Vibrational spectra of centrosymmetric molecules

JoumaL Ekvlgr

of golecuZar Sfrrrcrurz, 131(198S) Sciace Publishers B-V_, Ams+dam

ViBRATIoNAL

L%k’JXA

OF CENTFkOSYYM&ETRIG MOLECEXZS

Part HI_ meso-3,4KGbromohexane

i-3. A

..245-262 - F’rinted in The Netherlands

and meso-2,3,4&tekahromohexane

CROWDER

Depmfment (l&&iv&d

of Chemisfry.

WestT&us State

University.

Canyon,

Texas

79016

(U-S_&)

24 May 19Fs)

Liquid and s&id-state infra& and liquid-state Raman spectra were obtained for meso3,4dibromohexane and meao-2.3.4.5-tetrahromohesane‘I%e spectra show tie dihromotieme to erist as-2wo or more ccnformatio~_ in the neat liquid, including one that is cenizosymmeiric and one. thet is not centrosynunetric. The solid erkts only as one of tie This confc0xnation probably has all six two posible centrosymmetric conformstions. ~arbonx coplanar. The fetrahromc;7exa.ue ia a solid at rc,om temperatare and there is c&y one conformer present. The hck of coincidence of F&man and infrared bands show that conformer tomhave a center of symmetryNo additional conformers were found in dilute carbon disumde solution_ INTRODUCTION

It has recently been shown that meso-2,3~ibro~:o-l,~chloIulbutane [1] and meso-1,2,3,4-tetmbromobuk~e [Z] exist in the crystalline state in the centrosymmelzkal conformation with all carbolls %nd primary halogen coplanar. It has also been shown that meso-2,3d&romobut’ane crystalhzes in the centro~~mmetrical conformation [3] _ Vibrational spectra have now been obtained for meso-3,kIibromohexane and meso-2,3,4,S&rabromohexzme in order to compare the coizformational behavior of these compounds with the skructurahy similar compounds already studied_ EXPERIMEN’I’AL

meso-3,4-Dibromohexae was prepared by the stereospecific bromination of tiuns-313exene with CC& as solvent. The product was distilled and then redistilled, both times at reduced pressure. A fi-action boiling at 83-84”C/15 Torr each time was collected and. used. The spectra before and after the second distillation showed no differences, and gas chromatqraphic ana@rsis indicated the presence of only one compound_ meso-2,3,4.5-Telxabromohexane was prepared,_:in the same way as the dibromohexane, by bromination of tians,trans2,!Chexadiene. The productwas a white solid, wrich. was r~rystalhzed from CRC13~(mp. 180” C). 0022-2~60/35/~03.30

Q 1985

Elsevier

Science

Publishers

B-V_

246 were obtained with a Reclunan R-12 s_pectkphotometer m-1 FTIR spectrometer and Reman spectra vrere obtained &h a Beckman Model 700 spectrometer equipped with a Coherent R-ad$ai tioa Labs CR-2 Arron ion laser. The 488.0 nm line Y&SE&_ hhred

spectra

or a Nicolet

RESULTS

AND

DISCUSSION

Infmredspectraofmeso -3&dibromohex& (DBH) zire shown in Figs-l -and 2, and the Raman spectrun of the liquid is shown in Fig.3. The observed bands are listed ic Table 1. The C-Br &ret&kg frequencies of alI possible conformers of DRX would lie in the 500-700 cm-’ region. Liquid-state lR and Raman spectra show bands in this region at 670, 617, 559, 527 (IR) and at 673, 648, 625, and 533 cm-’ (R). Observation of this many bands indicates the presence of more than one conformation. In an effort to isolate bands due to one conformation, tied spectra were obtained for the solid, which was formed by cooling a liquid Glm to ca 80 K. The solidstate TR spectrum (Fig. 2) shows only the.550 cm-’ band in the C-Br stretch region. The z&appearance of the o+her bands in this region verifies the presence of rotational isomers_ Although there are a few coincidences of solid-state IR and liquid-state Raman bands, most of the solid-state lR bends do not have Ramen counterparts, including nearly all the strong IR. bands. It can therefore be concluded that the conformer preseu tinthesolid

I 1600

1400

1200

loo0

J

800

000

400

m-1

Fig_ 1. Partial

liquid&ate IR spectium of me60-3.4-dilxomoherze

at r&m tempmature.

247

%T

l-

1600

1200 CM-’

1400

Fig. 2. Partial solid-state

Fig_ 3_ Liquid&ate

1000

TR spectrum

Flaman

ape&rum

a00

600

400

of meso-3.4-dibromohexane

at ca

of meso-3.4dEbromohexme

80 K.

at morn

mperatule.

248 TABLE

1

R(liq.)

=oiq-)

1461m

1462s

IR(sol_)

RWq-)

Wliq-)

IR(M_II.)

953W 1465s

1422~1s

1288m

12019

1438ms 1383s 1356w 1344vw 132% 12936 1275m 1253~ 1205m 1191m 1153s

13ahs

903m

1342m 1315vw

P69m 807s

127Chs 12E5ms 12Olw

673m 648~s 625~s

1167ms

533s

lCY34m 105Oms

423m 388~

1139m 109ow 1056m

912rw 896mw

904w

884W

8848

866m ea805sh 793s 670w 617~ 5596 527m 48Om 420~

1033w 1001vw

1036ms

361m 3159 192vvs

550s 485m T

I 1036~ 1015w 997w

791s

JlO

n0

data !

has a center of symmetry. The very strong Reman band at 648 cm-’ must be the other C-Br sxetch band of this confo-rmer. It is absent in the IR spectrum, and the tw, C-Br stretches of the c1 conformer of 2,3dibromobukne were determined to be 547 (IR) and 642 cm-’ (El, calculated) [3] _ For XCHzCHBrCHBrCHIX (X = Cl, Br), the conformer analogous to conformer A (Fig- 4) is apparently the one present in the crystalline solid. Since the slxGghtchain hydrocarbons [4], n-all& chlorides 151, bromides [S], iodides [7], and other compounds also crystallize in the planar zigzag arrangement of carbon atoms, it is very likely that conformer A is the one present in the solid state of DBH. Coincidences of several Raman bands with liquid-state IR bands that disappear on solidification indicate either accidental coincidences for conformer B (Fig- 4) or the presen ce of a conformation that does not have a center of symmetry, such as conformer C, or both these possibilities Accidental coincidences will undoubtedly occur because of the large number of fundamental vibrations, but such coincidences are less likely in thf2 500-700 cm-' region. Observation of both IR and Raman bands at ca. 670, 650, aud 530 cm-’ indicates the presence of a conformer without a center of symmetry. Although the IR and Raman bands at ca. 620 and 530 cm-’ differ by 8 and 6 cm-l, respectively (617 and 625; 527 and 533 cm-’ ), it is assumed that, these are coincidences and that the differences are due to the uncertainty in the Raman values because of large slit width_

C Fig. 4. Most

probable

conformations

of mw-3,4-dl%romohexane.

Of the ten liquid&ate IX bauds that are absent in the solid-state spectrum, eight have Raman counterparts. This confirms the presence of at least one conformer that does not have a center of symmetzy. Th&e is insuf%zient evidence to conclude that the other centrosymmetric conforn~er is present. The IR. and Raman frequencies of DBH and the othertwo cer&osymmetric dihromo compounds (1,3) are all quite close (Raman = 635,642,647 cm-’ ; IR = 562, 547, 559 cm-’ )_ Examination of the normal vi.bratiDnsS~PWSthe frequency of the Raman-active C-Br stretching vibration should be higher than that of the m-active C-Br stretch because of the interaction of the Raman-active stretch with the middle C-C sWztch_ This i&era&ion is forllidden in the W. nzso-2.3.4,5-Tetmbromohexane (TBH) can titin two centrosymmetiical conformations analogous to those of tetrabromobutane (2). In one conformer, the six carbons are coplanar, and in the other conformer, four carbons and two bromines are coplanar. Jmed and Raman sp2ctra of the solid at room t&perature are shown in Figs_ 5 and 6, and the observed wavenumbers are listed in Table 2. Although there are a few coincidences of IX and Raman frequencies, the lack of coincidence of most bands shows the molecule ti have a center of symmetry. There are two exba bwds in the C-Br stretching region of the spectra. The weak Raman band at 696 cm-’ can be explained as the (Raman active only) combinrltion mode (191 + 504), and the m doublet at 670, 657 (1111-lis assurmedto be due to crystal field splitting. In order to check to see if the doublet at 670,657 cm-l is indeed due ti only one C-Br stretch, =d to check for the presence of otitir &_?formers in solution, IR spectra were obtained for a saturated solution of TBH in carbon dude. Although satumted, the solutio:~ was mute (ca l-2%),

250

1400

1600

1200

1000

BOO

600

400

CM-’

Fig_ 5. Partial solidstate IFt spectrum of meso-2-3,4,5_tetra!xomohsane tire

-

1600

1400

1200

1000

am

GO0

400

at room tempera-

I

200

0

CM-’

Fig. 6. Partial solid&ate temperature.

Raman speclnun

of meso-2.3,4.5-tetrabromohexzne

at room

251 TABLE

2

lnErar&andRaman

qQl-1 1447w

wavenumbersfor

-~ W-=L)

nt(6ol’n)

N-l-1

1380

922vw 88Qmw 706s

14516 144oms

1396vw 1384s

meso-2,3.4.5-tetrab~~~h~e .WSlLPl.)

IFt(&iol’n.)

98%

966

881s

875

696W

1349vw 133OwL.

670s 669

1206~ 1198m

1046mw 1026mw 1012w

1323m 12608 1209m

1318 1253 1203

657s 647s 67%

575

504S

1148m 1112mw 1045m

1145 1102 1042

312s 191v6 151

473m 401s t UO

T cEl3

1

and most bands absorbed only 5-10% (ca 90% ‘I’ with the background at 100%). It is evident, however, that there is only one band at the position of the solid-state CkBr doublet. In addition, there are no bands present that are absent in the solid-state spectrum, which indicates the presence in solution of only the one conformer that is present in the solid. This behavior is a little surprisiig, and differs from that of 1,2,3,4_tekabromobute (2), for which quite a few bands appeared in the solution spectra that were absent in the solid-state spectrum, show&g the presence of one or more additional conformers in solution_ An attempt was made to dissolve some TRH in a polar solvent (acetonitrile), but it wae ins;oluble_ The solid was also placed between two salt plates in a heated cell m an attempt to obtain an IR spectrum of the liquid above 180°C. However, ‘he solid sublimed before melting. A melting point had easily been obt&ecil with the compound in a capillary tube. The C-Y& &retcb.ing frequencies in TBH are higher than expected and are, in fxt, higher than in 1,2,3,4-~tmbromobuiane, even though all four bromides in ‘I’RH are bonded to secondary carbons and two of the bromides in TBB are bonded to primary carbons. ACKNOWLEDGEMEKTS The

Texas, work.

author is grateful to The Robert A. Wekh Foundation, Ho-us&on, and to the Killgore Research Center for .Gnancial support of this

252 RFiFSRENC63 1 2 3

G. A Crowder, J. Ramam sp-_. El (1979) 206. G.ACrowder.J.Raman S~,ectro~., 8 (1979) 320. R. G. Snyder, J_ Mol. Spectrosc_, 26 (1966) 273.

4 J. H. Schachtwhneider and FL G. Snydc, 6 R. G. Snyder and J. H. Schachtschneider, 6 7

G. A G_ A

Crowder Crowder

Spectrochim. Acta, 19 (1963) 117.

J. MoL Spech-osc_, 30 (1969) and M:. Jalilian, Can. J_ Spectrosc., 22 (1977) 1. ard S_ Ali, J. MoL Sh-uct, 25 (1975) 377_

290.