- Email: [email protected]
Far-infrared spectra and structure of small ring compounds. Ethylene carbonate, γ-butyrolactone, and cyclopentanone
JOURNAL
OF MOLECULAR
Far-Infrared Compounds.
SPECTROSCOPY
27, 285-295 (1968)
Spectra and Structure of Small Ring Ethylene Carbonate, Y-Butyrolactone, and Cyclopentanone
J. R. DURIG,
G.
L.
COULTER,
AND D.
W. WERTZ*
Department of Chemistry, University of South Carolina, South Carolina 29208
Columbia,
The infrared and Raman spectra of ethylene carbonate, r-butyrolactone, cyclopentanone, and cyclopentanone-d4 have been recorded over the frequency range of 33 to 1000 cm-‘. Assignments of the various ring deformations have been made on the basis of band types, Raman depolarization values, and frequencies. The data obtained on cyclopentanone is consistent with pseudorotation hindered by a barrier of 2.8 + 0.7 kcal/mole. INTRODUCTION
In recent investigations (1) of the far infrared spectra of some four-membered ring molecules, it has been shown that the out-of-plane skeletal ring vibration can have a potential function governing this motion which ranges from one nearly harmonic to one almost purely quartic. In addition, Pitzer and Donath (2) have stated that five-membered rings in which the skeletal torsional barriers are not all equivalent should display hindered pseudorotation as observed by Hubbard et al. (3) and Crowder and Scott (4) in the thermodynamics and far-infrared spectrum of thiacyclopentane, respectively. However, Pitzer and Donath (2) also predicted a barrier to pseudorotation in tetrahydrofuran of about the same magnitude as that for the thiacyclopentane, but Lafferty et al. (5) observed free pseudorotation in the far infrared spectrum of the tetrahydrofuran molecule. Therefore, as part of an overall plan to attempt to understand the underlying reasons for such drastic changes in the potential function governing these ring vibrations, we have initiated an investigation of the far infrared spectra of several five-membered ring compounds. Planar five-membered rings have two out,-ofplane skeletal bending vibrations which are expected to fall at rather low irequencies. These bending modes may be degenerate in the case of Dlhsymmetry. For molecules with CZ, symmet#ry, such as ethylene carbonate, one of the out-ofplane ring modes belongs to the A2 symmetry class and it, is only active inthc * Taken in part from the thesis of D. W. Wertz submitted istry in part,ial fulfillment of the degree of Ph.D. 285
to the Department
of Chem-
286
DURIG,
COULTER,
AND
WERTZ
Raman effect whereas the other ring mode belongs to the B2 symmetry species and should be active in both the infrared and Raman spectra. In order to characterize the low frequency vibrations of these five-membered ring molecules, we have recorded the far infrared and Raman spectra of ethylene carbonate, butyrolactone and cyclopentanone. EXPERIMENTAL
The samples of the five-membered rings under consideration were all obtained commercially; ethylene carbonate from Jefferson Chemical Company, y-butyrolactone from General Aniline and Film Corporation, and cyclopentanone from Columbia Organic Chemical Company. The a, a, a’, a’-d, cyclopentanone was prepared by the method of Angel1 et al. (6). Purification was carried out in the following methods. Ethylene carbonate was triply vacuum distilled and then zone refined. This resulted in a solid having a melting point range of 34.5’-35’C. The y-butyrolactone was distilled at a reduced pressure (10 mm Hg) . The resulting liquid had a slight yellow coloration which interferes with Raman analysis, so it was necessary to further purify it using a Wilkins-Aerograph model A90 P-3 vapor phase chromatograph with a lo-ft silicon rubber 30 column on a SO/SO chromosorb stationary phase. The chromatography was carried out at the following conditions: injector temp., 145”C, collector 18O”C, column temp. 12O”C, and detector at 200°C. The helium carrier gas flow was 30 ml/min. This gave a colorless liquid with a boiling point of 204°C. Cyclopentanone and Q! , a, a’, ar’-cyclopentanone-d4 were dried with MgSO, and vacuum distilled (10 mm Hg). The infrared spectra of all molecules were recorded from 3500-250 cm-’ using a Perkin-Elmer model 5’21 spectrophotometer. Atmospheric water vapor was removed from the spectrophotometer housing by constant flushing with dry nitrogen gas. The instrument was calibrated in the usual manner (7). All gas phase spectra were obtained using a Beckman IO-meter variable-path cell equipped with KBr windows. Settings of 6.4 and 8.2 meters were used. Due to the extremely low vapor pressure of r-butyrolactone and ethylene carbonate, the cell was heated at temperatures ranging from 70°-120°C. Sample quantities ranging from 50-250 ~1 were used and best results were obtained by directly injecting the sample into the cell. Infrared spectra of the pure liquids were obtained neat, between CsI plates and in solution with CC& at lo-20 ‘Z sample by volume. Far infrared spectra were obtained using a Beckman Model 11 Infrared spectrophotometer’ in the range 33-600 cm-‘. Atmospheric water vapor was purged from the instrument housing by continuous flushing with dry air. The instrument was calibrated with water vapor . Gas phase spectra were obtained with the Beckman 10 meter variable path cell at, settings of 6.4 and 8.2 m. The cell was equipped 1The far-infrared spectrophotometer Foundation Grant, GP-5827.
was purchased
with funds from a National
Science
FAR-INFRARED
SPECTRA
OF SMALL
RINGS
2ss
ivith polyethylene windows. Spectra of the liquids were also obtained in solutions of spectral quality benzene using Beckman 1 ml polyethylene liquid cells. These spectra were run from 600-33 cm-l using solutions of 10 and 20 % concenbration by volume. Raman data were obtained on t’he molecules by using a Cary model 81 Raman spect,rophotometer wit’h both the standard Toronto Arc exciting line at 4358 I% and t,he Spectra Physics r\Iodel 13.5 Helene gas laser’ (exciting line 6328 h). Spectra of t#he pure liquids were recorded with the standard Cary 7 mm cell and the 25 ~1 cell for laser spectra. For the region above 250 cm-l, a spectral slit width of 10 cm and slit height of 10 cm-’ n-as used with double slits. Below 250 c K’ single slits were needed in order to get close enough to the exciting line to see t,he low-lying ring puckering motions. Various sensitivities and scanning speeds were used to obtain suitable resolut’ion and presentations. Spectra were also obtained in the region 150 cm-‘- 3100 cm-’ with laser excitation to help correlate information on the low lying skelet’al modes. Depolarization values were measured by the met’hod of Cra\vford and Horwitz (8) and frequencies are espected to bc accurate to f2 cm-l. RESULTS
AND
DISCUSSlON
Ethylene Carbonat,e: Angel1 (9) has stated that the symmetry of the ethylene carbonate molecule is Cz, in t’he vapor and liquid phases and has assigned eighteen of the twenty-four fundamental vibrations. The six skeletal bending modes, however, were not assigned. Angel1 did observe a band at 717 cm-’ which he suggested might be the carbonyl bending mode, but, such a high frequency is out of line with the frequency for this motion in several four-membered rings. However, Angel1 found no other bands in the region 400-700 cm-l and concluded that] the skeletal bencling modes must lie below 400 en-l. ,4 n inspection of the Raman effect, of the pure liquid (Fig. 1Aj and the far infrared spectrum of a benzene solution (Fig. 2A) shows two very pronounced lines at’ 529 cm-’ and 217 cm-‘. There are also two very weal; IineF at, 440 cm-’ :tnd 484 cm-’ in t,he Raman spectrum. Angel1 does not observe the 717 cK’ band in the vapor and, consequent,ly, could not, assign the mode to a symmetry class, but he did observe t,he band to bc depolarized in a water solution and thus felt it \vas due to a motion in eit,her the Hl or B, symmetry class. Re-examination of the Raman dat,a of the pure liquid, however, showed the band t’o be strongly polarized. Consequently, WC have assigned this band t’o the skeletal bending mode of symmetry class A, and not to one of the C=O bending vibrations. This assignment is in agreement \vith \vork done by Dorris et al. (IO) in which an A-type band at 735 cm-l in virlylene cnrhon&e is assigned t’o the A1 skeletal bending mode. The band at 717 cm-l in ?The He-Ne GP-7079;
laser was purchased with funds from a National
Science Foundation
Grant,
600
700
a+#’
ml
300
400
200
-we” 8
700
I ,
600
500
4cc
I
300
260
FREQUENCY (Ctd)
FIG. 1. (A) Raman spectra cyclopentanone.
of 5 ml of ethylene
carbonate;
(B) r-butyrolactone;
and (C)
B I I
300
I I
I I
200
250
I
150
FREQUENCY (CM’; FIG. 2. Far-infrared 3) y-butyrolactone.
spectra
of 1574 solutions 289
in benzene
of (A) et,hylene
carbonate
and
290
DURIG,
COULTER,
AND
WERTZ
ethylene carbonate displays considerable asymmetry on the low frequency side, and we have tentatively assigned the shoulder at about 700 cm-’ in both the Raman and infrared spectra of the liquid to the B1 skeletal bending mode. The two carbonyl bending modes are expected to lie in the 4OG600 cm-’ region of the spectrum, but only one band is observed in this region-a medium intensity band at 529 cm-’ in the infrared spectrum and at 525 cm-’ in the Raman effect. Unfortunately, the band was not observed in the vapor and thus we cannot be certain whether the band arises from the in-plane or out-of-plane C=O bending mode or both. In the spectra of y-butyrolactone and vinylene carbonate, these two modes lie within 35 cm-’ of one another and are of comparable intensities. We have, therefore, assigned the 529 cm-’ band to both carbonyl bending vibrations. The two modes yet unassigned are the two out-of-plane ring bending vibrations or the ring “puckering” modes. The two modes may be described as an infrared inactive A, fundamental which is essentially a C-C torsion and a Bz mode which more closely resembles the puckering vibration of the four-membered rings. A weak band which is observed in both the infrared and Raman spectra of the liquid is found at 217 cm-’ and, due to its activity in the infrared, has been assigned to the Bz vibration. The A2 mode was not observed even in the Raman effect, but since its description is similar to a C-C torsion, it is felt that the band should lie very close in frequency to the other puckering mode and may well lie under the band at 217 cm-l in the Raman spectrum of ethylene carbonate. Two very weak bands are observed in the Raman at 440 cm-l and 484 cm-‘. The 440 cm-’ band is thought to be due to the first overtone of the Bz mode which would imply that the potential function governing this puckering motion must be very nearly harmonic. The 484 cm-’ band then might be assigned to either the first overtone of the other out-of-plane bending mode or a combination band result’ing from both modes being excited. Thus, the frequency of the A2 mode would be 242 cm-l or 267 cm-l respectively. y-Butyrolactone: Since this molecule has only one oxygen atom in the ring, it can have at most a plane of symmetry, thus all the skeletal bending vibrations are expected to be both infrared and Raman active. All six of the skeletal bending vibrations have been observed in the Raman effect (Fig. 1B) and far infrared spectrum (Fig. 2B). These six absorptions are readily assignable on the basis of assignments made for the ethylene carbonate molecule. Two Raman lines are observed at 674 cm-’ and 637 cm-’ and have been assigned to the skeletal bending vibrations which parallel the in-plane bending motions of the ethylene carbonate skeleton. The two carbonyl bending fundamentals are observed at .536 and 491 cm-’ in the Raman effect of liquid y-butyrolactone. The two out-of-plane bending vibrations of the ring are observed in the Raman effect, but they are so close to the exciting line that they appear as shoulders at about 224 cm-’ and 166 cm-l. The far infrared spectrum of a benzene solution of butyrolactone displays two
FAR-INFRARED
SPECTRA
OF SMALL RIK;c:S
291
very weak bands at 219 cm-’ and 160 cm-’ which must be assigned to the ring puckering fundamentals. Due to a lack of vapor phase data and observation of overtone bands of the two ring puckering modes, no conclusions could be drawl for the type of potential funcbions governing these vibrations. Cyclopentanone and (Y , a, a’, cu’-Tet~a~euter.ocyclopenta~~one: If the skeletotw of the two molecules are planar, they would possess CzUsymmetry and all six of the bending vibrations of t,he ring would be Raman active while only five would he infrared active. The more likely geometry would be that of a slightly bent, ring in which case all of the bending fundamentals would be bot’h Raman and infrared active. On the basis that the high frequency out-of-plane bending is Raman acfiw but infrared inactive, we have assumed Gels symmetry. It must be remembered, however, that if the ring is “puckered” only slightly, the dipole change during this motion would be expected to be very small and t’he infrared band extremeI\. weak. Figure 1C shows the Raman spectrum of cyclopentanone in which fiw hands have been observed at 711 cm-‘, 5% cm-‘, 472 cni?, GO cm-‘, and Wi cm-l. The line at 711 cm-l is in good agreement) mith lines observed at 717 cm ’ and 674 cm-’ in ethylene carbonate and y-butyrolactone, respectively, :~nd has, thus, been assigned to the skeletal bending mode of the A1 representation. Tlw cnrbonyl bending vibrations were observed to be very close in frequency to ow snot,her and thus we have assigned the two bands at 472 cm-’ :md 4.50 cnl-’ to these fundamentals. The band at 383 cm-l must then be assigned to the other illplane ring bending mode. The remaining Raman line at 236 cm-’ which is IIO~. observed in t,he infrared is then assigned to the A2 out-of-plane ring deform:ltio[l. The Raman spectrum of the tetr:adeuterocyclopentanonc show a strong band :It 63s cm-’ which, because of its intensity, is assigned to the A1 skeletal bending vibration which has shifted from illcm-’ upon deut’erat’ion. The R1 ring tleform:ltion shifts only slightly, to 566 cm--’ 111 . the deut’erated compound. The t\v(, carbonyl bending vibrations are assigned to the lines at 34-l cm--’ aud 391 cm ‘. A weak shoulder on the exciting line at, 228 cm-’ is then wcribed to the .I, ring deformat,ion. Figures 3h and 3B show the far-infrared spectra of the cyclopentanone :uld cyclopentanone-dl molecules, respectively. The spectrum of cyclopentanonc displays a strong absorpt’ion nhich consists of two bands at about, SO and 105 cm ’ and a much weaker band centered at about’ 93 cm-’ . A high resolution investigation done in l’rofessor R. C. l,ord’s laboratory revealed that actually the weal; band at 93 cm-’ proved to be composed of a series of strong Q branches het\vern 95 and 84 cn1-l with approximately n l.S cnlP1 spacing. Hubbard et al. (3 ) :rnd Crowder and Scott (4) employed a cosine based potential function to descritw the thermodynamics and far-infrared spectrum of thiacyclopentane, i.e. the) assumed that the pseudorotation was hindered by a barrier. Later, Pitzer and Donath (2) predicted a similar barrier for the cyclopentanone molecule. Therefore, we attempted to predict the frequencies of transitions observed in Profew)r
OURIG,
I
,
110
COULTER,
ANL, WERT%
B 1
1 /
90
t I
70
FREQUENCY (CM’) 3. Far-infrared spectra of gaseous (A) cyclopentanone and (B) cyclopentanone-dr Samples were examined at pressures of about 5 Torr and path lengths of 8.2 m. FIG.
.
Ilord’s laboratory by a cosine based function. The frequency of the 1 +-- 0 transition (95.0 cK1) was employed to calculate & value for the reduced moment of inertia con&mt, F, for a given value of the difference between X’lathieu eigenvalues, Ab, F = fi/Ab.
FAR-INFRAIWI)
SPECTRA
OF SMALL
RIN(:S
“9X
This value of F was used in an attempt to reproduce a frequency of the 2 +- I transition which would differ by about 2 cm-’ from the 1 +- 0 transition. The values of F and b were then further refined to obtain the best fit for the observed spacings. The values for the differences between Jlathieu eigenvnlues for t,hc transitions are uniquely specified by the value of the JInt,hieu pnramctc~r. s. \\.hich then yields the barrier height 1,’ = Fs. In this manner, a value for the reduced moment of inertia of 14.5 X lO_‘” g-cm’ and the height for the two-fold barrier of 3.5 lrcal/mole were obtained, and t’he first seven transitions below the barrier were then calculat,ed to be 95.0,92.!~,90.9. SS.7, 86.5, 84.1, and 51.7 cm-‘. Thus, it’ is seen that such a reduced moment of inertia and a two-fold barrier predicts a series of t,rnnsitions with approximnt.el\ 2 cm -I spacing which compares well with t’he first few levels reported b\- 1,ord ( I1 j). It would appear, therefore, t,hat the potential hindering t#he pseudorotation is nearly cosine in shape near the bot,tom of t,he well, but anharmonicities \vill probably cause a gradual divergence from the pure cosine potential furiction for the transitions originating from higher levels. Since the barrier height of 3.5 kcal, mole is simply a parameter used to obtain the best fit, we feel that this barrier height is the upper limit. Also, all of the transitions must lie belolv the barrier, ,so the lower limit must be near l.S kcal/mole. With these two limits of 3.5 and 1.S kcal/mole, we believe the barrier hindering pseudorotation in cyclopentanonc ia 2.7 f 0.8 kcal/mole which is in fair agreement with that predict’ed by I’itztsl and DonaOh (2) who calculated a barrier height of 2.5 kcal/mole from transferred potential constants. The speckurn of cyclopentanone-d4 is verysimilnrto that of cyclopentanone ~it’h broad bands around SO and 97 crC1. The high resolution spectrum of this region is far less pronounced than for cyclopentanow with the Q branches being very weak and diffuse (li). However, it appears that the 1 + 0 transition of the proposed hindered pacudorotor occurs near ~9 ml ’ and probably trails to lower frequency. Thus, it is believed that nhnt \ve have observed in our laboratory are the P and R branches of the out,-of-plane ring doformation, and, due to the very small half-width (-0.3 cm-‘), of t,heQ branches. the Q branch series is not pronounced. X summary of t)he fundamental frequencies of the skeletal bending modes fol t,his series of five-membered rings is given in Table I. ORSER\~ATIONS
BNI)
CONCLUSIONS
l’itzer and Donath ( 2 ) predict hindered pseudorotatjion in my saturated fivcmembered ring in which t,he t’orsional forces do not exa&ly cancel one another. Thus, they predicted barriers not only in thiacyclopentane and cyclopentanorw but also in t,etrahydrofuran and other molecules in which all five skelet,al atoms \\-ere not, ldent,ical. However, no effect#s of a barrier were observed in the far-infr:ired spectrum of either tetrahydrofuran (5) or 1, I<-dioxolnne ( 22), although tet,ra-
DURIG,
291
COULTER,
AND
TABLE FUNDAMENTAL
Symmetry&
Al B1 B1 Ba B2 ;12
FREQUENCIES
IN WAVENUMBERS
WERTZ
I OF THE SKELETAL
Ethylene Carbonate
-r-Butyrolactone
717 799 529 529 217 (217)h
674 637 536 491 224 166
Cyclopentanone
BENDING
MODES
Cyclopentanone-da
711 583 472 459 236 95c
* The symmetry of the vibration assumes Cz. symmetry for the molecule. b This assignment is tentative. c Far infrared vapor frequencies, all others are Ramau frequencies recorded liquid.
638 586 444 391 223 8gc
with the
hydrofuran was predicted to have a barrier similar to that for both thiacyclopentane and cyclopentanone. Therefore, it is quite evident that additional factors must be considered when predictions are made for the magnitude of barriers to pseudorotation in five-membered rings. Two of these factors are probably the ring strain and the relative effective masses of the ring substituents. The resultant force in the five-membered ring is probably a composite of all three of these factors, and at present there is insufficient data available to predict the relative effects of any one of them. It might be argued that a series of closely spaced Q branches could be explained on the basis of a slightly anharmonic potential instead of the cosine based function assumed for pseudorotation. However, in the case of a relatively low barrier (-1.S kcal/mole) the contribution to the entropy of a hindered pseudorotator differs from that of a harmonic oscillator of the same frequency by 1.5 e.u. at room temperature for cyclopentanone. Such a large entropy difference should be easily discernable by thermodynamic measurements. In the case of the high limit of the pseudodorotational barrier for cyclopentanone, the entropy difference amounts to only about 0.15 e.u. at room temperature which would be beyond the limit of thermodynamic detection. At 100 degrees above room temperat,ure t’he entropy difference becomes 0.3 e.u. which is still barely detectable by thermodynamists. Therefore, thermodynamic measurements could be used t#o distinguish between a pseudorotator and a relatively harmonic oscillator only in the case of a relatively low barrier. If one assumes the reduced moment of inertia of y-butyrolactone and ethylene carbonate to be the same as that of cyclopentanone, which seems reasonable when one considers that oxygen is nearly isobaric with a methylene group, then barriers to pseudorotation are calculated to be about 11 and 18 kcal/mole, respectively. Such high barriers would negate the effects of pseudorotation on the thermodynamic properties of these molecules at reasonable temperatures. Thus, in molecules with sufficiently high barriers, the
FAR-INFRARED
SPECTRA
motion can be treated as an ordinary about a most stable configuration.
OF SMALL
vibration
RINGS
in which the puckering
“35
oscillat8cs
ACKNOWLEDGMENT The authors would like to thank Professor Lord for communication of the far-infrared data on cyclopentanone prior to publication. The authors would also like to acknowledge the finalicia srlpport given this work by the National Aeronautics and Space Administration with Grant NGR-41-002-003-I. I~EC’EIVEU:
.January
31, 1968 REFERENCES
i. d.
2. 3.
4, 5, 6. 7. 8. 9. 10.
11. 18.
1:. I>LXIG ANU 1~. C. LORD, J. Chem. Phys. 46, 61 (1966); J. It. DUI~IG ANU A. C. MORRISSEY, ibid. 46, 4854 (1967) ; S. I. CHAN, T. R. BORGERS, J. W. RUSSELL, If. I,. S~I’RAUSS,AND W. D. GVINN? ibid. 44, 1103 (1966); J. R. DURIG ANI) A. C. X101
OF MOLECULAR
Far-Infrared Compounds.
SPECTROSCOPY
27, 285-295 (1968)
Spectra and Structure of Small Ring Ethylene Carbonate, Y-Butyrolactone, and Cyclopentanone
J. R. DURIG,
G.
L.
COULTER,
AND D.
W. WERTZ*
Department of Chemistry, University of South Carolina, South Carolina 29208
Columbia,
The infrared and Raman spectra of ethylene carbonate, r-butyrolactone, cyclopentanone, and cyclopentanone-d4 have been recorded over the frequency range of 33 to 1000 cm-‘. Assignments of the various ring deformations have been made on the basis of band types, Raman depolarization values, and frequencies. The data obtained on cyclopentanone is consistent with pseudorotation hindered by a barrier of 2.8 + 0.7 kcal/mole. INTRODUCTION
In recent investigations (1) of the far infrared spectra of some four-membered ring molecules, it has been shown that the out-of-plane skeletal ring vibration can have a potential function governing this motion which ranges from one nearly harmonic to one almost purely quartic. In addition, Pitzer and Donath (2) have stated that five-membered rings in which the skeletal torsional barriers are not all equivalent should display hindered pseudorotation as observed by Hubbard et al. (3) and Crowder and Scott (4) in the thermodynamics and far-infrared spectrum of thiacyclopentane, respectively. However, Pitzer and Donath (2) also predicted a barrier to pseudorotation in tetrahydrofuran of about the same magnitude as that for the thiacyclopentane, but Lafferty et al. (5) observed free pseudorotation in the far infrared spectrum of the tetrahydrofuran molecule. Therefore, as part of an overall plan to attempt to understand the underlying reasons for such drastic changes in the potential function governing these ring vibrations, we have initiated an investigation of the far infrared spectra of several five-membered ring compounds. Planar five-membered rings have two out,-ofplane skeletal bending vibrations which are expected to fall at rather low irequencies. These bending modes may be degenerate in the case of Dlhsymmetry. For molecules with CZ, symmet#ry, such as ethylene carbonate, one of the out-ofplane ring modes belongs to the A2 symmetry class and it, is only active inthc * Taken in part from the thesis of D. W. Wertz submitted istry in part,ial fulfillment of the degree of Ph.D. 285
to the Department
of Chem-
286
DURIG,
COULTER,
AND
WERTZ
Raman effect whereas the other ring mode belongs to the B2 symmetry species and should be active in both the infrared and Raman spectra. In order to characterize the low frequency vibrations of these five-membered ring molecules, we have recorded the far infrared and Raman spectra of ethylene carbonate, butyrolactone and cyclopentanone. EXPERIMENTAL
The samples of the five-membered rings under consideration were all obtained commercially; ethylene carbonate from Jefferson Chemical Company, y-butyrolactone from General Aniline and Film Corporation, and cyclopentanone from Columbia Organic Chemical Company. The a, a, a’, a’-d, cyclopentanone was prepared by the method of Angel1 et al. (6). Purification was carried out in the following methods. Ethylene carbonate was triply vacuum distilled and then zone refined. This resulted in a solid having a melting point range of 34.5’-35’C. The y-butyrolactone was distilled at a reduced pressure (10 mm Hg) . The resulting liquid had a slight yellow coloration which interferes with Raman analysis, so it was necessary to further purify it using a Wilkins-Aerograph model A90 P-3 vapor phase chromatograph with a lo-ft silicon rubber 30 column on a SO/SO chromosorb stationary phase. The chromatography was carried out at the following conditions: injector temp., 145”C, collector 18O”C, column temp. 12O”C, and detector at 200°C. The helium carrier gas flow was 30 ml/min. This gave a colorless liquid with a boiling point of 204°C. Cyclopentanone and Q! , a, a’, ar’-cyclopentanone-d4 were dried with MgSO, and vacuum distilled (10 mm Hg). The infrared spectra of all molecules were recorded from 3500-250 cm-’ using a Perkin-Elmer model 5’21 spectrophotometer. Atmospheric water vapor was removed from the spectrophotometer housing by constant flushing with dry nitrogen gas. The instrument was calibrated in the usual manner (7). All gas phase spectra were obtained using a Beckman IO-meter variable-path cell equipped with KBr windows. Settings of 6.4 and 8.2 meters were used. Due to the extremely low vapor pressure of r-butyrolactone and ethylene carbonate, the cell was heated at temperatures ranging from 70°-120°C. Sample quantities ranging from 50-250 ~1 were used and best results were obtained by directly injecting the sample into the cell. Infrared spectra of the pure liquids were obtained neat, between CsI plates and in solution with CC& at lo-20 ‘Z sample by volume. Far infrared spectra were obtained using a Beckman Model 11 Infrared spectrophotometer’ in the range 33-600 cm-‘. Atmospheric water vapor was purged from the instrument housing by continuous flushing with dry air. The instrument was calibrated with water vapor . Gas phase spectra were obtained with the Beckman 10 meter variable path cell at, settings of 6.4 and 8.2 m. The cell was equipped 1The far-infrared spectrophotometer Foundation Grant, GP-5827.
was purchased
with funds from a National
Science
FAR-INFRARED
SPECTRA
OF SMALL
RINGS
2ss
ivith polyethylene windows. Spectra of the liquids were also obtained in solutions of spectral quality benzene using Beckman 1 ml polyethylene liquid cells. These spectra were run from 600-33 cm-l using solutions of 10 and 20 % concenbration by volume. Raman data were obtained on t’he molecules by using a Cary model 81 Raman spect,rophotometer wit’h both the standard Toronto Arc exciting line at 4358 I% and t,he Spectra Physics r\Iodel 13.5 Helene gas laser’ (exciting line 6328 h). Spectra of t#he pure liquids were recorded with the standard Cary 7 mm cell and the 25 ~1 cell for laser spectra. For the region above 250 cm-l, a spectral slit width of 10 cm and slit height of 10 cm-’ n-as used with double slits. Below 250 c K’ single slits were needed in order to get close enough to the exciting line to see t,he low-lying ring puckering motions. Various sensitivities and scanning speeds were used to obtain suitable resolut’ion and presentations. Spectra were also obtained in the region 150 cm-‘- 3100 cm-’ with laser excitation to help correlate information on the low lying skelet’al modes. Depolarization values were measured by the met’hod of Cra\vford and Horwitz (8) and frequencies are espected to bc accurate to f2 cm-l. RESULTS
AND
DISCUSSlON
Ethylene Carbonat,e: Angel1 (9) has stated that the symmetry of the ethylene carbonate molecule is Cz, in t’he vapor and liquid phases and has assigned eighteen of the twenty-four fundamental vibrations. The six skeletal bending modes, however, were not assigned. Angel1 did observe a band at 717 cm-’ which he suggested might be the carbonyl bending mode, but, such a high frequency is out of line with the frequency for this motion in several four-membered rings. However, Angel1 found no other bands in the region 400-700 cm-l and concluded that] the skeletal bencling modes must lie below 400 en-l. ,4 n inspection of the Raman effect, of the pure liquid (Fig. 1Aj and the far infrared spectrum of a benzene solution (Fig. 2A) shows two very pronounced lines at’ 529 cm-’ and 217 cm-‘. There are also two very weal; IineF at, 440 cm-’ :tnd 484 cm-’ in t,he Raman spectrum. Angel1 does not observe the 717 cK’ band in the vapor and, consequent,ly, could not, assign the mode to a symmetry class, but he did observe t,he band to bc depolarized in a water solution and thus felt it \vas due to a motion in eit,her the Hl or B, symmetry class. Re-examination of the Raman dat,a of the pure liquid, however, showed the band t’o be strongly polarized. Consequently, WC have assigned this band t’o the skeletal bending mode of symmetry class A, and not to one of the C=O bending vibrations. This assignment is in agreement \vith \vork done by Dorris et al. (IO) in which an A-type band at 735 cm-l in virlylene cnrhon&e is assigned t’o the A1 skeletal bending mode. The band at 717 cm-l in ?The He-Ne GP-7079;
laser was purchased with funds from a National
Science Foundation
Grant,
600
700
a+#’
ml
300
400
200
-we” 8
700
I ,
600
500
4cc
I
300
260
FREQUENCY (Ctd)
FIG. 1. (A) Raman spectra cyclopentanone.
of 5 ml of ethylene
carbonate;
(B) r-butyrolactone;
and (C)
B I I
300
I I
I I
200
250
I
150
FREQUENCY (CM’; FIG. 2. Far-infrared 3) y-butyrolactone.
spectra
of 1574 solutions 289
in benzene
of (A) et,hylene
carbonate
and
290
DURIG,
COULTER,
AND
WERTZ
ethylene carbonate displays considerable asymmetry on the low frequency side, and we have tentatively assigned the shoulder at about 700 cm-’ in both the Raman and infrared spectra of the liquid to the B1 skeletal bending mode. The two carbonyl bending modes are expected to lie in the 4OG600 cm-’ region of the spectrum, but only one band is observed in this region-a medium intensity band at 529 cm-’ in the infrared spectrum and at 525 cm-’ in the Raman effect. Unfortunately, the band was not observed in the vapor and thus we cannot be certain whether the band arises from the in-plane or out-of-plane C=O bending mode or both. In the spectra of y-butyrolactone and vinylene carbonate, these two modes lie within 35 cm-’ of one another and are of comparable intensities. We have, therefore, assigned the 529 cm-’ band to both carbonyl bending vibrations. The two modes yet unassigned are the two out-of-plane ring bending vibrations or the ring “puckering” modes. The two modes may be described as an infrared inactive A, fundamental which is essentially a C-C torsion and a Bz mode which more closely resembles the puckering vibration of the four-membered rings. A weak band which is observed in both the infrared and Raman spectra of the liquid is found at 217 cm-’ and, due to its activity in the infrared, has been assigned to the Bz vibration. The A2 mode was not observed even in the Raman effect, but since its description is similar to a C-C torsion, it is felt that the band should lie very close in frequency to the other puckering mode and may well lie under the band at 217 cm-l in the Raman spectrum of ethylene carbonate. Two very weak bands are observed in the Raman at 440 cm-l and 484 cm-‘. The 440 cm-’ band is thought to be due to the first overtone of the Bz mode which would imply that the potential function governing this puckering motion must be very nearly harmonic. The 484 cm-’ band then might be assigned to either the first overtone of the other out-of-plane bending mode or a combination band result’ing from both modes being excited. Thus, the frequency of the A2 mode would be 242 cm-l or 267 cm-l respectively. y-Butyrolactone: Since this molecule has only one oxygen atom in the ring, it can have at most a plane of symmetry, thus all the skeletal bending vibrations are expected to be both infrared and Raman active. All six of the skeletal bending vibrations have been observed in the Raman effect (Fig. 1B) and far infrared spectrum (Fig. 2B). These six absorptions are readily assignable on the basis of assignments made for the ethylene carbonate molecule. Two Raman lines are observed at 674 cm-’ and 637 cm-’ and have been assigned to the skeletal bending vibrations which parallel the in-plane bending motions of the ethylene carbonate skeleton. The two carbonyl bending fundamentals are observed at .536 and 491 cm-’ in the Raman effect of liquid y-butyrolactone. The two out-of-plane bending vibrations of the ring are observed in the Raman effect, but they are so close to the exciting line that they appear as shoulders at about 224 cm-’ and 166 cm-l. The far infrared spectrum of a benzene solution of butyrolactone displays two
FAR-INFRARED
SPECTRA
OF SMALL RIK;c:S
291
very weak bands at 219 cm-’ and 160 cm-’ which must be assigned to the ring puckering fundamentals. Due to a lack of vapor phase data and observation of overtone bands of the two ring puckering modes, no conclusions could be drawl for the type of potential funcbions governing these vibrations. Cyclopentanone and (Y , a, a’, cu’-Tet~a~euter.ocyclopenta~~one: If the skeletotw of the two molecules are planar, they would possess CzUsymmetry and all six of the bending vibrations of t,he ring would be Raman active while only five would he infrared active. The more likely geometry would be that of a slightly bent, ring in which case all of the bending fundamentals would be bot’h Raman and infrared active. On the basis that the high frequency out-of-plane bending is Raman acfiw but infrared inactive, we have assumed Gels symmetry. It must be remembered, however, that if the ring is “puckered” only slightly, the dipole change during this motion would be expected to be very small and t’he infrared band extremeI\. weak. Figure 1C shows the Raman spectrum of cyclopentanone in which fiw hands have been observed at 711 cm-‘, 5% cm-‘, 472 cni?, GO cm-‘, and Wi cm-l. The line at 711 cm-l is in good agreement) mith lines observed at 717 cm ’ and 674 cm-’ in ethylene carbonate and y-butyrolactone, respectively, :~nd has, thus, been assigned to the skeletal bending mode of the A1 representation. Tlw cnrbonyl bending vibrations were observed to be very close in frequency to ow snot,her and thus we have assigned the two bands at 472 cm-’ :md 4.50 cnl-’ to these fundamentals. The band at 383 cm-l must then be assigned to the other illplane ring bending mode. The remaining Raman line at 236 cm-’ which is IIO~. observed in t,he infrared is then assigned to the A2 out-of-plane ring deform:ltio[l. The Raman spectrum of the tetr:adeuterocyclopentanonc show a strong band :It 63s cm-’ which, because of its intensity, is assigned to the A1 skeletal bending vibration which has shifted from illcm-’ upon deut’erat’ion. The R1 ring tleform:ltion shifts only slightly, to 566 cm--’ 111 . the deut’erated compound. The t\v(, carbonyl bending vibrations are assigned to the lines at 34-l cm--’ aud 391 cm ‘. A weak shoulder on the exciting line at, 228 cm-’ is then wcribed to the .I, ring deformat,ion. Figures 3h and 3B show the far-infrared spectra of the cyclopentanone :uld cyclopentanone-dl molecules, respectively. The spectrum of cyclopentanonc displays a strong absorpt’ion nhich consists of two bands at about, SO and 105 cm ’ and a much weaker band centered at about’ 93 cm-’ . A high resolution investigation done in l’rofessor R. C. l,ord’s laboratory revealed that actually the weal; band at 93 cm-’ proved to be composed of a series of strong Q branches het\vern 95 and 84 cn1-l with approximately n l.S cnlP1 spacing. Hubbard et al. (3 ) :rnd Crowder and Scott (4) employed a cosine based potential function to descritw the thermodynamics and far-infrared spectrum of thiacyclopentane, i.e. the) assumed that the pseudorotation was hindered by a barrier. Later, Pitzer and Donath (2) predicted a similar barrier for the cyclopentanone molecule. Therefore, we attempted to predict the frequencies of transitions observed in Profew)r
OURIG,
I
,
110
COULTER,
ANL, WERT%
B 1
1 /
90
t I
70
FREQUENCY (CM’) 3. Far-infrared spectra of gaseous (A) cyclopentanone and (B) cyclopentanone-dr Samples were examined at pressures of about 5 Torr and path lengths of 8.2 m. FIG.
.
Ilord’s laboratory by a cosine based function. The frequency of the 1 +-- 0 transition (95.0 cK1) was employed to calculate & value for the reduced moment of inertia con&mt, F, for a given value of the difference between X’lathieu eigenvalues, Ab, F = fi/Ab.
FAR-INFRAIWI)
SPECTRA
OF SMALL
RIN(:S
“9X
This value of F was used in an attempt to reproduce a frequency of the 2 +- I transition which would differ by about 2 cm-’ from the 1 +- 0 transition. The values of F and b were then further refined to obtain the best fit for the observed spacings. The values for the differences between Jlathieu eigenvnlues for t,hc transitions are uniquely specified by the value of the JInt,hieu pnramctc~r. s. \\.hich then yields the barrier height 1,’ = Fs. In this manner, a value for the reduced moment of inertia of 14.5 X lO_‘” g-cm’ and the height for the two-fold barrier of 3.5 lrcal/mole were obtained, and t’he first seven transitions below the barrier were then calculat,ed to be 95.0,92.!~,90.9. SS.7, 86.5, 84.1, and 51.7 cm-‘. Thus, it’ is seen that such a reduced moment of inertia and a two-fold barrier predicts a series of t,rnnsitions with approximnt.el\ 2 cm -I spacing which compares well with t’he first few levels reported b\- 1,ord ( I1 j). It would appear, therefore, t,hat the potential hindering t#he pseudorotation is nearly cosine in shape near the bot,tom of t,he well, but anharmonicities \vill probably cause a gradual divergence from the pure cosine potential furiction for the transitions originating from higher levels. Since the barrier height of 3.5 kcal, mole is simply a parameter used to obtain the best fit, we feel that this barrier height is the upper limit. Also, all of the transitions must lie belolv the barrier, ,so the lower limit must be near l.S kcal/mole. With these two limits of 3.5 and 1.S kcal/mole, we believe the barrier hindering pseudorotation in cyclopentanonc ia 2.7 f 0.8 kcal/mole which is in fair agreement with that predict’ed by I’itztsl and DonaOh (2) who calculated a barrier height of 2.5 kcal/mole from transferred potential constants. The speckurn of cyclopentanone-d4 is verysimilnrto that of cyclopentanone ~it’h broad bands around SO and 97 crC1. The high resolution spectrum of this region is far less pronounced than for cyclopentanow with the Q branches being very weak and diffuse (li). However, it appears that the 1 + 0 transition of the proposed hindered pacudorotor occurs near ~9 ml ’ and probably trails to lower frequency. Thus, it is believed that nhnt \ve have observed in our laboratory are the P and R branches of the out,-of-plane ring doformation, and, due to the very small half-width (-0.3 cm-‘), of t,heQ branches. the Q branch series is not pronounced. X summary of t)he fundamental frequencies of the skeletal bending modes fol t,his series of five-membered rings is given in Table I. ORSER\~ATIONS
BNI)
CONCLUSIONS
l’itzer and Donath ( 2 ) predict hindered pseudorotatjion in my saturated fivcmembered ring in which t,he t’orsional forces do not exa&ly cancel one another. Thus, they predicted barriers not only in thiacyclopentane and cyclopentanorw but also in t,etrahydrofuran and other molecules in which all five skelet,al atoms \\-ere not, ldent,ical. However, no effect#s of a barrier were observed in the far-infr:ired spectrum of either tetrahydrofuran (5) or 1, I<-dioxolnne ( 22), although tet,ra-
DURIG,
291
COULTER,
AND
TABLE FUNDAMENTAL
Symmetry&
Al B1 B1 Ba B2 ;12
FREQUENCIES
IN WAVENUMBERS
WERTZ
I OF THE SKELETAL
Ethylene Carbonate
-r-Butyrolactone
717 799 529 529 217 (217)h
674 637 536 491 224 166
Cyclopentanone
BENDING
MODES
Cyclopentanone-da
711 583 472 459 236 95c
* The symmetry of the vibration assumes Cz. symmetry for the molecule. b This assignment is tentative. c Far infrared vapor frequencies, all others are Ramau frequencies recorded liquid.
638 586 444 391 223 8gc
with the
hydrofuran was predicted to have a barrier similar to that for both thiacyclopentane and cyclopentanone. Therefore, it is quite evident that additional factors must be considered when predictions are made for the magnitude of barriers to pseudorotation in five-membered rings. Two of these factors are probably the ring strain and the relative effective masses of the ring substituents. The resultant force in the five-membered ring is probably a composite of all three of these factors, and at present there is insufficient data available to predict the relative effects of any one of them. It might be argued that a series of closely spaced Q branches could be explained on the basis of a slightly anharmonic potential instead of the cosine based function assumed for pseudorotation. However, in the case of a relatively low barrier (-1.S kcal/mole) the contribution to the entropy of a hindered pseudorotator differs from that of a harmonic oscillator of the same frequency by 1.5 e.u. at room temperature for cyclopentanone. Such a large entropy difference should be easily discernable by thermodynamic measurements. In the case of the high limit of the pseudodorotational barrier for cyclopentanone, the entropy difference amounts to only about 0.15 e.u. at room temperature which would be beyond the limit of thermodynamic detection. At 100 degrees above room temperat,ure t’he entropy difference becomes 0.3 e.u. which is still barely detectable by thermodynamists. Therefore, thermodynamic measurements could be used t#o distinguish between a pseudorotator and a relatively harmonic oscillator only in the case of a relatively low barrier. If one assumes the reduced moment of inertia of y-butyrolactone and ethylene carbonate to be the same as that of cyclopentanone, which seems reasonable when one considers that oxygen is nearly isobaric with a methylene group, then barriers to pseudorotation are calculated to be about 11 and 18 kcal/mole, respectively. Such high barriers would negate the effects of pseudorotation on the thermodynamic properties of these molecules at reasonable temperatures. Thus, in molecules with sufficiently high barriers, the
FAR-INFRARED
SPECTRA
motion can be treated as an ordinary about a most stable configuration.
OF SMALL
vibration
RINGS
in which the puckering
“35
oscillat8cs
ACKNOWLEDGMENT The authors would like to thank Professor Lord for communication of the far-infrared data on cyclopentanone prior to publication. The authors would also like to acknowledge the finalicia srlpport given this work by the National Aeronautics and Space Administration with Grant NGR-41-002-003-I. I~EC’EIVEU:
.January
31, 1968 REFERENCES
i. d.
2. 3.
4, 5, 6. 7. 8. 9. 10.
11. 18.
1:. I>LXIG ANU 1~. C. LORD, J. Chem. Phys. 46, 61 (1966); J. It. DUI~IG ANU A. C. MORRISSEY, ibid. 46, 4854 (1967) ; S. I. CHAN, T. R. BORGERS, J. W. RUSSELL, If. I,. S~I’RAUSS,AND W. D. GVINN? ibid. 44, 1103 (1966); J. R. DURIG ANI) A. C. X101