benzene

05x4~8539 7&0301 0343902 00.0

Spectrochlmlca Acta. Vol 34A. pp. 343 to 352 0 Pergamon Press Ltd 197X. Prmted m Great Bntam

Vibrational spectra and i.r. dicbroism of p-bis/dimetbylhydroxysilyl/benzene B. ZELEI, S. DOBOS Hungarian Academy of Sciences, Research Laboratory for Inorganic Chemistry, P.O.B.:H-1502 Budapest, 112. pf. 132, Hungary

and R. RIGHINI Institute

de Chimica Fisica, Laboratorio di Spettroscopia Molecolare, Universita di Firenze, Via G. Gapponi 9, Italy (Receioed

14 December 1976)

Abstract-Infrared spectra of p-bis/dimethylhydroxysilyl/benzene (PBDMHSB) were studied in solution, KI pellet and the oriented crystalline state. Raman spectra of this compound were measured in the solid phase and solution. An approximate assignment is given for the fundamental frequencies. Anomalous dichroism was observed for the CH out-of-plane vibrations of the benzene ring.

INTRODUCTION p-Bis/dimethylhydroxysilyl/benzene type silicon

compound, polymer,

the

monomer

at - 190°C the splittings are even more expressed. Two factors are known which may cause these splitis of the

a

disilanole-

tings in the i.r. spectra: the geometry of the three molecules located in the unit cell (their bond distances and angles) are slightly different from one side, and dynamic interaction may exist between their internal vibrations from the other one. In the first case the dichroic ratios calculated for the band components on the bases of the oriented gas model (that is for the three molecules separately) must coincide with the experimental values, while they could differ from each other in the second one. The dichroic ratios are to be studied in detail, in order to conclude which of the two factors is responsible for the splittings experimentally observed.

heat-resistant

poly/tetramethyl-p-silphenylene/silox-

Its preparation and specific properties have already been described [l-3]. In a previous paper [3] the empirical interpretation of the i.r. spectra was given. The aim of the present study is to supervise and complete the assignment formerly given and to conclude whether the oriented gas model can be satisfactorily used to explain the dichroic behaviour of the oriented crystalline PBDMHSB sample. According to the X-ray data [S], the silicon atoms are bonded to the benzene ring by single bonds without any double bond character (the mean value of the bond distance is 1.858 A) in this compound. For this reason, we can suppose that there is only a very low rotational energy barrier for the torsion of the dimethylhydroxysilyl group around the bond mentioned in the free state of the PBDMHSB molecule. So, in the vapour phase and nonpolar solvents the validity of the substituent C, and ring D,,, local symmetry is expected. However, the vapour spectra cannot be obtained because of the self condensation into oligomers above the melting point in vacuum. Spectra taken in nonpolar solvents prove the rightness of this supposition. On the other hand, spectroscopic data show the existence both free and associated molecules in polar solvents. Finally, in the solid state the oxygen atoms are moved out from the ring plane so that the molecular symmetry is lowered to Ci in the crystal: the unit cell contains three molecules which occupy three different sets of sites having Ci symmetry ane.

EXPERIMENTAL The model compound was prepared in the laboratory of the Hungarian authors according to [3]. Melting point: 138S”C. No allotropic transition was observed between - 190°C and the melting point. The i.r. spectra were registered on a Perkin-Elmer 225 type grating spectrophotometer from 4000 to 200 cm-‘. The frequency data are considered to be reliable better than 1 part in 2000. The solution spectra were run in a conventional liquid cell equipped with CsI windows. The solvents used were cycin spectrolo-&H, z > CS,, CHC13, Ccl, and (CH&CO scopic purity. The pathlength of the liquid cell was varied between 0.1 and 2.0 mm. The oriented polycrystalline films were grown from melt cooled between CsI or NaCl plates. In order to enhance the required orientation effect a temperature gradient was produced along the plates. The polarized i.r. spectra were measured using a PE AgBr grid polarizer. For the investigations at - 190°C a cryostate was used equipped with CsI windows. The Raman spectra were taken at room and liquid nitrogen temperature [12] on a Cary 81 type. Ar+ laser excited instrument (4880 A) at the University of Florence. The depolarization ratios were measured in CH,CN solution.

c51. In the solid phase spectra some of the bands are split into components at room temperature, however, 343

344

B. ZELEI,S. DOBOSand R. RIGHINI DISCUSSION

al.

[S]. The unit cell is triclinic, the space group is Pi, Ci. The dimensions are: a = 15.552 f 0.001 A, b = 10.251 k 0.0008 A, c = 6.457 f 0.001 A, c( = 83” 31’ + 0.5’, p = 97” 47’ k 0.8’, i’ = 92” 21’ _t 0.5’ and 2 = 3. The unit cell viewed from c* direction can be seen in Fig. 4. Three molecules, as the authors

If the kinetic interaction between the benzene ring and its substituents is ignored the ninety normal vibrations of the PBDMHSB molecule can be divided into the following symmetry species:

rring = 6Aa + Big + 3Baa + 5Bsg + 2A, t 5Biu + 5Ba, + 383, , * -Ly-/p. R.a. [15] i.a. [2] i.r.a. [ 131 r rubs,. = 17A’ + 13A” and they are active both in the i.r. and Raman spectra. The vibrational frequencies of the two substituents are expected to be very close, and they would perfectly coincide if the large and heavy silicon atoms could completely isolate them. The choice of the molecular axes can be seen in Fig. 1. VIBRATIONS

Raman-active

OF THE BENZENE

*

RING

modes

The A, vibrational modes are Raman spectrum, and the others expected region. The room temperature Raman powder like sample is shown in observed Raman-active frequencies Table 1. Infrared

Y(Bzu)

polarized appear

in the in the

Fig. 1. The choice of the axes of the PBDMHSB molecule.

spectrum of the Fig. 2, and the are collected in

uctive modes

The i.r. spectra of the pelleted and solved sample are shown in Fig. 3a, b, respectively. The assignment of the ir. bands to the crystal symmetry species has been made on the basis of the dichroism observed in the polarized light spectra and that of the crystal structure determined by ALEXANDER et

1

1600

know, are haidly ever found in a triclinic cell. This unusual structure is probably caused by the H-bond helix. Because the optical constants of this compound have not been determined so far, the dichroic ratios will be predicted according to the oriented gas approximation [6]. Using CsI plates two orientations can be obtained (named Face I and Face II) while between NaCl plates only the Face I grows. In the case of the Face I (see Fig. 5) the direction of the crystal growth is the shortest c axis, and the plane of oriented film coincides with the (110) or (100) crys-

/

1400

mm Frequency,

I

1000

803

cm-’

Fig. 2. The Raman spectrum of PBDMHSB in solid state

I

I

600

400

Vibrational spectra and i.r. dichroism

345

I00-

/ O. E I00-

§ I---

b 0

.ooo

3000

1400

1200 Frequency ,

i000

800

600

400

cm -I

Fig. 3. The i.r. spectra of P B D M H S B (a) in KI pellet (b) in solution of CC14, CS2, (CHa)2CO and cyclo-C6Hl2, respectively.

+ 1/2 0

I

I(O,O,I)

1/2)

C(5), I

d'

q I1

I I I

-I/21~ 0

O

I 0

II1(I/7, W2~I)

Fig. 4. The projection of the unit cell viewed along - c * axis from [5].

2C0

B.

346

ZELEI. S. DOBOS and

tallographic planes. In these two cases the calculated dichroic ratios are very similar in their values. The Face II (Fig. 6) would arise from the growth of the (111) plane, the mostly packed one, and the direction of the crystal growth coincides with the normal to the (111) plane, and the plane of the film so obtained is the (112) one. These inferences were supported by some elementary calculations, based on the oriented gas model according to the results. of HERRANZ and DELGAW [7]. The predicted dichroic ratios for the benzene ring vibrations are shown in Table 2. According to the predictions the main difference between these two orientations lies in the polarization of the BZUbands. In the spectrum of the Face I these bands are polarized perpendicularly, while in that of the Face II are practically depolarized. The B1, modes are expected to give perpendicular, and the BSu ones parallel bands in both orientations. The dichroic ratios, calculated for the three molecules of the unit cell separately, are different or even opposite. The spectroscopic data show that the ring CH

R.

RIGHINI

modes are split not only at liquid nitrogen, but at room temperature, too. Their components do not show the dichroism calculated for the three molecules separately. Similar anomalies have been observed by several authors so far. First, LA LAU [S] noticed two bands between 850 and 770cm-’ in the ir. spectra of p-dialkylbenzenes with long side chains, and assigned them to the umbrella mode of the benzene ring. He invoked the existence of a rotational isomerism, since he found the total intensity to be rigorously constant. Further evidence was provided by the fact that there is only one diffuse band in the liquid spectrum of p-di/hexadecyl/benzene, which by cooling suddenly divides into three sharp bands at 830, 805 and 780cm-‘. Lately, however, this phenomenon has been observed also in the i.r. spectra of p-dichlorobenzene, p-benzoquinone and fluorene [4,9, lo]. It is clear, that in all the mentioned cases the rotational isomerism cannot be responsible for the observed splittings. The authors have given different explanations. GREEN [4] regarded the exceeding Table I. Vibrational frequencies of p-W

SOlUll0ll

RaIlWn Powder

Inkared Solution

KI pellet

Face I

3240 us

3240 sh 3180”s

Dichr.

Face II

Dlchr.

3270 IS

1

313011s

II

3689 us

3llOsh 3052 w 3048 w-m

3048m

3052 w 3048w

2998 nv

2998 nw

3033 sh 3003 “W 2998 w

2957 m 2898 w

2955 m 2898 uw

2955 m-s 2898 w 2862 BW 1923 w

1919 w 1820 uw I662 uw

1908 uw 1820 uw 1662~

3033 m

2965 sh 2958 s 2898 us

1608 LW 1588s.~

IWVW I399 VW

I307 “W

1307 “W

1258 w

1254 w 1251 w w

1203w.p

Io!Nvs.p

1488~ 1437 lw

1395 w 1376 w-m

1394w 1376 m

1348~ 1324~

1346w 1327 w

1303w

I303 “W

2955 s 2898 w

comb. comb. comb.

1556~

I

1488 w 1443 w I408 sh 1396~ I380 m-s 1360~

I 1 I 1 < II I I

1332~

I

1305 VW 1259 us 1249 sh

i

1249 vs

1202 “W

I2cQ”W

1207~

1136~s IllOsh

1134us 1114sh

ll44sh 1138s 1135sh 1116sh

1144s 1140s ll35sh lll4sh

lO88sh IOtQm 1018~

109Om

I

1203 w

1088 “S

1013 Iv 970 VW

872 us

9oow 884 us 872 us

9M VW 883 us 872 VW

1488 w 1443 w 1407sh 1398 w 1378s 1369 w l35Ow 1328 VW

1252 L’S

II

872 uw

II i I

B,. R,. B;i

VI** BJ, ~2. A, comb. B,. comb.

I306 uw 1259 sh 1252 “S 1249 sh 1214~ 1209w

1092vs 1055 s 1018 w 971 w

A

2998 m

I524 uw 1488 VW 1437 ow

km,,.

3050m

1611 VW 1589 s 1554 VW

,524 uw

I

Approxlmale description

VIP%6,. 1’,6.,,. A’. A” “,..,p. A’. A”

• 347

Vibrational spectra and i.r. dichroism bands as combinations. According to LUNELLI and PECILE[9] they are caused by optical anomalies in the i.r. spectra of p-benzoquinone single crystal. The KRUSE'S [11] interpretation given for the polarized i.r. spectra of p-dichlorobenzene single crystal seems to be the most acceptable one. He assumed these bands to be induced by a strong interaction between the internal CH out-of-plane modes of the neighbouring molecules in the crystal. Really, we can assume that the interacting potential field influences mainly the vibrations of the light hydrogen atoms located at the periphery of the molecules. These splittings are not accounted for by either the site or the factor group symmetry. In the high resolution spectra, however, the splittings of the ring CC modes can also be observed. As an example in Fig. 7 the shape of the v CC, 19a band is shown as a funtion of the position of the polarizer. According to the calculated dichroic ratios for the three molecules one by one, the B1, bands are expected to be perpendicularly polarized and the components of the 19a band do not behave

so. Its splitting into components is probably caused by dynamic interaction existing in the unit cell and is responsible for the apparent shift of this band turning round the polarizer. The anomalous dichroism of the CH out-of-plane modes was observed in the spectrum of the orthorhombic fluorene, so it can not be the peculiarity of the triclinic cell. The fact that the benzene ring vibration of the p-bis/trimethylsilyl/benzene are not split in the polarized spectrum [13] reveals the role of the silanole groups in this phenomenon, but the exact reason is' not known. The fundamental frequencies of the benzene ring are collected in Table 3. VIBRATIONS

OF

THE

SUBSTITUENT

GROUPS

The assignment of these modes in the Raman spectrum is rather straightforward and does not require

dimethylhydroxysilyl/benzene in cmSolution

Raman Powder

Solution

Infrared

845w 836 w

836 w

835 sh 818 vs

K1 pellet

Face I

Oichr.

822 vs

835 sh 828 sh 821 vs 808 sh

il [I I[ L !!

818sh 803 vw 779 w 738 w, p 716vw 691 w

803 v w 780 w '

va6.,~. .4'. A'"

V28. B3.

"

808 sh

V4a, A'? VT, BSt,

"~'49, A"

!l? I!

785 sh 772 vs 738 sh

735 sh

735 vw

734 sh

I!

734 sh 713vw 690 m-s

£'[ 3_f

667 sh 663 s 661 sh 658 sh

± -> J I } l ± L

686 w

694 w

660 s

704m 687 w 668 sh 663 sh 661 s 658 sh

l 1] [I !l II 3_

502 m 497 s

498 m

VSO, A"

± ± -~ !I'[ 41 S

3_

(

II

468 s

±

472 s 413w 409 w

351 w

348ra

351 s

k

357s

±

298vw

296s

310sh 303 m 297 s 269 sh 258 s 256 s 218w 216m 212w 206w 204 vw 202 w

315m 307 m 294 m 269 w 265 s 254 s 223w 218 w 216w 206w 204 w 202 w

i;) ± '! ?

257 m 246 w 220 w

221 w

216w

202 w

202 vw

204 vw

v52, A'

vt++, B~f Y29. B3u

\

495s

254- w

VSI, A"

502 s

468 s

± -~ ]1 ± ~! !P ± ± II ± ± = II ± II ±

comb.? Vg, B2g

1

468 s

270 w

183 w 160w 145 w 120w 109w 92 w 74 w 40w

i} "

785 sh 767 vs

649 vs 635 m

375 w 355 w 330 w

850sh 836 sh 828 vs 822 sh

Approximate description

762 vs

661 VS

648 vs, p 634 w

Dichr.

769 vs 738 w 717w 688 w

Face II

V22, Blu v17, Aa v6, A~ vss, A' rts, B3~

]

£±

v54.ss. A }

~- !i it ~''~

vlo, B2g?

VSt,. A" VST. A"

t%s, A" CH3 torsions and ]altice modes

9. ZELEI.S. DOBOS and R. RIGHINI

348

IO” / I

0: 3600

3000

1500

1300

Frequency,

cm-’

0 950

900

850

800

700

750

650

500

Freauencv. cm-’

-IL-

0

I

I

360

3ko

3AO

3;o

260

3bo Frequency,

240

220

cm”

Fig. 5. The polarized i.r. spectra of the Face 1 (110) plane. The spectrum taken by light polarized parallel to the crystallographic c axis with broken line (I[ spectrum) and that by light polarized perpendicularly to the former direction with continuous line (I spectrum) are illustrated.

450

Vibrational

spectra

349

and i.r. dichroism

, 3000

3500

Frequency,

cm“

100

I’

,*-\

,’

‘\\\ :

/

,

/

1

0 950

650

900

750

600 Frequency,

700

650

450

500

cm _’

100

r

360

340

320 Frequency,

300

260

240

220

cm -’

Fig. 6. The polarized i.r. spectra of the Face II, (112) plane. The spectrum taken by light polarized parallel to the normal of the (111) plane with broken line (11 spectrum), and that by light polarized perpendicularly to the former direction with continuous line (I spectrum) are illustrated.

350

B. ZELEI, S.

DOBOS and

Table 2. The angles made by the transition dipole moments light in the II and I directions and the predicted dichroic

Face I

R. RIGHINI

of the vibrations with the electric vector of the polarized ratios for the Face I/(1 IO) plane/and Face II/(ll?) plane! Molecules I

I

II

111

56.42 0.3058

54.90 0.3306

I 17.07 0.2070

136.63 0.5284

49.85 0.4157

39.19 okQO7

0.5787

0.7955 0.5462

0.3446

&”

cd the un,t cell II 111

&.

R

II

III

91.14 O.OW4

96.74 0.0138

87.94 00013

32.38 0.7133

81 65 0.02

26 76 0.7973

120.32 0.2549

123.47 0.3041

62.62 0.21 IS

63.04 0 2056

154.92 0.8203

114.40 0. I707

0.0016

0.0454 0.0201

0.0062

3.4698

11.01.~7 ixii

4.6707

t,,SiC2

R*

I

II

\‘ssIc>

89.32 O.ooOl

92.04 OSQI 3

94.45 O.W60

34.49 0.6793

140.59 0.5969

31.62 0.7251

61.05 0.2343

56.1 0.3109

I

61.31 0.2304

141.7 0.6159

102.73 0.0485

30.28 0.7457

70.82 0.1080

50.80 0.3994

101 55 0.0401

89.21 o.Oca2

43 82 0.5206

92.03 0.0013

0.0002

0.0268 0.0.x3

0.008

6.289

I.494 3.655

IS.083

I250

I).iY7,, 1485

181.8

81.13 0.0238

128.90 0.3943

59.65 0.2554

126.33 0.3510

55.63 0.3187

32.32 0.7141

69.13 01270

34.94 0 6720

3.4734

Ol’4X EGG

3.9370

I B,.

&.

The same as in the case of Face I

0.07 I 7

0.0572 0.2952

1.0019

0.65 I9

y%&

1.5069

v,SiC, 67.44 0.1472

128.10 0 3808

74.65 0.0701

v,SiC, 20.71 0.8749

I3075 0.4262

13.77 0.9434

98.20 0.0204

46.18 0.4794

97.81 00184

1.0671 4.lLmO

23.529

102.04

0 YJ,,X 0.993

14.599

The same as in the case of Face I

R*

R

0.2390

7.U4JI 0.4239

0.0940

8.1037

R* = cos2y/cosz~ or cos2y~/cos2~ are the dichroic ratios for the molecules of the unit cell separately and for the Face I andzll, respective+ R = cos y/cos q5 or CDSy,.Js are the dichroic ratios for the unit cell according to the oriented gas model and for the Face I and Face II, respectively. The underlined R* values indicate that the molecule signed shows dichroism differing from the dichroism of the

other two ones and from that of the whole unit cell. y and yN are the angles made by the direction of the transition dipole moments and that of the crystal growth. C#Jis the angle made by the transition dipole moments and the experimental I direction, the latter is the same for both orientations (the nernendicular direction nearly coincides with the direction of the line going through the la and lb crystallographicai points).

further discussion: we only notice that the most intense and polarized band originates from the in-phase coupling of the irs SiCZ modes, and its frequency is lower than that of the corresponding out-of-phase i.r. active mode. Dichroic ratio values were only calculated for the skeletal stretching modes of the substituents (Table 2). Regarding the molecules of the unit cell separately, we expect in the Face1 three perpendicular, while in the Face II one parallel and two perpendicular v SiO band components, but one parallel component appears in the FaceI, too. The substituent skeletal bending modes are heavily mixed with each other and with the stretching modes. It is worth mentioning that the direction of the resultant of the transition moments of the 6 OSiC modes coincides with the perpendicular direction in both orientations, and so this mode gives perpendicular band in the spectra. The other deformational modes are split into three or more components due to the

crystal field effect, as well as to the difference in the geometry of the substituents. The fundamental frequencies can be found in Table 3. CONCLUSION

The following conclusions may be drawn for the present study: Because in the D2,, point group the two A, (out-of-plane) modes are forbidden, their appearance in the i.r. spectrum refers to the distortion of the benzene ring. As a consequence of the undistorted D,, ring symmetry, these two A, modes were not observed in the solution spectra of PBDMHSB, but in the solid phase spectra they appear due to the lowering in the ring symmetry and ‘in accordance with the X-ray data [5]. In spite of the fact that the symmetry of the benzene ring does not remain D,,, in the solid state, the dichroic behaviour of the i.r. active ring funda-

Vibrational spectra and i.r. dichroism

351

Table 3. Fundamental frequencies of p-bis/dimethylhydroxysilyl/benzene (a) Vibratronal Symmetry C,

modes

of the benzene f&

ring* No

Raman

lnlrared

3052 1589 1203 1088 738 375

(lOa) CH out-of-plane

930 717 220

(51 CH out-of-plane (4) CCC out-of-plane, (lObI CSi out-&plane.

puckering out-of-phase

17b) CH stretching (8b) CC stretching (3) CH in-plane bendmg’ 16b) CCC m-plane bending (9b) CSi in-plane bending.

in-phase

(17~~) CH out-of-plane (16~~) CCC out-ol-plane

2998 1488 1134 1013 468

(13) CH stretching 119al CC stretchinp (12) CCC in-plane-bending (Igal CH m-plane bendmg (2Oa) CSI stretching. out-&phase

3048 1376 1200 III4 296

(2Ob) CH (19bl CC (141 CCC (IBbl CH (IS) CSI phase

822 497 _

of the substituent

description

803

975 413/409t

frequencies

Approximate

(2) CH stretchmg in-phase (8a) CC stretchina (9a) CH in-planebendmg II) breathmg (6a) CCC m-plane bending 17a) CSi stretching. m-phase

3033 1611 1307 634 330

(b) Vibrational

in cm-’

stretchmg stretchrw stretch,“;. Kekule m-plane bending in-plane bending

(l7bl CH out-of-plane. umbrella (16b) CCC out-ol-plane I CSi out-of-plane. in-phase

(I )

groups* Infrared A

Symmetry

A”

3240 2955 2898 1437 I394 I249

2955 2898 1437 1394 1249

1060 872 835

835 76217793 694

66w49: 348 257 204 183: 160:

_ * Solid values. t Data from the polarized spectra. $ Raman data.

out-ol-

Approximate

description

OH stretching (associated) CH, awn. stretchina CH; s&t. stretchingCH, asym. deformation CH, asym. deformation CH, oym. deformation SiOH in-plane SiO stretching CH, rocking

dcfonnation

SIC1 asym. stretching SiOH out-of-plane deformation SIC2 sym. str&ching CSiO deformation Sic* sym. deformation CSiO SIC, Sic2 CH, CH,

asym. deformation asym. deformation delormation (torsion) torsion torsion

B. ZELEI. S. DOBOS and R. RIGHINI

352

gas model for D,, ring symmetry. Some of the CH out-of-plane modes show anomalous dichroism. The splittings of both the benzene ring and substituent vibrations may be observed, however, these are more expressed in the case of the substituent modes.

100

Acknowledgement-The authors would like to thank Prof. B. LENGYEL for his keen interest in this topic. One of us. S. Doeos, thanks Prof. S. CAL~FANOfor the kind hospitality at the University of Florence.

o\” i13

REFERENCES

t

E

6 f

0 3

Id90

Frequency, cm-’ Fig. 7. The crystal splitting of the Y CC, 19a band in the Face 1. The states of the polarizer were : --- 45”. parallel, -.-.-75”. . . ..lOS’. -135”, perpendicular and -.-165”, respectively.

mentals

dichroich

can be qualitatively characterised by the ratios expected on the basis of the oriented

[1] R. L. MERKER and M. J. SCOTT, J. Polymer Sci. 2A, 15 (1964). [Z] V. E. NIKITENKOV, Zhurn. Obsctch. C’hem. 9, 1666 (1965). [3] B. LENGYEL, S. DOBOS and B. ZELEI, Acta Chim. Sci. Hung. 47, 109 (1970). [4] J. H. S. GREEN,Specrrochim. Acta 26A, 1503 (1970). [5] L. E. ALEXANDER, M. G. NOR~HOLT and R. ENGMAN, J. Phys. Chem. 71, 4298 (1967). [6] J. W. ROCHLEDER and T. LUTY, Mol. Crysr. 5, 145 (1968). [7] J. HERRANZ and J. M. DELGADO. Spectrochim. Acra 31A, 1255 (1975). [S] C. LA LAU, J. Phys. Rad. 15, 623 (1954). [9] B. LUNELLI and C. PECILE, Spectrochim. Acta 29A. 1989 (1973). [lo] K. WITT, Specrrochim. Acta 24A, 1115 (1968). [11] K. M. M. KRUSE, Spectrochim. Acta 26A, 1603 (1969). [12] E. CASTELL~CCL Specrrochim. Acla 29A. 1217 (1973). [13] S. DOBOS, A. SZAB~ and B. ZELEI, Specrrochim. Acra 32A, 1393 (1976).