Raman spectra of solid and liquid chlorine monoxide, (Cl2O)

JOURNAL

OF MOLIK!UL.\R

Raman

SPECTROSCOPY

38, 476-482 (1971)

Spectra of Solid Monoxide,

and Liquid ( C120)

Chlorine

D. J. GARDINER University

Chemical Laboratory, Cambridge CBfZ lEW, England

Raman spectra of chlorine monoxide (C&O) have been obtained in the solid and the liquid phases. Polarization studies confirm the infrared assignment. INTRODUCTION

Chlorine monoxide has been t,he subject of a number of infrared studies (1-B); however, the assignment of t’he spectrum proved t)o be difficult as the posit,ion of the bending mode ~2 was in doubt’. In 1965 Rochkind and Pimentel (7) found the frequency of this vibration, and produced the now accept’ed assignment, (see Table I). EXPERIMENTAL Chlorine monoxide was prepared by passing dry chlorine over yellow mercuric oxide and trapping out t)he product at 195°K (8). The chlorine monoxide was purified by vacuum distillat’ion; where possible it was handled in darkness since it is easily photolysed. IJquid-phase Raman spect’ra were obtained by using a low temperature still designed by Ogden and Turner (9). The chlorine monoxide was distilled into this still under vacuum and kept there, gently refluxing, with COs/acetone coolant’. Solid-phase Raman spectra were obtained using the Coderg system shown in Fig. 1. A retractable copper t#ube was used to spray the sample from a trap at. 195°K onto the cold (77°K) copper block under vacuum. A Coderg P.H.I. Raman spectrometer was used with He/r\Te laser excitation. Polarization studies were made in the liquid phase using a vertically polarized exciting source and a half-wave plate. The specbromet,er was calibrated with carbon tetrachloride and indene; the frequencies quoted are believed t’o be accurate to fl cm-’ in the liquid phase and f0.5 cm-l in the solid pase except where st’ated ot’herwise. RESULTS TJiqui~

The temperature of t,he liquid chlorine monoxide in the still was somewhere between its boiling point) (275’K) and 195°K. Figure 2 shows the specka obtained 476

CHLORINE

MONOXIDE

RAMAN

TABLE

-_ I.R. (solid 1 (7 I (cm-‘) 630.7 296.1 670.8

Kaman (liquid) (cm-‘)

SP15CTKUM

-ITi

I Symmetry

Pl

Vibration

--

634 293 673

0.25 0.42 0.77

.I L .I1 I<,

“1 Y:! V8

\ 2o” -I\

FIG. 1. Experimental arrangement in the Coderg P.H.J. Raman taining apect,ra of solid samples condensed from the gas phase.

spectrometer

for oh-

using (i) vertically and (ii) horizontally polarized excitation. A-0 ot,her bands \vt’r(’ observed in the spectrum. Table I lists t’he observed frequencies and the depolarization ratios pl ; the solid state infrared frequencies (1) being included for comparison. High resolution spectra show isot,opic splittings; 635.2 cm-’ (Wl-O-Wl) and and 292.4 crn~-L 633.0 cm-l (35C1-O-37C1) for v1 , and 294.2 cm-’ (WI03G1) (V1-O-“7C1) for v., . The 37C1-O-37C1bands were not observed due to t,he 10~ concent,ration (6.7 %) of this isotopic species. The v3 band n-as so weak and broad that, no iaot,opic split,ting was observed.

The speckurn of the solid at 77°K is shown in Figure 3 and the high resolutiorl spectra of the ~1 and v3 bands are shown in E’ig. 4. Table II lists t,he frequencies observed, and the assignment. IYot all the 37C1-O-37C1bands were observed. A doublet having a splitting of 1 cm-’ at’ 60 f 1 cm-’ and B band at 65 rt 1 cm-’ were also observed and are thought to be due to lattice vibrations. DISCUSSION

In the optical system used in these experiments the depolarization ratio ~1 has values 0 < pl < 0.75 for a polarized (symmetric) band and ~1 = 0.73 for a depolarized (asymmetric) band. From Table I it can be seen that the observed de-. polarization ratios confirm the infrared assignment. It is noticeable in the Iluman sr)ectrum that the bending mode I+ is a comparativelystrong band, whereas in

GAKDINER

478

WAVENUMBER 250 1

300 I

350 I

600

650

I

700

1

250

300 I

350

600

I

t

650 I

700 I

I

(ii)

I

w

FIG. 2. Raman spectrum of liquid chlorine monoxide-(i) (ii) horizontally polarized excitation.

vertically

polarized

excitation,

the infrared spectrum it is weak. In the liquid Raman spectrum the asymmetric stretch is broad and weak. The solid state Raman spectrum shows three strong bands ~1 , vc , and vg , and a number of weak combination bands. The bending mode v2 consists of a single band at 303.1 cm-’ having a 35C1-O-37Cl isotopic component at 301.1 cm-’ which is revealed under high resolution. In the infrared spect,rum (7) the ~2 mode occurs at about 7 cm-l to low frequency of that in the Raman spectrum. The ~1 band appears as a doublet in the Raman spectrum (see Fig. 4), which is attributed to crystal field splitting. The low frequency component shows fine structure due to isotopic splitting-35C1-0-35C1 at 638.3 cm-‘, 35C1-O-3iCI at 627.0 cm-’ and 37C1-O-37C1at 635.3 cm-l. The high frequency crystal field split component at 636.1 cm-’ is too weak to detect any isotopic splitting. In the infrared spectrum, Rochkind and Pimentel observed the p1 band at 630.7 cm-‘, but de-

CHLORINE

MONOXIDE

RAMAN

SPECTRUM

4Z?

WAVENUMBER 250

300

350

I_

Boo

650

700

-

FIG. 3. Raman

9@)

950

1000

1250

I

spectrum

of solid chlorine

1300

1350

1400

I

monoxide

at 77°K.

WAVENUMBER 625

630

835

665

670

675

v d

c

b

a

FIG. 4. High

resolution

Raman

spectra

of vI and ~3 in solid

chlorine

monoxide

at 77°K.

tected no crystal field splitting. They only observed the isot,opic splitt,ing of this band in a matrix. However, in experiments with mixed C12W and C1,W the) did observe :i crystal field splitting which they attribute t.o 10~s of q,mmctry irl t,he unit cell.

GARDINER

480

TABLE

II

Raman (solid) (cm-l)

Vibration

Symmetry

301.1 303.1

I%

Al

VI

Al

B 1 B 1

625.3 627 .O

636.1 628.3 668.5

670.7 669.4 671.3 938.7 941.2

945.7

V% va +

YP

Cl-O-Cl 35 35

37 35

37 35 35

37 37 35

35

35

35

37

35

35

35 35

37 35

37 35 35

37 37 35

37 35 35

37 37 35

B1

Bl

1308b 1321.3 1327.1 1330.3 -8 Crystal field splitting. b & 1 cm-l.

The high resolution Raman spectrum of the vp band is also shown in Fig. 4. It appears as a quadruplet, (a) 671.3 cm-‘, (b) 670.7 cm-l, (c) 669.4 cm-l, and product-rule calcula(d) 668.5 cm-‘. Taking (a) to be the 35C1-O-35C1vibration, tions (10) put vs for 37Cl-O-z7C1 at 669.0 cm-‘. Although band (c) is in this position it is too intense to be solely due to 37C1-O-W1 which has a concentration of only 6.7%. This band, therefore, must arise mainly from crystal field splitting and thus bands (a) and (c) are assigned to t,he 35C1-O-36C1crystal field split components split comof va and bands (b) and (d) to their respective 35C1-O-37C1isotopically ponents. Rochkind and F’imentel observed a triplet absorpt#ion for v3 in the infrared spectrum consisbing of two intense bands at 670.8 cm-’ and 668.8 cm-l, and a weaker band at a slightly lower frequency. They assign the two intense bands, as in this work, to the crystal field split components of the 35C1-O-35Cl v3 vibration. They suggest, in con&a& t,o the above, t.hat t,he 37C1-O-3sC1 component of the band at) 670.S cm-’ is overlapped by the band at 668.8 cm-l as they did not resolve any other band between 670.8 cm-’ and 668.8 cm-’ attributable to 37C1-O35C] as is the case in the R.aman spectrum. It, will be noticed that the ergsbal field

CHLORINE

MONOXIIX

RAMAN SPIXTILUM

4Sl

TABLE III* Vibration Cl?0 (gas)

Correlation field sp!itting (cm-‘j

YI

698.3 630.7 636.1 296.4

Y?

303.1 669.1

1’3

671.3

TR K II1 & It IR & I<

-a Infrared

frequencies

from Ref. 6).

split, components of v3 both lie at the same frequencies in the infrared and tlw Raman spectra. It appears t,hat each of the fundamentals is split, into t,wo or t,hree component .q ‘7~ the crystal field, some infrared active and some Raman active, and in the case of v:, bot,h infrared and Raman actJive, as summarised in Table III. The split(ting cannot be “site group splitting” as none of t,he vibrations is degenerat,e; it, must, therefore, be “correlation field splitting”. The above observations allow some conclusions to be dramn concerning the unit. cell of solid chlorine monoxide. The occurrence of coincident correlation field split components for ~3 in t#he infrared and Raman spectra implies that the unit cell cannot have a centre of symmetry. The splitting of y1 into three components suggests that there are at least. three molecules per unit cell. Accepting that the complete spectrum has been observed, the only point, groups isomorphous wit,h t’he various factor groups which havt symmet,ry species with the required infrared and Raman activity arc D, , II, , f16 . C:#&) md D3h . It is possible, however, that there are a number of other components which are unobservable due to overlap with other band,< or low intensit\-. Without these components and, more importantly, without. the latjtice modes it, i,q not possible to construct a definite model of solid chlorine monoxide. In conclusion: the results of this work confirm t)he infrared assignmentj of ROC~Ikind and Piment,el and indicate for solid chlorine monoxide t)hat there are ai least three molecules in a noncentrosymmetric unit wll. ACKNOWLEDGMEPiTS The author would like t,o thank Dr. J. J. Turner and I)r. 1’. bZ. A. Sherwood helpful discussion, and the S.R.C. for a maintenance grant RECEIVED:

Sovember

9. 1970 REFERENCES

1. C. 11. B.ULEY .XXL)A. B. I>. CASSIE, Proc. Roy. Sot. A 142,129 (1933).

5. G. HETTNER,

11. POHLMAN, AND H. J. SCHUMACHER, Na/trrwisa. 23,114

(1035).

for their

482

GARDINER

S. R. POHLMAN AND H. J. SCHUMXHER, Z. Phys. 102,678 (1936). 4. W. G. PENNY AND G. B. B. M. SUTHEHL.\ND.Proc. Roy. Sot. A 166,654 (1936). 6. G. B. B. M. SUTHERLANDAND W. G. PENNY, Proc. Roy. Sm. A 166,678 (1936). 6. K. HEDBERG, J. Chem. Phys. 19,509 (1951). 7. 8. 9. 10.

M. M. ROCHKIND ,\ND G. C. PIMENTEL, J. Chem. Phys. 42, 1361 (1965). G. H. C.YDY, “Inorganic Synthesis,” Vol. V, 156 (1957). J. S. OGDEN AND J. J. TURNER, Chern. and Znd. 1295, (1966). G. HERZRERG, “Znfrared and Raman Spectra,” p, 228, Van Noskand Princeton, 1945.

N.J..