J . Phys. Chem. 1987, 91, 1205-1209
1205
A Direct Study of the Reactions of CH,(k ‘B,) Radicals with H and D Atoms T. Bohland, F. Temps,* and H. Gg. Wagner Max- Planck- Institut fur Stromungsforschung, 3400 Gottingen, West Germany (Received:July 28, 1986)
The following reactions of CHI radicals in their triplet electronic ground state 2 ’B, with hydrogen and deuterium_atoms were investigated at room temperature and pressures of a few mbar in an isothermal discharge flow reactor: CH2(X 3B1) + H(2S) CH(2JI) + H2(’Z)(la), 3 CH3(2A,”) (lb); CH2(X ’BI) + D(2S) CH(ZJI)+ HD(’Z) (2a), CD(211) + H2(’Z) (2b), --+ CHD(’Bl) + H(2S) (2c), Y CH2D(’A,”) (2d). CH2and CH radicals were detected with a far-infrared laser magnetic resonance (LMR) spectrometer while H and D atoms were monitored via their Lyman-a vacuum ultraviolet resonance absorption. Direct measurements of the pseudo-first-order decay of [CH,] in the presence of an excess of H or D yielded the rate constants k1(298 K) = (1.1 f 0.3) X 1014 cm3/(mol s) and k2(298 K) = (1.1 i 0.3) X 1014 cm3/(mol s). Under the conditions used the recombination channels l b and 2d are negligible. Thus, the measured values correspond to k l = kla and k2 = k2a + k2b+ kzc. When the value for kla is combined with literature data fo_rthe reverse reaction CH + H2 CHI + H (-la), the heat of formation of CH2 radicals was found to be AHf029s(CH2(X3B,))= 389 f 4 kJ/mol.
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Introduction The reactions between methylene radicals (CH,) in their triplet electronic ground state (X 3B1)with hydrogen and deuterium atoms
+ H(2S) CH2(% 3B,) + D(2S)
CH2(% 3BI)
-
products products
,
(1) (2)
are of great interest on one hand due to the importance of C H 2 radicals in combustion processes and on the other hand from a more fundamental point of view due to their nature as radicalradical reactions. In hydrocarbon combustion, especially in lower aliphatic fuel flames, reaction 1 plays a central role in the overall In particular, the reaction channel CH2
+ H + C H + H2
(la)
has been suggested as the primary source for C H radicals.’ Information on the rate and products of reaction 1 is therefore of importance for kinetic modeling of flames and for predictions of the product yields. Moreover, an investigatioh of reaction l a permits the determination of the heat of formation of C H 2 via the equilibrium constant. This fundamental quantity has been argued over for a long time.4 The interest in the reaction mechanism arises due to the existence of two fundamentally distinct pathways, i.e. direct H-atom metathesis vs. a complex mechanism. An analysis of these channels is not only of great importance for a broader theoretical understanding of radical-radical reactions, it also has considerable bearing for the unimolecular dissociation of CH3. Until recently, for the rate constant of reaction 1 only very indirect experimental data from kinetic analysis of rather complicated chemical systems were Reaction 2 had not yet been investigated. In an earlier publications we have reported on the results of a first direct study of reaction 1 em(1) Homann, K. H.; Schweinfurth, H. Ber. Bunsen-Ges. Phys. Chem. 1982, 85, 569. (2) Homann, K. H.; Wellmann, C. Ber. Bunsen-Ges. Phys. Chem. 1983, 87, 609. ( 3 ) Warnatz, J. Ber. Bunsen-Ges. Phys. Chem. 1983, 87, 1008. (4)Gaspar, P. P.; Hammond, G. S. In Carbenes, Vol. 11, Moss, R. A,, Jones, M., Eds.; Wiley: New York, 197.5; p 207. (5) Peeters, J.; Vinckier, C. Symp. (Int.) Combust., [Proc.],15th. 1974 1974, 969.
ploying laser magnetic resonance (LMR) detection of CH2. However, in those investigations H atoms could not be monitored directly but only via conversion with NO2 to OH. The present paper is concerned with a more accurate study of the kinetics of reactions 1 as well as reaction 2 at rmm temperature with direct detection of both reactants. C H 2 and C H radicals were monitored with the L M R while H and D atoms were observed via their Lyman-a vacuum ultraviolet (vacuum UV) resonance absorption. The temperature dependences of the reactions as well as the question of the reaction mechanism will be addressed in a later publication.
Experimental Section The reactions were investigated at room temperature in an isothermal discharge flow system coupled to an LMR spectrometer with an additional vacuum UV resonance absorption device. A detailed description of the setup has been given earlier.9 The flow reactor consisted of a Pyrex tube of 70 cm length and 4 cm internal diameter. It was internally coated with a thin film of Teflon (DuPont FEP 856-200). The system was equipped with a 1.2cm-i.d. moveable probe with a microwave discharge for the generation of atoms. A 15-cm-long photolysis cell was connected to the main flow tube at its upper end for production of radicals by photodissociation of a suitable precursor with an exciplex laser. The photolysis radical source and its use in connection with the flow tube technique have been described in detail earlier.IO The exciplex laser (Lambda Physik) was operated at a repetition rate of 5 Hz such that the residence time of the gas within the photolysis cell was short compared to the time between two laser pulses. Thus, the gas flow swept a pulse of radicals through the flow tube and the adjacent detection volume. The length of the radical pulse was typically a few tens of milliseconds, determined mainly by the flow velocity in the photolysis cell relative to the main tube flow velocity. The radicals were detected with an optically pumped LMR spe~trometer,~ which operates in the wavelength region between 40 pm IX I1250 pm. The LMR sample region was located intracavity between the pole caps of a 15-in. electromagnet (Bruker). The magnetic field was modulated at a frequency of 8 kHz. However, instead of slowly sweeping the main field over the spectroscopic transition to be detected, for use with the pulsed radical source the field was set constant at the maximum of the first derivative line shape. The periodic radical pulse gave rise to a periodic absorption of the far-IR laser radiation. The absorption signals were detected (9) Bohland, T.; Temps, F.; Wagner, H. Gg. 2.Phys. Chem., Neue Folge
(6) Lohr, R.; Roth, P. Ber. Bunsen-Ges. Phys. Chem. 1981, 85, 153. (7) Frank, P.; Bhaskaran, K. A,; Just, T. J . Phys. Chem. 1986, 90, 2226. (8) Bohland, T.; Temps, F. Ber. Bunsen-Ges. Phys. Chem. 1984, 88, 459.
0022-3654/87/2091-1205$01.50/0
1984, 142, 129. ( I O ) Bohland, T.;Temps, F.; Wagner, H. Gg. Ber. Bunsen-Ges. Phys. Chem. 1985, 89, 1013.
0 1987 American Chemical Society
1206 The Journal of Physical Chemistry, Vol. 91, No. 5, 1987
with a Ge bolometer (Infrared Laboratories) connected to a lock-in amplifier with a time constant between 3 and 10 ms. Under these conditions the lock-in output signal followed the temporal shape of the radical pulse. The area under the output signal pulse and, under constant flow conditions, the peak height are proportional to the radical concentration. To improve the signal-to-noise ratio (S/N) the lock-in output was fed to a signal averager (Tracor Northern) and accumulated over typically a few hundred laser shots. The vacuum UV resonance absorption cell for the detection of atomic species was located 8-cm downstream from the LMR sample region. Lyman-a resonance radiation was generated in a microwave discharge burning in H e (99.9996%, MesserGriesheim) with H2 present at the low impurity levels as provided by the manufacturer. The cell was equipped with a MgF, entrance window. A 50-cm focal length lens collected the radiation and focused it onto the entrance slit of a 0.2-m monochromator (Acton Research). Detection was performed with a solar blind photomultiplier (EM1 G-26 E314). In order to improve the S/N ratio the discharge was chopped with a 50% duty cycle at a frequency of 400 H z and signals were processed in a lock-in amplifier. All gases used were of the highest commercially available purities: He, 99.9999%; Ar, 99.9999%; H2, 99.999% (1% premixed in He); D,, 99.7% (1% premixed in He), all Messer-Griesheim. They were regulated with calibrated mass flow controllers (Tylan). C H 2 C 0 was obtained by pyrolysis of (CH,),CO and purified up to =99% by trap-to-trap distillations. The C H 2 C 0 flow was determined from the pressure rise in a calibrated volume. In all cases H e served as the main carrier gas. The pressure was measured directly within the LMR sample region with a pressure transducer (MKS). The usual corrections for viscous pressure drop along the reaction distance were applied. A small flow of Ar was added to the far-IR laser side arms to prevent loss of radicals and atoms by diffusion into the side arms and subsequent recombination.
Results Generation of Radicals and Determination of Absolute Concentrations. C H 2 radicals were generated in the side arm at the upper end of the reactor by exciplex laser photodissociation of CH2CO CH2CO
+ hv
-+
CH2
+ CO
Bohland et al. assuming a quantum yield of a(308 nm) = 1. The detection limit for C H 2 obtained by using the rotational transition at X = 158 pm in *-polarization, Bo = 0.323 TI5 after averaging over 4000 photolysis laser pulses at S/N = 1, was about 3 X IO8 radicals/cm3. H and D atoms were produced within the moveable probe in a microwave discharge of traces of H 2 or D2 in He ([H2],[D2]/[He] i lo4). Their absolute concentrations were determined via their Lyman-a resonance absorption which was calibrated by using the titration reactions H , D + NO2. The Lambert-Beer law was found to be fulfilled up to concentrations of [HI, [D] i 3 X lo-], mol/cm3, before line reversal became significant. The accuracy was estimated to be f20%. The detection sensitivities for S/N = l at a 4-s time constant were found to be 5 X lo9 and 3 X 1Olo atoms/cm3 for H and D, respectively. The concentrations of other atomic species, e.g. 0 atoms, were below the detection limits of the spectrometer ( 1Olo atoms/cm3 for 0 ) . Rate Constants for Reactions 1 and 2. The reaction system to be considered for the determination of the rate constants for reactions 1 and 2 is analogous to the one discussed previously.s In the absence of H or D atoms the fate of the CH, radicals is primarily determined by the heterogeneous reaction
+ HC2O
+
CH,
+ CO
(4)
could be found. Absolute CH2 concentrations were estimated from C H 2 C 0 photodissociation at X = 308 nm by using the known value for the absorption coefficient of CH,CO at this wavelengthI4 and (1 1) Ashfold, M. N. R.; Fullstone, M. A.; Hancock, G.; Ketley, G . W. Chem. Phys. 1981, 55, 245. (12) Langford, A. 0.;Petek, H.; Moore, C. B. J . Chem. Phys. 1983, 78, 6650. (13) Okabe, H. Photochemistry of Small Molecules; Wiley: New York, 1978.
(5)
+
CH2
+ CH2
-
-
C2H2
+ 2H
with k6 = 3.2 X lOI3 cm3/(mol s)I6 cannot play a significant role at low initial radical concentrations of [CH,], I 2 X lo-', mol/cm3. The reactions of CHI with undissociated CH,CO, H2, or D2 are negligible because of their high activation energies.I7 In the presence of a large excess of H or D atoms reactions 1 and 2 dominate the fate of the CH2 radicals. The following channels are possible: CH2
+H
---*
-
CH
+ H2
(la)
M
CH,
CH2
+
H
products
The wall rate constant was measured under similar conditions as in the present work by using the radical source 0 CH,CO CH2 + C 0 2 at low initial radical concentrations of [CH,], < 5 X mol/cm3 to be k , = 10-15 SKI. The mutual combination
(3)
The photolysis wavelength chosen was X = 193 nm rather than the more common 308 nm because of the higher absorption coefficient permitting the use of lower CH,CO concentrations. C H 2 C O are the major products with a qu_antum yield close to unity.I3 At X = 193 nm ig addition to the X ,B, ground state of C H 2 both the 5 'A, and b 'B, states may also be populated. However, in the presence of a large excess of the inert carrier gas He ([He]/[CH,CO] 1 2 . 5 X lo4 and [He] 2 5 X 10-8mol/cm3) any singlet state CH2 radicals are rapidly quenched within At
