Spectrochlmlca Acta, 1967, Vol. 23A, pp. 1609 to 1618. Pergamon Press Ltd. Printed in Northern Ireland

The in~ared and Raman spectra of hexafluoro-, hexachloro- and hexa b r o m o c y c l o propane*

FOIL A. I~'TT,T.ER a n d K E I ~ E T H O. H A R T ~ I ~ Mellon Inst i tute , Pit tsburgh, Pa., 15213

(Received 9 September 1966)

AbSta in - -The infrared spectra of cyelo-CaF6, -CaC1 ~, and -CaBr a were measured from 35 to 4000 cm -1, and Raman spectra with depolarization ratios were obtained. For c-CaCl 6 our data agree very well with those of ITo [Spectrochim. Acta ~ , 1581 (1966)], so they are not presented in full. Two new low frequency infrared bands were found which required a minor change in the assignments.

For c-CaF e and e-CaBr e the low frequency infrared spectra and the Raman polarizations are reported for the first time, and require numerous changes from previous assignments.

Dab symmetry is fully satisfactory, and most of the spectroscopically-active fundamentals of all three compounds have been identified.

INTRODUCTIOI~

THERE has been very little work on the vibrational spectra of the interesting compounds hexafluoro-, hexachloro-, and hexabromo-cyclopropane. Hv.zcxLv.~ et al. [1] measured a few of the infrared bands of the fluoro compound above 850 cm -1 with a prism instrument, and made a partial assignment which scarcely agrees at all with the one presented here. ITO [2] has made a much more comprehensive s tudy of all three compounds. His infrared measurements extended down to 400 cm -1 for the fluoro and bromo compounds, and down to 50 cm -1 for the chioro one. His Raman results for the fluoro compound are admit tedly incomplete, and he measured polarizations only for the chloro molecule. Thus it seemed tha t there were several areas in which we might obtain additional data for these substances. Our main contribution has been to add, for the fluoro and bromo compounds, the low infrared frequencies (<400 cm -1) and the Raman polarizations. This has led to some modifications of I to 's assignments.

EXPERIMENTAL DETArr,S AND RESULTS

Origin and properties of the samples c-C3F 8 is a gas, m.p. --80°C, b.p. --33°C. The sample was a gift from Dr. D. C.

England of E. I. du Pont de Nemours and Co. I t had been made b y the irradiation of cyclobutanone. A gas chromatographic analysis was performed on a silicone oil column operated at room temperature and at 0°C. Total impurities separated were less than two parts per thousand, and the highest impuri ty peak was about 1 ppt. The material seems to be quite stable; no change in the infrared spectrum of a gas sample at 170 mm was observed in over a week.

* This work was supported by the U.S. Army Research Office, Durham under grant ARO-D-31-124-G-594.

[1] J. HE~CKLE~, F. WACH~ and V. K~mHT, J . Phys. Chem. 69, 693 (1965). [2] 1~. ITO, Speetrochim. Aeta 22, 1581 (1966).

1609

1610 F .A. MILLER and K. O. HA~T~.A~

The c-CaC1 e and c-CaBr~ samples were very generously provided b y Dr. ][to. They were originally prepared by TOBEY and WEST [3, 4]. After recrystallization from CS2 both samples were colorless solids.

Infrared spectra

Infrared spectra were obtained from 35 to 4000 cm -~ with Becl~man IR-9 and IR-11 spectrophotometers. The spectral slit width (one-half the band pass) was less than 2 cm -1 everywhere.

The c-C3F~ was examined as a vapor in a 10 cm cell with pressures ranging from 1/2 to 180 torr. The spectrum of the solid was obtained down to 185 cm -1 with a conventional low-temperature cell fitted with CsI windows. The appropriate amount of c-CsF e gas was measured with a Hg manometer and then deposited slowly onto the cold CsI window. A thermocoup]e fixed in the window gave a temperature of 98°K. The sample was annealed to 112, 128, 151, and finally 155°K, where it sublimed off the window. The spectrum was unchanged by annealing.

c-C3C] s and c-C3Br e were examined as Nujol mulls, CsI and KB r pressed disks, and saturated CS~ solutions.

Raman spectra

Raman spectra were measured with a Cary model 81 Raman spectrophotometer, using Hg 4358/~ excitation. The spectral slit width was 10 cm -1. Qualitative polarizations were obtained by the usual two-exposure method using appropriate cylinders of Polaroid wrapped concentrically around the sample tube.

The c-C3F~ was examined as the liquid, using a cooling technique already described [5]. Briefly, the sample was contained in a 4 mm diameter Raman tube with a glass rod extension in place of the usual plane window. The tube was surrounded by a double-walled glass cylinder with an evacuated annular spacing. The rod projected through a stopper which closed one end of the cylinder. Cold nitrogen gas was blown into the jacket, and easily held the sample at --50°C.

The Raman spectra of c-CaC1 e and c-CaBr e were obtained from saturated CS~ solutions. For c-C3Brs, a large single crystal was also used (about 3 × 5 × 2 ram). I t gave a bet ter spectrum, but could not be used for polarization measurements.

Results

The results for c-C3F 6 and c-CaBr6 are given in Tables 1 and 2, and in Figs. 1-4. They are discussed in detail for each compound later. Raman frequencies are believed to be accurate to ~-2 cm -1, infrared ones to ~= 1 cm -1 for typical bands.

~)ISCUSSIOI~ OF RESULTS Choice of symmetry

D ~ symmetry is assumed, and is completely adequate. We found no evidence for any lower symmetry. Table 3 classifies the fundamental vibrations for D3h symmetry, and gives our frequency assignments for the three molecules.

[3] S. W. TOBEY and R. W~,sT, J. Am. Chem. Soe. 869 56 (1964). [4] S. W. TOBEY and R. WEST, J. Am. Chem. Soc. 86, 1459 (1964). [5] F. A. MILLER and I. M~TSUBA~A, Spectrochim. Acta 2~, 173 (1966).

T h e i n f r a r e d a n d R a m a n s p e c t r a o f h e x a f i u o r o . , h e x a c h l o r o - a n d h e x a b r o m o c y c Z o p r o p a n e 1 6 1 1

T a b l e I . I n f r a r e d a n d R a m a n s p e c t r a o f c y c l o - C s F 6

I .R . (gas, 25°C) I . R . ( s o l i d , - - 1 7 5 ° C ) R a m a n ( l i q u i d , - -50°C)

cm - t I n t e n s i t y * A R - P cm -1 I n t e n s i t y cm - t Intensi tyJ" Pol . Ass ignmen t +

195] 202} 2o9j 242] 251} 259)

282

497 l 503.} vw, b

542 ] 547.5~ s 553 )

611 vvw, b

747.5~ 752 ) vw

798 w

862] 865~ vs 869j 900 ] 907.5} vvw 915 )

975] 983} 995)

1008

1034 11111 I115J

1139 1188

1264 1272 1277.5} 1283 }

m 14 202 s ~T

w 17 261 m 257 65 dp ~zt

vw, b 364 36 p ~s 367 sh, ? ? ~1~?

350-560 m, v v b ? 419 w, sp 503 w, sp 505 52 dp

518 vw, b

In

v v ~

V-v~W

m

vw Vv~7

sh

VV8

11 545 s, sp

580 v v w , sp 622 / 625) vw

760 w

8031 w 807) sh 845 m

7 853 vs

15 908 v v w

939 w w 982~ 985) vw, sp

20 993 / 996J rn, sp

lO00 vvw, sh 1017 v v w

11111 1115) w 1126 v v w

1177.5 w, sp 1222 m, sp 1228 m, sp 1244 sh

11 1255

1269 1281 1287 1337 1345

v v s

sh sh sh

737 100 p

2 × 2 5 1 ~ 5 0 2 ( v ) 2 × 2 6 1 ~ 522 (s)

~10

251-}- 364~---615 (v) 2 6 1 + 3 6 4 = 6 2 5 (s)

251-}- 505~- 756 (v) 2 6 1 + 5 0 3 ~ 7 6 4 ( s )

1 2 7 8 - - 5 0 5 ~ 7 7 3 ( v )

2 5 1 + 548--~799 (v) 2 6 1 + 5 4 5 ~ 806(s)

863 19, b dp r 9

1280 I I ?

5 4 8 + 3 6 4 = 9 1 2 (v) 545-}- 3 6 4 ~ 9 0 9 {s)

2 0 2 + 7 3 7 ~ 9 3 9 ( s )

2 5 1 + 7 3 7 = 9 8 8 (v) 2 6 1 + 7 3 7 ~ 9 9 8 ( s )

2 × 505----- 1010 (v) 2 × 5 0 3 ~ 1 0 0 6 ( s )

251 + 865 = 1116 (v) 261 -~- 853 ~ 1114 (s)

1612 F . A . Mrr.r+mt~ and K. O. H A R T ~

Table 1 (cont.)

I . R . ( g a s , 25°C) I . R . (solid, - - 175°C) R a m a u (liquid, - - 50°C)

e m -1 I n t e n s i t y * A R - P c m -1 I n t e n s i t y om -1 I n t e n s i t y ~ Pol. Ass ignment~

1364~ 1369~ 1376 /

1392~ 1398~ w 1403J

1418~ 1422J w

1475~ 1481~ v w 1487]

1510 w 1530 v v w

1596

1639

1660 1666 1729

1759 1779

S 12 1361 m ~6

1370~ 1377} m

11 1401 w 543-}- 8 6 5 ~ 1413 (v) 545-}-853------1398 (s)

1417~ 1425) v v w

12 1 2 8 0 - } - 2 0 2 ~ 1482

1513 vw , b 2 5 1 + 1 2 7 8 ~ 1 5 2 9 (v) 2 6 1 + 1 2 5 5 ~ 1516 (s)

1540 v v w 1 2 8 0 + 201 ~--- 1541(s) 1560 v v w 1557 10 ~ ~1

v w 1589 v v w 865-}- 7 3 7 = 1602 (v) 853-}- 7 3 7 ~ 1590 (s)

v w 1612 v w 1278.}. 3 6 4 ~ 1642 (v) 1255.}. 3 6 4 ~ 1619 (s)

1632 v v w v v w , sh w 1653

~ 1 3 6 9 + 3 6 4 ~ 1733 (v) v v w / 2 × 865 1730 w w 1 5 5 7 + 2 0 2 ~ 1759 (v) w, b 1762 w, b 1278.}. 505~--- 1783 (v)

1 2 5 5 + 5 0 3 ~ 1758 (s) 1785 v v w 1 2 8 0 + 5 0 3 ~ 1783 (s)

1793 v v w , sh 1808~ 1 5 5 7 + 2 5 1 = 1808 (v) 1815} v v w 1 2 7 8 + 5 4 8 ~ 1826 (v)

1950 w, sp 2005 §/ 1981~ 2011 } w 12 1989J w 1278 + 737 ~ 2015 (v) 2017 } 1255 + 737 ~-- 1992 (s) 2005 § 2003 v w

2o15~ 2022) w w 2052 v v w

2105 v v w 1369 + 737 = 2106 (v) or 1557 + 548 = 2105 (v)

2125~ 2130~ w 16 2102 w 1278 + 865 = 2143 (v) 2141] 1255 + 853 ~ 2108 (s)

2122~ 2131J w 1280 + 853 ~ 2133 (s)

2154 v v w , b 2357 v v w 2411 w 2407 w 1557 + 865 ~ 2422 (v)

1560 + 853 ~ 2413 (s) 2525]~ w 1278 Jr- 1280 ----- 2558 (v)

2534 m 2529) w, sh 1255 + 1280 ~ 2535 (s) 2770 v v w

2804~ w 1557 + 1278 ~--- 2835 (v) 2827 w 2824J v w 1560 + 1255 ~ 2815 (s)

* s ~ s t rong, v s = v e r y s t rong, v v s = v e r y v e r y s t rong, m -~ m e d i u m , w = weak, v w ~ v e r y weak , v v w ---~ v e r y v e r y weak, sp ~ sharp , b = broad, sh ~ shoulder.

t R a m a u intensi t ies are re la t ive peak intensit ies. :~ (v) ~ v a p o r phase , (s) ~ solid phase . § 2005 c m - t is more intense t h a n expected for the P branoh of 2011 om -1.

The infrared and Raman spectra of hexafluoro-, hexachloro- and hexabromocyc~opropane 1613

Table 2. Infrared and Raman spectra of cyc~o-CsBr e, 4000-33 cm -1

I . R . (solid) R a m a u (solid) R a m a n (CS I so lu t ion) R e l a t i v e

e m -z I n t e n s i t y * c m -1 i n t e n s i t y c m -1 P o l a r i z a t i o n A s s i g n m e n t

89 19 ~14 118 v w 119 16 Vll

151 8 va 196 w 199 35 198 dp Vl0

226 2 224 dpT vls 256 100 254 p v 2 302 3 303 p 2 × 1 5 1 ~ 3 0 2

484 w, sh 492 s 494 2 494 d p V 9 520 530 w , b 662 w , b 683 m 719 vs 731 w, sh 758 v w 771 v w , b

854 17 857 864 sh 868 s

897 v w

4 9 2 ~ 1 9 6 ~ 6 8 8

Vs

6 8 1 - } - 9 0 2 7 7 1 d p ~1~

~8

1190 5 1191 p ~1

* See f o o t n o t e , T a b l e I .

Cyclo-CsFe Our infrared spectrum of c-OaF e is very rich in bands, while H~.ZOKL~.~ et a/. [1]

reported only nine frequencies. We did not find their 932 and 1172 cm -1 bands, and believe tha t they are due to impurities. Their frequencies are about 4 cm -1 lower than ours. ITO'S [2] spectrum is much more like ours, bu t again there are occasional large frequency differences. We did not observe his bands at 443 (vw), 573 (m), 960 (s), and 3634 cm -1 (vw), and suspect tha t they are also due to impurities.

In our gas spectrum the sharp spike at 1188 cm -1 is probably the Q branch of the strongest band of C~F 4. We estimate from its intensity tha t it represents roughly 1 ppt of C2F 4. None of the strong bands of other likely fluorocarbon impurities, such as CaFs, CgFe, CFaCF--CF2, or c-C4F s were present. This corroborates the gas chromatographic evidence that the sample is quite pure.

The infrared spectrum of solid c-CaFe is even richer than that of the gas. The

Fig. 1.

c

2BOO 2400 2000 1 8 0 0 1 6 0 0 1 4 0 0 1 2 0 0 I000 Frequency (cm "1)

800 600 400 200

I n f r a r e d s p e c t r u m o f cyclo-CaF s i n a 1 0 c m c e l l a t r o o m t e m p e r a t u r e .

A , 1 4 5 t o r r , B , 1 7 t o r r , C , 0 . 5 t o r r .

1614 F . A . MTr,r,-~R and K. O. HART~A~

Cyc lo -C3F 6

Roman

i 7

[

r

I I__l I ; _I I ~__ ~ I r I__L~ ~600 ~400 ~200 ~000 80O 600 400 200

Frequency (cm -1)

Fig. 2. R a m a n spectrum of l iquid cyclo-OsF e in a 4 mm tube at - -50°0. Slit w id th 10 ram. Period 20. Speed 0.5 cm-1/see.

1177.5 and 1337 cm -1 bands in the solid may be due to CsF 4. There is a question as to whether the complex at 995 cm -1 in the gas is due to one or two bands, and where the centers are. The solid does not really help. Although it shows a doublet, similar doublets are found in many other cases and probably are due to solid state effects.

In the Raman spectrum there is a strange feature, an extremely broad band extending from 350 to 560 cm -1. The strong 364 and 505 cm -1 bands are super- imposed on it, as shown in Fig. 2. Our tentative explanation is that this is a fluorescence band due to a trace of some impurity such as stopcock grease.

1. ~pecies a,'. The 737 and 364 cm -z Raman lines are readily selected for ~2 and ~3 because of their polarizations. There is no clear evidence for ~1. The only

C y c I ° - C 3 B r 6

800 1000

C y c l o - C3BrB

J _ L r f_ F 600 400 200

Frequency (cm -1)

Fig. 3. Infrared spec trum of solid cgc/o- CaBr 6 at room temperature . A, ~u jo l

mull; B, K B r disk.

1200 1000 800 600 400 20,D Frequency {crn -~)

Fig. 4. R a m a n spec trum of solid cyclooC3Br ~ at room temperature . Slit w id th 10 ram. Period 2.

Speed 0.5 em-*/sec. A, double slit; B, single slit.

The infrared and Raman spectra of hexafluoro-, hexachloro- and hexabromocydopropane 1615

Table 3. Fundamental vibrations and assignments for three hexahalocyc/opropanes

D ~ Schematic Spec ies A c t i v i t y N o . d e s c r i p t i o n CsF . CaC10 CsBr ,

a 1' R (p), ~ 1 C---C s t r . ~1557 1226 1190 2 C - - X s t r . L 737 420 256 3 C X z sc issors 364 235 151!

a l ~ - - , ~ 4 C X s t w i s t - - ~ a I ' - - , ~ 5 C X s w a g - - - - - - a2 ~ - - , I . R . (1[) 6 C - - X s t r . 1369 850 719

7 C X I r o c k 202 195 e ' R (dp) , I . R . ( .L) 8 C---C s t r . .~ 865 ~906 868

9 C- - -X s t r . t 1 2 7 7 . 5 t 6 0 7 494 10 C X I sc issors 547.5 315 199 11 CX~ w a g 251 182 119

e" R (dp) , - - 12 C - - X s t r . 1280 964 854 13 C X z t w i s t 505 ? 228 14 C X z r o c k 367? 130 89?

{ T h e m o d e s a r e p r o b a b l y b a d l y m i x e d .

observed possibilities for it are 1280 and 1557 cm -1. Unfortunately both were too weak for polarization measurements. We believe tha t one of these is ~1 (al) and the other ~1~ (e~) • We expect tha t ~1 and ~ may interact rather strongly, and indeed the 737 cm -1 value for ~2 is unusually low for either the C--C stretch or the C---F stretch. I t is therefore probable tha t ~1 is unusually high, and this suggests 1557 for it. The r.m.s, of 737 and 1557 cm -1 is 1215 cm -1, which is reasonable for either the C--C stretch or a C- -F stretch. Consequently we adopt 1557 cm -1 for ~1 in spite of its amazingly high value. I t should be emphasized tha t 1557 cm -1 cannot be explained as a binary combination of the observed bands and tha t it must be used to account for several sum tones.

2. Species a~ ~. These two fundamentals are infrared active only, and give parallel bands. We have calculated the moments of inertia assuming C--C -- 1.60 A, C- -F ~ 1.33 A (both as in c-C4Fs), and ~ F ~ C - - F ~ 109.5 °. This gives Z s ~ 356.9 and I® ~ I~--~ 312.9 amu-A 2. The parallel bands will then have P, Q, and /~ branches of about the same intensity, and a P-to-R separation of approximately 13.5 cm-L

The C---F stretch is ~6, and is expected to be between 1000 and 1400 cm -1 and to be intense. The two possibilities are 1277 and 1369 cm-L We prefer the latter because it matches the expected contour better. ~7 is a CFs rock, and the most reasonable choice is 202 cm -1.

3. Sloecies e'. This is the only species which is both Raman- and infrared-active. Again, as in al', it contains a C--C and a C- -F stretch which may interact. One of these, probably the former, is quite definitely 865 cm -1. I t is certainly an e' funda- mental because it appears in both spectra, is depolarized, and has a P - - R separation of only 7 cm-L The 1277.5 cm -~ band, which is the strongest one in the infrared spectrum, is a good possibility for the C F stretch.

We believe tha t the apparent coincidence of this band with the Raman line at 1280 cm -1 is accidental for the following reasons. First, the 1277.5 cm -1 vapor phase band shifts to 1255 cm -1 in the solid and one would expect the liquid frequency to lie between these values while, in fact, the liquid phase Raman frequency is higher than the gas phase frequency. Second, the only binary combination (of observed bands) which yields a band at 1480 is 202 ~- 1280 cm-L This combination

1616 F . A . MILLER and K. O. H A ~ T ~

is allowed if the 1280 cm -1 line belongs to the e" or a I' species, b u t no t i f i t is e'. Final ly the shift of the 2534 cm -1 sum tone to 2525 cm -1 upon freezing is reasonably expla ined as 1280 -~ 1277.5 cm -1 (vapor) and 1280 ~- 1255 cm -1 (solid). On the o the r ha nd i f i t is assumed t h a t 1280 and 1277.5 cm -1 represent the same v ibra t ion this over tone cannot be sat isfactori ly expla ined employing the observed frequencies; 2 × 1255 cm -1 predicts a solid phase f requency which is 15 cm -1 below the observed o n e .

There is only one o ther infrared gas band t h a t coincides wi th a R a m a n l i n e - 251 cm -1. Since i t is the lowest unassigned infrared f requency, we take i t for the CF s wag (~n) and look higher for the CF s scissoring mode (~1o). A good possibil i ty is the s trong band a t 547 cm -1. I t is not observed in the R a m a n spect rum, which suggests as" r a t he r t h a n e', bu t its P - - R spacing is a bi t too small for as". We therefore adop t i t for r~0.

4. Species e". These bands are Raman-ac t ive only and depolarized. F o r the C - - F s t re tch (~1~) the only observed choices are 1280 and 1557 em -1. Since the l a t t e r has a l ready been t aken for ~1, we assign 1280 cm -1 to ~ls. The band could also be expla ined as 737 W 545 = 1282 em -1, so the assignment is questionable.

Fo r the CF~ twist (~13) 505 cm -1 is a good choice. (We feel t h a t the weak infrared gas band a t 500 cm -1 is no t due to the same v ibra t ion because i t increases to 518 cm -1 in the solid. See the nex t section.) There is no good candidate for the remaining fundamenta l , ~14. The weak shoulder a t 367 em -1 is one possibil i ty; ano the r is the v e r y broad band ex tending f rom 350 to 560 cm -~. We t en t a t ive ly take 367 cm -~ for v14, bu t wi th no confidence.

5. l~emaining bands. There are numerous remaining infrared bands. Exp lana- t ions for m a n y of t hem as b inary combinat ions are given in Table 1; no a t t e m p t was made wi th t e r n a r y combinat ions. All sat isfy D3h selection rules. These selection rules allow 64 b inary sum tones in the infrared for c-CaXe, and abou t ha l f of t h em are observed in the gas phase for c-CsF6. Combinat ions involving cer ta in s y m m e t r y species are more preva len t t h a n others. Thus pract ica l ly all the a l ' ~ e' and e' ~- e' sums were observed, bu t ve ry few as" ~- e". The general ly good success in account ing for combinat ion bands increases our confidence in the assignments, especially in view of the numerous s y m m e t r y restr ict ions.

The spec t rum of the solid was as rich in bands as t h a t of the vapor . H o w ev e r some of the frequencies are considerably shif ted f rom the vapor values, and a t t imes this leads to some surprising correlations. Fo r example the gas band a t 497-503 cm -1 should be paired wi th 518 cm -1 and no t 503 em -1 in the solid. This is because the fundamenta l a t 251 cm -~ in the gas becomes 261 cm -1 in the solid. The i r over tones are t he n observed a t 500 and 518 cm -~ respect ively. Thus the 503 em -~ band in the solid is no t the over tone ; i t is p robab ly the Raman-ac t ive 505 cm -~ band appear ing weakly b y re laxat ion of the selection rules. Fundamen ta l s like 251,865, 1278, and 1369 cm -~ which show large shifts of thei r frequencies wi th change of s ta te usual ly have these shifts reflected in the sum tones, and this increases confidence in the assignments of the combinat ion bands. This is par t icu lar ly t rue for 251 cm -1, where the shift is 10 cm -1 upward on condensat ion.

I t was no t possible to eva lua te the two to ta l ly inact ive fundamenta l s f rom combina t ion tones.

The infrared and Raman spectra of hexafluoro-, hexachloro- and hexabromocyclopropane 1617

6. Summary. Of the fourteen fundamentals, we are missing frequencies for the two totally-forbidden ones, and values for three others are uncertain (~1, ~12, and ~,4). All the rest are believed to be certain. We have had to alter the earlier assignments because of our improved data ; we agree with H~.ICKLE~ et al. [1] on only two fundamentals, and with ITO [2] on five.

Gyclo-CaC1 e ITO [2] did a very good job on this compound, and we have only a little to add.

The frequencies which we observed are in substantial agreement with his, so we do not reproduce the complete list here. Our contribution is to add two new low infrared frequencies, a t 185 (vw) and 195 cm -1 (w) in the solid. In CS~ solution the former shifted to 182 cm -1 and is coincident with a Raman band at 181 cm -1 in CS~ solution. (Ito's Raman value of 177 cm -1 was for a different solvent.) This leads us to assign 182 cm -1 to the e' mode ~n, which is a CCl~ wag, and since 195 cm -1 has no Raman counterpart we assign it to ~(a2 ") which is a CC12 rock. For the other fundamentals we agree with Ito. We would change a few of his explanations for combination tones, but this scarcely warrants discussion or a table.

In this compound, as in c-CaFe, ~1 is weak and not measurably polarized. We have no value to suggest for ~13.

Cyclo-CaBr ~ Our results are given in Table 2. The infrared spectrum above 400 cm -1 was

obtained from both Nujol mulls and K B r disks. The frequencies measured by the two methods agreed within 1 cm -1. In order to determine whether the doublet at 864 and 868 cm -~ is due to a solid state splitting, this region was scanned using a saturated CS2 solution. The band maximum was clearly at 868 cm -1 with a weak shoulder about 856 cm-L

In our Raman spectrum the three lowest frequency lines were observed only for the single crystal, and therefore their polarizations could not be determined. The bands reported by I to at 395, 420, and 593 cm -1 are, we believe, due to CS~. Pure CS 2 and solutions of c-C3Br s in CS~ were scanned under identical, high gain conditions, and no differences were detected. We add the new Raman frequencies 89 and 302 cm -1 and polarization measurements.

In other respects the agreement of our data with I to 's is satisfactory. Nonetheless the new low-frequency infrared bands and the Raman polarizations lead to a number of changes in the assignments.

1. Species al'. The only polarized Raman lines are 1191, 303, and 254 cm -1. Since 303 cm -1 is weak and can be explained as 2 × 151 ---- 302 cm -~, we hesitate to take it as a fundamental. We assign 1191 and 254 cm -1 to the C---C stretch (vl) and the C- -B r stretch (~) respectively. Although 254 cm -1 seems very low for a C- -Br stretch, it can scarcely be assigned to ~3 because tha t would make ~3 in c-C3Br~ higher than in c-C3C1 ~ (235 cm-1), which is most unlikely. The selection of ~3 is deferred for a bit.

2. Species a~ ~. We take 719 cm -1 for the C- -Br stretch ~s. Another possibility is 866 cm -1, but this is higher than ~e in e-CaC16 (850), so again it is unlikely. For ~ we must look below the value for c-CaC1 e (195), and since there is no good candidate it is not assigned.

2

1618 F . A . MII~.R and K. O. HA~T~¢~

3. Species e'. The R a m a n - i n f r a r e d coincidences are 494, 199, a n d 119 cm -1, which we assign to the C - B r s t re tch (~) , the CBr 2 scissors (u10), and the CBr 2 wag (~u) respect ively . This leaves ~s, the C - - C stretch. There is a s t rong inf ra red b a n d a t 868 cm -1 which is still unass igned, and we a d o p t i t for ~s in spi te of the fac t t h a t i t is no t observed in the R a m a n spec t rum.

4. Species e". The 854 cm -1 b a n d is c lear ly ~12. Al though 854 cm -1 seems high for a C - - B r s t re tch there is p receden t for it, since an a n t i s y m m e t r i c s t re tch ing v ib ra t i on in pe rb romoe thy l ene is a t 880 cm -1. The shoulder a t ~ 8 5 7 cm -1 in the inf rared spec t rum of c-CzBr e in CS 2 solut ion m a y be this same v ib ra t ion appear ing b y b r e a k d o w n of selection rules. We t e n t a t i v e l y suggest 226 cm -1 for v13, 89 cm -1 for ~14, a n d 151 cm -1 for the still unass igned ~3. The 89 cm -1 b a n d could be due to a la t t ice mode, b u t no t 151 or 119 cm -1 because I t o observed these bands in solution.

5. Other comments. A few combina t ions are suggested in Tab le 2, all o f which sa t i s fy the selection rules. There are five bands , all w or vw, for which we could no t find b i n a r y combinat ions .

Comparison with perhalogenated ethylenes

I n the pe rha logena ted cyclopropanes a g rea t va r i a t ion was no t ed in the C - - X s t re tch ing frequencies and to a lesser ex t en t in the CX 2 scissoring. F o r ins tance the CBr s y m m e t r i c a 1' s t r e tch occurred a t 256 cm -1 while the a n t i s y m m e t r i c e u s t re tch

Table 4. Comparison of the frequencies of perhalogenated ethylenes and cyvlopropanes (cm -1)

C2F 4 c-CaF e C2C1 a c-CaC1 e C2Br 4 c-C3Br 6

C- -X sym. stretch 778 737 447 420 265 256 CX 2 scissors 394 364 235 235 144 151 C--X antisym, stretch 1340 1280 1000 964 880 854 C---X antisym, stretch 1337 1369 908 850 766 719 C---X sym. stretch 1186 1278 777 607 635 494 C---X~ scissors 558 548 310 315 188 199

was loca ted a t 854 cm -1. The change in the case of the C - - F s t re tch ing f r equency is p r o b a b l y due to in te rac t ion wi th the C - - C s t re tch, b u t this exp lana t ion is no t reasonable for the chloro a n d b romo compounds . While these v e r y large var ia t ions are unexpec ted , the p a t t e r n is no t unique and, in fact , i t closely paral lels the one obse rved in the pe rha logena ted e thylenes [6]. A compar i son of the a p p r o p r i a t e C - - X s t re tches and CX 2 scissoring of the two sets o f molecules in Tab le 4 shows surpr is ingly good correlat ion.

AcIcnowledgements---~e are greatly indebted to Dr. D. C. E~GLA~D for the generous gift of c-CsF 6, and to Dr. M. Iwo for the samples of c-CaCI a and c-CaBr e and for very kindly sending us the manuscript of his paper prior to publication.

[6] D. E. M.~r~, L. F~¢o, J. H. ~ and T. SHr~_a~OUCHI, J . Chem. Phys. 27, 51 (1957).