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Carbonyl cyanide O-oxide, the adduct of dicyanomethylene and dioxygen in argon matrices at 12 K
05&cas39/94 %.00+0.00 ~1993PeqmonPmsLtd
Spectmhimica AM, Vol. SOA, No. 2, pp. 209-218.19a( Printed in Great Britain
Carbonyl cyanide O-oxide, the adduct of dicyanomethylene and dioxygen in argon matrices at 12 K I. R.
DUNKIN”
and A. M$LUSKEY
Department of Pure and Applied Chemistry, University of Strathclyde, 295 Cathedral Street, Glasgow Gl lXL, U.K. (Received 23 February 1991; in final form 10 March 1993; accepted 16 March 1993)
Ahstraet-Photolysis of dicyanodiazomethane in Ar matrices has been studied. Dicyanomethylene appears to be the sole product in dilute matrices, and its UV absorption spectrum and a revised IR spectrum are reported. In O,-doped Ar matrices, dicyanomethylene reacts with 0, to give carbonyl cyanide O-oxide as a very photolabile adduct. Four IR bands and a broad UV absorption at 375 nm are assigned to this species. On further photolysis, the carbonyl oxide rapidly decomposes to give carbonyl cyanide, by O-atom expulsion. A strong product absorption at 585 nm is assigned to a charge-transfer complex involving the expelled oxygen, but the precise nature of this species remains unknown.
IN
1983 the first report was made from this laboratory of the direct observation of a matrix isolated carbonyl oxide, cyclopentadienone O-oxide (1) [l]. It was generated by the reaction of a carbene, cyclopentadienylidene, with Oz. Since then there has been a growing interest in the matrix chemistry of these reactive species, and some have also been studied by time-resolved spectroscopy in fluid solution. A recent review by SANDER lists 25 carbonyl oxides for which spectroscopic or theoretical data have been obtained
PI. Following the earliest reports, it was shown that carbonyl oxides undergo very facile photochemical decompositon by two possible pathways: ejection of an oxygen atom to give the corresponding ketone or aldehyde, or rearrangement via a dioxirane, e.g. compound 2, to an ester or lactone, e.g. compound 3, in which both oxygen atoms are retained [2,3]. hv -0
(1) hv
\ hv
(2)
(3)
The simplest carbonyl oxide is formaldehyde O-oxide but, despite theoretical interest in the molecule, it has so far eluded spectroscopic detection. The reaction of methylene with O2 in Ar matrices has been investigated by LEE and PIMENTEL, but they were unable to observe either the carbonyl oxide or dioxirane [4]. The main product was found to be formic acid, however, which does suggest the sequential intermediacy of both these species. A key question in the previous work on carbonyl oxides has been whether their electronic structures are best depicted as singlet polar diradicals, as in 1, or as zwitterions. The spectroscopic and theoretical evidence now seems to favour the polar * Author to whom correspondence should be addressed. 209
210
I. R. DUNKIN and A. MCCLUSKEY
diradical structure [2, S--7], but it is recognized that substituents could exert a profound influence. As part of our continued research into these species, therefore, it seemed worthwhile to study the effect of highly electron-attracting groups on the structure and reactivity of a carbonyl oxide. A report by SMITH and LERCMon the matrix isolation of dicyanomethylene [8] prompted us to study the reaction between this very electron deficient carbene and Oz. The present paper describes our study of the photolysis of dicyanodiazomethane in Ar matrices and the reaction of the resulting dicyanomethylene with 4. At an early stage, we found discrepancies between our IR spectrum of dicyanomethylene and that reported previously. This led us to investigate the identity of the carbene more fully than we had originally intended. We were eventually able to confirm that dicyanomethylene had been observed by the earlier workers, although we offer a partial revision of its IR spectrum and report its electronic absorption spectrum for the first time.
EXPERIMENTAL Equipment The matrix isolation equipment used in this work has been described in detail previously [9]. It consisted of a CsBr window in a metal holder, enclosed in a glass and metal vacuum shroud and cooled by an Air Products Displex model CSA 202 closed cycle helium refrigerator. The vacuum shroud was fitted with external KBr windows and one or two inlet ports. For the best results with W-vis spectroscopy, a CaFZ cold window was used in place of the usual CsBr window. Temperatures were measured by means of(i) a hydrogen vapour bulb and (ii) a Chromel-Au-O.07 atom% Fe thermocouple connected to an Air Products APD-B temperature controller, which also operated a small resistance heater in thermal contact with the cold window. The base temperature attainable was about 12 K. IR spectra were recorded on a Perkin-Elmer model PE 684 grating spectrometer interfaced with a Perkin-Elmer model 3600 Data Station. The spectrometer was calibrated regularly against polystyrene, and quoted band frequencies are accurate to better than + 2 cm-‘. UV-vis spectra were recorded on a Shimadzu model UV-250 spectrometer interfaced with a Hewlett Packard model HB86B computer via a Shimadzu OP-1 interface. Background spectra were run at the beginning of each experiment and all the spectra shown have the appropriate background subtracted. Photolysis was carried out by means of an Oriel200 W high pressure Hg arc, equipped with a water filter and, when needed, cut off filters or an Applied Photophysics high radiance ff3.4 grating monochromator. Owing to the high photosensitivity of carbonyl oxides, it was customary to carry out both photolysis and spectroscopic measurement in a darkened labaratory.
Materials Research grade Ar ( 299.9997%), CO ( 299.96%) and Or ( 299.97%) were obtained from BOC Ltd. Small quantities of Or were prepared as needed by subjecting low pressures of Or to the discharge from a Tesla coil, freezing at 77 K (liquid Nr), and pumping to remove residual Or. Tetracyanoethylene (TCNE) was a commercial sample used without further puritication. Carbonyl cyanide was obtained from TCNE by hydrogen peroxide oxidation to tetracy~oethylene oxide [lo], followed by reaction with di-n-butyl snlphide [ll]. (Warning: carbonyl cyanide reacts explosively with water to give HCN and COZ, and its use should be confined to an efficient fume cupboard.) Dicyanodiazomethane was prepared, following literature procedures, by bro~nation of malononitrlle 1121, reaction of the resulting ~bromomalono~t~le with anhydrous hydrazine 1131,and oxidation of the hydrazone with Pb(OAc)., (131. (Warning: dicyanodiazomethane is a potent explosive, thermally unstable above 75”C, and should be handled with the utmost care.)
Dicy~~~~methane was too involatile for its vapour pressure to be measured by simple manometry. Matrices were therefore formed by allowing part or all of a stream of host gas to pass over a sample of di~anodi~ometh~e contained in a glass side arm at room temperature or below, followed by deposition of the resulting gas mixture on the cold window at 12-20 K. Ar or other host gases were usually admitted to the system at rates of 0.1-0.3 1atm h-l, app~ximately 10
Carbonylcyanide O-oxide in Ar matrices
211
4z
a
1 .A M
(b)
I 2500
I
I
I
I
2000
I
I
I
I
v/cm-
I
I
I 1000
1500
I
I
I
II 500
*
Fig. 1. IR spectrum of dicyanodiazomethane (4) in Ar at 12K (a) after deposition, (b) after 45 min photolysis with d > 200 nm. Bands which remained unchanged during the photolysis belong to the atmospheric constituents CO, (C) and Hz0 (W), arising from a slight leak in the vacuum system.
times those adopted by SMITHand LEROI, and spray-on was terminated after about 1 h. Matrix ratios could not be determined, but the results indicate that good isolation was achieved.
RESULTSAND DISCUSSION Dicyanodiazomethane
in Ar matrices
In Ar at 12 K under the conditions that we adopted, dicyanodiazomethane 4 exhibited only six IR absorptions significantly above the noise level. These are shown in Fig. l(a) and listed in Table 1. Sm and LEROI [8] tabulated the IR absorptions of compound 4 only for the neat solid at 77 K, but our 12 K Ar-matrix spectrum appears to agree well with the 4 K Ar-matrix spectrum shown in the same paper. Comparison of the IR spectra shows, however, that much weaker absorptions of compound 4 were obtained in our experiments, even with comparable or greater amounts of Ar. We are thus confident that our matrices were considerably more dilute than those deposited by SMVIITH and LEROI. Photolysis of compound 4 in Ar at 12 K (A> 200 nm or 1= 255 + 10 nm) resulted in the disappearance of the IR bands of compound 4 and the appearance of just three significant new bands, which we attribute to the carbene, dicyanomethylene (5) (Fig. l(b) and Table 1). The strongest of the new bands at 1756 cm-’ agrees exactly with the strongest band of 5 reported by SMITHand LEROI, but the other two were not observed by these previous workers. Moreover, they identified two additional bands at 1158 and 392 cm-’ as belonging to 5, which do not appear in our product spectra. None of the bands observed by either group belong to the dimer of 5, tetracyanoethylene (TCNE), for which we recorded a separate Ar-matrix IR spectrum (Table 1). It is clear that in S~rrrr and LEROI’S experiments several other species were produced as well as 5, presumably as a result of the higher concentrations of their matrices. They reported a total of 16 product IR bands, most of which were not assigned, but they attributed three of these to 5 and another at 2045 cm-’ to the CN radical, because these alone disappeared on annealing. When we annealed matrices containing 5 to 35 K, no
212
I. R.
DUNKIN
and A. MCCLUSKEY
Table 1. IR absorptions of dicyanodiazomethane (4), dicyanomethylene (5) and tetracyanoethylene (TCNE) in Ar matrices at 12 K* IR absorptions (cm-‘) 5
4 2333 m 2131 m 2119 vs 1239 m 1209 517
TCNE
1916 1826 1756 m
2257 vs 2252 s 2221 m 1152 vs 1116 1082 956 m 929 912 798 767 670 576 556
* Bands were weak unless denoted vs very strong, s strong, or m medium.
were observed. The failure of 5 to dime&e to TCNE in our experiments is additional evidence of good isolation. The simplicity of our product spectra and the consistent behaviour of the three IR bands attributed to 5 in all further experiments (uide infra) provide strong support for the new assignments.
changes
CN
NC N2
)= NC (4)
hv -N2
> CN
:L
co
(5)
NC F NC
co
(6)
With less material deposited on the cold window it was possible to record UV-vis spectra of 4 and 5 in Ar at 12 K. The diazo compound (4) showed a single absorption with A,,,,= 255 nm. Irradiation into this band with monochromated light (A= 255 + 10 nm) resulted in its disappearance and the appearance of a new spectrum with a highly structured series of fairly sharp absorptions at 280-360nm and several weaker and broader absorptions at longer wavelengths (Fig. 2 and Table 2). These are all attributed to dicyanomethylene (5). The sharp absorptions at 280-360 nm form two distinct series (A and B in Fig. 2) each with separations of about 1650-1870 cm-‘. The excited state vibration involved could therefore be a counterpart of any of the three observed groundstate vibrations. The vibronic origin is probably too weak to appear in our spectrum. Dicyanodiazomethane
in CO and CO-doped Ar matrices
As further confirmation of the identity of compound 5, we investigated the photolysis of compound 4 in both pure CO matrices and Ar matrices doped with CO. In previous studies, CO has proved to be an efficient carbene scavenger [14], and the appearance of the expected ketene product (6) from the reaction of 5 and CO would provide additional strong evidence for the identity of the matrix photoproduct of substance 4. In pure CO at 12 K, compound 4 exhibited an IR spectrum very similar to that reported for Ar matrices, though the region at about 2160-2120 cm-’ was obscured by strong host-gas absorption. On photolysis (A= 255 + 10 nm) these bands diminished in intensity, virtually disappearing after several hours, but the bands assigned to the carbene 5 did not appear. Instead, as the bands of compound 4 diminished, synchronous growth was observed of a medium to strong band at 2185 cm-’ together with weak bands at 1202,953 and 566 cm-‘. The band at 2185 cm-’ occurs in the characteristic region for
213
Carbonyl cyanide O-oxide in Ar matrices
500
400
300 X/nm
Fig. 2. UV-vis absorpton spectrum of dicyanomethylene (5), obtained after complete photolysis (I. = 255 + 10 nm) of dicyanodiazomethane (4) in Ar at 12 K.
v(C=C=O), of ketenes; so the four new product bands are assigned to the expected ketene 6. In an Ar matrix at 12 K containing 1% CO, photolysis of 4 produced both the carbene 5 (with IR bands not significantly shifted from their frequencies in pure Ar) and the ketene 6 (y(C=C=O), shifted slightly to 2183 cm-‘). Annealing this matrix at 30-35 K resulted in a reduction in intensity of the bands due to 5 and synchronous growth of those due to 6. Thus the ketene is produced as a thermal adduct of CO and 5 even at these very low temperatures. This behaviour is in accord with that observed for other matrix isolated carbenes [ 141. Dicyanodiuzomethane
in O,-doped Ar matrices
Once satisfied about the identity of the carbene 5, we with Oz. Full arc photolysis (A> 200 nm) of compound 4 O2 resulted, after 60 min, in the complete removal of the a complex set of new absorptions (Table 3). Annealing
turned to a study of its reaction in an Ar matrix containing 10% IR bands of 4 and the growth of at 35 K yielded no new species.
Table 2. UV-vis absorptions of dicyanomethylene (5) in Ar matrices at 12 K* UV-vis absorptions (nm) 470 w br 445wsh 377 w br
357.7 A 348.3 B 335.7 A 326.9 B 316.5 A
314.1 sh 306.5 B 300.2 sh A 293.7 B
286.1 A
* Bands are denoted as br broad, sh shoulder, or w weak; or, where appropriate, assigned to series A or B (see Fig. 2).
I. R. DUNKINand A. MCCLUSKEY
214
Table 3. IR absorptions observed after photolysis of dicyanodiazomethane (4) in AI containing 10% 0, at 12 K, compared with those of carbonyl cyanide (8) in pure Ar (MR 11200) at 12K IR absorptions (cm-‘) 8
Photoproduct 2344 2280 2260
2238 >‘t 2233
1716 1707 m 1449 1445 1426 1375
Product assignment
8
co2
? ?
2235 2209 1916 1839 1826 1756 1739 1727 1709 1446
8 ? 5 ? 5 5 ? 8 8 8
1419
8
1308 1196
? ?
1136 1127m 1102 1096 m
1037 1006 804 798 793 767 744 733 721 s 714
Photoproduct 1143t 1137 1131 1112 1097 1089 1084 1074 1038
714 707 662
Product assignment 8 8 8 8 8 ? ? ? 8 + Oj?$
8 8 co2
655 * Bands of 8 were weak unless denoted s strong or m medium. t Likely matrix splitting. $ vj of OS and a weak band of 8 overlap at 1037-1038 cm-i, but relative intensities in the photoproduct spectrum suggested that both species could have been present.
Even at such a high O2 concentration, the product included an easily detectable proportion of the carbene 5. The major product was identified as carbonyl cyanide (8), by comparison with an authentic Ar-matrix IR spectrum (Table 3). All the stronger bands of compound 8 and many of the weaker ones were visible in the product spectrum, although some small shifts in frequency due to the presence of 10% O2 were noted. A number of bands in the product spectrum remain unassigned, however. Notable amongst these are bands at 2280 and 2260 and 2209 cm-‘, in the region where cyanide oxidation products such as RCNO, RNCO or ROCN are expected to have strong absorptions, and a band at 1839cm-‘, which is probably a carbonyl band. Subsequent experiments showed that, of these, only the 2260 cm-’ band could have belonged to a primary or secondary photoproduct. The others seemed to arise in photo-oxidation reactions at a late stage of the photolysis (uide in@). In order to try to identify some of the products formed in 10% O,/Ar matrices as a result of futher oxidation reactions, attempts were made to photo-oxidize compound 8. Irradiation (A> 200 or Iz= 255 f 10 nm) in 10% O,/Ar or 5% OJAr at 12 K, however, gave no discernible reaction. It is therefore possible that the photo-oxidations require the presence of O-atoms as well as OZ. Finally, it is possible that a small amount of ozone was formed in these experiments. The strongest band of O3 (Q 1038 cm-‘) [15,16] overlaps with a weak band of compound 8, but relative band intensities in some experiments suggested that both species could have been present. The formation of ozone as a photoproduct in carbene oxygen chemistry has been noted before [17], but under the conditions that we have used in this laboratory we have not observed it with any of the previously studied carbonyl oxides. It presumably arises from photo-induced O-atom transfer from the carbonyl oxide to OZ. Narrow band photolysis (2 = 255 f 10 nm) of similar matrices of compound 4 in 10% O,/Ar gave essentially identical results. No IR bands that could be assigned to the expected carbonyl oxide (7) were observed in any of these experiments.
Carbonylcyanide O-oxide in Ar matrices
215
When the oxygen concentration was reduced, the photoproduct spectrum became less complex. Narrow band photolysis (A= 255 + 10 nm) of 4 in 1% OJAr gave initially only the carbene 5 and small quantities of carbonyl cyanide (8). When the matrix was subsequently annealed at 35 K, four IR bands which had not been observed before appeared at 1290,1275,1264 and 1245 cm-‘. These disappeared again after only l-2 min further photolysis with light of rE> 345 nm (Fig. 3). At the same time, the IR absorptions of carbonyl cyanide (8), possibly the strongest band of ozone (y 1038 cm-‘) and the unassigned band at 2260 cm-’ grew. The four bands at 1290-1245 cm-’ were at all times very weak, but they appear to belong to an adduct of dicyanomethylene (5) and O2 which is also a precursor of carbonyl cyanide. In the light of all precedents [l-3, 5-7, 141, therefore, these can be assigned to the carbonyl oxide, carbonyl cyanide O-oxide (7) rather than the dioxirane (9). Prominent absorptions in the region 1100-1250 cm-’ are found in the IR spectra of compound 4, TCNE, 6 and 8, and can be attributed to skeletal modes of the dicyanomethylene group (principally v(C-C) and, in the case of 4, v(C=N)) [8]. P reviously studied carbonyl oxides have prominent IR bands close to WOcm-‘, assigned to ~(0-0) modes, and others in the region 14W-1300cm-1. The latter are diicult to assign with confidence, but there is evidence from “0 shifts of some contribution from v(C=O) [7,18]. It is likely that the four bands assigned to 7 have contributions from both dicyanomethylene skeletal modes and carbonyl oxide v(C=O). Unfortunately, ‘802, which could help in the assignment, is currently unavailable. The non-appearance of bands near 900 cm-’ in our spectrum of 7 must be attributed to the weakness of the spectrum overall. As a result we are unable to conclude that the cyano groups strengthen the O-O bond, as has been inferred for the trifluoromethyl group, which is also strongly electron withdrawing [2,18]. The band at 226Ocm-’ remained weak in the experiments in 1% OJAr and clearly belonged to a minor product. In analogy with the photochemistry of other carbonyl
Fig. 3. IR spectra in the range 1350-1200 cm-’ (a) obtained after nearly complete photolysis (L = 255 f 10 mn) of dicyanodiazomethane (4) in 1% OJAr at 12 K followed by annealing at 35 K, and (b) after a further 2 min photolysis with It> 345 nm. Spectrum (c) is the difference spectrum (a) - (b), showing bands which were removed during the second period of photolysis. Bands marked C arc assigned to carbonyl oxide (7). while those marked D belong to residual starting material 4.
I.R. DUNKIN and A. MCCLUSKEY
216
200
400
300
500
600
700
X/nm
Fig. 4. UV-vis absorption spectra (a) obtained after photolysis (I = 255 + 10 nm) of dicyanodiazomethane (4) in 2% O,/Ar at 12 K followed by annealing at 35 K, and (b) after a further 2 min photolysis of the resulting matrix with L > 345 nm. The weak broad band, which disappears on A>345 nm photolysis (C) is assigned to carbonyl oxide (7). it could possibly have been the carbonyl cyanate 10formed via dioxirane 9, but this species has not so far been characterized and an authentic sample is therefore unavailable.
oxides,
CN
CN
0.
02
): CN (5)
0’
CN A
d-
0
+o
k
CN
CN
(6)
(7) 1 hv ? I NC
hv
0
Y 0
(W ‘CN
The same process could be followed by UV-vis spectroscopy. In a typical experiment, the diazo precursor 4 in 2% OJAr was photolysed with monochromated light (L = 255 + 10 nm). Two products could be identified in the UV-vis spectrum: carbene (5), recognized by its characteristic series of absorptions at 280-360 nm, and a small amount of a species with absorptons at A,,,,= 425 sh and 585 nm. The weak broad absorption of 5 at 377 nm was also observed, but those at longer wavelengths were obscured by the other product’s absorption, cf. Fig. 4(a). Annealing the matrix at 35 K resulted in a reduction in intensity of the absorptions of 5 and the growth of a weak broad absorption with A,, = 375 nm (Fig. 4(a)). Further brief photolysis with light of 1~ 345 nm bleached this weak absorption, while that at A,,,,,= 585 nm grew considerably. Strong absorption at L < 320 nm, previously lying under other absorptions, also grew and obscured most of the remaining carbene bands (Fig. 4(b)). The weak 375 nm absorption observed on annealing almost certainly belonged to the same species observed in the IR spectrum in parallel experiments, and is thus assigned to the carbonyl oxide 7. The position of this absorption lies at the lower wavelength end of the range (ca 380-460 nm) observed for other carbonyl oxides [2]. The effect of the cyano groups is therefore to shift this electronic absorption to higher energy. A similar effect has been noted for the trifluoromethyl group, as manifested by the shift of 1, from 422 nm in benzophenone O-oxide and 387 nm in benzaldehyde O-oxide to 378 nm
Carbonyl cyanide O-oxide in Ar matrices
217
in trifluoroacetophenone O-oxide [2,18]. Carbonyl cyanide O-oxide has the shortest wavelength so far reported for this absorption of a carbonyl oxide. Carbonyl cyanide (8) [19,20] has an absorption band at about 250-350 nm, and ozone in Ar matrices (ca 7% 0,) has rZ, at 26Onm and no prominent maxima at longer wavelengths (M. Lynch, unpublished observations). Thus the formation of 8, with or without ozone, accounts for the growth in absorption below 320 nm observed when 7 was photolysed. We are unable to assign the product absorptions at 425 sh and 585 nm to any of the expected products, however. There is no IR evidence for a significant product other than 8, and we thus suggest that the strong absorptions in the visible may belong to a charge-transfer complex formed between O-atoms and 8 remaining together in the same matrix cage. Further investigation of this possibility will be undertaken.
CONCLUSIONS
Photolysis of dicyanodiazomethane (4) in Ar matrices has been studied in conditions of greater dilution than achieved previously [8]. It has been confirmed that the product is dicyanomethylene (S), but a revision of its IR spectrum has now been made (Table 1). In addition the UV absorption spectrum of 5 has been reported for the first time (Fig. 2 and Table 2). In the conditions used in this study, dicyanomethylene showed no tendency to dimerize to tetracyanoethylene, but a thermal reaction with CO yielding the corresponding ketene (6) did occur on annealing to 30 K or above, thus providing further support for the identity of 5. In O,-doped Ar matrices, dicyanomethylene reacted with O2 to give carbonyl cyanide O-oxide (7) as a very photolabile initial adduct. Four IR bands and a broad UV absorption at 375 nm are assigned to this species, but the weakness of all of them showed that it was impossible to build up substantial concentrations of 7 in the experimental conditions employed. No other absorptions attributable to 7 could be observed. The UV absorption of 7 has the shortest wavelength of all carbonyl oxides so far reported. On further photolysis, the carbonyl oxide rapidly decomposed at relatively long wavelengths (2 > 345 nm) to give carbonyl cyanide (8), possibly ozone, and a species with stong absorptions at 425 sh and 585 nm, which is tentatively identified as a chargetransfer complex between O-atoms and 8. A single unassigned product IR band at 2260 cm-’ might have belonged to the rearrangement product 10, but this could not be confirmed, and in any case represented only a very minor pathway. Acknowledgements-We thank the SERC for supporting this work with an Earmarked Studentship and equipment grants, and Mr M. Lynch for preparing some of the ozone.
REFERENCES [l] G. A. Bell and I. R. Dunkin, I. Chem. Sot., Chem. Commun. 1213 (1983). [2] W. Sander, Angew. Chem. Int. Ed. Engf. 29,344 (1990). [3] I. R. Dunkin and C. J. Shields, 1. Chem. Sot., Chem. Commun. 154 (19%). [4] Y.-P. Lee and G. C. Pimentel, J. Chem. Phys. 74,4851 (1981). [5] G. A. Bell, I. R. Dunkin and C. J. Shields, Specrrochim. Acta 41A, 1221 (1985). [6] I. R. Dunkin and G. A. Bell, Terrchedron41,339 (1985). [7] I. R. Dunkin, G. A. Bell, F. G. McCleod and A. McCluskey, Spectrochim. Acta 42A. 567 (19%). [8] W. H. Smith and G. E. Leroi, Spectrochim. Acta 2SA, 1917 (1969). [9] I. R. Dunkin and J. G. MacDonald, J. Gem. Sot., Perkin Trans. 2 2079 (1984). [lo] W. J. Linn, 0. W. Webster and R. E. Benson, J. Am. Chem. Sot. 87, 3561 (1%5). [ll] E. L. Martin, Org. Synrh. 51, 70 (1971). [12] W. J. Linn, Org. Synrh. CON. 5, 1007 (1973). [13] E. Ciganek, J. Org. Chem. 30,4198 (1%5). (141 M. S. Baird, I. R. Dunkin, N. Hacker, M. Poliakoff and J. J. Turner, /. Am. Gem. Sot. 103,519O (1981).
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[15] [la] [17] [18] [19] [20]
I. R. DUNKINand A. MCCLUSKEY L. Andrews and R. C. Spiker, J. Pl?ys. Chem. 76,3208 (1972). L. Brewer and J. L.-F. Wang, 1. Gem. Phys. 56,759 (1972). 0. L. Chapman and T. C. Hess, J. Am. Chem. Sot. 106,1842 (1984). W. W. Sander, 1. Org. Gem. 53,121 (1988). W. KemuIa and K. L. Wienchowski, Rocrniki Chem. 27,524,527 (1953). W. KemuIa and K. L. Wierzchowski, Ckm. Abs. 48,6253bd (1954).
Spectmhimica AM, Vol. SOA, No. 2, pp. 209-218.19a( Printed in Great Britain
Carbonyl cyanide O-oxide, the adduct of dicyanomethylene and dioxygen in argon matrices at 12 K I. R.
DUNKIN”
and A. M$LUSKEY
Department of Pure and Applied Chemistry, University of Strathclyde, 295 Cathedral Street, Glasgow Gl lXL, U.K. (Received 23 February 1991; in final form 10 March 1993; accepted 16 March 1993)
Ahstraet-Photolysis of dicyanodiazomethane in Ar matrices has been studied. Dicyanomethylene appears to be the sole product in dilute matrices, and its UV absorption spectrum and a revised IR spectrum are reported. In O,-doped Ar matrices, dicyanomethylene reacts with 0, to give carbonyl cyanide O-oxide as a very photolabile adduct. Four IR bands and a broad UV absorption at 375 nm are assigned to this species. On further photolysis, the carbonyl oxide rapidly decomposes to give carbonyl cyanide, by O-atom expulsion. A strong product absorption at 585 nm is assigned to a charge-transfer complex involving the expelled oxygen, but the precise nature of this species remains unknown.
IN
1983 the first report was made from this laboratory of the direct observation of a matrix isolated carbonyl oxide, cyclopentadienone O-oxide (1) [l]. It was generated by the reaction of a carbene, cyclopentadienylidene, with Oz. Since then there has been a growing interest in the matrix chemistry of these reactive species, and some have also been studied by time-resolved spectroscopy in fluid solution. A recent review by SANDER lists 25 carbonyl oxides for which spectroscopic or theoretical data have been obtained
PI. Following the earliest reports, it was shown that carbonyl oxides undergo very facile photochemical decompositon by two possible pathways: ejection of an oxygen atom to give the corresponding ketone or aldehyde, or rearrangement via a dioxirane, e.g. compound 2, to an ester or lactone, e.g. compound 3, in which both oxygen atoms are retained [2,3]. hv -0
(1) hv
\ hv
(2)
(3)
The simplest carbonyl oxide is formaldehyde O-oxide but, despite theoretical interest in the molecule, it has so far eluded spectroscopic detection. The reaction of methylene with O2 in Ar matrices has been investigated by LEE and PIMENTEL, but they were unable to observe either the carbonyl oxide or dioxirane [4]. The main product was found to be formic acid, however, which does suggest the sequential intermediacy of both these species. A key question in the previous work on carbonyl oxides has been whether their electronic structures are best depicted as singlet polar diradicals, as in 1, or as zwitterions. The spectroscopic and theoretical evidence now seems to favour the polar * Author to whom correspondence should be addressed. 209
210
I. R. DUNKIN and A. MCCLUSKEY
diradical structure [2, S--7], but it is recognized that substituents could exert a profound influence. As part of our continued research into these species, therefore, it seemed worthwhile to study the effect of highly electron-attracting groups on the structure and reactivity of a carbonyl oxide. A report by SMITH and LERCMon the matrix isolation of dicyanomethylene [8] prompted us to study the reaction between this very electron deficient carbene and Oz. The present paper describes our study of the photolysis of dicyanodiazomethane in Ar matrices and the reaction of the resulting dicyanomethylene with 4. At an early stage, we found discrepancies between our IR spectrum of dicyanomethylene and that reported previously. This led us to investigate the identity of the carbene more fully than we had originally intended. We were eventually able to confirm that dicyanomethylene had been observed by the earlier workers, although we offer a partial revision of its IR spectrum and report its electronic absorption spectrum for the first time.
EXPERIMENTAL Equipment The matrix isolation equipment used in this work has been described in detail previously [9]. It consisted of a CsBr window in a metal holder, enclosed in a glass and metal vacuum shroud and cooled by an Air Products Displex model CSA 202 closed cycle helium refrigerator. The vacuum shroud was fitted with external KBr windows and one or two inlet ports. For the best results with W-vis spectroscopy, a CaFZ cold window was used in place of the usual CsBr window. Temperatures were measured by means of(i) a hydrogen vapour bulb and (ii) a Chromel-Au-O.07 atom% Fe thermocouple connected to an Air Products APD-B temperature controller, which also operated a small resistance heater in thermal contact with the cold window. The base temperature attainable was about 12 K. IR spectra were recorded on a Perkin-Elmer model PE 684 grating spectrometer interfaced with a Perkin-Elmer model 3600 Data Station. The spectrometer was calibrated regularly against polystyrene, and quoted band frequencies are accurate to better than + 2 cm-‘. UV-vis spectra were recorded on a Shimadzu model UV-250 spectrometer interfaced with a Hewlett Packard model HB86B computer via a Shimadzu OP-1 interface. Background spectra were run at the beginning of each experiment and all the spectra shown have the appropriate background subtracted. Photolysis was carried out by means of an Oriel200 W high pressure Hg arc, equipped with a water filter and, when needed, cut off filters or an Applied Photophysics high radiance ff3.4 grating monochromator. Owing to the high photosensitivity of carbonyl oxides, it was customary to carry out both photolysis and spectroscopic measurement in a darkened labaratory.
Materials Research grade Ar ( 299.9997%), CO ( 299.96%) and Or ( 299.97%) were obtained from BOC Ltd. Small quantities of Or were prepared as needed by subjecting low pressures of Or to the discharge from a Tesla coil, freezing at 77 K (liquid Nr), and pumping to remove residual Or. Tetracyanoethylene (TCNE) was a commercial sample used without further puritication. Carbonyl cyanide was obtained from TCNE by hydrogen peroxide oxidation to tetracy~oethylene oxide [lo], followed by reaction with di-n-butyl snlphide [ll]. (Warning: carbonyl cyanide reacts explosively with water to give HCN and COZ, and its use should be confined to an efficient fume cupboard.) Dicyanodiazomethane was prepared, following literature procedures, by bro~nation of malononitrlle 1121, reaction of the resulting ~bromomalono~t~le with anhydrous hydrazine 1131,and oxidation of the hydrazone with Pb(OAc)., (131. (Warning: dicyanodiazomethane is a potent explosive, thermally unstable above 75”C, and should be handled with the utmost care.)
Dicy~~~~methane was too involatile for its vapour pressure to be measured by simple manometry. Matrices were therefore formed by allowing part or all of a stream of host gas to pass over a sample of di~anodi~ometh~e contained in a glass side arm at room temperature or below, followed by deposition of the resulting gas mixture on the cold window at 12-20 K. Ar or other host gases were usually admitted to the system at rates of 0.1-0.3 1atm h-l, app~ximately 10
Carbonylcyanide O-oxide in Ar matrices
211
4z
a
1 .A M
(b)
I 2500
I
I
I
I
2000
I
I
I
I
v/cm-
I
I
I 1000
1500
I
I
I
II 500
*
Fig. 1. IR spectrum of dicyanodiazomethane (4) in Ar at 12K (a) after deposition, (b) after 45 min photolysis with d > 200 nm. Bands which remained unchanged during the photolysis belong to the atmospheric constituents CO, (C) and Hz0 (W), arising from a slight leak in the vacuum system.
times those adopted by SMITHand LEROI, and spray-on was terminated after about 1 h. Matrix ratios could not be determined, but the results indicate that good isolation was achieved.
RESULTSAND DISCUSSION Dicyanodiazomethane
in Ar matrices
In Ar at 12 K under the conditions that we adopted, dicyanodiazomethane 4 exhibited only six IR absorptions significantly above the noise level. These are shown in Fig. l(a) and listed in Table 1. Sm and LEROI [8] tabulated the IR absorptions of compound 4 only for the neat solid at 77 K, but our 12 K Ar-matrix spectrum appears to agree well with the 4 K Ar-matrix spectrum shown in the same paper. Comparison of the IR spectra shows, however, that much weaker absorptions of compound 4 were obtained in our experiments, even with comparable or greater amounts of Ar. We are thus confident that our matrices were considerably more dilute than those deposited by SMVIITH and LEROI. Photolysis of compound 4 in Ar at 12 K (A> 200 nm or 1= 255 + 10 nm) resulted in the disappearance of the IR bands of compound 4 and the appearance of just three significant new bands, which we attribute to the carbene, dicyanomethylene (5) (Fig. l(b) and Table 1). The strongest of the new bands at 1756 cm-’ agrees exactly with the strongest band of 5 reported by SMITHand LEROI, but the other two were not observed by these previous workers. Moreover, they identified two additional bands at 1158 and 392 cm-’ as belonging to 5, which do not appear in our product spectra. None of the bands observed by either group belong to the dimer of 5, tetracyanoethylene (TCNE), for which we recorded a separate Ar-matrix IR spectrum (Table 1). It is clear that in S~rrrr and LEROI’S experiments several other species were produced as well as 5, presumably as a result of the higher concentrations of their matrices. They reported a total of 16 product IR bands, most of which were not assigned, but they attributed three of these to 5 and another at 2045 cm-’ to the CN radical, because these alone disappeared on annealing. When we annealed matrices containing 5 to 35 K, no
212
I. R.
DUNKIN
and A. MCCLUSKEY
Table 1. IR absorptions of dicyanodiazomethane (4), dicyanomethylene (5) and tetracyanoethylene (TCNE) in Ar matrices at 12 K* IR absorptions (cm-‘) 5
4 2333 m 2131 m 2119 vs 1239 m 1209 517
TCNE
1916 1826 1756 m
2257 vs 2252 s 2221 m 1152 vs 1116 1082 956 m 929 912 798 767 670 576 556
* Bands were weak unless denoted vs very strong, s strong, or m medium.
were observed. The failure of 5 to dime&e to TCNE in our experiments is additional evidence of good isolation. The simplicity of our product spectra and the consistent behaviour of the three IR bands attributed to 5 in all further experiments (uide infra) provide strong support for the new assignments.
changes
CN
NC N2
)= NC (4)
hv -N2
> CN
:L
co
(5)
NC F NC
co
(6)
With less material deposited on the cold window it was possible to record UV-vis spectra of 4 and 5 in Ar at 12 K. The diazo compound (4) showed a single absorption with A,,,,= 255 nm. Irradiation into this band with monochromated light (A= 255 + 10 nm) resulted in its disappearance and the appearance of a new spectrum with a highly structured series of fairly sharp absorptions at 280-360nm and several weaker and broader absorptions at longer wavelengths (Fig. 2 and Table 2). These are all attributed to dicyanomethylene (5). The sharp absorptions at 280-360 nm form two distinct series (A and B in Fig. 2) each with separations of about 1650-1870 cm-‘. The excited state vibration involved could therefore be a counterpart of any of the three observed groundstate vibrations. The vibronic origin is probably too weak to appear in our spectrum. Dicyanodiazomethane
in CO and CO-doped Ar matrices
As further confirmation of the identity of compound 5, we investigated the photolysis of compound 4 in both pure CO matrices and Ar matrices doped with CO. In previous studies, CO has proved to be an efficient carbene scavenger [14], and the appearance of the expected ketene product (6) from the reaction of 5 and CO would provide additional strong evidence for the identity of the matrix photoproduct of substance 4. In pure CO at 12 K, compound 4 exhibited an IR spectrum very similar to that reported for Ar matrices, though the region at about 2160-2120 cm-’ was obscured by strong host-gas absorption. On photolysis (A= 255 + 10 nm) these bands diminished in intensity, virtually disappearing after several hours, but the bands assigned to the carbene 5 did not appear. Instead, as the bands of compound 4 diminished, synchronous growth was observed of a medium to strong band at 2185 cm-’ together with weak bands at 1202,953 and 566 cm-‘. The band at 2185 cm-’ occurs in the characteristic region for
213
Carbonyl cyanide O-oxide in Ar matrices
500
400
300 X/nm
Fig. 2. UV-vis absorpton spectrum of dicyanomethylene (5), obtained after complete photolysis (I. = 255 + 10 nm) of dicyanodiazomethane (4) in Ar at 12 K.
v(C=C=O), of ketenes; so the four new product bands are assigned to the expected ketene 6. In an Ar matrix at 12 K containing 1% CO, photolysis of 4 produced both the carbene 5 (with IR bands not significantly shifted from their frequencies in pure Ar) and the ketene 6 (y(C=C=O), shifted slightly to 2183 cm-‘). Annealing this matrix at 30-35 K resulted in a reduction in intensity of the bands due to 5 and synchronous growth of those due to 6. Thus the ketene is produced as a thermal adduct of CO and 5 even at these very low temperatures. This behaviour is in accord with that observed for other matrix isolated carbenes [ 141. Dicyanodiuzomethane
in O,-doped Ar matrices
Once satisfied about the identity of the carbene 5, we with Oz. Full arc photolysis (A> 200 nm) of compound 4 O2 resulted, after 60 min, in the complete removal of the a complex set of new absorptions (Table 3). Annealing
turned to a study of its reaction in an Ar matrix containing 10% IR bands of 4 and the growth of at 35 K yielded no new species.
Table 2. UV-vis absorptions of dicyanomethylene (5) in Ar matrices at 12 K* UV-vis absorptions (nm) 470 w br 445wsh 377 w br
357.7 A 348.3 B 335.7 A 326.9 B 316.5 A
314.1 sh 306.5 B 300.2 sh A 293.7 B
286.1 A
* Bands are denoted as br broad, sh shoulder, or w weak; or, where appropriate, assigned to series A or B (see Fig. 2).
I. R. DUNKINand A. MCCLUSKEY
214
Table 3. IR absorptions observed after photolysis of dicyanodiazomethane (4) in AI containing 10% 0, at 12 K, compared with those of carbonyl cyanide (8) in pure Ar (MR 11200) at 12K IR absorptions (cm-‘) 8
Photoproduct 2344 2280 2260
2238 >‘t 2233
1716 1707 m 1449 1445 1426 1375
Product assignment
8
co2
? ?
2235 2209 1916 1839 1826 1756 1739 1727 1709 1446
8 ? 5 ? 5 5 ? 8 8 8
1419
8
1308 1196
? ?
1136 1127m 1102 1096 m
1037 1006 804 798 793 767 744 733 721 s 714
Photoproduct 1143t 1137 1131 1112 1097 1089 1084 1074 1038
714 707 662
Product assignment 8 8 8 8 8 ? ? ? 8 + Oj?$
8 8 co2
655 * Bands of 8 were weak unless denoted s strong or m medium. t Likely matrix splitting. $ vj of OS and a weak band of 8 overlap at 1037-1038 cm-i, but relative intensities in the photoproduct spectrum suggested that both species could have been present.
Even at such a high O2 concentration, the product included an easily detectable proportion of the carbene 5. The major product was identified as carbonyl cyanide (8), by comparison with an authentic Ar-matrix IR spectrum (Table 3). All the stronger bands of compound 8 and many of the weaker ones were visible in the product spectrum, although some small shifts in frequency due to the presence of 10% O2 were noted. A number of bands in the product spectrum remain unassigned, however. Notable amongst these are bands at 2280 and 2260 and 2209 cm-‘, in the region where cyanide oxidation products such as RCNO, RNCO or ROCN are expected to have strong absorptions, and a band at 1839cm-‘, which is probably a carbonyl band. Subsequent experiments showed that, of these, only the 2260 cm-’ band could have belonged to a primary or secondary photoproduct. The others seemed to arise in photo-oxidation reactions at a late stage of the photolysis (uide in@). In order to try to identify some of the products formed in 10% O,/Ar matrices as a result of futher oxidation reactions, attempts were made to photo-oxidize compound 8. Irradiation (A> 200 or Iz= 255 f 10 nm) in 10% O,/Ar or 5% OJAr at 12 K, however, gave no discernible reaction. It is therefore possible that the photo-oxidations require the presence of O-atoms as well as OZ. Finally, it is possible that a small amount of ozone was formed in these experiments. The strongest band of O3 (Q 1038 cm-‘) [15,16] overlaps with a weak band of compound 8, but relative band intensities in some experiments suggested that both species could have been present. The formation of ozone as a photoproduct in carbene oxygen chemistry has been noted before [17], but under the conditions that we have used in this laboratory we have not observed it with any of the previously studied carbonyl oxides. It presumably arises from photo-induced O-atom transfer from the carbonyl oxide to OZ. Narrow band photolysis (2 = 255 f 10 nm) of similar matrices of compound 4 in 10% O,/Ar gave essentially identical results. No IR bands that could be assigned to the expected carbonyl oxide (7) were observed in any of these experiments.
Carbonylcyanide O-oxide in Ar matrices
215
When the oxygen concentration was reduced, the photoproduct spectrum became less complex. Narrow band photolysis (A= 255 + 10 nm) of 4 in 1% OJAr gave initially only the carbene 5 and small quantities of carbonyl cyanide (8). When the matrix was subsequently annealed at 35 K, four IR bands which had not been observed before appeared at 1290,1275,1264 and 1245 cm-‘. These disappeared again after only l-2 min further photolysis with light of rE> 345 nm (Fig. 3). At the same time, the IR absorptions of carbonyl cyanide (8), possibly the strongest band of ozone (y 1038 cm-‘) and the unassigned band at 2260 cm-’ grew. The four bands at 1290-1245 cm-’ were at all times very weak, but they appear to belong to an adduct of dicyanomethylene (5) and O2 which is also a precursor of carbonyl cyanide. In the light of all precedents [l-3, 5-7, 141, therefore, these can be assigned to the carbonyl oxide, carbonyl cyanide O-oxide (7) rather than the dioxirane (9). Prominent absorptions in the region 1100-1250 cm-’ are found in the IR spectra of compound 4, TCNE, 6 and 8, and can be attributed to skeletal modes of the dicyanomethylene group (principally v(C-C) and, in the case of 4, v(C=N)) [8]. P reviously studied carbonyl oxides have prominent IR bands close to WOcm-‘, assigned to ~(0-0) modes, and others in the region 14W-1300cm-1. The latter are diicult to assign with confidence, but there is evidence from “0 shifts of some contribution from v(C=O) [7,18]. It is likely that the four bands assigned to 7 have contributions from both dicyanomethylene skeletal modes and carbonyl oxide v(C=O). Unfortunately, ‘802, which could help in the assignment, is currently unavailable. The non-appearance of bands near 900 cm-’ in our spectrum of 7 must be attributed to the weakness of the spectrum overall. As a result we are unable to conclude that the cyano groups strengthen the O-O bond, as has been inferred for the trifluoromethyl group, which is also strongly electron withdrawing [2,18]. The band at 226Ocm-’ remained weak in the experiments in 1% OJAr and clearly belonged to a minor product. In analogy with the photochemistry of other carbonyl
Fig. 3. IR spectra in the range 1350-1200 cm-’ (a) obtained after nearly complete photolysis (L = 255 f 10 mn) of dicyanodiazomethane (4) in 1% OJAr at 12 K followed by annealing at 35 K, and (b) after a further 2 min photolysis with It> 345 nm. Spectrum (c) is the difference spectrum (a) - (b), showing bands which were removed during the second period of photolysis. Bands marked C arc assigned to carbonyl oxide (7). while those marked D belong to residual starting material 4.
I.R. DUNKIN and A. MCCLUSKEY
216
200
400
300
500
600
700
X/nm
Fig. 4. UV-vis absorption spectra (a) obtained after photolysis (I = 255 + 10 nm) of dicyanodiazomethane (4) in 2% O,/Ar at 12 K followed by annealing at 35 K, and (b) after a further 2 min photolysis of the resulting matrix with L > 345 nm. The weak broad band, which disappears on A>345 nm photolysis (C) is assigned to carbonyl oxide (7). it could possibly have been the carbonyl cyanate 10formed via dioxirane 9, but this species has not so far been characterized and an authentic sample is therefore unavailable.
oxides,
CN
CN
0.
02
): CN (5)
0’
CN A
d-
0
+o
k
CN
CN
(6)
(7) 1 hv ? I NC
hv
0
Y 0
(W ‘CN
The same process could be followed by UV-vis spectroscopy. In a typical experiment, the diazo precursor 4 in 2% OJAr was photolysed with monochromated light (L = 255 + 10 nm). Two products could be identified in the UV-vis spectrum: carbene (5), recognized by its characteristic series of absorptions at 280-360 nm, and a small amount of a species with absorptons at A,,,,= 425 sh and 585 nm. The weak broad absorption of 5 at 377 nm was also observed, but those at longer wavelengths were obscured by the other product’s absorption, cf. Fig. 4(a). Annealing the matrix at 35 K resulted in a reduction in intensity of the absorptions of 5 and the growth of a weak broad absorption with A,, = 375 nm (Fig. 4(a)). Further brief photolysis with light of 1~ 345 nm bleached this weak absorption, while that at A,,,,,= 585 nm grew considerably. Strong absorption at L < 320 nm, previously lying under other absorptions, also grew and obscured most of the remaining carbene bands (Fig. 4(b)). The weak 375 nm absorption observed on annealing almost certainly belonged to the same species observed in the IR spectrum in parallel experiments, and is thus assigned to the carbonyl oxide 7. The position of this absorption lies at the lower wavelength end of the range (ca 380-460 nm) observed for other carbonyl oxides [2]. The effect of the cyano groups is therefore to shift this electronic absorption to higher energy. A similar effect has been noted for the trifluoromethyl group, as manifested by the shift of 1, from 422 nm in benzophenone O-oxide and 387 nm in benzaldehyde O-oxide to 378 nm
Carbonyl cyanide O-oxide in Ar matrices
217
in trifluoroacetophenone O-oxide [2,18]. Carbonyl cyanide O-oxide has the shortest wavelength so far reported for this absorption of a carbonyl oxide. Carbonyl cyanide (8) [19,20] has an absorption band at about 250-350 nm, and ozone in Ar matrices (ca 7% 0,) has rZ, at 26Onm and no prominent maxima at longer wavelengths (M. Lynch, unpublished observations). Thus the formation of 8, with or without ozone, accounts for the growth in absorption below 320 nm observed when 7 was photolysed. We are unable to assign the product absorptions at 425 sh and 585 nm to any of the expected products, however. There is no IR evidence for a significant product other than 8, and we thus suggest that the strong absorptions in the visible may belong to a charge-transfer complex formed between O-atoms and 8 remaining together in the same matrix cage. Further investigation of this possibility will be undertaken.
CONCLUSIONS
Photolysis of dicyanodiazomethane (4) in Ar matrices has been studied in conditions of greater dilution than achieved previously [8]. It has been confirmed that the product is dicyanomethylene (S), but a revision of its IR spectrum has now been made (Table 1). In addition the UV absorption spectrum of 5 has been reported for the first time (Fig. 2 and Table 2). In the conditions used in this study, dicyanomethylene showed no tendency to dimerize to tetracyanoethylene, but a thermal reaction with CO yielding the corresponding ketene (6) did occur on annealing to 30 K or above, thus providing further support for the identity of 5. In O,-doped Ar matrices, dicyanomethylene reacted with O2 to give carbonyl cyanide O-oxide (7) as a very photolabile initial adduct. Four IR bands and a broad UV absorption at 375 nm are assigned to this species, but the weakness of all of them showed that it was impossible to build up substantial concentrations of 7 in the experimental conditions employed. No other absorptions attributable to 7 could be observed. The UV absorption of 7 has the shortest wavelength of all carbonyl oxides so far reported. On further photolysis, the carbonyl oxide rapidly decomposed at relatively long wavelengths (2 > 345 nm) to give carbonyl cyanide (8), possibly ozone, and a species with stong absorptions at 425 sh and 585 nm, which is tentatively identified as a chargetransfer complex between O-atoms and 8. A single unassigned product IR band at 2260 cm-’ might have belonged to the rearrangement product 10, but this could not be confirmed, and in any case represented only a very minor pathway. Acknowledgements-We thank the SERC for supporting this work with an Earmarked Studentship and equipment grants, and Mr M. Lynch for preparing some of the ozone.
REFERENCES [l] G. A. Bell and I. R. Dunkin, I. Chem. Sot., Chem. Commun. 1213 (1983). [2] W. Sander, Angew. Chem. Int. Ed. Engf. 29,344 (1990). [3] I. R. Dunkin and C. J. Shields, 1. Chem. Sot., Chem. Commun. 154 (19%). [4] Y.-P. Lee and G. C. Pimentel, J. Chem. Phys. 74,4851 (1981). [5] G. A. Bell, I. R. Dunkin and C. J. Shields, Specrrochim. Acta 41A, 1221 (1985). [6] I. R. Dunkin and G. A. Bell, Terrchedron41,339 (1985). [7] I. R. Dunkin, G. A. Bell, F. G. McCleod and A. McCluskey, Spectrochim. Acta 42A. 567 (19%). [8] W. H. Smith and G. E. Leroi, Spectrochim. Acta 2SA, 1917 (1969). [9] I. R. Dunkin and J. G. MacDonald, J. Gem. Sot., Perkin Trans. 2 2079 (1984). [lo] W. J. Linn, 0. W. Webster and R. E. Benson, J. Am. Chem. Sot. 87, 3561 (1%5). [ll] E. L. Martin, Org. Synrh. 51, 70 (1971). [12] W. J. Linn, Org. Synrh. CON. 5, 1007 (1973). [13] E. Ciganek, J. Org. Chem. 30,4198 (1%5). (141 M. S. Baird, I. R. Dunkin, N. Hacker, M. Poliakoff and J. J. Turner, /. Am. Gem. Sot. 103,519O (1981).
218
[15] [la] [17] [18] [19] [20]
I. R. DUNKINand A. MCCLUSKEY L. Andrews and R. C. Spiker, J. Pl?ys. Chem. 76,3208 (1972). L. Brewer and J. L.-F. Wang, 1. Gem. Phys. 56,759 (1972). 0. L. Chapman and T. C. Hess, J. Am. Chem. Sot. 106,1842 (1984). W. W. Sander, 1. Org. Gem. 53,121 (1988). W. KemuIa and K. L. Wienchowski, Rocrniki Chem. 27,524,527 (1953). W. KemuIa and K. L. Wierzchowski, Ckm. Abs. 48,6253bd (1954).