The vibrational spectra and structure of cyclooctane

Specbxhimica

Acta,

1959, No. 12, pp. 1103

The vibrational

to 1117.

Pergamon

Printed

in Northern

Ireland

spectra and structure of cyclooctane*

H. E. BELLIS? University

Press Ltd.

andE.

of Connecticut, (Received

J. SLOWINSKI, Storm,

JR.

Connecticut

19 August 1959)

Abstract--The Raman and infrared spectra of cyclooctane have been obtained under several conditions of temperature and state of aggregation. The infrared spectra of gaseous, liquid and solid cyclooctane contain essentially the same bands. No unequal dependence of band intensities on temperature is observed in either the Raman or infrared spectrum of the liquid. This behavior is taken to imply that only one structural isomer of cyclooctane is present under ordinary conditions. Division of the vibrational spectrum into regions associated with the group frequencies present in the molecule and comparison of the spectrum in each region with that predicted for the various possible models allows one to select the “tub” form (point group D,,) as the most An assignment of frequencies is made on the basis of the probable structure of cyclooctane. “tub” moclel.

Introduction ALTHOUGH the spectra and structures of several of the cyclic saturated hydrocarbons have received considerable attention in recent years, relatively little has been published on the eight-membered ring, cyclooctane, C&H,,. The infrared spectrum that is available is that given by COPE [l] and was obtained for characterization of the substance. GODCHOT [2] and GOUBEAU [3] reported the Raman spectrum, though neither author included any polarization data. LIVINGSTON and coworkers [4] carried out an electron diffraction study on cyclooctane with inconclusive results. The crown model, symmetry Ddd, seemed most compatible with their data, but a mixture of rotational isomers, with a large percentage of crown form present, could not be excluded. LORD and LIPPINCOTT [5] studied the infrared and Raman spectra of the compound and their preliminary results also implied the existence of rotational isomers. cycloHexane, similar to cyclooctane but with two fewer carbon atoms in the ring, has been extensively studied [6-91 and the chair configuration, symmetry D3d, seems well established. In this work we have attempted, by a study of the infrared and Raman spectra of cyclooctane, to determine the most probable structure of the.molecule. To aid in the interpretation of the data a complete and parallel study of cyclohexane was

l] 21 31 41 51 61 71 81 91

* Presented in part at the Symposium on Molecular Spectroscopy, Columbus, Ohio, June t Present address, Electrochemicals Division, du Pont Co., Buffalo, New York. A. C. COPE, Msss. Inst. of Tech., Cambridge, Mass. Private communication. M. GODCHOT, E. CANALS and G. CAIJQUIL, Compt. rend. 194, 1547 (1937). J. GOIJBEAU, Ann. Chem., Justus Liebigs 567, 214 (1950). R. N. LIVINGSTON, Purdue University, Lafayette, Ind. Private communication. E. R. LIPPINCOTT, University of Maryland, College Park, Md. Private communication. A. LANGSETH and B. BAK, J. Chem. Phys. 8, 403,415 (1940). R. S. RASMUSSEN, J. Chem. Phys. 11, 249 (1943). D. A. RAMSAY and G. B. B. M. SUTHERLAND, PTOC. Roy. Sot. (London) A 190, 245 (1947). C. W. BECKET, K. S. PITZER and R. SPITZER, J. Am. Chem. Sot. 69, 2488 (1947).

1103

1957.

H. EBELLIS

SLOWLNSKI, JR.

mdE.J.

also made. Though we include no data on cyclohexane here, since our results are very similar to those available, w.e will refer to its spectrum and structure determination where they pertain to the problem of the structure of cyclooctane.

Experimental i3ample preparation A 200 ml sample of research grade cyctooctatetraene was obtained from the The General Aniline and Film Corp., and was purified by vacuum distillation. infrared spectrum of the product on comparison with the spectrum of LIIPPINCOTT and LORD [lo] indicated about 2% styrene as impurity. The purified material was reduced with hydrogen at low pressure over a 5% Pd on carbon catalyst according to the method of ZIEGLER and WILMS [ll]. The product was separated from the catalyst and solvent and unsaturated impurities were removed. Fractional distillation of the sample resulted in 99% pure cyclooctane as determined by vapor phase chromatography. The physical constants of this sample, used in all subsequent Raman studies, are b.p. 150°C at 745 mm Hg; m.p. 13.9”C; d, 0.8356 g/ml; ni” l-4576. For the infrared studies the above sample was further purified by vapor-phase chromatography. By a method previously described [12] the cyclooctane fraction was trapped as it came from the column and a small sample of chromatographicallypure material was thereby obtained. The sample of cyclohexane was Eastman Kodak spectral grade and was purified The final sample analysed 99.5 per cent pure by vapor by fractional distillation. phase chromatography. cycloHexanone and cyclooctanone, used in some of the spectral studies, were obtained from Eastman Kodak and K and K Laboratories, respectively, and were purified by fractional distillation. Infrared

spectra

The infrared spectra from 2-15 ,u were obtained in our laboratory with a Perk&Elmer Model 21 instrument with sodium chloride optics. The region from 15-34 /J was obtained on a Perkin-Elmer Model 13 instrument with a cesium bromide prism. * Wavenumber calibration of our instrument was made on the basis of published frequencies of polystyrene him. Absorption peaks are believed to be accurate to f3 cm-l, or better from 300-1700 cm-1 and &6 cm-l, or better between 1700 and 3000 cm-l. For all liquid samples other than the purest cyclooctane, standard infrared cells were used. The cell, of thickness 0.05 mm, made to hold the small volume of pure cyclooctane has been described [12]. The spectrum of solid cyclooctane was obtained in a double cell which could be cooled to -50°C with a stream of dry nitrogen. Spectra were taken for a sample frozen at -50°C and at 10°C intervals up to room temperature. The infrared spectrum of the liquid was measured at * Courtesy

of M. Tobin,

Olin

Mat&son

Laboratories,

New

Haven,

Corm.

[lo] E. R. LIXTINCOTT and R. C. LORD, J. Am. Chmn.Sot. 73, 3370 (1961). ill] K. ZIEQLER and H. Wnxs, Ann. C&w&., .h&~, ~ieb@ 567 (1950). [12]

H. E. BELLIS

and E. J. SLO~~NSKI,

J. Chem.

Phya.

1104

25, 794 (1966).

The vibrational

spectra and structure

of cyclooctane

several temperatures. between room temperature and 120°C. The oven used encompassed both beams of the spectrophotometer and was electrically heated. The vapor spectrum in the infrared was obtained in a 4.5 cm gas cell. A small amount of liquid was injected into the cell, which was then evacuated and placed in the oven. The temperature was raised until a sufficiently intense gas spectrum was obtained. Raman spectra The Raman spectra were obtained photographically in our laboratory with a Hilger E 615 glass spectrograph of aperture f = 5.7. Hg 4355 A excitation was used with a filter of saturated sodium nitrite and eosine plus a Wratten 2A filter. Background intensity was low and no lines arising from Hg 4047 B excitation were identified. Kodak 103J plates were used and exposure times varied from 30 min to 6 hr, with a slit width of from 0.10 to 0.15 mm. Calibration was made by comparison The with an iron spectrum exposed above and below the Raman spectrum. Raman frkquencies are believed to be accurate to f3 cm-l below 1700 cm-l and +6 cm-l above 1700 cm-l. Intensities of lines were estimated by eye on an arbitrary scale with 10 assigned to the strongest observed Raman line. The method of EDSALL and WILSON [13] was used to obtain polarization data. Polaroid cylinders with the plane of polarization parallel and perpendicular to the axis of the Raman tube were prepared and suitably masked. Nearly identical times of exposure were used in the two experiments, which were made consecutively on the same plate.

Experimental results The observed infrared and Raman spectra for liquid cycZooctane are given in Raman lines, of which eleven appear to be Table 1. There are a total of thirty-six polarized. In the infrared spectrum forty-three bands are observed. The infrared spectrum of the solid was essentially identical with that of the liquid. In the rather weaker spectrum of the vapor sixteen bands were observed. Almost no structure was apparent in any of the bands in the infrared spectrum of cyclooctane vapor, although the vapor spectrum of cyclohexane showed considerable structure. Intensities of infrared bands in the Table are qualitatively assigned according to the following convention for bands as observed in a liquid cell of O-125 mm thickness : below 5 per cent transmission (!Z’), very strong; 5-20 per cent T, strong; 20-50 per cent T, medium; 50-90 per cent T, weak;, and greater than 9.0 per cent T, very weak. Temperature studies

If rotational isomers are present in cyclooctane they might be detected spectroscopically in several ways. One would expect possible dependence on temperature of band intensities in the infrared or Raman spectrum; changes in concentration of isomers in accord with the Boltzmann distribution law should be reflected in unequal dependence of band intensities on temperature. On solidification of the [13]

J. T. EDSALL

and E. B. WILSON,

JR., J. Chem.

Phys.

1105

6, 124 (1938).

H. E. BELLIS Table

1. Vibrational Raman (liq) (cm-l)

215 243 292

spectrum

and

and E. J. SLOWINSKI,

frequency

hfrared (cm-l)

(liq)

329 (1) D 369 (4) P (0) (0) (3)

434 w 444 w

IS

(2) D

671

(1) D

D

480 m mm 697w

698 (8)

P 719vw

735 (0) P 759 (7) P 792 (3)

D

848 (2) D 869 (0) 952 (6) D 979 (5) D 981 w 1013 vw 1030 1044 loss u!x

(0) (1) D (5) D (4)

1126 1140 1164

(3) p (0) (0)

10449

@81

m

1llOvw

1245

113 m 1164~~ 1216 vw 1mrn 1mrn 1282 m 1325 VW 1mrn

(0)

1442 1462

(10) D (3) P

(D,,

(979 - 767 = 212) CCC bending CCC bending CCC bending CCC bending (953 - 515 = 438) (1290 - 848 = 442) CCC bending CCC bending (1442 - 759 = 683) (1467 - 767 = 700) CC stretching (1085 - 369 = 716) (2 x 369 = 738) CC stretching CC stretching CC stretching (292 + 515 = 807) CH, rocking (3 x 292 = 876) (1462 - 515 r 947) CC stretching CC stretching (698 + 292 = 990) (767 + 243 = 1010) CH, rocking CH, rocking CH, rocking CH, twisting (792 -+ 329 = 1121) CH, rocking CH, twisting (792 -j- 369 = 1161) (698 + 516 = 1214) CH, twisting CH, twisting CH, twisting, E CH, (1085 + 243 = 1328) CH, wagging

A,, B, CH, wagging

(4) p

LB2

of cyclooctane

1348 1261 1447 1461 /425 1733 1805

VW s vs s m w VW

E (1044 + 292 = 1336) E CH, wagging B,, E CR, bending A,, E CH, bending B, CH, bending E (979 + 767 = 1746) E (1044 + 759 = 1803)

1106

tub

model)

kssigrlment

E B, E B, A, E B, B, E B, E A, E A, A, E E E B, E E B, B, E E B, E E B, E A, B, E E E E A, E B,

(1) D (1) D (3) D

435 446 @j

assignment

JR.

wagging

The

vibrational

spectra,and Table

Raman (liq) (cm-l)

Infrared (cm-l)

1940 2125 2230 2280 2400 2670 2688 2855 2907 2925

(2) P (8) P (10) P (10) P

structure

of cyclooctane

1 (co&L)

(liq)

E B, E E E E A,

480 = 1947) 848 = 2138) + 767 = 2234) + 979 = 2269) + 979 = 2421) + 1287 = 2648) 1345 = 2690) A,, B,, B,, E CH sym. stretching A, (2 x 1442 = 2884) A,, B,, B,, E CH asym. stretching E (2850 + 243 = 3093) E (2920 + 293 = 3213)

w w w w w

m

=VS

WV!3 3080 w 3200 w

(1467 (1290 (1467 (1290 (1442 (1361 (2 x

+

+

sample one might expect that a transition to a single isomeric form would simplify the spectrum. No such effects were observed. There was no unequal variation of band intensities with temperature as the liquid was heated from room temperature to 12O’C. The infrared spectrum of solid cyclooctane at -5O’C was substantially the same as that of the liquid. The above evidence implies the absence of detectable amounts of rotational isomers in cyclooctane. The attempt was therefore made to interpret the observed spectra on the basis of a single molecular structure. The fact that a reasonably simple interpretation on that basis is possible would appear to support the view that the existence of rotational isomers in cyclooctane at about 25°C is unlikely.

Interpretation Preliminary

selection

of the spectra

of fundamentals

Since cyclooctane is such a large molecule, possessing some sixty-six degrees of vibrational freedom, it is necessary, before proceeding with the structure determination, to make a tentative selection of bands associated with fundamental frequencies of vibration. Studies of the spectra and vibrational analyses of cyclopropane [14], cyclobutane [15], cycZopentane [16], cyclohexane [9], and our spectra of cyclohexane indicate that with our intensity conventions fundamental bands in the infrared spectrum of cyclooctane may be expected to have an intensity of medium or greater and that Raman bands will have an intensity of 1 or more. Also, fundamental vibrations appear to occur only in the regions O-1500 cm-l and 2750-3200 cm-l for these cyclic hydrocarbons. [la] [16] [16]

A. W. BAKER and R. C. LORD, J. Chem. Plya. 23, 1636 (1965). G. W. RATHJENS, N. K. FREEMAN, W. D. Gwm md K. S. PITZER, (1953). F. A. MILLER md R. G. INSKEEP, J. Chem. Phys. IS,1519 (1950). 1107

J. Am.

Chews. Soo. 75,

5634

H.

E. BELLIS

and

E. J. SLOWINSKI,

JR.

Application of the above empirical rules to cyclooctane results in the estimate that its vibrational spectrum contains about twenty infrared fundamentals and about twenty-six Raman fundamentals, of which nine are polarized. If we consider that a coincidence between infrared and Raman bands occurs ivhen the bands fall within 6 cm-1 of one another below 1700 cm-l and within 12 cm-l of one another above 1700 cm-1 and that at least one member of the coincident pair has sufficient intensity to be considered a fundamental band, there are twenty coincidences in the vibrational spectrum, and of these five involve polarized lines.

conciusions drawn

from general nature of vibrational

spectrum

Since the cyclooctane molecule is so large, no definite’structure can be selected on the basis of the general character of the vibrational spectrum. However, the presence or absence of coincidences in the spectrum can presumably be determined and used to eliminate some of the possible symmetries from further consideration. As noted above, twenty coincidences were observed, with eleven of them occurring below 1200 cm-l, where resolution is good and chance coincidences less likely than at the higher frequencies. This implies that structures in which all coincidences are forbidden are highly unlikely for cyclooctane. In the case of cycbohexane, where the accepted point group does forbid any coincidences, out of twelve observed vibrational frequencies below 1200 cm-l only one has an apparent coincidence. If the existence of coincidences in the vibrational spectrum of cyclooctane is accepted, cyclooctane camrot belong to any of the following point groups, since they do not allow for any coincidences: Dsh, Csh, S,, Dbd, D4,,, C4k, Dtl, C,, and Ci. It is noted that the D,, point group, to which the crown form of the molecule would belong, must be eliminated on this basis.’ Several other point groups appear to be unlikely for cyclooctane on the basis of the total number of lines observed in the vibrational spectrum. Those point groups of very low symmetry have predicted spectra containing about twice as many lines as were observed. Since it seems improbable that such a large fraction of lines would not be seen, the following symmetries are eliminated: CzV, D,, C, and C,. At this point eight point groups remain as possible symmetries for cyclooctane. They are CBV, C,, D,, D,, Cdv, C,, S, and D,,. Similar.reasoning applied to the overall vibrational spectrum of cyclohexane also leaves eight possible point groups as symmetries for that molecule. Conclz&ons

drawn

front analysis

of group vibrations

The difficulties in attempting to determine the structures of large molecules, particularly hydrocarbons, on the basis of their overall vibrational spectra are obvious. The spectral regions in which carbon-hydrogen stretching and bending frequencies occur are very narrow; with many such frequencies active, overlapping of several bands caused by the same group vibrations in different classes tends to occur. Limited resolution, especially in the C-H stretching region, also makes detection of individual vibrations difficult. The net result is that at the higher frequencies the number of bands present is much less exactly known than at the lower frequencies, where the spectral range of the group frequencies is larger and resolution is better. Final determination of the most probable symmetry of 1108

The vibrational

spectm and structure

of cyclooctsne

cyclooctane was therefore carried out on the basis of those group frequencies occurring at the lower end of the vibrational spectrum, below 1400 cm-l. In Fig. 1 are sketched the general forms of the vibrations in each of the vibrational classes for a planar, Dsh, model of cyclooctane. These vibrations follow in the usual way 11’71from the D,, character table given in Table 2. The normal vibrations in each class are considered to consist primarily of the vibrations of

A,, CC stretching R CH, bending CH (sym.) stretching

A,,

B,, CCC bending CH, bending CH (sym.) stretching

El,

El, R

CH, CH, CH

rocking twisting (asym.)

CH,

CH,

Es, CCC bending CH, rocking CH, twisting CH (asym.) stretching

CH,

CC stretching CH, bending CH, wagging CH (sym.) stretching

E,, CCC CH, CH, CH

Aa” CH, IR CH

wagging

Es, CC stretching CH, wagging

twisting

E,, IR stretching

A,,

twisting

bending rocking twisting (asym.)

Fig. 1. Vibration

stretching

B,,

rocking (asym.)

stretching

CCC bending CH, rocking CH (sym.) stretching

E,, CC stretching R

CCC bending CH, bending CH, wagging CH (sym.) stretching

Es, CC stretching CCC bending CH, bending CH, wagging CH (sym.).stretching

_

of czJclooct&ne De, model.

groups of atoms within the molecule. These group vibrations may consist of skeletal modes, in which the CH, groups move as units, and modes in which the CH, group vibrations are important. Our notation is that of KOHLRAUSCH [18], and below each of the diagrams are the group vibrations which would be associated with that class and the spectral activity of that class. Using the diagrams and [17] G. HERZBERO, Infrraredand Raman Spectra of Polyokwnic York (1945). [18] K. W. F. KOHLRAUSCH, Ranaanspekcen p. 215. Edwards,

1109

Moleculea

Ch.

2.

Van Nostrand,

Ann Arbor, Mich. (1946).

New

H. E. BELLIS

and E. J. SLOWINSKI,

JR.

the spectral activities of each of the vibrational classes, one can quickly predict the vibrational spectrum of the D,, model of cyclooctane on a group vibration basis. This information is made more useful by noting the spectral regions in which one expects to find each of the group vibrations. From the reported vibrational analyses of the normal alkanes [19, 201 and the cyclic hydrocarbons [9, 14-161 spectral regions can b$ obtained for each type of group vibration. C-H bondstretching vibrations are seen to occur from 2800-3200 cm-l; CH, bending modes Table Daj,

I

2C,l

2~3

2. Character

2~7~4 4c,*

2~33

A- 10

1

1

1

1

1

iyo A 2u B 18 B 1U fz;;

1 1 1 1 1

111 1

11 1 1

-1 -1 -1 -1

1 1 1 1 1

-1 -1 -1

l-l 1 1 1 11

E 10 E lU E 20

;

$

i

7;

2 2

::: E su

2 2

-2

-q2 0

-42

-2 -2 0 0

1

1

-1.1--l -1 -1 -1 1

-1 1 1 -1

0

2

1/20 d2

-2 2 -2

00 0

for

4c,"

1

0 0 0

042-2 0

table

point

group

2q

i

2s,2

1

1

1

-1-l 1 -1 -1 1 -1

-1 1 -1 1

D,,

1

1

-1-l-l-l 1 -1 -1 1 -1

1 -1 -1 1 0

o-2 0

1

-1

Vi-$

42 2

2~3~3

1/21

O-42 0

-2

0

Oh

40,

1 1

1 -1

-1

1 2

1 -1

1 1

1 -1

40,

-1 -1

1 -1 1 -1 1

2 2

i 0 0

0 0 0

2

0

0

appear from 1400 to 1500 cm-l; CH, wagging and twisting fundamentals from 1170 to 1400 cm-l; CH, rocking and C-C bond-stretching modes from 670 to 1170 cm-l; C-C-C bond-angle bending modes are active below 670 cm-l. Using the above spectral ranges, which fit the previously analysed cyclic hydrocarbons quite well, one can predict the number of bands to be expected in each of the spectral regions once the analysis of group vibrations has been made. The predictions for the D,, point group appear in Table 4. For structures of symmetry lower than D,, the predictions of vibrational spectra on a group frequency basis were obtained by referring to the correlation table given in Table 3. This table, essentially that of WILSON et aZ. [2l], gives the vibrational classes into which the classes of point group Dsh transform when the symmetry of the molecule is decreased by changing the atomic positions. The correlation table and the spectral activities of the vibrational classes for each point group allowed construction of Table 4. In Table 4 are included all point groups not already eliminated, plus D,, and C,,. A table analogous to Table 4 was prepared for the cyclohexane molecule. [19] [ZO] [21]

H. TSCHAMLER, J. K. BROWN, E. B. WILSON, 1956).

J. Chem. Phys. 22, 1845 (1964). N. SgEPPARD ad D. M. t%KPSON, Tram. J. C. DEOIUS and P. C. CROSS, MoZecuZar

1110

Roy. Sot. VilibTatiom

(London) A 247, 36 (1964). Ch. 6. McGrmv-Hill, New

pork

z

w

c

(II,,+

A2

A 21

E E2 .3E Es

-Go

E 2u

E 30

E 3u

refers

E2

El

E 1U 2

El

El

E 10

3

Dzd’ to butterfly

Es

E

E2

El

Bl

B

B

B

B

A

A

A

A

ca

3. Correlation

2u

B

to tub model;

B2

B 1U B2

*1

B 10

B 20

Al

278

A

A2

1U

A

Al

10

A

c 8V

Table

model).

Es

Es

E2

E2

E1

El

B2

B2

Bl

Bl

A2

A2

Al

Al

D3

table

for

E,

E1

E2

E2

E 1

Es

Al

Bl

-42

B2

B2

A2

Bl

Al

D4d

the

D4

E

E

B,

+

B,

E

E

E

E

223

E

E

A

A

A

A

E

E

2A

2B

E

E

B

A

B

A

B

A

+

D2d

E

E

A,+4

B,

E

E

Bl

Al

B2

A2

B2

A2

Bl

B A

Al

(cyclooctane)

A

84

A

A

A

c4

B, + B,

4v

its subgroups

2B

c

and

B, + B,

E

E

-4

A2

A2

Al

Al

A2

A2

Al

of a group

B, + B,

E

E

Al

Al

-42

A2

A2

A2

Al

Al

species

B,

‘E

BlfB2

Bl+B2

Al

A2

A2

Al

Al

A2

A2

Al

C 21,

E

E

A, +A,

Bl

Bl+

A,+A,

fB2

B, + 33, A,+.&

E

B2

A2

Bl

Al

J32

A2

Bl

Al

%i’

B2

B

:

s E?,

E 0, .

il

Ii w

d 8 R 0

5f 4 8 z c. g L

H

=: tsJ

+

group

* R = totalRamanline8;

Observed

c 2u

%i’

D2d

84

c4

c 4u

D4

D4a

Point

-

-

-

RP

33

22

14

7

60

8

8

8

4

00

20

42

42

6

0

4

4

4

0

C PC 00

group

2

8

6

5

6

6

6

5

3

3

3

3

1

3

2

1

2

2

2

1

1

1

1

1

3

7

3

3

4

4

4

3

2

1

2

2

RRPIRCPC 2 1 1

IR

2

7

3

3

4

4

4

2

0

1

2

2

0

0

1

1

0

-

-

in BpECtrLmI

40

51

62

40

20

31

16

91

10

12

12

80

10

R

2

2

4

4

2

RP

bends:

5

10

5

6

8

8

4

6

2

3

4

2

IR 100

2

0

4

4

0

C = total

51

10

50

6

8

8

40

4

00

20

42

20

63

51

62

62

31

5

2

3

4

4

RP

coincidences:

113

16

10

11

12

12

113

113

R

8

13

7

6

8

8

7

5

3

3

4

4

IR 2

1

5

0

0

4

4

3

PC = total

9

13

7

6

8

8

7

40

00

20

42

42

C PC 00

CH rocking CC Btr&Ching (670-1170 cm-l)

of cyclooctane

C PC

CH twisting CH wagging (1170-1400 cm-l)

infrared

vibration8

= total

1

3

0

0

2

2

2

0

0

CH bending (1400-1500 cm-l)

4. Predicted

= totalpolarizedRamanlines;

2

14

16

6

7

8

113

4

12

8

6

4

12

8

102

4

12

6

4

3

51 62

4

62

IR 2 4

2

RP

62

31

10

*R

CH stretching (2800-3200 cm-‘)

Table

poIarized

7

10

7

6

8

8

8

7

4

2

2

2

1 .

R

1

4

2

1

3

2

2

1

1

0

0

0

2

8

,4

3

5

4

4

2

0

0’0

0

0

0

4

0

0

3

2

2

0

0

0

0

0

coincidences.

2

8

4

3

5

4

4

3

2

0

0

0

RPIRCPC 0 0 0

CCC bending ( <670 cm-l)

The

vibrational

spt&ra

and

structure

of cplooctane

Structures eliminated by consideration of the overall vibrational spectrum of cyclohexane again showed poor agreement between predicted and observed spectra. Further elimination of structures was possible, however, on the basis of poor agreement below 1400 cm-l. The two point groups D,, and D,, have predicted spectra which best agree with that observed, with the exception that for point group Da,, some coincidences are predicted, where for point group D,, there are no coincidences allowed. Below 1400 cm-l the D,, model predicts five Raman coincidences for the eight predicted infrared fundamental absorption bands. In this region six principal infrared bands are observed, and only one of these can be considered to have a Raman coincidence. This evidence, plus other rather more minor disagreements between predicted and observed spectra, allows one to select the D,, model as being the most probable structure of cycZohexane. The parallel study of the spectrum of cydohexane was very useful, in that features observed in the course of its vibrational analysis gave a general implication of features to be considered reasonable in the rather more complicated spectrum Among the features observed in the cyclohexane analysis are: of c@ooctane. (1) the separation of fundamental from harmonic bands is consistent with our intensity convention, except that CH, wagging modes in the infrared are somewhat weaker than expected; (2) considerable overlapping of C-H stretching and of CH, (3) chance coincidences below 1400 cm-l occur for two bending modes occurs; out of eleven observed infrared bands; (4) all predicted polarized fundamental bands below 1400 cm-l are observed in the Raman spectrum of cyclohexane. Considering now the predicted spectra for the various molecular symmetries as given in Table 4, the situation regarding coincidences seems clear. Below 1400 cm-l of out twenty-six observed fundamentals there are sixteen apparent coincidences. To attribute all sixteen to chance appears to be unreasonable in view of the nature of the spectrum of cyclohexane, and hence all structures which completely forbid coincidences are eliminated as before. Again, structures of symmetry C,, and below predict spectra about twice as rich as observed and are therefore ruled out. Of the eight symmetries still under consideration, those associated with point groups Csv, C, and D, can be eliminated on the basis of the observed spectrum below 670 cm-l. These models allow no polarized Raman lines and no infrared bands in this region. One polarized Raman line and two infrared bands are observed. Also, seven Raman lines are observed where two are predicted. Between 670 and 1170 cm-l these models also predict a simpler spectrum than is observed. Point groups CdV, C, and S, predict at least five polarized. coincidences below In the 1170-1400 cm-l region, where one polarized Jl70 cm-l and one is observed. Raman line is observed, point groups C, and S, predict four such lines and point group C,, no polarized lines. This evidence makes it appear that these point groups are unlikely for cyclooctane. Three structures remain for consideration. Two are in point group Dzd, a “tub” form and a “butterfly” form, and one in point group D,, a distorted crown form (see Fig. 2). In the region below 1170 cm- l, these structures, of all those examined, have predicted spectra which best agree with that which is observed. The principal difference between predicted spectra for the two point groups is 9

1113

H.E.

BPLLIS and E.J.

SLOWINSKI, Jn.

that the D,, models have predicted spectra in which all infrared bands have Raman coincidences, whereas the *spectrum for the D, model indicates that of fourteen predicted infrared fundamentals below 1400 cm-l four will not have Raman coincidences. In this region of fifteen observed infrared bands, thirteen have Raman coincidences, and the two bands that do not, occur sufficiently close to strong Raman lines to be possibly overlapped by them and hence unresolved. Though the D, distorted’crown model cannot be completely excluded on this basis, the spectral evidence seems to indicate that one of the D,, models is more likely.. The main point of disagreement between the predicted spectra for the D,, models and that observed occurs in the 1170-1400 cm-l region, where three

Did,

crown

D,,, distorted

Fig.

crown

Dee+, tub

2. Some possible

models

DOdl butterfly

of cyclooctane.

Raman lines are observed and ten are predicted. Models consistent with the observed spectrum in this region show elsewhere very poor agreement with that spectrum and had to be eliminated. We presume that the several CH, twisting and wagging modes are either too weak to be seen or overlap considerably. Since the vibrational spectrum of cycbohexane contains only two Raman lines in this region, with two predicted, the likelihood of overlapping of bands could not be confirmed by the cyclohexane results. The choice between the two D,, models for cyclooctane rests on two pieces of evidence. The tub model predicts that one polarized Raman line associated with C-C-C bond angle bending will be observed, where the butterfly model requires two such lines. One polarized Raman line, at 369 cm-l is observed. Similarly, in the C-C stretching region the tub model predicts two polarized Raman lines and the butterfly model only one. In the observed spectrum there are two strong polarized Raman lines, at 69s and 759 cm- l. These two bands appear to be associated with similar modes and presumably both are due to C-C bond stretching vibrations. This evidence seems to favor the tub model for cyclooctane. The other information comes from a study of the Raman spectrum of cyclooctanone. If this molecule possessesa plane of symmetry about half its Raman lines should be polarized. If there is no element of symmetry all its Raman lines should be polarized. If the symmetry of the ring in the ketone is the same as that of the unsubstituted molecule, then if cyclooctane has the tub form there will be no plane of symmetry in the ketone, and all lines in the Raman spectrum should be polarized. If cycbooctane has the butterfly form a plane of symmetry can exist in the ketone and half the lines might be polarized. The polarization spectrum of cycZooctanone was obtained, and though it was of relatively poor quality, all Raman lines appeared to be polarized. 1114

The

vibrational

spectra

and structure

of qclooctane

This evidence, though not conclusive, seems to indicate that the tub model of We have therethe molecule, in point group .Dzd, is most probable for cyclooctane. fore made an assignment of vibrational frequencies on the basis of the tub model.

Assignment of vibrational frequencies The predicted vibrational spectrum of the tub model of cycbooctane is given in Table 5. Since the spectrum is so complex, assignment of many of the frequencies, especially at the higher frequencies, must be regarded as tentative. In the Raman spectrum below 670 cm-l only the line at 369 cm-l appears to be polarized. This is evidently the A, CCC parallel bending vibration. The diffuse Raman lines at 480 and 515 cm-l have definite infrared coincidences and correspond Table Symmetry

5. Predicted

-

spectrum

-

for

4

4

classes

tub

model

of cyclooctane

-

Bl

Bz

E

1 1 1 1 1 1 1 2 R 9

1 1 1 1 1

2 2 2 2 2 2 2 2 R, IR 16

C-H stretching C-H stretching CH, bending CH, wagging CH, twisting CH, rocking C-C stretching C-C--C bending Spectral activity* Total

1 1

(sym.) (asym.)

1 1 1 1 2 1 RP 9

* RP

= Raman

active,

lines

polarized;

1 1 1

1 1

1 0

-I

I = inactive;

2 I s

1 1 1 R, IR 8

R = Raman

active;

IR

= infrared

active.

to the B, and E perpendicular ring-bending vibrations. On the assumption that the B,-vibration should be slightly more intense in the Raman spectrum (derived from a Raman active mode in Dsh) the 480 cm-l line is assigned to that’mode and the 515 cm-l line is assigned to class E. No coincidences are observed for either the 329 or the 671 cm-l Raman lines. The Bl perpendicular ring-bending mode is taken as 329 cm-l since 671 cm-r seems too high for such a vibration. A B, and an E parallel bending vibration are expected in the vicinity of 300 cm-l. Cn the strength of its intensity the line at 292 cm-l is almost certainly one of these. Since a line appears in the Raman spectra of cyclooctane derivatives at 243 cm-l (cycZooctanone, 242 cm-l; cyclooctene, 240 cm-l; methyl cyclooctane, 246 cm-l) this line is selected as the other fundamental. The stronger line at 292 cm-l is assigned to the E-class on the basis of overtone assignments. The 243 cm-l line is then assigned to the B,-vibration. In the region between 670 and 1170 cm-l three strong polarized Raman lines are observed. The lines at 698 and 759 cm-l are assigned to the CC stretching frequencies: in cyclohexane this frequency occurs at 802 cm-l. The third polarized 1115

H.E.

BELLIS and E. J.SLOWINSEI,JR.

line, at 1126 cm-l, must be due to the CH, rocking vibration, which in q/dohexane was assigned at 1155 cm-l. . Bands appearing in the infrared and Raman spectra at 790, 853 and 955 cm-l are possibly CC stretching frequencies in the B,and &‘-classes. A strong infrared band at 767 cm-l, whose Raman counterpart is presumably masked by the polarized line at 759 cm-l, should be included in that group. The strong R,aman line at 979 cm-l is most easily assigned as a B, CC stretching mode, since its infrared coincidence is weak. The Bz CC stretching frequency should occur near 979 cm-l with the Raman line having good intensity, since both of these modes arise from the Raman-active E,, mode in the D,, model. The B,-vibration is therefore taken to occur at 953 cm-l. A CH, rocking vibration in cyclohexane at 848 cm-1 implies that the cyclooctane band at 853 cm-l is also due to a rocking mode; this band in the vapor spectrum shows what appears to be some PQR-structure, is moderately strong, and is reasonably assigned to the B,-class. With that assignment the bands at 767 and 792 cm-l are taken to be associated with the two CC stretching frequencies in class E. The two remaining class E rocking modes most likely occur at 1044 cm-l and 1085 cm-l, with some Fermi interaction possibly present. The remaining rocking vibration, in class B,, could be assigned at either 1030 or 1107 cm-l. In cyclohexane a similar inactive twisting vibration has been assigned at 1109 cm-i [9]; on this basis the 1107 cm-l band is also taken to be due to a twisting mode, also in class B,, and the 1030 cm-l band is then assigned as the B, rocking vibration. In the region 1170-1400 cm-l the CH, twisting modes are assigned at lower frequencies than the CH, wagging modes. There is one observed polarized Raman line, and it is assigned to the A, wagging mode. There is no‘polarized line that can be assigned to the A 1twisting mode; on the basis of its intensity and broadness the 1290 cm-i band is presumed to consist of overlapping bands, one of which is due to the A, twisting vibration. In order to keep the B, twisting frequency near that of the related B, mode, assigned at 1107 cm-l, the infrared band at 1134 cm-l is selected for that mode. The bands at 1230 and 1257 cm-1 are now assigned to the two remaining twisting vibrations in class E. The strongest wagging band in the infrared occurs at 1361 cm-1 and is probably in class E, by virtue of arising from an infrared-active vibration in the Dsl, model, If we presume tha.t the B, and B, wagging vibrations, which arise from the same degenerate vibration in the D,, model, are close to one another, then assignment of the 1341 cm-1 band to the B, mode and the 1345 cm-l band also to the B, mode seems reasonable. The other E wagging band is assumed to overlap the 1287 cm-l band; such an assignment for this vibration is implied by some of the observed overtone bands. In the region above 1400 cm-l definite assignment of frequencies is in most cases impossible, since very extensive overlapping of bands tends to occur in both the infrared and Raman spectra. In the CH, bending region the polarized line at 1462 cm-l is assigned to class A, and is assumed to contain an overlapping band in class E, coincident with the 1467 cm- l band in the infrared spectrum. The strongest Raman line in the region, at 1442 cm-l, is probably in part caused by the bending vibration in class B, and in part by the other class E bending vibration, which should also give rise to the most intense infrared band in that region. The B2 vibration, predicted to be weaker in the infrared, is assigned at 1475 cm-l, 1116

The

vibrational

spectra

and

structure

of cyclooctane

In the CH stretching region poor resolution forces assignment of all symmetrical stretching modes to 2555 cm-l and all asymmetrical stretching modes to 2925 cm-l, by analogy with cyclohexane. An assignment of the overtone bands consistent with the above assignment of fundamentals is given in Table 1. Acknowledgement--The writers grant in support of this work.

are very

grateful

1117

to the National

Science

Foundation

for a