Matrix isolation infrared spectroscopic characterization of the 1:1

J. Phys. Chem. 1989, 93, 279-282

279

Matrix Isolation Infrared Spectroscopic Characterization of the 1:l Complexes of HF and HCI with 18-Crown-6 and Related Cyclic Polyethers Bruce S . Ault Department of Chemistry, University of Cincinnati, Cincinnati, Ohio 45221 (Received: April I , 1988)

n e matrix isolation technique has been employed to characterize for the first time molecular complexes of crown ethers with neutral guest molecules in an isolated, solvent-free environment. The codeposition of HF and HCI with a series of cyclic ethers, ranging in size up to 18-crown-6, gave rise to the formation of 1:l hydrogen-bonded complexes. These were characterized by their infrared spectra, which were dominated by intense, broad absorption due to the hydrogen halide stretching motion in the complex. For example, the complex of H F with 18-crown-6 had a major absorption at 3290 cm-’, while the HCI analogue absorbed at 2375 cm-I. These band positions are quite near the absorptions for the complexes of H F and HCI with (CH3)20,suggesting that interaction of H F and HC1 with these cyclic polyethers is to a single oxygen atom in the ring and is not a cooperative interaction, as has been observed for alkali-metal cation complexes of these ethers;

Introduction The ability of crown ethers, cryptands, podands, and related macrocycles to act as effective complexing agents is well-established and represents a field of great chemical and biological importance.’-8 The first synthetic species in this class were the cyclic polyethers, better known as crown ethers, with the general formula c-(CH2-O-CH2),. Crown ethers form very stable molecular complexes with alkali-metal and alkaline-earth cations, as well as somewhat weaker complexes with neutral molecules. The high stability of these complexes has been attributed to a cooperative binding to several if not all of the ring oxygens. In most cases, the interaction of a neutral guest molecule with a crown ether has been postulated to occur through a hydrogen-bonding intera~tion;~ one group has obtained indirect evidence for an intermediate neutral complex between HBr and crown ethers in solution.1° Kebarle and co-workers” recently determined the gas-phase proton affinities of a series of crown ethers and found that the values were considerably higher than for simple ethers. (The PA for 18-crown-6 was found to be 230 kcal/mol, compared to 191 kcal/mol for (CHJZO). Singh and Kollman have combined a b initio methods with molecular mechanics to model this interaction and conclude that “the polyethers 12-crown-4 through 18-crown-6 achieve their impressive M A relative to dimethyl ether by dipole alignment only, without significant contribution from proton sharing”.I2 Little is known about the neutral guest-crown ether complexes, particularly in comparison to the cation complexe~.”’~ Infrared (1) Weber, E.; Vogtle, F. In Topics in Current Chemistry; Springer-Verlag: New York, 1981; Vol. 98. (2) Izatt, R. M.; Christensen, J. J., Eds. Synthetic Multidentate Mucrocyclic Compounds; Academic: New York, 1978. (3) Izatt, I.; Christensen, J. J., Eds. Progress in Macrocyclic Chemistry; Wiley: New York, 1979; Vol. 1. (4) Hubberstey, P. Coord. Chem. Rev. 1981, 34, 1. ( 5 ) Black, D. St. C.; Hartshorn, A. J. Coord. Chem. Rev. 1972, 9, 219. (6) Chock, P. B.; Titus, E. P. Prog. Inorg. Chem. 1973, 18, 287. (7) Christensen, J. J.; Eatough, D. J.; Izatt, R. M . Chem. Rev. 1974, 74,

spectroscopy should be a particularly effective tool for the study of such complexes, in that hydrogen-bonded complexes have distinctive infrared spectral features.16 However, such studies in solution have been hampered by the complexity of the systems studied and by large spectral bandwidth^.'^.'^ Matrix isolation, on the other hand, is noted for small bandwidths as well as a noninteracting environment. A wide range of hydrogen-bonded complexes involving simple acids and bases has been characterized in inert matrices,’e22 including an extensive series of HF complexes with Lewis bases.2325 Infrared spectra of crown ether complexes in low-temperature matrices should provide new insights into the structure and stability of the complexes and may provide evidence of the degree of cooperativity between oxygen atoms in the complex. With this background in mind, a study was undertaken to investigate the complexes of HF and HCI with a series of crown ethers and related cyclic compounds.

Experimental Section All of the experiments in the current study were carried out on conventional matrix isolation equipment which has been decribed previously.26 HCI and HF (both Matheson) were introduced into the vacuum line from lecture bottles and purified by freezethaw cycles at 77 K. HF adsorbed strongly to the surface of the stainless steel manifold, making accurate determination of the sample concentration difficult. DC1 (Merck) was handled in a manner similar to HCI; exchange with residual impurities in the vacuum line led to the presence of HC1 in the DCl samples, with a typical D / H ratio of 2-3. 1,4-Dioxane (Fisher) was introduced into the vacuum line as the vapor above the liquid and diluted to an appropriate ratio with argon. 1,3,5-Trioxane and 12-crown-4 did not have sufficient room-temperature vapor pressure to be handled in this manner. Instead, for each the pure material was placed in a glass finger attached to a needle valve and joined to the argon deposition line near the entrance to the vacuum vessel. The needle valve was opened sufficiently to allow a small flow of the vapor pressure of the material to be entrained

351.

~

(8) Melson, G. A., ed. Coordination Chemistry of Macrocyclic Compounds; Plenum: New York, 1979. (9) DeBoer, J. A. A.; Reinhouldt, D. N.; Harkema, S.;van Hummel, G. J.; deJong, F . J. Am. Chem. SOC.1982, 104, 4073. (10) Shchori, E.; Jagur-Grodzinski, J. J . Am. Chem. SOC.1972,94,7957. (11) Sharma, R. B.; Blades, A. T.; Kebarle, P. J . Am. Chem. SOC.1984, -106. - - , 510.

(12) Singh, U. C.; Kollman, P. A. J. Compt. Chem. 1986, 7 , 718. (13) DeJong, F.; Reinhouldt, D. N. Adu. Phys. Org. Chem. 1980,17,259. (14) Grootenhuis, P. D. J.; Uiterwijk, J. W. H. M.; Reinhouldt, D. N.; van

Staverren, C. J.; Sudholter, E. J. R.;Bos, M.; van Eerden, J.; Klooster, W. T.; Kruise, L.;Harkema, S.J. Am. Chem. SOC.1986, 108, 780. (15) Vogtle, F.;Sieger, H.; Mueller, W. M. Top. Curr. Chem. 1981, 98, 107.

(16) Pimentel, G.

e.; McClellan, A.

~

~~

L. The Hydrogen Bond; W. H.

Freeman: San Francisco, 1960. (17) Mosier-Boss, P.; Popov, A. I. J. Am. Chem. SOC.1985, 107, 6168. (18) Savoie, R.; Rodrigue, A.; Pigeon-Gosselin, M.; Chenevert, R. Can. J. Chem. 1985, 63, 1457. (19) Ault, B. S.; Pimentel, G. C. J . Phys. Chem. 1973, 77, 57. (20) Barnes, A. J. J . Mol. Struct. 1983, 100, 259. (21) Ault, B. S. J . Phys. Chem. 1979, 83, 837. (22) Truscott, C. E.; Auk, B. S.J . Phys. Chem. 1984, 88, 2323. (23) Andrews, L. J. Mol. Struct. 1983, 100, 281. (24) Andrews, L. J. Phys. Chem. 1984,88, 2940. (25) Andrews, L.; Johnson, G. L.; Davis, S . R. J. Phys. Chem. 1985,89, 1710. (26) Ault, B. S. J . Am. Chem. SOC.1978, 100, 2426.

0022-365418912093-0279$01 .50/0 0 1989 American Chemical Society

Ault

280 The Journal of Physical Chemistry, Vol. 93, No. 1, 1989

in the argon stream and deposited into the matrix. While this manner of deposition did not allow for quantitatively accurate concentrations, reproducible samples could be prepared, as judged by the intensity of the absorption bands of the parent material. 18-Crown-6, the least volatile of the ethers studied here, is a solid at room temperature. The solid material was placed in a small Pyrex Knudsen cell, which was in turn placed in a resistively heated oven within the vacuum vessel, and directed a t the cold window. The rate of evaporation and deposition of 18-crown-6 was controlled by the voltage applied to the heating coil of the oven. Temperatures slightly above room temperature were required to produce moderately intense infrared absorptions of parent 18crown-6. Matrix samples were deposited in twin-jet mode (hydrohalic acid/argon from one manifold and ether/argon from the second manifold) for 20-24 h, at which time final infrared spectra were recorded. An IBM 98 FTIR was used throughout, at 1-cm-' resolution, typically averaging 2000 interferograms for the final spectrum. In some experiments, the sample was then warmed to around 35 K and recooled, followed by the acquisition of additional spectra.

Results Prior to any deposition experiments involving the crown ethers and the hydrogen halides, blank experiments were conducted on each reagent alone in argon. The spectra of Ar/HF and Ar/HCl matrices were in good agreement with past spectra obtained in this laboratory and with literature spectra.1e23 While the concentration of the Ar/HF samples was not accurately determined, the intensities of the well-known monomer, dimer, and trimer bands2' gave a good indication of the concentration. Infrared spectra were recorded for each of the crown ethers in argon matrices, and these were then used for comparison to spectra taken after codeposition of the crown ether and hydrogen halide. HF Reactions. Samples of Ar/HF were codeposited with samples of Ar/1,&dioxane (C4H*O2)= 500 in several experiments, with varying HF levels. In all of these experiments, the dominant new spectral feature was a very intense, broad absorption centered at 3302 cm-'. In the higher yield experiments, this band was nearly fully absorbing and had a width a t half-maximum of approximately 60 cm-'. The intensity of this band was observed to vary directly as the level of HF in the matrix was changed (and the dioxane concentration was held constant). In addition, a number of weak-to-medium intensity product bands were observed at 754, 801, 832, 904, 1047, 1085, 1101, 1310, 1333, and 2964 cm-'. These, too, varied with the HF level in the sample. Except for the bands between 750 and 810 cm-', all of these bands fell within 5-15 cm-' of bands due to parent 1,Cdioxane. A number of experiments were conducted employing HF and 12-crown-4, over a wide range of concentrations of each reagent. As with the above system, a dominant product band was observed in each experiment at 3249 cm-I. This band was very intense and had a width at half-maximum of approximately 80 cm-I. Also, the intensity of this band varied directly with the apparent concentration of each of the reagents, as judged by the intensity of the parent bands. The lower energy region of the spectrum was very cluttered, as a consequence of the complicated spectrum of 12-crown-4 (28 atoms, 78 normal vibrations). Also, the bandwidths of the parent bands of 12-crown-4 were broader than those for 1,4-dioxane. Consequently, only two distinct product bands were observed in this region of the spectrum, both weakly, at 725 and 1042 cm-'. A number of deposition experiments involving H F and 18crown-6 were carried out, also over a wide range of concentrations in argon. A single, broad intense product band was observed in these experiments, centered at 3290 cm-I. This band varied with the level of HF in the sample, as well as within the level of 18-crown-6. The low-energy spectral region was very complex due to parent 18-crown-6, and no clear product bands were observed. Figure 1 shows infrared spectra, in the H-F stretching region, of the products of the deposition of HF with 1,4-dioxane, 12-crown-4, and 18-crown-6.

I

I

3800

I

I

.

*

I

,

,

3200 3800 ENERGY (crn-l)

L

,

,!

3200

Figure 1. Infrared spectra from 3000 to 4000 cm-l of the complexes of H F with cyclic ethers. Trace a shows a blank spectrum of H F in argon, while trace b shows the spectrum obtained after deposition of this same Ar/HF sample with 18-crown-6. Trace c shows the product after deposition of H F with 1,4-dioxane, while trace d is the infrared spectrum of the products of the codeposition of H F with 12-crown-4. TABLE I: Position' of the Hydrogen Stretching Mode Y, for 1:l Complexes of HF, HCI, and DCI with Crown Ethers and Related Cyclic Ethers base HF HCI DC1 PAd 3954 2880 2090 1,4-dioxane 3302 2400 1880 193.8 1,3,5-trioxane 2580 12-crown-4 3249 2320 22 1 18-crown-6 3290 2315 1119 230 dimethyl ether 3350b 2302c 191 "Band positions in cm-l. bFrom ref 25. CFrom ref 19. dProton affinity of the base, from ref 11 and 29.

HCI Reactions. Samples of Ar/HCl were codeposited with each of the crown ethers listed above, in every case over a range of concentrations of HCl and crown ether. The codeposition of HCl with 1,Cdioxane led to a very broad (300-cm-' width a t half-maximum), moderately intense absorption centered at 2400 cm-'. In addition, a relatively strong band was noted at 873 cm-', near an intense parent absorption a t 879 cm-I, and a weaker doublet at 830, 835 cm-'. When HC1 was codeposited with 1,3,5-trioxane, a broad intense feature was observed centered a t 2580 cm-', with slight submaxima noted as well. When HCl was codeposited with 12-crown-4 into argon matrices, the dominant feature was a very broad, moderately intense absorption at 2320 cm-'. When this sample was subsequently annealed, this band grew considerably in peak intensity. Finally, HCl was also codeposited with 18-crown-6 in several experiments, and a similar strong, broad absorption was observed centered at 2375 cm-I. In all of these experiments (other than HC1 with 1,4-dioxane), no product absorptions were noted in the low-energy region, probably due to the considerably complexity of the spectrum of the parent base. Samples of Ar/DC1 was codeposited with a sample of 1,4dioxane in several experiments; in each some residual HCl was present from exchange, such that the above product band near 2400 cm-I was observed. In addition, a somewhat broad (60 an-'), moderately intense product band was noted near 1880 cm-' in each experiment. In addition, the product absorptions near 835 and 873 cm-I were both present, although whether they were due to HCI or DCl could not be readily determined. No additional product bands were noted in the DCl experiments. Samples of Ar/DCl were also codeposited with 18-crown-6; besides the product absorption at 2375 cm-' due to the residual HC1, a distinct new absorption was observed at 1779 cm-'. This band was

Complexes of HF and HC1 with 18-Crown-6

The Journal of Physical Chemistry, Vol. 93, No. 1, 1989 281

Z

0 a

LL

0 m

m d

I

. 3200

,Y

I.

.

.

.

2800 2400 2000 E N E R G Y (cm-1)

.

.

I

1600

Figure 2. Infrared spectra from 1500 to 3400 cm-' of the complex of 12-crown-4 with HCl (trace b) and an HCl/DCl mixture (trace c), compared to a blank spectrum of HC1 in argon (trace a). moderately intense, and somewhat broad, although not as much so as those in the HCI experiments. No product bands were observed in the low-energy region. Figure 2 shows infrared spectra arising from the deposition of HCl and HCl/DCl mixtures with 12-crown-4, while Table I tabulates the position of the dominant spectral feature for each of the HF, HCl, and DC1 systems presented above.

Discussion For all of the systems studied here, the spectra contain new absorptions which cannot be attributed to either the parent hydrogen halide or crown ether, and consequently they must be assigned to a product species. These product absorptions were dominated for each system by a very broad, intense feature on the low-energy side of the parent hydrogen halide fundamental, a result which is indicative of hydrogen-bond formation. In addition, for a number of the systems studied, weaker product absorptions were detected within 5-15 cm-' of certain absorptions of the parent crown ether. This suggests that the base is slightly perturbed in the product species but retains its structural integrity, a conclusion which also points to the formation of a molecular complex. Moreover, the interactions of HF and HC1 with a variety of simple bases containing a single oxygen donor atom have been well-studied by a number of research groups,"25 and in every case evidence for an isolated, hydrogen-bonded complex was obtained. In view of the spectral characteristics observed here and the result of previous researchers, assignment of the product absorptions to a hydrogen-bonded complex for each system is clearly appropriate and thus made. The stoichiometry of the complex is not as readily determined, particularly due to the considerable width of main spectral feature. Nonetheless, since these experiments were conducted in the concentration range of approximately 1000/ 1/ 1 and since previous studies have led primarily to 1:l complexes (even at significantly higher hydrogen halide concentration), formation here of a 1:l complex is most likely. Moreover, only a single set of product absorptions was noted as the relative concentrations of the acid and the base were varied over a relatively wide range, although one cannot rule out some overlapping structure within the very broad absorption above 2000 cm-' in each system. In particular, the same set of product absorptions was observed whether the hydrogen halide was in excess or the crown ether was in excess. These facts all point to formation of an isolated 1:1 hydrogen-bonded complex between the hydrogen halide and the crown ether for each system, although one cannot completely rule out formation for a small amount of 2:l complex, with absorption also lying within the band envelope of the broad, high-energy absorption. This report, then, marks the first observation of a molecular complex of any type involving a crown ether under isolated, solvent-free conditions.

The most notable spectral characteristic of a hydrogen-bonded complex is a shift to lower energy of the hydrogen stretching mode us of the acid subunit in the complex.16 In many cases, this absorption is also intensified and broadened relative to the parent acid absorption. For each system studied here, exactly such an absorption was noted, for example, the 2375- and 3290-cm-I bands in the HCl and HF complexes with 18-crown-6. Moreover, in the two systems where DCl was employed, a counterpart absorption was observed shifted to lower energy. The ratio vH/vD ranged from 1.28 to 1.34, characteristic of an anharmonic hydrogen motion, which further supports assignment of the dominant spectral feature to the hydrogen stretching motion of the hydrogen halide in the hydrogen-bonded complex. The magnitude of the shift (which will be discussed in more detail below) was in each case comparable to previously studied complexes. In addition to a shifted hydrogen stretching motion, hydrogen-bond formation leads to the creation of two librational or bending modes of the hydrogen bond, in- and out-of-plane (corresponding to rotations for the free hydrogen halide). These occur at much lower energies but are potentially observable, particuarly for the HF complexes. Only in the HF/ 1,4-dioxane system were likely candidates for the librational modes detected, namely, the sharp absorptions near 754 and 801 cm-I. These were the only product absorptions which did not fall near parent modes of 1,Cdioxane, and they are relatively close to the librational modeszs of the HF-O(CH& complex at 684 and 801 cm-I. Consequently, they are assigned to the two librational modes of HF in the hydrogen-bonded complex with 1,4-dioxane. These modes should be observable as well with the larger systems 12-crown-4 and 18-crown-6. However, the spectral congestion was much greater for these systems, with the large number of vibrational modes for the parent crown ether, as well as generally broader parent bands. No absorptions were noted in these experiments which could be attributed to the HF librational modes. While the corresponding librational modes for HCl complexes have been observed for some systems, they typically fall a t lower energies and are relatively weak. No product absorptions were noted in the present study which could be attributed to an HCl or DCl librational mode. The base subunit in a hydrogen-bonded complex is also perturbed slightly upon complex formation. Shifts of particularly sensitive modes are on the order of 5-20 cm-', while less sensitive modes often do not shift clear of the absorption envelope of the parent base. For several of the systems studied here, perturbed base modes were observed, most notably the 1,Cdioxane system. Since each absorption fell near a corresponding absorption of parent 1,Cdioxane, assignment can be made directly by analogy to the parent. For the larger bases, few if any perturbed modes were detected. However, these systems were characterized by broader parent absorptions, so large shifts would be needed to resolve the product absorptions from the parent bands. Also, the spectrum below about 1200 cm-' in each case was quite complex, with overlapping parent bands. Consequently, the perturbed base modes might well have been hidden by the numerous parent modes. Kebarle reported" proton affinities for the crown ethers which were substantially higher than for single oxygen atom donors such as (CH3)20. Researchers in the hydrogen-bonding fieldzoy2' have often taken proton affinity as a measure or predictor of the strength of hydrogen bonding between an acid and base subunit. While this argument would predict considerably greater shifts for HF and HCl complexed to the crown ethers than to (CH3)20,this was not observed to any significant degree, as shown in Table I. Consequently, to the degree that the magnitude of the shift of the hydrogen halide stretching motion can be taken to reflect the strength of a hydrogen-bonding interaction, the hydrogen bonding of HF or HC1 to a crown ether resembles closely the interaction of these acids with simple ethers, and cooperativity between oxygen atoms in the crown ether does not appear important. This conclusion is consistent with a previous solid-state studyz8 of the (27) Ault, B. S.; Steinback, E.;Pimentel, G.C. J . P h p . Chem. 1975, 79,

615.

J . Phys. Chem. 1989, 93, 282-291

282

interaction of H 2 0with a crown ether, where it was noted that the water molecule is too small to fill the whole macrocyclic cavity and is therefore not coordinated by all the oxygen atoms. It is, nonetheless, difficult to rationalize the present results with the measured proton affinities of Kebarle and co-workers. The best explanation, perhaps, revolves around the calculations of Singh and Kollman12which suggest that dipole alignment of the oxygens in the ring to the proton accounts for the increased proton affinity. In the current case, with a polar but neutral hydrogen halide guest, this dipole alignment should be less, given that the operative forces are dipole-dipole, rather than ion-dipole. The best comparison to make, if the data were available, would be binding energies of the hydrogen halides to dimethyl ether compared to the crown ethers. (28) Helgesen, R.C.; Tarnowski, T. L.; Cram, D. J. J . Org. Chem. 1979, 44, 2538.

(29) Lias, S. G.; Liebman, J. F.; Levin, R. D. J . Phys. Chem. Ref Data 1984, 13, 695.

One major difference between the spectra of the complexes of H F and HC1 with (CH3)zO and the current complexes is the bandwidth of the hydrogen stretching motion. For example, the bandwidth of the HF stretching mode in its complex with 12crown-4 was approximately 80 cm-', while for the HF.0(CH3)2 complexZ5it was 13 cm-'. For the HCI complexes reported here, bandwidths were on the order of several hundred wavenumbers, again considerably greater than for HCl with simple ethers. This additional broadening either may reflect a range of conformations of the hydrogen-bonded complex, which would then average out to yield a very broad absorption, or may reflect some mobility of the hydrogen halide in the complex. Acknowledgment. The author gratefully acknowledges support of this research by the National Science Foundation under Grant C H E 87-21969. Registry No. HF, 7664-39-3; HC1, 7647-01-0; D1,7782-39-0; Ar, 7440-37-1; 1,4-dioxane, 123-91-1; 1,3,5-trioxane,110-88-3; 12-crown-4, 294-93-9; 18-crown-6, 17455-13-9; dimethyl ether, 115-10-6.

A Flash Photolysis-Shock Tube Kinetic Study of the H Atom Reaction with 02: H 0 2 + OH 4- 0 (962 K 5 T 5 1705 K) and H 4- O2 Ar -+HOP 4- Ar (746 K 5 1 5 987 K)

+

+

A. N. Pirraglia?**J. V. Michae1,f.s J. W. Sutherland,f and R. E. Klemm*.f Department of Chemical Engineering and Applied Chemistry, Columbia University, and Department of Applied Science, Brookhaven National Laboratory, Upton, New York 11973 (Received: April 25, 1988)

-

-

Rate constants for the reactions H + O2 OH + 0 (1) and H + O2 + M H 0 2 + M (2) were measured under pseudo-first-order conditions by the flash photolysisshock tube technique that employs the atomic resonance absorption detection method to monitor [HI,. Rate data for reaction 1 were obtained over the temperature range from 962 to 1705 exp(-16132 & 276 cal K, and the results are well represented by the Arrhenius expression k l ( T ) = (2.79 & 0.32) X mol-'/RT) cm3 molecule-' s-I. The mean deviation of the experimentally measured rate constants from those calculated by using this expression is &16%over the stated temperature range. The recent shock tube data of Frank and Just (1693-2577 K) were combined with the present results for kl(7') to obtain the following Arrhenius expression for the overall temperature exp(-16439 & 186 cal mol-'/RT) cm3molecule-' s-'. The mean deviation span (962-2577 K): k,(T) = (3.18 f 0.24) X of the experimentally measured rate constants from this expression is &15% over the entire temperature range. Values for the rate constant for the reverse of reaction 1 were calculated from each of the experimentally measured k l ( T )values with expressions for the equilibrium constant derived by using the latest JANAF thermochemical data. These k-'(T) values were also combined with similarly derived values from the Frank and Just data. This combined data base showed that k-'( T ) was essentially constant between 962 and 2577 K with an average value of 2.05 X lo-'' cm3molecule-' s-' and a one standard deviation uncertainty of 0.42 X lo-" cm3 molecule-' s-'. Kinetic results were also derived for reaction 2 from the difference between the experimental first-order [HI, decays and the corresponding calculated k,( T ) values. The temperature span over which k2 data could be determined was limited to 746 K IT I987 K. Although these rate data exhibit a slight negative temperature dependence, the magnitude of the uncertainties in the k2 results and the limited temperature span that could be covered preclude the calculation of reliable Arrhenius parameters. Instead, a simple average value may be used to represent cm6 molecule-2 s-', where the error limit is given at the one standard deviation this rate constant, k2 = (7.1 f 1.9) X level. All the results obtained are compared with those of previous investigations.

Introduction

The reactions between atomic hydrogen and molecular oxygen

+0 2 H +0 2 +M H

-+

OH + 0

-+

HO2

+M

(1) (2)

are among the most important elementary reactions in gas-phase combustion. Reaction 1 is the major branching step in the Hz/O2 mechanism.' and this mechanism is an essential subset of the

* Author to whom communications should be addressed. Columbia University.

* Brookhaven National Laboratory.

Present address: Argonne National Laboratory Chemistry Division, Argonne, IL.

0022-3654/89/2093-0282$01 .50/0

hydrocarbon oxidation mechanism as well.*+ Reaction 2 is a chain terminating Step in the H2/02 mechanism in the lower range of combustion temperatures and it is in direct competition with reaction 1 . ' ~ ~ (1) Bradley, J. N. Flame and Combustion Phenomena; Methuen: London, 1969' and references therein' (2) Wagner, H. Gg. Fourteenth Symposium (InternationaI) on Combustion; The Combustion Institute: Pittsburg, 1973; 27. (3) Khandelwal, S. C.; Skinner, G. B. In Shock Waves in Chemistry; Lifshitz, A., Ed.; Dekker: New York, 1981. (4) Westbrook, C. K.; Dryer, F . L. Eighteenth Symposium (InternationaI) on Combustion; The Combustion Institute: Pittsburgh, 1981, p 749. ( 5 ) Bittner, J. D.; Howard, J. B. Nineteenth Symposium (International) on Combustion; The Combustion Institute: Pittsburgh, 1982; p 21 1. (6) Westbrook, C. K.; Dryer, F. L. Prog. Energy Combust. Sci. 1984, 10, 1 and references therein.

0 1989 American Chemical Society