G. SCHATZ AND M. KAUFMAN
3586 energy on going from I to U.l9 This is expected since the s-trans conformer should have the lowest dipole moment with the dipole moment of the transition state being intermcdiate. The steric interaction between the tert-butyl group and t,he carbonyl oxygen should tend to increase the strans conformer enwgy compared to the methyl formate in the same way as found for amidesaZoOn the other hand, the inductive effect from the teyt-butyl group should tend t o increase the torsional barrier through an increased double bond character in the 0C ( 0 ) bond It is thus not possible to predict how the barrier should change from methyl formate to tertbutyl formate: we believe, however, that the steric effect dominaces ( c j . amidee21). The energy difference between thle s-trans conformer and the transition state, as well as the encrgy difference between the s-trans and s-cis conformers, Phould thus be smaller for tert-butyl formate than for methyl formate, which is in good agreement with the present results and those of Subrahmanyam and P ~ ~ e r c y . ~ " From a compariscn of the barriers to internal rotation in formal es, nitrates13amides,z and nitrosamines4 it is obvious that the substitution of the carbonyl carbon in amides and formates with nitrogen increases the barrier by some 2 kcal/mol, possibly due to nitrogens greater electronegativity. In the same way it is seen
that the barrier height is reduced by more than 10 kcal/mol when an oxygen is substituted for the amino nitrogen in amides or nitrosamines. Both these effects are in agreement with the idea that it is the degree to which the resonance structure (2) contributes to the resonance hybride that primarily determines the barrier height. The contribution from the resonance structure (2) will decrease with increasing electroneg-
0
x-Y
(3-
//
ativity on X and decreasing electronegativity on Y.
Acknowledgments. We wish to thank Dr. E. Wennerstrom for helpful discussions and Dr. W. Egan for valuable linguistic criticism. This work was supported by a grant from the Swedish Natural Science Research Council. (19) This change is less than the error limits given in Table 111, but as can be seen from Figure 2, the r values for the s-cis barrier of sample I are signifioantly higher than the T values for ,thes-cis barriers of sample 11. (20) L. A. Laplanohe and M. T. Rogers, J. Amer. Cham. Soc., 86, 337 (1964). (21) R. M. Hammaker and B. A. Gugler, J. Mol. Spectrow., 17, 356
(1965).
Chemilumineseence Excited by Atomic Fluorine by G . Schatz and M. Kaufman"' Frick Chemical Laboratory, Princeton University, Princeton, New Jersey Publication costs assisted by the O&e
08640 (Received July 11, 1972)
of Naval Research
Visible or ultraviolet luminescence is observed in the reactions of atomic fluorine with many organic and inorganic molecules. I n most cases, the radiation is produced by reaction with fluorine atoms generated by electric discharge through either CF4 or F2-Ar mixtures, indicating that the luminescences are due neither to impurities nor to the very exothermic reactions of Fz. Some emissions, such as OH(A X) in the reaction with HzO, NH(A X) with NH1, and Clz(h --c X) with Cl,, are best explained as due to atomic combination. 1n other cases, this mechanism can be precluded by thermochemical arguments. With hydrocarbons, CH, CZ, CF, and CF2 bands have been observed. The presence of oxygen is apparently not necessary t o produce the CN emission, CO Cameron bands are one of the impurity-caused luminescences which have been observed in the hydrocarbon reactions. They are generated only when the atomic fluorine is produced from CFI and are possibly due to the reaction between atomic oxygen and CF.
-
Introduction The emission Qf ljght of variable intensity and color is certainly on? of the most striking features of flames, the Of the intensity and lzibution of this light ha3 long been employed to furThe JournaZ os Pkysical Chemistry, Vol. 76, N o . $4, 197%
-f
ther our understanding of these combustion systems. Such investigations are most profitable when there exists prior lrnowledge of the mechanism producing the (1) Correspondence should be addressed t o Department of Chemistry, Emory University, Atlanta, Ga. 30322.
CHEMILUMTNESCENC~E EXCITED BY ATOMIC FLUORINE emission, since in most flames, explosions, etc,, complicating factors, such as temperature gradients, diffusion, and wall effects, make it ext,remely difficult to unambiguously assign mechanisms. Much of the emission from flarnes is clue to chemiluminescence, ie., specific reactionii vchich produce products in radiating excited states. For flames employing oxygen or air as the oxidizer, the mechanism generating many of these emissions has beer thstablished by studying atomic flames, produced by adding reagents to flowing streams containing atomic oxygen or hydrogen. I n the present work, lumiriescences generated in atomic fluorine flames have been investigated to acquire greater understanding of combustion systems in which Fz or fluorine-containing compounds are employed as oxidizing agents. Chemiluminescence has also played a key role in the study of the kinetics of some of the simplest chemical reactions, those involving atoms. By far the most popular method of measuring the concentration 01 gas phase N, 0, and has been by titration with a stable compound which. rcacts rapidly and specifically with the atom according do a known stoichiometry.2 I n chemiluminescentj titrations, there is a change in the color or intensity sf: the luminescence a t the equivalence point. To datc, for atomic fluorine, no chemiluminescent titration has been devised which does not gave erroneous results in the presence of F2.3 Undoubtchdly, ths laex of availability of such a simple method for measuring fluorine atom concentrations has seriously inhibited 1% ork on atomic fluorine chemistry. Luminescence from fluorine-containing flames4” and discharges4b h*3s been previously investigated, and there has been exi,ensive work on infrared radiation ,~ diproduced in reactions of atomic f l u ~ r i n eprimarily rected toward laser development. The present paper, however, is the first to report on the visible and ultraviolet radiation produced in systems excited by atomic fluorine. ~
x
~
e~ ~
~~
~ c e
~~
t
i~
~~
~
One of the most troublesome problems that arises in the study of claemiluminescence is emissions produced by impurities. I n this work, spectral features assigned to CN, OB, CO, NO, and N F often appeared when the constituent elements of these molecules were not inten tionally b e q introduced into the system. Contaminants are particularly difficult to avoid when handling atomic fluorine, since this corrosive atom reacts with Pyrex and quartz, liberating atomic oxygen, 8 s well a8 a number of stable molecules. Fortunately, coating these materials with Teflon greatly reduces the rate a t which I;hey are attacked. As an aid in identifying emissions that are caused by impurities, me have reacted each substrate with fluorine atoms prodiiccd by an electric discharge through both CF, and F2-Ar mixtures. These two sources of fluo-
3587 rine atoms present somewhat different purification problems. Although discharging pure F2 would not introduce unwanted elements into the system, commercially available Fz (Matheson Co.) contains an appreciable air impurity. Due t o the similarity of the boiling points of Fz, 02, and Nz, removal of this impurity in a continuous flow system is exceedingly difficult and was not attempted. Thus, unpurified F2 was diluted five- to tenfold with Ar and discharged with 50-100 W of 2450-MHz power in an Evenson-type cavity6 on an alumina section of a flow tube, as suggested by Rosner and A l l e n d ~ r f . ~As anticipated, bands due to oxygen and nitrogen impurities were often excited by this fluorine atom source. The alternative source of atomic fluorine, discharged CFq (Matheson Co.), could be readily purified by outgassing a t liquid Nztemperature. It has been shown that a microwave discharge through CF4 in an alumina discharge section of a Teflon-protected flow system produces primarily atomic fluorine and CzFe.’ In addition to undissociated CF4, higher fluorocarbons, and a small amount of F2, there may also be some CF2 present in this source of fluorine atoms. Differences in the spectra excited by thc t,wo fluorine atom sources may result from their different concentrations of F and Fz, as well as from their different im[F]cF~ = purity concentrations. Generally, [F]E~-A~/ 2-5 and [ F 2 ] ~ z -[ ~FrB / ]= ~~ 3-30. 4 A t a total pressure of 1.0 Torr, a typical pressure of atomic fluorine is 100 p for F2-Ar and 30 p for CF4. In addition, some product of atomic fluorine reactions usually reacts with Fz, regenerating the fluorine atom. This chain process results in a further increase of the eflective atomic fluorine concentration in the E’Z-Ar source. It is notcworthy that many of these reactions of F2are more exothermic than the original atomic fluorine reaction and thus may be responsible for the observed luminescence. On the other hand, Fz could eliminate emissions by effectively scavenging an active intermediate. The experimental arrangement is sbou-n in Figure 1. The alumina tube, in which FZ-Ar or is discharged, (2) H. hfelville and B. G. Gowenlock, “Experimental Xethods in Gas Reactions,” Macmillan, New York, N. Y.,1964, p 248, F. KanEman, J . Chem. Phys., 28, 992 (1958); P.Harteck, R Reeves, and G. Mannella, ibid., 2 9 , 608 (1958). (3) D. E. Rosner and W. D. Allendorf, J . Phys. Chem., 75, 308 (1971). (4) (a) R. A . Durie, Proc. Roy. Soc., Ssr. A , 211, 110 (1952); G. Skirrow and H. G. Wolfhard, ibid., 232, 78 (1955); (b) for example, B. A. Thrush and J. J. Zwolenik, Trans. Faraday Soc., 5 9 , 582 (1963).
(5) J. C. Polanyi and D. C. Tardy, J . Chem. PhUs., 51, 5717 (1969); J. H. Parker and G. C. Pimentel, ibid., 51, 91 (1969); IT W. Chang, D. W. Setser, M. 3.Perona, and It. L. Johnson, Chem. Phys. Lgtt., 9, 587 (1971); H. W. Chang, D. W. Setser, and M. J. Perona, J . Phys. Chem., 75, 2070 (1971); N. Jonathan, C. M. Melliar-Smith, and D. H. Slater, J . Chem. Phys., 5 3 , 4396 (1970). (6) F. C. Fehsenfeld, K. M . Evenson, and a.P. Broida, Rev. Sei. Instrum., 36, 294 (1965) ; coinmercidly available from the Opthos Instrument Co. (7) C. E. Kolb and M . Kaufman, J . Phys. Chem., 76, 947 (1972). The Journal of Physical Chemistry, Vola76, N o . $4,1978
G. SCHATZ AND M. KAUFMAN
3588
of bands attributed to impurities varied considerably, undoubtedly owing to differences in the concentration of the impurity. Table I : Summary of Luminescences Excited by Atomic Fluorine Reecltent
co, coz, 80% Hz, H2S, NOz NzO, CClr, CFaCl HzO NHa
Figure 1. Flow system: A, bulbs for purifying reagents; B, helical traps; C, alumina tube; D, microwave cavity; E, Teflon connector; F, light-tight enclosure; G, light traps; H, AI foil. reflector; J, McLeod gauge; K, thermocouple gauge; L, movable inlet tube; M, Teflon O-ring connector; N, trap.
is joined by a Teflon connector to a section of the flow system containing tmo light traps and the region where the luminescence is viewed. This section is easily removed for Frelquent cleaning and recoating with Teflon. Reactants m e added through a movable inlet, which is positioned for mnxinium spectrometer response. Pressure is measured with either a n!kLeod or thermocouple gauge and gcnerally falls in the range 0.5-2.0 Torr, The apparatus does not contain provisions for measuring flow rates. However, some of the more interesting reactions are aurrently being investigated with a niore sophiutioaLed flow system. Spectra are recorded with a Jarrell-Ash, 0.25-m, Ebert monochromator (f/3.5) , having an ultimate resolution of (:a. 3 A and employing a photomultiplier detector sensitive from 1800 to 6200 A. For the weaker spectra, an aluminum-foil reflector is employed, and the phototube is cooled with nitrogen chilled to Dry Ice temperature. A nitrogen purge of the monochromator and a Xjpectrosil viewing section of the flow tube are USE$ when investigating the spectral region elow 2000 A. Wavelength calibration is from known rres in the spectrum and by comparison with a lowpressure niercury lamp. The entire apparatus is installed in a light-tight hood, and suitable shielding is employed E ~ Othat light from the discharge does not enter the monochromator.
esults Table I summarizes the luminescences excited by reacting atomic fluorine with various substrates. Except for the few cases noted, the emissions are produced with both discharged CF4 and discharged F2-Ar as the fluorine atom source. The intensity refers to the strongest line of each band system, with no allowance made for variation of the sensitivity of the spcctrometer, which decreases rapidly below 2000 and above 6000 Weak (w) emissions could usually be recorded only with resolution of ca. 50 b. The intensity
A.
The Journal oj’PFglaica2 Chemistry, VoZ. 76, No. 94, 107l
Cl2 Bra CFaBr CFaI Hydrocarbonsb
Emiasians doteoteda
None None None OH(A X, v‘ = 0,l)m NH(A X, v’ = O,l)s, NF(b + X, v‘ = 0)w C12(A -3 X, v’ = 6-12,14)m BrF(O+ X, v’ = 1-9)w BrF(O++ X, v‘ = 2-7)w IF(O+-3 X, 21’ = 0 - 6 ) ~ CF(A + X, v’ = 0,l)mc and (B -+ X v’ = O)m,c CFz CO(A -+ X, O’ = 2-10)mC,d and (a XI v’ = 0-5)111,~~~ C2(d .-+ a, v’ = O - ~ ) S , CH (A -+XI V’ = O)S, C3wc, OH(A+ XI U’ = 0 - 3 ) ~ CN(B ,~ -3 X, -+
-+
-+
M,C
.-t
V’ =
0-4)vd
a Intensities: s, strong; m, medium; w, weak; v, variable. CHb, C2H2, CzHr, CzHs, propylene, 1,3-butadiene, benzene. FZ-Ar fluorine atom source only. Due to impurity. CF, fluorine atom source only.
b
Discussion Although we will speculate on mechanisms for some of the observed luminescences, such speculations are a t this stage based primarily on thermochemical arguments and on analogy with ot,her chemiluniinescent systems. Reliable mechanisms can only be arrived a t after detailed study of the dependence of emission intensities on total and partial pressures in the system. Among the substrates for which no luminescence was observed, CO, COz, NzO, CCL, and CF&l react too slowly with atomic fluorine a t 25” and pressures of a few Torr to generate appreciable amounts of light.* Fluorine does not have a great affinity for electronegative elements, such as oxygen or chlorine. The more electropositive elements in these reactants are protected, either by strong multiple bonds to nonreactive atoms or sterically, as in CF$l and CCL ( and MacLean, however, have suggested that, at flame temperatures, atomic fluorine is able to displace chlorine from such per halo me thane^.^) Not all of the ‘honluminescent” reactions are slow, however. The (8) These results have been established by brief studies of these reactions which employed moleoular beam analysis, a new technique described in M. Kaufman and C. E. Kolb, Chem. lnstrum., 3, 175 (1971). A niore detailed investigation of the reaction with CCl4 has been reported in ref 7. (9) K. H. Homann and D. I. MacLean, Combust. FEame, 14, 409 (1970); J. Phys. Chem., 75, 3645 (1971).
CHEMILUMINESCENCE ‘EXCITEDBY ATOMICFLUORINE addition of atomic fluorine to SO2 to produce FzSO2 is H, and H Fz, quite rapid,a as are the reactions F which produce vibrationally excited H F and are the basis for H F 1asers.j The combination of hydrogen and fluorine atoms does not provide sufficient energy to populate the first excited electronic state of HF. It is interesting to compare the rapid reactions of atomic fluorine with the hydrides, HzO, HZS, and NH3. In the H28systelm, thr A state of OH can energetically be populated by three-body combination of 0 and H atoms, these being generated by a sequence such as
+
+ HzO +H F + OH F 4-811 --+ HF + 0 0 + OR + + H o + R 4-(14) OH* + ( n q F
0 2
+
(1) (2) (3)
(4) Reaction 3 , which involves two intermediates, probably does proceed ai, an appreciable rate in this system, since its rate coxrstnnt is ca. l0l3 cm3/mo1-sec,lo and oxygen atoms do not r5act with the major components &, F2, and HF). Atomic combination has been proposed for OH(A + X) emission in combustion reactions employing Q 2 . l1 The mechanism is also in accord wii h t hp absence of SH(A 4 X) emission in the BzS reaction, since combination of a sulfur atom with a hydrogen :%tomprovides 5 kcal less energy than that nececsary to populate the A state of SH. Durie has observed SH(A -3 X) emission from a €IzE’, flame contaminatcd tl ith sulfur-containing compounds. H o w Jer, at flame temperatures, sufficient thermal energy is available to form the A state. Sulfur is deposited in the reaction of atomic fluorine with HzS, and its formation could be initiated by a reaction analogous to ( 3 ) . ‘Phe reaction with iY€& completes the pattern, 4nce here the observed A(311) -,-X(lZ;-) bands of KH c m also be excited by combination of thermal atoms. Both Clz and Ear2 produce yellow luminescence when added to atomic fluorine. The glowing region in these reactions extends many centimeters downstream of the halogen inlet. In the el2case the emission is due to the A(%+) -+ X(lZ) transition of C12, strongly suggesting that the mecianism producing the luminescence is1’ ----f
F + 6 1 2 --+ ClF
el
+ C1
+ C1 + (31) --+ C12(AJITo+)+ (SI)
(5)
(6) Since recombination of atomic chlorine would be slow a t our pressures, the Luge extent of the emitting zone is consistent with reaction 5 being very fast.I3 The intensity of the (~mibionwas measured with a photocel) filter combination Yencsitive in the range 4500-6200 A and was found LO maximize a t a C1, flow very close to that which just removcd all the atomic fluorine.14 We have also noted that Fz concentrations, measured mass
3589 spectrometrically, are hardly affected by addition of sufficient Clz to maximize the emission, indicating that the reaction C1
+ Fz
--t
C1F
-+ F
(7)
must be quite slow at 25”. These observations suggest that the reaction with C12 may be of some use as a chemiluminescent titration for atomic fluorine in the presence of molecular fluorine. In the Br2 reaction, the yellow color is due to the 3n[0. --s X(l2) transi9. tion of BrF, with bands originating from 1 5 v’ I Brz emission from atomic bromine combination occurs a t wavelengths longer than 6500 Al5 and would not be detected by our photomultiplier. The 0’ 4 X transition of the corresponding interhalogen is also observed in the reaction of atomic fluorine with CF3Br and CFJ. There is considerable evidence that these reactions do not proceed by halogen displacement a t room temperature. l6 Thus, either free Br and I atoms are formed by secondary reactions in these systems or the emission is produced by a mechanism other than atomic combination. Characteristic luminescence is found for the reaction of atomic fluorine with hydrocarbons, as well as for molecules such as methyl bromide and ethylene oxide, which contain other elements in addition t o carbon and hydrogen. The visible light from these systems is the brightest of all the reactions we have investigated, with the most intense bands being those of CW( A X) and Cz (Swan system). The emissions are similar to those observed by Durie from flames of organic suband also resemble the spectra of stances burning in FZ4& organics burning in O2 or air.11 In the latter case, CH bands are generally ascribed t o reactions of oxggencontaining species. l7 In our atomic fluorine reactions, hou-ever, CH emission was most prominent in runs with minimum oxygen contamination (as measured by the intensity of OR emission), and thus oxygen-containing -jc
(10) D. D. Drysdale and A. C. Lloyd, Oxid. Combfist. Rev., 4, 157 (1950). (11) A. G. Gaydon, “The Spectroscopy of Fhmes,” Wiley, New York, N . Y., 1057. The reaction may proceed by three-body recombination or by preassocintion as suggested by S.Ticktin, G. Spindler, and €1. I. Schiff, Discuss. Faraday Soc., No. 44, 218 (1967). (12) L. W. Bader and E. A. Ogryzlo, J . Chem. Phys., 41, 2926 (1964) ; M. A. ,4.Clyne and J. A. Coxon, Proc. Roy. Xoc., Ser, A , 298, 424 (1967); M. A . A. Clyne and D. H. Stedman, Tmns. Faraday Soc., 64, 1816 (1968). (13) J. Warnatz, N.Gg. Wagner, and C. Zetzsch, Ber. Bunsenyes. P h , ~ sChem., . 7 5 , 119 (1971). (14) The fluorine atoms were simultaneously monitored by molecular beam analysis; see ref 7 and 8. (15) D. €3. Gibbs and E. A. Ogryzlo, Can. b. Ckem., 43, 1905 (1965). (16) I. 0. Leipunskii, I. I. Morozov, and V. L. Tal’roze, Dokl. A k a d . Nauk S S S R , 198, 1367 (1971); J. Bozzelli, C. Kolb, and .M. Kaufrnan, t o be published. (17) P.H. X.ydd and W. I. Foss, Eleventh Symposium on Combustion, The Combustion Instituk, 1967, p 1179; J. Peeters, J. F. Lambert, P. Hertoghe, and A . Van Tiggelen. Thirteenth Symposium on Combustion, The Combustion Institute, 1971, r) 321; see ref 11. The Journal of Physical Chemistry, Vol. 76,IVO.3.4, 197.9
G. SCHATZ AND M. KAUFMAN
3590 species are probabkr not necessary for its production. An oxygen-free reaction, that of two excited CH2 radicals, has recently been proposed by Quickertls to explain CH emission in the 0 CzHz system. Siirprisinglv, ai wavelengths shorter than 3000 8,the emission from hydrocarbon reactions is dramatically different depending on whether FZ-Ar or CF, is used as the source of atomic fluorine. With F2-Ar, the A -+ X and 13 -+ X transitions of CF, as well as bands of GF2, are detected when fairly high concentrations of Fzare employed. Xeither the A state (123 kcal/mol) nor the B state (141. kcal/mol) of CF (Do = 110 kcal/ is accessible by combination of thermal, groundstate carbon and fluorine atoms. Thus, this mechanism can bs rulecl out, even though there is some heating of the flow tube at high F, partial pressures. Although CF bands were not reported in Durie’s study of F, flames, they xere observed by Sliirrow and Wolfhard in the emission from ClFs-supported flame^.^" The reaction between discharged F r A r and hydrocarbons also produces ~ t r o n gCO fourth positive impurity bands, extentling t o the short-wavelength limit of the spectrometer CF, GF?, and (20fourth positivc bands were never observed using discharged CF,. Instead, in the region 1930-2760 8, SOITIF: 40 bands were generated by reaction with many hydrocarbons. Except for a few very weak features, all these bands can be assigned to the Cameron system, a(Q) X(l2), of CO. This is a surprising conclusion, since the a(311)state of CO has a lifetime of approximately low2sec and is easily quenched by a variety of dianiagnetic and paramagnetic collision. partners.19 However, the fit to the Cameron bands is quite good consrdering the quality of the spectra. Further confidence in this assignment is gained from the observation tliai, no shifts or splittings of bands are observed when CN2D2is substituted for CH4 as the substrate. Since thc intensity of the Cameron bands is not greatly affected by different methods of purifying the reagents, we must conclude that they are mainly due to the sniaii amount of oxygen that is liberated by tvall reactions. The bands are not produced by atomic fluorine formed by discharging F2-Ar mixtures. Thus,
+
-+
The Journal of Phgsical Chemistry, Vol. 76, N o . 2-4, 1072
a reactant unique to the CF4 system must be necessary for their appearance. The reaction
0
+ CF
CO*
4-F
(8)
provides an appealing mechanism for this lumincscence because of its simplicity and the fact that it can populate the a(%) state, but not the .A(%) state of CO. Some 15 kcal of vibrational excitation of the CF would be necessary, however, to excite levels up to v’ = 5 , from which emission is observed. We have thus shown that visible and ultraviolet luminescence is quite a common occurrence in systems excited by atomic fluorine. In most cases these emissions can be produced with both discharged CF4 and discharged F2-Ar mixtures as the source of fluorine atoms, indicating that they are due neither to reactions of impurities nor to the very exothermic reactions of Fz. One can also rule out a mechanism involving energy transfer from excit,ed F a molecules produced by fluorine atom recombination, analogous to the current explanation for many of the emissions excited by “active” nitrogen,20 since in the fluorine case this would not supply sufficient energy to generate visible light. While in several instances, three-body combination of the constituent atoms of the emitting molecule seems to be a reasonable mechanism for the emission, in other cases this can be precluded by thermochemical or kinetics arguments. Further investigations directed toward understanding these interesting phenomena are currently underway.
Acknowledgment. The authors would like t o thank J. Bozzelli, B. Hertzler, and Dr. C. E. Kolb for considcrable aid with these experiments. This work was supported by the Office of Naval Research under Contract KO. N0014-67-A-0151-0013. (18) K. A. Quickert, J . Phys. Chem., 7 6 , 825 (1972). (19) G. M. Lawrence, Chem. Phzis. Lett., 9 , 575 (1971); T. 5. Wanchop and H. E’. Broida, J . Chem. Phys., 56,330 (1972); T.G. Slanger and G. Black, ibid., 5 5 , 2164 (1971) ; T. 6. James, J . Mol. Spectrosc., 40, 545 (1971). (20) A. H. Wright and C. A , Winkler, “Active Nitrogen,” Academic Press, New York, N. Y., 1968.
