High-pressure infrared spectra of 1,4-benzoquinone and tetrafluoro-1,4-benzoquinone

SPECTROCHIMICA ACTA PART

ELSEVIER

A

Spectrochimica Acta Part A 52 (1996) 755-760

High-pressure infrared spectra of 1,4-benzoquinone and tetrafluoro- 1,4-benzoquinone Jianfang Wang, D.F.R. Gilson* Department ()[' Chemistry, McGill University, 801 Sherbrooke St., W. Montreal, Que. H3A 2K6, Canada

Received 7 August 1995; revision accepted 11 November 1995

Abstract

The pressure dependence of the infrared spectra (600-1800, 2500-3600 c m 1) has been measured for 1,4-benzoquinone(2,5-cyclohexadiene-l,4-dione) and fluoranil (2,3,5,6-tetrafluoro-2,5-cyclohexadiene-l,4-dione). No phase changes occur in either compound over the pressure range from ambient pressure to 50 kbar in benzoquinone and to 40 kbar in fluoranil. The pressure dependence of the C H stretching frequencies in benzoquinone appear to be normal but the C = C and C = O stretching modes show a much higher dependence on pressure in fluoranil than in benzoquinone. Keywords: Benzoquinone; Fluoranil; High pressure; Infrared

1. Introduction

The quinoid structure is important as an electron acceptor group, particularly for its involvement in various biological processes and in the formation of charge transfer complexes. The tetrahaloquinones, fluoranil and chloranil, are commonly used as dopants in organic conducting materials and complexes of chloranil can exhibit insulator-conductor transitions at low temperature. At room temperature, benzoquinone [1,2] and chloranil [3 6] are isomorphous, with monoclinic crystal structures, space group P21/a, with two molecules per unit cell. The fluoranil crystal structure is not isomorphous [7,8] but benzoquinone and fluoranil *Corresponding author.

both differ in behaviour from the tetrachloroanalogue for which a displacive phase transition occurs at 94 K [6,9 t2]. This transition involves a doubling of the c-axis and a change in space group from P2~/a to P2~/n [6]. No phase transitions have been reported for benzoquinone, where the crystal structures at room temperature and at 113 K are the same [1,2], or for fluoranil. Phase transitions in solids that may not be observable by lowering the temperature can sometimes be induced by the application of high pressure. The phase transition in chloranil has been investigated by following the behaviour of the low frequency R a m a n spectrum as a function of pressure [11] and in the present work we describe the effect of high pressure on the infrared spectra of benzoquinone and fluoranil.

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JianJang Wang, D.F.R. Gilson / Spectrochimica Acta Part A 52 (1996) 755 760

The vibrational spectra of benzoquinone, and of various isotopically substituted species, have been studied extensively [13-23] and there has been considerable uncertainty over the assignments of the vibrational frequencies. The more recent studies have focussed on the re-assignment of the vibrations in the Raman spectra [23] and on calculations of the vibrational spectra by ab initio methods [24-26]. The infrared and Raman spectra of the room temperature solid phases of chloranil and fluoranil have been reported and analysed by normal coordinate methods [27,28]. 2. Experimental Benzoquinone and fluoranil were obtained from Aldrich and purified by repeated sublimation under reduced pressure at room temperature. Infrared spectra were recorded on a Bruker IFS48 FT spectrometer at 1 cm 1 resolution with a liquid-nitrogen-cooled MCT-D326 detector. High pressure spectra were obtained using a High Pressure Diamond Optics (Tucson, Arizona) diamond anvil cell placed on the translation stage of a Bruker model 8590 microscope and aligned with the aid of a Sony video system. The antisymmetric N - O stretching mode of sodium nitrate (0.1% in NaBr) was used as the calibrant [29]. The samples, plus calibrant, were contained in stainless-steel gaskets of 200 /tm thickness and no pressure transmitting fluid was used. The spectra of benzoquinone were measured at pressures up to 50 kbar and up to 40 kbar for fluoranil. 3. Results and discussion The observed frequencies for benzoquinone and fluoranil, their assignments, and the pressure dependences are given in Tables 1 and 2, respectively. Figures 1 and 2 show the pressure dependence of the frequencies of selected peaks, including the C = C and C = O stetching vibrations. There were no clear breaks or changes in slope in the pressure dependences of any vibrations that would indicate the existence of a phase transition in either compound, and thus their behaviour is quite different from chloranil.

A factor group analysis for solid benzoquinone has been given by Lunelli and Pecile [19]. The molecular symmetry is approximately C2h (there is some distortion from true symmetry); the molecules occupy sites with C~ symmetry and each u-mode should be split into a doublet. The au modes are inactive in the free molecule but a peak appears in the solid state spectrum of benzoquinone at 964 cm-~ and is possibly the inactive au fundamental, v22. The crystal structure of fluoranil is monoclinic, space group P21/c, with two molecules per unit cell [7,8]; the molecular symmetry remains C2h (neglecting small distortions). The doublets caused by factor group splitting were also observed for fluoranil, but the splittings are smaller than those seen in the benzoquinone specTable 1 Pressure dependence of the infrared frequencies of benzoquinine Frequency/cm 746 835 869 905 946 964 1075 1086 1114 1182 1291 1307 1313 1345 1366 1386 1505 1508 1593 1601 1675 1679 2947 2965 2983 3018 3061 3074 3254 3307

~ dv/dP/(cm 0.40 0.03 0.13 0.21 0.12 0.19 - 0.07 ( 0.26 0.33 0.09 0.40 } 0.36 0.28 0.18 0.38 0.00 0.18 0.26 0.29 0.08 } 0.17 0.51 0.54 0.51 0.32 0.23 0.54 0.24 0.26

t kbar

I) Assignmentsa vl4, blu, C - C str

V28, b3u, C - H bend vl3, blu, C - C str v7, a u, C H bend v21, bzu, C H bend

V20, b2u, C - C str vi2, btu, C H bend

vt9, b2u, C-q2 str vii , blu , C-~) str

V~o, blu, C H str vls , b2u , C - H str

~Assignments based on Refs. [19] and [26].

Jiatllbng Wang, D.F.R. Gilson / Spectrochimica Acta Part A 52 (1996) 755 760

757

Table 2 Pressure dependence of the infrared frequencies of fluoranil Frequency,cm 632 737 993 1013 1046 1086 1099 1223 1290 1317 1333 1357 1387 1516 1648 1656 1676 1694 1744 1761 2707 3049 3380

~

dv/dP/(cm

I kbar

~)

0.13 - 0.03 0.21 0.15 0.33 0.05 0.41 0.15 0.59 -- b 0.38 0.56 0.47 -- 0.040 0.57 0.50 0.30 0.75 0.78 O.77 1.02 0.63

Assignment'~ v~, b~u, CCC b e n d + C yes, b3., CCO bend v21>, beu, C F str v~> blu, C C s t r + C

F str

F bend

v~9, C C str v~l, b~u, C F str

FR FR Vls, b2u, C C str v m, blL,+ C--O str

"Assignments from Refs. [26] and [27]. bShoulder, peak positions not accurate.

tra. The usual behaviour for factor group splittings is that the splitting increases with increasing pressure, but this effect is small in the spectra of fluoranil. A pair of lines occurs at 1648 and 1656 cm +~ in the infrared spectrum of fluoranil and, with increasing pressure, the higher frequency peak increases in intensity while the 1648 c m peak decreases until, at about 11 kbar, the latter has disappeared. This behaviour is typical of Fermi resonance [30]. The high frequency combination bands in the 2947-2983 cm-~ region in benzoquinone and above 2707 cm-J in fluoranil, generally show much higher dv/dP values, as expected. There has been some uncertainty over the assignment of the C = C and C = O stretching modes in the quinones although it is now clear that the carbonyl stretch is at higher frequency. Boesch and Wheeler [26] have compared the X-ray and electron diffraction molecular structures of benzoquinone, fluoranil, and chloranil with the results of different quantum chemical calculations and

discussed the trends in bond lengths and vibrational frequencies. The carbonyl stretching motion is much more localized than other vibrations, and thus serves as an indicator of bond strength. The increase in the C-~-O stretching frequency from benzoquinone to fluoranil parallels the decrease in bond length and indicates a stronger bond and higher force constant. The calculations indicated that the C = C bonds should be longer in the haloquinones, as a result of interactions with the p-orbitals of the substituents, but this is not well supported by the experimental values. In the present study we find that the major difference between benzoquinone and fluoranil is that the C = C and C ~ O stretching motions in fluoranil have much higher dv/dP values than in benzoquinone. Both normal coordinate and quantum calculations show that there is extensive mixing between the ring bending motion and C - F bending motions in fluoranil and thus comparisons of other dv/dP values for fluoranil and benzoquinone are not particularly useful. Instead we

JianJang Wang, D.F.R. Gilson / Spectrochimica Acta Part A 52 (1996) 755-760

758

can compare the values with related vibrations in other molecules. For comparisons between the pressure dependences of different frequencies, the logarithmic pressure dependence, din v/dP, can be used, since this is proportional to the mode Gruneisen parameter, 7~, via the equation 7~= - Vdv~/(v~dV) = - (l/tc,)(dln v~/dP)

180o

~o0

(1)

where V is the crystal volume and K~ is the isothermal compressibility. Since the quinone structure is a diene and is not aromatic, the values for dv/dP and d l n v / d P for the C = C stretching frequency in the quinones can be compared with those measured for norbornylene (NBE), norbornadiene (NBD), and bicyclooctene (BCOE), as examples of molecules with strained double bonds [31 33], see Table 3. In these compounds, the effects of differences in compressibility can be seen in the change from high 7 values in the more

~6oo "7 ~® e=

~soo

140o

i

1300

1700

12oo

I

I

I

I

I

I

I

5

10

15

20

25

30

35

Pressure, 1600

40

kbar

Fig. 2. Pressure dependence of the infrared peaks of ftuoranil for the region 1200-1800 cm - ~.

!

¢ ....

1500

•

..Q

E E >

1400

ee

1300

1200 0

I 5

I 10

I 15

I 20

I 25

Pressure,

I 30

I 35

i 40

I 45

50

kbar

Fig. 1. Pressure dependence of the infrared peaks of benzoquinone for the region 1200-1700 cm - i.

compressible disordered phases to lower values in the less compressible ordered phases. Clearly, the pressure dependence of the C = C stretching frequency in benzoquinone tends to be at the low end, and fluoranil at the high end of the range for these examples. Since C H stretching vibrations are anharmonic, they tend to exhibit high dv/dP values, particularly in the more compressible disordered phases. The dv/dP values of the infrared-active C - H stretching modes in benzoquinone are similar to those of the vinyl group C - H stretches in NBE, NBD and BCOE. The pressure dependence of the C - F stretching frequency in 1fluoroadamantane [34] is more than twice the value of the C - F stretch in floranil, but is not strictly comparable since sp 3 hybridization is involved in the adamantane derivative.

Jian/ang Wang, D.F.R. Gilson / Speetrochimica Aeta Part A 52 (1996) 755 760

759

Table 3 Comparisons of logarithmic pressure dependences v/cm i

dv/dP/(cm -I k b a r ')

dlnv/dP/(kbar

3062 3074 3059 3048 3064 3073 3102 3060 3069 3097 3042 3025 3028 3042

0.22 0.53 1.1 0.58 0.44 1.2 0.9 0.4 1.2 0.99 0.9 0.2 0.44 0.82

0.7 1.7 3.6 1.9 1.4 3.9 3.0 ~ 1.0~ 4.0 3.2" 3.0 0.7 1.4 2.7

1593 1676 1570 1569 1574 1560 1574 1618 1618

0.20 0.50 0.24 0.10 0.35 0.49 0.43 0.42 0.31

1.6 3.0 1.5 0.8 2.2 3.1 ~ 2.7 ~' 2.6 1.9

1676 1694

0.24 0.30

0.5 1.8

993 729

0.21 0.46

2.1 6.2

i x 104)

C H stretching j?equencies Benzoquinone NBE

phase I phase 11

NBD

phase I phase II

BCOE phase I phase II

C-C stretching jrequencies Benzoquinone Fluoranil NBE phase I phase II NBD phase l phase I1 BCOE phase 1 phase 11

C-O stretching jhequencies Benzoquinone Fluoranil

C F stretching JJ'equencies Fluoranil Fluoroadamantane ~Involved in Fermi resonance.

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