Spectrochimica Acta, Vol. !ZEA, pp. 1467 to 1478. Pergamon Press 1972. PrInted in Northern Ireknd

Raman and infrared spectra of tetramethy1enecyc2obutane4 and tetramethylenecycZobutane=ds*

FOIL A. MILLER, FRED R. BROWN and KEE H. RHEE~

Department of Chemistry, University of Pittsburgh Pittsburgh, Peuueylvsnia 16213

(Receiuerl 28 October 1971)

Ab~~TetramethylenecycEobu~e-d, esd tetmmethylenecyclobutaue-de have been ieoleted for the first time. They m white crystrtlline solids clt -22’C!, end they mpidly polymerize upon werming to mom temperature. Infrared datta m reported for the vapors & room temper&we md for the solida at -190%, and Raman measurements for several cooled solutions and for the solids below -160°C. The d&e m completely compatible with a plauar riug &nd Ddn symmetry but with no other possible symmetry. Assignments are made for 23 of the 24 allowed fumkmeutals .

INTRODUCTION

THE STRUCTURE of tetramethylenecyclobutane, or [4]-radislene, is shown in Fig. 1. This compound is the second member of a series called the radialenes. [n]-Radialenes are carbocyclic systems cont&ing 71 cross-conjugated exocyclic double bonds. [4]-Radialene was first reported by GRIFFIN and PETERSON in 1962 [la, lb], but it has not heretofore been isolated.

Recently the vibrational spectrum of [3]-radialene or trimethylenecyclopropane was studied in our laboratory [2]. The results showed that the molecule is unques- tionably planar. We have now succeeded in synthesizing pure tetramethylene- cycZobutane_d,, and -ds (henceforth abbreviated TMCB-d,, and TMCB-&). A vi- brational analysis shows that these molecules are also planar and have symmetry

D4w

1. Preparation EXPERIMENTAL

TMCB-d, w&s prepared by a modification of the method of GRIFFIN and PETER- SON [la]. The reaction sequence is shown in Fig. 2. For their final reaction step, GRIFFIN snd PETERSON dehydrobrominated IV with sodium ethoxide in ethanol. The resulting TMCB-d,, was then codistilled with the ethsnol. The product was never isolated; it was always in a solvent. Our dehydrobrominetion was accom- plished by adding IV slowly to molten KOH-water eutectic mixture at 16OY!, sweeping the system with helium, and collecting the products thus formed in a

* From & theeis submitted by Fred R. Brown iu partial fulfllhnent of the requirements for the Ph.D. degree at the University of Pitteburgh, 1971.

t Permanent eddress: Community College of Allegheny County, Boyce Campus, Monroeville, Peuusylv~L.

[la] C. W. GSIFFIN end L. I. PETERSON, J. Am. Chem. Sot. 84,3399 (1962). [lb] G. W. GRIFFIN and L. I. PETIERSON, J. Am. Chem. Sot. S&2268 (1963). [2] K. H. RHEE asd F. A. MILLER, Spectrochim. AC&Z %‘A, 1 (1971).

1467

1468 FOIL A. WE, FRED R. BROW and ti H. Rmm

H H

‘C/

II H C H

kc’ ‘c c/ / \J =\ H H

/\ H H

TETRAMETHYLENE CYCLOBUTANE

Fig. 1. Structure of tetramethyleneoyclobutane. In claeeifying the vibrations, the 2 and y axes were taken through the C=C bonds and the z axis perpendiculm

to the ring.

I-I

M COOMe

h* Iig SBSOA

MeOOC I-I I

COOMe COOMe

H

Q

H H H

dOOMe bOOMe II

CH,OR CH,OH

C&OH CH,OH III

CH,Br CH,Br

Q

H,C CI-&

H H KOIi

H H - 160%

Ix

J32C CH2

CH,Br CH,Br V

IV

Fig. 2. Reaction sequence for preparing tetramethylenec~cZobutane.

series of cold traps. The ilrst trap was at -22’C (Ccl, slush), the second at -81°C (dry ice in 60% CCl, and 60% CHCI,), and the third at -190°C (liquid nitrogen). This procedure is analogous to DORKO’S method for the preparation of (3)-radialene [3]. By adjusting the helium flow rate properly, TMCB-d,, was isolated in the first of the cold traps. Approximately 100 mg was obtained from each run.

Completely deuterated TMCB was synthesized by exactly the same route, exoept that LiAlD, was substituted for LiAlH, in the reduction of II.

TMCB-d, and --da are white crystalline solids at -22°C. The solids are unstable at room temperature so samples were stored in evacuated ampoules in liquid nitrogen.

[3] E. A. Domxo, J. Am. Chn. Sot. 87, 6518 (1966).

R- and infrared spectra of tetramethylenecycZobutaue.ds 1469

2. Infrared 8pectra

The i.r. spectra were measured over the 33-4000 cm-l range with Beckman IR-11 and IR-12 spectrophotometers. The spectral slit width was less than 2 cm-l everywhere.

Vapor phase spectra for both compounds were obtained in a conventional 10 cm gas cell equipped with KBr windows. TMCB was sublimed into the gas cell and condensed against the wall of the cell with liquid nitrogen. The sample was allowed to warm up to room temperature and thereby fill the cell with vapor. No attempt was made to measure the pressure but it was no more than a few torr. After approxi- mately 30 min decomposition was found to occur ; a whitish deposit was formed on the walls of the cell. Also new i.r. bands began to appear, especially a very strong one at 866 cm-l in TMCB-d, and at 686 cm-l in TMCB-ds. A simultaneous decrease in intensity of the original i.r. bands occurred. The identity of the decomposition product(s) was not determined. Vapor phase spectra were recorded only above 400 cm-1 because there was not enough sample for low-frequency measurements.

The i.r. spectra were also recorded for the polycrystalline solids at -190°C in order to avoid this decomposition. The low temperature cell was of conventional design [4]. Thick samples were deposited on a transparent cold plate held at about -190°C. The entire range of 33-4000 cm-l was examined by using either a poly-

ethylene or a KBr support plate and windows. The i.r. frequencies are believed to be accurate to rt 1 cm-l for the solid. Because

of the instability of the vapor, vapor phase frequencies were not measured on an expanded scale for all bands. Those vapor phase frequencies believed accurate only to f3 cm-l are marked by an asterisk in Tables 1 and 2 ; the others are thought to be accurate to f 1 cm-l.

3. Raman spectra

Raman spectra were obtained with a Spex Ramalog system described elsewhere [2]. In brief it consists of a Spex model 1401 double monochromator and an ITT FW-130-S20 detector cooled to -20%. The source was a Carson Laboratories Model 300 argon ion laser. Approximately 760 mW of 4SSOd radiation (measured at the laser output) was employed. Only d.c. amplification was used in this work. Raman frequencies are believed to be accurate to f2 cm-l for bands not marked “broad” or “shoulder”.

The samples were sealed in thin-walled 1 mm i.d. melting point capillaries. Raman spectra were obtained for the solids below - 150°C. Depolarization measure- ments were made on Ccl, and C!,H,OH solutions held just above their freezing points. Depolarisation measurements on the melt were attempted, but the samples immediately began to decompose. The samples were held at the desired temperatures by supporting them in an evacuated jacket and blowing cold nitrogen gas over them. This device is described elsewhere [6].

When measurements were made on Ccl, solutions, a 30% transmission filter was placed in the exciting beam to avoid vaporizing the solution. No decomposition was noted in the solid or in any of the solutions.

[4] E. L. WAUNEB tmd D. F. HORNIU, J. Chma. Phy8.18,296 (1960). [6] F. A. MIUER and B. M. -Y, Appl. Spectry $34,291 (1970).

1470 FOIL A. l&mm, FRED R. BROW and b H. RHICE:

Table 1. Tetramethylenecyclobutane-do. Raman and ix. spectra (om-I)

Reman Infralwd AE&pment

Solid (-19OW) 1nten.t ccl, p Vapor (RT) Solid (- 190%) Inten.

73 90

106.6

9 Lattice mode 10 Lattioe mode

100 Lattice mode

230 3s 244 1 26

360 366 1

2 3

600 18 620 <l 683 28 732 1 761 3

867 891

1120

11 20

<1

1167 119s

-1301 N1316

1390

1397

1424 1434 1

1681 1604

1662 1681 1723

2064 2179 2329

Cl 66

<1 <I

16

sh

6 14

1 3

14 97 10

1 2 2

N. E.

N. E.

232$ t

N. E.

697$ ?

681: 0.08

119q 0.76

*13s1 VW

VI7 R&T Reel?

1367 sh, vw 1371 w

1396 dp Vl8

1399P 1404Q m 1408R 1

1409.6 s

1421 r % 1463 w

*IS36 w, b 1195 + 266 = 1460 Impurity

*161Sw,b 1609 w 1639 dp 1683 0.08 1726 P

2 x 806 = 1610 Impurity

V11 VI

*794 m

a74 Pe 889 884 Q vs 894 893 R B 903

‘1162

208 213 1 222 227 1

266

779 “VW

803 m 806 m 808 1 m

1132 1167.6

w 1162 I

S

w

m

“S

“S

m

w w

w

VlO

%1

VI0 %I

v*

891 + 237 = 1128

1783 1789 > :

1800 1807

891 + 892 = 1783

892 + 1160 = 2062 1434+ 761= 2186

Raman and infrared epectra of tetrs4lethylenecycZobutrure-de 1471

Table 1. (ConGwd)

RW IIlfr8lWd Assignment

&lid (-19O’C) Intat Ccl, P Vapor (RT) Solid (-190°C) 1&3ll.

2390 <1 2x 1195=2390 2407 <l 1602 + 751 = 2413 2972 2, eh 2988 7 2987 0.27 Vl 3020 2 Vll 3053 <1 1602 + 1390 = 3052

*3031 w 111 l 3117 m 3082 m %a

3084 14 3087 0.75 VI6

8, m, w = strong, medium. weak, v = very, b = broad, sh = shoulder. p, dp = polarized, depolarized. RT = Rooti te~pereture, N. E. = not exnmizwd.

+ Aaoumy only f3 cm-‘. t Relative peak intensitiee. unoorrected for imtrument response. p = depola+ation ratio. For depolarized lines. p = 0.75 f 0.03. $ C,H,OH solution

Table 2. TetramethylenecycZobutane-d,. Raman and ix. spectra (cm-l)

Raman

8olid (- 15O’C) Intcm.t Ccl, p

Infrared A@mmnt

Vapor (RT) 8olid (-190°C) Inten.

58 2 98 64

IQ6 209 >

313

16 11

581 609 646

1

8 559 3

13

703 2

724 19 715 dp

1030 1050 1077

191 198 >

m

Lattioe mode Lattice mode

VlO

1 3 1044 1

WV

dp

211 229

500

W

m

VW

673.5 685.5

691 1

714

801

807 I 844.

847 I 894

\

899 I -930

975 -1025

W

W

““B

VW 609 + 195 = 804

W 646 + 195 = 841

-

VVW

m

-

do hp.

703+ 229 = 932

724 + 313 = 1037

V11 %

1472 FOIL A. MILLER, FRED R. BROWN asd KEE H. REEE

Table 2. (Continued)

RfUIl8ll Infrared

Mid (-160°C) Intemt CCI, p Vapor (RT) Bolid (-190%) lien. Assignment

1103 eh. vvw *1091 w 1110 > 8

1126 1

1143 30

1623 11 1668 100 1728 Cl 1786 <1 1613 <1 1849 <l

2183 <1

2216 6

2248 <1 2282 <1 2324 8

1144 dp

1618 dp 1667 P

2216 P

2326 dP

1127

1133P 114OQ m

I 1146R 1142

1287 1318 1406 1410 1 1442 1447 1 1638 1680

2174

2210

l 2216 l 2234 Iw

2234

2323 w 2340 1 w

VW

vs

VW

VW

w

w

VW

w

VVW

VVW

-

V17 688 + 609 = 1297

714 + 609 = 1413

714+ 724= 1438

Impurity Impurity %l VI

1143 +- 646 = 1791 lllO+ 714= 1824 1142 + 714 = 1666 1110 f 1077 = 2187 1632 + 661 = 2184 1142 + 1077 = 2219

Vl

VI7

Vll 2 x 1142 = 2264

V18

%I

Footnotee: 8ee Table 1

RESULTS

The spectra are shown in Pigs. 3 and 4. Data for TMCB-d,, are given in Table 1

and for TMCB-d8 in Table 2. Table 3 lists the fundamental vibrations and our assignments. HERZBERG’S conventions have been used throughout [6]. In classifying the vibrations, the z and y axes were taken through the C==C bonds and the z axis perpendicular to the ring. Teller-Redlioh product rule results are given in Table 4.

ASSIGWMENTS

The initial assumption was that TMCB-dO and TMCB-d8 are planar with symmetry D oh’ This was found to be valid. Point group Da contains a center of symmetry and therefore the rule of mutual exclusion holds. A few i.r.-Raman coincidences are observed, particularly for the deuterated molecule, but these are reasonably ex- plained as accidental coincidences. The classification of the fundamentals is 4tz,, + lal,, + 3aB, + 2a,, + 4b, + lb,,, + 4b,, + 3b,, + 3e, + 7e,. The selection rules, which are given in Table 3, are such that 15 modes are Raman active, four polarized

[f3] a. HEWBEM3, I@mred and Raman Spectra, Vm Nostmnd, New York (1946).

1473

200 2400 ,600 ,400 IO00 600 200 , I! I.1 II IIll 1

A I

! I

Roman solid, - JSO’C

I I t I I

y--+-t+ I f !

1 I I , , , , , , ( , , , 200 2400 ,800 1400 ,000 600 eon

cd’

l?ig. 3. Raman spectra of ~t~ethyl~~oZobu~e -&o and -&. Solids at low temperature. 4 cm-* slits. Note 2 x scale change at 2000 om-I.

200 2400 ,600 ,400 ,000 600 200 i I I

.

1 B

. w, I

I I &ypd w=,

!

CL96 Infrored gas, 25%

11 I i t I I 11 11 ) 1

,200 2400 ,600 t 400 f 000 600 200 lxn-’

Lj

Fig. 4. Infrared spectra of tetmmethylene&obutane -d, and -de. Seotiona A and C; Gas, 10 cm cell. Pressure unknown (vapor pressure at 26%). Sections B and

D: Solids at -190% Note 2x soale change at 2000 cm-l.

and I1 depolarized. Nine modes are ix. active, two parallel and seven perpendicufar, Thus 24 of the 32 modes are spectroscopically allowed.

Frequencies for the solid rather than the vapor are used in making the a&gnments so that the i.r. and Ii&man vdaes will be for the same physical state. The vibrational assignments for TMCB-d, will be discussed first, followed by the results for TMCB-da.

1474 FOIL A. MILLER, FRED R. BROW and KEE H. REEE

Table 3. Fundamental vibrations of tetrsmethyleneoyolobutane-do and -d,

(% symmetry)

Assignmente (IXII-~) (solid at-ate values)

Speaitw A&i&y No. Sohematio description d,, 4

a,, -. i.r.(ll)

b 1# R(dp), -

b 1” -,- b II Wdp). -

b I”

%

-,-

R(dpL -

-, i.r.( 1)

R(P), -

-1-

-.-

1 CH, sym. str. 2 c=cstr. 3 CR, scissoring 4 Rii str. 6 CH, twist 6 CH, antiaym. str. 7 CH, rook 8 C=CH, rock 9 CH,wag

10 C=CH, wag 11 CH, sym. str. 12 c=c str. 13 CH, scissoring 14 Ring deform&ion 16 CH, twist 16 CR, antisym. str. 17 Ring str. 18 CH,rook 19 C=CH, rock 20 CH, wag 21 C=CH,weg 22 Ring puokering 23 CH, wag 24 CH, twist 26 C=CH, wag 26 CH, antisym. atr. 27 CH, sym. str. 28 C=C str. 29 CH, scissoring 30 Ring 31 CH,rook 32 C=CH, rook

2988 2216 1681 1668 1434 1077 683 646 - - - - - - -

894 714 210 196

3020 2248 1662 1623 1390 1060

600 661 - -

3084 2324 1196 1143 867 703 367 313 - - - - - -

891 724 761 609 237 203

3082 2332 3031* 2234 - -

1409 1142 1160 1110

806 688 266 229

-

l Vapor value.

Table 4. Product rule ratios for tetramethylenecyclobutane-d9 and -d,

Species Theoretiaal Observed a/o Differonce

al, 0.600 0.620 +a.0 4u 0.707 - - ato 0.646 - -

%I 0.734 0.742 1.1 bl, 0.600 0.613 2.6 b I” 0.707 - -

b I# 0.600 0.618 3.6 b 8” 0.707 - -

% 0.646 0.664 3.6 % 0.269 - -

Assumed dimensions

Cd 1.34 A C-C 1.486 A =C-H 1.07 A <Ha 120° <C-G-c 900 Planar

Calculated momenta of inertia: emu A*

I# = I” 201 239

1, 402 478

Raman and infrared spectra of tetramethylenecyclobutene-ds 1475

1. Rawuzn-active fundamentala of TMCB-d,

(a.) Species a,, (pohixed). Three of the four a,, fundamentals are readily identified by their polarizations. The CH, symmetric stretch v1 is 2988 cm-l, and the C--r! stretch Y% is 1681 cm-l. Although vs is not polarized, it can be assigned to 1434 cm-i because this value is so characteristic of CH, scissoring. The intensity and polarization of 683 cm-l indicate that it is vat the ring breathing mode.

(b.) Species br., bzs, and e, (depolarized). There are eleven depolarized Raman- active fundamental vibrations. Two of these are CH, stretches, vu and vie. Two candidates are found at 3084 and 3020 cm- l. The 3084 cm-l band is assigned to vi6 and 3020 cm-l to vi1 because antisymmetric CH, stretches are usually higher than symmetric ones. The C=C stretch via is certainly the depolarized band at 1662 cm-i. The CH, scissoring via is assigned to 1390 cm-i on the basis of intensity, depolarization, and frequency. Two other assignments can be made with little di%culty vi7, a ring stretching frequency, to 1195 cm-1 and vss, the CH, degenerate wag to 891 cm-l. The latter is based on proximity to the i.r. band at 894, which is certainly a CH, wag.

Still unassigned are a ring mode vi*, CH, modes via and va4, and C=CH, modes vlo and vza. Candidates with reasonable intensity are found at 857, 751, 600, 357 and 237. The ring deformation via is assigned to 600 cm-l because of its small shift upon deuteration to 561 cm-l. (See next section on TMCB-ds.) The two C==CH, modes, a rock vlo and a wag vaS, are expected to be low in frequency. In Iximethylenec~cZopropane, the Raman-active C=CH, rock was assigned to a very weak band at 340 cm-l, while the Raman-active C=CH, wag was found at 236 cm-l. Therefore vro is assigned at 357 and vzl at 237. The 237 band appears as a doublet at 230 and 244 cm-l in the solid but is a single band in solution.

This leaves two vibrations unassigned : v18 (a CH, rock) and vz4 (a CH, twist). vls is assigned to 857 and vz4 to 751. If the converse were used, vss and v,,, both in species e,, would be 891 and 857 and would thus be separated by only 34 cm-i. We believe this is too close for two CH, fundamentals in the same species and prefer the alternative assignment.

2. Infiared-aetive fundamentals of T_MCB-d,,

(a.) Species aBn (P arallel bands). There are two vibrations in species a,,: vo, a CH, wag, and vlO, a C=CH, wag. The CH, wag is certainly the very strong band at 894 cm-l; it has the appropriate P-Q-R structure in the vapor phase. This is only 3 cm-l higher than the Raman-active CH, wag va3. We consider this to be a case of accidental near-degeneracy. The C==CH, wag v10 is assigned at 210 cm-l, which is near the Raman-active one, Q, at 237. Unfortunately this region was not examined in the vapor so no contour was obtained.

(b.) Species e, (perpendicular bands). There are seven fundamental vibrations in this species. Both vz8 and va, are CH, stretches. Two bands appear in the vapor at 3117 and 3031, while only one band appears in the solid at 3082. We assume that this is the counterpart of 3117, and that for some reason the lower one has been greatly weakened. The band at 3082 (3117) is assigned to yea, while the second band in the vapor at 3031 is assigned to v2,. The degenerate C=C stretch poses a difficult problem, and discussion will be deferred until a later section. The CH,

e

1476 FOIL A. MILLER, FRED R. BFLOWN and KEE H. REEE

scissoring is assigned to 1409, the ring mode to 1160, the CH, rock to 806, and the C=CH, rock to 265 cm-r.

3. Raman-active fundamentals of T&CB-$

(a.) &e&es ars (polarized). The CD, stretch Ye, and the C=C stretch yz, are as- signed to the strongly polarized bands at 2216 and 1668 cm-l respectively. These were the only bands found to be polarized in the entire spectrum. The CD, scissors V, is assigned to the weak band at 1077 cm-l with reservations. The ring breathing mode is attributed to 646 cm-l because that is the strongest band in this region of the spectrum. The shift of 37 cm-l from TMCB-d, is reasonable.

(b.) &peciea b,, b,, ati es (deplardzed). The medium intensity band at 2324 cm-r is assigned to or,, and the weak band at 2248 to err. The C--C stretch ylz is con& dently assigned to the depolarized band at 1623 cm-l, while the CD, scissors z19 is attributed to the depolarized band at 1050 cm-l. As noted previously, the ring deformation y14 is taken as 661 cm-l. Reasonable candidates for the two lowest depolarized fundamentals, the C=CH, rock yle and C=CH, wag yag, are found at 313 and 203 cm-l respectively. These are 43 and 34 cm-l lower than the corre- sponding bands of the light compound. The ring stretch Q, is assigned to a strong band at 1143 cm-l.

The three remaining fundamental vibrations are difficult to assign. Three reason- able candidates are at 724, 703 and 609 cm- 1. These are assigned respectively to Q, vls and v2Q These choices are supported by the product rule calculations to be mentioned later.

4. Infrared-active fwndamentab of TMCB-4

(a.) Species az,, (parallel bad). These two vibrations are unequivocally assigned to 714 cm-l (vs) for the methylene wag and to 196 cm-l (m) for the C=CD, wag.

(b.) Sjpecies e, (pef+pendicular bad). The two C-D stretches are easily identified as 2332 and 2234 cm-l. The CDs scissoring and C-C stretch are very close to one another and may be badly mixed. At the present time the scissoring is assigned to 1142 and the C-C stretch to 1110 cm-l. A reasonable candidate for Q, the CD, rock, is the doublet centered at 688 cm-l. The C=CD, rock is reasonably attributed to 229 cm-l. No satisfactory candidate for the degenerate C=C stretch vas could be found, as was the case for d, also.

6. Other bands

Solid TMCB-d, and -de exhibit intense Reman bands at 106 and 98 cm-l respec- tively. These are too low to be fundamentals, so they are assigned as lattice modes. Most of the remaining bands for both compounds can be satisfactorily explained as binary combination tones as shown in Tables 1 and 2. A bothersome exception is the doublet of medium intensity at 1800-1807 for d,.

OTHER COMMENTS 1. h0amt rule calculations

Theoretical Teller-Redlich Product Rule ratios have been calculated by the method outlined in HERZBERG [6]. The moments of inertia used are given in Table 4. This table also compares the theoretical Product Rule ratios with those

Raman Bnd infrared 8peCtr8 of t&remethylen~&butane-d,, 1477

oalculated from the vibrational assignments. Good agreement is observed. The small discrepancies in Table 4 could be due to differences in the anharmonioities in the two molecules, the use of solid state frequencies, and errors in the dimensions assumed for calculating the moments of inertia.

2. Totally symmetric C=C stretch

The totally symmetric C--r! stretch is found at 1681 cm-i (1668 in the deuterated compound). This frequency is slightly higher than normal for R,C=CH, (1660- 1630 cm-l), but considerably lower than that for trimethylenecycZopropane (1800 cm-l). The high value for the latter compound was rationalised by assuming that the C--r! stretching vibration has to do extra work against the C-C bonds of the ring beoause of the interior angle of 60”. In tetramethylenecycZobutane the interior angle is 90°, and therefore less work must be done which results in a decrease in frequency. The downward trend continues for R,C=CH,, for whioh the corre- sponding angle is near 120’.

3. Degenerate C=C stretch (a~, q,)

Aa mentioned earlier, the assignment of the degenerate C=C stretch poses a difficult problem. TMCB-d,, vapor shows a moderately intense i.r. band at 1767 cm-l, but nothing else between 1450 and 2000 cm-l. In the solid this band appears as a doublet centered at 1786 cm-i. Neither vapor nor solid TMCB-$ shows any trace of i.r. absorption between 1680 and 2000 cm-l.

Two possibilities exist for the 1786 (1767) band of TMCB-d,,: (1) It could be vaS, with its counterpart in TMCB-& too weak to observe, or, (2) it could be a com- bination tone. In fact the sum of the i.r.-active and Raman-active CH, wagging modes, vB + veg, is promising: talc. 894 + 891 = 1785, obs. 1786. In TMCB-u$ the values are: talc. 714 + 724 = 1438, obs. 1444. This would account for the absence of the band in TMCB-& in the 1700 cm-l region. An analogous combin- ation tone has been observed with moderate intensity in the related compounds trimethylenecycZopropane [2] and 1,2- and 1,3dimethylenecycZobutane [7]. Further- more it is well known that methylene out-of-plane wags of oleflns give exceptionally intense overtones and sum tones, and that these usually exhibit a positive snharmon- ioity. Finally, 1786 (1767) seems too high for the C=C stretch. We therefore conclude that it is a sum tone, and that for some reason the C=C stretch is extremely weak. It is disappointing that it must be left unassigned in both molecules.

4. Other po88ible molecukw 8ymmetrie.a

The only other reasonable molecular configuration is that of a puckered ring, The symmetry for this would be D,. This symmetry would have 28 Raman-active and 16 i.r.-active fundamental vibrations, with all of the i.r. modes being coin- cident with Raman ones. Too few coincidences occur for this to be an acceptable choice. Any other possible symmetry allows for even more coincidences and is therefore also excluded.

5. Com~&on with other cyclobutane deriuatiues

It is well known that the planarity or non-planarity of the cyclobutane ring is determined by the resultant between two rather large opposing forces: the ring

1478 FOIL A. MILLER, FRED R. BROWN and KEE H. RHEE

strain, which tends to keep the ring planar, and the repulsion between substituents on adjoining ring atoms which tends to make it pucker. In cyclobutane itself the ring is puckered, so the repulsions are slightly dominant. Because the barrier to in- version is only 518 f 5 cm-l [8], the ring inverts rapidly at room temperature. Replacing a CH, in cyclobutane by a C=CH, group or by a C=O group would be expected to favor a planar ring because (1) it increases the ring strain, and (2) it

reduces the amount of H. . . .H repulsion. Observations are compatible with this simple picture. Methylene cyclobutane still has a puckered ring, but the barrier is lowered to 168 f 10 cm-l [9, lo]. In cyclobutanone the barrier is only 8 cm-l, and the ring is effectively planar [II-131. When two such groups are present in 1,3- substitution the ring seems to be definitely planar. Examples are l,%limethylene- cyclobutane [7] and cyclobutane-1,3-dione [14].

It might seem at first that tetramethylenecycZobutane would surely be planar. However the answer is not quite so obvious because now the hydrogen atoms again lie in opposed or eclipsed positions. Their repulsions might fold the ring. This actually does happen for perchlorotetramethylenecyclobutane. Its ring is folded in both crystal and solution, as shown by recent X-ray [15] and vibrational spectra studies [lS]. The former has given a dihedral angle of 26.5”. Consequently it is useful to have established that tetramethylenecyclobutane is planar.

Ackraowledgements-This work was supported by the National Science Foundation under Grant GP-9260. The purchase of the spectroscopic equipment was aided by NSF Instrumen- tation Grant GP-8287, and additional aid was provided by an NSF Science Development Grant GU-3184

[7] F. A. MILLER and F. R. BROWN, to be published. [S] F. A. MILLER and R. J. CAXWELL, Xpectrochim. Acta 27A, 947 (1971). [9] L. H. SHARPEN and V. W. LAUFLIE, J. Chem. Phy8.49,3041 (1968). [lo] T. B. MALLOY, F. FISEIXZ and R. H. HEDUES, J. Chern. Whys. 62, 6326 (1970). [ll] J. R. Duma and R. C. LORD, J. Chem. Phy8.45, 61 (1966). [12] T. R. BORQERS and H. L. &RAUSS, J. Chew Phya. 46, 947 (1906). [13] L. H. SHARPEN andV. W. LAURIE, J. Chem. Whys. 49,221 (1968). [14] F. A. MILLER, F. E. KIVLAT and I. MATSUB-, Spectrochim. Acta 24A, 1523 (1968). [15] F. P. VAN REMOORTERE and F. P. BOER, J. Am. Chem. Sot. 92, 3356 (1970). [16] F. A. MILLER and D. FINSETH, work in progress.