J . Phys. Chem. 1986, 90, 2450-2462
2450
scant attention in the literature. Much of the thermochemistry is uncertain. A condensation product, C4N+,was observed in the reaction of C3+ with H C N . The exothermicity shown in Table VI1 is based on a determination of a very uncertain appearance potential for C4N+ by electron impact on C4N2.47Low signal intensities for C3+ prevented an accurate rate determination. One other measurement of the C3H+reaction rate with H C N has been reported by Raksit and B ~ h m e , who ~ ' noted efficient formation of the collision-stabilized association product H2C4N+. Because collisional stabilization is too slow to be observed in our work, the rate coefficient and branching ratios we report pertain to a bimolecular process. We have assumed, as noted earlier, that the appearance of the association product at low pressures indicates stabilization by radiative loss from the intermediate complex. In Table VI1 we report that C3H+reacts with H C N to give two products. The proton-transfer channel is endothermic by about 7.5 kJ mol-'. If we assume that the rate of this reaction can be calculated using the proton affinity difference between C3 and H C N and that the reverse reaction proceeds at the collision rate at 300 K, then the observed reaction should proceed at about 1/20 the collision rate. This is consistent with the value of the measured rate constant. Both the C4+ and C4H+ions react more rapidly than do their C3+counterparts with HCN. In both cases an association product was observed together with other reaction products. The structures of the C5N+ and HC5N+ species are not known, and both cyclic and acyclic structures are possible. Conclusions It is evident from this study that hydrocarbon cations of the type C,H,+ have a marked tendency to undergo condensation
reactions with C2H2and H C N and that the reactivity generally increases with decreasing y . In several cases, notably the reactions of C4H2' and C4H3+with C2H2and of C3HC,C4+,and C4+with HCN, an association product is observed at pressures as low as 1X torr in the ICR cell. To survive the many milliseconds between formation and detection, some stabilization of the association product must have taken place within the ICR cell, and we have attributed this stabilization process to energy loss by photon emission. The stability of these association adducts suggests that they are probably bound chemical species rather than loosely bound adducts formed through electrostatic interaction. Many of the reactions studied are relevant to combustion and astrochemical environments. The detection of C2H2and H C N in the atmosphere of Titan suggests the presence of a whole range of organonitrogen molecules produced by sequential ion-molecule reactions, and we plan to investigate further some of these reactions. In the combustion zones of unsaturated hydrocarbons, particularly at pressures greater than 0.02 torr, rapid association reactions of the primary ion give efficient routes for converting small hydrocarbon molecules into large unsaturated hydrocarbon molecules.
Acknowledgment. This paper presents the results of one phase of research carried out at the Jet Propulsion Laboratory, California Institute of Technology, under Contract No. N A S 7-918, sponsored by the National Aeronautics and Space Administration. Registry No. C2Hz,74-86-2; HCN, 74-90-8; C', 14067-05-1;CH', 24361-82-8; CHZ', 15091-72-2;CHp', 14531-53-4;CH4+*,20741-88-2; C2', 12595-79-8; CZH', 16456-59-0; C2H2'*, 25641-79-6; C2H3', 14604-48-9;C2H4+*,34470-02-5;C2H3+, 14936-94-8;C3+, 12595-80-1; C,H3+, 26810-74-2;C,', 64886-35-7;C4H4+*,59699-48-8.
Infrared Chemllumlnescence and Laser- Induced Fluorescence Studies of Energy Disposal by Reactions of F and CI Atoms with H2S (D2S), H2Se, H20 (D,O), and CH,OH B. S. Agrawallat and D. W. Setser* Department of Chemistry, Kansas State University, Manhattan, Kansas 66506 (Received: August 28, 1985; In Final Form: January 15, 1986)
The vibrational energy disposal to HF (DF), HCI (DCI), SH (SD), and C H 3 0 and relative product formation rate constants were measured for the title reactions. The experiments were done in a fast-flow reactor with infrared emission and laser-induced fluorescence to observe nascent product state distributions for the diatomic hydride products. The C H 3 0vibrational and HF (DF) rotational distributions are partially relaxed, but the nascent distributions for these cases were estimated. Laser-induced fluorescence measurements showed that the SH, SD, and CH30 fragments received only 0.02-0.03 of the available energy. The (fV(HX))values for group VI (group 16) hydrides, 0.45 (F + HzS), 0.44 ( F + D2S), 0.48 ( F + H2Se), 0.42 (F + D20), 0.41 (CI H2Se), and 0.37 (C1 D2S), are similar but distinctly smaller than for F and CI atom reactions with group IV and VI1 (group 14 and 17) hydrides. The (fR(HX)) values, 0.18 (F H2S), 0.19 ( F + H2Se), and ~ 0 . 1 4(F + D2S), are typical for H-abstraction reactions with similar cross sections. The F + HzO/DzO systems were used to characterize the extent of secondary reactions in the fast-flow reactor. For high reagent concentrations and longer reaction times, S2 was observed in the F/C1 + H2S systems by LIF.
+
+
I. Introduction The energy disposal to HF and HCl from hydrogen abstraction reactions by F and CI atoms has been systematically studied by infrared chemiluminescence (IRCL) in a fast-flow reactor (FR) at 300 K in our laboratory.'" Considerable IRCL data also exist from cold-wall, arrested-relaxation (AR) studies for certain F and C1 atom reaction^.^'^ For the hydrides of group IV and VI1 a large fraction (0.5-0.7) (group 14 and 17) of the available energy is released as HF vibrational energy and a smaller, and more variable, fraction is released as rotational 'Present address: Center for Laser Studies, University of Southern California, University Park DRB-17, Los Angeles, CA 90089-1 112.
0022-3654/86/2090-2450$01.50/0
+
energy.I6*l7 A C O ~ S ~ ~ S has U S developed ~ ~ . ~ ~ for the energy disposal from F + NH3/ND3 and Cfv(HF)) is -0.45; this increases to (1) Smith, D. J.; Setser, D. W.; Kim, K. C.; Bogan, D. J. J . Phys. Chem. 1977, 81, 898.
(2) Sung,J. P.; Malins, R. J.; Setser, D. W. J. Phys. Chem. 1979,83, 1007. (3) Wickramaaratchi, M. A.; Setser, D. W. J . Phys. Chern. 1983,87, 64. (4) (a) Manocha, A. S.; Setser, D. W.; Wickrammaaratchi, M. A. Chem. Phys. 1983, 76, 129. (b) Wategaonkar, S.; Setser, D. W., to be submitted to J. Phys. Chem. ( 5 ) Agrawalla, B. S.;Manocha, A. S.;Setser, D. W. J . Phys. Chem. 1981, 85, 2873. (6) Tamagake, K.; Setser, D. W.; Sung, J. P . J . Chem. Phys. 1980, 73, 2203. (7) Bogan, D. J.; Setser, D. W. J. Chem. Phys. 1976, 64, 586.
0 1986 American Chemical Society
The Journal of Physical Chemistry, Vol. 90, NO. 11, 1986 2451
Energy Disposal by Reactions of F and C1 Atoms
to IRCL is needed to characterize the energy of the radical and
TABLE I: Thermocbemical Data" (kcal mol-') reaction
+
F HI0 F+OH F + D20 F+OD F + HIS F + D2S F H2Se F + CHSO-H F + GeH, F CHI C1 H2S CI D2S CI + H,Se CI HI
+
+ + + +
Doo (ref) 118.0 f 0.2 (26) 101.3 f 0.5 (27) 120.0 f 0.2d 102.6 f 0.5 (27) 90.0 f 2.0 (26) 91.4 f 2.0d 76.0 f 2.0 (28) 99.4 f 1.0 (29) 78.6 f 2.0' 103.6 f 0.5 (30) 90.0 f 2.0 (26) 91.4 f 2.0d 76.0 f 2.0 (28) 70.4 f 0.1 (26, 27)
-Moo 17.3 f 0.2 34.0 f 0.5 16.9 f 0.2 34.3 f 0.5 45.3 f 2.0 45.5 f 2.0 59.3 f 2.0 35.9 f 1.0 56.7 f 2.0 31.7 f 0.5 12.3 f 2.0 12.1 f 2.0 26.3 f 2.0 31.9 f 0.1
E," 2.0 f 0.3( -0.5 2.0 f 0.3( -0.5 ~ 0 . 7 ~ -0.7' -0.5 -1.0 -0.5 1.1 f 0.3g 1.2 f 0.3" 1.2 f 0.3" 0.2 f 0.2* 0.8 f 0.2'
(E) 21.1 f 0.4 36.0 f 0.8 20.7 i 0.4 36.3 f 0.8 47.8 f 2.0 48.0 f 2.0 61.6 f 2.0 38.7 f 1.0 59.0 f 2.0 34.6 f 0.7 15.3 f 2.0 15.1 f 2.0 28.3 f 2.0 34.2 f 0.2
"The average available energy, ( E ) , was calculated from Do"(HF) = 135.3, Do'(DF) = 136.9, Doo(HC1) = 102.3, Doo(DC1) = 103.5 kcal mol-' from ref 27; (E) was calculated from AH,,' 3RT E,. *Unless indicated otherwise, the activation energies were estimated from the magnitudes of the rate constants relative to cases with known E,. cThe E, for H 2 0 was estimated by comparison to F CH, and F + His; however, direct meas u r e m e n t ~ give ~ * ~0.8 kcal mol-'. dCalculated by applying zero point corrections to Do0(H20)and Do'(H2S). 'Reference I l c and comparison to F CHI. fFrom this work, see text. ZSee ref 32b and 32c. "From comparison of k(C1 + H2S) with k(C1 + H2Se) and k(C1 HI). 'From the highest observed HCI(u,J) level from an AR study of CI HI, ref 31.
+
+
+
+
+
+
+
0.55 for F + PH3. The energy disposal for F B2H6 seems to resemble that for F + CH4., In this work we report a systematic study using both IRCL and laser-induced fluorescence (LIF) of the energy disposal for F and C1 atom reactions with hydrides of several group VI (group 16) elements. The IRCL and LIF measurements for the F + H2S (D2S) reactions permit assignment of (fv(HF)), Vk(HF)), and (fv(SD)) and estimates of cfR(SD)) and (fT).Although less detail is obtained for the reactions with H2Se, H20, and CH,OH, the data still permit a detailed discussion of the reaction dynamics. A preliminary description of the LIF measurements for S D and C H 3 0 has been published.'* Earlier IRCL studies6~11~'s of F + H2Ssuggested that the HF(u) distribution was rather flat in contrast to the sharply peaked HF(u) distributions from group IV and VI1 (group 14 and 17) hydride molecules. This observation implied that the dynamics for hydrogen abstraction from HIS, and perhaps from H,Se, might differ from more typical H abstraction reactions in an interesting way. However, this work shows that the energy disposal pattern from F and CI with group VI (group 16) hydride molecules still is dominated by mixed energy release on a repulsive surface; the cfv(HF)) and (fv(HC1)) are high and the energy released to the radical product is small. Since most radical fragments from H abstraction reactions have small infrared transition probabilities, an alternative technique (8) Bogan, D. J.; Setser, D. W.; Sung, J. P. J . Phys. Chem. 1977,81, 888. (9) Polanyi, J. C.; Wodall, K. B. J . Chem. Phys. 1972, 57, 1574. (IO) Nazar, M. A.; Polanyi, J. C. Chem. Phys. 1981, 55, 299. (11) (a) Dill, B.; Heydtmann, H. Chem. Phys. 1980, 54,9. (b) Dill, B.; Hildebrandt, B.; Vanni, H.; Heydtmann, H. Chem. Phys. 1981,58, 163. (c) Dill, B.; Heydtmann, H. Chem. Phys. 1978,35, 161. (d) Ibid. 1983,81,419. (12) (a) Sloan, J. J.; Watson, D. G.; Wright, J. S . Chem. Phys. 1979, 43, 1. (b) Ibid. 1981, 63, 284. (c) Donaldson, D. J.; Parsons, J.; Sloan, J. J.; Stolow, A. Chem. Phys. 1984, 85, 47. (d) Donaldson, D. J.; Sloan, J. J.; Goddard, J. D. J . Chem. Phys. 1985, 82, 4524. (13) Beadle, P.; Dunn, M. R.; Jonathan, N . B. H.; Liddy, J. P.; Naylor, J. C.; Okuda, S. J . Chem. SOC.,Faraday Trans. 2 1978, 74, 2158. (14) Beadle, P.; Dunn, M. R.; Jonathan, N . B. H.; Liddy, J. P.; Naylor, J. C. J . Chem. Soc., Faraday Trans. 2 1978, 74, 2170. (15) Wickramaaratchi, M. A.; Setser, D. W.; Hildebrandt, B.; Korbitzer, B.; Heydtmann, H. Chem. Phys. 1985, 94, 109. (16) (a) Holmes, B. E.; Setser, D. W. In Physical Chemistry of Fast Reactions, Vol. 2, Smith, I. W. M., Ed.; Plenum: New York, 1980. (b) Bogan, D. J.; Setser, D. W. In Fluorine-Containing Free Radicals-Kinetics and Dynamics of Reaction, Root, J. W., Ed.; American Chemical Society: Washington, DC, 1978. (1 7) Agrawalla, B. S.; Setser, D. W. In Gas-Phase Chemiluminescenceand Chemi-Ionization, Fontijn, A,, Ed.; North-Holland: Amsterdam, 1985. (18) (a) Agrawalla, B. S.; Setser, D. W. J. Phys. Chem. 1984,88,657. (b) Agrawalla, B. S . Ph.D. Dissertation, Kansas State University, 1984.
LIF is one method. The radical fragments from group IV (group 14) hydrides do not have suitable states for LIF and the spectra for N H 2 and PH2 are complex. The OH,'9320OD,21SH,22-24and C H 3 0 2 5radicals do have suitable transitions, and the LIF measurements for SH, SD, and C H 3 0 radicals, plus the IRCL measurements for H F (or HCl), provide a rather complete view of the energy disposal for the H abstraction of group VI (group 16) hydrides. We expect these results to be a good description, with allowance for special dynamical effects when necessary, for direct H abstraction reactions in general. The relevant thermochemical data for the F(C1) group VI (group 16) hydride reactions are listed in Table I. The 21.1 and 20.7 kcal mol-' available energies26 for F + H 2 0 and D 2 0 , respectively, can give only HF(u=l, J=12) and DF(u=l, J=19 and u=2, J = l l ) . Formation of HF(u=2, J=O) and DF(u=3, J=O) is endoergic by 1.1 and 3.4 kcal mol-', respectively, for the above thermochemistry, which is based upon an E, of 2.0 kcal mol-'. Since the secondary steps, F O H and OD, can yield HF(u13) and DF(u54), respectively, these systems are used to discuss the importance of secondary reaction in the fast-flow reactor in the Appendix. Hydrogen abstraction from the hydroxyl group of C H 3 0 H / C D 3 0 H by F atoms provides an example with more available energy to complement the H 2 0 case.15 The H2S/D2S and H2Se reactions with CI and F atoms provide the best cases for IRCL studies because several HF and C1 vibrational levels are within the allowed thermochemical range. The thermochemical limit with C1 atoms for H2S, D2S, and H2Se correspond to H C l ( u I l ) , DCl(u12), and HCI(u13), respectively.
+
+
11. Experimental Techniques
-
The experiments were done in a 4-cm-diameter flow reactor (the bulk flow velocity was 180 m s-') constructed from Pyrex tubing. The only modification relative to earlier s t u d i e P was the addition of 2-cm-diameter baffle arms perpendicular to the observation window for the LIF studies. A sketch of the flow reactor is shown in ref 17. The 2-cm-diameter arms with a small Ar purge flow through the arms gave unrelaxed HF(u) distributions from F CH4. However, larger diameter arms apparently caused turbulence in the flow, as judged by the small degree of HF(u) relaxation for the test reaction, F + CH,. An array of light baffles was placed inside the arms; all interior surfaces were painted black. Microwave discharge (60 W, 2450 MHz) in mixtures of Ar/CF4 and Ar/C12 was used for F and C1 generation, respectively. The discharged gas flow made one right angle turn that included a Wood's horn to reduce the scattered light in the observation zone. Typically, 0.1-1.0 l m o l s-l flows of CF, (or C12) with -700 lmol of Ar were used. For these conditions -50% dissociation is commonly reported, and the [F] and [Cl] concentrations are reported as being equal to the CF, or C12 flows, ~. which corresponds to (2.5-25) X 10" molecules ~ m - Recently we have measured33the [F] in a very similar apparatus using the
+
(19) Mariella, Jr., R. P.; Lantzsch, B.; Maxson, V. T.; Luntz, A. C. J . Chem. Phys. 1978, 69, 541 1. (20) Silver, J. A,; Dimpfl, W. L.; Brophy, J. H.; Kinsey, J. L. J . Chem. Phys. 1976, 65, 1811. (21) Murphy, E. J.; Brophy, J. H.; Arnold, G.S.; Dimpfl, W. L.; Kinsey, J. L. J . Chem. Phys. 1981, 74, 324, 331. (22) Hawkins, W. G.; Houston, P. L. J . Chem. Phys. 1980, 73, 297. (23) (a) Tiee, J. J.; Wampler, F. B.; Oldenberg, R. C.; Rice, W. W. Chem. Phys. Lett. 1981, 82, 80. (b) Tiee, J. J.; Ferris, M. J.; Wampler, F. B. J . Chem. Phys. 1983, 79, 130. (24) Friedl, R. R.; Brune, W. H.; Anderson, J. G. J . Chem. Phys. 1983, 79. 4227. (25) (a) Inoue, G.; Akimoto, H.; Okuda, M. Chem. Phys. Lett. 1979,63, 2113. (b) Ibid. J. Chem. Phys. 1980, 72, 1769. (c) Powers, D. E.; Hopkins, J. B.; Smalley, R. E. J. Phys. Chem. 1981,85, 271 1. (d) Sanders, N.; Butler, J. E.; Pasternack, L. R.; McDonald, J. R. Chem. Phys. 1980, 48, 203. (26) Darwent, B. de B. Natl. Stand. ReJ Data Ser., Natl. Bur. Stand. 1970, No. 31. (27) Huber, K. P.;Herzberg, G. Molecular Spectra and Molecular Structure IV. Constants of Diatomic Molecules; Van Nostrand-Reinhold: New York, 1979. (28) Dixon, D. A,; Holtz, D.; Beauchamp, J. L. Inorg. Chem. 1972, 11, 960. 1
~~
2452 The Journal of Physical Chemistry, Vol. 90, No. 11, 1986
Agrawalla and Setser
TABLE 11: HF( v ) and DF( v ) Distributions and the High J Componentso reaction F + H2S
F + D,S
F
+ H2Se
F
+ GeH4
press., time,b torr ms 0.65 0.10 0.26 0.15 0.13 0.35
[CFd, molecules 0.15 0.19 0.2 1
[RH13 l o i 2 molecules cm-3 P, 1.1 24 (0.35) 0.88 30 (0.37) 1.o 28 (0.39)
HF(0) distributions” p2 p3 p4 32 (0.20) 33 (0.05) 11 (0.03) 31 (0.24) 29 (0.11) 10 (0.03) 33 (0.25) 30 (0.1 1 ) 09 (0.02)
total
P5
P6 high J” 0.18 0.22 0.23
0.65 0.26 0.13
0.10 0.15 0.35
1 .o 0.58 0.61
3.0 4.0 4.5
21 (0.24) 23 (0.30) 24 (0.26)
20 (0.17) 20 (0.10) 22 (0.04) 15 (0.03) 02 19 (0.23) 21 (0.12) 21 (0.05) 14 (0.03) 02 22 (0.21) 22 (0.13) 19 (0.06) 11 (0.03) 02
0.12 0.16 0.15
0.65 0.26 0.13
0.10 0.15 0.35
0.83 0.5 0.28
2.4 1.8 1.o
15 (0.57) 21 (0.57) 21 (0.57)
21 (0.45) 27 (0.30) 26 (0.12) 11 (0.07) 02 23 (0.48) 25 (0.37) 21 (0.20) 10 (0.13) 01 23 (0.54) 26 (0.39) 20 (0.20) 10 (0.15) 01
0.29 0.37 0.39
0.65 0.26 0.13
0.10 0.15 0.35
0.75 0.57 0.36
1.1 0.82 0.48
12 (0.74) 11 (0.71) 14 (0.57)‘
18 (0.71) 28 (0.33) 40 (0.04) 02 17 (0.67) 29 (0.43) 40 (0.05) 03 19 (0.60) 28 (0.37) 37 (0.09) 02
0.32 0.33 0.33
“ T h e number in parentheses is the fraction with J 2 8; the last column gives the fraction of the total steady-state distribution with J > 8. The x . , 2 i P Lis normalized to 100. b T h e time for the gas to flow from the reagent inlet to the center of the observation window. CTheemission intensity from c = 1 was quite low at 0.13 torr and the 0.26-torr data are more reliable.
ClZtitration technique and found that the F-atom concentration was two times the CF, concentration. Thus, the concentrations quoted here probably are lower limits, but within a factor of 2, of the true [F] and [Cl]. The hydride reagents, typically diluted 50-fold with Ar, were introduced 1.5 cm upstream of the center of the observation window via an inlet ring with numerous small holes. The approximate reaction time at 0.6-torr Ar pressure was 0.1 ms. The CF,, C12, and H2S were obtained from Matheson; H2Se and D 2 0 (99.75% isotopic purity) were purchased from Scientific Gas Products and J. T. Baker, respectively. Methanol was from Fischer Scientific Co. (Spectral Grade), and C D 3 0 H was from Merck Sharp and Dohme. The D2S was synthesized following the procedure of B a ~ d l e r . ~The ~ isotopic purity of DzS was checked via the relative intensities of the HF/DF infrared emission spectra. We obtained CLJVu(HF)/CJVu(DF)= 0.08 f 0.01 from three different measurements. Since the rate constant ratio k(F+HzS)/k(F+DzS) = 1.4 (see Results section), the [H2S]/[DzS] is 0.06 f 0.01 with an isotopic purity of 94%. Since the IR absorption spectrum showed H D S rather than H2S impurity, [HDS]/[D,S] = 0.12 f 0.02. The D2S samples were handled in vacuum lines and storage bulbs that had been seasoned with DzO. The reagent samples were purified by freeze-pumpthaw cycles before the reagent/Ar mixtures were prepared and stored. The IRCL was recorded with a Digilab (FTS-20) Fourier transform spectrometer equipped with a liquid-N2-cooled InSb detector and a CaF2/Fe203beam splitter. A quartz cutoff filter (-2000 cm-I) was placed in front of the detector to reduce the thermal background. The spectral response of the detection system from 1800 to 8000 cm-’ was obtained by recording a 1000 “ C black-body spectrum and a quartz-iodine standard lamp spectrum. The resolution, typically 1.0 cm-’, gave resolved H F / D F and HCl/DCl rotational lines. A tunable flashlamp-pumped dye laser (Chromatix Model CMX-4) equipped with doubling crystals was used for the LIF measurements. A 0.3-m McPherson monochromator with a Hammamatsu R212UH PMT was used to identify the emission spectra; the total fluorescence signals for vibrational population measurements were recorded through narrow band-pass filters with another R212UH PMT. A boxcar (PAR Model 162/164) was used for signal averaging. Since our CMX-4 laser ( 1-ws pulse (29) Engelking, P. C.; Ellison, G. B.; Lineberger, W. C. J . Chem. Phys. 1978, 69, 1826.
(30) Weissman, M.; Benson, S. W. J . Phys. Chem. 1983, 87, 243. (31) Maylotte, D. H.; Polanyi, J. C.; Woodall, K. B. J. Chem. Phys. 1972,
-57. , .1547 -. .
(32) (a) Walther, C. D.;Wagner, H. Gg. Ber. Bunsenges. Phys. Chem. 1983, 87, 403. (b) Foon, R.; Kaufman, M. Prog. React. Kinet. 1975, 8, 81. (c) Jones, W. E.; Skolnik, E. G. Chem. Rev. 1976, 76, 563. (33) Habdas, J.; Setser, D. W., submitted for publication to J. Phys. Chem. (34) Baudler, M. In Handbook of Preparative Inorganic Chemistry, Vol. 1, Brauer, G.,Ed.: Academic Press: London, 1963; p 134.
width, 6-cm-’ band-pass) did not scan in the ultraviolet, excitation spectra could not be recorded and the fluorescence was identified from the wavelength-resolved emission spectra. The intensity of the LIF signals was limited by the laser intensity, Le., the transitions were not saturated. The data reduction methods for both IRCL and L I F measurements were of standard form. The height of each rotational line of the IRCL spectrum was divided by the Einstein coefficientj5 and detector response factor to obtain the relative population, NuJ, of each rotational level. The relative vibrational population, N,, was obtained by summing over all J levels of a vibrational level. Since the reaction time is sufficiently short that the differential rate law description is valid,’.” C , N , can be plotted vs. reagent concentration for constant At and [XI to obtain the HX(u21) formation rate constants relative to a reference reaction. The normalized vibrational distribution, P,,had a f10% reproducibility for each u level for nominally identical experiments. For the LIF observation of SH/SD, the R, + RQ21band head X,u”=O) transition was pumped and the (A,u’=O of the (A,u’=O X,u”=l) fluorescence intensity was monitored to determine No, whereas the band head of the (A,u’=O X,u”=l) transition was pumped and the (A,u’=O X,u”=O) emission intensity was measured to determine N I . The density of rotational lines in the two band heads are quite similar, but a slightly higher fraction of u = 1 molecules was excited than u = 0, and the LIF intensity from u = 0 was increased by a factor of 1.1 to account for this difference.’8b Except for this factor the ratio of populations in the c0” and ul” levels is given by
-
+-
-
-
Nu,,, ( u , ~ . ~ , . c. )l~, , / P i l , , -=--Nuo~t (u,,,,,,,) $c/ PEc Too,uO,,
(1)
The frequencies are the excitation wavelengths (cm-I); the r‘ and P are the observed fluorescence signals and relative laser intensities and the T terms denote the transmission of the interference filters. The Franck-Condon factors cancel and are not required to obtain the population ratio for this particular LIF excitation scheme. The populations in the u3 normal mode of C H 3 0 were determined by exciting the u” = 0, 1, and 2 levels to the u’ = 2 level of the CH,O(A) state and the u’ = 2 u” = 4 fluorescence then was observed. The relative populations in the u” = 0 and 1 levels is given by
-
UL2,UI,, &,/P:]Jfqc2,L.,,,, NLo,, u~~~~~~~ c c / P t c 4u;o,8t
Nu,,’ -
(2)
A similar relation holds for the N.,2,t/Nuo,t ratio. The FranckCondon qdd,,are required in this case; the other quantities are the same as for eq 1. (35) Oba, D.; Agrawalla, B. S.; Setser, D. W. J . Quant. Spectrosc. Radiat. Transfer 1985, 34, 283.
The Journal of Physical Chemistry, Vol. 90, No. 11, 1986 2453
Energy Disposal by Reactions of F and C1 Atoms
F + H2S
I
=
I /I/
-
Figure 1. Part of an HF(Au=l) spectrum, 1-cm-’resolution, from F +
-
=
p v t w
H2S showing the R branch heads from high J levels of the (1 0) and (2 1) bands. The spectrum was recorded at 0.65 torr, At = 0.15 ms and [H2S] and [CF,] = 4.0 X loi2 and 1.5 X 10l2 molecules cm-),
respectively. The number of scans for this spectrum was 128, but more scans (256 or 512) were required for lower reagent concentrations.
initial I
\
1
I
I
r$ye ’ p.8:e=kk&~-o-o~
0
0-c-
-0-0-0
J LEVEL Figure 2. The flow reactor HF(u,J) distribution from the F + HIS reaction at 0.65 torr, [H2S] = 1.0 X loi2molecule cm-3and [CF,] = 2.1 X 10” molecules ~ m - ~ the; vibrational distribution is P1-P4 = 24:32:33:11. The plots with the squares show the AR data of Dill et al. (ref 1 IC)with PI-P4 = 26:33:27:14. The estimates of the nascent rotational distributions (solid lines) were obtained by transferring the 300 K
component back to the high J envelope. The thermochemical limit for each u level is denoted by the arrows (the thermochemicallimit for v = 1 is J = 25). 111. Experimental Results A . HF(v,J) Distributions from F H2S. The F H2S reaction was studied in detail; the HF(u11) formation rate constant was measured and the HF(u) distribution was checked for possible secondary reaction and/or relaxation for a variety of conditions. Although the majority of the HF(u,J) emission intensity was from the 300 K Boltzmann populated rotational levels, considerable emission from high levels ( J 1 8) also was observed, as shown by the HF spectrum in Figure 1. The HF(u,J) distribution deduced from this spectrum is shown in Figure 2 and a summary is given in Table 11. As is common for F atom reactions,16*” the HF(u,J) rotational distributions extend to the thermochemical limit for several u levels. Our limit for Doo(HSH) is 191.0 kcal mol-’, which agrees with the literature result of Table I. The dependence of the HF(u) distribution upon [H,S] and [CF,] for At = 0.1 ms is shown in Figure 3; the distribution is nearly constant for [H2S]12=0.2 X l o i 2to 2 X 10l2 molecule . ~ r n and - ~ [CF,] = 0.1 X 10 to 1.1 X 1OI2 molecule ~ m - ~For
+
+
=
’:
0.3-7 - 0 / v1
50.1-7
- -
0
+
+
I
v=3
v1
T
I
+ v4 I
I
2454
The Journal of Physical Chemistry, Vol. 90, No. I I I986
Agrawalla and Setser
~
F+PH3 .-
- - -0-
q
~
~
TABLE 111: Nascent HF(v)/DF(v)/HCl(v) Distributions from F (CI) Reactions
3’
HX(u)IDX(c) distributions“,* exptl P , P2 P3 Pa P5 PA technique
reaction
F + H,S
F + D2S‘ F + H2Se
24 22 23 26 25 19 14 29 34 13 42 40 70
32 37 25 33 32 20 21 35 55 36 51 51 30
33 31 34 27 33 22 27 24 11 42 7 9
11 10 18 14 10 22 26 9
15 11 3
FR FR AR AR FR FR FR AR FR FR FR AR FR
2 1
ref this work 6 6
llc 15
this work this work lld 15 15 this work Ild this work
hv2 -0.-
*.
--
----_
F + CD3O-H F+CH3O-D C1 + H2Se C1 + D2Sd
\
Figure 4. The HF(u) distributions from F + PH3 (the [F] and [PH,] were 3.2 X 10l2 and 1.5 X 10l2 molecules cm”) and H2S (the [F] and vs. reaction time [H,S] were 2.0 X 10l2and 1.5 X 10l2molecules at 0.65 torr. The kinetic simulation of the F + H2S system with the same [F] and [H2S]and a nascent distribution of PI-P4 = 0.24:0.32:0.33:0.11 is shown as solid points; see text for other details of the calculation. Solid and dashed lines are drawn through the calculated and experimental points, respectively.
important at our [HF] concentrations. In order to check the various possibilities, numerical integration of a set of rate equations for the H2S system was done. The scheme included the F + H2S reaction ( k = 1.6 X cm3 molecule-‘ s-I with the P, of Table V), and assumed values for the F S H reaction ( k = 5 X lo-’’ cm3 molecule-’ s-’ and P0-P4 = 40:25:20:10:05), HF(u) radiative decay,35and vibrational relaxation by H2S3’*and F.37b The calculated and experimental results are compared in Figure 4, and the agreement is reasonable, considering the uncertainty in the rate constants for relaxation and for the F + S H reaction. Calculations in which k(F+SH) was set to zero suggest that H2S collisions are responsible for nearly all of the HF(u) relaxation. Assignment of the observed HF(u) distribution at 0.1-ms reaction time as the nascent distribution was based on the constancy of the P, vs. [H2S] and [F] plots of Figure 3 . However, the calculations and the data of Figure 4 show that the observation time could not be extended beyond 0.2 ms without relaxation effects becoming important. The calculations suggest that extrapolation to zero time might give the better estimate for the nascent P I ; however, the extrapolated result would be the same as the steady-state result to within our experimental uncertainty. The HF(u) distributions from this work and earlier studies are compared in Table 111. The FR results of Wickramaaratchi et aI.,l5 which were the preliminary experiments of the present study, are in agreement with our final results. The two AR studies6*’IC report higher P4values. Adjustment for slightly different Einstein coefficient^^.^^ reduce P4 only to 0.17 and 0.13. Tamagake et al. noted a shift in populations to lower u levels with decreasing reagent flow and selected intermediate flow data as representative of the H2S primary reaction. The apparently higher P4 may be an artifact of the choice of the “best” data. Since our HF(r;) distributions depend sensibly on [H2S], [CF,], and At and satisfy other criteria (as discussed in section 1V.A.) for nascent product distributions, we favor the current FR data as the nascent HF(c) distribution. B. DF(c’,J) Distribution f r o m F + DzS. The F + D2S reaction, ( E ) = 48.0 kcal mol-’, can yield DF(u16); however, the maximum allowed rotational level was not observed for any c‘ level, although
9
“These distributions are normalized so that x.L.21PL = 100. bThe reproducibility of the P, values in this work was &lo%, except for CI + D2S for which it was 4~20%. ‘See text for the corrections used to obtain this distribution from the observed steady-state distribution. “This distribution was obtained by extrapolation to low reagent concentration (see text); the uncertainty in these P, values is f2096. F
4-
,
+ D2S
t
+
(37) (a) Kwok, M. A,; Cohen. J. J. J . Chem. Phys. 1974, 61, 5221. (b) Bott, J . F. J . Chem. Phys. 1984, 81. 245.
, ,*-e*
I
J LEVEL
+
Figure 5. The DF(u,J) distributions from F D2S at 0.26 torr and 0.15-ms reaction time. The [CFJ and [D2S] were 1.0 X 10l2 and 3.0 X 1 O I 2 molecules c ~ n - respectively. ~, The vibrational distribution was 22:20:21:20:15:02 for this particular experiment. The thermochemical limits for u = 4, 5, and 6 are denoted by arrows; the limits for u = 1, 2, and 3 are J = 36, 32, and 29, respectively. The estimates of the nascent rotational distributions were extended to J = 26 for c = 1 and 2, although the emission from these lines were not actually identified because of overlap by stronger lines.
the observed distributions for u = 5 and 6 were only 2-3 levels below the thermochemical limits. The best DF(u,J) distributions, obtained at 0.26 torr of Ar, are shown in Figure 5 and the vibrational distributions (with the high J fractions) are summarized in Table I1 for 0.13, 0.26, and 0.65 torr. Although there is no strong minimum in the DF(u,J) rotational distributions, the 300 K Boltzmann population for J = 8 to 12, is very low and we retained J 2 8 as the definition of the DF(high J ) component. The small increase in the high J fraction at lower pressure resembles that found for F H,S. Our estimate for the initial DF(u) rotational distributions is shown in Figure 5 . Although the DF(u) distributions extend to about the same J range as for HF(u), the DF rotational energy is considerably smaller (the comparisons for the highest observed rotational levels of H F (DF) are -9700 (4750) and 8300 (5000) cm-I for u‘ = 1 and 2), because the DF rotational constant is about half that for HF.
+
Energy Disposal by Reactions of F and C1 Atoms
The Journal of Physical Chemistry, Vol. 90, No. 1I , 1986 2455
v- 5
F+H2Sa v.5
I
F
+GeH4
1
Initial
. . J LEVEL
J LEVEL
Figure 6. The flow reactor HF(u,J) distribution from F + H,Se at 0.65 torr with [H,Se] = 8.5 X 10" molecules cm-I and [CF,]= 9.2 X 10" molecules the vibrational distribution is 15:21:27:26:10. Since P6 is only 01, it is not shown. The plots with the data shown as squares are the AR results;IldPI-P, = 29:35:24:09:03;see text for explanation of the high P1-P3 values. The solid lines are estimates of the nascent rotational distributions. The thermochemical limits for u = 3, 4, and 5 are denoted by arrows; the limits for u = 1 and 2 are J = 31 and 27, respectively.
Since the available energy would permit formation of DF(u,J) in much higher J levels, an angular momentum constraint for formation of DF(1ow u, high J)seems likely. The DF(u) distributions were systematically studiedIsbvs. [D2S] = 22:2022:2014:02 was essentially invariant and [CF,] and for [D2S] = (0.6-5.0) X loi2molecule cm-3 and [CF,] = (0.4-2.0) X lo1, molecule ~ m - ~ At. higher [D2S] and [CF,] some DF(u) relaxation was observed. A broad DF(u) distribution is expected; but, the apparently larger PI than P2 is suspicious. The HF(u) emission from F + H D S (impurity in the D2S sample) could be examined and PI-P4 was 29:32:30:09 rather than 24:32:33:11. This could be a real difference between H2Sand HDS or it may imply that relaxation (or impurities) affected the D2S sample. A possible impurity that could affect the D F yields is DzO, since the reagent lines were treated with D 2 0 prior to flowing the D2S to the reactor. Corrections based on comparing the best HF(u) distribution from F + H2Swith the HF(u) distribution obtained from the D2S sample were applied to estimate the possible effect upon the DF(u) distribution. The fractions of the distribution that must be moved from the HF(u-1) to HF(u) levels were first calculated so that the P,(HF) obtained in the D2S experiments were equal to the P,(HF) from H2S. A plot of these fractions vs. fv was made and interpolated to obtain the fractions for DF(u=1-5) and these were applied to the experimental DF(u) distribution. The corrected DF(u) distribution was Pl-P6 = 19:20:22:22:15:02. Although this distribution is within the experimental uncertainty of the observed distribution, the corrected distribution will be used as the nascent DF(u) distribution in Tables I11 and V, because we have no physical basis for expecting P l / P 2 > 1.0 for DF(u) given the distribution for F + H2S. C . HF(u,J) Distributions from F + H2Se. The available energy, 61.6 kcal mol-I, can yield HF(u16) and emission from the maximum allowed rotational level was observed for u = 4, 5, and 6. Thus, the IRCL data confirm28Doo(H-SeH) I76 kcal mol-I. Figure 6 shows the steady-state distributions, as well as estimates of the nascent rotational distributions. The high J fractions, see Table 11, are larger than for F + H2S, and there is a modest increase in the high J fractions as the Ar pressure
Figure 7. The HF(u,J) distributions from F + GeH, at 0.65 torr with [CF,]and [GeH,] = 0.76 and 1 . 1 X l o i 2 molecule cm-), respectively; PI-P5 = 12:18:28:40:02. The solid curves are estimates of the nascent rotational distributions, which were obtained by moving the 300 K Boltzmann component back to the high J envelope. The thermochemical limits for u = 2-5 are denoted by arrows; the limit for u = 1 is J = 30.
is reduced. The HF(u) distribution was constant for [H2Se] = (0.2-2.2) X 10l2 molecule cmY3and [CF,] = (0.15-3.0) X 10l2 molecule ~ r n - ~The . average HV(u) distribution from these experiments, P1+6 = 14:21:27:26:11:01,was assigned as the nascent HV(u) distribution. Experiments were done with several H2Se/Ar mixtures and the distributions were reproducible. Dill et studied H2Se by the AR technique. Figure 6 shows that our Pl-P3 values are lower with smaller 300 K Boltzmann rotational components for these levels. This is very surprising considering the nature of the two experimental techniques. A probable explanation for the difference is the presence of H, impurity in the H2Se sample used by Dill et al., who took the gas directly from the commercial cylinder. Our H,Se sample contained a large amount of noncondensible gas, probably H,, which was carefully removed before the H,Se/Ar mixtures were prel~~ pared. The H2 in the H,Se sample would give H F ( ~ 1 3 ) with low rotational excitation and explain the larger HF(u53) populations in Dill et al.'s work. The F GeH4 reaction was studied to observe steady-state rotational distributions in the flow reactor from a group IV (group 14) hydride molecule with an ( E ) similar to F H2Se. Spectra were recorded at 0.13, 0.26, and 0.65 torr; the HF(u,J) distributions at 0.26 torr are shown in Figure 7 and the high J fractions are summarized in Table 11. The HF(u) distribution from GeH, is more sharply peaked than for H2Se or H2S. The maximum in the u = 1, 2, and 3 high J distributions occurs at larger J and the fractions are somewhat larger for GeH, than for H2Se. However, the total high J fraction is nearly the same for the two reactions because P4is higher with lower ( E R ) for GeH4. Our estimate of the nascent rotational distributions in Figure 7 are based on the variation of the FR distributions with pressure and on some AR data.38s39 The thermochemistry for GeH, will be considered when the 0 + GeH, reaction is d i s c u ~ s e d . ~ ~ . ~ ~
+
+
(38) (a) Sung, J. P.; Setser, D. W. J . Chem. Phys. 1978, 69, 3868. (b) Kim, K. C.; Setser, D. W.; Bogan, C. M. J . Chem. Phys. 1974,60, 1837. The bimodal nascent rotational distribution reported here is now viewed as an artifact of the model used to account for rotational relaxation. (39) Wickramaaratchi, M. A,; Tamagake, K.; Setser, D. W., to be submitted to J . Chem. Phys. (40) Agrawalla, B. S.;Setser, D. W., to be submitted to J . Chem. Phys. (41) Almond, M. J.; Doncaster, A. M.; Noble, P. N.; Walsh, R. J . Am. Chem. SOC.1982, 104, 4717.
2456
The Journal of Physical Chemistry, VoL 90, No. 1I , 1986
Agrawalla and Setser
ni
TABLE IV: Product Formation Rate Constants for F and CI Reactions at 300 K
r
?
iI
reaction F + CHI
rate constant,b re1 rateo 10-” cm3 constant molecule-’ s-)
7.2 f 1.2 6.6 f 0.4
1.O
+ H20 1.1 f 0.3 + CD,O-H 4.8 f 1.4 + CH3O-D 2.6 f 0.8 + H2S 1.9 f 0.2 14 i 2 16 f 2 F + D2S 1.4 f 0.2 10 f 2 11 f 2 F + H2Se 2.3 f 0.9 17 f 6 18 f 7 CI + H I 1.o 15.5 f 0.8 CI + H2Se 3.3 f 0.4 51 f 6 59 f 7 6.3 f 0.5 47 C1 + H2S “The F + CH4,* and CI + HI46 reactions were used for reference F F F F
241’
2L 2”
k 5’
ref 42a 42b 32b, d[F]/dr 15, H F ( u t 0 ) 15, DF(u2O) this work, HF(u21) this work, HF(u2O) this work, H F ( u 2 I ) this work, HF(u2O) this work, H F ( u 2 I ) this work, HF(u2O) 46 this work HCI(u2.1) this work, HCI(u20)
cm3molecule-’ s - l ,
with rate constants of 7.2 X IO-” and 1.55 X respectively. bSee Table V for the Po values.
A
I 3300
I
3200
3100
A
I 34cO
I
I
3500
3600
/O*
084 a48
Vl 0
Figure 9. Plots of LIF intensity, Is,, for SD(u” = 0 and 1) excitation from F + D2S vs. [D2S] at constant [CF,] = 4.1 X 10” molecules cm-I and vs. [CF,] at constant [D2S] = 7.4 X 10” molecules respectively. The measurements were made for 0.65 torr and 0.13-ms reaction
time. SD(A,O-X.P)
-L.-..d
1
3100
32W
3300
3400
A
3500
36W
3700
- -
Figure 8. (a) Laser-induced CH30(A2AI,u9/= 1+X2E,uj”) fluorescence spectrum (1-nm resolution) from excitation of u i = 1 ujll = 0 at 310.4 nm. The bands marked with stars probably b_elongto u3‘ = 1 u2“ = 1, vi‘ progression. (b) Laser-induced CH30(A2,~3’=2-+X,~3’’) fluorescence spectrum from excitation of u3‘ = 2 u,“ = 0 at 304.2 nm. (c) The S D and SH(A22+-X2n) LIF spectra for a monochromator resolution of 2 nm. The SD(0-1) and ( 0 - 2 ) fluorescence were obtained from transition ) (322.8 nm). excitation of the Rl + RQ21band head of the (0 The SD(0-0) fluorescence was obtained from ( W l ) excitation (343.7 nm). The SH(O-+I) fluorescence was obtained from excitation of the R,+ RQ21 band head of the ( 0 4 ) transition (323.7 nm). Some accidental S2(B-X) fluorescence observed from SD(OI-I ) excitation also is shown.
-
D. The HF(v)/DF(v)Formation Rate Constants. The HF( v l l ) formation rate constants for F + HzS, D2S, HzSe, and the m e t h a n o l ~ were ’ ~ measured relative to F CH4 for which the 300 K rate constant4zis (7.2 f 1.2) X lo-” cm3 molecule-’ s-I. Plots of [HF,vLl] vs. [RH] were made for constant [F] and At; the slopes give the relative rate constants. Examples of these plots can be found in ref 17. The relative rate constants for HF(vT1) formation from HzS:D2S:HzSe are 1.9 f 0.2:1.4 f 0.2:2.3 f 0.9. With allowance for HF(u=O), the total HF formation rate constants (in cm3 molecule-’ s-l) become kHZs= (1.6 & 0.2) X 1O-Io, koIs = (1.1 & 0.2) X 1O-Io, and kH2Se = (1.8 f 0.7) X The
+
(42) (a) Clyne, M. A. A.; Nip, W. S . Int. J . Chem. Kinet. 1978, 10, 367. (b) Clyne, M. A. A.; Hodgson, A. Chem. Phys. 1983,79,351. (c) Clyne, M. A. A.; Hodgson, A. J . Chem. Soc., Faraday Trans. 2 1985, 81, 443.
rate constant values are listed in Table IV and the estimates of P,(HF) are given in the Discussion section. There is an order of magnitude difference in the rate constant between H2S and H 2 0 , which supports our assignment of a larger activation energy for HzO in Table I. The kinetic isotope effect for F H2S/D2Sis 1.4 f 0.2, which is just the mass effect, similar kinetic isotope effects have been found for CH4/CD4and NH3/ND3., The k(F+CD30-H) previously was reportedI5 as (4.8 f 1.4) X lo-” cm3 molecule-] s-] and k(F+CH,O-D) as (2.6 f 0.8) X lo-]’ cm3 molecule-I s-I, which corresponds to an isotope effect of 1.8. E . LIF Measurements of SD, S H , and C H 3 0 Vibrational Distributions. The LIF spectra of SD and SH from F DzS and H2S are shown in Figure 8c. The poorer signal-to-noise ratio for SH is due to the extensive predissociation of SH(AZZ+p’=O). For the 2-nm resolution of these spectra, only the SH(A,’Z’-+ 2113/2 and 22f+2111,2) bands, separated by -380 cm-], are observed. Parts a and b of Figure 8 show the CH30(A2AI,u3’= 1-+X2E,v3”) and CH30(A,v3’=2tX,v3’’) progressions following excitati_on of (A,u3”=1) and (A,v3’=2), respectively, from CH30(X,u3/1=O). The side bands (marked with asterisk) probably correspond to v3’ = 1 v2” = 1, v3” and v3’ = 2 u2” = 1, v3“ progressions; these bands also were observed by Ino_ue et al.25a3b Vibrational relaxation did not occur in the CH,0(A,2A,) state since e@ssion was observed only from the v3’ level excited. The CH30(A,v3’=2) decay times at 0.1, 0.3, 0.5, 1.0, and 1.4 torr pressure were measured and the fluorescence lifetime was invariant over this pressure range with a mean value of 2.1 & 0.2 ws. This result is in agreement with Ebata et al.43(2.2 ps) who monitored
+
+
-
(43) Ebata, 69, 27.
-
T.; Yanagishita, H.; Obi, K.; Tanaka, I. Chem. Phys. 1982,
The Journal of Physical Chemistry, Vol. 90, No. 11, 1986 2451
Energy Disposal by Reactions of F and C1 Atoms I
a4t
01)
42
0 4 0 . 6
CO
0.8
Total Pressure (torr)
Figure 10. Variations of N , / N , (upper three plots) and N2/N0(lowest plot) ratios of CH30(v3”)as a function of Ar pressure. The three independent experiments for N , / N , are shown on different scales for clarity. The N , / N o and N2/Noplots closest together were from the same experiment. The [CF,] and [CH,OH] for the three experiments were constant, 3.0 X l o t 2and 2.0 X lot2molecules ~ m - respectively. ~,
vjl = 0, 1, and 2, but Inoue et aLZ5reported a somewhat smaller value (1.5 ks) for v3’ = 3. Plots in Figure 9 of IsDvs. [D2S] and [CF,] for u” = 1 and 0 demonstrate first-order behavior in [D2S] and [CF,] up to 2.4 X 10l2 and 2.0 X 10l2 molecule ~ m - respectively. ~, Hence, the mean ratio for these experiments, Pl/Po = 0.15 f 0.05, was assigned as the nascent SD(u’9 distribution from F D2S. This ratio also was invariant to change in Ar pressure from 0.1 to 1.0 torr. Attempts were made to assign a P1(SH)/Po(SH) for the H2Sreaction. The LIF signal from SH(u”=l) was poor and the S/N ratio was -1; however, the signal was observable from SH(u”=O) with a S/N ratio of -5. The apparent P,/Po ratio of 0.07 f 0.03 is probably an upper limit to the true distribution, since the noise for SH(u’=l) was as strong as the signal. Since SeH(A) is highly predissociated, LIF studies of SeH were not attempted. The LIF measurements for CH30(u3” = 0, 1, and 2) vs. Ar pressure, Figure 10, demonstrate vibrational relaxation. Extrapolation of these plots to zero pressure gave estimates for the nascent P,/Poand P2/P0ratios. The average Pl/Povalue from three experiments was 0.24 f 0.07; the single P2/P0data set extrapolated to 0.04 f 0.02. An analysis of the data was done to estimate the rate constant for CH30(v;I) relaxation; the scheme included formation of CH@(V3” = 0 and 1) with Pl/Po= 0.24 and relaxation of CH30(V3”=1) by Ar. The variation of P,/Po with pressure corresponded to k,, = (1.5 f 0.5) X lo-’, cm3 molecule-’ s-I. A higher initial Pl/Poratio (0.32) with k,, = 2.0 X 10-l2 cm3 molecule-I s-’ also would fit the data. The extrapolated Pl/Po= 0.24 f 0.07 and P2/P0= 0.04 f 0.02 values were used as the nascent CH30(v3”) distribution; although, slightly higher values would be acceptable. In He carrier no CH30(v;I= 1) would be observed at 0.15 torr; however, the LIF signal for CH30(v3”=O) was large. As would be expected, CH30(u3”) relaxation is more rapid in H e than in Ar. One factor that could affect the observed CH,O vibrational distribution is the occurrence of the CH30 CH20H isomerization r e a c t i ~ n . Although ~ ~ ~ ~ ~ this isomerization is 10 kcal mol-’ exoergic, the activation barrier is -35 kcal mol-]. Since
+
-
-
the highest observed CH,O vibrational energy was less than 3v3”, the average internal energy of C H 3 0 is well below the isomerization threshold and the isomerization reaction should not have affected the measurements. F. HCl(u) Distribution and Rate Constant for CI + H2Se. The HCl(u) formation rate constant and the HCl(u) distribution were measured. The HCl(u) distribution was the same at 0.13,0.26, and 0.65 torr for [Cl,] = [H2Se] = 2.0 X l o t 2molecules ~ m - ~ , which showed that HCl(u) relaxation by Ar is not occurring. The HCl(u) distribution, P1-P3 = 42:51:07, was constant for [H2Se] = (0.5-7.5) X 10l2 molecule cm-3 and for [Cl,] = (0.5-8.0) X 10l2 molecule ~ m - and ~ , the HCl(u) emission intensity was first order in both [H2Se] and [Cl,] for this concentration range. Hence, PI-P3 = 42:51:07 was taken as the nascent HCl(u) distribution. A recent AR studylld gave PI-P3 = 40:51:09 in agreement with our FR results. Emission from high J levels was not observed even at 0.1 3 torr, Le., the HCl(u) rotational distributions were 300 K Boltzmann. The AR resultslld do show partially arrested rotational distributions with significant populations in the J = 10-19 levels for u = 1 . The HC1 rotational relaxation is sufficiently fast that populations in the high J levels are difficult to observe in the flow reactor.2 The AR data were used to estimate nascent rotational distributions; the J,,, values of 21 and 16 for u = 1 and 2 closely match the thermochemical limit.Ild The HCl(u21) formation rate constant was measured relative cm3 molecule-’ s-I);,~ to C1 + H I ( k = (1.55 f 0.08) X kH se/kHI Was 3.3 f 0.4 and kH2Se(U>-1)= (5.1 f 0.6) X lo-’’ cm3 molecule-’ s-I. The C1 + H2Se rate constant was the largest observed in this work. Since only HCl(uI1) can be formed,’Ic we made no attempt to study C1 H2S. However, the H2S C1 rate constant (6.3 X lo-” cm3 molecule-’ s-l) is known from independent The large increase in the rate constant for H2Se vs. H2S contrasts with F atom reactions for which kHIS= h2se ~ H ~ Q . G . DCl(v) and S D ( v ) Distributions from C1 D2S. The available energy, 15.1 kcal mol-’, can yield DCl(u12); however, IRCL measurements of C1 D2S were difficult because the DCl(u) emission is at the edge of the response curve for the InSb detector and quartz filter. Replacing the quartz filter by a band-pass filter (1800-2200 cm-I) resulted in saturation of the detector for the sensitivity required to observe the DCl emission. Therefore, the quartz filter was used even though only a few lines of the DCl(2-1) band could be recorded. Rather high [Cl,] and ~ , necessary to obtain [D2S], (0.5-2.0) X lOI3 molecule ~ m - were suitable spectra because the DCl(u) Einstein coefficients are -4 and -20 times smaller than for HCl(u) and HF(u), respecti~ely.~~ Plots of the DCI(u) distributions vs. [D2S]and [Cl,] showed some dependence on both [D2S] and [Cl,], and the nascent DCl(u) distribution was estimated from extrapolation. The result was P I P 2 = 0.7:0.3 with an uncertainty of f20%. The LIF signals for SD(u”=l) and SD(u”=O) were first order with [D2S] and [ a 2 ]up to 3 X 10l2and 2 X 10l2molecules ~ m - ~ , respectively. The PI/Poratio for this concentration range was 0.05 f 0.02. The LIF intensity for SD(u”=l) was low (the S/N was only 1.5) from C1 D2S, and the F D2S reaction was used as a prereaction to set the laser wavelengths. No LIF could be observed from SH(u”=l), and the Pl(SH)/Po(SH) from C1 H2S cannot be reported. H . LIF Measurements of S2(u”). For the high reagent concentrations in the F + H2S/D2S and C1+ H2S/D2Ssystems, LIF from S2(B32,--X3Z.J was obtained; see Figure 11. In fact, excitation of S2(X,u”=O-6) was relatively easy. Qualitative studies in the F + H2S system gave an approximate S2(u”) distribution of Po-P6 = 48:24:12:08:(05):02:01; the P4value was obtained by interpolation. The possible dependence of the S2(u’9 distributions on Ar pressure, [H2S], or [CF,] was not evaluated in this preliminary study. The s2(u”=o-4) levels were observed from the
+
+
+
+
-
+
+
+
(44) Adams, G. F.; Bartlett, R. J.; Purvis, G . D.Chem. Phys. Lett. 1982, 87, 311.
(45) Batt, L.; Burrows, J. P.;Robinson, G . N . Chem. Phys. Lett. 1981, 78, 467.
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Agrawalla and Setser
TABLE V: Summary for F and CI Atom Reactions with Group VI (Group 16) Hydrides
HX(u)/DX(u) distributionsu
reaction ( ( E ) ,kcal mol-’) F
+ H2S (47.8)
F + D,S (48.0) F
Po
+ H2Se (61.6)
+ GeH4 (59.0) F + D20 (20.7) F + H 2 0 (21.1) F + CD,O-H (38.7) F + CH,O-D (39.0) F + CH,O-H (38.7) CI + H2Se (28.3) CI + D2S (15.1) CI + H2S (15.3)
14’ 09‘ 10’ 06‘ 0gb 05‘
F
05’ 16b 23‘ 14
