J . Phys. Chem. 1989,93, 598-608
598
Matrix Photochemistry of Hypofktorous Acid, HOF: Oxygen Atom Transfer and Other Reactions Evan H. Appelman,* Chemistry Division, Argonne National Laboratory. Argonne, Illinois 60439
Anthony J. Downs,* and Christopher J. Gardner Inorganic Chemistry Laboratory, University of Oxford, Oxford, OX1 3QR, England (Received: May 2, 1988)
The infrared spectrum of the hypofluorous acid molecule, HOF, trapped in different solid matrices at low temperatures shows that the vibrational fundamentals u1 and u2 are perturbed to extents that vary with the basicity of the adjacent molecule, D, and the appearance of absorptions attributable to librational motions of the D...H-OF unit implies the formation of specific hydrogen-bonded complexes in some cases. As the matrix concentration of HOF increases, aggregation occurs to give [HOFI2 and higher multimers. Exposure of HOF isolated in an Ar matrix to broad-band ultraviolet radiation results in photodissociation with the formation of two products thought to be 0--.[HFI2and 02.-4HF]2in proportions dependent on the concentration of HOF. Photolysis of HOF trapped in a reactive matrix results in oxygenation of the matrix molecules, X, and complexation of the product XO by HF; e.g. X = N2,CO, 02,or CH,. The CO, 02,or CHI matrices also yield on photolysis small but significant amounts of FCO’, 02F,or CH,...HF, respectively, products that signal the intervention of fluorine atoms. Photolysis of an Ar matrix including HOF together with a potential substrate, Y, at concentrations >1% results exclusively in oxygenation, and not fluorination, to give YO (Y = PF,, AsF3, or H20). The results are discussed in relation to the mechanisms likely to govern the matrix photochemistry of HOF.
I. Introduction The first positive evidence of hypofluorous acid came from the infrared spectra of solid nitrogen matrices containing a mixture of H 2 0 and F2;’ hence it appeared that UV photolysis of the mixture gives rise to the adduct HOF. HF. Subsequent research established that neat hypofluorous acid can be prepared by the controlled thermal reaction between H20 and F2.2 As a result the HOF molecule is now relatively well characterized in terms of its IR,”s4m i c r o w a ~ ephotoelectron,6 ,~ photoionization,’ and ’H and I9F NMR* spectra. Whereas hypofluorites like C F 3 0 F function typically as electrophilic fluorinating agents and as general reagents for photofluorination at carbon centers? hypofluorous acid acts predominantly as an oxygenating or hydroxylating agent. Certainly this is the mode of its response to many aromatic substratesI0 and to H 2 0 and H202in reactions 1 and 2, which appear to be implicated in the fluorine-water reaction.”
-
HOF HOF
+ H20(1)
+ HOOH(aq)
-
-
+ HF O2 + H F + H 2 0
HOOH
(1) (2)
Gaseous H O F shows intense absorption at wavelengths shorter than 300 nm, with no observed maximum at X > 200 nm.12913 (1) Noble, P. N.; Pimentel, G. C. Spectrochim. Acta, Part A 1968, 24A, 797. (2) Studier, M. H.; Appelman, E. H. J . Am. Chem. SOC.1971,93,2349. Appelman, E. H. Acc. Chem. Res. 1973, 6, 113. (3) Appelman, E. H.; Kim, H. J . Chem. Phys. 1972, 57, 3272. (4) Goleb, J. A,; Claassen, H. H.; Studier, M. H.; Appelman, E. H. Spectrochim. Acta, Part A 1972,28A, 65. (5) Kim, H.; Pearson, E. F.; Appelman, E. H.J . Chem. Phys. 1972,56, 1. Pearson, E. F.; Kim, H. [bid., 1972, 57, 4230. (6) Berkowitz, J.; Dehmer, J. L.; Appelman, E. H. Chem. Phys. Lett. 1973, 19, 334. (7) Berkowitz, J.; Appelman, E. H.; Chupka, W. A. J. Chem. Phys. 1973, 58. 1950. (8) Hindman, J. C.; Svirmickas, A.; Appelman, E. H. J. Chem. Phys. 1972, 57, 4542. (9) Shreeve, J. M. Adu. Inorg. Chem. Radiochem. 1983, 26, 119. (10) (a) Appelman, E. H.; Bonnett, R.; Mateen, B. Tetrahedron 1977,33, 21 19. Migliorese, K. G.; Appelman, E. H.; Tsangaris, M.N. J. Org. Chem. 1979,44, 171 1. Andrews, L. E.; Bonnett, R.; Appelman, E. H. Tetrahedron 1985, 41, 781. (b) Appelman, E. H.; Thompson, R. C.; Engelkemeir, A. G. Inorg. Chem. 1979, 18, 909. (11) Appelman, E. H.; Thompson, R. C. J . Am. Chem. Soc. 1984, 106, 4167.
0022-3654/89/2093-0598$01.50/0
Although UV irradiation is known to cause photodecomposition, no steps had been taken previously to explore the photochemistry of the molecule. In the experiments reported here we have studied the effects of UV photolysis on HOF molecules isolated in a solid matrix at 10-20 K. The course of photolysis has been charted by measuring the IR spectrum of the matrix, in which the host or CH,; may be either inert, e.g. Ar, or reactive, e.g. N2,CO, 02, we have also investigated the effect of doping the matrix with a potential reagent, e.g. EF3 ( E = P or As), H 2 0 , or H202. The products thus identified are discussed in relation to the light they shed on the mechanisms of the primary and any secondary reactions. The research was motivated partly by recent interest in the role played by HOF in stratospheric fluorine chemistry.12 An estimate in the order of hours has been given for the maximum photolysis lifetime of HOF in the stratosphere. Since typical vertical mixing times in the stratosphere are in the order of months to years, HOF is scarcely likely to compete with HF as a reservoir or sink for fluorine atoms. However, the fate of fluorine in the stratosphere remains poorly understood compared, say, with the fate of chlorine. We were anxious also to seek direct evidence of unfamiliar molecules that may play important mechanistic roles in the thermal reactions of HOF and F2. Such molecules include, for example, the species (i) HOOF, which has been postulated as an intermediate in reaction 3 and (ii) HOOOH, which has been postulated as an intermediate in reaction 2.”
F2 + H202(aq)
1 ‘C
O2
+ 2HF(aq)
(3)
11. Experimental Section
Apparatus. The cryogenic equipment used at Oxford has been described e1~ewhere.l~Matrices were deposited on a CsI window cooled typically to 13-25 K by means of a Displex refrigerator (Air Products, Model C S 202) inside a shroud maintained at a pressure less than lo-’ Torr. The temperature of the window was measured with a chrome1 vs iron-doped gold thermocouple or with a hydrogen vapor bulb and was varied by adjusting the voltage (12) Elliott, S . Atmos. Enuiron. 1983, 17, 759. (13) Appelman, E. H., unpublished work. (14) (a) Hawkins, M.; Downs, A. J. J . Phys. Chem. 1984,88, 1527. (b) Hawkins, M.; Downs, A. J. J . Phys. Chem. 1984,88,3042. (c) Hawkins, M.; Almond, M. J.; Downs, A. J. J . Phys. Chem. 1985, 89, 3326.
0 1989 American Chemical Society
The Journal of Physical Chemistry, Vol. 93, No. 2, 1989 599
Matrix Photochemistry of Hypofluorous Acid
TABLE I: Wavenumbers ( a ) and Assignments of Infrared Absorptions between 4000 and 200 cm-’ Exhibited by Matrices Containing HOF at 15-20 K Ar matrix N2 matrix CO matrix‘ O2 matrix CH4 matrix‘ assignment F ! cm-l intense isb cm-’ intense ! i cm-’ intense molecule mode 3.6 cm-1 intense i.6 cm-I intense
vq
3778 3756 3720 3572 3553 3537
wm ms ms ms
3536 3550
wm
3515
vw
3516
W
S
3460
W
348 1 3420 3365 3342 2680 1626 161 1 1594 1430 1396 1358 1350 1342 888
S
3470
S
3550
S
3532 3520 3481
ms ms
vw
I
3525
S
3490
m
3462 3415
m W
vw ms vw
ms wm ms
2685 1756
W
1365
ms
1385
sh
1350
S
882
ms
ca. 380
m
W
1393
W
1359
vs
1360
s
1350
vs
886
wm
88 1 387
W
884
ms
ms
W
vs ms ms
‘Absorptions due to solid matrix not listed. bError limits f 2 cm-I. CKey: s, strong: m, medium; w,weak; sh, shoulder: v, very. dSee ref 19. e M , matrix molecule. applied to a 20-W heater wound around the second stage of the refrigerator. The infrared spectra of the matrices were recorded in the range 4000-200 cm-’ with either a Perkin-Elmer Model 225 or a Perkin-Elmer Model 580A spectrophotometer. The spectral resolution varied from 0.5 to 2 cm-I. Sample photolysis was carried out with the radiation derived from a Hanovia “Uvitron” 100-W high-pressure mercury arc. A 4-cm water filter served to reduce the amount of infrared radiation reaching the matrix. Chemicals. Hypofluorous acid was prepared in milligram amounts by pumping molecular fluorine over water at 0 OC in a single passe2 This method produced H O F in rather low yield (ca. 1% of theoretical) but had the merit of affording a product virtually free of HF, as testified by the IR spectrum of the matrix-isolated vapor species (see below). Argon, nitrogen, carbon monoxide, oxygen, and methane gas (grade “X”)were used as received from the British Oxygen Co. Trap-to-trap distillation in vacuo of commercial phosphorus trifluoride (Strem Chemicals) gave a product judged to be substantially pure on the basis of its vapor pressure and the IR spectrum of its vapor. Arsenic trifluoride was prepared by the reaction of fluorosulfonic acid with arsenic(II1) oxide;15 after fractional condensation of the crude trifluoride in vacuo, the infrared spectrum of the vapor showed it to be free from significant impurity. Xenon difluoride was prepared by exposure to sunlight of a Pyrex bulb containing an equimolar xenon-fluorine mixture, and purified by vacuum sublimation.16 Hydrogen peroxide was derived from a commercial aqueous sample (B.D.H., AnalaR, 30%, (w/v)) by evaporation under reduced pressure; hydrogen peroxide-d2 was similarly produced after equilibration of an isotopically natural sample of the peroxide with an excess of D 2 0 (99.8 atom 9% deuterium, Norsk Hydro). The I R spectra of matrices doped with H202 indicated that the samples of peroxide included HzO, typically in the proportions H2O2:H20> 1O:l. Procedure. A glass vacuum line with a grease-free “handling” section was located adjacent to the matrix-isolation assembly and was connected to the inlet tube of the assembly via FEP tubing. Matrix samples were prepared by slow deposition, a needle valve (15) Engelbrecht, A.; Aignesberger, A.: Hayek, E. Monarsh. G e m . 1955, 86, 470. (16) Williamson, S.M. Inorg. Synth. 1968, 11, 147.
being used to regulate the flux of matrix gas (typically 2-3 mmol h-I) passing through an FEP U-tube containing a sample of HOF held at ca. -135 OC. From the U-tube the gaseous mixture passed through an FEP nozzle to impinge on the CsI window maintained at 13-25 K. In these circumstances the matrix composition could be estimated only roughly. A second dopant could be introduced into the matrix either (i) by premixing with the matrix gas (e.g. PF3), or (ii) from a glass ampule communicating with a glass tube concentric with the FEP nozzle, the rate of deposition of this component being controlled by maintaining the ampule at an appropriate (low) temperature (e.g. AsF3, H 2 0 , or H202). 111. Results and Discussion
( a ) IR Spectra of Matrices Containing HOF. The first experiments involved the preparation of condensates containing HOF isolated in each of the following solid hosts: Ar, N2,02,CO, and CH4. The composition of each matrix was typically in the order matrix gas:HOF = 500-1OOO:l. Details of the IR spectra exhibited by these matrices are given in Table I, and representative portions of the spectra are illustrated in part A of Figures 1-5. In each case the spectrum was dominated by three prominent regions of absorption comprising (i) one or more intense bands at 3400-3600 cm-I, (ii) one or two intense bands at 1350-1400 cm-I, and (iii) a weaker band at 880-890 cm-I. Our results are consistent with the results of earlier matrix experiments,’~~ and there can be little doubt that the three regions of absorption are associated with the three vibrational fundamentals of the HOF molecule. By way of comparison, we note that the fundamentals of the gaseous molecule occur at 3578.5, 1354.8, and 889.0 ~ m - l . ~ With dilution to produce, for example, a matrix with the proportions matrix gas:HOF = ca.2000:1, the spectrum consisted of just a single sharp band in each of the three regions 3400-3600, 1350-1400, and 880-890 cm-I. Hence we were able with some confidence to identify the matrix-isolated monomer HOF and to show that the monomer was the predominant guest species in the more concentrated matrices. With N2as the host the results thus obtained were entirely consistent with those reported previ~usly;~ otherwise this is the first study to show how the vibrational properties of the H O F monomer vary from one matrix to another (see Table I). With HOF isolated in a CH4 or C O matrix the IR spectrum also included a broad band of medium intensity in the region 300-400 cm-’. Analogy with the vibrational properties of H F c~mplexes’~J* leads us to ascribe this additional band to
600
The Journal of Physical Chemistry, Vol. 93, No. 2, 1989
the librational motion of a unit [M],.-.HOF in which there is specific hydrogen-bonding between the H O F monomer and one or more adjacent matrix molecules M. Increasing the concentration of HOF in the matrix resulted in the appearance of additional bands, although these bands were relatively slow to appear in the spectrum of a matrix with a relatively strongly interacting host like CO. Most of our matrices differed from those described previously4 in that they contained little or no H 2 0 i 9or HF,i7,18*20 as attested by the absence of IR bands characteristic of these molecules in one form or another. Only the Ar matrices appeared to be contaminated with detectable amounts of these species (see Table I). Accordingly, we conclude that the additional IR bands displayed by the more concentrated matrices arise mainly from aggregates of the type [HOF],. Such species we believe to be responsible for the bands at 3550, 3516, 3460, and 1393 cm-I exhibited by our N 2 matrices; some of these bands coincide with features identified in the earlier matrix exp e r i m e n t ~ 'but , ~ attributed to species like [H20], and HOFsHF. To test our conclusions we carried out experiments with N, matrices including HOF and doped with various concentrations of water, while otherwise keeping unchanged the conditions of deposition. As the concentration of water increased, the IR bands characteristic of H 2 0 in different degrees of aggregationi9 were observed to grow; simultaneously the band at 3550 cm-I gained slightly in intensity, but the ones at 3536, 3516, 3460, and 1393 cm-' maintained the same relative intensities. Matrix-isolated [H20], gives rise to an absorption near 3550 cm-',I9 but the results of the doping experiments lend support to the assignments given in Table I. The presence of some water in the more concentrated Ar matrices was signalled by the appearance of bands of weak-tomoderate intensity attributable to H20or [H20],.19 To determine the origins of the bands in the spectra of these matrices, we again resorted to experiments in which the matrix was deliberately doped with H 2 0 or D20. The result of the doping was to increase dramatically the intensities of the absorptions at 3365, 3342, and 1430 cm-I at the expense of the other bands associated with HOF or HOF-containing species. Since the wavenumbers of these three bands did not change measurably when D 2 0 was substituted for H 2 0 , and since their relative intensities remained roughly constant, we infer that they originate in vibrational transitions of an H O F molecule coordinated to H 2 0 (or D20). The appearance of not one but two bands in the 0-H stretching region invites various explanations involving, for example, matrix site effects or possibly the presence of more than one conformer of the complex I. The O---H-0
\
H 9 H
F
I
Ar matrix also displayed bands at 3537 and 1358 cm-' virtually coincident with v 1 and v2 of the H O F monomer isolated in an N, (17)Andrews, L.J . Mol. S t r u t . 1983,100, 281;J . Phys. Chem. 1984, 88,2940. Barnes, A. J. J . Mol. Srrucr. 1983, 100, 259. (18)(a) Andrews, L.;Johnson, G. L. J . Chem. Phys. 1982, 76,2875. (b) Andrews, L.;Johnson, G. L. J. Chem. Phys. 1983,79,3670. (c) Johnson, G. L.; Andrews, L. J . Am. Chem. SOC.1982, 104, 3043. (d) Johnson, G.L.; Andrews, L. J . A m . Chem. SOC.1983,105, 163. ( e ) Andrews, L.;Johnson, G. L.; Kelsall, B. J. J. Chem. Phys. 1982,76,5767;J . Phys. Chem. 1982,86, 3374. ( f )Andrews, L.;Johnson, G. L. J . Phys. Chem. 1982,86,3380.( 9 ) Andrews, L.;Johnson, G. L. J . Phys. Chem. 1984,88,5887.(h) Andrews, L.;Kelsall, B. J.; Arlinghaus, R. T. J . Chem. Phys. 1983, 79, 2488. (i) Arlinghaus, R. T.; Andrews, L. J . Phys. Chem. 1984,88,4032; J . Chem. Phys. 1984,81,4341;Inorg. Chem. 1985,24, 1523. (j)Andrews, L.;Arlinghaus, R. T.; Johnson, G. L. J . Chem. Phys. 1983, 78, 6347,'6353. (k) Andrews, L.;Johnson, G. L.; Davis, S. R. J . Phys. Chem. 1985,89, 1710. (1) Davis, S. R.; Andrews, L. J . Mol. Spectrosc. 1985, 1 1 1 , 219. (m) Hunt, R. D.; Andrews, L. J. Chem. Phys. 1987,86,3781.(n) Andrews, L.;Withnall, R.; Hunt, R. D. J . Phys. Chem. 1988,92,78. (19) Redington, R. L.;Milligan, D. E. J . Chem. Phys. 1962,37, 2162. Tursi, A. J.; Nixon, E. R. J. Chem. Phys. 1970, 52, 1521. Ayers, G.P.;Pullin, A. D. E. Spectrochim. Acta, Part A 1976,32A, 1629. Fredin, L.;Nelander, B.; RibbegArd, G. J . Chem. Phys. 1977,66,4065. (20) Andrews, L.;Johnson, G. L. J. Phys. Chem. 1984,88,425.Andrews, L.;Bondybey, V. E.; English, J. H. J . Chem. Phys. 1984,81,3452. Hunt, R. D.; Andrews, L. J . Chem. Phys. 1985,82,4442.
Appelman et al. I .
Y,
1080 , 36110
,,
3200 ,
A
3zW
V"
,
c-
-1
1Lw , l2lio
A
lM0 ,
800 ,
600
io00
mo
6'03
,
LW ,
200
Loo
io0
l A
i LhO
I "IH F
'
36W
'
'
uno
m,VA ,
'
c-
Figure 1. Regions 4OOC-3000, 150&1200, and 1000-200 cm-' in the IR spectrum of an Ar matrix with the composition Ar:HOF = ca. 300:l deposited a t ca. 20 K: (A) after deposition; (B) after photolysis with broad-band UV radiation for 45 min. Bands marked in black represent photoproducts.
matrix. Accordingly we assign these to the corresponding vibrations of the weak complex N2--.HOF formed between H O F and N2impurities. All the remaining bands can then be assigned to H O F or its oligomers. Hence the relative complexity of the spectra of Ar matrices arises from a significantly greater degree of self-aggregation and complexation with matrix impurities; this in turn reflects the weaker HOF-matrix interactions prevailing in Ar matrices. Similar complications, including the formation of complexes with H20and N2impurities, have been experienced in recent studies of H F isolated in N e or Ar matrices.20 We have not carried out detailed studies on the features due to H O F oligomers. However, the conditions of our experiments, with matrix gas:HOF 3 500:1, were such as to limit aggregation mainly to species like the dimer [HOFI2. With increasing H O F concentration we note the appearance of two bands in the 0-H stretching region; these occur at 3490-3553 and 3462-3481 cm-', usually on the low-energy side of v 1 of the monomer. Simultaneously a band appears in the H-0-F deformation region at 1365-1396 cm-' to high energy of v2. Although these bands vary in intensity according to the nature and concentration of the matrix, they maintain roughly the same relative intensities. Hence we believe that they are associated with the dimer [HOF],. Matrices giving evidence of substantial aggregation also show weaker bands near 3515 and 3420 cm-I, which may be ascribed to higher oligomers, e.g. [HOFI3. The vibrational properties of [HOF], appear therefore to resemble those of [HF],.20 Allied to the observation of two absorptions in the 0-H stretching region, this leads us to favor for [HOFI2an "open" structure like I1 rather than the cyclic structure I11 used as a basis for interpreting the IR s p e c t r u m of solid HOF.,l c
F---- H-0
F
I1
I11
( b ) Broad-Band UV Photolysis of Ar Matrices Containing HOF. We have found unmistakable evidence of photodissociation of matrix-isolated H O F on exposure to UV radiation. Thus the IR spectrum of an Ar matrix containing HOF at high dilution and exhibiting initially only the bands characteristic of the H O F
A
(21)Appelman, E.H.; Wilson, W. W.; Kim, H. Spectrochim. Acta, Part 1981,37A. 385.
The Journal of Physical Chemistry, Vol. 93, No. 2, 1989 601
Matrix Photochemistry of Hypofluorous Acid TABLE II: Wavenumbers ( i )and Assignments of Infrared Absorptions between 4000 and 200 em-' That Develop upon Broad-Band UV Irradiation of an Ar Matrix Containing HOF at Ca. 20 K
assignment 1.' cm-'
intens*
absorber
mode
3196 3161 340
w-m
02. .[HF], (A) O.*.[HF]2 (B) O...[HF], (B)
v(H-F) v(H-F) vlib(O.-.H-F)
'Error limits f 2
-
S
s, br
Key: s, strong; m, medium; br, broad.
cm-I.
monomer underwent marked changes when the matrix was irradiated with broad-band UV light. With the decay of the absorptions associated with H O F we observed the simultaneous appearance and growth of three new absorptions, viz. a weak feature at 3796 cm-I, an intense one at 3767 cm-I, and a relatively broad one at ca. 340 cm-I. Photolysis for more than about 2 h using the unfiltered UV output of a 100-W Hg arc resulted in the extinction of the HOF bands. Photolysis of an Ar matrix richer in HOF resulted in the decay of all the features associated with HOF and its derivatives ([HOF],, H,O-HOF, and N,.HOF) and the simultaneous development of the three bands at 3796, 3767, and 340 cm-'. No additional bands were observed on photolysis of the more concentrated matrix, but the band at 3796 cm-' gained markedly in intensity with respect to that at 3767 cm-'. Accordipgly we are left in no doubt that these two bands have their origins in different photoproducts A (3796 cm-') and B (3767 cm-I). The changes brought about by photolysis are illustrated in Figure 1; spectroscopic details of the photoproducts, with their possible identities, are listed in Table 11. The new bands at 3796 and 3767 cm-' must correspond to vibrational fundamentals of HF molecules in one form or another. Andrews and his c o - ~ o r k e r s have ~ ~ , exploited ~ ~ , ~ ~ matrix isolation, with FTIR detection, to elegant effect in characterizing the interactions of H F with a wide variety of donor partners D, as revealed through the formation of complexes of the types D...HF and D. .[HF],, inferences about the stoichiometry and geometry of such complexes being endorsed by experiments with both protonated and deuteriated forms of HF. The precedents thus established point to just such species as the absorbers A and B responsible for the bands at 3796 and 3767 cm-I, respectively. The energies are too low to be attributed to the v(H-F) modes of monomeric HF units; instead they invite assignment to the corresponding modes of dimeric [HF], complexed by a weak base D. Andrews et al. have reported that the strong IR band characteristic of [HF], in an Ar matrix at 3825.5 cm-' is red-shifted to 3787.7 cm-' by complexation with an N2 molecule to give N2. -[HF]2,17v18reflecting the superior proton affinity of N, compared with that of Ar.,, The proton affinity of O2 (423 kJ) is intermediate between those of Ar (377 kJ) and N2 (477 kJ),,, and so the wavenumber of the strong IR band associated with v(H-F) of the complex O2...[HFl2, as formed in solid Ar, should lie between 3825.5 and 3787.7 cm-'. Hence A can be plausibly identified as O,...[HF] Support for this assignment is provided, moreover, by independent e ~ p e r i r n e n t involving s~~ UV photolysis of an Ar matrix containing H202and F2 and which gave as the most prominent feature attributable to a photoproduct an absorption at 3795 cm-'. Such results, allied to the effect of increasing the matrix concentration of HOF, afford circumstantial evidence that 02..-[HF], is a primary product derived mainly from the photodissociation of two adjacent H O F molecules, as in the dimer [HOF],:
-
-
,.
(4) We are left therefore to suggest the origin B of the second and more intense high-frequency band (at 3767 cm-I) that developed (22) Beauchamp, J. L. In Interaction between Ions and Molecules; AusIoos, P., Ed.; Plenum: New York, 1975; p 415. (23) Appelman, E. H.; Downs, A. J.; Gardner, C. J., to be submitted for publication.
on photolysis of H O F isolated in an Ar matrix. This does not coincide in energy with the v(H-F) mode of any weak complex of the type D.-.HF or D--.[HF], that has been definitively characterized to date.'7J8 Thus, the possibility that B is 0,. .HF we can discount, since Ar matrices incorporating this complex are reportedlSmJ' to feature a v(H-F) absorption at 3917 cm-l. Apart from the low-frequency feature near 340 cm-I, there was no hint of an absorption due to any other potential photoproduct, 02F2,28929 or e.g. 'OH,24 H,02,25 H02*,26*OF,2702F',28,29 The most plausible interpretation is that B is a complex of the type D. .[HF], involving a donor species D with no detectable IR absorption and with a proton affinity slightly greater than that of O2 (Le. >423 kJ).,, The only obvious candidate is an 0 atom, which has a proton affinity of 457 or 647 kJ, according to whether it is in its electronic ground state (3P) or its first excited state (1D).30 By a process of elimination, therefore, we are led tentatively to propose that B is O.--[HF],. The IR band at ca. 340 cm-' observed to develop on photolysis exhibited a growth pattern suggesting that it too belongs to B. The energy is typical of the librational mode of a complex in which the H F molecule is coordinated to a weak oxygen base like CO, or N20.17J8For example, the corresponding transitions of the species C 0 2 . .HF and N20. .HF, each isolated in an Ar matrix, appear at 3 13 and 324.7/327.9 cm-I, respectively. Support for this identification of the photoproducts A and B comes from independent experiments by Andrews et a1.18ndesigned to explore the effects of broad-band UV photolysis on the weak .HF isolated in solid Ar at ca. 12 K. Such photolysis complex 03-. brings about the decay of the IR bands due to 0 3 * - . H Fand the growth of bands in the v(H-F) region at 3917, 3856, 3796, and 3767 cm-' attributable to the species 0,- .HF, 0. -HF, 0,. [HF],, and 0...[HFI2, respectively. It may seem surprising that our photolysis experiments with H O F gave little or no hint of a band at 3856 cm-' suggesting the formation of 0...HF, which might be expected to be the primary photodissociation product of monomeric HOF. However, the reaction
-
-
-
-
hu, Ar matrix
(5) HOF (0.* *HFJmatrixcage is expected to be highly exothermic; in the conditions prevailing in our experiments (Ar matrices, 15-20 K) the 0 atom is likely therefore to escape from the immediate site of its formation3I and ~ ~ ~ * ~ ~of the HF molecule too may well be m ~ b i l i z e d . ' ~Formation 0...[HF], may then be seen as the outcome of migration and subsequent trapping of the photochemically generated 0 atoms and/or H F molecules within the matrix. There is also the possibility that 0. .[HF], may, like 0,. .[HF],, originate in the photodissociation of the dimer [HOF], in reaction 6, which differs from (4) in that the combination of 0 atoms to form O2molecules is forestalled by escape of some of the 0 atoms from the matrix cage. e
hv
(6) [HOFI 2 (0. * [HFI 2Jmatnx cage + 0 The magnitude of v(H-F) associated with 0. -[HF], suggests that the 0 atom is in its electronic ground state (3P) and not in an excited state, or indeed in the form of an Ar('D)O exciple^.^^ H F complexes of reactive atoms or radicals are not without precedent. For example, Andrews et al. have presented spec-
-
(24) Aquista, N.; Schoen, L. J.; Lide, D. R., Jr. J. Chem. Phys. 1968,48, 1534. (25) Lannon, J. A.; Verderame, F. D.; Anderson, R. W., Jr. J. Chem. Phys. 1971. 54. 2212. (26) Smith, D. W.; Andrews, L. J . Chem. Phys. 1974,60,81. Jacox, M. E.; Milligan, D. E. J. Mol. Spectrosc. 1972, 42, 495. (27) Andrews, L.; Raymond, J. I. J. Chem. Phys. 1971, 55, 3070. (28) Spratley, R. D.; Turner, J. J.; Pimentel, G. C. J. Chem. Phys. 1966, 44, 2063. (29) Jacox, M. E. J. Mol. Spectrosc. 1980, 84, 74. (30) Wagman, D. D.; Evans, W. H.; Parker, V. B.; Halow, I.; Bailey, S. M.; Schumm, R. H. "Selected Values of Chemical Thermodynamic Properties"; NBS Tech. Note (US.) 1968, No. 270-3. Moore, C. E. "Atomic 1949, No. 467. Energy Levels"; NBS Circ. (US.), (31) Perutz, R. N. Chem. Rev. 1985, 85, 77. H. (32) Fournier, J.; Mohammed, H.; Deson, J.; Maillard, D.Chem. Phys. 1982, 70, 39.
TABLE 111: Wavenumbers ( i )and Assignments of Infrared Absorptions between 4OOO and 200 ern-' That Develop upon Broad-Band UV Irradiation of an N2Matrix Containing HOF at Ca. 20 K
assignment cm-'
E:
3857 3798 3777 3740 3690 2262 2240 1276 ca. 360
intensb
absorber
mode
u(H-F) u(H-F) u(H-F) u(H-F) u(H-F) u(N=N) u(N=N) u(N=O) ~lib(N20..*HF)
vw
mw W
ms, br vw, br W
m W
m, br
'Error limits f 2 cm-I. bKey: s, strong; m, medium; w, weak; br, broad; v, very.
:---+--- I I , .i-F
I id00
Appelman et al.
The Journal of Physical Chemistry, Vol. 93, No. 2, 1989
602
'
BW
'
Ym
2800
'
2100
'
2000 Vtcm\ lLm
'
1XC -'4'
800
'
600
'
W m -
Figure 2. Regions 4000-2000, 1500-1200, and 1000-200 cm-' in the IR spectrum of an N2 matrix with the composition Nz:HOF = ca. 400:l deposited at ca. 20 K: (A) after deposition; (B) after photolysis with broad-band UV radiation for 162 min. Bands marked in black represent photoproducts.
troscopic evidence to show that hydrogen or halogen atoms interact with H F in solid Ar to give complexes of the type X - s H F (X = H, F, C1, or Br).'8n*33 Jacox has also given good grounds for believing that fluorine atoms react with CH3X molecules (X = H,34aF,34bor OH3&) in solid Ar to give HF complexes of the radicals 'CH2X. (c) Broad-Band UV Photolysis of N2 Matrices Containing HOF. Broad-band UV irradiation of an N2 matrix containing H O F resulted in the appearance and growth of the new IR bands listed in Table I11 at the expense of the bands associated with the parent molecule. The changes in the IR spectrum are illustrated in Figure 2. Five of the new bands occurred in the region 3650-3900 cm-I, characteristic of u(H-F) for H F or [HF], coordinated to a relatively weak b a ~ e . ' ~ The - ' ~ growth patterns of these bands in different experiments leave no doubt that they originate in more than one absorber. Thus, the very weak band at 3857 cm-' appears after photolysis for about 1 h but disappears on further photolysis, while the band at 3798 cm-l is proportionately more intense when the matrix contains a relatively high concentration of HOF. That the most prominent feature in this region of the spectrum, viz. the absorption at 3740 cm-I, is relatively broad indicates an absorber dependent for its formation on a diffusion-controlled process. However, the most striking aspect of the photolyzed N2matrix is the appearance of new bands at 2262,2240, and 1276 cm-I, which must be due to the molecule (33) Hunt, R. D.; Andrews, L.J . Chem. Phys. 1985,82,4442. Hunt, R. D.; Andrews, L., private communication. Ault, B. S. J . Chem. Phys. 1978, 68, 4012. (34) (a) Jacox, M. E. Chem. Phys. 1979, 42, 133. Johnson, G. L.; Andrews, L. J . Am. Chem. SOC.1980,102, 5736. (b) Jacox, M. E. Chem. Phys. 1981, 59, 199. (c) Jacox, M. E. Chern. Phys. 1981, 59, 213.
N20.14a*35With the relatively dilute matrices we used, N 2 0 was generated only in low concentration (N2:N20 > 1000:1), and it is not surprising therefore that the third fundamental of NzO, expected to occur near 590 ~ m - ' escaped , ~ ~ detection. The only other band to appear on photolysis occurs at 360 cm-' corresponding, we presume, to the D..-HF librational mode of a complex in which HF is coordinated to a weak base D. Published matrix studies of H F complexes of N2I7J8lead us to infer that species of the type N2. -[HF], (n = 1,2, . ..) are not formed on photolysis of H O F isolated in an Nz matrix. Instead the major photoproduct appears to be one in which HF is coordinated to a base stronger than N2. This base we believe to be N2017,18a formed by transfer of an 0 atom photolytically generated from H O F to an N, molecule. The vibrational properties of the 1:1 complex N z O .HF isolated in an Ar matrix have been reported previously.'8a This complex is probably responsible for the weak bands at 3857 and 2262 cm-I. The strong u(H-F) band at 3740 cm-' we ascribe to the complex N20.. -[HF],. As N 2 0 is more basic than N2 but less so than CO,,, the more strongly IR-active v(H-F) mode of N,O...[HF], is expected to have an energy intermediate between those of the corresponding modes of N2. -.[HF], and CO..-[HFI2. For [HF],, N,...[HF],, and CO... [HF], isolated in an Ar matrix, the relevant energies are 3825.5, 3787.7, and 3645.7 cm-I, r e s p e ~ t i v e l y . ' ~ ~Provided ' ~ * ~ ~ that the energy of the transition is relatively insensitive to whether the supporting matrix is N, or Ar, it seems perfectly reasonable therefore that the corresponding transition of N,O.. .[HF], should occur at 3740 cm-I. A much weaker band at 3777 cm-', which appears to grow in unison with the feature at 3740 cm-I, may correspond to the more weakly IR-active u(H-F) mode of the same complex. On the other hand, the behavior of the band at 3798 cm-' is reminiscent of that of the band very similar in energy that appears on photolysis of H O F in an Ar matrix (q.v.). Again we associate this with the 0, complex O2..-[HFl2. The weak broad band at 3690 cm-', which becomes discernible only on prolonged photolysis, is probably due to an oligomer containing more than two H F molecules, e.g. N 2 0 . .[HF],. In the complex N20...HF trapped in an Ar matrix, the stretching modes of the N 2 0 are blue-shifted by 23 and 29 cm-' with respect to the parent Although our weak band at 2262 cm-l is blue-shifted by 26 cm-l, the most conspicuous IR band of N,O [v(N=N) at 2240 cm-'1 is blue-shifted by only 4 cm-', while v(N=O) is actually red-shifted by 15 cm-I. Since the complex N,O. ..[HF], has not been characterized previously, it is difficult to know how well this reflects the properties of the unperturbed complex. We are conscious that N2 and N 2 0 are quite similar in their base strengths,,, and in these circumstances the nature of the matrix may well have a significant influence on the properties (possibly even the structure) of the complex. With such a small blue-shift of the v(N=N) mode, it would be quite hard to distinguish between complexed and uncomplexed N 2 0 , but in the region of v(N=O) there was no hint of an absorption at 1291 cm-' corresponding to the uncomplexed N 2 0 molecule.35 Hence it appears that the product of oxygen atom transfer-in this case N20-is invariably complexed by one or more H F molecules. The UV photolysis of H O F presumably accounts for the N 2 0 detected by Noble and Pimentel in their pioneering experiments on the photolysis of H 2 0 and F2isolated together in an N2 matrix.' Although the IR spectra of their matrices indicated the presence of 03,OF2, and 02F', as well as some unidentified impurities, the intensities of the bands are consistent with the supposition that H O F is the principal source of the oxygen atoms responsible for the formation of N20. (d)Broad-Band UV Photolysis of CO Matrices Containing HOF. Photolysis experiments analogous to those described in the preceding sections were carried out with a solid C O matrix doped with HOF. After irradiation for about 50 min, the matrix displayed an IR spectrum devoid of any bands associated with H O F
-
-
-
(35) Laane, J.; Ohlsen, J. R. In Progress in Inorganic Chemistry; Lippard, S. J., Ed.; Interscience: New York, 1980; Vol. 27, pp 465-513.
Matrix Photochemistry of Hypofluorous Acid
The Journal of Physical Chemistry, Vol. 93, No. 2, 1989 603
TABLE I V Wavenumbers (2) and Assignments of Infrared Absorptions between 4000 and 200 cm-' That Develop upon Broad-Band UV Irradiation of a CO Matrix Containing HOF at Ca. 20 K
a,@cm-I 3835
assignment absorber mode C02.. .HF u(H-F) CO-.-HF u(H-F)
inten$ w, br w, br vs mw
3151
2347 1855 1791 1019 66 1 627
TABLE V Wavenumben (I) and Assignments of Infrared Absorptions between 4000 and 200 cm-' That Develop on Broad-Band UV Irradiation of an O2 Matrix Containing HOF at Ca. 20 K
co2
m ms
vw
u(C=O) u(C=O)? u(C-F) y2
6(0=C-F)
@Errorlimits f 2 cm-I. bKey: s, strong;, m, medium; w, weak; br,
broad; v, very. L
, 3 m , 3 m , 2403 ~
,
2aw
S
mw, br mw, br mw mw
02F'
u(0-0)
mw
v(H-F) u(H-F) U]
~
u3
1
w w, br
mw, br
mw, br
02F-..[HF],? O2F.-.[HF], or O,...[HF],
40-F) 40-F) ulib modes
02F'
d(0-0-F)
02F'
}
vw
+ ~3
~1
Os***[HF]. . .. (n = 1, 2, ...) O3-..[HF], (n = 1, 2, ...)
sh }
378
u(H-F)
+
e[
vw
702 694 583 563 464 440
mode u(H-F) u(H-F) u(H-F)
.HF 0.* *HF 03...HF 02***[HF]2 0 2 . * .[ HF], 0 2 . * HF], O3*.*[HF],(n = 1, 2, ...) 02.
mw m m
1039 1028
,
assignment absorber
intensb
3909 3860 3806 3193 3700 3397 2108 1496 1103
y3
FCO' FCO. * * [HF],? FCO' COZ FCO'
vw
:~i cm-'
@Errorlimits f 2 cm-I. bKey: s, strong; m, medium; w, weak; sh, shoulder; br, broad; v, very. l k
'do0
'
36bo
'
Zoo
'
lab0 ' 21W
'
26m
'
l&Ot'cm.lV llTn
'
1m '
ldio '
em
'
li
Av
W F ,15I
7po
-
(36) Frdin, L.; Nelander, B.; RibbegArd, G. J. Mol. Spectrosc. 1974, 53, Guasti, R.; Schettino, V.;Brigot, N. Chem. Phys. 1978, 34, 391. (37) Milligan, D. E.; Jacox, M. E.; Bass, A. M.; Comeford, J. J.; Mann, D. E . J. Chem. Phys. 1%5,42, 3187. Jacox, M. E.J. Mol. Spectrosc. 1980, 80, 257.
L
5 p 0 1
I
I
Figure 3. Regions 4000-1700 and 1400-600 cm-' in the IR spectrum of a CO matrix with the composition C0:HOF = ca. 500:l deposited at ca. 20 K: (A)after deposition; (B) after photolysis with broad-band UV radiation for 48 min. Bands marked in black represent photoproducts.
and containing in their place the new bands listed in Table IV. Representative spectra are illustrated in Figure 3. The product bands included only two relatively weak features (at 3835 and 3757 cm-I) attributable to u(H-F) for coordinated HF or [HF],. Much more conspicuous were the product bands at 2347 and 661 cm-' clearly identifiable with the formation of C02.36 Weaker bands at 1855, 1019, and 627 cm-' coincide with the principal IR absorptions of the FCO' radical,37 originating presumably in the scavenging of photogenerated F atoms by the C O matrix,31 as in similar experiments involving the UV photolysis of transN2F2, NF,, OF,, or OCF2.37 The only other feature that appeared on photolysis was a very weak absorption at 1791 cm-I. The origin of this we have been unable positively to identify. The more obvious possibilities like OCF, and [OCF], can be eliminated by comparison with the IR spectra reported in earlier studies of these species. Plausible alternatives are the hypofluorite radical FOCO' or the FCO' radical perturbed by coordination to one or more H F molecules. Neither of the u(H-F) bands tallies exactly in energy with the values reported elsewhere"J* for the complexes CO.-.[HF], and C02...HF isolated in Ar matrices. On the assumption that changing the matrix from Ar to C O causes v(H-F) for these complexes to be red-shifted by about 34 cm-I, we suggest tentatively that the absorber responsible for the band at 3835 cm-l is CO,. .HF and that responsible for the band at 3757 cm-I is COS .HF. ( e ) Broad-Band UV Photolysis of 0,Matrices Containing HOF. Similar experiments were carried out with a solid O2 matrix
,
36bo
A 3100 '
v
li00
'
I
13"0q,,;~lba
'
sba
'
ico
'
sbo
'
3 0
Figure 4. Regions 4000-3000 and 1500-300 cm-I in the IR spectrum of an O2matrix with the composition 02:HOF = ca. 300:l deposited at ca. 20 K: (A)after deposition; (B) after photolysis with broad-band UV radiation for 45 min. Bands marked in black represent photoproducts.
containing HOF. After broad-band UV irradiation for 45 min, the IR bands characteristic of HOF and its aggregates had decayed markedly while the new bands listed in Table V had appeared. Portions of representative spectra are shown in Figure 4. The most prominent band to appear at wavenumbers
