Infrared spectra and normal coordinate analysis of tetra-atomic sulfur and selenium halides

Jottrnal of Molecthr Structttre

449

Elsevier Publishing Company, Amsterdam. Printed in the Netherlands.

INFRARED SPECTRA AND NORMAL COORDINATE TETRA-ATOMIC SULFUR AND SELENIUM HALIDES

ANALYSIS

OF

ROBJZRTOFORNERfS** AND CURT E. HENNIES

Imtimte of Aeromwtical

Technology,

S. Jose dos Campus, S.P. (Braziil

(Received August 12th, 1969)

ABSTRACT

The infrared spectra (220-700 cm-- ‘) of S&I,, S,Br, , Se&l, and Se,&, , and the Raman spectrum of S,BrCI, are reported. Assignments proposed for C, symmetry are supported by a normaI coordinate analysis using Urey-Bradley and Valence Force fields, Sets of force constants obtained by a least squares adjusting procedure are given,

INTRODUCTION

The Raman spectra of S2C12, SZBr2, Se,Cl, and SezBr, were obtained by Stammreich and Fornerisl in the middle 1950’s. Previous spectroscopic work, which included only S&It, was summarized there. The results of these studies were not sufficient to establish whether these molecules have a planar (CZV) or nonplanar (CZ) structure because of uncertainties in the observations of the Raman polarizations. X-ray and electron diffraction resuits, available at the time only for SZCIZ, were also inconclusive. Later, an electron diffraction study of SZCIZ and S,BrZ by Hirota showed that these molecules have a C, structure with an azimuthal angle of approximately 83”. Using the spectroscopic data of Stammreich and Fomeris, Hirota carried out a force constant calculation and suggested a different assignment for the observed frequencies. The far infrared spectrum of S2Br2 was obtained by Bradley et aL3. These authors proposed a still different interpretation for the observed spectrum of this molecuIe. Some time ago one of the present authors4 studied the infrared spectra of the sulfur and selenium halides out to 45 fr. The fundamental bands that fall beyond this wavelength were thus not observed. * Part of a thesis presented by_C.E. Hen&s to the Department of Physics, LA-T., for the MSc. degree (Feb. 1967). ** Present addresses: Department of Chemistry, University of Pittsburgh, Pittsburgh, Pa. C. E. Pennies, Dept. de Fisica, Fat, de Filosotia, Ciencias e Letras, Univ. de S. Paula, S.P., Brazil. .i. h-.foi.Sirucrure, 5

(i97ti) 449460

450

R. FORNERIS,

C. E. NENNIES

Although these incomplete results were thought at the time not to be worth publishing they also showed that the assignment proposed by Stsmmreich and Forneris needed to be revised. Renewed interest in these molecules is shown by recent infrared and Raman studies6’*. Very complete spectral results and detailed discussion of assignments supporting the C, symmetry -were given by Hendra and Park6 and by Frankiss’. Late in 1966, when computing facihties became available to us, a complete normal coordinate analysis of the vibrational spectra of the sulfur and selenium halides was carried out. As a result, modifications of the former assignments were proposed. They pointed, in agreement with the newer results of Hendra and Park and of Frankiss, to C, symmetry for these molecules. This paper presents our normal coordinate analysis. Our infrared results are also given, since some additional features observed in our spectra were not previously reported. A Raman spectrum of &BrCl is also given in spite of the fact that only four out of six expected bands were observed. We believe it worth reporting since our calculated values agree very well with the observed ones and, besides, an explanation is proposed for the non-observation of the two missing bands.

EXPERIWNTAL

PROCEDURES

AND

RESULTS

Infrared absorption spectra were obtained between 220 and 700 cm-l using two Perkin-Elmer spectrometers: a double beam instrument with CsBr optics and a single beam instrument with Csf optics. The molecules were studied as pure liquids and in CS2 solutions. The preparative methods were the same as previously describedr. Special cells were constructed using polyethylene film. To avoid buckling of the polyethylene the cells were pressed between CsI plates. Raman spectra of equimolar liquid mixtures of SzClz and S2Br, were obtained with helium 6678 A excitation using the helium technique9 and a grating spectrograph. Four new bands were observed (in addition to those of SzClz and S,Br,) and are attributed to the &&Cl moIecule. These bands are listed in Table 6Infrared frequencies are accurate to +2 cm- ’ and Raman frequencies to +3 cm-l. Table 1 lists the observed infrared frequencies together with the interpretation for C, symmetry. The results agree well with those of Hendra and Park6 and of Frankiss’. Some additional features in our spectra, but not reported by these authors, are the following. A splitting of the bands assigned to the v, and v5 modes of S,Brz was observed at 351 and 365 cm -r for the pure liquid. An analogous splitting was also observed by Frankiss’ in diluted solutions in benzene, For the selenium compounds the corresponding bands were not resolved. Bands interpreted as 2v, were observed for all molecules. As pointed out by Hendra and Park6, the corresponding 2v, bands for the selenium compounds might coincide with imf. Moi. Sfrztcfure, 5 (f970) 449-460

SULFUR TABLE

AND

SELENIUM

451

MONOHALIDES

1

O~~WED

mmt~mo

237 414 436 448 536

sPEcTRA

(22%700

&Brz

se2 Cl2

340 351 365 sh 529

252 355 355 286 575 700

cm-‘)

0~ SULFUR

Se2 Br2

AND

SELENIUM MO~~OHALID=

Interpretation 1’6 2V3

(204) 260 260 286

VS v2

2;: 2y’s

B From Raman spectrum,ref. 1. purity bands. Two harmonics (interpreted as 2v, and 2v,) were observed in the infrared for Se&I,. The infrared band reported by Bradley et al. 3*8 for S2Br2 at 302 cm-l was not observed. This was interpreted as due to Br, 5 and agrees with the recent results of Hendra and Park6 and of Frankiss7. A corresponding band observed in the Ramau spectrum of S2Br, by Stammreich and Forneris’ and by Bradfey et al.’ was also shown by these authors’-’ to be due to dissolved bromne.

ASSIGNMENT

Our assignment of the fundamentals, based on C, symmetry, is shown in Table 2 and agrees with that of Hendra and Park6 and of Frankiss7. For an XZYZ molecule of C, symmetry we expect six modes of vibration, four of class A, polarized in the Raman, and two of class B, depolarized. A11 are infrared active. The numbering of the fundamentals folIows Herzberg I*. The values given in this table TABLE

2

FuNDAMENTAL

Species

VIB~~ONALF~~~CI~OFSUL~RA~S~~N~UMMOND~AL~D~

Vibration

(Cm-')

S2Cf2

&h

Se&&

Se, Brz

Description

540 449 206

529 365 172

288 367 130

292 265 107”

102

66

87

50

436 240

351 200

367 146

265 118’

X-X stretch X-Y stretch Y-X-X bend + torsion torsion+Y-X-X bend X-Y stretch Y-X-X bend

(CT21

A

B

a From ref. 6. L Mol. Structure, S (1970) 449460

452

R. FORNERIS,

C. E. HENNIES

are ?rom Stammreich and Forneris’ and from Table 1, except two bands of Se,Br, that are from Hendra and Park6. The assignment has been discussed in great detail by these authors and by Frankiss and need not be repeated. Our normal coordinate analysis supports it entirely.

NORMAL

COORDINATE

ANALYSIS

Morino and Mizushima” were the first to present a set of calculated force constants for S,CI, . They employed a quadratic potential function with five force constants, independent of the azimuthal angle, and obtained an expression for the symmetrical frequencies that did not include the torsional vibration. Since their assignment of the fundamentals was very different from that of the present paper, their results will not be discussed further. Other calculations were carried out for &Cl, and S2Br, by Hirota’ using the Raman frequencies of Stammreich and Fornerisl and assuming a potential function of the Urey-Bradley type. We were able to reproduce Hirota’s results almost exactly. However, our present assignment differs from the one used by Hirota since v2 and v5 differ numerically for S2Br2, and are reversed for S2C12. To our knowledge, no force constant caiculations have been published for the selenium monohalides. We employed the F-G matrix method of Wilson” for our calculations, using two different potential energy functions_ Refined sets of force constants for S2C12 and S,Br2 were computed by a Ieast squares procedure as explained by Mann et aLI3 and by de Altr-I4 . Relative amplitudes of vibration were also calculated for each normal vibration for all molecules.

Our internal coordinates

Fig.

1. Internal

J. Mol.

coordinates

Structure, 5 (1970)

of X,Y,. 449-460

were taken as the infinitesimal

changes in the mol-

SULFUR

AND

SELENIUM

453

MONOHALIDES

ecular parameters shown in Fig. 1. The bond lengths and angles used are given in

Table 3. The symmetry coordinates are the folIowing: TABLE

3

GEOMETRICAL

PARAMETERS

Molecule

Pammeter rl, r2 CA>

SzCf2

2.07 2.24 2.24,2.07 2.13 2.24

=

SzBr2 = S2BrCI b SeXI, c Se2Br2 ’

1.97 1.97 1.97 2.28 2.28

106” IO6 106 106

83” 83 83 83 83

106

n Ref. 2. b Same parameters as for sulfur choride and bromide. c interatomic distances are the sum of the covalent radii; angles are the same as for the sulfur monohalides.

S1

=

s2

=

s3

=

AR ‘A@, 42 +W~

S, = AT -w$

s5

=

+42)

&

=

-I=A(rr -r2) 42 +4,

-42)

The most general potential energy function for this type of molecule of C2 symmetry contains 13 force constants and is given by:

2V = KI(AR)2+K2(rA~)2-t-K~((Ar,)2+(Ar2)2)+ K4 ((rA#p,)” + (rW2)‘) -I- 2K4AfWrt

+ Ad +

2K~l,(AR)(rAz)+2K,(AR)((rA~~)+ Wbrp2))+ 2K8(W(Arl + A&--2K$ W&NW+ (Wd@dH2K,,((rA~‘r)(Ar2)+(rAcjb2)(Ar,))+2K~t(Ar1)(Ar2)+ 2Kt2(rA~t)(rA~2)+2K~3(rA~)((rAdji)+(rA~2)~The force constants K&._, are the principal ones. KS and KS are related to the stretching between bondkd atoms, and K2 and KG with the bending of the angles.

The remaining K’s are interaction force constants. Since only six fundamental frequencies are available for each molecuie, only two of the interaction force constants K, _ I s can be determined from these calculations. As we will show below, we have attempted to estabiish which of these interaction constants is more significant in these molecules. J.

Mol.

Structure,

5 (1970) 449-460

454

R. FORNERIS,

A Urey-Bradley Tts complete form is:

C. E. HENNIES

type of potential function was also used in our calculations.

2V = (fil,t-2t~F$_2s~F)(AR)2+(K~r~~F~~~1F)~(Ar,)2(Ar~)2)+ + (H-S~S,E;‘+~,~,F)((~,A#,)~-(~,A#~)~)-~-~~(V,A~)~~ +(-t,r,Ffts,s,~)((AR)(Ar,)t-(AR)(Ar,))f +p(toslF~r-s~flr’)((AR)(r,A~~)+(AI;l)(r,A9,)>+ +~,(f,s,F~--s,t,F)((Ar,)(r,A~,)t(Arz)(r~A~2)).

with p. = (W)% so = (R-r

P =

(r/W,

r, = CrR)%

~0s ~~/g, si = (r-R

to = r sin $/q,

tx = R sin 4jq,

cos 4)/q, q = R2 --I+‘--2Rr

cos #L

Three different FORTRAN XI programs and an IBM 1520 computer were used in our calculations. The first program calculated initial sets of force constants for different assignments of the fundamental frequencies, all the interaction force constants being set equal to zero. A second program calculated sets of frequencies for the General Valence and Urey-Bradley Force Fields (G.V.F.F. and U.B.F.F.), using the four principal and two interaction force constants. For the G.V.F.F., provision was also made to compute the frequencies for varying values of K, and Ka. Fixed values were in this case given to KZ and K4. The third program contained an additionai least squares procedure that adjusted the sets of force constants to give the calculated frequencies with a relative deviation of 0.1 % from the observed values. The calculations were performed according to the following steps:

Starting sets were established for the principal force constants Kr _+, setting the interaction constants equal to zero. Several different assignments were tried for the normal modes. None proved satisfactory besides the one presented in this paper, Our first attempt was, obviously, a trial to reproduce the assignment of Stammreich and Forneris’, Taking as a typical example the SzCll molecule, we found that the value obtained for KI (1.9 mdyn A-“) was too small compared ta the corresponding value of 2.7 mdyn A-r for other molecules of the S2X, type. OR the other hand, two sets of values result for K3 and K4 in this approximation. Their values are expected to be very close to each other. However, widely different values were obtained with Stammreich and Forneris’s assignment, and even the introduction of two of the remaining interaction force constants did not bring the values of Ka and K4 into coincidence, The other X,Y, molecules behave, for this .E Mol. Stmcfwe,

5 (1970)

449460

SULFUR

AND

SELENIUM

455

MONOHALIDES

assignment, in an analogous manner. This assignment was therefore and we confined our attention to the assignment of Table 2.

(B) Determination

abandoned

of the initial set of force constants

Using the K,_, starting sets for the adopted assignment of fundamentals (Table 2), frequencies were calculated for molecular models of C, symmetry having different angles. Values tried included 4 = 106”, z = 83” and Cp= 90”, o = 90”, as well as combinations of these values. For all angles within the above limits, and for the same set of Kl + force constants, the calculated frequencies changed very little for all molecules. This led us to assume, in our calculations for the selenium monohalides, the same angles as the ones observed by electron difraction’ for the sulfur monohalides. The choice of the two most significant interaction force constants among the nine available was made as follows. The frequencies for each molecule were calculated using the corresponding starting set K, _-4 plus one of the interaction force constants, giving it a value of 0.10 mdyn A-‘. This process was repeated for each one of the nine interaction force constants. In this way we were able to establish that, for the G.V.F.F., K, and KS made the greatest contribution for S,Cl, and SJ3r2 while KS and Kl 1 were more significant for Se,Cl, and SezBr,. A similar calculation was carried out for the U.B.F.F. Although very good results were obtained for the sulfur monohalides, it was not possible to find with U.B.F.F. a set of force constants which reproduced satisfactorily the observed frequencies for the selenium monohalides. In the following, only the final results for the U.B.F.F. will be given. A trial-and-error adjusting process was then applied to the corresponding sets of force constants. It was possible to calculate values for the frequencies of the SzC12 and S2Br2 molecules within 2 % of the observed values. The corresponding results are given in Table 4 for the G.V.F.F. For the selenium monohalides, however, no easy adjustment could be carried out. To obtain a similarly satisfactory agreement between calculated and observed frequencies for these molecules we proceeded as follows. Making use of the fact that the principal force constants Kl and K3 (in the G.V.F.F.) are concerned with the stretching modes while K2 and K4 are concerned with the bending modes, we chose a pair of values of K2 and K4 which reproduced the three bending modes for Se&I, and Se,Br,. Then, giving to K3 successively a value between 1.000 and 2.500 mdyn A-’ in 0.250 mdyn A steps, we calculated the fundamental frequencies for all values of Kl between 0.500 and 3.000 mdyn A-‘. For K, and K4 we used values of 0.057 and 0.190 mdyn A-‘, respectively, for Se&I, and values of 0.035 and 0.140 mdyn A-l for Se,Br, . In Fig. 2 are reproduced as an example some of the typical curves obtained for Se,CI, _ For other values of K2, K, and KS, similar curves were obtained and are, obviously, contained in the space defined between the extreme curves of this figure. Analogous curves were also obtained for Se,Br,. From the curves one can see the relation J. Mol.

Structure,

5 (1970)

449460

456

R. FORNERIS,

C. E. HENNIES

4

TABLE

~A~~LA~DANDOBSER~DFREQUEN~~ESANDF~RCECONS~ANTSF~RSULFURANDSELMI~~M~NOHALIDES

USING

THE G.V.F.F. AND S&r2

S2CZ&m-')

U.B.F.F. (cm-

I)

Se+ Cf2 (cm-

‘)

SeJ3r2(cm-1)

Calc.

Ohs.

Calc.

Obs.

Caic.

Ohs.

Calc.

Obs.

541.9 448.9 436.0 240.0

540 449 436 240

530. i 363.9 354.4 200.0

529 365 351 200

288.7 373.4 362.2 145.7

288 367 367 146

294.5 269.0 253-l 120.0

292 265 265 118

207.3 102.9

206 102

173.0 65.7

172 66

130.5 87.2

130 87

101.7 52.2

107 50

G.

V.F.F. (mdyn A- I)

Kl

2.610

2.500

1.880

1.740

Kz K3

0.066 2.022

0.070 1.750

0.049

0.036

1.920

1.700

KO KS & Kll

0.256 0.200 0.098

0.258 0.080 0.090

0.160 0.040

0.155 0.030

0.050

0.050

U.B.F.F. (mdyn ii-‘) KS

2.460

2.460

HZ

0.065

0.060

1.870 0.210 0.190 -0.019

1.560 0.150 0.160 -0.016

K,

H F F

Frequencies (cm-‘1

400-

b -367

b

3EC

300-

-288

200-

'J,b

'150--146 -130

700

a,b a.b

87

t

I

I .looo

Fig. 2. Behavior

1.500

of calculated frequencies for Se2C12 as a function of the principal force constant G.V.F.F. (without interaction force constants) and Kz = 0.057 and (a) For K3 = 1.000 mdyn A-‘; (b) for Ks = 2.500 mdyn A-‘.

KI, using a simplified K4 = 0.190 mdyn A-‘. J. Mot,

Structure.

2.000 K,(mdynlA)

5 (1970)

44%460

SULFUR

AND

SELENIUM

457

MONOHALIDES

four principal force constants ri-, _-4 and the stretching and deformation frequencies. It is also apparent that there is no set of these principal force constants capable of reproducing satisfactorily all the six observed fundamental frequencies. It was possible in this way to choose a starting set of K1 --4 and, by giving values to K, and K1 1.,to compute sets of frequencies whose values agree with observed values within 2 OA.These resuhs are also in Table 4. Similar calculations were carried out for the U.B.F.F. also. existing

between

the

We appfied this adjusting procedure for the SzCiz and S,Br, molecules only, It served as an additional support to the assignment. We reached agreement between the calculated and observed frequencies of Iess than 0.1 %, both for the G.V.F.F. and U.B.F.F. (Table 5). We did not carry out the same adjusting procedure for the Se&i2 and SezBr, molecules, the former adjustment that gave a 2 % deviation between calculated and observed frequencies (Table 4) seemed to be sufficient, since the broad bands of Se&& at 367 cm-’ and of SezBrz at 265 cm- 1 TABLE 5 CALCULA~DNNDAMENTALFREQUENC~ES BY THE LEAST SQUARES

AND

S*C&fC~

-x

Calc.

Ohs.

Calc.

Obs.

540.0 49.0 436.0 240.0 206.0 102.0

540 449 436 240 206 102

529.0

529 365 351 200 172 66

1

FORCE

CONSTANXSOF

S2C&

AND

SZBrz,

ADJUSTED

H?Zl-HOD

S,Bf&x?x-

365.0 351.0 200.0 172.0 66.0

‘)

G.KF.F.(mdynA-') JG rz; K3

Ko KS K6

2.610

0.068 2.023 0.260 0.180 0.098

2.387

0.071 1.710 0.264 -0.053 0.067

U.B.F.F. (mdynA-') KR

i? H’

F" F

2.490

2.460

0.066 1.875 0.257 0.046 -0.005

0.061 1 s37 0.236 0.032 -0.003 .T. Mol. Sfrucfure, 5 (1970) 449-460

458

R.

FORNERIS,C.E.HENNIES

were not resolved experimentally*. For this reason we thought it to be pointless to bring the agreement down to 0.1 % in this case. We expect, by analogy with the sulfur monohalides, that the splitting in each of these bands should be between 6 and 14 cm-l. The exact amount is, however, unknown. Potential energy distribution The potential energy distributions in terms of symmetry coordinates were computed according to the method of Morino and Kuchitsu’5 for alI our molecules except the chlorobromide (Table 6). One sees that the approximate description of the vibrational modes usuaIIy applied to these molecuIes needs to be revised in part. The v3 and v4 modes, commonly described as Y-X-X bend and azimuthal angle TABLE 6 POTENTIAL

ENERGY

DI.STRIBUTION

FOR xzY2

MOLECULES

A

1

S2Cl2

S, s, s3 s4

0.85 0.11

0.04 0.00

B

2

3

4

0.06

0.06 0.01 0.63 0.29

0.02 0.00 0.27 0.71

0.85 0.09 0.00

SS S.5

S2Br2

0.90 0.07

0.03 0.00

Se2C12

Se2Br2

0.89 0.03 0.08 0.00

0.64 0.37 0.01 0.00

0.04 0.78 0.17 0.01

0.24 0.73 0.03 O.UCl

0.29 0.63 0.09 0.00

0.06 0.08 0.49 0.37

0.06 0.01 0.65 0.28

0.05 0.01 0.66 0.25

5

6

0.95 0.05

0.01 0.99

0.86 0.14

0.09 0.91

0.99

0.01

0.01

0.99

1.00 0.03

0.00 0.97

0.03 0.00 0.33 0.64

0.02 0.00 0.26 0.72

0.01 0.00 0.24 0.74

* From polarization observations Hendra and Park6 have almost been able to resoIve the band of Se2C12_ 3. Mol. Structure, 5 (1970)449460

SULFUR

AND SELENIUM

459

MONOHALIDES

torsion, respectively, are realIy highly mixed modes of these same bending and torsional modes. This happens for all our mole&es.

On the other hand, for SezBr,,

the v1 mode appears to be a mixture of the Se-Br stretching modes.

RAMAN ~PEGTRUM AND~01~32CONSTANTS 0~ S,BrCl As already mentioned, only four of the expected six Raman bands of S,BrCI were observed in equimolecular mixtures of S2CIZ and S213r,, The observed bands are given in Table 7. In all our spectra of the mixturesthese bands always appeared accompanied by the strongest bands of SaCI2 and SzBrz. TABLE 7 OBSERVED AND

CALCULATEDVIBRATIONFUNDAMENTALS,AND~OR~EC~~~~TANTS~OR

Vibration

Obs.

Calc.

Approximate

(CS)

(OK’)

(cm

description

535

534.9 426.5 370.7 225.5 185.0 84.4

-

225 180 87 G. V.F.F.

Kl K2

& KS KS Ki

U.B.F.F.

(mdyn A-‘)

2.498 0.069 1.866 0.262 0.063 0.082

- ‘)

& K K u F’ F

SJ3rCl

S-S stretch S-Cl stretch S-Br stretch S-S-Cl bend S-S-Br bend torsion (mdyn .&- ‘)

2.415 0.063 1.706 O-246 0.039 -0.004

The &BrCl moIecule should possess C, symmetry, and all six vibration fundamentals are expected to give polarized Raman bands. All shouId also be infrared active. We calculated the fundamental vibration frequencies of the S,BrCI molecule in the same way as for the sulfur and selenium monohalides. The geometrical parameters are in Table 3. For these caIcuIations we built a set of force constants by simply taking the arithmetic mean of the corresponding force constants of S&l, and S2Br2 given in Table 5, Both G.V.F.F. and U.B.F.F. sets of force constants were used. The calculated values for the G.V.F.F. are in Table 4. Corresponding values for the U.B.F.F. are not reproduced since the maximum deviation from the catculated vafues of Table 5 is less than 2 cm-l. J. Mol. Structure,5 (1970) 449-460

460

R. FORNERIS,

C. E. HENNIES

As we can see in Table 7, there is excellent agreement between the calculated and observed values of the frequencies. It is also immediately apparent why we failed to observe two of the fundamental bands of this molecule. Their calculated values show that these bands, 426.5 and 370.7 cm-‘, are expected to fali close to the strongest Raman bands of ‘: _C12and SzBr,, located, respectively, at 436-449 and 356 cm- ‘_ We were unable to resolve these two bands from the corresponding intense companion bands of the parent molecules which were always present in our spectra.

ACKNOWLEDGEMENIS

The experimental results presented here were obtained by one of the authors (R-F.) in the laboratories of Professor Jean Lecomte and Professor N. Stammreich. This author is greatly indebted to them for the facilities provided. The authors are also grateful to Professor F. A. Miller for usefu1 comments and to the Fund. de Amparo a Pesquisa do Est. de S. Paulo, S. P., Brazil, for support of the computer work.

REFERENCES 1 H. STAMMREICA AND R. FOKNEKIS,Spectrochim. Acta, 8 (19.56) 46.

2 3 4 5 6 7 8 9 10 11 12 13 14 15

E. HIROTA,BU& Chem.&?c..k?pm,31 <1958) 130. E. B. BRADLEY, C. R. BENNETT AND E. A. JONES,5’pectrocfzfm~Acta, 21 (1965) 150.5. R. FORNERIS, unpublished results. C. E. PENNIES, Master’s Thesis, Institute of Aeronautical Technology, S. J, Campos, Brazil, February, 1967. P. J. HENDRAAND P. J. D. PARK, J. Chefs. Sue., A (1968) 908. S. G. FRANKISS.J. &&I. Strttcture, 2 (1968) 271. E. B. BRADLEY,C. A. FRENZEL AND M. S, MATHUR, J. Chern.P&s., 47 (1967) 4325; 49 (1968) 2344. H. ST.&~&REICH, Spectrachim, Acta, 8 (1956) 41. G. HERZBERG,Malecuiar Spectra and Molecular Stracture, Vol. If, Van Nostrand, New York, 1945, p. 271. Y. MORIN~ AND S. MIZUSHIMA,Sci. Papers Inst. Phys. Chem. Res. Tokyo, 32 (1937) 220. E. B. WILSON, JR.,3, Citem. Ph_vs_,7 11939) 1047; 9 (1941) 76. D. E, MANN, T. SHSMANOU~~, J. H. MEAL AND U. FANO, J. Chem. Phys.. 27 (1957) 43. G- DE ALII, Ann. C/&z. (Rome), 53 (1963) 948. Y. MORINO AND K. KUCHITSU, J. Chem. Pftys., 20 (1952) l&09.

J. IWOK.Sfructttre, 5 (1970) 449-460