frequencies are correlated with those of the closely related molecules, CFz-N=N, CH2-CH=CH, CF2CH=CF, and O=C-CH=CH. Changes in potential constants of ring bonds in going from unsubstituted molecules to fluorine-bearingones correlate with changes in bond lengths. Interpretable CF bond characteristics are also found in these molecules. I

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Introduction Fluorine-bearing C3rings are of current interest because of the marked electronic effect of fluorine substituents on these systems. This effect is evident in the changes in carbon-carbon bond lengths in the C3 rings. Laurie and co-workers have obtained detailed structures for 1,l-difluorocyclopropane and for 3,3-difluorocyclopropene.1~z Compared to the corresponding unsubstituted hydro0022-3654/78/2082-1056$01 .OO/O

I

carbons, the CC bonds adjacent to the fluorine-bearing carbons are shorter and the CC bonds opposite are longer. Ab initio molecular orbital calculations correlate these effects of fluorine substitution with changes in strength of the ring bonds.3 One might also expect to find a correlation with changes in potential constants for the ring bonds. Thus, the CC stretching constants for the adjacent CC bonds would be strengthened and the CC stretching

0 1978 American

Chemical Society

Vibrational Study of 3,3-Difluorocyclopropene

constants for the opposite CC bonds weakened. In addition, sympathetic changes in the bond lengths and potential constants for the CF bonds should be apparent. With the objective of securing well-determined potential constants for the influence of fluorine substitution on the full range of bond types in C3 rings, we have undertaken a vibrational study of three species: 1,l-difluorocyclopropane, 3,3-difluorocyclopropene, and the monofluorocyclopropenyl cation, along with various deuterated modifications. Within this set of molecules are the strained single bond, the strained double bond, and the strained delocalized (aromatic) double bond. The present paper reports on vibrational spectroscopy and normal coordinate calculations for 3,3-difluorocyclopropene and its two deuterated modifications. A parallel study of 1,l-difluorocyclopropane is nearing completion. It has also proved possible to convert difluorocyclopropene to the monofluorocyclopropenyl cation. 3,3-Difluorocyclopropene is a new molecule. Its synthesis and characterization are described. Gas-phase, matrixphase, and crystal-phase infrared spectra and liquid-phase Raman spectra have been recorded and used to obtain virtually complete vibrational assignments of the 45 fundamental vibration frequencies of the three isotopically varied species. Fundamentals for difluorocyclopropene are compared with those for similar modes in the isoelectronic molecule, difluorodiazirine (CFz-N=N),4 in cyclop r ~ p e n e ,in~ cyclopropenone,6 and in 1,3,3-trifluorocy~lopropene.~ A potential function of a modified valence type has been fitted to the full set of frequencies for 3,3-difluorocyclopropene and its two deuterated species. The extent to which various normal modes can be described as simple group frequencies is examined. Potential constants are compared with those of related molecules including cyclopropene, diazirine,8 arid difluorodiazirine. For use in these comparisons it has been necessary to recompute potential constants for some of these molecules. The expected influence of fluorine substitution at the apex carbon atom on the potential constants for the bonds in the ring is found. Lengths and potential constants for CF bonds are also examined.

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Experimental Section Synthesis and Purification. 3,3-Difluorocyclopropene was prepared by dehydrohalogenation of l-chloro-2,2difluorocyclopropane. The reactant chlorodifluorocyclopropane was synthesized by reaction of vinyl chloride with CF2, which was produced by the thermal decomposition of perfluoropropylene ~xide.~JO Vinyl chloride (30 mmol; source, Matheson Co.) and 15 mmol of perfluoropropylene oxide were sealed in a 500-mL, cylindrical Pyrex vessel, and the gaseous mixture held at 200 "C for 8 h. The yield of cyclopropane was about 50%. Bulbto-bulb distillation from -70 "C to Iiquid nitrogen temperature removed coproduct CF3COF and most of the unreacted vinyl chloride as well as minor by-products. Further purification of the chlorodifluorocyclopropane was accomplished by gas chromatography at 60 "C on a 4-m tricresylphosphate (TCP)-on-firebrickcolumn. Relative elution time of the cyclopropane to vinyl chloride was 4.0. To dehydrohalogenate the chlorodifluorocyclopropane, it was vacuum distilled rapidly and repeatedly at room temperature through a 40-cm tube filled with 20-30 mesh Ascarite (NaOH on asbestos). This treatment also removed residual CF3COF. The Ascarite was not pumped dry, because moist Ascarite was more effective in the dehydrohalogenation reaction. Heating the column to 50


1057

"C did not improve yields appreciably, presumably because Ascarite also causes decomposition of the product cyclopropene.ll When it became too dry, Ascarite was dampened by pulling room air through it. Ten passes of a sample of the chlorodifluorocyclopropane through the Ascarite column gave conversions of about lO%.l2 A fraction rich in the more volatile cyclopropene was then vaporized off and fractionated on the TCP gas chromatography column at room temperature. Higher column temperatures caused loss of cyclopropene. Relative elution time of the cyclopropene to the cyclopropane was 0.6. Finally, the cyclopropene was dried by passing it through a column packed with Pz06. Difluorocyclopropene was stored without loss at liquid nitrogen temperature. It was also stored successfully in the gas phase at room temperature in sealed flasks at pressures below 50 Torr, provided the sample was very pure and flask was rigorously dry. Vapor pressures of difluorocyclopropene were measured from -40 to 10 "C and extrapolated on the basis of the Clausius-Clapeyron equation to a normal boiling point of 34 "C. 3,3-Difluorocyclopropene-d, was prepared by rapid exchange of difluorocyclopropene in DzO that was about 3 M in NaOD. In a typical experiment 0.5 mmol of the cyclopropene were condensed into a 50-mL flask containing 4 mL of frozen DzO solution. The DzO was then thawed as rapidly as possible while the flask was being shaken and the cyclopropene was quickly drawn off without refreezing the D20. Partly deuterated cyclopropene samples were prepared with D z 0 / H 2 0mixtures. After exchange the cyclopropene was dried again. All of the transfers described in this Experimental Section, including those to the gas chromatograph, were made on a good vacuum system. Mass spectra confirmed the identity of the difluorocyclopropene and its deuterated modifications.13 For the undeuterated molecule principal peaks (and relative intensities) are as follows: 77 (3) 13CCzF2H2, 76 (61) C3F,H2 and l3CC2FzH,75 (100) C3F2H,74 (3) C3F2,57 (25) C3FHz, 56 (6) C,FH, 55 (3) CSF, 50 (7) CFZ, 31 (45) CF, 26 (22) CzHz. The dz molecule gave a comparable spectrum. For the l-chloro-2,2-difluorocyclopropane the base peak (intensity 100) was at 77 for the C3F2H3fragment; the parent peak of 112 for C3FzH336C1 was only 0.3. Other intense peaks were 51 (43) CFzH and 27 (21) C2H,. The chlorodifluorocyclopropane has a rich infrared spectrum with principal bands in the gas phase (cm-', intensity, band shape): 3065 w A, 1470 vs B?, 1365 m B?, 1300 s C?, 1215 vs A, 1020 m A, 990 s B?, 950 s A, 890 w C, 730 w B, 695 w A, 585 w C, 480 w A, 405 vw A?, and 350 vw B?. Analytical gas chromatograms from the TCP column showed that all samples used for vibrational spectroscopy had purities greater than 99%. Mass spectral analysis gave 3% dl for the d z sample that was used for infrared spectroscopy. The sample used for Raman had a higher isotopic purity as judged from the infrared spectrum. Mass spectral analyses for the dlsamples gave 63% do, 33% dl, and 4% dz for the sample that was hydrogen rich; and 4% do, 39% dl, and 57% d z for the other sample that was deuterium rich. The latter two analyses for dl samples were checked within 1-2% by infrared experiments in which samples of the majority impurity, do in one and d2 in the other, were placed at measured pressures in a cell in the reference beam to give nearly pure dl spectra. Spectroscopy. Infrared spectra were recorded on a Perkin-Elmer 621 spectrometer in cells equipped with cesium iodide windows. Cells for gas spectra had 10-cm

1058



FREQUENCY

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Figure 1. Infrared spectrum of 3,3-difluorocyclopropene in the gas phase.

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path lengths. Solid-phase spectra were obtained in a conventional low temperature cell by spraying gas samples onto a CsI plate cooled to liquid nitrogen temperature and annealing by temperature cycling. For matrices a Cryogenics Technology Model 20 Cryocooler was used. Dilution ratios Were 2OOO:l with either argon or nitrogen as the matrix gas (both Matheson Co. research grade). Deposition rates of matrices were about 3 mmol/h. Gas-phase infrared spectra under survey conditions are shown in Figures 1, 4, and 7; and matrix-phase spectra in Figures 2 , 5 , and 8-13. Results of solid-phase spectra are given as parts of the detailed assignments in Tables I1 and 111. Frequencies are good to f l cm-l, as read from scans at 1/3 the normal slit program (with appropriate changes in other parameters) and expanded scale. The spectral slit width

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is then less than 1 cm-' throughout the spectrum. The matrix spectra in Figures 8-13 were recorded under these higher resolution conditions and show blips from the frequency marker. Raman spectra were recorded on a Spex Ramalog 5 spectrometer with excitation by filtered 5145-A light from a Coherent Radiation CR-5 argon-ion laser. Power level at the sample ranged from 100 to 250 mW. Liquid samples were sealed in capillaries and held at about -80 "C in a Harney-Miller type cell. At room temperature samples discolored and fluoresced. Figure 3 gives the spectrum of the undeuterated species, and Figure 6 the spectrum of the d2 species. No Raman spectra were attempted on the dl species because of the large do or d2 content of the samples. For the survey spectra shown in the figures the

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Figure 4. Infrared spectrum of 3,3-difluorocyclopropene-d, in the gas phase.

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spectral slit width was 5 cm-l. Tabulated frequencies, which were read from expanded scale spectra recorded with a 2.5-cm-' spectral slit width, are accurate to fl cm-l. Mass spectra were recorded on a Hitachi RMS-4 spectrometer.

Assignments The predictions for the infrared and Raman spectra of 3,3-difluorocyclopropene and its deuterated modifications are r d , , d 2 (CZu) = 6al lA(22), PI + 4bl [c(28), dpl + 2al (dP) + 3b2 W 7 ) , dPl and rd,(C,) = 10a'(A-C, p) + 5a"(B, d p )

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TABLE I : Moments of Inertia (amu A2P

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The numbers following the A and C band-shape designations for gas-phase infrared spectra are the calculated RP spacings for the do m~lecule.~ The QQ spacing is given for the B-type band, which had a well-defined QQ structure

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The Journal of Physical Chemistry, Vol. 82,

No. 9, 1978

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in the calculated spectrum. Deuteration does not markedly change the predicted spacings. Polarized Raman bands are designated p, and depolarized ones dp. Table I gives the principal moments of inertia of the three molecules. The undeuterated molecule has K = -0.29 and p* = 0.71 and is thus a rather asymmetric top. In the do and d2 molecules the axis of the least moment of inertia, I,, is coincident with the C2 symmetry axis. The I, axis also lies in the plane of the ring. This plane is considered to be the principal symmetry plane of the molecule and the plane in which the bl modes are symmetric. Although band shapes for a' modes of the monodeuterated molecule may range from A- to C-type, one can expect higher molecular symmetry to be reflected in the shapes of bands for modes that involve principally atoms other than hydrogen or deuterium. 3,3-Difluorocyclopropene-doand -d2. Details of the assignments for the do and d2 molecules are presented in Tables I1 and 111, respectively. al Modes. For the do molecule three obvious A-type bands with 22-24-cm-l R P spacings are found at 1598, 1343, and 500 cm-l in the gas-phase infrared spectrum (Figure 1). Confirming two of these are the polarized Raman bands at 1600 and 499 cm-l (Figure 3). A weak Raman feature occurs at 1316 cm-l. Strong, polarized Raman bands at 3153 and 760 cm-l reveal two more al fundamentals, even though the corresponding infrared bands are complicated by overlapping features. The PR separation of 22 cm-I for the principal band in the 3150-cm-l region in the gas-phase infrared spectrum supports an A-type description. In the infrared spectrum in the 770-cm-l region there are two distinct Q branches of comparable intensity. These two bands are attributed to Fermi resonance between 2v8 = 786 cm-l and v5 In the liquid-phase Raman spectrum an intense band with a shoulder on the high-frequency side is found in this region. In the argon matrix spectrum (Figure 2), the lower frequency component of the doublet has a significantly greater intensity. In the nitrogen-matrix and the crystal-phase spectra (Table 11),the higher frequency component is of even lower relative intensity. The marked sensitivity of the relative intensities of the two components to small frequency shifts caused by changes in phase is consistent with the Fermi resonance interpretation. Small frequency changes could cause sizable changes in extent of mixing of the combination tone and fundamental. We have taken the lower frequency of the two gas-phase Q branches as the better approximation to the fundamental. The sixth al fundamental of the do molecule is centered at the weaker of the two Q branches near 950 cm-l in the gas-phase infrared spectrum. Twice the QP spacing of this

band is 22 cm-' in agreement with an A-type band shape, and the Raman band, although weak, is definitely polarized. As discussed in a subsequent paragraph, the more intense, overlapping band proves to be a C-type band due to a bl mode. For the d2molecule five of the six al fundamentals give easily recognized A-type shapes to bands in the gas-phase infrared spectrum (Figure 4). These bands are at 2429, 1509, 1339, 657, and 487 cm-l. Polarized Raman bands (Figure 6) support each of these assignments. However, for the 1339-cm-l band, as in the Raman spectrum of the do molecule, the corresponding Raman band is quite weak. There can be no doubt about the assignment of a fundamental near 1340 cm-l since an intense, A-type band appears in the infrared in this region for all three isotopic species. From the Raman spectrum it is obvious that the intense, strongly polarized band near 800 cm-l is the sixth al fundamental for the d 2 molecule. A weak, apparently depolarized feature just above this intense band in the Raman spectrum corresponds to the C-type band which overlaps v4 in the gas-phase infrared and appears as a distinct feature in the argon-matrix (Figure 5) and crystal-phase (Table 111) infrared spectra. a2Modes. Assigning the two a2 modes of the do molecule is not straightforward, even though the Raman spectrum (Figure 3) appears to contain two obvious candidates: the depolarized bands a t 522 and 393 cm-l which have no counterparts of significance in the infrared spectrum. However, from group frequency comparisons even the higher of these frequencies is not high enough to be a candidate for v7, which is principally symmetric flapping of the CH bonds. As a guide, one notes that trifluorocyclopropene has its lone CH flapping frequency at 788 cm-l.I A weak Q branch is in the gas-phase infrared at 883 cm-l corresponding to a weak, apparently depolarized Raman band at 897 cm-I and cannot be explained as a combination or difference tone. These observations along with the findings for the other two isotopic species have led us to accept the 8 8 3 - ~ r n feature -~ as v7, made weakly infrared active by some perturbation. In the Raman spectrum of the d2 molecule an unmistakable, though weak, depolarized band, is at 740 cm-l. This corresponds to a weak Q branch in the gas-phase infrared, which cannot be satisfactorily explained as a combination or difference tone. We assign v7 at 732 cm-l for the d2molecule and take the weak infrared activity of this mode in both do and dz species as further support for the assignments. Difficulty in observing the symmetric CH twist in the Raman spectrum has been encountered before in cyclopropenes. No spectral evidence was found for this mode in cyclopropenone-do or -d2.6 At best it gives a weak feature in



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the spectra of cyclopropene-do and -1,2-d2.5 The two Raman-observable, infrared-unobservable bands of the do molecule described in the previous paragraph are both candidates for the one other a2 fundamental. A similar situation is found in the spectra of the d2 molecule where the 470- and 352-cm-l Raman bands have only weak features in the crystal phase as infrared counterparts. The gas-phase spectrum of the dl molecule is the key to assigning these two pairs of bands. As seen in Figure 7, the band in the 350-cm-l region is clearly B-type and therefore due to an out-of-plane fundamental. In contrast, the two bands in the 500-cm-l region in Figure 7 are A- or C-type and thus clearly due to in-plane fundamentals. Evidently, v7 is at 393 and 352 cm-l in the do and d2 molecules, respectively. The bands at 522 and 470 cm-', which are assigned as bl modes, are discussed further in subsequent paragraphs. Additional support for assigning v7, which is largely CF2 torsioning, in the 350400-cm-l region comes from the corresponding assignment at 340 cm-l in 1,l-difluorocyclopropane. The lowest frequency bl mode in this cyclopropane is at 535 cm-l.14 b2 Modes. These assignments are taken up next because they are uncomplicated and help set the stage for the assignments of the bl modes. In the gas-phase infrared spectrum of the do molecule obvious B-type bands are at 680 and 416 cm-l (Figure 1). Though somewhat distorted, the intense band at 1094 cm-I must also be B-type. QQ spacings in these three bands are 7 cm-l, in agreement with the calculated band shapes. The 416-cm-l band has an obvious Raman counterpart (Figure 3). That the 1094-cm-l band is an antisymmetric CF2 mode is consistent with the low intensity in the Raman spectrum and the substantial decrease in frequency in going from gas-phase to condensed-phase spectra (Table 11). Given the well-defined B-type band shape in the gas-phase infrared spectrum, no doubt exists about assigning vi4 at 680 cm-l. For the d2molecule good B-type band shapes are present at 1093 and 587 cm-l in the gas-phase infrared (Figure 4). A distorted B-type band is at 356 cm-l. Weak Raman features (Figure 6) correspond to the two higher frequencies and a well-defined Raman band corresponds to the lowest frequency. As in the spectrum of the do molecule, a large shift occurs in the 1093-cm-l band upon going to condensed phases. bl Modes. In the gas-phase infrared spectrum of the do molecule, reasonably intense C-type bands are found partly overlapped at 3128, 1131, and 968 cm-l (Figure 1). Although a useful RP spacing cannot be secured for the 3128-cm-l band, twice the RQ spacing for 1131-cm-l band is 30 cm-I and the R P spacing for the 968-cm-l band is 27 cm-l. These spacings are substantially larger than those for A-type bands and are consistent with the calculated value of 28 cm-l. Weak, depolarized Raman bands are observed for each of these three transitions (Figure 3). Although we did not select the 1131 cm-l as a bl fundamental in our first assignment of this spectrum, other proposals such as using one of the Q branches in the 770-cm-l region proved unsatisfactory when the spectra of all three isotopic species were considered. Furthermore, there is no satisfactory explanation for the 1131-cm-l band in the infrared as a combination or difference tone. The Raman band might be explained as the combination tone, v5 + v g = 1161 cm-l, however. This assignment for the antisymmetric CCC stretching mode correlates well with the assignment of this mode at 1091 cm-l for the isoelectronic molecule, difluorodiazirine, as shown in Table VI. The corresponding fundamental for cyclopropenone is a t 1161 cm-l, Table VI, and may also be supportive of

The Journal of Physical Chemistry, Vol. 82, No.

9, 1978 1063

the present assignment. By a process of elimination in the spectra of the do molecule and an unambiguous assignment for the dl molecule (vide infra), the Raman band at 522 cm-I must be due to the fourth bl mode. For the d2 molecule only one C-type band is apparent in the gas-phase infrared spectrum, and this feature at 833 cm-l is partly overlapped (Figure 4). An approximate R P spacing of 30 cm-l is obtained by doubling the RQ spacing. A bl transition must be in the 2320-cm-l region. Because the gas-phase infrared spectrum is complex and weak in this region, we have taken the 2322-cm-l value of the depolarized Raman line for this mode. Following the pattern of the assignment for the do molecule, the third bl transition is the Raman band at 470 cm-l (Figure 6), which does have a weak counterpart in the crystal-phase infrared spectrum (Table 111). As for the do molecule, the fourth bl fundamental, vl0, is a problem. Given the assignment for the do molecule, the missing bl transition of the d2 molecule must be in the 1000-llOO-cm-l region. In the gas-phase infrared spectrum about half of this region is blanketed by the intense band centered at 1093 cm-l. However in the argon-matrix spectrum (Figure 5), a band of medium intensity appears at 1051 cm-l, which could be the 13C counterpart of ~ 1 3 .A feature is nearby at 1039 cm-l in the crystal-phase spectrum, and the Raman has a weak band a t this frequency. However, this Raman band may be due to ~ 1 3(b2), which would undergo a substantial decrease in frequency in the liquid phase. Both bands could also be due to v g ~ 1 =4 1074 cm-' shifted to lower frequency by Fermi resonance with ~ 1 3 . An alternative assignment for vl0 is keyed to the depolarized Raman band at 1103 cm-l. However, this frequency can be explained as the combination tone, v7 + vi5 = 1098 cm-l (BJ. The corresponding infrared band, shifted to lower frequency as in the do species, would be the weak feature at 1081 cm-l in the matrix. We have selected this latter assignment in order to be consistent with the parallel Raman activity and frequency shifts in the spectra of the d o molecule in this region. The assignments of all of the fundamentals of the do and d2 molecules are summarized in Table VIII. 3,3-Difluorocyclopropene-dl. Table IV gives the details of the spectra and assignments for the dl molecule. Figures 8-13 are for argon matrices. Spectra of do-rich and d2-rich samples of the molecule have been juxtaposed so that bands due to the dl molecule can be easily recognized as the bands of significant intensity common to the two spectra. a' Modes. In the gas-phase infrared spectrum of the dl molecule (Figure 7), bands with A-type or C-type contours are easily found for nine of the ten a' fundamentals. These bands are at 3148,2371,1557,1341,953,823,718,505, and 478 cm-'. RP spacings range from 21 to 26 cm-l in accord with predictions for these band types. Fermi resonance is apparent in the 1550- and 950-cm-' regions. Centered at 1532 cm-* is a band due to v5 vl0 = 1542 cm-l (A') or v7 vg = 1541 cm-' (A') with intensity almost equal to that of the 1557-cm-l band, which we have used for the fundamental. In the argon matrix spectrum (Figure 8) the higher frequency component is decidedly more intense. For the other Fermi resonance doublet, the higher frequency Q branch in the 965-cm-l region can be explained as 2vlO = 956 cm-l (A'). In the argon matrix spectrum (Figure 10) the higher frequency component is less intense. Hence, we have used the 953-cm-l Q branch for the fundamental frequency. Remaining to be assigned is a frequency in the 1100-cm-l region corresponding to the antisymmetric CCC stretch at 1131 cm-l in the domolecule.

+

+

+

1064

The Journal of Physical Chemistry, Vol. 82, No. 9, 1978 I

O

O

t

'

I

I

I

1

'


I 4

20

olu /I

c

l

1

l

l

l

,

l

CM-I

1550

A

DH

I

;

3

I

l

1500

l

I

I

I

I

I

I

!

1300

1350

Figure 8. Infrared spectra of 3,3-difluorocyclopropene-d, in 2000: 1 argon matrix: upper trace, d,-rich sample; lower trace, do-rich sample. About 7 mmol were bounce-sprayed on the cold window. For most of the spectra in this series a correction of +1 cm-' must be applied. do-rich

I

O

80-

-

DE-rich

*

O

-

"

"

'

~

~

~

~

~

~

-

D2-rich

80 60 I$

-

c 8

-

40-

20-

L

0

I I1 L I

-

/I

20

I

c t

I I

I -1I

0

1 I

I

I

1000

I

I

I

C M"

I

I

950

Flgure 10. Continuation of Figure 8. do-richdisplaced downward 20%. Flgure 9. Continuation of Figure 8. do-rich displaced downward 30%.

As in the spectrum of the d2 species, this region of the spectrum is dominated by the intense band due to the antisymmetric CF2 mode. Detailed features in the gasphase spectrum of the dl in this region must be viewed with suspicion since the sample from which the spectrum was obtained had approximately twice as much do as dl. Compensating with do in the reference beam left the spectrometer starved for energy in this region. While the argon matrix spectrum (Figure 9) does not suffer from the blanketing problem of the gas-phase spectrum, it does have a number of bands due to do and d2 species. Furthermore, as discussed above, it is likely that features in this region are related to the intense and antisymmetric CF2stretching fundamental by Fermi resonance or effects in the matrix due to incomplete isolation or direct matrix effects. With these reservations in mind and for consistency with the assignments for the other two isotopic species, we have assigned the tenth fundamental to the weak matrix feature a t 1121 cm-l. An alternative explanation as Fermi resonance is also given in Table IV. The other possibility for the antisymmetric CCC stretch is the stronger feature a t 1064 cm-l in the matrix, which has been assigned as the

20

800

700

Figure 11. Continuation of Figure 8 . dodisplaced downward 2 0 % .

13C counterpart of ull. This choice for the elusive antisymmetric CCC stretch in the dl species, as well as the corresponding ones for the do and d2 species, was favored by the normal coordinate calculations. The two other versions that were evaluated with normal coordinate calculations were as follows: (a) (do) 1131 cm-', (d2)1051 cm-l, and (d,) 1064 cm-l; (b) (do)1060 cm-l, (d,) 1051 cm-l, and (d,) 1064 cm-l. Each of these versions gives comparable agreement with the isotope product rules.

The Journal of Physical Chemistry, Vol. 82, No.


9, 1978 1065

(Frequencies in cm-' ) TABLE IV: Assignments of Infrared and Raman Spectra of 3,3-Difluorocyclopropene-dl Infrared Gas Fre qa

ab

3148(- 26)

0.06

267 0 2429 2371( 23) 21 60 3055 1910 1645 1557(21) 1532( 22) 1510 1440 1341(24)

0.03 vw 0.2 0.2 0.1 0.1 0.2 1.1 0.82

0.2 8.0

Assignment Band shape

Ar matrixC freq, intd

SYm species

Freq

C?

€and

v1

2418 w A

do impurity?

v2

fund v4 + v7 '4 + '8

2v4,

1552 s 1526 m

A-C A

v3 f

v5

VS

1095(8)

8.0

B?

1075 vs 1064 s

VI1

A

VI0

A'

' 6

v3

A

'10

fund v7 + v 9

1328 1232 1223

1121 w

-t

+ V8 Fermi resonance with d , impurity

v4

B?

vi1 v1s

v7

1509 1443

0.1 0.1 1

a' A' A' A' A' ' A' a'

2v7 fund

1542 1541

1230 1220 1120

+

v11

v3

1331 vs 1320 s

A'

d, impurity

2164 2059 1922 1908 1646 A A

a'

3150 2682 2429

+

'12

a' A' A' A' a' A" a' '

'14

+ v9 fund V8

1118

'13

9'

fund )C?

VI1

1091

'10

A' '

'13

Fermi resonance with v l l ? 965

0.4

C

955 w

953

0.4

C

951 m 827 w

1069 956

' 6

'6 v12

829 823(21) 718(26) 61 3( 7 ) 505(21) 478(21) 403( 7 ) 351( 7)

1.3 0.38 0.73 0.26 0.23 0.20 0.15

A

819 s 716 m 613 m 504 m 478 w 405 w 352 w

C B A A B B

A" A'

'15

2VlO Fermi resonance with v 6 fund fund VI0 f

a' a' a' ' a" a' a' a' a' a' ' a"

VIS

fund fund fund fund fund fund fund

v7 V8 '13

v9

v10 VI4

VIS

a Values in parentheses are RP spacings for A-type and C-type bands, and QQ spacings for B-type. A-type and C-type designations are approximate for this molecule of C, symmetry. b-d See Table 11.

41

- 20

600

500

v

Figure 12. Continuation of Figure 8. &rich displaced downward 20%.

HZ

Modes. For the five a'' modes the gas-phase infrared spectrum (Figure 7) has three obvious B-type bands, located a t 613,478, and 351 cm-l. All have && spacings of 7 cm-l. To be consistent with the spectra of the do and d2 species, the band centered near 1095 cm-l must be another a" fundamental even though the detailed shape of the band cannot be relied upon. The assignment for CH flapping which should be in the 800-cm-l region is problematical. That this band, which should be infrared active, is not easily found is a disappointment in view of a"

0

1

1

I

I

I

I

I

I

I

I

l

450 400 CM-' 350 Figure 13. Continuation of Figure 8. For d,-rich trace the ordinate has been expanded X 5. d,-rich displaced downward 70%.

the difficulties in assigning the corresponding modes in the do and d2 molecules (vide supra). However, in the gasphase spectrum the band due to the CH flapping mode is overshadowed by the relatively intense and distorted band centered at 823 cm-l. Furthermore, a weak but convincing peak is in the argon matrix spectrum at 827

l

1066



TABLE V: Isotope Product Rule Checks on the Assignments Diff, a, b, a, b,

Calcd

Obsd

%

0.507 0.539 0.736 0.736

0.524 0.550 0.743 0.738

3.2 2.0 1.0 0.2

of the highest frequency antisymmetric CF2 mode. As already noted, the close correlation of the somewhat doubtful antisymmetric CCC stretching mode of difluorocyclopropene with comparable modes in difluorodiazirine and cyclopropenone strengthens this assignment and the related assignments for the d2 and dl species. Lack of close correlation of this mode with one in cyclopropene itself could well be due to the influence of the fluorine substituents on the CC force constants, as discussed below. The marked asymmetry in the trifluorocyclopropene molecule, as well as electronic and kinetic coupling effects of the third fluorine, could explain the lack of close correlation with a single mode in this case. Most of the other fundamentals for trifluorocyclopropene correlate well with those of the difluorocyclopropene. The secure assignment of 788 cm-l for the out-of-plane CH flap in trifluorocyclopropene supports the more doubtful assignments for the symmetric (D)HC=CH(D) flapping modes in difluorocyclopropene and cyclopropene.

Dif f , Calcd

Obsd

%

a'

0.517

0.540

4.5

a"

0.746

0.731

-2.0

cm-l (Figure 11). It has been taken as the fifth a" mode. The complete assignment of fundamentals for the dl molecule is summarized in Table VIII. S u m m a r y of t h e Assignments. The assignments of fundamentals of all three isotopic species of difluorocyclopropene are summarized in Table VI11 aIong with the results of the normal coordinate calculations. The overall assignment for the three isotopic species satisfies the Rayleigh rule.15 Table V gives the checks with the isotope product rules which are satisfactory when allowance is made for the use of some condensed-phase frequencies and for some frequencies which are not corrected for Fermi resonance. Although many of the weaker spectral features have been omitted from Tables 11-IV, it was possible to explain all of them satisfactorily as binary combinations. We believe that this complete assignment of the fundamentals of the three isotopically substituted difluorocyclopropenes is substantially correct. Assignments for which some doubt remains experimentally are given in italics in the summary in Table VIII. The two areas of uncertainty, which are discussed in detail above, are the set of antisymmetric CCC stretching modes and the set of highest frequency CH and CD flapping modes. Table VI gives correlations of the observed fundamentals of 3,3-difluorocyclopropenewith comparable modes in the related molecules: difluorodiazirine, cyclopropenone, cyclopropene, and trifluorocyclopropene. The correlation with the isoelectronic molecule, difluorodiazirine, is quite close for every comparable mode with the mild exception

Normal Coordinate Calculations Refinement f o r Difluorocyclopropene. Normal coordinate calculations for difluorocyclopropene and its deuterated modifications were done with a set of 15 internal coordinates and an 18-parameter, valence-type potential function. The nonredundant set of internal coordinates consisted of seven bond-stretching coordinates: a C=C, two C-C, two CH, and two CF stretching; and eight bond angle-bending coordinates: two C=CH and four CCF angle-bending and two out-of-plane CH wagging. The last pair of coordinates were defined such that both were positive when bending to the same side of the plane of the ring. Geometric parameters, which were derived from microwave spectroscopy, are given in Table I.* The computer programs used for the calculations were adaptations of those developed a t the University of Minnesota.16 Refinement calculations were carried out simultaneously on all three isotopic species of difluorocyclopropene. All 45 assigned frequencies, including the several doubtful

TABLE VI : Correlation of the Fundamentals of 3,3-Difluorocyclopropenewith Those of Related Molecules (Frequencies in cm-l) Approx descriptiona a1 vi

vc= c

'2 "3

S'CCC

'4

ss CH

'5

S'CF, "CF,

' 6

4- S'CF,

+ S'CCC

a,

'

3150 1598 1343 946 7 69 5 00

v7

SYCH rCF,

883 394

'9

avCH

3128 1131 968 523

h. -1 'IO "11

'iz

avccc a6CH PCF,

b,

1563 1282

3098 1483 1026 853

3158 1656 1110 905

?

920

788 570/380

3068 1161 892

3124 769

( 3 157) 1160/854 (993) 281

805 5 00 448 1091 544

1011

3157 1793 1404 993 764 500

1103 + a6 C F , 1094 1248 aYCH 680 788 570 (788) 5701380 '15 a6CF, f a'CF, 416 481 Reference a s = symmetric; a = antisymmetric; v = stretch; 6 = bend; p = rock; y = out-of-plane bend; and r = torsion. 4e. b, and b, symmetry species are reversed in this reference. Nitrogen atoms should replace the double-bonded carbons in the approximate descriptions. SS C F ~ T, C F , , and a6CF from ref 4b; corresponding values in ref 4a are 502, 451, and 481 Reference 6. All assignments are from liquid phase spectra, from which one expects lowered frequencies for CH cm-'. Reference and CO stretching modes and raised frequencies for CH bending modes in this hydrogen bonded substance. 5a. e Reference 7. '13

"14

a'CF,

The Journal of Physical Chemistry, Vol. 82, No. 9, 1978 1067


TABLE VII: Potential Constants for Difluorocyclopropenea Difluorocyclopropene refinement OverlayC Zeroorder DisperDisperconstn Constn sion Constn sion Kc=cb 8.48 8.61 0.29 8.02 0.32 3.9gh 4.97 0.31 5.75 0.25 Kcc 0.01 5.27 0.04 4.98g 5.35 KC, 4.53 0.18 4.53 0.19 KCF 4.1lt 0.01 0.55 0.01 0.51 HCCH 0.42g 0.11 1.83 0.05 2.05 HCCF 0.4g 0.32 0.02 0.31 0.01 YCH 0.35' Kcc/HccH, 0.0 -0.21 0.04 -0.34 0.04 0.62 0.05 0.82 0.10 KCC/HCCF O.0 0.66 0.06 0.50 0.06 KCClHCCFe 0.0 0.03 0.01 0.083 0.008 HCCH/HCCH 0.0 KCF/'YCHd HCCF/?'CHd HCCF/Y C H ~ YCHIYCH

0.0 0.0 0.0

0.0

-0.16 0.071 0.037 -0.57

0.05 -0.12 0.023 0.04 0.022 0.04 0.017 -0.05

0.02 0.01 0.01 0.01

1.47 0.33 1.21 0.19 0.0 0.92 0.07 0.79 0.05 0.0 0.30 0.09 0.56 0.06 0.0 a Stretching constants in mdyn A - ' ; bending constants in mdyn A radm2; stretch-bend interaction constants in K for stretching, H for bending, and y for mdyn rad-'. Other potential constants from out-of-plane flapping. the overlay calculation are Kc=c = 9.99 (0.23), K-c = 5.35 (0.30), K C F = 6.16 ( 0 . 3 8 ) , J - I c c ~ =1.03 (0.08), K c F / K G c = 1.68 (0.25), Kc,/K,c= -0.38 (0.13), HCCH/HC,F(ccCOmmOn)= 0.030 (0.02),HccH/HccF (opposite) = 0.052 (0.03), H C C F / H C C F (CF common) = 0.19 (0.06). Also HccF/ycH (Opposite)= H C C F / ~ C H (CC common). Most are for trifluorocyclopro ene alone. See text. d c-c common. e c-c opposite. ?Angles J. L. Duncan.and diagonally opposite. Reference 19. and G. R. Burns, J. Mol. Spectrosc., 30, 253 (196.9). L. H. Ngai and R. M. Mann, ibid., 38, 322 (1971). Reference 7. N. C. Craig and J. Overend, J. Chern. Phys., 51, 1127 (1967). KCF/KCF HCCF/HCCFd HCCF/HCCFf

'

J

ones, were given unit weight. Table VI1 gives the final set of 18 potential constants along with the corresponding set of zero-order values. Seven of the potential constants were diagonal elements in the F matrix due to the seven distinct types of valence bond-stretching and bond-bending coordinates. The remaining eleven were off-diagonal, interaction Constants. Due to the scarcity of potential constants for molecules related to difluorocyclopropene, the set of zero-order potential constants (Table VII) was assembled from a mixture of valence-type and Urey-Bradley-type potential constants. Each least-squares refinement started from the zero-order quantities for diagonal potential constants and values of zero for each of the off-diagonal potential constants. Final values of refined potential constants were insensitive to the initial, zero-order values and thus were not distorted by the admixture of Urey-Bradley constants. Refinement of the diagonal potential constants alone gave a poor fit to the observed frequencies, including an unacceptably small value of the potential constant for CF stretching. The tendency for CF stretching constants to go to low values in fluoro-C3 rings has been noted before.17 This decrease in the CF stretching constant, which is accompanied by an increase in the CCF bending constant, is discussed in more detail below. Off-diagonal potential constants were tested singly or in small groups and were used in subsequent refinements only if a given off-diagonal constant improved the fit of

the calculated frequencies to the observed ones and was well defined as indicated by a small statistical dispersion. In Table VI1 these interaction constants are arranged in three groups: (1)those that directly influence modes that are symmetric with respect to the plane of the ring (al and bl), (2) those that directly influence out-of-ring-plane modes (az and b2),and (3) those that influence both types. After addition of the CF stretch-stretch interaction constant, the CF stretching constant refined to a more reasonable value. Other interaction constants found to be significant were Kc-clHccH, Kc-c/Hcc~ (two types), HCCHIHCCH, KCFIYCH, HCCF/YCH (two types), YCH/YCH, and HccF/HccF (two of three types). K is for stretching, H is for in-plane bending, and y is for out-of-plane flapping. Though smaller in magnitude and longer range than most and of the other interaction constants, the HCCF/YCH KCF/yCH constants were important in achieving a good fit of the a2 and bz modes, respectively. The small constant yCH/yCH was also important. Off-diagonal potential constants that were tried and discarded were HccF/HccF (CF bond in common), Kc-c/ KCF,Kc-c/KcH, and KCF/HCCF. In the early stages of the investigation of interaction constants Kc-c/KcF seemed important, but, after the addition of the two Kc-c/HccF interaction constants, this stretch-stretch constant became poorly defined and was removed. Kc

Vibrational Assignments and a Potential Function ... - ACS Publications

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