J . Phys. Chem. 1990, 94, 8 128-8 134
8128
are predicted to be stable to loss of an electron, as well as to loss of X-. Thus, these three compounds are the only potential candidates for high-energy density species. Acknowledgment. This work was supported in part by grants
from the Air Force Office of Scientific Research (87-0049) and the National Science Foundation (CHE86-40771). The computer time made available to M.S.G. by the North Dakota State University Computer Center and by the Minnesota and San Diego Supercomputer Centers is gratefully acknowledged.
Laser-Induced Fluorescence Study of Methoxy Radical Formation from the Reactions of F(2P) Atoms with CH,OH, CD,OH, and CH,ODt Denis J. Bogan,*J Myron Kaufman,**sClifford W. Hand,I William A. Sanders," and B. E. Brauersb Chemistry Division, Code 6180, Naval Research Laboratory, Washington, D.C. 20375-5000, and Department of Chemistry, The Catholic University of America, Washington, D.C. 20064 (Received: July 18. 1989; In Final Form: May 21, 1990)
-
Kinetic aspects of the formation and consumption of methoxy radicals in the reactions of F(,P) atoms with CH30H, CH30D, X and CD30H have been studied in a flow reactor at 300 K. Laser-induced fluorescence from the transition (A ,Al ,E) was used to monitor the concentration of methoxy. Reaction of F with CH30H forms both CH30 (la) and CH20H ( 1 b) radicals, but CH20H reacts quickly with F, to regenerate fluorine atoms (12): F(,P) + CH30H HF + CH30 (la); F(,P) + CH30H HF + CH20H (1 b); F2 + CH20H F + FCH20H (12). The effect of this chain reaction sequence is that, in the presence of excess F2 and CH30H, each F atom initially present leads to one CH30 radical. The role of excess F, was independently established as follows: methoxy was not observed from the reaction, CI + CH30H CH20H HCI, but appeared when F2 was added downstream of the CH30H inlet. The fractional yield of methoxy, I'(CH,OH+CH,O), in the elementary reaction of F with methanol can be obtained from the ratio of CH30 LIF signals in the absence and presence of F2,respectively. We find r(CH30H-.CH30) = 0.4 f 0.1. From the deuterium-labeled reactants we find the kinetic isotope effect in reaction la to be kGH/keD = 1.1 0.6, and that in reaction l b to be kc-*/kc+ = 1.5 f 0.6. On the basis of our data and data of others for exothermic H abstraction by F(,P), OH(211), and CN(,Z), we propose a general correlation of the magnitude of the primary kinetic isotope effect with the location of the transition state on the potential energy surface.
-
-
I. Introduction Abstraction of hydrogen atoms from molecules by ground state fluorine (2P) atoms is a general method for production of free radicals.14 When methoxy radicals are prepared by this method, hydroxymethyl radicals (CH20H) are produced in a competing reaction ~ h a n n e l . ~ - ' ~ F(2P) + C H 3 0 H products (1) F(2P) + CH,OH F(2P) + C H 3 0 H
-
-
-
HF
+ CH30
AH = -32 f 1 kcaI/mo116,'8 ( l a ) HF
+ CH20H AH = -41 f 3 kcal/mol"
(lb)
No other product channels of the reaction have been observed in mass spectrometric s t ~ d i e s . ~ JIt~is,J therefore ~ unlikely that any products other than CH30, C H 2 0 H , and H F are formed in primary processes at room temperature. Assuming this to be true, the fractional yields of each product channel, for the case of C H 3 0 H , are defined by
= 1 - I'(CH30H-CH30) 'Experimental work done at Naval Research Laboratory. *Permanent address: Catholic University. 8 Permanent address: Department of Chemistry, Emory University, Atlanta, GA 30322. Permanent address: Department of Chemistry, University of Alabama, Tuscaloosa. -. .-....- , AL 35487. . 1' Permanent address: Office of the Dean, U S . Coast Guard Academy, New London, CT 06320. %Permanentaddress: Chemical Abstracts Service, 2540 Olentangy River Road, Columbus, OH 43210. ~
-
+
With appropriate substitution of D for H, this notation will also serve for isotopically labeled molecules. For simplicity the generic notation rlawill be used to indicate the methoxy yield from any isotopic methanol by reaction I . Reaction 1 is important for several reasons. If its kinetics can be properly understood, it offers a potentially useful means to prepare methoxy radical for kinetic and spectroscopic studies. Methoxy is the simplest and most important member of the class of alkoxy radicals and is important in combustion ~ h e m i s t r y l ~ , ~ ~ ( I ) Bogan, D. J.; Setser. D. W. ACS Symp. Ser. 1978, 66, 237. (2) Manwha, A. S.; Setser, D. W.; Wickramaaratchi, M. A. Chem. Phys. 1983, 76, 129. (3) Jacox, M. E. Rev. Chem. Intermed. 1985, 6 , 77. (4) Dulcey, C. S.; Hudgens, J. W. J . Chem. Phys. 1986, 84, 5262 and references therein. (5) McDonald, R. G.; Sloan, J. J.; Wassell, P. T. Chem. Phys. 1979, 41, 201. ( 6 ) Inoue, G.; Akimoto, H.; Okuda, M. Chem. Phys. Lett. 1979,63, 213. (7) Inoue, G.; Akimoto, H.; Okuda, M. J . Chem. Phys. 1980, 72, 1769. (8) Dill, B.; Heydtmann, H. Chem. Phys. 1980, 54, 9. (9) Hoyermann, K.; Loftfield, N. S.; Sievert, R.; Wagner, H. Gg. Proceedings of the 18th Symposium (International) on Combustion; The Combustion Institute: Pittsburgh, 1981; 831. ( I O ) Jacox, M. Chem. Phys. 1981, 59, 213. ( I 1 ) Dulcey, C. S.; Hudgens, J. W. J . Phys. Chem. 1983, 87, 2296. (12) Meier, U.; Grotheer, H. H.; Just, Th. Chem. Phys. Lett. 1984, 106, 97. (13) Wickramaaratchi, M. A.; Setser, D. W.; Hildebrandt, H.; Korbitzer, B.; Heydtmann, H. Chem. Phys. 1985, 94, 109. (14) Bogan, D. J.; Sanders, W. A.; Eaton, H. G.; Kaufman, M. J. In Lasers as Reactants and Probes in Chemistry; Jackson, W. M., Harvey, A. B.. Eds.; Howard University Press: Washington, DC, 1985; pp 461-472. (15) McCaulley, J. A,; Kelly, N.; Golde, M. F.; Kaufman, F. J . Phys. Chem. 1989, 93, 1014. (16) Batt, L.; McCulloch, R. D. Int. J . Chem. Kinet. 1976, 8, 491. (17) Dyke, J. M.; Ellis, A. R.; Jonathan, N.; Keddar, N.; Morris, A. Chem. Phys. Lett. 1984, 1 1 1 , 207. (18) Baker, G.; Littlefair, J. H.; Shaw, R.; Thynne, J. C. J. J . Chem. Soc. 1965, 6970. (19) Pollard, R. T. In Comprehewiue Chemical Kinetics; Bamford, C. H., Tipper, C. F. H., Eds.; Elsevier: Amsterdam, 1977; Vol. 17, p 249. (20) Griffiths, J. F. A h . Chem. Phys. 1986, 64, 203.
0022-3654/90/2094-8 128$02.50/0 Q 1990 American Chemical Society
Methoxy Radical from Reactions of F(2P) Atoms
The Journal of Physical Chemistry, Vol. 94, No. 21, I990 8129
in the reactor to allow the observation of chemiluminescence (CL) along the reactor axis. Melted fluorocarbon wax was flowed over the interior of the cleaned and dried Pyrex flow reactor until all of the exposed surface was thoroughly wetted. The excess was drained and the coating was allowed to solidify at room temperature in the laboratory. The newly coated reactor was conditioned by discharging a flowing 1% F2 in He mixture at 1 Torr for at least 10 h before collecting kinetic data, giving a negligibly small F atom wall recombination rate. Conditioning had to be done only once provided that the reactor was kept under vacuum when not being used for kinetic measurements. If the reactor was vented to atmospheric pressure for more than a few minutes, some degree T e f l o n Sliding Seal of reconditioning was needed. Precautions were taken to eliminate traces of oxygen from the CL TO gas entering the discharge. Technical grade helium was passed Pump through a 77 K trap containing a mixture of 80-100 mesh molecule sieve 5A and activated charcoal. Between kinetic runs the trap was heated to 650 K and evacuated with a diffusion pump -. f12 T e l e s c o p e to remove the trapped impurities. This procedure minimized but \/ / 6 Filters did not eliminate the broad-band steady-state CL superimposed on the LIF signal. Even the highest purity commercially available helium gave a much larger C L interference than technical grade Monitor u Monitor helium purified by the 77 K trap. Figure 1. Schematic of the discharge flow reactor with laser-induced Fluorine was stored in a 60-L stainless steel tank as a mixture fluorescence and chemiluminescence detection. of 0.001-0.01 mole fraction F2 in He. Mixtures were prepared by diluting a commercial 10% F2in He mixture with pure He that and atmospheric chemistry.21,22 There is also a need for funhad been passed through the 77 K trap. The F atom mole fractions damental understanding of reactions of small organic molecules used in experiments were on the order of The desired that could be involved in area-selective dry etching techniques used concentrations were prepared by on-line merging, upstream from in the fabrication of microelectronic devices, because, as stated the discharge, of a measured flow of the mixture from the 60-L in ref 23, "It is important to try to understand the chemistry that tank with a measured flow of purified He. Ordinary stainless steel would limit the lifetime of resists".23 The reactions of F atoms tubing and needle valves proved to be satisfactory for handling with small organic molecules are among the important members the F2/He mixtures. Very stable reagent flows were required, of this class. and a system described elsewhere5' was used to control and Reported values of the methoxy yield from reaction 1 range measure gas flow rates to within f0.2%. from 0.25 to 0.81. This range of more than a factor of 3 is Reagent grade C H 3 0 H was outgassed by several freezedistressingly large for an important reaction that has been studied pumpthaw cycles and was then vacuum-distilled into a stainless by several groups with several different techniques.5~8~9~12~13~15,55 steel tank. The deuterium labeled methanols were treated in the We have studied the reactions F(2P) C H 3 0 H , CD30H, and same way. C D 3 0 H was purchased from Merck Isotopes, and CH30D, using laser-induced fluorescence, LIF, to detect the C H 3 0 D was prepared by reacting D 2 0 with an excess of Mgmethoxy radicals. The methoxy yield by channel la was measured (OCH3)2. The purity and isotopic composition of labeled samples in each case. From these yields the primary H/D kinetic isotope were verified by mass spectrometry. Mixtures with mole fraction effects at the hydroxyl and methyl positions, kGH/kGD and X = 0.05 methanol in He were stored and metered from a 60-L kC-H/kc+, respectively, were determined. stainless steel tank. A chain reaction sequence plays an important role in the overall Methanol was carried to reactor inlet 3, Figure 1, by a 1/4-in. kinetics of the F methanol system when excess F, is present. Teflon tube. A small plume of chemiluminescence could be seen It is critical to our method of measuring methoxy yield that the at the point where methanol entered the F + He stream. A Teflon chain reaction chemistry be understood; therefore we also studied baffle (not shown in Figure 1) at inlet 3 greatly reduced the the reaction of F2 + hydroxymethyl radical in the presence of interference of the plume with the LIF signal, and also created excess methanol. local turbulence that helped to promote rapid mixing of the 11. Experimental Section reactants. After the introduction of methanol, the gas stream flowed around a 90-deg bend and through an additional distance All reactions were studied in the discharge flow system shown of 10 cm to the detection point. in Figure 1. The reactor was a Pyrex tube of 2.2 cm i.d. The LIF excitation source was the frequency doubled output Concentric reagent inlets were provided by a Teflon probe system of a Chromatix CMX-4 tunable dye laser, operated at 5-20 pulses that could be moved axially to vary the time between reagent per second. The LIF signals were observed through a filter mixing and fluorescence observation. Fluorine atoms were genconsisting of 4 mm thicknesses, each, of Corning 7-51 and Hoya erated by passing a mixture of F2 and H e through an alumina UV-32 glass filters. This combination transmitted 0.40 of the discharge tube inside a 2450-MHz microwave cavity. A highfluorescence in the range 340-390 nm, but less than of the capacity pumping system consisting of a mechanical forepump laser light in the range 295-300 nm. There was no detected and Roots type booster pump was used to give mass flow rates scattered light signal in any of the kinetic runs. up to 8400 L/min STP, at a typical pressure of 1 Torr. The LIF signals were peak and noisy and were further processed The baffle arms for reducing scattered laser light shown in by a gated integrator. The period of integration, approximately Figure 1 follow the design of Butler." Each baffle arm was swept 5 K S , was chosen to slightly exceed the radiative lifetime of the by a slow flow of He to prevent reactive species from entering excited m e t h o ~ y . ~ Figure , ~ ~ . ~2~illustrates the relationships bethe dead space. A quartz window was placed at the 90-deg bend tween the LIF and laser monitor signals and the gate pulse. In both panels the laser monitor pulse has been filtered (time constant (21) Pitts, J. N. Jr.; Finlayson, B. J. Angew. Chem., Inr. Ed. Engl. 1975, To
Inlet 3
-
632
L L Y
+
+
14, 1.
(22) Nicolet, M. Revs. Geophys. Space Phys. 1975, 13, 593. (23) Morgan, R. A. Plasma Etching and Plasma Technology, Plasma Technology I; Elsevier: New York, 1985. (24) Butler, J . E. Appl. Opt. 1982, 21, 3617.
(25) Ebata, T.; Yanagishita, H.; Obi, K.; Tanaka, 1. Chem. Phys. 1982, 69, 21. (26) Agrawalla, B. S.:Setser, D. W. J . Phys. Chem. 1986, 90, 2450.
8130 The Journal of Physical Chemistry, Vol. 94, No. 21, 1990
Bogan et al.
TABLE I: Measurements of Methoxy Yield from F + CH30H [ FIO, l o i i cm-3 1.9 9.3 4.6 2.3 2.8 3.6 1.1 3.5 1.8 3.1 3.5 1.6 1.9 3.1 1.3 1.5
P(total), Torr 1.04 1 .oo 0.99 0.96 0.76 0.97 0.98 0.96 1.01 0.94 0.94 0.93 0.98 0.98 1 .oo
0.99
[CH,OH],,
[F2101
ioi4 cm-’
1 OI5 cm-)
1o4 s-I
I O2 s-I
kl’lk,‘
1.9 2.0 1.6 I .5 2.9 0.35 0.23 0.59 1.2 0.29 0.47 0.59 1.1 1.1 3.0 2.0
0.87 0.95 0.87 1.1 I .9 I .1 0.75
1.3 1.4 1.1
0.57 2.8 1.4 0.69 0.84 1.1 0.33 1.1 0.54 0.93 1.1 0.48 0.57 0.93 0.39 0.45
228 50 80 160 240 22 48 37 160 22 29 85 I30 80 540 290
GATE
1.1 1.1 1.1
1 .o 1.2 1.1 1 .o 1.1
1.1
ki’.
1 .o
2.0 0.24 0.16 0.4 1 0.83 0.20 0.32 0.4 1 0.76 0.76 2.1 1.4
ria* 1 0.52 f 0.09 0.41 f 0.02 0.42 f 0.04 0.42 f 0.06 0.57 f 0.15 0.29 f 0.06 0.26 f 0.08 0.40 f 0.09 0.49 f 0.07 0.37 f 0.06 0.54 f 0.04 0.32 f 0.03 0.32 f 0.02 0.36 f 0.02 0.40 f 0.06 0.39 f 0.10 AV 0.41 f 0.09
corr to LIF (added F,) 0.19 0.006 0.04 0.16 0.22 0.10 0.13 0.2 I 0.27 0.33 0.10 0.20 0.17 0.07 0.37 0.22 0.16 f 0.08
I
i
LASER
303.5
298.2
295.5
nm Figure 3. Portion of the excitation spectrum of CH,O measured by laser-induced fluorescence in the region 340-400 nm. The displayed spectrum spans 900 cm-I. Data were taken at a scan rate of 0.1 cm-’/s, 10 laser shots/s. and 50 shots/display point. LIF
LASER
Figure 2. Laser output, gate, and laser-induced fluorescence and their time relationships. The upper panel displays the gate and laser pulses. The lower panel compares the laser-induced fluorescence and the exciting laser pulses for five consecutive shots. The early part of the LIF trace is the desired signal and the later part, which is randon in time, is undesired chemiluminescence.
The precision of the titration was &IO%. Within this limit, the results showed that the F2 was 100% dissociated in the discharge and that the amount of F atom recombination was negligible over the 50 cm length of the inlet portion of the reactor. All of the data reported here were obtained at room temperature with a total pressure in the reactor of 1 Torr. Linear flow velocities were 10-16 m/s, with forepump only, and 25-40 m/s, with the booster pump. The partial pressure of F atoms was typically in the range 0.005-1 .O mTorr. The distance from the discharge to the methanol inlet was about 1 m, which corresponds to a transit time of 20-100 ms. 111. Data and Kinetic Analysis
The kinetic analysis is complicated because the LIF signal yields a relative, not absolute, measure of the concentration of methoxy radicals. No direct method for detecting hydroxymethyl radicals = 2.5 ps) to remove high-frequency noise. Signals from several was available. However, the relative methoxy radical concenlaser shots are superimposed in the lower panel. The shot to shot tration can be used for the present purpose, depending upon certain variation of the LIF signal demonstrates the need for the gated assumptions. integrator. The known fast rates of F atom reaction^'^.^^,^^ and the conThe concentration of methoxy was monitored by measuring the finement of most of the chemiluminescence to a tight plume at fluorescence in the filtered range 340-390 nm, following excitation the methanol inlet suggest that the rate of reaction is limited by of the 3; vibronic band of the (A 2A, X 2E) tran~ition.~.~’ This the rate of mixing and that there is a complex interplay between band is centered near 298.3 nm for both C H 3 0 and CD30. Figure turbulence, diffusion, and reaction in the region of the probe tip. 3 shows part of the LIF spectrum, which was verified as C H 3 0 by comparison to the work of Inoue, Okuda, and A k i m ~ t o . ~ , ~ ’ The large excess of methanol ensured the virtually complete disappearance of F as soon as mixing occurred. We attempted The F atom concentrations in the reaction zone were measured to observe the growth of methoxy as a function of distance between by using the chemiluminescent titration procedure of Schatz and the methanol inlet and detector. In all cases we observed a Kaufman,28 based on the reactions monotonic decay of methoxy as distance from the inlet to detector F + Cl2 --c FCI + CI (3) increased. This was true even within the chemiluminescent plume, although data within the plume are probably unreliable due to C12(b 311$,)+ M C1 + C1 + M (4)
-
-
(27) Inoue, G.; Okuda, M.;Akimoto, H. J. Chem. Phys. 1981.75.2060.
(28) Schatz, G.; Kaufman, M . J. Phys. Chem. 1972, 76, 3586. (29) Foon, R.; Kaufman, M.frog. React. Kinet. 1975.8, 81. (30) Smith, D. J.; Setser, D. W.;Kim, K. C.; Bogan, D. J. J. Phys. Chem. 1977. 81, 898.
The Journal of Physical Chemistry, Vol. 94, No. 21, 1990 8131
Methoxy Radical from Reactions of F(2P) Atoms competition between reaction and mixing. Under the present conditions the plume was about 3 cm long, corresponding to a time interval of 1 2 x s. Wickramaaratchi et a1.s value, kl = 6.9 X IO-'' cm3 molecule-' s-I,l3 where k l = k,, + klb,was used for estimating the pseudofirst-order rate constants for F atom decay, k,' (=kl[CH30H],), in the experiments reported in Table I. These are compared to estimated pseudo-first-order rate constants for methoxy decay, cm3 kg/ (=k,[F],), by reaction 6. An estimate of k6 = 3 X F CH30 H2C0 H F (6)
-
+
+
molecule-I s-I was used, assuming reaction on every collision. As shown in Table I, the estimated ratio kl'/k,' varied from 22 to 540 and estimated k,' 1 1600 s-I in all cases. Reaction 6 is actually less important than suggested by this ratio. Reaction 1 remains pseudo first order with methanol in excess, k,' being essentially constant, until all F atoms are consumed. Reaction 6 is not in fact pseudo first order. Its initial rate, described by kg/, will rapidly decrease to zero as F atoms are consumed by methanol. A reasonable expression for the final methoxy radical concentration, [ ~ H @ ] F I N Acan L , be obtained by considering reactions la and 1 b and neglecting all others. Then the decay rate of the F atom concentration would be given by -(d[FI/dt) = kl[Fl[CH3OHl, [FI = P I 0 exPl-~,~CH,OH1,4 (7) The resulting growth rate for methoxy radical is d[CH,O]/dt = kla[CH30H][F], e ~ p ( - k ~ [ C H ~ 0 H ] ~ t ) When integrated and evaluated from t = 0 to t = infinity, this expression gives [CH301FINAL = [FIO(kla/kl! (8) The practical definition o f t = infinity is the point when F atoms are consumed and no more methoxy is formed. However reaction 6 should not be neglected. Defining Y = exp{-k,'t] leads to d[CH30]/dt = -(d[CH30]/dY)k,' exp{-k,'t) = (k,,' - k6[CH30])[F], exp(-k,'r], and
Integration and evaluation from t = 0 to t = infinity results in the elimination of time as a variable and yields the expression kia'
- exP(-k,[F'lo/ki'))
(10)
k6
Since the LIF intensity at the viewing window is proportional to the methoxy concentration, the semiempirical expression [CH301FINAL a LIFFINAL= c ( 1 - expl-P[FIol) ( 1 1) can be used, under pseudo-first-order conditions, with methanol in excess. C and p are constants, and C embraces apparatus sensitivity as well as k l i and k,. We observed exponential growth curves of LIF vs [F],, having the form of eq 11, using fixed values of [CH3OHIoand [F2I0,and a fixed distance from methanol inlet to detector. An example is shown as the lower curve of Figure 4.
The above analysis was used for the experiments reported in Table I, for which [F], < 10I2atom ~ m - ~At. these concentrations, and for the present experimental conditions, methoxy radical decay was less than 5% in 20 cm of reactor length. It was not necessary to consider its bimolecular self-reactions. In other experiments, where it was our intent to study the bimolecular self-reactions, concentrations of [Flo > 5 X 10l2molecules were used.31 Determination of the fractional methoxy radical yield in reaction 1 requires a further kinetic analysis of the reaction scheme. If (31) Bogan, D. J., to be submitted for publication.
Figure 4. C H 3 0 laser-induced fluorescence intensity as a function of initial F atom concentration. For the upper trace, excess F, was present, and for the lower trace it was absent. The smooth curves are leastsquares fits to eq 11. The reagents were introduced at inlets shown in Figure 1; inlet 1 for F, inlet 2 for F1, and inlet 3 for CH,OD. Full scale on the abscissa corresponds to [F], = 1 X IOd Torr.
a large concentration of F, is introduced into the reactor downstream from the discharge, through inlet 2, along with methanol introduction at inlet 3, C H 2 0 H radicals formed by reaction 1 b react to regenerate F atoms32+33 by the process F2
and the time-dependent F atom concentration by
[CH30I =
LIF
+ CH20H
-
FCH20H
+F
AH = -70 kcal/mol (12)
The fluoromethanol product may contain excess vibrational energy and could undergo unimolecular decomposition to form formaldehyde and HF, but its fate is unimportant to this analysis, provided that it does not yield any additional F atoms. If [F2] is large enough, reaction 12 is the dominant loss process for hydroxymethyl radical. With a sufficient excess of F2 and methanol, every F atom that fails to produce a methoxy radical by reaction l a is recycled for another chance. The total amount of methoxy produced in excess F2 is increased by a factor of (I'(CH3OH-+CH3O))-', (where I' is defined in eq 2a), compared to the amount produced in the absence of added F2, all other conditions being the same. The reactions la, lb, and 12 comprise a chain mechanism for methoxy formation. Reactions l b and 12 are propagation steps and reaction la is the termination step. The factor {I'(CH30H+CH30)J-' is the chain length, the ratio of propagation events to termination events. On the basis of this argument, the LIF intensity with added F, was plotted as a function of [F], and fitted to eq 11. As the upper curve of Figure 4 shows, the equation fits these data well. The experimental branching fraction was then taken to be equal to the ratio of the asymptotic C values (eq 11) of the LIF intensity, giving
In all cases the value of the parameter /3 (eq 11) was constant within f 10% regardless of the presence or absence of F2. The addition of more terms for consumption of F atom would not change the form of eq 11, although it would change the value of C. Equation 11 would still be a valid description of the relative values of methoxy concentration. Once the methoxy growth curve (Figure 4) was established as having the form of eq 11, growth curves were no longer taken. Instead, repeated measurements of LIF(no F2)/LIF(added F2) were made and averaged, using very large ratios of [CH,OH],/[F], and [F2],/[F],,. These data are reported in Table I. The mean value of I'(CH30H-CH30) was 0.4 f 0.1 at room temperature. (32) Dainton, F. S.; Lomax, D. A.; Weston, M . Tram. Faraday Soc. 1962, 58, 308. (33) Kondratiev, V . N. Rate Constants of Gas Phase Reactions; US. Department of Commerce: Washington, DC, 1972; COM-72-10014.
8132
The Journal of Physical Chemistry, Vol. 94, No. 21, 1 5190
1-
I I
TABLE II: H/D Kinetic Isotope Effects for the Reactions of F(*P) Atom plus Methanol and Data from Which They Are Derived“
h
h
U
Bogan et al.
U
O
reactant
I
CH30H CHjOD CD30H
w
r(CH,OH+CHjO) r(CHjOD+CH30) r(CD,OH+CD30) k(C-H)/k(C-D) = k(0-H)/k(O-D) =
1
8
I
Figure 5. C H 3 0 laser-induced fluorescence intensity as a function of added F2in the reaction of CI atom with methanol. The reagents were introduced at inlet 1 for CI,inlet 2 for methanol, and inlet 3 for F2. The points represent IO experimental determinations, four of which are superimposed at the origin. The smooth curve is a least-squares fit to eq 11. Full scale on the abscissa corresponds to [FJO = 40 X IOd Torr.
We obtained independent evidence for the role of reaction 12 as a chain-propagating step by initating the sequence of reactions with CI atoms rather than F atoms. It has been reported that CI abstracts the C-H hydrogen from methanol (rather than the 0-H) with a selectivity factor greater than 30 at 300 K.Iz When CI reacted with a large excess of methanol, in pure He carrier gas, no C H 3 0 signal was observed. When Fz was added, downstream of the methanol injector, the growth of a methoxy signal was observed. The curve was fitted to the empirical equation of exponential growth to an asymptote: I = IFINALI~ - ex~(-c[FzlN where c is a constant. This is shown in Figure 5. The asymptotic value of IFlNAL was identical with that obtained when the reaction was initiated by replacement of the initial concentration of CI atoms with an equal concentration of F atoms, all other conditions being unchanged. These observations support the essential assumption needed for the present branching ratio analysis, namely, that the chain mechanism, reactions la, lb, and 12, is driven to completion. The reaction, CI F2 CIF F, has a rate constant of k < cm3 m~lecule-~ s-I under our conditionsz8and is too slow to be significant. There is no evidence in the present study to support a significant rate constant for the reaction: F2 + C H 3 0 F + C H 3 0 F (14)
+
+
+
-
For the sake of argument, if an upper limit of 20% depletion by reaction 14 is assigned to the data point in Figure 5 at F2 = 36 pTorr, then, using the reactor residence time of 0.05 s, an upper limit estimate of k i 4 I3 X cm3 molecule-’ s-I is derived. The primary kinetic isotope effects were determined by studying the reactions F + C H 3 0 H and either F C H 3 0 D or F CD30H, in separate kinetic runs under otherwise identical conditions. The data used in these determinations are shown in Table 11. The kinetic isotope effect ratios are defined by
+
-kC-H-
- klb(CH3OH)
kC-D
k I b(CD3OH)
This Work relative rate constants k l = kla + k l b I.O@ = 0.36 f 0.05 + 0.64 f 0.05 xd = Ad + 0.64 f 0.05 yd = 0.36 f 0.05 + Bd
+
at the methyl position, and ~ - H kla.(CH,OH) -k = ~ S D kia(CH3OD) at the hydroxyl position. The quantities on the right-hand side of the equations were not directly observed, but instead rlavalues were measured. In addition, the relative values of the total rate constants, k , , for each isotopic reactant must be known, and it must be assumed that the secondary isotope effect is negligibly small. The vibrational frequencies governing the secondary isotope effect (all of them except C-H, C-D, 0-H, and 0-D stretches)
= 0.36 f 0.05 = 0.35 f 0.09 = 0.46 f 0.06 1.5 f 0.6‘ 1 . 1 f 0.6‘
Work of Heydtmann and Setser Research Groups” absolute rate constants. IO-” cm3 molecule-I
reactant
CHjOD CD30H
k , = k , , + kl,
6.9 f 1.3 = 2.6 f 0.8 7.6 f 1.6 = 4.8 f 1.4
s-I
+ 4.3 f 1.0 + 2.8 f 0.8
r(CHjOD-CH30) = 0.38 f 0.14 ~ ( C H ~ O D A C H ~ O= D 0.62 ) f 0.19 r(CDjOH-CDj0) = 0.63 f 0.23 r(CD,OH+CD,OH) = 0.37 f 0.13 k(C-H)/k(C-D) = 1.5 f 0.6‘ k(0-H)/k(O-D) 1.8 f 0.8‘
“Rates for pairs of isotopic reactants were measured on the same day under otherwise identical conditions. This is why r(CH30H+CH,O) differs slightly from the table I value. bAssumed normalization condition. CErrorsare 1 u based upon the data precision. Error propagation was calculated according to Wilson.3s Independent errors combine as the sum of orthogonal vectors. For products and quotients the relative errors are vector summed. For sums and differences the absolute errors are vector summed. d ~y., A, and B are experimentally undetermined quantities for which we can solve, see text, section 111. The solutions arex = 0.98 f 0.16, A = 0.34 f 0.17, y = 0.78 f 0.15, B = 0.42 f 0.16. are expected to be nearly identical in the respective reactants and transition states, and the entropies of rotation will be changed very little by the substitution of D for H.34 By setting k , = 1.00, for the CH30H isotope, as a normalization condition, and by assuming no secondary isotope effect, the array of equations shown in Table I1 can be solved, where x,y, A , and B are defined by their positions in the array. Using the definitions of the branching fractions (eqs 2a and 2b and the experimental measurements of Table 11, the solution for the four unknowns can be obtained. rIa(CH3OD-+CH30) = 0.35 f 0.09 = {X - (0.64 f 0 . 0 5 ) ) / ~
x = 0.98 f 0.16, A = 0.34 f 0.17 r,,(CD,OH4CD30) = 0.46 f 0.06 = (0.36 f 0.05)/y y = 0.78 f 0.15,
B = 0.42 f 0.16
The isotope effects and their l u error limits, calculated according to Wilson,35 are kc-H/kc-o = (0.64 f 0.05)/(0.42 f 0.16) = 1.5 f 0.6 k&H/kwD = (0.36 f 0.05)/(0.34 f 0.17) = 1.1 f 0.6
IV. Discussion IV.1. Data Reliability and Comparison to Other Work. The right-hand column of Table I shows the magnitude of a correction which was subtracted from LIF(added F2). The mean value of this quantity was 0.16 of LIF(added Fz), and it represents a spurious methoxy signal found when no Fz passed through the discharge. The spurious signal was observed only when (a) the discharge was lit and high-purity He was flowing through it, (b) F, was introduced through inlet 2, and (c) methanol was intro(34) Benson, S. W. Thermochemical Kinetics, 2nd ed.;Wiley: New York, 1976; p 5 1 . (35) Wilson, E. B. Jr. An Introduction to Scientific Research; McGrawHill: New York, 1952: p 272.
Methoxy Radical from Reactions of F(2P) Atoms duced through inlet 3. The signal was found to be eliminated either by extinguishing the discharge or by shutting off the flow of F,. Use of the high-speed booster pump minimized this problem. When the booster was not used, reaction times were increased by a factor of 2.5 and the spurious signal was as large as 0.5 of the magnitude of LIF(added F2). These observations suggest that some species, produced in the discharge, reacted to produce methoxy when both F2and methanol were present. Excited metastable He atoms are believed to be quenched with high efficiency by collision with the reactor walls. Use of the high-speed booster pump would allow He metastables to survive for a longer distance downstream from the discharge. The fact that the magnitude of the spurious signal decreased with use of the booster pump suggests that He metastables are an unlikely source. We note also that, for the experiments reported in Table 1, [F], was on the order of 5-10 parts per million with respect to the He carrier gas. We believe that ppm level impurities, of this same order of magnitude, in the discharged He are the most likely source of the spurious signal. It is also a possibility that impurities at this level were released from the walls of the discharge tube when the discharge was lit. Any atom reaction forming hydroxymethyl radical would then form F atom by reaction 12, and this would lead to the formation of methoxy radical by reaction la. Oxygen is the most likely candidate.36 Another possibility is the direct formation of F from F2 by a reactive atom, with oxygen again a potential andi id ate.'^ In the early stages of this work,I4 concentrations of [F], > 5 x 10l2atom were necessary to observe methoxy radicals with a reasonable signal to noise ratio. This concentration corresponds to 100 or more ppm with respect to the He carrier gas. Spurious signals were not detected in these experiments. The flow rate of He was nearly constant in all experiments. Therefore, if the spurious signals were due to a ppm level impurity in the He, they would remain nearly constant in magnitude, regardless of the value of [F],, and probably below the limit of detection at high values of [F],. Their observation would be a function of [F], and detection sensitivity. Data in Table I show a weak inverse correlation between the size of the spurious signal corrections and [F],, with correlation coefficient = -0.62, consistent with the proposed explanation. The 10-50 times greater sensitivity achieved with the gated integrator allowed us to use the very large ratios of [CH,OH],/[F], and [F2],/[F], shown in Table I. This gives greater confidence that the chain reaction scheme proceeded to completion so as to ensure the validity of eq 11 and allows the use of methoxy radical concentrations low enough to ensure the absence of significant bimolecular self-reaction. A general conclusion to be drawn from these observations is that caution is indicated when the electrical discharge method is used to produce very low concentrations of atoms. The discharge is indiscriminate in making atoms and other reactive species. The 1 u error limits reported in I'(CH30H-+CH30) = 0.4 f 0.1 are based upon the precision of the measurements. The actual error is almost certainly larger than this. We cannot assess the magnitudes and directions of any systematic errors. It is interesting to note that most other studies of reaction 1 report a higher methoxy yield than our value. This is shown in Table 111. The 1 u errors in the H / D kinetic isotope effects reported in Table I1 are believed to be reasonably correct because of partial cancellation of any systematic errors. The variation in the reported rate constant measurements is about as large as the variation in the reported branching fractions, about a factor of 3 in both cases. It seems clear from all reported measurement^^^^^^^^ that k , is within a factor of 3 of the gas kinetic collision rate. IV.2. Reaction Dynamics and Kinetic Isotope Effect. Infrared chemiluminescence studies of the highly exothermic hydrogen(36) Keil, D. G.; Tanzawa, T.; Skolnik, E. G.; Klemm, R. B.; Michael, J. V. J . Chem. Phys. 1981, 75, 2693. (37) Clyne, M. A. A,; Watson, R. T. J. Chem. SOC.,Faraday Trans. 1 1974, 70, 1109.
The Journal of Physical Chemistry, Vol. 94, No. 21, I990 8133 TABLE 111: Experimental Measurements of Methoxy Fractional Yield in the Reaction F(") Methanol" reactant fractional yield of methoxy ref CH3OH 0.41 f 0.09 this work 0.35 f 0.09 CH3OD CD3OH 0.46 f 0.06 CHSOD 0.5 5 CHSOH 0.20 f 0.03 86 CH3OD 0.30 f 0.03 CD3OH 0.24 f 0.02 CD3OD 0.33 f 0.04
+
CH,OD CD30H
0.252,'; 0.402;:
9
CH3OH CH3OD CDSOH CH30D CD30H
0.59 f 0.06 f 0.14 f 0.23 f 0.07 f 0.08
12
0.38 0.63 0.81 0.69
13 15
"The fractional yield of methoxy, as defined in the text, is T(CH30H-.CH30) for C H 3 0 H reactant, r(CH30D-+CH30) for C H 3 0 D reactant, r(CD30H-+CD30) for C D 3 0 H reactant, and r(CD30D-+CD30) for CD30D reactant. Refer to the references given above for full details. bHeydtmann's group has published a subsequent report in collaboration with Setser and coworkers; see ref 13 and this table.
abstraction reactions of F(2P) with m e t h a n ~ I , ~ ~and * J a~ wide *~~ variety of polyatomic hydride^,',^.^ have been reported. The HF(u > 0) populations observed in these experiments offer considerable insight into the reaction dynamics. The observable consequences of a highly exoergic hydrogen abstraction reaction, taking place on a potential energy surface (PES) with a very small and very early barrier, extend to the kinetic isotope effect on the bulk rate constant. This is less widely recognized than the effects on vibrational energy disposal. These effects are related, originating from the features of the PES. The reaction Hz F H F H has been studied in great detail and interpreted with trajectory calculations on theoretical PES'S.^^-^' Sixty-six percent of the energy released appears as H F vibration, with population inversion in the lower levels. This has been attributed to the large exoergicity with a very early transition state (TS), occurring at large H-F s e p a r a t i ~ n . ~ ' . ~ ~ Heidner and co-workers have measured rate constants for F + H2 and D2 over the temperature range 295-765 K.47 They obtained a temperature-dependent kinetic isotope effect, KIE, of k(F+H,)/k(F+D,) = (2.1 f 0.8) exp[(9 f 126)/7'l, and an E, of 1182 cal/(deg mol) for H2. The key point for discussion is their finding of virtually no difference in the activation energies of the two isotopes; that is, E,,D = Ea,H. Bender and co-workers have developed a collinear ab initio PES for the F + Hz reaction that is in quantitative agreement with experimental data.40 It is easily recognized from their perspective plots that the potential in the H-H coordinate, with rH-F fixed at 1.54 A (the TS distance), is essentially the same as the H-H potential at the longest rH-F that they calculated (2.5 A). Hence the H-H bond in the TS has essentially the same potential and
+
-
+
(38) Polanyi, J. C.; Wong, W. H. J. Chem. Phys. 1969,51, 1439. (39) Muckerman, J. T. J . Chem. Phys. 1972, 56, 2997. (40) Bender, C. F.; ONeil, S. V.; Pearson, P. K.; Schaefer 111, H. F. Science 1972, 176, 1412. (41) Polanyi, J. C.; Schreiber, J. L. J. Chem. Soc.,Faraday Discuss. 1977, 62, 267. (42) Polanyi, J. C. Science 1987, 236, 680. (43) Holmes, B. E.; Setser, D. W. In Physical Chemistry of Fasr Reactions; Smith, I. W. M., Ed.; Plenum Press: New York, 1980; Vol. 2, pp 83-2 14. (44) Johnson, R. L.; Kim, K. C.; Setser, D. W. J . Phys. Chem. 1973,77, 2499. (45) Bogan, D. J.; Setser, D. W. J . Chem. Phys. 1976, 64, 586. (46) Agrawalla, B. S.; Setser, D. W. J. Phys. Chem. 1984, 88, 657. (47) Heidner, R. F.; Bott, J. F.; Gardner, C. E.; Melzer, J. E. J. Chem. Phys. 1980, 72, 4815.
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Bogan et al.
The Journal of Physical Chemistry, Vol. 94, No. 21, 1990
This simple view of the effects of the PES does not provide any bond strength as in an isolated Hz molecule. This is a limiting information about the ratio of the Arrhenius preexponential case situation that will be unchanged by isotopic substitution. factors, AH/AD,or of quantum mechanical tunneling. These terms Consequently, the zero-point energy level difference between the must be considered to obtain a complete interpretation. reactants and TS is essentially the same for the reactions of both We would expect PES’S of similar shape, for other exothermic H2 and D,. The theoretical calculations of Bender et al.40are R’ HR’ R H R” reactions, to lead to similar behavior. consistent with the observation of Heidner et aL4’ that Ea,Di= Ea,H. The KIE for H abstraction from various primary, secondary, and The temperature dependence of the KIE of F + methane has tertiary C-H (C-D) bonds by OH, over a wide temperature range, not been reported, but at 300 K, kc-H/kc-D= 1.6 f 0.2 has been has been studied and summarized by T ~ l l y . Similar ~~ studies found.48 for CN abstracting H have been conducted by Hess, Durant and The reactions of polyatomic hydride molecules (RH) with F Tully.” They have noted the trends that would be predicted from are expected to result in observations of product vibrational energy our postulate that the primary KIE is strongly correlated with disposal and KIE that resemble those of the F + H2 reaction. This the location of the TS. expectation has been verified for F + methane.48 This work, and the works of other^,'^.^^,^^ verify it for F + methanol. V. Conclusion In the general F + HR case, the R group is expected to carry Methoxy radical formation from the reactions of F(zP) atoms very little of the reaction exoergicity except when there is bonding with isotopic methanols has been studied by laser-induced rehybridization or a significant release of geometric strain after fluorescence. We have found that methoxy radical accounts for the breaking of the R-H Using LIF, Agrawalla and about 40% of the products of reaction 1 and hydroxymethyl radical Setser found that the methoxy radical product of the F + methanol for 60%. Thus the reactivity per H atom is greater for the less reaction carried 2-3% of the reaction e x o e r g i ~ i t y . ~ ~ . ~ ~ exothermic of two highly exothermic product channels. This is To the (apparently) considerable extent that results from an interesting and unexplained effect. three-body calculations can be extended to F + HR reactions, Deuterium substitution has little influence on the rate constants near-equal activation energies for HR and DR and large product at either the methyl or the hydroxyl position. The work of othvibrational energy both result from an exoergic PES with a low e r ~ ~ is, in ~ agreement. , ~ ~ , ~ ~ early barrier. The study of the reaction F2 + C H 2 0 H in excess methanoli4 The often-quoted semiclassical limit for a primary KIE is shows that the reaction system F(2P) atom plus 100-fold or larger kc-H/kc-D = 7 at 300 K, due to ( E , D- ea,^) = 1.2 kcal/mol for excesses of F2 and methanol is a useful method of preparation a C-H vibrational frequency of 3000 cm-I, assumes identical of methoxy radical. With (10” I [F], (atoms cm-’) I5 X 10l2), zero-point energies for C-H and C-D in the TS,49and identical this system proceeds to completion in less than 5 ms. The hyArrhenius A factors. Since all bound C-H potentials will show droxymethyl radical is removed efficiently by reaction with the a change in zero-point energy upon deuteration, this limit contains excess FZ. Under these conditions, the final concentration of an implicit assumption of a flat (or repulsive) C-H potential with methoxy radicals is expected to be essentially equal to the initial no bond strength at the TS. This is correct only in the limiting concentration of F(,P) atoms. case of a highly endothermic reaction with a very late barrier. New measurements of product yields and relative rate constants For a very exothermic reaction with a very early barrier occurring for the reactions of F plus methanol (and deuterated methanols) on a three-body collinear PES, the quantity (Ea,D- Ea,H)is equal have recently been reported by Khatoon and H ~ y e r m a n n .They ~~ to zero. As the surface is changed from exothermic with early found yields (for C H 3 0 H ) of 0.60 for methoxy radical and 0.40 barrier to endothermic with late barrier, this quantity will increase for hydroxymethyl radical, kc-H/kc-D= 1.02, and k@.,/kSD = monotonically to its maximum value. The later the TS, the larger 0.88. the KIE, until the semiclassical limit is reached. This generalization is a logical extension of the view of dynamics on potential We thank Dr. J. E. Butler for providing us energy surfaces pioneered by Hammondso and P ~ l a n y i . ~ ~ . ~ ’ . ~Acknowledgment. ~ with a set of baffles24and Dr. J. V. Michael for a thorough reading of the manuscript and for helpful suggestions.
+
(48) Rowland, F. S.; Williams, R. L. J . Phys. Chem. 1971, 75, 2709. (49) Laidler, K.J. Chemical Kinerics, 3rd ed.;Harper & Row: New York, 1987; pp 434-436. (50) Hammond, G.S.J . Am. Chem. SOC.1955, 77, 334. (51) Sanders, W. A.; Bogan, D. J.; Hand, C. W. Reu. Sci. Instrum. 1986, 57, 3059. (52) Garrett, B. C., Truhlar. D. G. f n t . J . Quant. Chem. 1987. 3 1 , 17.
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+
(53) Tully, F. P. Chem. Phys. Lett. 1988, 143, 510. (54) Hess, W. P.; Durant, Jr., J. L.; Tully, F. P. J. Phys. Chem. 1989, 93, 6402. ( 5 5 ) Khatoon, T.; Hoyermann, K. Ber. Bunsen-Ges. Phys. Chem. 1988,92,
669.
