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10331
J. Phys. Chem. 1992,96, 10331-10334
Rate Constant for the Tunmellng Reactlon H2 4- D Mixture at 4 K
H 4- HD in the Solid D2-H2
Susumu Illtamura, Hiroyuki Morikita, and Kenji Fueki
Tetsuo Miya**
Department of Applied Chemistry, Faculty of Engineering, Nagoya University, Furo-cho, Chikusa-ku, Nagoya 464-01, Japan (Received: July 17, 1992)
Amounts of D and H atoms, produced by y-radiolysis of D2-n-H2(1 mol %) mixtures, were measured at 4.2 or 1.9 K by ESR. The amounts of D atoms decay upon storage of the irradiated sample at 4.2 K up to 114 h, while those of H atoms are nearly constant. The results were interpreted in terms of competition of a tunneling reaction n-H2 D H HD with combination reactions of D and H atoms. Similar decay behaviors of D and H atoms were also observed in D2-pH2(1 mol 96) mixtures at 4.2 K and D2-n-Hz(1 mol %) mixtures at 4.5 K. The rate constant for the tunneling reaction n-H2+ D + H HD was estimated from a kinetic treatment of the decay behavior. It was found that the rate constant is expressed by two ranges of values: 2.7 X 10-'-4.9 X 10-' cm3mol-' S-I and 3.9 X 10-4-2.6 X lC3cm3mol-' s-'. The different values of the rate anstant arc related probably to internal rearrangementsof reactive spbcies in solid hydrogen at ultralow temperature. The experimental rate constant for the tunneling reaction n-Hz + D H + HD was compared with the theoretical ones reported previously.
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Introduction The role of quantum mechanical tunneling in a hydrogen atom-molecule reaction, which can be considered as a prototype bimolecular reaction,has been important in the theory of chemical kinetics. Since almost all thermally-activated processes are suppresed at ultralow temperaturea near 4.2 K,only the tunneling reaction can be revealed at these temperatures. Miyazaki and his group haw studid in detail the tunneling reactions of hydrogen atoms in solid hydrogen.' The following results were obtained by them. 1. A large isotope effect, i.e., k(D+H2)/k(H+D2) > 4 X 104, was observed in the tunneling reactions of D(H) atoms in D2-H2 mixtures at 4.2 K. 2. Direct evidence was obtained for the tunneling reaction HD D H D2 at 4.2 and 1.9 K. 3. The absolute rate constants were measured at 4.2 K for the tunneling reactions HD D--+ H D2 and Hz + H -L H + H2 4. Quantum diffusion of H atoms in solid H2was interpreted in tams of a new mechanism that H atoms migrate by repetition of the tunneling reaction H2 + H H + Ha. 5. The rate constant for the tunneling reaction pH2(J=O) + H H pH2(J=O) is about 3 times larger than that for the tunneling reaction 0-H2(J=1) H H + H2(J=0,1).2 6. The a n a l p of ESR spin-flip lines3*'and ESR line widthss indicate that H atoms in solid H2 are trapped in substitutional sites, while H and D atoms in solid HD and D2 are trapped in interstitial octahedral sites. The above experimental results have stimulated theoretical studies on tunneling reactions H2(HD) H(D) at ultralow temperatures. 1. The absolute rate constantswere calculated for the tunneling reactions H2(HD) + H(D).6.7 2. Theoretical studies**9of the effect of rotational quantum states (J = 0, 1) on the tunneling reaction H2 + H H + H2 explained qualitatively the experimental resultsS2 The experimental values' of rate constants for the tunneling reactions HD + D .-,H + D2and H2 H H + H2 are similar to the theoretical values!*7 Though a rate constant for the tunneling reaction H2 + D H + HD was calculated theOreticall~P9~ the rate constant for this reaction was not measured previously. In this study the rate constant for this reaction will be obtained experimentally and compared with the theoretical value. Recently it was painted out that chemical reactionsin the solid phase cannot be represented by a simple homogeneous kinetics, but by a timedependent rate constant.I0 Since the reaction of hydrogen atoms in solid hydrogen is the simplest reaction system
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To whom correrpondence should be e d d r d .
0022-3654/92/2096-1033 1 $03.00/0
in the solid state, the analysis of the mechanism of the tunneling reactions in solid hydrogen will reveal the characteristics of reactions in the solid phase.
Experimental Section Normal hydrogen (n-H,) and D2 were more than 99.999 and 99.5 mol % pure, resp6ctively. n-H2consists of 75%orthohydrogen (eHJ and 25%parahydrogen @HJ. Neat pH2 was synthesized by passing normal hydrogen gas through a column of 13X m0lecular sieve at 14 K.2 The synthesized p H 2 contains 0-H2 at a concentration less than 5 mol 96. Hydrogen was sealed in a fused quartz sample tube and solidified by rapid oooling of the sample tube from room temperature to 4.2 K. The sample was irradiated at 4.2 or 1.9 K with y-rays from a 6oco source to a total dose of about 1.5 my. The H and D atoms, produced by radiolysis, were measured at 4.2 or 1.9 K by a JES-FE2XG ESR spectrometer at a low microwave power level that docs not result in saturation of the signals of the hydrosen atoms. The amounts of the hydrogen atoms were obtained by double integration of signals with a digitizer-personal computer system. The errors in the estimate of the amounts are about 10%. In this study, we have evacuated the vacuum part of the cryostat during storage of the sample at 4.2 K and could prolong the time of measurement up to 114 h. The temperature of 4.5 K was obtained by illuminating the sample in the liquid helium with an IR lamp. The temperature of the sample was measured by a thin Au-Chromel thermocouple (0.012 cm in diameter) inserted into solid hydrogen. When the IR illumination was cut off, the temperature of the sample was cooled quickly to 4.2 K in several seconds. The temperature of 1.9 K was obtained by pumping the helium vapor in a cryostat with a rotary pump." RfSdtS When a D2-n-H2(1 mol 96) mixture is irradiated with y-rays, D and H atoms are produced and they can be measured by ESR. Figure 1 shows the relative yields of D atoms, depicted by squares, and H atoms, depictcd by circles, upon storage of the irradiated mixtures at 4.2 K. The values are the mean values of two runs. The initial concentrations of D and H atoms are 2.0 X lo-' and 1.2 X lW7 mol respectively, which were determined by use of the G value (5.4) of D and H yields in the radiolysis of the D2-n-H2(1 mol 96) mixtures at 4.2 K.I2 Figure 2 shows the relative yields of D atoms, depicted by squares, and H atoms, depicted by circles, upon storage of the y-irradiated D2-pH2(1 mol 4%) mixtures at 4.2 K. The initial and 1.3 X concentrations of D and H atoms are 2.3 X mol ~ m - respectively. ~, Figure 3 shows the relative yields of D atoms, depicted by squares, and H atoms, depicted by circles, upon storage of the (8
1992 American Chemical Society
10332 The Journal of Physical Chemistry, Vol. 96, No. 25, 1992
Miyazaki et al.
TABLE I: Relative Yields of H At-
in Total Ykldr d H a d D Atom after ~-1mdirti0a O f DZ-WH~(1 mol%) Ln
'O!-----7
[Hl/([Hl + [Dl) 4.2K 1.9 K 0.39 0.32 0.36 0.33 0.01 3-0.024
sample A sample B simuld valueo
oValucs are simulated under the condition that kT = 3.9 X 104-2.6
x io-' cm3 mol-' s-l. ~
ob
2s
50 7'5
100
Storage time I hr Figure 1. Effect of storage of 7-irradiated D2-n-H2(1 mol 4%) at 4.2 K. The initial concentrations of D and H atoms are 2.0 X and 1.2 X lW7mol respectively. (0) D atoms; (0)H atoms; (---) simulated yields of D atoms (see text); (-) simulated yields of H atoms (see text).
storage of the yirradiated D2-n-H2(1 mol 96) mixtures at 4.2 K. The amounts of the D atoms ( 0 )decrease gradually, while those of the H atoms (0)are nearly constant. All possible elementary reactions are represented as follows. D2, H2 *- D, H (1) D+D+D2
(2)
H+H+Hz
(3)
H+D-,HD H2 + D 4 H + HD D2
'0
w 10 20 30 40 50 62.6 Storage time / hr
F'igure 2. Effect of storage of y-irradiated D2-pH2( 1 mol %) at 4.2 K. The initial concentrations of D and H atoms are 2.3 X lW7and 1.3 X lW7mol respectively. (0) D atoms; (0)H atoms; (-- -) simulated yields of D atoms (see text); (-) simulated yields of H atoms (see text).
+H
4
(4)
(5)
D + HD
(6)
D atoms, produced by radiolysis (reaction l), migrate through solid deuterium and combine with other D atoms (reaction 2) or H atoms (reaction 4). Similarly H atoms combine with other atoms (reactions 3 and 4). If only these combination reactions take place at 4.2 K, both H and D atoms should decrease during the storage of the sample. The constant yields of H atoms indicate that a formation process of H atoms competes with the combination reactions. H atoms can be produced by a tunneling reaction D H2 D H + HD (reaction 5 ) . The reaction D2 H HD (reaction a), however, cannot occur at ultralow temperature because of its endothermic process. Then, the amounts of D and H atoms are expressed by the following two equations.
+
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+
d[DI -- -2k2[DI2 - k4[Hl [Dl - kT[H21 [Dl
(7)
-d[HI dt - -%[HI2 - k4[Hl [Dl + kT[H21 [D]
(8)
dr
u
OO
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y-irradiatedD2-n-H2(1 mol 96) mixtures at 4.5 K, the temperature of which was produced by IR illumination of the sample. 7-hradiation of the sample and ESR measurement were done at 4.2 K. The values are the mean values of three runs. The initial concentrations of D and H atoms are 1.8 X lo-' and 6.0 X mol ~ m - respectively. ~, Table I shows relative yields of H atoms to total yields of H and D atoms, [H]/([H] + [D]), after y-irradiation of D2-n-H2(1 mol %) mixtures at 4.2 and 1.9 K. D&plwioa
where k2, k3, k4, and kT are the rate constants for the reactions 2,3,4, and 5, respectively. Since H (D) atoms combine with other H (D) atoms without any activation energy, it is assumed here that they combine with other atoms at every encounter. The combination reaction can be regarded as a diffusion-controlled reaction. Then,
5 10 15 Storage time I min
Figure 3. Effect of storage of y-irradiated D2-n-H2(1 mol %) at 4.5 K. The initial concentrations of D and H atoms are 1.8 X lo-' and 6.0 X lo"mol an-),r&pectively. ( 0 )D atoms; (0)H atoms; (- -) simulated yields of D atoms (see text); (-) simulated yields of H atoms (see text). ESR spcctra of D and H atoms were measured at 4.2 K.
mb ChtBllrtfOrtbeR d Hz + D+ H + HD. Figure 1 shows the relative yields of the D and H atoms upon
+
4 r r ( D ~+ OD)
(9)
k3 = ~ X ~ ( D+HDH)
(10)
k4 = k r ( D + ~ DD)
(1 1)
k2
where D D and DHare diffusion coefficients of D and H atoms, respectively, and r is the contact distance between two atoms in a combination reaction. If kDand kH are denoted here as 4 r r b and 4.rrDH,respectively, q s 7 and 8 are represented by
-d[D1 = dt
~ ~ D [ -D(kHI +~ k ~ ) [ H[Dl l - k~[Hzl[Dl
(12)
d[H1 (13) ~ ~ H [ -H(kHI +~ kd[HI [Dl + k~[Hzl[Dl dt Initial concentrations of D atoms ([DIo) and H atoms ([Hlo) at the beginning of the storage were determined by ESR measurement. If kb kH,and kTam given, the amounts of D and H atoms can be obtained by integration of the two simultaneous differential equations (12) and (13) with a computer. Now, we will discuss the three typical cases. -I
Tunneling Reaction H2
+D
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H
+ HD
The Journal of Physical Chemistry, Vol. 96, No. 25, 1992 10333
TABLE Ik CompvisOn of Rate Constants for Tun~~eling Reactions at 4.2 K
k. cm3 mol-' s-I theor
Sato et al. reaction D H + HD H H HzC
+ + HD + D
n-H, n-H,
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+
+
+ H + Dzc
exptl 0.27-0.49d 26 21 1.9 x 10-3 2.3 x 10-3
10
Truhlar et al.' I
I1 7.6 X 1V2 3.9 x 10-2
3.9 x 10-2
4.4 x 10-4
116
1.2 0.46
2.4 x 10-3 1.4 X
2.7 x 10-3
1.0 x 10-3
18 13
'Quoted from ref 6b. 6Quoted from ref 6a. CQuotedfrom ref 7. dThe values were obtained at 4.5 K. 'Quoted from ref 2.
Case 1 : D and H atoms diffuse through D2 solid by the same diffusion coefficient; that is, kD = kH. Case 2 D atoms can diffuse through Dz solid, whereas H atoms cannot diffuse; that is, kD> kH= 0. In this case, D atoms can migrate by repetition of a tunneling reaction D2 + D D + DZ, while H atoms cannot migrate through the D2 solid. The possibility for this case was discussed previously in detail." Case 3: H atoms diffuse faster than D atoms, that is, kH > kD. Since D atoms decay in pure D2solid,I3 a diffusion coefficient of D atoms is not zero. For instance, we take the following rate constants: kD = 1.7 cm3 mol-I s-', kH = 0 (case 2), and kT = 5.9 X lo4 cm3 mol-' s-', where the initial concentrations of D and H atoms are 2.0 X and 1.2 X mol ~ m - respectively, ~, in Figure 1. Then, the amounts of D and H atoms, calculated from eqs 12 and 13, are shown by a dashed line and a solid line, respectively. These lines coincide with the experimental values. We can estimate the possible range of kTby fitting the simulated lines to the experimental yields of D and H atoms within experimental errors. In case 1 where kD = kH,kTis ranged from 1.2 x lo-' to 1.8 x cm3mo1-l s-'. In case 2 where kD> kH, kTis ranged from 3.9 X 10-4to 7.3 X 10" cm3mol-' s-I. In case 3 where k H > km kT Values for kH >> l m k are ~ V ' h d y the same as those for kH look,. Thus, kT Values were determined in the range of kHwhere kD C kHC lookD. kT is ranged from 2.1 X 1 0 - ~ to 2.6 x IC3om3 mol-' e'. In conclusion, kTis ranged from 3.9 X 10" to 2.6 X cm3 mol-' s-l in all three cases. Two Rate Constants for the Tunneling Reaction H2+ D H HD. When D2-n-H2(1 mol %) is irradiated with y-rays at 4.2 K, H atoms are produced preferentially. The relative yields of H atoms ([H]/([H] + [D])) amount to 0.36-0.39, which exceeds a mole fraction (0.01) of H2 in the sample (cf. Table I). The preferential formation of H atoms was also observed by the photolysis of Dz-n-H2-HI(0.05 mol %), where ultraviolet light can be absorbed only by hydrogen iodide.I4 Thus, it was concluded previously that the preferential formation of H atoms in the radiolysis of D2-n-Hz mixture8 is not due to the excitation, charge, and proton transfers from D2 to H2, but to a tunneling reaction H2 + D H + HD (reaction 5 ) of D atoms.14 In the previous section, the rate constant for the tunneling reaction 5 was estimated as 3.9 X 10-4-2.6 X l C 3 cm3mol-' s-I. Since the time of y-irradiation is 2 h, the yields of H atoms after the irradiation can be estimated by use of the rate constant (3.9 X lO"2.6 X lo-' cm3mol-' s-I) for reaction 5 , shown in Table I. The simulated yields (0.013-0.024) of H atoms are much smaller than the experimental yields (0.36-0.39). The large initial yields of H atoms suggest that H atoms are produced by a fast tunneling reaction H2 + D H + HD during y-irradiation, which was not taken into account in the above simulation. Now, we will discuss the fast tunneling reaction. First, it was reported previously that the temperature of solid hydrogen immersed in liquid helium may increase about 1.3 K during y-irradiational2 There is a possibility that the preferential formation of H atoms during y-irradiation at 4.2 K may be c a d by a slight increase of temperature of the sample. In order to examine this possibility, the D2-n-H2(1 mol 96) mixtures wcre irradiated at 1.9 K and the ESR spectrum was measured at 1.9 K. Table I shows that the selective formation of H atoms is also observed at 1.9 K. In the y-irradiation at 1.9 K, the temperature of the sample
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dots not exaxd 4.2 K during the irradiation. Thus the preferential formation of H atoms is not due to the bulk temperature increase of the sample during y-irradiation. However, the temperature increase in a local region, such as a spur, may be plausible during y-irradiation. Second, recently it was reported that the rate constant for the a tunneling reaction p H 2 H H + H2 is larger than that for the tunneling reaction 0-H2 H H + H2at 4.2 K.?Since n-H2 consists of 75%0-H2and 25%pH2, there is a possibility that the tunneling reaction p H 2 + D H + HD takes place much faster H HD does and that the fast tunneling than 0-H2 D reaction of D atoms may correspond to the reaction with pH2. Figure 2 shows the behavior of D atoms, represented by squares, and H atoms, representedby circles, in the y-irradiated D2-pH2( 1 mol %) mixtures at 4.2 K. The time dependence of the yields of D and H atoms in the D2-pH2(l mol %) mixtures is roughly similar to that in the D2-n-H2(1 mol 96) mixtures shown in Figure 1. The yields of D and H atoms can be estimated from eqs 12 and 13 by a treatment similar to that in the D,-n-H2( 1 mol 96) mixtures. The calculated yields of D atoms, represented by a dashed line, and H atoms, represented by a solid line, were ob tained under the following conditions: [DIo = 2.3 X lo" mol [HI, = 1.3 X mol anm3, kD= 2.6 cm3mol-' s-', kH= 0,and kT = 5.8 x lo4 Cm3 mol-' S-'. In all cast8 where kH < kD, kH = kD, or kH > kD, kT in the D~-PHZ( 1 mol 96) mixtures is w e d from 4.4 X 10" to 4.1 X cm3 mol-' s-I, which is the same order of magnitude as that (3.9 X 10-4-2.6 X lW3cm3mol-' s-I) in the D2-n-H2(1 mol 9%) mixtures. Therefore, the rate constant for the tunneling reaction p H 2 D H + HD is similar to that for the tunneling reaction 0-H2 D H + HD, though the rates of these two reactions cannot be compared exactly because of ambiguity of the simulation. Since a preferential formation of H atoms is also observed in the y-irradiation of the D2-pH2( 1 mol %) mixtures, the fast reaction p H 2 D H HD may occur also in this mixtures. Third, when the D2-n-H2( 1 mol 96) mixture is irradiated with y-rays at 4.2 K and then the irradiated mixture is stored at 4.5 K, the amounts of D atoms, depicted by squares, and H atoms, depicted by circles, are shown in Figure 3. The yields of D atoms decrease upon the storage at 15 min, while those of H atoms increase slightly. The results can be explained by reactions 1-5, namely, eqs 12 and 13. The yields of H atoms increase by reaction 5, which competes with combination reactions (reactions 2-4). The amounts of D and H atoms are estimated from eqs 12 and 13 under the following conditions: [D], = 1.8 X lW7 mol ~ m - ~ , [HI, 6.0 X lo-' mol ~ m - kD ~ , kH 46 cm3 mol-' S-', and kT = 3.7 X 10-' cm3mol-' s-'. The estimated yields of D atoms, represented by a dashed line, and H atoms, represented by a solid line, agree qualitatively with the experimental values (cf. Figure 3).15 h Where kH < km kH = kb OT k~ > k ~ k, ~ ranged k from 2.7 X 10-1to 4.9 X 10-' cm3mol-' s-I, which is much larger than the value (3.9 X 10-4-2.6 X lV3 cm3mol-' s-') obtained by the storage of the irradiated samples at 4.2 K. The large value of kT obtained at 4.5 K may correspond to the rate constant for the fast tunneling reaction n-H2 + D H + HD during y-irradiation. computsollfo Rate constant8for the TuImebg Reaction 0-H2+ D H + HD witb Theoretical V i l w a It was concluded in the previous section that the tunneling reaction n-H2
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J. Phys. Chem. 1992,96, 10334-10339
10334
+ D H HD consists of a fast reaction and a slow one. The fast reaction occurs during y-irradiation at 4.2 K or at 4.5 K after the irradiation with the rate constant of 2.7 X IO-'-4.9 X lo-' cm3 mol-' s-I. A slower reaction is observed at 4.2 K after the irradiation with the rate constants of 3.9 X 104-2.6 X lO-' cm3 mol-' s-I. A physical meaning of two types of reactions is not clarified as yet. A plausible explanation is given as follows. The local motion of H2 molecules in solid D2 may be somewhat more fret at 4.5 K or during y-irradiation. Thus a D atom and an H2 molecule can take a mutual orientation suitable for the reaction, resulting in the fast tunneling reaction. At 4.2 K without irradiation, however, the motion of an H2 molecule is restricted and thus the tunneling reaction takes place slowly. It is well-known that the rate constant for reaction in the solid phase at low temperature is not expressed by a single value when the rate of internal rearrangements of reactive species competes with the rate of reaction.I0 Since the theoretical calculation of the rate constant for the tunneling reaction n-H2 + D H + HD is based upon the model that a H2molecule and a D atom take an orientation suitable for the reaction,the larger value of the two experimental rate constants derived here should be compared with the theoretical rate canstant. In Table 11, the experimental rate constant for the tunneling reaction n-H2 D H HD is compared with those calculated by others.69' The rate constants for other tunneling reactions reported previously are also shown there. The experimental rate constants for the three types of tunneling reactions agree qualitatively with theoretical values, if we take into consideration the following situations. First, the theoretical rate constants were calculated for tunneling reactions for the gas phase, while the experimental values were obtained in the solid phase. Second, the theoretical rate constant for a tunneling reaction at ultralow temperature is affected significantly by a slight change of a potential-energy surface at very low energies. A potential-energy surface for a hydrogen atom-molecule reaction in the range of very low energies is slightly ambiguous. Third, though the experimental rate constant for the tunneling reaction HD D H + D2was obtaiied directly from the experimental observations, the rate constants for the tunneling reactions n-H2 + D H +
-
+
-+
+
-
-
-
HD and n-H2 + H H + H2weft obtained based on the model that the tunneling reactions compctcs with combination reactions of H (D) atoms. In summary, the rate constant for the tunneling reaction n-H2 + D H + HD at an ultralow temperature has been estimated for the first time from experimental results. The rate constant is expressed by two ranges of values: 2.7 X 10-'-4.9 X IO-' cm3 mol-' s-' and 3.9 X lW-2.6 X lo-' cm3 mol-' PI, which will be related probably to rearrangement of reactive s p i e s in the solid hydrogen at an ultralow temperature.
-
Acknowledgment. This work was supported in part by a Grant-in-Aid for ScientificResearch from the Japanese Ministry of Education, Science and Culture. RWhy No. H2, 1333-74-0; atomic D, 16873-17-9.
References md Notes (1) Miyazaki, T. Radial. Phys. Chem. 1991,37,635, and related papers cited therein. (2) Miyazaki, T.; Hiraku. T.; Fueki, K.; Tsuchihashi, Y . J. Phys. Chem. 1991, 95, 26. (3) Miyazaki, T.; Iwata, N.; Fueki, K.; Hase, H. J. Phys. Chem. 1990,91, 1702. (4) Miyazaki, T. Chem. Phys. Left. 1991, 176,99. ( 5 ) Miyazaki, T.; Morikita, H.; Fueki. K.; Hiraku, T. Chem. Phys. h i ? . 1991, 182, 35. (6) (a) Takayanagi, T.; Masaki, N.; Nakamura, K.;Okamoto, M.; Sato, S.;Schatz, G. C. J. Chem. Phys. 1987,86,6133. (b) Takayanagi, T.; Sato, S.J. Chem. Phys. 1990, 92, 2862. (7) Hancock, G. C.; Mead, C. A.; Truhlar, D. G.; Varandas, A. J. C. J. Chem. Phys. 1989, 91, 3492. (8) Takayanagi, T.; Masaki, N.J. Chem. Phys. 1991,95,4154. (9) Bowman, J. M. J. Phys. Chem. 1991, 95,4921. (10) Plonka, A. Pmg. Rem?.Kine?. 1991,16, 157, and related papers cited
therein.
(11) Miyazaki, T.; Lee, K. P.; Fueki, K.;Takeuchi, A. J. Phys. Chem. 1984, 88, 4959. (12) Miyazaki, T.; Kato, M.; Fueki, K. Radia?.Phys. Chem.1990,36,501. (13) Miyazaki, T.; Iwata, N.; Lee, K. P.; Fueki. K. J. Phys. Chem. 1989, 93. 3352. (14) Miyazaki, T. Bull. Chem. Soc. Jpn. 1985, 58, 2413. (15) A slight discrepancy at storage time less than 5 min may indicate the
presence of another faster tunneling reaction H2+ D
-
H
+ HD.
Klnetlcs of Pyrolysis of a Coal Model Compound, 2-Plcdlne, the Nltrogen Heteroaromatlc An8bgue of Toluene. 1. Product Dlstrlbutlons Andrew Terentis, Alan Doughty, and John C. Mackie* Department of Physical and Theoretical Chemistry, University of Sydney, NS W 2006, Australia (Received: July 29, 1992; In Final Form: September 10, 1992) The pyrolysis of 2-picoline in dilute mixtures with argon has been investigated using the single-pulse shock tube and was found to decompose over the temperature range 1300-1550 K,at an average residence time of 800 p.s and uniform pressure of 14-16 atm. The major products observed w m acetylene, methane, hydrogen, HCN, and cyanoacetylene. Over the studied range of mixture compitions (0.06-0.20 mol 4% of 2-picoliie) the overall rate of disappearanceof 2-picdine obeyed first-ordex s-' and E,& = 98 f 7 kcal kinetics. Arrhenius parameters for disappearance of picoline were found to be Adb = 1017.4*'.1 mol-'. From the distribution of observed products it is concluded that the principal initiation reactions were analogous to those known to occur in toluene, the hydrocarbon analogue of 2-picoline, and were found to be. C-C bond fission to yield the o-pyridyl and methyl radicals and C-H fission to yield H atoms and 2-picoly1, the N-containing analogue of benzyl. Major products were observed from decomposition of both the o-pyridyl and the 2-picolyl radicals. Cyanoacetylene arises principally from secondary reactions of o-pyridyl. A product with m l z = 91 was observed at the lowest temperatures at which 2-picoline decomposition could be detected. It has been identified as 1-cyanocyclopentadieneand arises from loss of H from the 2-picolyl radical. Other products arising from secondary decomposition of 2-picolyl at higher temperatures include HCN and cyclopentadienyl radicals. Introduction
The evolution of NO, from the combustion of coal and heavy fuels is a very complex proass, with the yield of NO, depending both on the combustion conditions and the origin of the coal.'-' The principal source of NO, in the combustion of these fuels is 0022-3654/92/2096- 10334t03.OO/O
fuel-bound nitrogen (FBN). The conversion of FBN to NO, is thought to occur through pyrolysis of volatilized FBN to yield NO, precursors, which then react with oxygen to yield NO,. This study of the pyrolysis of 2-picoline is part of an attempt to understand the mechanism of evolution of NO, precursors 6 1992 American Chemical Society
