Photochemistry of an ozone-bromine complex; the IR spectrum of matrix isolated Br2O

Journal of Molecular Structure,

157 (1987)

Elsevier

Amsterdam

Science

Publishers

B.V.,

l-15 -

Printed

PHOTOCHEMISTRY OF AN OZONE-BROMINE SPECTRUM OF MATRIX ISOLATED Br,O*

S. D. ALLEN,

Department (Received

M. POLIAKOFF

of Chemistry,

in The Netherlands

COMPLEX;

THE IR

and J. J. TURNER

University ofNottingham,

Nottingham, NG7 ZRD, (Gt. Britain)

8 July 1986)

ABSTRACT Cocondensation of O,/Ar and Br,/Ar mixtures gives IR evidence for a weak O,/Br, complex which is shown by “0 substitution to have an asymmetric structure. As in previous work photolysis leads to new species one of which has two IR bands; detailed comparisons suggest these are the vg (weak, 623.4 cm-‘) and v, (strong, 526.1 cm-‘) bands of a perturbed BrOBr molecule. GENERAL

INTRODUCTION

(BY J. J. T.)

Everyone who has worked in George Pimentel’s laboratory has experienced the same sense of exhilaration. I had the great privilege of working in Berkeley for two years, 1961-1963, a time which certainly shaped my own scientific attitudes and plans. It seemed appropriate therefore to comment on those early years, and on appropriate developments therefrom, as well as to present some recent scientific results which relate to my own research in GCP’s laboratory. In 1961-63 matrix isolation was in full flower in Berkeley but other exciting work was also in progress. Ken Herr was building the infrared (IR) rapid scan instrument with enormous dedication; this work led to the first successful IR spectra of transient gas phase species (CF,) [l] and illustrates GCP’s perception of problems that are really worth tackling. This is exemplified by the number of current workers who, with the advantage of laser technology, are obtaining IR spectra of unstable species. Jerry Current and later Jerry Kasper were attempting the IR emission studies that led to the first chemical laser [2]. Immediately after the demonstration of laser action in ruby by Maiman [ 31, Ric Spratley was set to work to build a ruby laser for Raman spectroscopy. All this time GCP was heavily involved in CHEM STUDY, basically a complete rethink of the way to teach Chemistry in High Schools. This led to an interesting collaboration with GCP: we made a film together entitled “Inert(?) Gas Compounds” which, although the acting was not of Oscar *Dedicated

to Professor

0022-2860/87/$03.50

George

C. Pimentel.

0 1987

Elsevier

Science

Publishers

B.V.

2

standard, did tell an interesting story of how we got into noble gas chemistry. I had spent some time trying to examine the IR spectrum of NH3 in a Xe matrix, looking for restricted rotation. This work had hardly progressed largely because, with the liquid hydrogen cryostat, it was impossible to deposit Xe at the optimal temperature; the result was cracked and scattering matrices. However, it meant that I was the only person in the laboratory with a cylinder of Xe so that immediately after the announcement of Xe compounds [4], GCP set me to work on matrix studies. Following successful matrix syntheses of the known XeFz and XeF4 we were able properly to characterise KrF, for the first time [5]. GCP’s interest in compounds of what were then still called “inert” gases should come as no surprise to those who recall that in an elegant paper in 1951 he had developed the 3-centre molecular orbitals for ICl; etc., and predicted on isoelectronic arguments that noble gas dihalides should exist [6] . One striking feature of these matrix experiments was the ubiquitous presence of IR bands which were eventually assigned to OzF and OzFz, produced because of O2 impurity in Fz. On my return to England, Ric Spratley continued this work [7], and he and GCP developed a simple but effective molecular orbital treatment for these and a whole range of molocules [8]. One link with those early days is that Jeremy Burdett and I recently published an extended treatment of such systems [9]. It is now perhaps worth commenting on some technical developments which bear a direct relationship to those earlier days and which will be discussed at greater length by others. In matrix isolation, undoubtedly the most important advance was the introduction of closed-cycle refrigerators which permit easy control of temperature and moreover are less frightening that manhandling 17 litres of liquid hydrogen! Secondly, FTIR with its increased sensitivity has made it possible to probe interactions and follow reactions not previously possible, although FTIR introduces its own complications (see later). Two other developments in matrix isolation in which we have been involved are polarisation studies and IR laser chemistry. For a number of organic and organometallic photochemical processes, matrix photolysis with plane-polarised UV light, followed by IR and UV/visible spectroscopy with plane-polarised light provides both structural and mechanistic information [lo]. This is a development of photoselection described by Albrecht [ll]. Interestingly, but unsurprisingly, when this was reported to GCP he commented that he had tried similar experiments himself in the very early days of matrix isolation! The ability of a matrix to provide a cold frozen environment offers the best hope for observing mode-selective IR photochemistry, the goal of much of the work of IR photochemistry in all phases. With the advent of IR lasers, particularly CO and COz, it has proved possible to test this hypothesis and although extremely difficult to prove there is evidence of success. Our own work [12] on Fe(CO)4 and that of Frei and Pimentel [13] on F,/hydrocarbon reactions are probably the most detailed investigations carried out in this area.

3

The original work of Pimentel and Herr [l] on time-resolved IR was taken up spasmodically by one or two people but real progress had to wait for the development of lasers. GCP’s aim has always been to get the whole of the relevant IR spectrum in one shot by very fast scanning or diode arrays rather than by a point-by-point method at a series of selected wavelengths; the latter method, although splendid for kinetic measurements, is much more difficult for complete spectral collection because of the difficulty of accurately reproducing the conditions every flash. With some exceptions most current work is of the point-by-point variety whether this involves the frequency or interferometric domain. Moore and co-workers [14] and Weitz and co-workers [15] (both ex-GCP people) have probed CH2 and metal carbonyls, respectively, in the gas phase. Our own work [ 161 uses either flash lamp or excimer laser for the generation of organometallic transients in solution with the resulting transient monitored (like Weitz) by a CW line-tunable CO laser. One of the advantages of the noble gas environment for matrix isolation is that, with one or two notable exceptions, (e.g. XeFz [5] ), it provides an inert environment for spectroscopy, structure and photochemistry. Two specific disadvantages are the ubiquitous “matrix splittings” and the inability to obtain real stability data. For species which are not “very” unstable and requirement is less rigorous, liquefied noble hence where the “isolation” gases provide an environment which may be seen as a “half-way house” between the frozen matrix and conventional solvents. Moreover, the liquid noble gas provides an almost perfect spectroscopic solvent and this particular aspect has been utilised by, among others, Ewing [1’7] and Bulanin [18], both also associated with GCP. For kinetic measurement as a function of temperature, it is necessary to have cells capable of withstanding up to 20 atm pressure and possessing sensitive temperature control. Maier et al. [19] at Los Alamos developed such cells and, in collaboration with him, we have examined the photochemistry, spectroscopy and kinetics of a wide range of organometallics [20]. We consider now some work from our laboratory, which, although forming only a small part of our recent activities, does provide a direct link to 1961-63. INTRODUCTION

TO BROMINE

OXIDES

The binary compounds of halogens and oxygen are a fascinating set possessing unusual structural and chemical properties [21]. Because of instability, many were originally characterised in low temperature matrices, not surprisingly several by GCP and colleagues, (e.g. OOF [7], ClOO, (C10)2 [22] ). Until recently, one notable gap in structural information has been for bromine oxides largely because none of the compounds are stable. Campbell et al. [23] were the first (in 1968) to describe IR spectra, including isotopic oxygen data, for solid Br*O. The spectra are reproduced in Fig. 1. Apart from solid state splitting effects, the spectra were easily ascribed to a bent triatomic molecule with bands at 587, 504 and 197 cm-‘, assigned

4

cm-’

Fig. 1. IR spectra of solid annealed Br,O obtained on (a) Perkin-Elmer 521 and (b) R.I.I.C. FS 720, spectrometers; (c) expanded spectrum of Br,O prepared with oxygen containing -25% ‘80,. The dotted lines are unannealed samples. (With permission from ref. 23.)

in that paper to v3, v1 and v2 respectively. Some time later, Pascal [24] obtained both IR and Raman data for solid Br,O confirming the earlier work. There has also been considerable work on the vibrational spectra of the solid higher oxides of bromine (Brz03, Br204) but structural conclusions have had to be somewhat tentative [24-261. In a note published in 1978, Tevault and Smardzewski [27] isolated BrOO in a matrix by codeposition of Br atoms (from discharged BrJAr) and an OJAr mixture. Shortly afterwards they published a paper [28] in which BrO, OBrO, BrOBr and BrBrO were identified in matrices by reaction of atomic or molecular bromine with atomic or molecular oxygen. There was a significant difference in the IR spectrum of BrOBr from the early work [23, 241. In particular, they observed only one band at 526.1 cm-‘, with the corresponding “0 band at 501.4 cm-‘; applying the usual rules [29] this isotopic shift gives a lower limit to the BrOBr bond angle of 87”. They pointed out (quite correctly) that the earlier [23] solid state isotopic data implied meaningless BrOBr bond angles, although small errors in the quoted solid state frequencies could easily account for this. They also suggested that the mechanism of generation of the various Br/O species via photolysis of O3 involved motion of photogenerated oxygen atoms through the matrix. However, this seems unlikely in the light of O3 photolysis experiments by Downs et al. [ 301 and Andrews and co-workers [31], in which 0 atom transfer occurs via a weak complex formed between O3 and the substrate trapped as adjacent species in the same matrix cage. There are thus several questions about 03/Br, photolysis experiments: what is the mechanism of the primary photochemical step?; what is the complete IR spectrum and structure of Br,O?; why are solid state and matrix spectra so different? This brief paper provides a (partial) answer to some of these questions.

5

EXPERIMENTAL

Oxygen was used as supplied by BOC (Research Grade) or Messer Griesheim (W. Germany, 99.9% pure); 1802 was supplied by Prochem (99% “02 ; 1% 1602). Bromine, supplied by BDH (98% pure), was triply-distilled and stored over PZ05 in vacua. Ozone was prepared by electric discharge (2000 V, 80 mA) in oxygen (1 Torr). Considerable caution is needed to prevent explosions of condensed ozone; full details are given elsewhere [25]. The matrix equipment includes an Air Products Displex CS202 with CsI or CsBr windows. OJAr and Br,/Ar mixtures were sprayed on separately through PTFE needle valves. A Nicolet 7199/MX3600 FTIR system was used to record IR spectra. Photolysis was carried out with a 500 W Xe arc lamp with Kodak Wratten filters. RESULTS

AND

The 03/Brz

DISCUSSION

weak complex

and itsphotolysis

The photolysis of OJBr, mixtures in low-temperature matrices is achieved with long wavelength light (h > 580 nm). A pertinent first question is, what happens when ozone is photolysed alone in an argon matrix? The IR spectrum of O3 in matrices has been thoroughly studied by Andrews and Spiker [32] and Green and Ervin [33]. Our own experiments with O3 in argon (see Table 1 for frequencies) revealed a dramatic demonstration of the importance of choosing the correct level of “zero-filling” when using an FTIR instrument. The degree of “zero-filling” is given by n in the equation TABLE

1

Frequencies

(cm-‘)

Assignment

of the vj banda of 1a0/‘80 0,

scrambled

0,

spectra

Ref.

32

16/16/16

1040.0

16/16/18 18/16/18 16/18/16

Ref.

33

This work 1039.16

1026.2

1025.63

1025.18

1017.1 1006.5

1016.70 1006.17

1016.33 1005.84

18/18/16

992.0

991.63

991.23

18/18/18

982.8

982.35

982.02

the most

intense

component

in Ar matrices 0, /Br,

1039.69

aQuoting

and OJBr,

of matrix

split band.

spectrum

This work 1029.60 1020.37 1012.72 ‘1006.98 996.53 986.58 979.06 -973.10

6

NTP = 2” X NDP where NTP is the number of transform points and NDP is the number of data points in the interferogram. Most textbooks and interferometer manuals stress that “for photometric accuracy” three degrees of zero-filling (i.e. n = 3; NTP = 8 X NDP) should be used, but most spectra are quite satisfactory when NTP = 2 X NDP. However, it is essential that 3 degrees of zero-filling are used in those cases where resolution, linewidth and band separation are similar in size. Figure 2 shows the matrix-split vj IR band of O3 in Ar (1:lOO). The effect of changes in zero-filling are very striking. With the same interferograms but increasing NTP, the “resolution” improves until with NTP = 8 X NDP, the spectrum is virtually identical with a spectrum recorded with 0.12 cm-’ resolution. Figure 3 shows the same v3 band of O3 in Ar both before and after a few hours photolysis with h > 580 nm. The only effect is that the arrowed bands (which are not present at high dilution of O3 in Ar) disappear: there is no change in the principal matrix-split features. The conclusion is that, although h > 580 nm radiation will undoubtedly decompose O3 into O2 and 0, the 0 atom has insufficient energy to escape from the matrix cage, and recombination occurs. Shorter wavelength light will eventually lead to destruction of the 03. The arrowed bands are presumably dimer/polymer bands and their photolytic removal is due to, say, [O,] 2 -+ [ 302] .

DEGREE OF ZERO-FILLING

cm-l Fig. 2. IR spectra (Nicolet 7199A with 1280 computer) of vg band of 0, in solid Ar (1: loo), illustrating effects of zero-filling. Traces A-D were all produced by transforming the same interferogram (NDP = 16 K): (A) NTP = NDP; (B) NTP = 2 X NDP; (C) NTP = 4 x NDP; (D) NTP = 8 x NDP; (E) shows a higher resolution spectrum of the same sample for comparison (NDP = 128 K, NTP = 256 K).

a

b

Fig. 3. IR spectra

of OJAr

(l/100)

(a) before

and (b) after long wavelength

photolysis.

When 03/Ar and Br,/Ar mixtures are condensed together the usual O3 bands are seen together with a principal new feature, a band at 1030 cm-’ which disappears on long wavelength photolysis, leaving the O3 band unaffected. This band is clearly the v 3 band of O3 in an 03/Br, complex and compares with similar observations [31] for O,/ICl. There appeared to be very little, if any, frequency perturbation of the v1 and v2 bands of OS, although the intensity ratios are changed considerably viz, v3: v2: vl in O3 1:0.05:0.007, and in the 03/Br2 complex 1:0.2:0.1. Further evidence about the 03/Br2 complex comes from isotopic data. Figure 4(a) shows the IR spectrum in the region of the v3 bands of O3 in both free and complexed form when O3 partially enriched with “0 (160: 180 - 1.6:l) is cocondensed with Br2/Ar; Fig. 4(b) shows the spectrum of the unphotolysed O3 which remains after long wavelength photolysis; Fig. 4(c) shows a subtraction spectrum which is thus the spectrum of the 03/ Br, complex. Table 1 lists the frequencies of both O3 and the 03/Br2 complex. In the v1 and v2 region, because the frequency shifts between O3 and 03/Br2 complex are negligible, the results are less clearly interpreted. The patterns in Fig. 4 show clearly that each band of ozone arising from a symmetrically isotopically substituted O3 molecule (i.e. 16/16/16-16/18/16, etc.) gives rise to an equally displaced single band in the complex. However, from the asymmetrically substituted species (16/16/M, 16/18/M) two bands are formed; this correlation is also given in Table 1. This is surely

8

WnVENUMBERS

and scrambled, enriched 0, (160: Fig. 4. IR spectra of matrix of Ar containing Br, “0 -1.6:1) in region of vg band of 0,: (a) after deposition, (b) after photolysis, h > 580 nm, (c) subtraction spectrum (b) minus (c).

because the 03/Br, complex of symmetry so that possible

/“2 o/“\o-&-B~

is antisymmetric structures are

with respect

to the O3 plane

etc.

o/“\o/Br

rather than

op\o I

or

o/O\0 \ / Esr-Br

This is analogous to the IC1/03 complex studied by Downs et al. [30] and Andrews and co-workers [ 311. The IR spectra of the products formed on photolysis of the OJBr, complex are much the same as those described by Tevault et al. [28]. In

9

brief, long wavelength photolysis generates IR bands assigned to BrBrO which can be converted with (and a weak complex with Br2, BrBrO-Brz) Nernst glower radiation to a species with a strong band at 526.1 cm-‘, assigned by Tevault et al. [28] to BrOBr. This is the principal photochemical route, but it should be noted that the greater sensitivity of FTIR revealed several other weak features; isotopic data ( 78Br/a*Br; 160/180) showed several of these features to be due to species with Br-0 and/or Br,NO o groups with intensity ratios varying with the matrix ratio of 03/Br,/Ar. These species arise from complexes such as (OS),/ Br,, 03/(Br&, etc., but it was not possible to provide a complete structural assignment: full details of the spectra are given elsewhere [25]. IR Spectrum of BrOBr As mentioned earlier, although the solid spectrum shows two and 504 cm-‘) [23, 241 assigned to Br-0 stretching vibrations, spectra show only one strong IR band (526 cm-‘). We have put of effort into searching for the second stretching band and Fig.

bands (587 the matrix a great deal 5 shows the

a

WAVENUt3BERS

Fig. 5. JR spectra of Br,O generated by photolysis of OJBr, matrices in solid Ar and obtained by computer subtraction. (a) ‘60,/Br,/Ar (l/1/200), (b) ‘80,/Br,/Ar (l/1/200), (l/1/200). The high frequency bands assigned to Br,O are arrowed. (c) ‘6*‘80,/Br,/Ar The region marked with an * in (a) is an artefact due to computer subtraction.

10 TABLE

2

Frequencies

(cm-‘)

of bands assigned

Br160Br

Br”OBr

587,504,197 592,506 526.1 623.4, 526.1

553,476 501.4 592.1,

to BrOBr Ref.

501.2

solid solid matrix matrix

23 24 28 This work

best evidence we have obtained. There is thus reasonable, but not overwhelming, evidence that both stretching vibrations of BrOBr have been observed (the frequencies are given in Table 2) and from the relative intensities in the matrix might be assigned: 623, vl and 526 u3. However, there are several reasons why the reverse assignment is to be preferred. An approximation to the separation of vl and us for a symmetric triatomic molecule ABA with ABA = @ is given by

where f,.,.t is the stretch-stretch interaction constant and f,., is the principal stretching force constant; g,; and g,., are the corresponding g matrix elements.* From the known data for OF* and Cl*0 (see Table 3) we have: OFz: vl - v3 predicted = 75 cm-‘, observed 100 cm-‘; ClzO: vl -- v3 predicted = -65 cm-‘, observed -40 cm-‘. For Br,O, assuming the two stretching fundamentals to be at 623 and 526 cm-‘, and also assuming a bond angle of 113”, the ratio f,.,l/f,.,. can be calculated assuming either v1 > v3 or v3 > v I, i.e. Au = - + 100 or -100 cm-‘; for v1 > v3 f,.,,/f,, -0.5 while for v3 > vl f,.,,/fr, -0.16 compared with ratios of 0.21 and 0.15 for OF* and Cl,0 respectively. Thus, both the Au pattern (OFz, 100 cm-‘; C120, -40 cm-‘; Br,O, -100 cm-‘) and the f,t/f,.,. ratio suggest the assignment v3 = 623 and v 1 = 526 cm-‘. Further support for this assignment comes from the ‘*O isotopic data. The shift in v3 for BAB on symmetric isotopic substitution [39] gives a lower limit (BAB) and upper limit (BAB) to the bond angle BAB via the equation

*We are grateful

to Professor

I. M. Mills for very helpful

discussions

about

this approach.

11

TABLE 3 Spectroscopic

and structural data for OF, and Cl,0

Bond length (A ) Bond angle Frequencies of fundamentals (cm-‘)

Force constants (mdyn A-‘)

OF,

Ref.

Cl,0

Ref.

1.405 103”4’ V, 928 v2 461 v1 831 f(OF) 3.39 f(OF, OF) 0.81

34 34 35a 35 35 36 36

1.70 111” 630.7 296.4 670.8 f(OC1) 2.75 f(OC1, OCl) 0.41

37 37 3gb 38 38 38 38

aGas phase. bSolid. TABLE 4 Lower limit to X0X bond angle (degrees) from va isotopic shift [ 29 ]

OF* Cl,0 Br,O

Assignment la

Assignment 2b

Experimentale

98 104 113

84 84 87

103 111 -

%orrect assignment for OF,, Cl,0 (Table 3); V, < vj for Br,O. bReverse assignment for OF,, C1,O; vI > vg for Br,O. ‘See Table 3.

where MA, MB are the masses of A and B in the isotopically substituted molecule. For BrzO the Br isotope shift (79Br + 8’Br) is too small to resolve; ‘60 + 180 substitution gives a lower limit to the bond angle. Table 4 gives the data for FzO, Cl,O, Br,O with alternative assignments for the u1 and v3 vibrations. The indication is that a better pattern of results is obtained with the assignment v1 < v3 for BrzO. Where does one expect the frequencies of Br,O to appear? Table 5 shows frequencies and force constants for a number of relevant compounds containing O-X bonds; attention is drawn to appropriate ratios. The ratio of u3 values for X0X and YOY (X, Y = halogen) will be given by (ignoring anharmonicity)

v3

(X0X)

v3

(YOY)

=

(

1

21Mx Sin2 +

MO

=fY

(

1 +M

sin’ 0

x0x -

V(OX)

- f(OX,

owl

YOY [f(OY) 2 >

- f(OY,

OVI

2

>

where, f(OX), f(OY) are the principal and f(OX, OX), f(OY, OY) the interaction stretching force constants. For OFz and ClzO, using the appropriate frequencies and bond angles, this relationship predicts a ratio of (f(OF) - f(OF, OF))/(f(OCl) -f(OCl, OCl)) of

12 TABLE

5

Frequencies,

force Ref.

OX Species OF 39 Cl0 40 BrO 28 OCIOBr-

constants

and ratios for some halogen/oxygen Ratio

Frequency (cm-‘)

18”;;.5:-1.21 729.9 _c_

1.16

species

Force constant (mdyn _&-I)

5.41 4.66 4.18

______ -

Ratio

% __1____

1.16 1.11

41 42

HOX Species HOF

43a

HOC1

44

3537.1 (3493)” (-v(OH)) 1359.0 (1394)” (bend) 886.0 (885)” (-v(OF)) 3581 (-v(OH)) 1239 (bend) 7 1.22 729 (-V(OC1)) 1164

HOBr

44

(bend)

3590 (-v(OH)) 626 (-v(OBr))

1.16 >

(6.81)a ( 0.96)a (4.37)a 7.104b 0.775

1.10

3.980 7.143c 0.702

1.11

3.594

X0X Speciesd FOF

ClOCl

928 831 461 631 671 296

(v,) (~2) (~2) (~1) (~3) (~2)

1.24

3.95 0.81

(f(OF)) (f(OF, OF))

2.75 0.41

(f(OCl)) 1 (f(OCl,

1.44

OCl))

aThe figures in parentheses are from ref. 43(a), for HOF . . . HF in a matrix; unbracketed figures are from ref. 43(b), where HOF was completely matrix isolated. There is essentially no difference in the v(O-F) vibration. The force constant calculation assumed a bond angle of 104”. bThe force constant calculations assumed a bond angle of 113”, compared with a known angle of 103 * 3”. ‘Assuming a bond angle of 110”. dSee also Table 3.

1.34, identical to the value obtained from the force field solutions. To apply similar arguments to Cl*0 versus BrzO in order to predict the position of v3 for BrzO, we need to make assumptions about the force constants and structure of BrzO. On the basis of the other X- 0 species a reasonable assumption is that (f(OC1) - f(OC1, OCl))/(f(OBr) - f(OBr, OBr)) = 1.10-1.20; assuming BrOBr = 113”, this predicts v,(Br,O) = 597-572 cm-‘. (For a bond angle of 110” the range is 588-563 cm-* and for 120” the range is 618-591 cm-‘.) It is interesting that there is a band for solid Br,O at 587 cm-‘; for the matrix data the assignment of v3 to 623 cm-’ rather than 526 is also indicated. There are however the problems of the relative intensities of the bands of matrix and solid Br,O and the large solid-matrix shift in frequency. Table 6 lists some appropriate intensity ratios. On the proposed assignment for BrzO

13

TABLE

6

Intensity

ratios in the IR for us/v,

V,IV, OF, Cl,0

Matrix Matrix

Cl,0 Br,O Br,O

Solid Matrix Solid

6.0 -1.0 <0.2 -0.1 -1

Ref. 45 46 38 This work 23,24

it is perhaps surprising that v1 should be much more intense than v~. However, for solid Cl20 v1 is also much more intense than v3 and Rochkind and Pimentel [38] commented that the relative intensities of the fundamentals of solid Cl20 were very sensitive to Cl2 impurities. We have demonstrated above that BrBrO and BrOBr are produced from a Br2/03 weak complex. The BrBrO and BrOBr will therefore share the matrix cage with O2 and it is quite conceivable that interaction between O2 and BrOBr has an effect on the band intensities. However, the interaction cannot be very strong or else the “0 substitution would lead to a more complex spectrum for Br20. The matrix shift for Br,O is much larger than for OF2 and Cl,0 (Table 7). The obvious experiment, which would remove the possible complication of Br,O/O, interaction, is to prepare Br,O and slowly cocondense it with excess Ar onto a cold window. We have tried this apparently trivial experiment many times and were quite unable to obtain convincing spectra: the BrzO readily decomposes so that matrix sites contain Br, and O2 and it would also appear that the interaction between Br,O molecules is sufficiently strong for them to be condensed as (Br,O), in the matrix. We hope others may be tempted to try such experiments and have more luck! In conclusion, we can say that we have probably observed the two stretching vibrations of matrix isolated BrzO but that the problem is not completely resolved. TABLE

7

Matrix shifts (cm-‘)

for X0X

species v (matrix) Ail,

OFza Cl,0 Br,O aShift

+5 +7 +36 from

liquid

Raman.

-

I, (solid) AG,

+4 +10 i22

Ref. 34,35 38,46 23, this work

14 ACKNOWLEDGEMENTS

We thank Drs. J. L. Pascal, G. Davidson and J. S. Ogden for helpful discussions. This work was supported by the SERC. REFERENCES 1 2 3 4

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