Polarized infrared spectra

Journal of Molecular Elsevier

Scientific

POLARIZED

Structure,

Publishing

95 (1982)

Company,

INFRARED

KIVINEN

and MAR’ITI

Fundamental

in The Netherlands

vibrations

of some mono-

University

of Helsinki, Fabianinkatu

35,

RASANEN

Department of Physical Helsinki 17 (Finland) (Received

Printed

OVASKA

Division of Chemistry, School of Pharmacy, SF-001 70 Helsinki 17 (Finland) MARKKU

-

SPECTRA

Part 1. Stretched polymer method. and d&substituted benzenes

ANTI7

141-150

Amsterdam

5 March

Chemistry,

University

of Helsinki, Meritullinkatu

1 C, SF-001 70

1982)

ABSTRACT Polarized IR spectra of five benzene derivatives (four of C,, symmetry and one of Dzh symmetry) have been recorded using the stretched polymer method. It has been found that the values of the dichroic ratios depend systematically on the symmetry class of the vibration in question. The IR-active vibrations can thus be unambiguously divided into the correct symmetry classes. The general validity of the method, as well as its value for vibrational analysis, is discussed. INTRODUCTION

It is well known that the IR spectrum of a compound contains valuable information about the molecular structure. It is easy nowadays to produce high quality spectra of compounds in different phases. An unambiguous vibra. tional assignment is, however, often quite difficult even for relatively simple organic molecules and using all the available information. In certain cases normal coordinate analysis has proved to be valuable. Additional information can, at least in principle, be obtained from the polarized IR absorption spectra of oriented samples. Orientation can be achieved in a variety of ways. Uniaxial systems such as nematic crystals, liquid bilayers and stretched polymers can be used [l-4] . The stretched film method is especially suitable for studying polymers [ 5, 61. The degree of orientation varies depending on the method used. The molecular orientation in a single crystal is perfect. In the case of a single crystal, however, the perturbation caused by its surroundings changes the vibrational states of a single molecule drastically. It is also quite difficult to obtain single crystals thin enough for IR transmission spectra.

0022-28fjO/82/0000-0000/$02.75

0 1982

Elsevier

Scientific

Publishing

Company

142

The stretched polymer method has been much used in electronic spectro~. scopy for assigning electronic bands [ 3 and refs. cited therein] . Only in a very few studies has the method been used in the IR region [ 7-121. In polymeric substances the number of vibrational bands is enormous. In practice, however, the spectra may be relatively simple and have regions where the absorbance is sufficiently low that the polymer can be used as matrix material in IR studies. Polyethylene is especially suitable in this respect. The polarized spectrum of polyethylene is shown in Fig. 1. It is found that IR spectra of high quality can be obtained by dissolving the sample molecules into a polymer matrix and orienting them by stretching the film. The molecules used possess rather high symmetry (C,, and DZh in this study) and their vibrational spectra are well understood. The geometry of these derivatives can readily be changed by varying the substituents. In the present paper the results for some substituted benzene molecules obtained by using stretched polyethylene matrices are reported. EXPERIMENTAL

The following benzene derivatives were used: bromobenzene, 1,4-bromofluorobenzene, 1,4-bromochlorobenzene, 1,4-dibromobenzene and 1,4bromoiodobenzene. These compounds were of highest purity, obtained from Fluka AG, Switzerland, and were used without further purification. The chloroform used was of p.a. grade from E. Merck A. G., Darmstadt. Lowdensity polyethylene films of 0.20 and 0.50 mm in thickness were from Pekema Oy (Kulloo, Finland) and Etola Oy (Helsinki, Finland), respectively. The liquid compounds were introduced into the polymer sheets by soaking the film strips in the liquid at room temperature for a suitable time (from 5 min to about 20 h). The solid compounds were first dissolved in chloroform. After several experiments suitable concentrations were obtained. Usually five to seven spectra at different concentrations were recorded for every compound. Most spectra were recorded with the 0.20 mm thick sheet, the 0.50 mm thick polyethylene being used for the weakest bands.

4000

3000

2000

1600 WAVENUMBER

1200

800

400

(CM-‘)

Pig. 1. Polarized IR spectrum of an LDPE sheet of thickness 0.20 mm stretched by 300%: (-) electric vector of polarized incident radiation parallel to the direction of stretching; (---) electric vector perpendicular to the direction of stretching.

143

After introducing the sample into the polyethylene, the film was stretched by about 300% using a laboratory constructed stretching machine. The draw ratio was determined by measuring the distance between two ink marks printed on the sample before stretching and was defined as X(%) = lOO(Z - lo)/ lo (o/o), where 1, is the initial length between the ink marks (typically 1.0 cm) and 1 is the length after drawing (typically 4.0 cm). The stretched film was clamped onto a film holder, which was designed to fit the sample cell holder of the Beckmann VLT-2 variable-temperature unit. The IR spectra were recorded in the region 4000-200 cm-’ with a PerkinElmer 577 spectrophotometer. A scanning time of 60 min was used. All the spectra were recorded at liquid nitrogen temperature. This was necessary in order to prevent the evaporation of the sample out of the film. Sharp bands were also obtained due to the low temperature. The spectrometer was equipped with a wire grid polarizer mounted on a AgBr substrate. The stretching direction of the film was oriented at 45” with respect to the entrance slit of the spectrometer and the two spectra were recorded with the polarizer at 45” and 315” orientation as recommended in ref. 13. RESULTS

Results from dichroism measurements have usually been expressed either in terms of a dichroic ratio, R = AIL/Al, or the reduced dichroism AA/A = 3(A,, - Al)/(A,, + 2AJ, where AlI and Al are the absorbances measured with light polarized parallel and perpendicular respectively to the stretching direction [ 141. The dichroic ratio is used in the following. Asthe spectra were recorded on a transmission scale, the transmission values are first changed to absorbances in order to obtain the A,, and Al values. The experimental setup ensures that A ,, and Al refer to the same matrix area. Thus A ,, and Al refer to the same number of solute molecules, irrespective of spectral band pass (slit width) and deviations from uniform solute distribution. The absorbance values are, however, affected by the fact that the base line may vary to some extent with polarization (see Fig. 1). This effect was eliminated manually as far as possible. The origins of the orientation of molecules dissolved in aligned polymer matrices are still not fully understood [ 141. With larger molecules it is a general rule that the longest molecular dimension tends to become oriented parallel to the polymer chains. For “disk-like” molecules it has been suggested that all directions in the molecular plane are equally oriented [ 151. A molecular fixed coordinate system (x, y, z) as illustrated in Fig. 2 was used in this case, the z axis being the CZ symmetry axis. The relationship between the various symmetry species and the molecular axes is also illustrated. Except for 1,4-dibromobenzene, all the halogenobenzenes studied have C,, symmetry and the following representation: llA, + 3A, + 6B, + lo&, of which the A ,, B, and B2 modes are active in the IR. 1,4-Dibromobenzene belongs to the DZh point group and has the representation 6A, + B1, + 3&s

144

z

(A,;B,,) f

x

X

(B1;B,,) Fig. 2. Molecular fixed coordinate axes. On the axes are marked the symmetry which the respective coordinates belong in the C,, and Dzh point groups.

species to

+ 5B3, + 2A, 4 5B1, -I 5B2, + 3B,,. In this case only BIU, Blu and Bsu are IR-active. The experimental results are presented in Tables l-3. The numbering of the vibrations as well as the assignments are primarily those of Green [ 161~ It should be noted that the choice of the molecular axes differs from that used by Whiffen [17]. The R values given in parentheses in Table 1 are to be considered approximate. Usually these bands are weak and/or polyethylene absorbs in the same region. The dichroic ratios are in the range 1.7-3.0 for A, vibrations, 0.250.60 for B, vibrations and 0.75-0.95 for Bz modes. (The few exceptions which were noted are discussed below.) The dichroic ratio is thus closely related to the symmetry species of the vibration in question. Typical examples of the spectra are shown in Figs. 3(a)-(c). It is an important consequence of CZVand D Ih symmetry that it is possible to measure dichroic ratios for three perpendicularly polarized absorptions of a molecule. Unfortunately the y polarized vibrations in these molecules are usually very weak and difficult to measure. However, it is thought that, except for 1,4-bromoiodobenzene, the R, values are reasonably accurate. The three Ri values are not independent in a uniaxial orientation distribution but are given by the equation (see ref. 3) K,+K,+K,=l where Ki = Ri/(2 f Ri) i = X, y, z Table 3 shows the experimental Ki values and their sum, which is reasonably close to the theoretical value, i.e. unity.

145 TABLE

1

The observed dichroic ratios for the fundamental according to their symmetries symmetry

Vibrationb

vibrations

Wavenumber Iit.a (cm-l)

InLDPE (cm-')

of bromobenzenes

classified

Dkbroic ratioC

Average dichroic ratiod

1.78 1.75 1.78 (1.64) 0.517 0.461 0.507 (0.833)

Bromobenzene Al A1 A, -4, Bl B, B, B2

1021 669 314 903 681 458 615

670 314 902 681 454 610

1.73-1.86 (3) 1.71-1.77 (3) 1.66-1.91 (3) (1.64)(1) 0.463-0.559 (3) 0.4514.477 (3) 0.49M.530 (3) (0.833)(1)

1231 1156 1064 596 824 495 323 1400

1232 1155 1066 595 823 494 322 1401 1087 414

1.9s2.13 (3) 1.80--2.11(6) 1.99-2.16 (6) 1.75-2.08 (6) 0.3694.377 (2) 0.356-0.440 (6) 0.511-0.523 (2) 0.812--0.839 (2) 0.6944.718 (2) 0.86eO.968 (2)

2.07 1.91 2.06 1.97 0.373 0.399 0.517 0.826 0.706 0.914

1087

'18 "24

1010 496 812 480 1390

1091 1071 1012 498 813 477 1390

2.21-2.61 (5) 2.32-2.64 (5) 2.07-2.59 (5) 2.24-2.81 (4) 0.291-0.366 (4) 0.296-0.385 (4) 0.760--0.761 (2)

2.46 2.47 2.34 2.52 0.326 0.338 0.761

“20 “21 “22 “28 “29 “26

1003 424 807 473 1100

1072 1008 430 810 472 1102

2.26-3.01 (5) 2.2w2.55 (5) 2.253.06 (4) 0.233-0.387 (5) [email protected] (4) (0.707) (1)

2.50 2.39 2.69 0.308 0.333 (0.707)

“6 “8 “10 *16 “18 “27

1000 397 803 470 1100

1072 1002 401 805 467 1103

2.59-3.21 (6) 2.29-2.73 i3j 2.79-3.06 (2) 0.264-0.309 (6) 0.28S-O.383 (8) (1.28-1.35) (2)

2.94 2.58 2.93 0.292 0.339 (1.32)

“8 (b) “9 Cl-J) z::

(‘;:

;:;

y)

“19 & “29 (s)

1,4-Bromofluorobenzene A, A, -4, AI Bl B1 B, B* B2 B2

“5 *L5 “I “IO “16 *18 “19 “24 “17 "29

1 ,C-Bromochlorobenzene -4, A* A, AI B, B, B2

“6 “7 “8 “10 “16

1,4-Dibromobenzene BIU BIU B,U B 3u B 3u B XI 1,4-Bromoiodobenzene A, A* A, B, B, B2

aTaken from refs. 16 and 17. bWhiffen’s symbol is given in parentheses [ 171. ‘The number of measurements is given in parentheses. dValues given in parentheses are approximate.

146 TABLE

2

Average dichroic in Fig. 2

ratios

for different

symmetries.

Rx

___---_____-__~Bromobenzene 1,4-Bromofluorobenzene 1,4-Bromochlorobenzene 1,4-Dibromobenzene 1,4-Bromoiodobenzene TABLE

x, y and z refer to the directions

__I.

0.495 0.417 0.332 0.319 0.319

shown

RZ

RY (0.833) 0.815 0.761 (0.707) (1.32?)

1.77 1.99 2.44 2.52 2.84 -

3

The orientation

factors

for bromobenzenes

Bromobenzene 1,4-Bromofluorobenzene 1,4-Bromochlorobenzene 1,4-Dibromobenzene 1,4-Bromoiodobenzene

----__

KX

KY

I&

K,+ KY+ K,

0.198 0.173 0.142 0.138 0.138

0.294 0.290 0.276 0.261 (0.396)

0.469 0.499 0.550 0.558 0.587

0.962 0.961 0.968 0.956 (1.12)

DISCUSSION

As the spectra are recorded well below the melting points of the compounds, the question arises as to the state of the molecules dissolved in a polymer matrix. It is believed that the spectral data can be treated using the oriented gas model approximation. For instance, there is no sign of crystal field splitting. A typical example is given by Fig. 4, where the v19 band of bromobenzene is shown in different phases. The frequency values for the PE matrices are close to previously reported literature values (Table 1). The environment of molecules in polyethylene matrices is thus, to some extent, analogous to that in low-temperature inert gas matrices [ 181. Asymmetric

dihalogeno

benzenes

From a total of 30 fundamental frequencies of the asymmetric dihalogenobenzenes, 12 belong to carbon-hydrogen vibrations and are partly obscured by polyethylene absorptions. This is especially true in the case of CH stretchings, although CH bendings can usually be measured. Of the remaining vibrations some also occur at regions obscured by polyethylene or their intensity may be too low (at the concentrations used) for dichroic ratios to be measured. As stated above, the three A2 modes are not active in the IR. Table 1 shows that the number of absorptions for which it has been possible to measure the dichroic ratios, with a reasonable degree of accuracy, varies from six to ten for different compounds.

147

Fig. 3. (a) Polarized IR spectrum of bromobenzene in a stretched’0.20 mm thick LDPE sheet. The peak positions of the bands polarized parallel and perpendicular to the stretching direction are marked by I1and 1, respectively. The positions of the vibrations of bromo. benzene as given by Whiffen [ 171 are indicated by numbered arrows at the top of the spectrum. (b) Same as in (a) but with a 0.50 mm thick sheet and a greater concentration of bromohenzene. (c) Polarized spectrum of 1,4-bromofluorobenzene in a 0.20 mm thick sheet in the region 150@-1700 cm-‘. For 1,4-bromochlorobenzene, 1,4-bromofluorobenzene and 1,4-bromoiodobenzene the results in Table 1 are consistent with the assignments given by Green [ 161. In addition the following observations were made. For 1,4-bromofluorobenzene there are two bands in the region 15891599 cm-’ [see Fig. 3(c)]. The band at 1599 cm-’ is weak and has an R value of about 0.95 which indicates B2 symmetry, while that at 1589 cm-’ is medium strong and has an R value of about 1.7, common to A 1 bands. Green assigns the CC stretching vibrations v4 (A,) and ~23 (B2) a wavenumber of 1589 cm-l [ 161. According to our polarization measurements it seems reasonable to assign the 1599 cm-’ band to v23 and the 1589 cm-’ band to v4. The CH in-plane bending vibration of 1,4-bromofluorobenzene at about 1012 cm-’ givesa poorlyresolved doublet, with peaks at 1016 and 1012 cm-‘. The R value of this doublet is about 2, which is in accordance with the A, symmetry assignment (us).

Fig. 4. The v,~ iribration of bromobenzene: ethylene (at 77 K); (c) solid (at 77 K).

(a) liquid (at room

temperature);

(b) in poly-

Bromo benzene and 1,4-dibromobenzene In the case of 1,4dibromobenzene (D Zh symmetry) only 13 modes (5BlU, 5BzU and 3B,,) are IR-active (five of these are CH modes). It was possible to determine the dichroic ratio of eight of these. The results are consistent with the assignment given by Green [ 161. For bromobenzene the results obtained largely agree with Whiffen’s assignment [ 171. In addition to the absorptions given in Table 1 there are fairly strong bands at 1580 and 1070 cm-‘. The former, according to Whiffen, represents an overlapping of v4 and vz3 (h and I according to his notation) and the latter an overlapping of v7 and v2s (4 and d). Dichroic ratios of 1.59 and 1.79, respectively were obtained. This indicates that the AI vibrations predominate in both of these bands, but in the former the ~23 vibration of B2 symmetry also contributes to the R value. In the region 1150-1180 cm-’ there are two CH in-plane bending vibrations, one of AI and the other of B2 symmetry. In monosubstituted benzenes these are assigned so that the B, vibration appears at a lower wavenumber (e.g. Whiffen [ 171). The dichroic ratios of these bands are difficult to measure, partly because the bands are weak and partly because polyethylene has a weak but very dichroic band near 1175 cm-‘. It seems, however, that in bromobenzene the v27 band at 1159 cm-’ has an R value of close to 2, whereas the vg band at 1175 cm-’ has a considerably smaller R value. This contradicts the usual assignment. Another contradiction to Whiffen’s assignment is the band at 989 cm-l which should be a B, vibration (Whiffen’s j band). Although the band is quite weak, it clearly has positive dichroism, R about 1.8, which suggests that it is of AI symmetry [see Fig. 3(b)].

149

Dichroic

values

In Table 2 are listed the average values of R for the various symmetries of the bromobenzenes. It can be seen that as thepara substituent is changed from hydrogen to iodine the R, values increase and the R, values decrease quite monotonously. This is, of course, to be expected: the longer the molecule the better it tends to be oriented. An orientation triangle [ 31 is drawn in Fig. 5. Since the points are located relatively close to the lower side of the triangle (for which K, = K,), the orientation can be said to be mainly rod-like. On going from 1,4-bromoiodobenzene [point (5)] to bromobenzene [point (l)] the molecular orientation gradually diminishes. CONCLUSIONS

The results obtained in this study clearly indicate that the stretched polymer method is also applicable to the IR region. The values of the dichroic ratios are both accurate and reproducible enough to allow classification of the fundamental modes into the correct symmetry species. In many cases this could be the additional piece of information required for unambiguous assignment of vibrations. The most severe limitations of the method are the obscuring effect of the polyethylene vibrations and the low solubility of some compounds in PE. The former may be avoided, to some extent, by using deuterated samples or deuterated polyethylene. The low solubility may be overcome in some cases by using polymers other than PE. Work is in progress to investigate these factors.

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0 Kz

Fig. 5. The orientation triangle with the experimental (Ky, KZ) values: (I) bromobenzene, (2) 1,4-bromofluorobenzene, (3) 1,4-bromochlorobenzene, (4) 1,4-dibromobenzene, (5) 1,4-bromoiodobenzene. Point (5) is calculated from the experimental K, and Kz values.

150

In this study only the behaviour of the fundamental modes is considered. The stretched polymer method is, however, also suitable for studying overtone and combination bands, although a more sensitive spectrometer than presently at our disposal is required. At the time of writing we became aware of a new paper by Radziszewski and Michl [ 191. The principle used by these authors is analogous to that used in this study. Their results agree closely with ours and clearly indicate the importance of polarized IR data in spectral assignments. ACKNOWLEDGEMENT

This work was supported

by a grant from The Academy

of Finland.

REFERENCES 1 B. Norden, Appl. Spectrosc. Rev., 14 (1978) 157. 2 J. Michl and E. W. Thulstrup, J. Chem. Phys., 72 (1980) 3999. 3 E. W. Thulstrup, Aspects of the Linear and Magnetic Circular Dichroism of Planar Organic Molecules, Springer-Verlag, New York, 1980. 4 E. Meier, E. Sackmann and J. G. Grabmaier, Applications of Liquid Crystals, Springer Verlag, New York, 1975, p. 21. 5 R. Zbinden, Infrared Spectroscopy of High Polymers, Academic Press, New York, 1964. 6 B. Jasse and J. L. Koenig, J. Macromol. Sci., Chem., Cl7 (1979) 61. 7 J. Kern, Z. Naturforsch., Teil A, 17 (1962) 271. 8 H. Jakobi, A. Novak and K. Kuhn, Z. Electrochem., 66 (1962) 863. 9 T. Tsunoda and T. Yamaoka, J. Polym. Sci. Part A, 3 (1965) 3691. 10 N. S. Gangakhedkar, A. V. Namjoshi, P. S. Tamhane and N. K. Chaudhuri, J. Chem. Phys., 60 (1974) 2584. 11 R. T. IngwaIl, C. Gilon and M. Goodman, J. Am. Chem. Sot., 97 (1975) 4356. 12 J. Jo& and B. Norden, Spectrochim. Acta, Part A, 32 (1976) 427. 13 Wire grid polarizers and mounting kits for infrared spectrometers, Perkin-Elmer brochure No. L82, July, 1973. 14 A. Davidsson, Thesis, University of Lund, 1977. 15 B. Norden, Chem. Ser., 1 (1971) 145. 16 J. H. S. Green, Spectrochim. Acta, Part A, 26 (1970) 1503. 17 D. H. Whiffen, J. Chem. Sot., (1956) 1350. 18 W. J. Orville-Thomas, in A. J. Barnes, W. J. Orville-Thomas, A. MiiIIer and R. Gaufres (Eds.), Matrix Isolation Spectroscopy, D. Reidel Publishing Company, Dordrecht, 1981, p. 1. 19 J. G. Radziszewski and J. Michl, J. Phys. Chem., 85 (1981) 2934.