Ultraviolet absorption spectra of tellurium monochloride and tellurium monobromide

JOURNAL

OF

MOLECULAR

Ultraviolet

37, 314-320 (l!)il/

SPECTROSCOPY

Absorption Spectra of Tellurium Monochloride and Tellurium Monobromide G.

Department

A.

OLDEHSHAW

of Chemistry,

AND Ii.

The Unisersity,

llonr~o~ Hm11, United Kinycr’our

Band systems of TeCl and TeBr have been photographed in nbsorptioll following flash photolysis of TeC12 and TeBrs The vibrational analyses of t,he spectra are discussed and the following equations for hand head frequencies derived. For TeCl:

P=

142 624 144 298

+ 455~’ -

386~” c111-~.

+ 314 (21’+

!S) - 0.5 (c’ + ‘8,’ - 267 (D” + ‘5, + 0.7 (Y +

j,i,P en-i.

The transitions are thought to arise from t,he 211jl/ra~1 211~~:! levels of the grormd states of TeCl and TeBr. INTRODUCTION

Spectroscopic studies of the monohalides of tht, 1group O’B elermmts have beeu relatively few, owing to the transient nature of molecules of this type, and have been almost exclusively concerned with the halogen monoxides. The elect’ronic spectra of ClO, BrO and IO consist, of single systems of red-degraded bands in the near ultraviolet or visible regions. They were first observed in emission from flames (1-5) but the initial vibrational analyses were uncertain due to difficulties in the assignment of vN. Later studies of the absorption spectra, which were photographed following flash phot,olysis of mixtures of the halogens and oxygen, allowed more complete vibrational and rotational analyses to be JWformed (S-8). These investigations, t,ogether with a more extensive study of the emission spectrum of IO (9), showed t,hat the trnnsitjion involved in each case is dist.ances, vibrational conprobably 211i-X211i , and values of the interatomic More recently, the elect,ron stants, and dissociation energies were obtained. resonance spect(rum (10) and microwave spectrum (11, 12) of Cl0 have been analysed, giving improved values of the rotational, spin-orbit coupling and hyperfine constants. A progression of diffuse bands in the region W-422 nm, observed in the flash photolysis of sulphur monochloride, has been tentatively assigned to the ‘I&:! 314

UV SPECTRA

OF TeCl AND TeBr

315

2&/2 t#ransition of SC1 analogous to those of the halogen monoxides (13). The electron resonance spectra of BrO (14), IO, SF, and SeF (16), and the infrared spectrum of OF trapped in argon and nitrogen matrices (16), have also been reported. We have succeeded in photographing the ultraviolet spectra of TeCl and TeBr during the flash photolysis of tellurium dichloride and tellurium dibromide vapors. The transitions involved are of a different type from those already known for the halogen monoxides and SCl. EXPERIMENTAL

METHODS

Mixtures of gaseous TeClz and nitrogen, and gaseous TeBrz and nitrogen, were subjected to flash photolysis. The apparatus was similar to that described previously (17). The reaction vessel was 0.47 m long and constructed of Vitreosil. TeC12 and TeBr, are too involatile to allow effective photolysis of the vapors at room temperature, so both the reaction vessel and photolysis lamp were enclosed in a furnace which could be heated to 400°C (18). The pressure of the dihalide in the vapor phase was controlled by introducing a small quantity of liquid to the reaction vessel, adding the required pressure of nitrogen, and adjusting the furnace temperature. Since the spectra of TeCl and TeBr occur in the same region as the continuous absorption of the parent compounds, the adjustment of the pressure of the dihalides was critical, too small a pressure resulting in inadequate photolysis and too large a pressure in complete absorption of the photographic flash in the region of interest. Both TeClz and TeBrz were obtained from K and K laboratories and were redistilled in vacua before use. B.O.C. ‘oxygen free’ nitrogen was used without further purification. Spectra were photographed using both a Hilger medium quartz spectrophotograph and a Hilger Littrow spectrograph (E 742.1). Measurements of wavelength were made, on photographs taken with the latter instrument, by comparison with iron arc lines, using a Joyce-Loebel Mark 3 microdensitometer. Spectrograms taken at short delay times in the photolysis of both Tech and TeBrz showed the band system of Tea lying between 390450 nm (19) and lines due to atomic Te, in addition to the new band systems of TeCl and TeBr. RESULTS

The spectrum of TeCl shown in Fig. 1 was photographed with the Littrow spectrograph during the flash photolysis of TeClz mixed with 100 mm nitrogen at 140°C. The flash energy used was 1600 J and the time delay 2 I.csec.The spectrum of TeBr, recorded under similar conditions in the flash photolysis of TeBrz , but using a temperature of 15O”C, is shown in Fig. 2. The simple vibrational structure of the spectra indicates that the absorbers are diatomic and the fact, that, different spectra are observed after photolysis of TeClz and TeBrz excludes the possibility that they are due to Tez . The Spectrum of TeCl Only nine violet-degraded bands were recorded. Further experiments were

performed using the medium yuart$z spectrogrnpll in an at,tempt’ to observe additional bands to shorter wavelengths but absorption by the parent compomltl prevent’ed this. Some of the bands appear diffuse but the variation of diffuseness with U’and v” indicates t,hat t,his is probably largely due to isotopic spreading. The weak background also necessitated t,he use of :I relatively wide slit (0.03 mm). There is therefore no definite evidence of I)reclissoci:Lt,ion in the bands. The bands fall into two groups, labelled a and h in Fig. 1, and t,he wavclrngt.hs and frequencies of the heads are recorded in Table I. The frequencies of t hc heads of system b are less accurate t,han those of system a oning to t’lle dificultJ

Ultra-violat Frc;. 1.

Ultraviolet

~pectrw

of T.Cl

spect,nlm

of TdII

235 I

L

191

4

0,

090 system b

Ultra-violet

FIG. 2. IJltraviolet,

spectrum

spectrum

of Tel31

of

T&r

UV SPECTRA

317

OF TeCl AND TeBr

TABLE

I

B.~ND HEADS OF TeCl __ System

Band

Xair (nm)

Y (cm-l)

a a a a %

0,l 070 190 290 071 371

236.675 234.536 232.060 229.646 227 632 229.318

42 42 43 43 43

; b

320 OF0 I,0

225.677 227.301 223.488

43 981 44 298 44 731

TABLE

II

DESLANDRES TABLE OF TeCl System a

0 0 1 2 3

42 624

385

455 43 079 453 43 532 449 43 981

387

-__

239 624 079 532 917 594

BANDS (cm-l)

1

0

42 239

44 298 443 44 731

____System b

381

1 43 917

43 594

of making measurements against a very weak background. Deslandres schemes of the two systems are shown in Table II. The absence of bands with yn > 1 precludes a complete vibrational analysis but the following constants of system a are derived from Table II: yoo = 42 624 cm-‘, , -1 AGllz = 455 cm ,

AGri, = 386 cm-‘.

of Table II shows that within the rather large error of measurement, the vibrational intervals in system b are the same as those in system a. It is therefore tentatively concluded that a and b are two component subsystems of the same transition, separated by 1674 cm-‘.

Examination

The Xpectrum of TeBr This bears a close resemblance to the spectrum of TeCl, occurring in a similar wavelength region and consisting of violet-degraded bands. As in the case of TeCl, the bands can be divided into two groups, those lying above 236 nm,

OLl)II:RI’IHAW

AN11 ROBINSON

TABLE;

II I

BAND HE.\DS OF T&r Band

’1

0, 1

2-12,mi

.11 150

241.298

-41 430 I1 i44

2,0

237.71B

$2 055

0,3

23ti ,029

239.484

0,2

234.570

42 355 42 IjlH

0,l 0,o

233.121

42 883

231 .688

43 148

111

231,439

.I3 1%

190 2.0

23o.oui

43 4ii2

228.374

43 77-l

3 ,0

226 758

4-l OY(i

4,O

225.175

44 396

“c;i,c*(cm--’ 1

1 x5

42 883

267

4:3 195

314 43 462

Y (cn-

0,o 1 ,o

0 43 148

x;,i, (ml)

312

312 43 774 312 44 0% 310 44 39fi

labelled a, and those below 236 nm, labelled 6. Bands of system h are more numerous and intense t,han those of system a. The increase of diffuseness in bands well-removed from the system origin again indicates isotopic spreading rather than predissociation. Measurements of band head frequencies are shown in Table III, and Table IV

UV SPECTRA OF TeCl AND TeBr gives Deslandres schemes for both systems. syst,em b can be represented by the formula Y = 43 125 + 314.2(v’ + 44, -

267.4(v”

+ x)

+ 0.7(v”

The

-

319

frequencies

0.5(v

of the bands

of

+ 44)”

+ $$)” cm-‘.

The four bands of system a can be represented within the error ment by a formula which is identical except for t’he substitution 41 406 cm-’ for Te. A comparison between observed and calculated frequencies is included in Table III. It appears reasonable to assume a and b of TeBr represent two components of a single system wit,h rated by 1719 cm-‘.

of measureof the value band head that groups origins sepa-

DISCUSSION The similarities between t’he observed band systems of TeCl and TeBr are very marked. Both sets of bands lie in the same spectral region and are violetdegraded. Each system consists of two components and the separations between these are similar, 1674 cm-’ for TeCl and 1719 cm-’ for TeBr. The ground st#ate of t,he molecules is expected to have the configuration . . . (w7r)4(xfJ)2(mr)“, %IIi. Rotat’ional analyses of the halogen oxide spectra by Durie and Ramsay (8) have shown that the ground state is ‘II, and the fact that it is inverted has been confirmed in the case of Cl0 by the analysis of the electron resonance spectrum by Carrington, Dyer, and Levy (10). The separations between, and relative intensit,ies of, systems a and b in both TeCl and TeBr suggest t,hat t,hey correspond to t,ransitions from the *II 1,2 and *II312components of the ground state t,o a common upper state (B) which has a spin splitting considerably smaller t(han that of the ground state. The derived molecular const’ants of TeCl are then as follows: ITor XYI, T, = 0 and 1674 + 0 cm-‘. AGliz = 386 cm-‘. For B, To = 44 298 and 44 298 +

b cm-‘,

AG/z = 45.5 cm-‘. In the case of TeBr, l’or X*II,

the paramet,ers

are:

T, = 0 and 1719 +

w, = 267 cm-‘,

c cm-‘, w,x, = 0.7 cm~‘.

320

OLDERSHAW

AND

ROBINSON

For B, Z’,, = 43 125 and 43 1% + c CC ‘,

b and c represent. the spin-orbit coupling of the B states. The upper states have been labelled B since the K--X transitions are cled~ different from the A*&X%I transitions of t,he halogen monoxides. The latter lie to longer wavelengths and, in conkast to the R-X systems, involve reddegraded bands and a decrease in oe on excitat,ion. It, seems likely that in thch B st)ates one of the UT electrons has been promoted to a less arkibonding orbital.

The authors thank the Royal Society and t,he Scielrcr Research Colulcil for gratits flrl equipment,. K.R. is indebted to the Sciellce Researrh Coilncil for a, maintenance award RECEIVED:

July 20, 1970

1. W. M. V.UDY.\, Proc. Indian dcad. Sci. &xl. d 6, 122 (1937). ZVor. Zndian Acad. &:i. Secl. A 7, 321 (1938). 3. R. II. Co~,ls~~.\.v.\ND A. (+. GIYUON, Discuss. Fawday/ Sot. 2, 1tiG (1947,1. .\SU A. (;. (:aydolt. I\:a(ure 161, 242 (1948). 4. CT. P~NETIEB 5, E. H. COLlCMAN, ii. G. (;i\YDON,.\sI)w. %I. vmy.\, ;\'c/tu?,e 162, 108 (lQ#). 6. G. PORTICR, Discuss. Faraday Sac. 9, 60 (1950). 7. C;. HISIUBEKG .\ND II. A. Rxl~~.\Y, Disc~m. Farat&/ Sot. 9, 80 (1950). 8. I). A. DT;RIE .\ND I). A. R.LMs.\Y, Can. .I. Phys. 36, 35 (1958). 9. It. A. I)UI. H. LEVY. J. (‘hcrn. Phys. 47, 1756 (1967). 11. T. AMANO, E. I~IRoT.\, IND Y. Mz’uN~, J. Xol. S~ccl~osc. 27, 257 (1968). I,$. T. AM..\NO,9. S.~ITO, I<. HIROT,\, .\ND T. MORINO, J. :Vlol. SpectTosc. 30, 275 (1969). 13. R. J. DONOV.IN, 1). HUSMN, .WD 1'.T. JXKSON, Trans. Furaclay SOC. 64, 1798 (196X). 14. A. C~RRINGTOX .~ND I). H. L~:vy. ./. Chenz. Phq~. 44, 12!)X (1966). 15. 8. C.~RKINGTON, G. N. Crrrut~r:, I’. N. I)yx~t, 1). II. Ll:vY, .\NU T. .A. MII,LXIL. (,‘/icu( Comnurn. 641 (1967). 16. A. AKIULL, It. R. REIXH.\I~D, :\NI) L. 1’. LAKSOS, J. .4,rlc’r. (‘/reul. Sot. 87, 1016 (lQ(iS/. 17. CT.A. OLDERSH.~\\-~NJI K. ROMNSON, .J. Mol. Speclrosc. 32, 469 (1969). 18. (;. A. OLDERSHAW .UVD K. ROBINSON, Trans. Faraday Sac,. 64, 2256 (1968). “Spectra of IXatomic Molecules,” 2nd Ed.. Van Nostrand, New York. 19. G. HE:KZUERG,

2. W. M. VIIYYI,

1950.