Spectax~himlca Acta, 1967, Vol. 23A, pp. 891 to 908. Pex~iamcm Press Ltd. Prlnte4 in Northern Ireland

Tor~Aonal frequencies in the far infrared--V. Torsions around the C--C single bond in some

benzaldehydes, htrtural, and related compoundst

FOIL A. MTI.T.ER, W. G. FATELt~Y and R. E. WXTXOWS~ }dellon Institute, Pittsburgh, Pennsylvania 15213

(Rewlved 20 Apr/~ 1966; eev/,se, d 8 J'uZy I966)

Abs t~c t~The infrared spectrum between 33 and 400 em -z has been examined in the vapor and liquid phases for the following 26 compounds: o-, m-, and iv-F, C1, Br, and CHs benzalde- hydes; pyridine.2-, -3-, and 4.aldehydes; acet~phenone and its o-, m-, and p- F derivatives; furan-2-aldehyde; and several monofluorostyrenes and -nitrobenzenes. Torsional frequencies were sought, and have been assigned for all but the last two groups of compounds.

For all the men-substituted benzaldehydes, for a few of the or~ho ones, and for furau-2- aldehyde evidence has been found for the presence of two ro ~tational isomers in the vapor. The parameters ]71 and V2 of an approximate potential function for the internal rotation have been evaluated. In the me~ benzaldehydes the O-~s rotamer is the more stable one, whereas in the o~ho compc~ands and in furan-2~aldehyde it is the O.tran~ one.

For meta-fluorobenzaldehyde a temperatu_~e-dependence study of some bands in the mid- infrared has confirmed the exlst~nce of two rotamers, and has given 0.5 keal/mole as an approx- imate value for t~heir energy difference in CDCIs solution.

1. INTRODUCTION

I~ a previous paper we have ~scussed the torsion around the C--C single bond I t

in conjugated molecules of the type ==C--C-~, for which butadiene and glyoxal serve as protypes [I]. The present work extends this study to the C--C torsion in some conjugated ar--omatic compounds: benzaldehydes (I: X = F, C1, Br, and CHa), pyridine aldehydes (II), aeetophenones (III), and furan-2-aldehyde (IV).

H~c.~O H ~-,,, C/i~O HsC~-.c~O H~,c~CHz O~N~,/O

I I~ III I~ ~ 3ZI

The torsional frequencies have been measured in the far infrared spectrum for both

This work was supported in part b y t h e U.S. Air Force under Contract No. AF 33(657)- 11142 with the Air Force Systems Command, Wright-Patterson Air Force Base, Ohio; the National Science Foundation under Contract No. GP-5050; and the National Institutes of Health under Gran~ No. GM-11815.

[I] W. G. F A ~ Y , R. K. ~ A ~ s , F. A. M~.T.~ and R, E. W r ~ o w s ~ , S ~ . A ~ 231 (1965).

891

892 FOIL A. MILLER, W. G. FATELEY and R. E. WITKOWSKI

vapors and liquids, and an effort has been made to evaluate the potential function. Several styrenes (V) and nitrobenzenes (VI) were also examined, but the torsional bands could not be found.

There has been some earlier work on internal rotation in these and similar molecules, but nearly all of it has been on condensed phases. We have already emphasized that if one is to deduce a torsional barrier which is due only to forces wit&n a molecule, it is absolutely necessary to make observations on gaseous samples [l]. Strikingly different results are obtained in liquids and solutions due to the influence of neighboring molecules. Specific examples will be mentioned later in Section 4(B). All of the barrier results given here will be from gas phase data.

SILVER and WOOD [2] measured torsional frequencies in the far infrared for benzaldehyde, o-, m-, and p-chlorobenzaldehyde, and o-, m-, and P-tolualdehyde as liquids or solutions. ANET and AHMED [3] have used nmr to evaluate AF*, the free energy of activation for internal rotation, for benzaldehyde, p-methoxybenz- aldehyde, and p-dimethylaminobenzaldehyde in solution. KARABATSOS and VANE [4] have made nmr coupling constant measurements on numerous benzaldehydes, pyridine aldehydes, and furan-2-aldehyde in solution, including several compounds treated here. The internal rotation in furan-2-aldehyde has been studied by nmr [5] and by microwave spectroscopy [6], the latter being the only pertinent gas- phase measurement on these compounds of which we know.. These various earlier results will be compared with ours at appropriate places later in the paper.

2. EXPERIMENTAL

Commercial samples were used after checking their mid-inf?ared spectra against reference spectra. All of the vapor-phase data were obtained with a Beckman IR-I 1 far infrared spectrophotometer. Because of the low vapor pressure of the samples, a multiple reflection cell was used at either 8.2 or 10 m path. The fluoro- benzaldehydes and furan-2-aldehyde were run at room temperature. For all the other compounds the cell was warmed. To observe the torsional bands, which are usually weak, the cell was heated to about 96” and the sample reservoir was held somewhere between 50 and 87’ depending on the vapor pressure.

The 4 mm thick polyethylene windows were perilously close to their yield point at 95’, so for this temperature they were replaced by 1.6 mm polypropylene. The latter has the added advantage of being transparent at 73 cm-r, where polyethylene has a temperature-sensitive band, but unfortunately at this thickness polypropylene absorbs too strongly above 160 cm-l to be useful there. Heating the cell changed the 0 percent transmission line because of emission, but this caused no real trouble.

Liquid phase spectra were also obtained on all compounds, using both the

[Z] H. C. SILVER and J. L. WOOD, Trame. Faraday Soo. 60, 5 (1964). [3] F. A. L. ANET and M. A-, J. Am. C&m. Sot. 86, 119 (1964). [4] G. J. KARABATSOS and F. M. VANE, J. Am. Chem. Sot. 85, 3886 (1963). [a] K. I. DAHLQVIST and S. FORBEN, J. Phy.c Chem. 60, 4062 (1966). [S] F. M~NNIC+, Physikeliache Institut der Univereitit Freiburg i. Br. Abstract 410 at 8th

European Congres.9 on Moleoula;r Spectroswgy, Copenhagen, August (1966).

Torsional frequencies in the far i&axed-V 893

Table 1. Observed infrared bands

Compound Gea Liquid %m - Yua.

VW

m-F-benzaldehyde

p-F-benzaldehyde

o-Cl-benzaldehyde

m-Cl-benzaldehyde

p-Cl-benzaldehyde

o-Br-benzaldehyde 101 W

m-Br-benzaldehyde

Ekizaldehyde 111 (8)

o-F-benzeldehyde e 130-13 -0.23

-96 104 108.S

-114 I 190 200 210 I 264 406 440

II

ah w m

W,8h

m

Q3.6 200 270 328 387 417 426 >

88 103 141

m

In

W. b

230 231 > 280 423

m

W

B

94 106 (8)

81.5 157 178 303 l

m

W

W

VW

225 W

326 m

90 sh 107 m 140 f 10 w, vb

133 -0.20 (a)

16Q -0.10

210 * .

126 f 6

192 202 208 235 l

l

113 -200

l

l

.

.

124 f 6 -0.20

162 203

248

l

*

124 s. b (8)

104 w 179 196 308 366

120 f 6 176 239

130 143 160 182

-0.16

-0.01

-0.21

- 0.20

-0.27

-0.20

-0.26

894 FOIL A. MILLER, W. G. FATELEY and R. E. WITKOWSXI

Table 1. (Cont.)

Compound Gas Liquid

m-Br-benzaldehyde (conk%) 224 8 -268 w

320 xv 430 8

p-Br-benzaldehyde 73.5

68 1

w 81

140 f 10 w, b

o-tolualdebyde

m-tolualdehyde

p-tolueldehydo

Pyridine-2-aldehyde

Pyridiw-3-eldehyde

Pyridine-4-aldehyde

Acetophenone

o-F-ecetophenone

m-F-scetophenone

p-F-ecetophenone

Furan-2-aldehyde

222 308 358

430

85 247

-436

93 108.6 189.6 227 236 >

89.6 166 178 189 197 1 111

(a)

110 (a)

102 (8)

w * m

w

w, b m w

w m 8 * * In

48 & 2 a, b 189 VW 220 m

42 f 2 m, vb 216 8 (0)

47f 2 8, b (c)

61 & 2 B, b (c)

128 VW 134 VW 146 m

199 8 213 “B 236 vs 246 B, sh > 280 “B

237 * * *

92 & 2 -0.26

176

300

382 *

116 f 6 vb * *

-0.36

129 f 3

+ *

1142 4vb

-0.19

-0.27

*

130f 3 (a)

130f 3 (a)

126& 3 (a)

* * l

-0.20

-0.20

-0.26

-70 *

*

l

157 w 175 w

216 YS

252 m

301 “S

Table 1. (&md.)

Torsional frequencies in the far infrared--V 896

Compound Gas Liquid

styrene 204 194 I 213 440

o-F-styrene 161 f 5 183 (?) 236

268 I 266

(d)

m-F-styrene l

l

p-F-styrene l

o-F-nitrobencene 430

m-F-nitrobenzene 162 214 223 > 376 420

S

vs

s ah

s

VW, b

m

s

m 8

1

l

l

l

l

~216 s 260 s, sh

(s)

w, m, s = week, medium, strong. v = very; b = broad; sh = shoulder. * Not examined in this region.

(a) 70-150 cm-’ only. (0) 3L-80 cm-’ only. (d) 140-320 cm-l only. (e) 33-300 cm-’ only.

IR-11 and a far infrared interferometer manufactured by Research and Industrial Instruments Co. (model FS-520). The results from the two instruments agreed well. The interferometer was not used for the vapors because the longest gas cell available for it was one meter, which was insufficient to give the torsional absorptions.

Table 1 lists the observed bands. The frequency range was 33 cm-l to at least 400 cm-l unless indicated otherwise in the table. The frequency accuracy of both instruments is much better than 1 cm-l, but some of the bands are so broad that the uncertainty in choosing the origin is several times this.

Table 1 also gives the fractional difference between the gas and liquid frequencies for most of the torsions. The gaseous frequencies are always lower than the liquid ones, which is the general behavior of far infrared bands [7]. It will be seen that the differences amount to 20-25 percent which is as large percentagewise as the shifts produced by strong hydrogen bonds (although opposite in sign)!

Figure 1 illustrates the bands which have been assigned to the torsion in twelve substituted benzaldehydes. (For benzaldehyde itself, see [l].) The spectra are not pretty because they were very difficult to obtain. Each band has been run several times, and we are convinced that the reported absorptions are real.

[7] W. G. FATELEY, I. MATSUBARA, and R. E. WITKOWSKI, S~ochim. Acta SO.1461 (1964).

896 FOIL A. MILLER, W. G. FATELEY and R. E. WITKOWSKI

XT

0-b o-CH,

2 94

VI04 I IO5

I 08.5 m-F ( m-Cl (

_LT;

126 110 94 76 118 .I02 86 70 116 110 86 70 I18 102 66 70 cm-'

Fig. 1. Observed torsional bands for the substituted benzaldehydea. Upper curves: empty cell. Lower curves: sample. A. Halo benzaldehydes: 10 m path. Fluorine compounds were run at 23°C. For all the others the cell WM 95 & 3"C, and the sample reservoir was between 62” (o-Cl) and 87” (p-Br). B. Tolualdehydes:

8.2 m path. Cell, 88”; sample reservoir, 6043°C.

3. THEORY

The goal of this work was to evaluate the potential hindering the internal rotation. The theory for calculating the barrier height from the observed frequen- cies has been presented in our earlier paper [l], and only the needed equations will be repeated here. The potential energy is expressed as the series

V(a) = g (1 - COSW) (1)

where a is the angle of interrml rotation, and is taken as zero at the most stable conformation. The problem is to evaluate the first three I’, parameters. It is

assumed that terms beyond n = 3 are negligibly small. When only one torsional frequency is observed, some simplifying assumption

has to be made. Customarily the assumption is that the potential function in the region of the torsional transition is harmonic. Then

(2)

Torsional frequencies in the far infrared--V 887

where v* = v,+ 4v,+9v,

5 = observed torsional transition in cm-l

P(in cm-l) = h/87r2cI, = I&862/1, (in amuAz)

I, = reduced moment of inertia for the torsion

(3)

(4)

A. Asymmetricauy-substituterl benzatiebydes

Consider tist the general case of an asymmetrically substituted benzaldehyde, such as o&o-chlorobenzaldehyde.

What can be predicted qualitatively about the relative importance of V,, V,, and V,?

(i) VP -The factors giving rise to VI must of course have’s periodicity of 2?r in a. An obvious possibility is steric interaction. In addition there may be a small contribution from hydrogen bonding between the halogen atom and the aldehydic hydrogen atom. These effects will be most important in the o&o derivatives, much smaller for the meta ones, and zero for the para ones.

(ii) V,. -The two-fold barrier arises mainly from the overlap between the rr orbitals of the carbonyl bond with those of the aromatic ring. This coefficient is therefore expected to be relatively important regardless of whether the substituent is o&o, meta, or para.

(iii) V8. -The three-fold barrier is probably very small, as judged from the very few values which have been deduced for analogous compounds. (cf. acrolein [l].) We have been unable to conceive of any reason for an appre- ciable three-fold barrier, and are inclined to think that this term playe only the role of a small correction. We therefore take the further step of aasum- Cng that V, is zero. Then Equation (3) becomes

v* = VI + 4V, (5)

B. Symmetrically 8Ub8titUted benzaldehydea

For para derivatives in which the substitutent itself is symmetrical-an atom, or the group -CN, -NO, or -CH, for example -the series of Equation (1) is simpli- fied even more. Symmetry then requires that V(a) = V( a &- r). Consequently VI = V, = 0, and only the V, term is left. Equations (2) and (3) reduce to

V2 = F2/4F. (symmetrically-substituted rings only)

This would also hold for any symmetrical multiply-substituted benzaldehyde, such as 3,6-dimethyl, 2,tLdifluoro, etc. (We did not examine any multiply-substituted

F

898 FOIL A. MILLER, W. G. FATELEY and R. E. WITKOWSKI

benzaldehydes because of their low vapor pressure.) For all of these cases, one observed frequency then suffices to evaluate V,.

C. F values

To calculate F for each substance, we have had to estimate bond distances and angles. For the benzaldehyde framework the dimensions obtained by QUADE and LIN [8] from a microwave study were used. A computer program has been

Table 2. Unsubstituted and para-substituted comnounds

Estimated F

(cm-l)

Vapor atorsion (cm-l)

Liquid f Va

(cm-l) (kcal/mole) l

Benzaldehyde p-F-benzaldehyde p-Cl-benzaldehyde p-Br-benzaldehyde p-CH,-bensaldehyde Pyridine-4-aldehyde Acetophenone p-F-acetophenone

1.891 111 1.743 93.5 1.687 81.5 1.025 73.5 1.649 89.5 1.892 102 0.5255 48 f 2 0.5253 51 * 2

4.66 133 6.68 3.58 113 5.23 2.81 104 4.58 2.37 92 f2 3.72 3.47 11,4 & 4 5.03 3.83 126 f 3 6.00 3.1 - - 3.5 - -

l For illustration only. Not valid barriers.

developed which requires only the coordinate positions and masses of the atoms, and which gives the center of mass, the orientation of the three principal axes, the principal moments of inertia, and F. The values of F are given in Tables 2,3, and 6. We have no way of knowing how accurate these values are because they are based on guesses about the molecular geometry. For the o&o and meta compounds the calculation has an added complication. Presumably the two planar conformations are more stable than any nonplanar ones (at least for the meta compounds). Following SILVER and WOOD, we designate them O-c& and 0-trans:

O-&3 O-tram

They have different F values, and our estimates for both forms are given in Tables 3 and 6.

4. DISCUSSION OF THE FAR INFRARED RESULTS FOR BENZALDEHYDES

A. lBenzaldehyde and its para derivative8

For these compounds a single band, usually with a pronounced & branch and detectable P and R branches, was observed (Fig. 1) . (For p-bromobenzaldehyde the band at 73.5 cm-l is not due to polyethylene; polypropylene was the window

[S] C. R. @JADE, Ph.D. thesis, University of Oklahoma (1962). masrtu&n Abstr. a8, 1710 (1962).

Tab

le

3.

Met

a-su

bst

itu

ted

co

mpo

unds

Co

mp

ou

nd

Rel

ati

ve

4 p

op

ula

tio

n,

(cm

-l)

Asa

ign

ed

VI

V,

Sta

ble

N

(s

tab

le)

VII

I&*

0-t7

YU

l.4

O-C

iS

Ch

oic

e V

tOT

SiO

n

to

(kee

l/m

ole

) ro

tam

er

N (

met

aa

tsb

le)

am

sx

(kca

l/m

ole

)

m-F

-ben

zald

ehy

de

1.69

7 1.

875

A*

B

m-C

l-b

enza

ldeh

yd

e 1.

826

1.87

1 A

*

B

m-B

r-b

enza

ldeh

yd

e 1.

680

1.86

7 A

*

B

wL

’H,-

ben

eald

ehy

de

1.68

6 1.

860

A’

B

Py

rid

ine-

J-a

ldeh

yd

e 1.

896

1.88

6 A

B

m-F

-ace

top

hen

on

e 0.

446

0.44

0 A

B

96

108.

6

96

O&

8 10

8.6

o-tm

w

94

0-t?

OM

10

6 O-CM

94

105

90

107

90

107 93

10

8.6

93

108.

6

O-C&l

0-tr

aw

0-t?

WU

O-CM

O-C

M

0-tm

w

o-tr

aw

048

O-C%8

0-tr

aw

110

110

47

41

o-tr

on8

O-C&¶

0-tY

CW

t&

0-C

i8

1.37

4.

14

3.04

4.

20

0.66

4.

04

2.16

4.

30

I.44

4.

02

4.16

4.

14

1.71

4.

09

3.33

4.

16

v*/

4

4.66

4.

69

3.64

3.

69

O-&9

10

O&

CZ

W

176

O-G

.4

2.5

0-t?

a?U

20

0-k

7

0-h

an

a

280

O-C

M

11

o.tr

a?w

10

6

94”

46’

4.86

92O

20’

4.

38

96’

8’

4.77

96’

0’

4.99

l P

refe

rred

ch

oic

e.

Q

DO0 FOIL A. ~KILGER, W. C. FATE-Y and R. E. WITKOWSKI

material.) Calculated values of V,, obtained from the vapor frequencies by Equa- tion (B), are given in Table 2. They decrease in the order H > F N CH, > Cl > Br. Puru-substituents can influence the barrier height only through electronic inter- actions, not sterically. We had hoped that V, would provide a useful measure of these interactions-that for example, it might be a measure of the double bond oharacter of the C-CHO bond. Such is apparently not the case, because no series of ortho-para directing ability, Hammett’s u constants, electronegativity, etc. places the substituents in the above order. We cannot see any reason for this particular sequence.

B. Effect of phy8ical state

These symmetrically-substituted compounds offer a good example of why barriers from condensed-state data should be mistrusted. It has already been noted that the torsional frequencies in the vapor are 20-26 percent lower than those in the liquid state. Since the frequency is squared in calculating the potential barrier (Equations 2 or 6), very serious errors are introduced if vapor values are not used. Some illustrative examples are included in Table 2.

C. Meta-8ub8tituted benzaldehydes

It is significant that for each of the me&substituted benzaldehydes there are at least two observed low-frequency infrared bands separated by 6-16 cm-l (Table 1). There are four nossible causes for the “extra” band.

(i)

(ii)

(iii)

(iv)

it may be due to aiother fundamental vibration. This seems most unlikely, for no other fundamental frequency is expected below 160 cm-l. Also, no second band was observed for any of. the para derivatives. It may be due to a difference tone, although we have been unable to find any definite assignment for one. It may be due to a hot band, and specifically to the 2 c 1 torsional trans- ition. In principle a low-temperature study should provide a test for (ii) and (iii), but in practice the vapor pressure is so low that it is impossible to do this in the gaseous state, and in the liquid the band is single and very broad so the experiment is unpromising. In any event we think it likely that the first few hot bands would be almost superimposed on the funda- mental, and not some 10 cm-l away, by analogy with the torsions in other conjugated molecules. In acrolein the torsion is very nearly harmonic [9] (up to v = 6 or 7 [lo]), and in fluoroprene it is harmonic to within the accuracy of microwave intensity measurements [ll].

There may be two rotational isomers co-existing in appreciable concentra- tions and having slightly different torsional frequencies. These are presumed to be 0-trans and O-&s (see 3(C) earlier). The best support for this is the fact that at least two bands are observed for all the meta derivatives, and that these are just the derivatives which may have two stable rotamers. For the para compounds symmetry requires that the two stable coplanar

[D] J. C. D. BRAND and D. G. WILLUM SON, Dku&m Faraday Soo. 86, 184 (1963). [lo] C. C. Coeraw, National Research Council of Canada, Ottawa, private communication. [II] D. R. LIDE. JR., J. CJwn. Phya. 87, 2074 (1962).

Tomionel frequenoie4s in the far infnued-V 901

forms be indistinguishable. For the tn-tlro compounds the sterio effeot may be large, so that only one of the plan= forms may have an appreciable population. For the rneta compounds the steric e%kcts are much smaller. There may then be two rotamere with nearly the same energy, so that each has sufficient population to be observed. Even if their energies were identically the same, their torsional frequencies would be different because they have different P’s.

a-

Fig. 2. Potential curve for the torsion in m-Br-benzaldehyde (to wale). V(a) (koal/ mole) = 1.44 (1 - oos a)/2 f 4.02 (1 - cos2a)j2. A: i3rst term. B: eeoond

term. c: sum. v, iEJ a8aumed to be !zero.

We cannot rigorously eliminate explanations (ii) and (iii), but (iv) is suoh an interesting possibility that we should like to consider it further. Some discussion will now be devoted to the implications of this interpretation.

It is assumed that the potential for the internal rotation is given adequately by the iirst two terms of the series of Equation (1). It is further assumed that there are two potential wells, one at a = 0 and one at a = w, with an energy difference of Vi between their minima (see Fig. 2). For each well, V* = P/P, but:

I%., = vi + 47, (6)

v,*_, = -VI + 4v,. (‘1

!&is result is obtained by recalling that near the minimum of the potential well, if it is harmonic in this region,

V = V* a*/4 [l, Equation 61. (8) Therefore

PV 0 aas min = v*/2. (9)

(PV/aa*) is obtained from Equation (I), evaluated at a = 0 and a = V, and put in (9) to give Equations (6) and (7).

The explicit assumptions made above may, of course, not be correct. In addition there are several implicit ones. It is possible that the stable conformations are not

902 FOIL A. MILLER, W. G. FATELEY and R. E. WITKOWSKI

the coplanar ones where u = 0 and r, but may have the -CHO group twisted slightly out of the plane of the benzene ring. (This would be most likely for a 2,6-disubstituted benzaldehyde with bulky groups.) Also there need not be a minimum at rr. Indeed, for our model there will be a minimum only if 4V, > V,. There will be a maximum if 4TrZ < V,.

Returning now to Equations (6) and (7) and an attempt to evaluate V, and V, separately, we are confronted by the problem of four possible alternative solutions. Either of the two observed frequencies can be assigned to a given rotamer (say the 0-truns one), so that two V*‘s will result from (2). One of these ‘v*‘s can be attrib- uted to a = 0 and the other to a = T, or vice versa. These latter two options always give the same Vz, but one gives a positive V, and the other its negative. The negative value merely means that we chose the wrong rotamer as the stable form. Thus there are really only two sets of results, not four. They are given in Table 3.

The B assignments for all the molecules can be eliminated on the basis of relative populations. For the two-term potential function, if there is a metastable form it will be at a = f r. The energy of the minimum of the metastable potential well is just V, (Fig. 2). This will also be very nearly equal to the energy difference between the v = 0 levels in the two wells. Therefore the relative populations of the v = 0 states is

N (stable)

N(metastable)

= eV,lH’ (10)

For the B assignments these ratios come out to be 176, 20, 280, and 105 for m-F-, -Cl-, -Br-, and -CH,-benzaldehyde respectively. These population ratios should be compared with the relative intensities of the two bands, which appear to be roughly 1O:l for the m-F and m-CH, compounds and 3:l for the m-Cl and m-Br ones (the higher frequency being the more intense). Although the intensity per mole of the 0-cis and 0-trans rotamers need not be identical, they will probably not differ more than three-fold. Thus the population ratios for choice B are incom- patible with the observed intensity ratios, especially for the bromo and methyl compounds. We therefore reject choice B. The ratios for A are acceptable. The conclusion is that in each case the stable rotameiis the 0-cis one. It has the highest torsional frequency, and is more stable than the O-tram rotamer by V,, which is 1.4,0.7, 1.4, and 1.7 kcal/mole for the fluoro, chloro, bromo, and methyl compounds respectively.

Figure 2 givesthe potentialcurve toscaleforthetorsioninm-bromobenzaldehyde. The position of the maximum in the potential curve is obtained from

cos a,, = - VI/4 VS.

Knowing G, one can then calculate V,, from Equation (1). V,, is the barrier for transition from the stable to the metastable rotamer. The values of sax and V,, are included in Table 3. It will be seen that G, is between 92 and 96” for each of the above four compounds, and that V,, is 4-5 kcal/mole.

KARABATSOS and VANE’S [4] nmr results led them to conclusions about the stable rotamer which disagree with ours in two out of three cases, as summarized in

Torsional frequencies in the far infrared-V 903

Table 4. A difference in state is involved, and we do not know whether this causes the disagreement or whether there is an error in one of the methods. It will be seen later that for furan-Z-aldehyde these authors disagree with the results of three other types of experiments, so their results are suspect.

Table 4. Meta-substituted compounds: comparison with other results

Compound

KARABATSOS andVm~[4] This work (solution) (gas)

% in % in Stable rotamer stable form Stable rotamer stable form

m-Cl-benzaldehyde na-Br-benzaldehyde m-CHs-benzaldehyde

0-trans 0-tra?&Y

o&a

60-70 o-&3 70 60-70 o-tie 85-90

970 O-&S 90

D. O&m-substituted benzaldehydes

In these compounds steric effects surely will be important, and V, may well be larger than V,. One or both of the rotamers may even be non-planar. We shall assume that our approach is still valid, however.

For o-fluoro- and o-chlorobenzaldehyde two bands were observed (Fig. 1). In the o-fluoro compound 96 cm-l could be the P branch, but it is stronger enough than the R branch so that some other assignments must at least be considered. Let us assume that for both compounds the bands are due to two different rotamers. The results are given in Table 6. Again we prefer the A choices because the relative populations are much more reasonable. Note that now, however, the more stable rotamer is the O-tram one. This is exactly the opposite of what was found for the nzeta benzaldehydes. It is nonetheless reasonable. Steric considerations favor the form with the oxygen turned away from the halogen. Also with the aldehyde hydrogen thus turned toward the halogen there is the possibility of a weak intramolecular hydrogen bond. It is surprising, however, that V, is not larger for these orth compounds than for the meta ones. Again, we remind the reader that &ho-fluorobenzaldehyde may really have only one band.

For o&o-bromo- and o&o-methyl benzaldehyde only one torsional frequency was observed. These are just the compounds having the largest substituents, and presumably steric effects make the O-&s form relatively unstable.* The best we can do with these compounds is to calculate P* = V1 + 4V, from Equation 2. One has to assume which of the two rotamers, 0-cis or 0-trans, is the more stable because the value of P is different for the two. Although we believe it is 0-trane, we have calculated for both rotamers, and have tabulated the results in Table 5. They are given as I’*/4 for more realistic comparison with V2 values.

E. Methyl torsions

In none of the methyl-containing compounds (including the acetophenones dis- cussed later) did we observe the methyl torsion. This is not surprising. Methyl torsion bands are characteristically very weak, and at these low vapor pressures we would not expect to detect them. In addition, for pra-tolualdehyde the

l The van der Waals radii are F = 1.36, Cl = 1.80, Br = 1.96, and CH, = 2.00 A.

904 F o ~ A. ~ . ~ . ~ , W. G. F , ~ and R. E. Wrr~owsx~

Torsional frequenoies in the far infrared--V 90s

frequency will be extremely low because the barrier is six-fold. The same will also be true for meta-tolualdehyde to a good approximation.

F. Cornme?& on Silver and Wood’s paper [23 The barrier values for several benzaldehydes given by these authors were

obtained from liquid-state frequencies before it was fully realised how large an effect a ohange of state could cause in the far infrared. (They included a warning on this point, however.) SILVER and WOOD had only one datum for each compound, so they used a one-term potential function which is periodio in rr. The barrier parameter which they call V, is, in our notation, v*/4 = (IrJ4) + V,.

5. DISCUSSION OF RESULTS FOR OTHER COMPOUNDS

A. Furan-t-aldehyde (furfural)

ALLEN and BERNSTEIN [12] were the first to show, from a study of several sets of doublets in the infrared and Raman spectra, that this compound exists as an equilibrium mixture of two rotational isomers. They found no temperature

cx / ’ @0 0

&,H

L O a

004fmi3 00-c&l

dependence for the gas (AH f! 0), but for the liquid there was a small dependence corresponding to an energy difference of the order of 1 kcal/mole.

KARABATSOS and VANE’S nmr study of coupling constants in solution [4] led to the result that the molecule exists completely in the 00-tie form. DAELQVIST and FORSEN [6] studied the nmr temperature-dependence of dimethyl ether solutions, and they found on the contrary that the 00-trans form is the more stable, with AH = 1.05 kcal/mole. They point out that there may be large systematic errors in this method, so that a check of the numerical result by an independent method is very desirable.

Very recently M~NNIU [6] has reported a microwave and far infrared study of the molecule. He found that in the vapor the molecule is planar, and that the two rotamers co-exist with the 00-trans form being the, more stable. He reported V, = 0.08, I’, = 8.67, and V, = 0.91 kcal/mole. (Note that steric hindrance is probably much smaller in furan-2-aldehyde than in the o&o substituted benzalde- hydes, so it is reasonable that v1 is smaller. We are BUrpriBed at the relatively large value of V,, however.) Inexplicably MijNNIo’8 infrared frequencies for the vapor (175 cm-l for the 00-trans form and 160 cm-l for the 00-cis form) differ greatly from our observed absorptions (128, 134, 145, 199, . . . cm-i). We therefore include our calculated results in Table 5.

The relative intensities of the 128, 134, and 145 cm-l bands are very roughly 1: 1: 20. We feel that 145 cm-i must be used for the torsional frequency of one rotamer because of its intensity. This then leaves two possible pairs of frequencies: 128 and 146, or 134 and 146 cm-l. For each pair the higher frequency can be

[12] (3. ALLEH and H. J. BERNSTEIN, Can. .7. Chem. 33, 1086 (1966).

906 FOIL A. MILLER, W. G. FATELEY and R. E. WITKOWSKI

attributed to either OO-trans or to OO-cis, and the lower value to the other. There are consequently four possibilities, designated A-D in Table 5. Set A is preferred on the basis of relative populations. It attributes 134 cm-l to the 00-cis rotamer and 145 to 00-trans. The latter is the more stable by 2 kcal/mole. We do not have a good explanation for the 128 cm-l band.

B. Pyridine aldehydes

These compounds contain a -CHO group substituted o, m, or p to the nitrogen of a pyridine ring. Only one torsional frequency was observed for the 2- and 3- derivatives, so we could do no more than calculate V* for them. The results are given in Tables 2, 3, and 5.

From nmr measurements on pyridine aldehydes in solution [4], it has been suggested that pyridine-2-aldehyde is 70-85 percent ON-trans, and that pyridine- 3-aldehyde is about 70 percent ON-trans. We found no evidence for two rotamers.

C. Acetophenones

Acetophenone and its o, m, and p fluoro derivatives were studied. V, or V*/4 was found to be 3.1, - 3.0, - 3.5, and N 3.5 kcaljmole respectively (Tables 2, 3, and 5). The torsional bands are very broad, which leads to a considerable uncertainty in these values. The results are surprising because the value of the o-fluoro compound is lower than that for the m- and p-fluoro ones. An nmr study of many o&ho subsitituted acetophenones [13] (but not o-fluoro) suggests that these molecules are not planar in the ground state. If so, this would lower their barrier relative to the meta and para derivatives, and explain the above result. JONES et a,?. [la] studied the infrared carbonyl stretching band near 1700 cm-l for these four compounds. They found no evidence for two rotamers, although o-chloro- and o-bromoacetophenone did show some.

D. Styrenes

No definite torsional frequency could be found for styrene and its o, m, and p-fluoro derivatives, although higher-frequency absorptions were found as listed in Table 1. For the three fluoro compounds there was general broad absorption from 33 to about 80 cm-1 in liquid samples, but the gases did not show any such absorption bands.

E. Nitrobenzenea For nitrobenzene itself and its p-fluoro derivative the torsion is unfortunately

infrared-forbidden, although Raman-active. In nitrobenzene it has been assigned at 139 cm-l by STEPHENSON et al. [la]. This seems far too high in view of the masses involved and the values of the torsional frequencies in the benzaldehydes.

Ortho- and meta-fluoronitrobenzene were studied from 33 to 500 cm-l as vapors at lOO”C, but no torsional frequency could be assigned. Other absorptions are listed in Table 1.

[13] K. S. DEA.XI and J. B. STOTEERS, Can. J. C&m. 48,479 (1985). (141 R. N. JONES, W. F. FORBES and W. A. MTJELZER, Can. J. Chem. 86,504 (1957). [16] C. V. STEPHENSON, W. 5. WILCOX and W. C. COBURN, JR., Spectrwhim. Acta I?, 933 (1981).

Torsional frequencies in the far infrared-V 907

6. TEMPERATURE-DEPENDENCE STUDIES ON WFLTJOROBENZALDEHYDE IN THE MID-INFRARED

A. Introduction At the time the preceding experimental work was being done, we were unaware

of any other observation of distinguishable rotamers in aromatic aldehydes. (We have since learned of the work on furan-2-aldehyde.) We therefore sought other evidence to confirm the existence of two rotamers in equilibrium at observable concentrations. Since the torsional absorption of m-fluorobenzaldehyde seemed to give the best evidence for two rotamers, P ve concentrated on this compound.

t Fig. 3. Temperature-dependent infrared g bands in vn-fluoro-benzaldehyde, CS, %

solution. 0” a

1258

-60° 4% I 1250 13oc

The first step was to examine the C=O stretching region around 1700 cm-l, and indeed there are two bands there. This may mean two different rotctmers, but the second band may also be due to a sum tone, probably in Fermi resonance with the fundamental. To test the latter possibility, the bsnds were examined in a variety of solvents. The relative intensities of a Fermi resonance pair are often very sensitive to the solvent [16]. The pure liquid, and solutions in CS,, Ccl,, CHCl,, and CDCl, were used. The ratio of the peak intensity of the 1720 to the 1700 cm-1 band varied from 0.6 to 1.4. Thus there is a reversal of intensities, but the results are really inconclusive.

It was then decided that a temperature dependence study of the mid-infrared spectrum should be made. A preliminary survey showed that not only the 1700 pair, but also a pair near 1250 and 1260 cm-l, are temperature-sensitive. They are shown in Fig. 3. The latter pair is, in fact, better separated and more accurately measurable than the 1700 cm-l one. The relative intensities of its two bands invert in going from f27 to -60°C. For both pairs the tempersture dependence is completely reversible and reproducible. The lower-frequency member of each pair becomes relatively more intense as the temperature is lowered, showing that it belongs to the more stable rotamer (if this is indeed the origin of the pair).

[IS] af. the C=O doublet in cg(&pentanone. C. L. fh3ELL et a.!., Spectrochim. Acta 15, 926 (1969).

908 FOIL A. Mmraq W. G. FATELEY and R. E. WITKOWRXI

B. Experimktal procedure Measurements were made with a Beckman IR-9 instrument, using a spectral

split width (one-half the band pass) of 0.8 cm-i at 1200 cm-l and 1.2 cm-l at 1700 cm-l and a very slow scan speed. CDCI, was found preferable to CS, as a solvent. Measurements were made at seven temperatures between +28 and - 70% in a variable temperature liquid cell manufactured by the Limit Corp. Both band areas and peak intensities were used. The area measurements gave considerably more scatter due to the difficulty of correcting for band overlaps, and it was con- cluded that peak intensities gave better results.

C. Reeults The ratio of the peak intensities for a pair of bands was used as a measure of the

equilibrium constant for Rotamer 1 z Rotamer 2. The logarithm of this ratio was plotted against l/T, a straight line was drawn through the points, and AH was derived from its slope. The results for m-fluorobenzaldehyde are given in Table 6.

Table 6. AH for m-fluorobenzaldehyde

Band pair Solvent (cm+) (.i$OL3,

C% 1248 and 1258 570 f 100 CDCI, 1261 and 1261 450 f 100

1702 and 1718 760 f ?

In CDCla solution, AH is about 0.6 kcal/mole. The estimated accuracy of flO0 cal is only a guess. Differences of f20 cal can be obtained from the same data merely by making different choices for the straight line.

If the two pairs of bands are due to two rotamers in equilibrium, they should give the same AH. They do not; 460 and 760 caljmole are obtained in CDCl,, and are rather far apart. However the 1702-1718 cm-r pair overlap badly so that we do not know the accuracy of AH and cannot be sure that there is an inconsist- ency. At any rate there are two pairs of bands which do show a reversible temper- ature dependence. Fermi resonance is unlikely, and the most reasonable way to account for them is by an equilibrium between two forms. We suggest that these are the O-G& and the 0-trane rotamers.

From the potential curve (Fig. 2) it is seen that AH = V, (aside from a small correction for the difference in zero point energy). Vr, for 7n-fluorobenzaldehyde in the gas phase, is 1.4 kcallmole (Table 3). Better agreement with the solution value is not expected; AH’s often differ by a kcal in different phases [17].

Note added in proof

We have now measured the torsions in two deuterated benzaldehydes. The results are:

%Jrlion (@=) va (k&/mole)

Benzaldehyde 111 cm+ 4.92 Benzaldehyde-d, (CDO) 104.6 5.13 Benzaldehyde-d, 102 f2 4.99

[17] 1,2-diohloro- and 1,2dibromoethane provide good example& S. MIZUSHIMA, L%UC&W of MO&CU&S and Inted Rot&ion, p. 41. Academia Press, New York (1964).