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Hydrogen abstraction by fluorine atoms: F + HX and F + DX (X = I, Br, Cl)
Volume 57, number 3
1 August1978
HYDROGEN ABSTRACTION BY FLUORINE ATOMS: F f HX and F f DX (x = I, Br, Cl) E. -BERG,
AJ. GRiMLEY
and P.L. HOUSTON
Department of BremistJy, Cornell University.
Ithaca,New York 14853, USA Received19 April 1978
Absolute reactiorfratesfor F z HX and F + DX (X = I, Br, Cl) havebeen obtainedby mordtoringthe rise time of HF @F) vibrationalfluorescencefoliowingmultiphotondissociationof SF6 in mixturesof HX @X) and argon.The cross stations for reactionarc, in units of lo-l6 cm’, 4.37,5.26, and 1.16 for HI, HBr, and HCl, respectively.The isotope effects k&kDx, are 1.29 i 0.14, I.29 IT0.18, and 1.38i 0.29, respectively.
1. Introduction Hydrogen abstraction by fluorine atoms [1,2], particularly from HI, HBr, and HCl, is of importance in understanding the pumping mechanisms of HF chemical lasers [3-5]_ Relative reaction rates to individual vibrational levels of the HF product have :low been measured in several laboratories [3-l l] and compared to the results of classical trajectory calculations [8,12]. However, very little information is available in the literature concerning the total absolute reaction rates. In this paper the technique of laser photolysis/infrared fluorescence 1131 has been used to determine the total reaction rates for F + HX and F + DX where X = I, Br, Cl_ These rates are discussed in terms of the reaction mechanism proposed by Mei and Moore [14] to explain the high value of the cross section and the H/D isotope effect observed in the Cl f HI reaction.
2. Experimental
A detailed description of the experimental apparatus will be reported elsewhere [ 15]_ Briefly, the output of a Tachisto TEA CO, laser was focused by an antireflection coated germanium lens of 5 cm focal length into a cell containing 0.03-O. 1 torr of SF6, O-O.4 torr of HX or DX (X = I, Br, Cl), and S-6 torr of argon. Infrared emission from the vibrationally excited HF or DF reaction product was observed through a quartz
window with an HgGe detector. An interference filter held at 77 K and centered at 3.03 pm with a bandwidth of 1.24 pm was used to isolate the HF fluorescence. DF fluorescence was observed through a filter centered at 3.82 urn with a bandwidth of 0.8 pm. The time constant of the detector and its associated electronics was roughly 200 11s. Fluorescence signals were digitized by a transient recorder (Biomation 805) and averaged ia a hardwired signal analyzer (Northern 575A) before being transferred to a computer (Prime 400) for further analysis and storage. A nonlinear least squares fitting routine was used to fit a function of the form amp [exp(-t/Td) - exp(-t/r,)] to the signal, where rd and rr are the deey and rise times, respectively. SF6 and the hydrogen halides were obtained from
Matheson at the following minimum purities: SF6(99.8%J HCl(99.9%), HBr(99.8%), Hi(98%). The deuterium halides were obtained from Merck at the following minimum atom percents deuterium: DCl(99%), DBr(98%), DI(98%). All gaseswere further purified by freezepump-thaw cycles. HBr, HI, and DI were withdrawn from bulbs immersed in a -95°C bath to avoid any Br2 or I2 impurity. Before using any of the deuterium halides, the entire vacuum system was passivated with 8 ;orr of D,O fcr several hours and then evacuated to
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Time (psec) Fii I_ Di&ize3 S&MI of HF fluorescencefrom the F f HI reactionfollowingumltiphotondissociationof 0.05 torr of SF6 in 0.1 ton of Hi and 5.2 tourof argon,The signalis the avew from 8 laserpulses.
3, Remits Fig. 1 displays a typical HF fluorescence signal observed folIowing the photolysis of 0.050 torr of SF6 in O-100 torr of HI and 5.2 torr of argon. The signal is the average from 8 laser pulses_ Sirmlar averaged sign&s, ah with S/N > 30, were obtained foilowing photoIysis of SF6 in HBr, HCl, DI, DBr, and DCL No signals were observed in the HF (DF) spectral region when either the SF6 or the HX (DX) was omitted from the reaction mixture. The observed signals were independent of the SF6 pressure over the range 0.03-U.l torr. As a check on the isotopic purity of the DX, we observed the HF fluorescence from photolysis of SF&X/Ar mixtures through a filter which passed radiation from ah possrbIe HF product states (2.7 arm center, 0.86 irm bandpass). The amplitude of *zheHF signal compared to the amplitude of the DF signai after correction for radiative Zifetime, filter transm%sion, and detector response indicated that the ratio of HF/DF was Iess than 3%. In ail cases the signah were accurately represented by a function of the form signal = amp - fexp(-f/r& - eX~-q%rj] wfit%?S rd and or are the decay and rise times, respectively. In fig 2 we have plotted rgi versus the partial pressure of HI(circles), KBr (triangles) and HCI (squares) at constant: SF6 and argon pressure. The . sIopes of the linear fits give the rate constants for the 374
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Pressure (mtarrf Fig. 2. Ptot of the inverser&e time for HF ffuorescenceversus pressureof HI &I&S), HBr (triangks) and HCI (squares) for constalltpzssures of SF6 and argon_
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(JXEMICAL PHYSICS LETrERS
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be in the range of 033 to O-41_ An unambiguous experimentalmeasurementof the branchingratio is difficult because of the possibility of vibrational-to-electronic (V + E) energy transfer: HF(u) I- I s HF(u’) f I*. k-3
0
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210 260 Pressure tmtorr)
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Fig. 3_ Plot of the inverse rise time for DF fhorescence versus pressure of DI (circles), DBr (triangles) and DCI (squares) for ccnstantpressures of SF6 and argon.
respectiveF + HX reactions. Siiar data are presented in fig. 3 for the F + DX reactions. A summary of the observed rate constants is given in table 1.
4. Discussion
AR attempt was made to determine the branching ratio between t&e reactions F+HI%F(V)+I,
Cl)
F+m?HF(u)+I*,
(2)
where I = I(5 2P3,z) and I* = i(5 2P,&.
Considerable
controversy has arisen over tie value of k2/(kl + k2). Sung and Setser IlO] have found that k2/(kl f k2) < OXJO while Burak and Eyal [ 161 have suggested that k&k1 + kq) might be es high as 0.5. From an information theoretical analysisof the HF vibrational distributicn measured by Jonathan et al. [6], Dinur et al. [17] have determined that k2/(klf k2) should
(3)
Coombe and Pritt [18] have measured the rate at which E + V transfer from I” to HF(u = 0) produces I f HF(u = 2). From their rate constant and the equilibriumconstant for reaction (3) with u = 2 and u’ = @, it may be calculated that k3@, u’ = 2,0) = 3.25 X lo4 s-l torr-‘. Bates for other Au = u - u’ = 2 processesshould be similar. Unless the partialpressuresof HF(u) and I are kept extremely low, the bigh magnitude of k3 may cause I* to be produced by reaction (3) on the time scale of observationin a flow system such as that used in ref, [lo] or ref. 1161. That this disadvantagemay be overcome by using a pulsed F atom source has been previously suggested [ 163. Therefore, we looked carefully for an I* signal using an I&b detector and an interferencefilter centered at 1.3 17 m with a 0.11 m bandpass. Although a signalwas observed in this spectralregion, it is likely that much of the signalis due to HF overtone fluorescencepassedby the filter. 23 rotational lines of theHF3+1bandand16linesofthe2+0bandare passed by our I* filter. Since similarsignalsin this spectral region were seen from photolysis of either mixtures of SF, in HI or mixtures of SF, in H,, it is clear that the major fraction of the observed signalis due to HF ratherthan to I*_ Because of the large difference in radiativelifetimes, overtone fluorescence from even a small number of HF states may totally overwhelm any fluorescencefrom I*. Ahhough the expected I* signal should disappearmuch more slowly than the HF(u) signal, the signal-to-noiseratio in this experiment did not allow us to make an accurate determinationof the amount or even the presence of any I * fluorescence. As a consequence we were unable to determine a branching ratio between reactions (1) and (2). An accurate value of the branchingratio might be obtained by coupling the pulsed F atom source used in this work with detection of I and I * by vacuum ultraviolet absorption. The laser photolysis~infraredfluorescence technique [13] for measuringrate constants is known to be direct and accurate. Nonetheless, the present extension of this technique to ‘he use of multiphoton photolysis deservessome comment. Three independent laboratories 375
CHEWCAL PNYSKCSLETPEPS
Vohme 57, mlm~r 3
[19-Z] using two different techniques have shown that the F atoms produced by multiphoton dissociation of SF6 recoil with very little translaticnal excitation_ Quick and Wittig [ZO] and presses et al_ [;?I1 hnve shown that the v&es for the rate of the F + Hz reaction obtained by the muhiphoton dissociation/ infrared fluorescence techique using SF6 are in good agreement W;th Literature values for the reaction of thermai ff uorine atoms with Hz_ In our experiments we have taken the further precaution of using an argon buffer gas at pressures such that the F atom wih experience at least 10 colhsions with argon before it colIices with HX or DX_ A second potential problem with the multiphoton dissociation deserves same consideration_ As vibration to translation (V + T) energy t_rznsferoccurs from SF6 to the gas mixture, the temperature in the fluorescence re$on of the cell will rise. The method presented in this paper will provide us&xl rate constants only for reactions which are rapid compared to the rate of V + T energy ~transfer.The rate of V + T transfer from SFG(vs = 1) to argon is well known [22,23] _ Based on this rate and the pressure of argon in our experiments,
I August 1978
V -+ T transfer would take about 90 ~.ls_In contrast, most of our data is taken under conditions for which the reaction is complete in less than IO flcrs_ Furthermore, we would expect any change of temperature on the time scale of reaction to cause a curvature in the plots of r--1 versus pressure. Such curvature is not observed (figs. 2 and 3). Finally, the rise rate of the HF fluorescence from the F f HI reaction varied by < 5% when the SF6 pressure was varied in the range 0.030_10 torr and when the laser energy tluence was varied in the range of 4-35 J/cm’_ Since the amount of any heating by V + T transfer will be proportional to the amount of energy absorbed, and since the amount of energy absorbed varies strongly with pressure and energy fluence, the absence of any effect on the rise rate can be taken to mean that little if any V + T heating occurs on the time scale of the reaction_ We feel confident, therefore, that the results reported in this paper are accurate, room temperature rate constants_ A summary of the F i HX and F + DX absolute rate constants obtained in this work is presented in table 1_ Because of the relatively small isotope effects, we have not corrected the F f DX rates for the < 3%
Absdute rates for F + HX and F f DX tX = I, Br, U)
System
this wotk F*HI F+DI F+HBr
F+DBr F+HQ
??+DC.E
From relativeratesa)
Absolute rates ts-’ torr”)
(8.72 * 0.22) x io5 (6.77 f 0.58) X 10’ (1.09 -F0.07) x lo6
f&47 i 0.66) x IO5 (2.67 f 0.28) x 10’
(1.93 c 0.22) x IO5
literature
2_r4x
2,0x 3.5 x 3.9x 8.0 x
10~
105 lo5 lo5 los
literature
8.3 x 10’ 7.7 x lo5 6-l X 10’ b)
;:i; 1111 171 2271 $281 (291
!S x IO5 1.3 x lo5 1.3 x lo5 b)
a) CalcuMed from Iiteratwz vahe of k(HIlfk(HX) aswming &HI) = 8.72 X 10’ s-l torr-x _ b) Taken fkomref. 171 without correction for new ffF transition probabilities[3I ]_ 376
Ref.
f::f WI 173
CHEMICAL PEYSICS LETIERS
Volume 57, number 3
HX impurity. Such a correction would fall within the stated error limits. The rate constants obtained in this work are in reasonable agreement with those obtained by less direct techniques, as shown in the third and fourth columns of table 1. Our observation that the rate for F + HI is slightly slower than the rate for F f HBr is at variance with the observations made by fluorescence yields in flow systems [7,10,1 I] _ Since the fluorescence yield technique involves a correction for radiative lifetime and an integration of the wavelength dependence of the fluorescence, we do not feel that the slight discrepancy is unreasonable_ Our method is based on a direct observation of the formation rate of products and should give accurate results. It is interesting to compare our results for the F + HX (DX) reactions with literature values of the cross section and isotope effect for the Ci C I-TX(DX) and Br + HX (DX) reactions. Table 2 presents a comparison for all of the measured X’ + HX (DX) abstraction cross sections. With the exception of F + HI, the reaction cross section increases as the HX bond becomes weaker. While this effect may be due simply to a lowering of the barrier to reaction or an increase in the number of available product states as the reaction becomes more exothermic, Mei and Moore 1141 have taken the view that for Cl + HX the high values for the cross section and the isotope effect imply a mechanism involving interhalogen attractive forces [24-261. In this view, the Cl atom is attracted to the iodine end of HI and reaction is completed following rotation of the hydrogm atom to a position between the two halogens. Support for this mechanism comes from the non-Arrhenius behavior of the rate constants for both Cl + HI and Cl i HBr as a function of temperature. The Cl + HI reaction actually shows a slight inverse temperature depenT&e 2 Reactioncrosssectionsandisotope effects 2, Atom
Diatom HI
Fb) a c)
Br d)
4.27 (1.29) 33.2
2.79
(1.84)
1 August 1978
dence above 300 K_ If this attractive mechanism is responsible for all of the abstraction reactions in table 2, then it is somewhat surprising that the cross section for F + HI is not larger than that for Cl f HI. The attraction between F and HI should be even greater than that between Q and HI [24-261. One possiile explanation is that at room temperature the F f HI cross section has already begun to deviate significantly from Arrhenius behavior_ On the other hand, the H/D isotope effects for the F + HX reactions, while larger than that predicted by trajectory calculations on, for example, an FHCl LBPS surface [ 121, are not as large as those found for the Cl + HX reactions. Further experiments now in progress in our laboratory on the temperature dependence of the reaction rates will be necessary to assessthe importance of the attractive mechanismintheF+HXsystems.
5. Conclusion The cross section for the reaction between F and HX (X = I, Br, Cl) increases from Cl to Br but decreases from Br to I. Hydrogen abstraction is about 13 times more favorable than deuterium abstraction for these reactions. Although these results could indicate the presence of a mechanism involvvg attraction between the fluorine atom and the halide end of HX, furLher investigation of the temperature dependence of the reaction rates is needed to assessthe importance of such a mechanism. Acknowkdgement We gratefully acknowledge support of this work by the Air Force Office of Scientific Research (AFOSR78-3513) and by the Standard Oil Company of Ohio. References
Bl3r
HCl
5.26 (1.29)
1.16 (l-38)
1.44 (1.5)
2) Crosssectionin A2 (&&bx). b, This work. Cl From ref. [ 321.
d, From ref. [33].
Cl1 R Foon andM. Kaufinan,Progr. ReactionKinetics8 (1975) 81. [2] S.H. MO,E-R Grant,F.E. Little, R.G. Manning, C-k Mathis, G.S. Werre and J-W. Root, *m: Fluorinecontaining free radicals, e& J.W. Root, ACS Symposium series66, Washington(1978) p_ 59. [3] S. Bittenson,Ph.D. Dissertation,Universityof W-&n, Madison(1977). 377
VoIume 57, number 3
CHEMICAL
PHYSICS LETTERS
[4] R-D- Coombe, G.C. Pimentel and &f-J_Berry, IEEE J. Quantum EIectron. QE-9 (1973) 192. [S] M_J_ Berry, in: MokcuIar energy transfer, eds. RD. Levine and J_ Jortner (Wiley, New York. 1976) p_ 114 [6] N. Jonathan, CM_ Melliar_SmIth. D. Tii and DH_ Slater, Appl. Opt 10 (1971) 1821. [7] N. Jonathan. CM_ MeUiar_Smith, S. Okuda, D_H_ SIater and D. Thnlin, MoL Phys. 22 (1971) 561. [8] AM-G. Ding, LJ. Kirxh, D.S. Perry, J.C. Po’hmyi and J-L Schreiber. Discussions Faraday Sot. 55 (1973) 252_ [9] LJ. Kirsch and J-C_ Pohinyi, J. Chem. Phys. 57 (1972) 4498. [lo] JP. Sung and D-W_ Setser, Chem. Phys. Letters 48 (1977) 413. [ 111 D J_ Smith, D-W_ Setser. KC_ Kim and D J. Bogan, J. Phys. Chem. 81 (1977) 89%. [ 121 J. Santamaria and D_L_ Bunker, Chem. Phys. 23 (1977) 243. [ 131 FJ. Wodarczyk and C.B. hfoore, Chem. Phys. Letters 26 (1974) 484_ (141 C-C. Mei and C-B. hfoore, J. Chem. Phys. 67 (1977) 3936. [I51 E- Wibzberg. Li_ Kovaienko and P_L_ Houston, in preparation_ 1161 I- Burak and hf_ EyaI, Chem. Phys. Letters 52 (1977) 534. 1171 U. Dinur. R- Kosloff. R-D_ Levine and MJ. Berxy. Chem. Phys. Letters 34 (1975) 199. 1181 R-D- Coombe acd AT_ Ritt Jr., J- them- Phys. 66 (1977) 5214_
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[ 191 MJ. Coggiola, P_A. Schulz, Y.T_ Lee and Y-R Shen, Phys. Rev_ Letters 48 (1977) 425. [20] CR Quick and C. Wittig, Chem- Phys. Letters 48 (1977) 420. [21] J.Bf. Presses, RR Weston Jr_ and G_ Flynn. Chem. Phys. Letters 48 (1977) 425. 1221 J.L Steinfdd, I. Burak, D-G. Sutton and A-V. Nowak, J. Chem. Phys. 52 (1970) 5421. [23] RD. Bates Jr.. J.T. Knudtson, G-W_ FIynn aud AM. Ronn, Chem. Phys_ Letters 8 (1971) 103_ [24] Y-T_ Lee, J-D. BfcDonaId, R.B. LeBreton and D_R Herschbach, J. Chem.‘Phys. 49 (1968) 2447. [25 ] Y.T. Lee, P-R LeBreton, JJ). hfcDonaId and D-R Herschbach, J. Chem. Phys. Sl(l969) 455. 1261 J.hf. Farrar and Y-T_ Lee. J. Am. Chem_ Sot. 96 (1974) 7570. 1271 J. Wamatz, Ph.D. Dissertation, George-August University, G&tingen (1968). [281 J. Wamatz, H.G. Wagner and C. Zetsch, Report T-02401 92410/01017 to the Fraunhofer Geselfschaft (1972). (291 T-L._ Pollock and W.E. Jon-. Can. J. Chem. Sl(1973) 2041. [39J K-L. Kczlpa and J. Wanner, Chem. Phys Letters 12 (1972) 560. [311 RX SiIeo and TA_ Cool, J. Chem. Phys. 65 (1976) 117. Bergmann and CJ3. Moore, J. Chem. Phys. 63 (1975) [321 K_ _.^ 643.
[33] R Bergmann, S-R Leone and C-B. Moore, J. Chem. Phys. 63 (1975) 4161.
1 August1978
HYDROGEN ABSTRACTION BY FLUORINE ATOMS: F f HX and F f DX (x = I, Br, Cl) E. -BERG,
AJ. GRiMLEY
and P.L. HOUSTON
Department of BremistJy, Cornell University.
Ithaca,New York 14853, USA Received19 April 1978
Absolute reactiorfratesfor F z HX and F + DX (X = I, Br, Cl) havebeen obtainedby mordtoringthe rise time of HF @F) vibrationalfluorescencefoliowingmultiphotondissociationof SF6 in mixturesof HX @X) and argon.The cross stations for reactionarc, in units of lo-l6 cm’, 4.37,5.26, and 1.16 for HI, HBr, and HCl, respectively.The isotope effects k&kDx, are 1.29 i 0.14, I.29 IT0.18, and 1.38i 0.29, respectively.
1. Introduction Hydrogen abstraction by fluorine atoms [1,2], particularly from HI, HBr, and HCl, is of importance in understanding the pumping mechanisms of HF chemical lasers [3-5]_ Relative reaction rates to individual vibrational levels of the HF product have :low been measured in several laboratories [3-l l] and compared to the results of classical trajectory calculations [8,12]. However, very little information is available in the literature concerning the total absolute reaction rates. In this paper the technique of laser photolysis/infrared fluorescence 1131 has been used to determine the total reaction rates for F + HX and F + DX where X = I, Br, Cl_ These rates are discussed in terms of the reaction mechanism proposed by Mei and Moore [14] to explain the high value of the cross section and the H/D isotope effect observed in the Cl f HI reaction.
2. Experimental
A detailed description of the experimental apparatus will be reported elsewhere [ 15]_ Briefly, the output of a Tachisto TEA CO, laser was focused by an antireflection coated germanium lens of 5 cm focal length into a cell containing 0.03-O. 1 torr of SF6, O-O.4 torr of HX or DX (X = I, Br, Cl), and S-6 torr of argon. Infrared emission from the vibrationally excited HF or DF reaction product was observed through a quartz
window with an HgGe detector. An interference filter held at 77 K and centered at 3.03 pm with a bandwidth of 1.24 pm was used to isolate the HF fluorescence. DF fluorescence was observed through a filter centered at 3.82 urn with a bandwidth of 0.8 pm. The time constant of the detector and its associated electronics was roughly 200 11s. Fluorescence signals were digitized by a transient recorder (Biomation 805) and averaged ia a hardwired signal analyzer (Northern 575A) before being transferred to a computer (Prime 400) for further analysis and storage. A nonlinear least squares fitting routine was used to fit a function of the form amp [exp(-t/Td) - exp(-t/r,)] to the signal, where rd and rr are the deey and rise times, respectively. SF6 and the hydrogen halides were obtained from
Matheson at the following minimum purities: SF6(99.8%J HCl(99.9%), HBr(99.8%), Hi(98%). The deuterium halides were obtained from Merck at the following minimum atom percents deuterium: DCl(99%), DBr(98%), DI(98%). All gaseswere further purified by freezepump-thaw cycles. HBr, HI, and DI were withdrawn from bulbs immersed in a -95°C bath to avoid any Br2 or I2 impurity. Before using any of the deuterium halides, the entire vacuum system was passivated with 8 ;orr of D,O fcr several hours and then evacuated to
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Time (psec) Fii I_ Di&ize3 S&MI of HF fluorescencefrom the F f HI reactionfollowingumltiphotondissociationof 0.05 torr of SF6 in 0.1 ton of Hi and 5.2 tourof argon,The signalis the avew from 8 laserpulses.
3, Remits Fig. 1 displays a typical HF fluorescence signal observed folIowing the photolysis of 0.050 torr of SF6 in O-100 torr of HI and 5.2 torr of argon. The signal is the average from 8 laser pulses_ Sirmlar averaged sign&s, ah with S/N > 30, were obtained foilowing photoIysis of SF6 in HBr, HCl, DI, DBr, and DCL No signals were observed in the HF (DF) spectral region when either the SF6 or the HX (DX) was omitted from the reaction mixture. The observed signals were independent of the SF6 pressure over the range 0.03-U.l torr. As a check on the isotopic purity of the DX, we observed the HF fluorescence from photolysis of SF&X/Ar mixtures through a filter which passed radiation from ah possrbIe HF product states (2.7 arm center, 0.86 irm bandpass). The amplitude of *zheHF signal compared to the amplitude of the DF signai after correction for radiative Zifetime, filter transm%sion, and detector response indicated that the ratio of HF/DF was Iess than 3%. In ail cases the signah were accurately represented by a function of the form signal = amp - fexp(-f/r& - eX~-q%rj] wfit%?S rd and or are the decay and rise times, respectively. In fig 2 we have plotted rgi versus the partial pressure of HI(circles), KBr (triangles) and HCI (squares) at constant: SF6 and argon pressure. The . sIopes of the linear fits give the rate constants for the 374
240 -
72000” z 5 c
i60-
0
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120
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Pressure (mtarrf Fig. 2. Ptot of the inverser&e time for HF ffuorescenceversus pressureof HI &I&S), HBr (triangks) and HCI (squares) for constalltpzssures of SF6 and argon_
VaAme
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(JXEMICAL PHYSICS LETrERS
1 August 1978
be in the range of 033 to O-41_ An unambiguous experimentalmeasurementof the branchingratio is difficult because of the possibility of vibrational-to-electronic (V + E) energy transfer: HF(u) I- I s HF(u’) f I*. k-3
0
I
I
70
I
140
I
I
210 260 Pressure tmtorr)
I
I
350
420
I
Fig. 3_ Plot of the inverse rise time for DF fhorescence versus pressure of DI (circles), DBr (triangles) and DCI (squares) for ccnstantpressures of SF6 and argon.
respectiveF + HX reactions. Siiar data are presented in fig. 3 for the F + DX reactions. A summary of the observed rate constants is given in table 1.
4. Discussion
AR attempt was made to determine the branching ratio between t&e reactions F+HI%F(V)+I,
Cl)
F+m?HF(u)+I*,
(2)
where I = I(5 2P3,z) and I* = i(5 2P,&.
Considerable
controversy has arisen over tie value of k2/(kl + k2). Sung and Setser IlO] have found that k2/(kl f k2) < OXJO while Burak and Eyal [ 161 have suggested that k&k1 + kq) might be es high as 0.5. From an information theoretical analysisof the HF vibrational distributicn measured by Jonathan et al. [6], Dinur et al. [17] have determined that k2/(klf k2) should
(3)
Coombe and Pritt [18] have measured the rate at which E + V transfer from I” to HF(u = 0) produces I f HF(u = 2). From their rate constant and the equilibriumconstant for reaction (3) with u = 2 and u’ = @, it may be calculated that k3@, u’ = 2,0) = 3.25 X lo4 s-l torr-‘. Bates for other Au = u - u’ = 2 processesshould be similar. Unless the partialpressuresof HF(u) and I are kept extremely low, the bigh magnitude of k3 may cause I* to be produced by reaction (3) on the time scale of observationin a flow system such as that used in ref, [lo] or ref. 1161. That this disadvantagemay be overcome by using a pulsed F atom source has been previously suggested [ 163. Therefore, we looked carefully for an I* signal using an I&b detector and an interferencefilter centered at 1.3 17 m with a 0.11 m bandpass. Although a signalwas observed in this spectralregion, it is likely that much of the signalis due to HF overtone fluorescencepassedby the filter. 23 rotational lines of theHF3+1bandand16linesofthe2+0bandare passed by our I* filter. Since similarsignalsin this spectral region were seen from photolysis of either mixtures of SF, in HI or mixtures of SF, in H,, it is clear that the major fraction of the observed signalis due to HF ratherthan to I*_ Because of the large difference in radiativelifetimes, overtone fluorescence from even a small number of HF states may totally overwhelm any fluorescencefrom I*. Ahhough the expected I* signal should disappearmuch more slowly than the HF(u) signal, the signal-to-noiseratio in this experiment did not allow us to make an accurate determinationof the amount or even the presence of any I * fluorescence. As a consequence we were unable to determine a branching ratio between reactions (1) and (2). An accurate value of the branchingratio might be obtained by coupling the pulsed F atom source used in this work with detection of I and I * by vacuum ultraviolet absorption. The laser photolysis~infraredfluorescence technique [13] for measuringrate constants is known to be direct and accurate. Nonetheless, the present extension of this technique to ‘he use of multiphoton photolysis deservessome comment. Three independent laboratories 375
CHEWCAL PNYSKCSLETPEPS
Vohme 57, mlm~r 3
[19-Z] using two different techniques have shown that the F atoms produced by multiphoton dissociation of SF6 recoil with very little translaticnal excitation_ Quick and Wittig [ZO] and presses et al_ [;?I1 hnve shown that the v&es for the rate of the F + Hz reaction obtained by the muhiphoton dissociation/ infrared fluorescence techique using SF6 are in good agreement W;th Literature values for the reaction of thermai ff uorine atoms with Hz_ In our experiments we have taken the further precaution of using an argon buffer gas at pressures such that the F atom wih experience at least 10 colhsions with argon before it colIices with HX or DX_ A second potential problem with the multiphoton dissociation deserves same consideration_ As vibration to translation (V + T) energy t_rznsferoccurs from SF6 to the gas mixture, the temperature in the fluorescence re$on of the cell will rise. The method presented in this paper will provide us&xl rate constants only for reactions which are rapid compared to the rate of V + T energy ~transfer.The rate of V + T transfer from SFG(vs = 1) to argon is well known [22,23] _ Based on this rate and the pressure of argon in our experiments,
I August 1978
V -+ T transfer would take about 90 ~.ls_In contrast, most of our data is taken under conditions for which the reaction is complete in less than IO flcrs_ Furthermore, we would expect any change of temperature on the time scale of reaction to cause a curvature in the plots of r--1 versus pressure. Such curvature is not observed (figs. 2 and 3). Finally, the rise rate of the HF fluorescence from the F f HI reaction varied by < 5% when the SF6 pressure was varied in the range 0.030_10 torr and when the laser energy tluence was varied in the range of 4-35 J/cm’_ Since the amount of any heating by V + T transfer will be proportional to the amount of energy absorbed, and since the amount of energy absorbed varies strongly with pressure and energy fluence, the absence of any effect on the rise rate can be taken to mean that little if any V + T heating occurs on the time scale of the reaction_ We feel confident, therefore, that the results reported in this paper are accurate, room temperature rate constants_ A summary of the F i HX and F + DX absolute rate constants obtained in this work is presented in table 1_ Because of the relatively small isotope effects, we have not corrected the F f DX rates for the < 3%
Absdute rates for F + HX and F f DX tX = I, Br, U)
System
this wotk F*HI F+DI F+HBr
F+DBr F+HQ
??+DC.E
From relativeratesa)
Absolute rates ts-’ torr”)
(8.72 * 0.22) x io5 (6.77 f 0.58) X 10’ (1.09 -F0.07) x lo6
f&47 i 0.66) x IO5 (2.67 f 0.28) x 10’
(1.93 c 0.22) x IO5
literature
2_r4x
2,0x 3.5 x 3.9x 8.0 x
10~
105 lo5 lo5 los
literature
8.3 x 10’ 7.7 x lo5 6-l X 10’ b)
;:i; 1111 171 2271 $281 (291
!S x IO5 1.3 x lo5 1.3 x lo5 b)
a) CalcuMed from Iiteratwz vahe of k(HIlfk(HX) aswming &HI) = 8.72 X 10’ s-l torr-x _ b) Taken fkomref. 171 without correction for new ffF transition probabilities[3I ]_ 376
Ref.
f::f WI 173
CHEMICAL PEYSICS LETIERS
Volume 57, number 3
HX impurity. Such a correction would fall within the stated error limits. The rate constants obtained in this work are in reasonable agreement with those obtained by less direct techniques, as shown in the third and fourth columns of table 1. Our observation that the rate for F + HI is slightly slower than the rate for F f HBr is at variance with the observations made by fluorescence yields in flow systems [7,10,1 I] _ Since the fluorescence yield technique involves a correction for radiative lifetime and an integration of the wavelength dependence of the fluorescence, we do not feel that the slight discrepancy is unreasonable_ Our method is based on a direct observation of the formation rate of products and should give accurate results. It is interesting to compare our results for the F + HX (DX) reactions with literature values of the cross section and isotope effect for the Ci C I-TX(DX) and Br + HX (DX) reactions. Table 2 presents a comparison for all of the measured X’ + HX (DX) abstraction cross sections. With the exception of F + HI, the reaction cross section increases as the HX bond becomes weaker. While this effect may be due simply to a lowering of the barrier to reaction or an increase in the number of available product states as the reaction becomes more exothermic, Mei and Moore 1141 have taken the view that for Cl + HX the high values for the cross section and the isotope effect imply a mechanism involving interhalogen attractive forces [24-261. In this view, the Cl atom is attracted to the iodine end of HI and reaction is completed following rotation of the hydrogm atom to a position between the two halogens. Support for this mechanism comes from the non-Arrhenius behavior of the rate constants for both Cl + HI and Cl i HBr as a function of temperature. The Cl + HI reaction actually shows a slight inverse temperature depenT&e 2 Reactioncrosssectionsandisotope effects 2, Atom
Diatom HI
Fb) a c)
Br d)
4.27 (1.29) 33.2
2.79
(1.84)
1 August 1978
dence above 300 K_ If this attractive mechanism is responsible for all of the abstraction reactions in table 2, then it is somewhat surprising that the cross section for F + HI is not larger than that for Cl f HI. The attraction between F and HI should be even greater than that between Q and HI [24-261. One possiile explanation is that at room temperature the F f HI cross section has already begun to deviate significantly from Arrhenius behavior_ On the other hand, the H/D isotope effects for the F + HX reactions, while larger than that predicted by trajectory calculations on, for example, an FHCl LBPS surface [ 121, are not as large as those found for the Cl + HX reactions. Further experiments now in progress in our laboratory on the temperature dependence of the reaction rates will be necessary to assessthe importance of the attractive mechanismintheF+HXsystems.
5. Conclusion The cross section for the reaction between F and HX (X = I, Br, Cl) increases from Cl to Br but decreases from Br to I. Hydrogen abstraction is about 13 times more favorable than deuterium abstraction for these reactions. Although these results could indicate the presence of a mechanism involvvg attraction between the fluorine atom and the halide end of HX, furLher investigation of the temperature dependence of the reaction rates is needed to assessthe importance of such a mechanism. Acknowkdgement We gratefully acknowledge support of this work by the Air Force Office of Scientific Research (AFOSR78-3513) and by the Standard Oil Company of Ohio. References
Bl3r
HCl
5.26 (1.29)
1.16 (l-38)
1.44 (1.5)
2) Crosssectionin A2 (&&bx). b, This work. Cl From ref. [ 321.
d, From ref. [33].
Cl1 R Foon andM. Kaufinan,Progr. ReactionKinetics8 (1975) 81. [2] S.H. MO,E-R Grant,F.E. Little, R.G. Manning, C-k Mathis, G.S. Werre and J-W. Root, *m: Fluorinecontaining free radicals, e& J.W. Root, ACS Symposium series66, Washington(1978) p_ 59. [3] S. Bittenson,Ph.D. Dissertation,Universityof W-&n, Madison(1977). 377
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CHEMICAL
PHYSICS LETTERS
[4] R-D- Coombe, G.C. Pimentel and &f-J_Berry, IEEE J. Quantum EIectron. QE-9 (1973) 192. [S] M_J_ Berry, in: MokcuIar energy transfer, eds. RD. Levine and J_ Jortner (Wiley, New York. 1976) p_ 114 [6] N. Jonathan, CM_ Melliar_SmIth. D. Tii and DH_ Slater, Appl. Opt 10 (1971) 1821. [7] N. Jonathan. CM_ MeUiar_Smith, S. Okuda, D_H_ SIater and D. Thnlin, MoL Phys. 22 (1971) 561. [8] AM-G. Ding, LJ. Kirxh, D.S. Perry, J.C. Po’hmyi and J-L Schreiber. Discussions Faraday Sot. 55 (1973) 252_ [9] LJ. Kirsch and J-C_ Pohinyi, J. Chem. Phys. 57 (1972) 4498. [lo] JP. Sung and D-W_ Setser, Chem. Phys. Letters 48 (1977) 413. [ 111 D J_ Smith, D-W_ Setser. KC_ Kim and D J. Bogan, J. Phys. Chem. 81 (1977) 89%. [ 121 J. Santamaria and D_L_ Bunker, Chem. Phys. 23 (1977) 243. [ 131 FJ. Wodarczyk and C.B. hfoore, Chem. Phys. Letters 26 (1974) 484_ (141 C-C. Mei and C-B. hfoore, J. Chem. Phys. 67 (1977) 3936. [I51 E- Wibzberg. Li_ Kovaienko and P_L_ Houston, in preparation_ 1161 I- Burak and hf_ EyaI, Chem. Phys. Letters 52 (1977) 534. 1171 U. Dinur. R- Kosloff. R-D_ Levine and MJ. Berxy. Chem. Phys. Letters 34 (1975) 199. 1181 R-D- Coombe acd AT_ Ritt Jr., J- them- Phys. 66 (1977) 5214_
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[ 191 MJ. Coggiola, P_A. Schulz, Y.T_ Lee and Y-R Shen, Phys. Rev_ Letters 48 (1977) 425. [20] CR Quick and C. Wittig, Chem- Phys. Letters 48 (1977) 420. [21] J.Bf. Presses, RR Weston Jr_ and G_ Flynn. Chem. Phys. Letters 48 (1977) 425. 1221 J.L Steinfdd, I. Burak, D-G. Sutton and A-V. Nowak, J. Chem. Phys. 52 (1970) 5421. [23] RD. Bates Jr.. J.T. Knudtson, G-W_ FIynn aud AM. Ronn, Chem. Phys_ Letters 8 (1971) 103_ [24] Y-T_ Lee, J-D. BfcDonaId, R.B. LeBreton and D_R Herschbach, J. Chem.‘Phys. 49 (1968) 2447. [25 ] Y.T. Lee, P-R LeBreton, JJ). hfcDonaId and D-R Herschbach, J. Chem. Phys. Sl(l969) 455. 1261 J.hf. Farrar and Y-T_ Lee. J. Am. Chem_ Sot. 96 (1974) 7570. 1271 J. Wamatz, Ph.D. Dissertation, George-August University, G&tingen (1968). [281 J. Wamatz, H.G. Wagner and C. Zetsch, Report T-02401 92410/01017 to the Fraunhofer Geselfschaft (1972). (291 T-L._ Pollock and W.E. Jon-. Can. J. Chem. Sl(1973) 2041. [39J K-L. Kczlpa and J. Wanner, Chem. Phys Letters 12 (1972) 560. [311 RX SiIeo and TA_ Cool, J. Chem. Phys. 65 (1976) 117. Bergmann and CJ3. Moore, J. Chem. Phys. 63 (1975) [321 K_ _.^ 643.
[33] R Bergmann, S-R Leone and C-B. Moore, J. Chem. Phys. 63 (1975) 4161.