Far infrared spectra of ring compounds

.lOI!KNAL

OF

MOLIW~~LAR

Far

SPECTI~OSCOPY 39, 340-344 (1071)

Infrared

VI. Spectrum

Spectra

of Ring

and Conformation

Compounds

of 1,4Xyclohexadiene’

J. T,AANE~ASD R. C. LORD ,Spectroscopy

Laboratory,

Massachusetts

Institute

oj’ Technology.

(‘ambridge,

d1as.s. 02139

The absorption spectrum of 1,4-cyclohesadiene vapor in the range 60-250 cm-1 shows only one band, which is of type C with Q-branch peaks of decreasing intensity in the order 107.7,108.4, 107.0, 100.35, 105.75, 105.2, and 104.7 cm-l. These are assigned to the BzILring-puckering vibration of a L),h structure having a planar ring. A potential function computed a priori from the torsional and angle-bending potentials appropriate to this vibration gives fairly good quantitative agreement’ with the ohserved frequencies.

The far infrared spect’ra of certain four-membered (I-5) and five-mrmbcred (6, 7) ring molecules have been valuable for distinguishing between planar and nonplanar conformations and also for measurement of the potential-energy barrier to inversion in nonplanar cases. Thus, for example, the ba.rrier to inversion of the nonplanar cyclopentene molecule (6) has been measured as -0.66 kcal/mole and the dihedral angle in the equilibrium conformation found to be 23”. Although the mid-infrared and Raman spe&ra (8) of 1,4cyclohexadiene and its deuterium analog have been r&her conclusively interpreted to show that this molecule has a planar ring (I& struct,ure), we have invest.igat,ed it,s far infrared spectrum to see whet,her evidence could be found for t,he existence of a small barrier at’ the planar conformation similar to the barrier in cycloptlnt,ene. The spectrum of a dried commercial sample of 1 ,&yclohexadiene (Ii. and I\. I,aborat,ories, Plainview, N. J.) was examined at high resolution in an evxcunt ed double-beam instrument (9) over the range CO-250 cm-‘. The only absorption found was that of a band at 108 cm-‘, although path lengths up t 0 10 m :It R pressure of 8.5 Torr were used (Fig. 1). This band arises from the I&,‘ ring-puckering mode, in \\-hich the ring atoms move approximately as shown: + -c 2

\cII

II -Y

(1)

/+

1 This work was supported earlier grants). * Present, address: tion, Texas 77843.

by the i’Gat,ional Science

Ilepartment

of Chemistry, 340

Foundation

((‘;rant

Texas A. and hI. University,

(:P-13473

alld

College

Sta-

FAR

INFRARED

I 90

OS0

SPECTRA

I loo

OF RING

/ I IO

WAVE NUMBER

COMPOUNDS

I 120

I 130

341

140

IN CM-’

FIG. 1. Type-C’ absorption bands of 1,4-cyclohexadiene 4 m; pressure, 8.5 Torr; spectral slit width, 0.4 cm-‘.

vapor

at 108 cm-‘.

Pat’h length,

The most interesting feature of the band is the collect,ion of seven components of the central &-branch. These components are interpreted as the various vibrational transit.ions O-l (108.4 cm-‘), l-2 (107.7), 2-3 (107.0), 3-4 (106.35), 4-5 (105.75), 5-6 (105.2) and 6-7 (104.7). Thus the vibration is only slight.lg anharmonic, and since the more int,ense peaks, which arise from the lower energy t.he potential function is less steep t,han levels, have the highest frequencies, quadratic in the range covered by these transitions. From the small degree of anharm0nicit.y it may be concluded either that, t,he barrier t,o inversion of the boat form is very high (so that the vibration is taking place at the nearly quadratic minimum of a deep potent’ial well) 01’that there is zero barrier (planar form). If t,he former is the case, the barrier must, be several times higher than the topmost level (~740 cm-‘) observed, in view of the very slight anharmonicity up to this level. In fact, if one adopts t’he often-used form of the puckering potential (3, 4-Y) V = A (2” - SZ”), where 22 represents the displacement of each aliphat,ic carbon at.om out of the pIane of the other four carbons, very good agreement is obtained with the observed frequencies. The and 20, the equilibrium calculated barrier is close to 3400 cm-’ (9.7 kcal/mole) value of 2, is equal to about 0.3% A.” However, because both t,he barrier height and the calculated positions of t,he carbon at,oms are in conflict wit,h t,he spectroscopic evidence (8, IO), we believe the second alternative (planar form) is preferable. The recent electron diffraction work of Oberhammer and Bauer (11) on 1 ,;fcyclohexadiene assigns the molecule a dihedral angle of lTi9.3” with Z0 = 0.115 A. 0

3 The value of 0.385 A depends on the maguitrlde calculated

to be 129 amu. However,

the dependence

of the reduced mass, p, which on g is very slight (~(“6); see (6).

was

312

LAANE

Although t,hey do not, int,erpret rium configuration, t,heir value

their results as compatible \vit’h a planar equilibof 20 is much too small to be consistent with t’hc

above high-barrier

pot,ential

potential

funct,ion

for t#he ring-puckering

function.

observed

if &is dihedral split,ting

tion of numerous WC prefer equilibrium

Moreover,

it seems to us unlikely

mode would be so nearly

spectrum

is extremely

of t’he levels below the barrier transitions

to ascribe

1 ,kyclohexadiene structure

LORI)

angle were near 159”. In cyclopenkne,

angle is lT,S”, t’he far infrared inversion

ANI)

above

the mild

t’o the nature

configurat.ion.

irregular

anharmonicity

of the puckering

of the potemial

system

qualitatively

I

vibration

in

leading

to t,he planar

potential

of the planar

as in Fig. 2. When

I

the ring-puckering

I

I

/.I cv-

o--__-_-__---_

E

(6), due bot,h t’o the

t.he barrier.

T

7

as

where the dihedral

of 232 cm-’ and to the observa-

The origin of the ring-puckering

may be described

that, the

harmonic

;3000-

z

2

v-.z--,

E b 2000w d IOOO5 > O----

L

J--...

-i

6000 t ; 5000 E U-Z4--,

y4000 2 p3000 /

'2000 -I

l,o,o,ot, , 0.4

0.3

,\i-/,_,, 0.2

,

0.1 0 ,+0.1 to.2 t-o.3to Z in A

PIG. 2. Potential cwves for the Bt, ring-puckering potent,ial due to t,orsion about t.he C-C single bonds; ‘angle strain; (bot,tom) sum of these two.

model of 1,Gcyclohexndiene: (top) (center) pot,ent,isl due to C-CW,C

FAR

INFRARED

SPECTRA

OF 111X(: COXSPOuKlI>S

343

coordinate Z is zero, t.he molecule is planar; at nonzero values of Z, it, is puckered into a boat struct’ure. The upper curve shows the form of the torsional potential result,ing from the barrier to internal rotat.ion about the CX single bonds. This potential is at. a minimum in the planar configurat,ion and reaches a maximum when one of the CH bonds of a CH2 group eclipses t,he adjacent olefinic CH bonds. This is calculated t!o occur at 2 = f0.55 A. These CH, , CH interactions should have roughly the magnitude of the CH, , CH interaction in the propylene molecule, where the barrier (12) is 693.4 cm-‘. Since there are four such interactions in 1,4-cyclohexadiene, the total torsional barrier is est,imated to be BOO cm-‘, or about X kcal/mole. The center curve in Fig. 2 represents the potent.ial fun&on due to the angle st,rain in the molecule. In a planar six-membered ring t,he average angle is 1120”. The methylene carbons in 1,4_cyclohexadiene would prefer to have tetrahedral angles and t’herefore have a higher potential energy at, 2 = 0 t,han at, Z = ho.174 A, where the angles achieve the tetrahedral configuration if the double-bond angles are kept at 120”. At larger absolute values of Z these angles become smaller still and a quartic potential of the type found in smaller ring molecules is txpect,ed. The total potential function resulting from t.he above t,wo cont.ribut.ions is shown in the lower curve. The torsional potent’ial is clearly the predominant one as far as the lower vibrational levels are concerned. The angle-strain potential, which has a maximum at Z = 0, does not change rapidly enough at small Z to cause a maximum in the overall potent.ial, but simply combines with the torsional potent,ial to lower t’he net curvature of t,he total potential and to change its anharmonicity. The observed anharmonicity varies wit,h vibrational quantum number. Thus it appears that t.he t.ot.al potent,&1 function, which by symmet,ry is an even funct,ion of Z, must contain more than quadratic and quartic terms to be consistent with this fact. However, the approximate form of the total potential is quadrat.ic near 2 = 0, and t.he quart,ic cont,ribut,ion of t,he angle strain will not be appreciable u&l vibrational levels some distance above those observed (perhaps 200 cm-’ higher) are reached. The tot,al pot,ent,ial of Fig. ‘J combined with a reduced mass of 129 amu leads t,o a spacing of about 104 cm-‘, in satisfactory agreement \vith the observed values. hTo at,t,rmpt was made to evaluate the anharmonicity, but. Fig. 2 shows that the effect of the angle- strain potential would be to cause positive anharmonicit’y at, large values of v. There is some suggestion of this in the fact that the observed transition at) 104.7 cm-l terminates the series of Q-branches somewhat abruptly. Obscuration of t,he next, Q-branch may result from a convergence of t,he branches as the anharmonicity begins t,o change sign. Furt.her work is in progress on the related molecules 1 ,-l--dioxacyclohexin2,5-diene, 1,4-dioxn-cvclohex:~-~-~Ile, and os:L-c~cloh~sn-“-rrlc‘. RECEIVED: February S, 1971

REFERENCES I. A. I).\NTI, W. J. LWFERTY, 0. 8. I. CHIN,

AND 1%. C. LORD, J. Chem. Phys.

33, 294 (lUC,O).

T. Ii.. BOHGEHS, J. W. I~USSELL, H. L. STR.\USS, ASI) W. T). (:KIXN,

J. (‘h(,~i

I’hys. 44, 1103 (1966). ,3. J. 1:. Ikrt~r, .tND It. C. LORD, J. Chem. Php. 46, 61 (196ci). 1. T. H. BURGERS .\ND H. L. STRAUSS, J. Chem. Ph!/.s. 45,947 (1966). 5. J. L.\.\NE \NI) 1:. C. LORD, J. Chew. Phys. 48, 1508 (1968). N. J. L.\.\sE ANI) I<. C. LORD, J. Chem. Php. 47, 4941 (1967). 7. T. UED.\ AND T. SHIKYNOUCHI, J. (Them. Ph!ys. 47, 4042 (1967). 8. H. D. STIDHAM, Spectrochim. Ada 21,23 (19G5). .!I. T. &I. H.\Ro .\NI) N. C. LORD, J. Opt. Sov. 7, 589 (1968). 10. II. W. ($.\RI+ISCH, JR., AND M. G. GRIFFITH, J. d,,ler. Chew SUC. 90, 3590 (19ciS). 11. H. OREHHAMMER AND S. H. BAUER, J. Avaer. Chew. Sot. 91, 10 (1969). 23. I). I?. LIDE ANI) I). E. MANN, J. Chern. Phys. 27,868 (1957).