J . Phys. Chem. 1990, 94, 4946-4951
4946
Kinetic Studies of the Reactions of H,CN and D,CN Radicals with N and H Fred L. Nesbitt,+ George Marston,* and Louis J. Stief* Astrochemistry Branch, Laboratory for Extraterrestrial Physics, NASAIGoddard Space Flight Center. Greenbelt, Maryland 20771 (Received: November 2, 1989; In Final Form: February 8 , 1990)
Absolute rate constants for the reactions of H2CN and D2CN with N atoms have been measured by using the discharge flow-mass spectrometry technique between 200 and 363 K. No isotope effect was observed. The results are as follows (IO-" cm3 s-l): kl(200 K) = 3.9 f 2.2, k1(298 K) = 4.4 k 1.4, kl(363 K) = 6.7 k 2.0. Quoted uncertainties reflect statistical errors at the 95% confidence level. The reactions of D2CN with H and with D atoms were also studied. Analysis of the data from these experiments was difficult, but a lower limit for the rate constant for this reaction of 7 X IO-" cm3 s-I was obtained at 298 K, no isotope effect being observed. Ignoring isotopic variations, the products are represented by NH + HCN and H2 + HCN for reactions with N and H, respectively. This study represents the first determination of the rate constants for these reactions. The significance of these processes in models of the atmospheres of Titan and Jupiter and in the chemistry of circumstellar and interstellar clouds is discussed; their role in laboratory experiments on active nitrogen/hydrocarbon reactions is also considered.
Introduction
The kinetics and mechanism of the elementary reactions of the H2CN radical are of considerable interest due to their possible role in such diverse systems as the atmospheres of Titan' and Jupiter,2 H C N formation in 0-rich circumstellar clouds3 and interstellar clouds! and active nitrogen/hydrocarbon Extremely limited information is available on H2CN kinetics; only H2CN and H2CN + NO have been the reactions H2CN studied.8 I n particular, information is needed for the reactions
+
N
+ H2CN
-
products
(1)
H
+ H2CN
-
products
(2)
and
in order to model the chemistry of these complex systems. In previous publicationsg-" we reported the temperature dependence of the rate constant and branching ratios for the reaction
N
+ CH3
-
-+
HCN
+ H2
+ 2H H,CN + H
HCN
(3a) (3b) (3c)
We demonstrated that the reaction is very rapid (k3 = 8.5 X lo-" cm3 s-l at 298 K)9-10 and that the principal channel is that leading to formation of H2CN." In the present experiments we used this rapid reaction to generate the H2CN radical. Absolute rate constants for reactions 1 and 2 or their isotopic variants were measured at 200-363 and 298 K, respectively, by using the discharge flow-mass spectrometry technique at I-Torr total pressure. The present study represents the first determination of these rate constants. Experimental Section
All experiments were performed in a Pyrex flow tube -60 cm long and 28 mm in diameter. The flow tube was coupled via a two-stage stainless steel collision-free sampling system to a quadrupole mass spectrometer (Extranuclear Laboratories Inc.) that was operated at low electron energies ( e 2 0 eV) to minimize fragmentation. Ions were detected by an off-axis channeltron multiplier (Galileo Electro Optics Corp.). The flow tube has a Pyrex movable injector for the introduction of one of the reactant gases. A more thorough description of the apparatus has been presented previously.'* Helium carrier gas was flowed at between 700 and 850 pmol s-I into the reaction vessel which was maintained at I-Torr total 'Research Associate, Chemistry Department, The Catholic University of America, Washington, DC 20064. *NAS/NRC Postdoctoral Research Associate. Present address: Physical Chemistry Laboratory, Oxford University, South Parks Road, Oxford 0x1 342,United Kingdom.
0022-3654/90/2094-4946$02.50/0
pressure. The linear flow velocity ranged from 1600 to 2700 cm s-l. Gas flows were measured and controlled by electronic flow meters. The flow tube was cooled by pumping ethyl alcohol that had been chilled in a low-temperature bath/circulator (Neslab Instruments, Inc., Model ULT-80) through a jacket surrounding the flow tube. Alternatively, the flow tube was heated by pumping silicone oil (Aldrich Chemical Co., Inc.) that had been heated in a high-temperature bath/circulator (Neslab Instruments, Inc., Model EXACAL EX-250 HT). The temperature was varied from 200 to 363 K. Side arms, at the upstream end of the flow tube, contained microwave discharges (70 W, 2450 MHz) for the production of atomic nitrogen and atomic fluorine. Nitrogen atoms were generated in the microwave discharge of dilute mixtures of N 2 (]-IO%) in helium. A recombination volume (440 cm3) with a minimum residence time of 60 ms was placed after the nitrogen discharge to reduce the amount of excited N2p r e ~ e n t . ~Fluorine ,'~ atoms were generated in a microwave discharge of dilute mixtures of CF4 (0.1%) in helium. A similar recombination volume (440 cm3) was placed after the F atom discharge to reduce the amount of CF3 and CF, reaching the flow t ~ b e . ~ The * ' ~F atom discharge tube was uncoated alumina that was coupled via Teflon unions to Pyrex tubing. For the study of reaction 2, H and D atoms were generated in a microwave discharge of dilute mixtures of H2 or D2 (0.05%) in helium. The microwave discharge was mounted on the movable injector. The discharge regions, the injector, and the flow tube were left uncoated. All the glass surfaces were thoroughly cleaned and treated with dilute ( 5 % ) HF. The H2CN radical was produced by the reaction sequence F CH4 CHj HF kA(298 K) = 8.0 X IO-" cm3 s-I (ref 14) (4) CH, + N H,CN H k3(298 K ) = 8.5 X IO-" cm3 s-I (ref 9) (3c)
+
-
+
+
(1) Yung, Y. L.; Allen, M. A.; Pinto. J. P. Asrrophys. J . , Suppl. Ser. 1984, 55, 465. ( 2 ) Kaye, J . A,; Strobel, D. F. Icurus 1983, 5 4 , 4176. (3) Conrad, M . P.; Schafer, H. F. Nuture 1978, 274, 456. Nejad. L. A. M.; Millar, T. J. Mon. Nor. R. Astron. SOC.1988, 230, 79. (4) Langer, W. D.; Graedel, T. E. Astrophys. J., Suppl. Ser. 1989,69, 241, Prasad. S . S . ; Huntress, W. T. Astrophys. J., Suppi. Ser. 1980, 43, 1 ( 5 ) Safrany, D. R. Prog. Reacr. Kinet. 1971, 6, I . (6) Froben, F. W. Ber. Bunsen-Ges. Phys. Chem. 1974, 78, 1984. (7) Borrke, P. J.; Mile, B. J . Chem. Soc., Chem. Commun. 1980, 395. (8) Home, D. G.; Norrish, R. G . W. Proc. R. Sot. London 1970, A3l5.
301. (9) Stief, L. J.; Marston, G.;Nava, D. F.; Payne, W. A,; Nesbitt, F. L. Chem. Phys. Lett. 1988, 147, 570. (10) Marston, G.;Nesbitt, F. L.; Nava, D. F.; Payne, W. A.; Stief, L. J. J . Phys. Chem. 1989, 93, 5769. ( 1 1 ) Marston, G . ; Nesbitt, F. L.; Stief. L. J . J . Chem. Phys. 1989, 91, 3483. ( 12) Brunning, J.; Stief, L. J . J . Chem. Phys. 1986, 84, 4371. ( 1 3 ) Plumb, 1. C.; Ryan, K. R. Int. J . Chem. Kiner. 1982, 14, 861.
8 I990 American Chemical Society
Reactions of H 2 C N and D2CN with N and H
The Journal of Physical Chemistry, Vol. 94, No. 12, I990 4947
The radical D2CN was generated in the deuterated analogues of these reactions. Although reaction 3 is very rapid, the subsequent reaction of H,CN with N is also fast. Consequently, it is not possible to isolate the production of H2CN from its reaction. As will be discussed later, this problem complicates carrying out the experiments and also analyzing the results. A crucial part of these experiments was the determination of the absolute N atom and H atom concentrations. Nitrogen atoms were titrated by the fast reactionI4 N
+ NO
-
N2 + 0
kS(298 K) = 3.4 X lo-" cm3 s-I (5)
The nitrogen atom concentration was determined by measuring the decrease in the NO' ion peak at m/e = 30 (electron energy 13.5 eV) when the N 2 discharge was initiated. The N concen- [NO]di,,,,. As discussed tration is given by [N] = [NO]discoff previously,I0 a number of precautions were taken to avoid systematic errors in the measurements of [N]. The nitrogen atom wall loss was measured by performing the N O titration at various injector positions. The results obtained gave a wall loss of 5 8 s-1. Two methods were used to calibrate for H and D. The methods for H are briefly summarized here; the methods for D are similar and are described as methods 1 and 2 in our study of the products of the N CD3 reaction." The first method for H consisted of monitoring the decrease in the H 2 s.ignal at m / e = 2 (electron energy 18 eV) when the hydrogen discharge was initiated. The H concentration is given by [HI = 2([H21diron- [H21discon).This method measures [HI at the downstream end of the flow tube and would be subject to error if there were wall recombination of 14 to re-form H2. A second method was tried in which NO, was added through the movable injector to scavenge H atoms by the rapid reactionI5
6--
4t c
t
1
+
H
+ NO2
-
OH
+ NO
k6(298 K) = 1.4 X
cm3 s-l (6) Under these conditions, the change in H2 signal is a reflection of the H atom concentration at the injector tip, and by varying the injector position, we obtained a wall loss rate constant of about 20 s-l at 298 K from these experiments. Measurements in the presence and absence of N O 2 were in agreement when the wall loss was taken into account. The H2CN radical was monitored at m/e = 28 (electron energy 12 eV) while D2CN was monitored at m/e = 30 (electron energy 18 eV). For H,CN a lower electron energy was required in order to minimize interference from vibrationally excited ground state N,; for D2CN there was no such interference and higher electron energies could be used. Previous publications have discussed the presence of vibrationally excited N 2 in these systemsl09" and the superior detection of D,CN compared to H2CN. Radical concentrations were obtained from computer simulations. The concentration of F was obtained by monitoring the decrease in the CH4 signal at m/e = 16 (electron energy 16 eV) when the CF, discharge was initiated. The concentration of F atoms was obtained from the equation [F] = [CH4ldirOrr-[CH4Idiron. The concentration of methyl radicals generated in reaction 4 was equal to the F atom concentration. Typically [CH,], was -7 X IO" ~3117~~.
Helium (99.999%, Air Products) was dried by passage through a molecular sieve trap at 77 K before entering the flow system. CF, (99.9%, Matheson) was degassed at 77 K. N O (99%, Matheson C.P.) was purified by successive distillation at 143 K and collected at 77 K . N O 2 (99.5%, Matheson) was left to stand overnight mixed with excess 0, to oxidize lower oxides. The NO, was then trapped out at 77 K and the 0, pumped off. N, (99.999%, Scientific Gas Products UHP), CH, (99.97%, Matheson UHP), CD4 (99%, MSD Isotopes), H, (99.999%, Matheson (14) DeMore, W. B.; Molina, M. J.; Sanders, S. P.; Golden, D. M.; Hampson, R. F.; Kurylo, M. J.; Howard, C. J.; Ravishankara,A. R. Chemical Kinetics and Photochemical Data for Use in Statospheric Modeling. Evaluation No. 8, JPL Publication 87-41, 1987. (15) Michael, J . V.; Nava, D. F.; Payne, W. A,; Lee, J . H.; Stief, L.J . J . Phys. Chem. 1979, 83, 2818. and references therein.
L
TIME/ms
Figure 1 . Typical first-order logarithmic decay plots for reaction 1: (0) T = 200 K, [N] = 4.58 X 10l2~ m - (~0. ) T = 298 K,[N] = 7.70 X 10I2 cm-'.
UHP), and D2 (99.5%, MSD Isotopes) were used without further purification. Results Reaction of H2CN and D2CN with N . In these experiments, the N and F atoms were admitted at the back of the flow tube and C H 4 was added through the sliding injector. The H2CN concentration is determined mainly by the reactions N CH3 H2CN H (3)
+
H2CN
-+
+N
-
+
products
(1)
Assuming [N], >> [CH,],, this simplified mechanism can be solved exactly and yields the solution [H,CN] =
k3' [CH3Io (exp(-k,'t) - exp(-kl't)) ki' - k3'
(7)
where k,' = ki[N], and k3' = k,[N],. This model can be made more accurate if k,' and k{ include contributions for the wall losses of H,CN and CH,, respectively. In the limit when exp(-k,'t) > k,' or if k, is a little larger than k, and t is very large. As a first approximation, our data were treated in the usual first-order manner. The signal due to H2CN at m/e = 28 was monitored as a function of the distance between the tip of the sliding injector and the sampling pinhole of the mass spectrometer. The distance was converted to time by dividing by the axial velocity of the gas in the flow tube, and the natural logarithm of the H2CN signal was plotted as a function of time. The form of these plots was as expected: an initial nonlinear region, where generation of H,CN dominates over its reaction with N , was followed by a linear region, the slope of which gave the pseudo-first-order rate constant. A small correction (2-8%) was made to these rate constants for axial diffusionIoJ6to give the corrected rate constants, (16) Lewis, R. S.;Sander, S. P.; Wagner, S.;Watson, R. T. J . Phys. Chem. 1980, 84, 2009.
4948
The Journal of Physical Chemistry, Vol. 94, No. 12, 1990
TABLE I: Reactions Used To Simulate N + HzCN' reaction k(200 K ) k(298 K ) k(363 K ) ref N+CH,6.4 X 8.5 X 1.4 X IO-" I1 HCN + H2 -+HCh+2H 0 4.3 x 10-12 0 11 H,CN + H 5.8 X IO-" 7.2 X IO-'' 1.3 X IO-'' I 1 N + H2CN 3.9 X IO-" 4.4 X IO'" 6.7 X IO-" this work N H + HCN C H I + CH, C,H, 5.0 X IO-" 5.0 X IO-" 5.0 X IO-" I H+CH,2.6 X IO-" 2.6 X IO-'' 2.6 X IO-" I CH4 ( 1 Torr) N + NH N, + H 1.6 X 1.9 X 2.1 X I H2CN + wall 180 240 I20 this work
-
-+
H
products wall products
+
20
20
20
this work
"Second-orderrate constants in units of cmd s"; first-order rate constants in
units of s-].
Nesbitt et ai. TABLE 11: Rate Data for the Reaction D,CN (H,CN) K' [ N] / I Oi2 kco:/s& [N]/l0l2 I .37 130 5.83 2.39 215b 6.54 3.64 175 8.47 4.58 28 1 8.95 5.16 346b
-
of 60 f 70 s-,. The quoted uncertainties here reflect statistical errors a t the 95% confidence level. This will the case throughout unless specifically identified otherwise. Two things are apparent. First, k,' is not very much greater than kl'; the rate of formation of H2CN is about twice the rate of decay. Second, the reaction time, t , cannot be considered very long as the decays were measured on time scales of the order of the buildup period. Despite the good linearity displayed in Figures 1 and 2, it is clear that the pseudo-first-order approach is not applicable. The observed data were treated by using a numerical integration technique. A model using the reactions listed in Table I and rate constants taken from the literature13l1was used to calculate H2CN decay rate constants at a number of N atom concentrations. Three N atom concentrations, covering the range of values employed in the experiments, were used. The decay rates were calculated by using two reaction times; the two times covered the range used to obtain the linear portions of the decays in the experiments. Some simulations were carried out using a number of different times and showed that the semilog plots were linear, as observed experimentally. The decay rate constants were then compared with those expected on the basis of the least-squares best-fit line from Figure 2; they were not compared with the individual experimental decay rates. The rate constants for the reactions of H2CN with N and with the flow tube walls were varied, and the goodness of fit was determined by minimizing x2 in the expression (9)
where ke,jare the experimental pseudo-first-order rate constants and kc,iare the calculated rate constants. The agreement between the first-order rate constants obtained from the simulation and the second-order plot was better than 10%. On the basis of this analysis, the following room-temperature rate constants were estimated: k , = 4.4 X IO-" cm3 s-I and k8 = 240 s-'. The measured pseudo-first-order rate constants were substantially underestimated when compared to those expected if the rate of formation of H2CN were very fast compared to its decay. However, the extent of the underestimation is almost independent of [ N ] and k , is only slightly affected; the wall loss of H2CN is considerably higher than expected on the basis of the pseudofirst-order analysis. Two methods were used to assess the errors in the rate constants. In the first method, sensitivity coefficients showing the dependence of k, and k, on changes in the individual values of k,' used in this simulation (Le., those obtained from the second-order plot) were estimated. These coefficients were then combined with the
kco;/s-' 283 296 363b 499
"D2CN experiment except where noted. *H2CNexperiment. c k i (200 K) = (3.9 f 1.4) X IO-" cm3 s-l, intercept = 90 f 80 S-I (pseudo-first-order analysis). k,(200 K ) = (3.9 f 2.2) X IO-" cm3 s-l, k8 = 180 f 130 5-l (modeled; see text). TABLE 111: Rate Data for the Reaction H2CN (DzCN) + N at 298 K' [ N ] / 1 0l2 c ~ n ' ~ k,,,"/s'l
k,,,. These had an average uncertainty of *lo%. Experiments were also performed with the deuterated radical, D2CN. as the minor species; no isotope effect was observed. Figure 1 shows some typical plots used to obtain k,,,, and Figure 2 is a plot of k,, vs [N] at 298 K. From Figure 2, the apparent rate constant for reaction 1 is (4.9 f 1.0) X IO-" cm3 s-I and the intercept indicates a rate constant for the reaction H 2 C N + wall products (8)
+ N at 200
2.13 2.47 2.87 3.89 3.91 3.96 4.95 5.41 5.49 6.09
[N]/l0I2
kco:/s'I
6.19 7.70 7.7 I 7.75 8.69 10.1 12.0 12.0 13.1
352 565 415 253 48Ib 56Sb 702 62Sb 72Sb
183 300 132 219 319 264 233b 309 319 358
"H2CN experiment except where noted. bD2CNexperiment. c k l (298 K ) = (4.9 f 1.0) X IO-" cm3 SKI, intercept = 60 f 70 s-I (pseudo-first-order analysis). k,(298 K ) = (4.4 f 1.4) X IO'", k , = 240 f 60 SKI(modeled; see text). TABLE IV: Rate Data for the Reaction D2CN + N at 363 [ N ] / 10I2 C I I - ~
kwr/ s-I
[N]/IOi2cm-3
2.46 2.92 4.38 4.51
91 154 184 363
6.92 7.92
10.6
K O
kcor/s-l 397 480 66 1
"k'(363 K ) = (6.7 f 1.6) X IO-" cm3 s - I , intercept = -45 f 100 s-I (pseudo-first-order analysis). k1(363 K ) = (6.7 f 2.0) X IO-" em's-',
k , = 120 i 180 s-I (modeled; see text).
standard error in the points to give the errors in k , and k 8 . Uncertainties were also estimated in the following way. The simulations were repeated using extreme values for the slope and intercept of Figure 2, as determined by the 95% confidence limits. The values obtained from k , and k8 were then used as upper and lower limits for the rate constants. For k , the two methods agreed to within IO%, although for k8 the agreement was only to about 50%. The average of the two methods was used to give the following (95% confidence limits):
k , = (4.4 f 1.4) X lo-', cm3 S K I , k , = 240 f 60 s-I The uncertainty in k , is thus about 40% higher than that in the slope obtained from Figure 2. Experiments were also carried out at 200 and 363 K; only the N + DICN reaction was studied at the higher temperature. These results were analyzed in exactly the same way as the room-temperature data. Differences between the analytical and numerical analyses were observed only as changes in the wall loss rate constants. The results at the three temperatures are k,(200 K ) = (3.9 f 2.2)
X
IO-" cm3 s-l
k,(298 K) = (4.4 & 1.4) X lo-" cm3 s-I k , ( 3 6 3 K ) = (6.7 f 2.0)
X
IO-" cm3 s-,
Quoted uncertainties reflect statistical errors at the 95% confidence level. A further uncertainty of about 15% arises from systematic errors. The data for reaction 1 are summarized in Tables 11-IV. The wall loss rate constants ( k , ) showed no systematic change with temperature. However, it should be noted that the room-
Reactions of H2CN and D2CN with N and H
1t 800
1
0
N
+
HzCN
N
+
DzCN
1
*/
c
600
1
-9.5
-I F
1 t
1
t
1
i
-
8
r" i
400
200
I/ t
-105
1
t/
I
r t
1
/ *
k
1
-11
1
0
I
2
temperature experiments, many of which were carried out first, gave the largest value for k8 (240 s-I), while the experiments at 363 K were performed last and gave the lowest value (120 s-l). It appears that the flow tube wall became less reactive to H2CN over the time the experiments were done. Figure 3 shows the rate data displayed in an Arrhenius plot. I n view of the uncertainty in the rate constants and the small number of data points, it is not possible to make accurate estimates for the Arrhenius parameters of this reaction. However, in the temperature range 200-363 K, the rate constants can be obtained with about 50% accuracy from the expression
---
+ H2CN NH + HCN AH = -142 kJ mol-{ AH = -293 kJ mol-' N + H2CN N 2 + CH2 AH = -356 kJ mol-' N + H2CN H2C=N=N N + H2CN c-CH2N2 AH = -322 kJ mol-'
(la) (lb) (IC) (Id)
A previous publication" from this laboratory showed that, at least
at room temperature, reaction l a was the major channel. This conclusion was based both on the rate of formation and the yield CH3 system. The rate constant for the of HCN in the N reaction leading to HCN formation was found to be (5.2 f 1.8) X 10-I' cm3 s-' at 298 K, in excellent agreement with the apparent value (4.9 f 1.0) X IO-" cm3 s-' obtained by the usual pseudofirst-order analysis based on H2CN and DzCN decay. The yield of HCN in the N + CH, system was shown to be 0.90 f 0.15, indicating that HCN is formed with near unit efficiency by the reaction sequence N + CH3 H2CN + H (3c), N + HzCN NH HCN ( l a ) . Reaction of D2CN with H a n d D. These experiments proved very difficult to carry out and were performed only at room temperature. The deuterated radical, D,CN, was used rather than H2CN as this species was much easier to detect. D2CN was
+
+
-
,
1
i I
1
,
l
0
6
10
Figure 3. Arrhenius plot for reaction 1.
TABLE V: Rate Data for the Reaction D,CN ([HI or [D])"/1012 2.44 6.56 4.26 4.19 3.22 2.25 5.24
k,,/s-l 443 875 1060 765 592 467 985
+ H (D) at 298 Kc
([HI or [D])"/lO1* 3.976 6.09b 2.216 I .86b 6.656 4.576 5.766
k,,/s-l 39 1 I I64 375 346 913 513 813
'H atom experiment except where noted. b D atom experiment. ck2(298 K) = ( 1 . 3 f 0.6) X cm3 s-', intercept = 130 & 130 s-',
(10)
A number of thermodynamically accessible channels exist for reaction 1:
N
l
( IOOO/T)/K-'
Figure 2. Summary plot of the pseudo-first-order rate constant k,, vs [N] for reaction I at 298 K.
exp(-200/T) cm3 s-'
l
t
[N]/lOLZ c K 3
k , ( T ) = 1.0 X
1
1
-
generated at the back of the flow tube by admitting N, F, and CD, through the side arms, while H atoms were added through the sliding injector. This arrangement was adopted to prevent F atoms from being destroyed by reaction with H2. The signal at m / e = 30 was maximized by varying the flow of N2 and CF,. Best signals were obtained with relatively low concentrations of N ((-5-8) X lo1' cm-)) and F ((-2-3) X 10" present in the flow tube. CD4 was present in large excess ( - 1.5 X I O l 3 to ensure rapid conversion of F to CD3. D2CN concen) , consequently the radical trations were very low ( 1Olo ~ m - ~and was difficult to detect. Decays were measured and analyzed by using the normal pseudo-first-order approach, corrections for axial diffusion'0J6 of 2-7% being made to first-order rate constants. The corrected first-order rate constants had an average uncertainty of f13%. H atom concentrations were between 2 X 10I2and 7 X IO'* c r f 3 . Experiments were also carried out with D atoms as the excess reactant: on systematic difference between the H and D experiments was observed. The data for reaction 2 are summarized in Table V. Figure 4 shows a summary plot of the data. From the slope of the line in Figure 4 a value for k2 of (1.3 f 0.6) X cm3 s-I is obtained. The quoted uncertainty here reflects statistical errors at the 68% confidence level. Unfortunately, computer simulations reveal difficulties in the analysis described above. The main problem is that the reaction generating DzCN is not completely over by the time the H atoms
-
4950
1200
r
1000
800
B
I
I
,
I
\
Nesbitt et al.
The Journal of Physical Chemistry, Vol 94, No. 12, I990
H +
1
D
1
DZCV
+ DzCN
/
i
I
-
i
600
/
i
/
/
100 /’
0
-
-
L i
>
2
0
1
8
i [HI oi [I)] ) 1 0 ‘ c mi ? Figure 4. Summary plot of the pseudo-first-order rate constant k,,, vs [HIor [D] for reaction 2 a t 298 K
are added. Admitting a larger concentration of N atoms obviates this problem but destroys the signal in two ways. Not only is D2CN destroyed more quickly by the larger concentration of N , it is also formed earlier and thus has more time to react with N and the walls of the flow tube. A large wall loss for DzCN further aggravates the problem, as a steady state is set up and addition of H simply perturbs this steady state. However, these experiments were carried out after the N H 2 C N experiments and, as discussed in the previous section, the wall loss is probably quite low What is clear is that when these difficulties occur, the measured rate constant underestimates the true value. For this reason we have taken the lower limit determined by the errors in the slope in Figure 5 as a lower limit for the rate constant, and we write
+
k2(298 K)
>7
X
cm3 s-I
Attempts were made to determine the products of the reaction between H and D2CN. Two possible channels are H H
+ DZCN
+ D,CN
-
+
HD
Two potential difficulties in this work arose from the methods of formation and detection of the H2CN radical. Interference at m / e = 28 from excited nitrogen formed in the discharge reduced the sensitivity for H2CN when compared to D2CN. However, sensitivity was very good for D2CN, and in the absence of interference, electron impact mass spectrometry is probably as good a detection method for H,CN as it is for D2CN. Furthermore, preliminary work in this laboratory indicates that photoionization mass spectrometric detection of H2CN may be sensitive even in the presence of the products of a nitrogen discharge. The method used for the formation of H,CN creates more serious problems. To start with, both of the radical‘s precursors are labile and so must be prepared in situ. Furthermore, N atoms react very rapidly with H2CN, reducing its concentration and making the extraction of kinetic parameters from the raw data rather difficult. It is these difficulties which give rise to the large uncertainties in our rate constants. Alternative methods of detection and generation of methyleneamidogen have recently been reviewed.’’ The absence of kinetic isotope effects in reactions 1 and 2 is of some interest. H / D substitution can give rise to large changes in rate constants (up to a factor of 10 at room temperature) as a result of differences in reaction activation barriers. Reactions without a barrier are not expected to show large isotope effects. Our lower limit of 7 X lo-” cm3 s? for the H + DzCN reaction at room temperature is very large and can be compared with the cm3 s-l calculated from a hard-sphere gas collision value of model. Reaction must occur on virtually every collision; i.e., the reaction has no barrier which is consistent with the absence of an isotope effect. It can be further concluded that the H + H2CN reaction will have very similar rate parameters to those for the H + D2CN reaction. The reaction of N atoms with HzCN appears to have a small activation barrier while no H atom kinetic isotope effect is detected. These observations are consistent with the reaction proceeding via a transition state with C-H bond lengths very similar to those in H2CN. Such a transition state will occur if the rate-limiting step in the reaction is the formation of a N-CH2N complex. In such a mechanism, part of the exothermicity of the reaction is expected to be distributed in a Boltzmann-type way among the vibrational modes of the products. A second possibility is that the reaction proceeds via direct abstraction on an attractive potential. In this case, considerable vibrational excitation in the N-H product may be observed. Infrared chemiluminescence experiments may distinguish between the two mechanisms. Comparison of the results for the reactions of H,CN or D2CN radicals with N and H can be-made with similar reactions of other small free radicals with these atomic species. In the case of the N atom reactions, the rate constant at 298 K for H2CN or DzCN N is about half that for CH, N ( k = 8.5 X IO-” cm3 comparable to that for O H + N ( k = 5.0 X IO-” mi3s-I),I8 and 50-100% greater than those for NO N (3.4 X IO-’’ cm3 s-’)I4 and C H i- N (2.1 X l V i l cm3 s-I).l9 Thus, all these N atom reactions have rate constants within a factor of 2 of 5 X lo-]’ cm3 s-’ at 298 K; they also have rather weak or negligible temperature dependences. Comparison with H atom reactions is difficult as many are additions involving a third body. However, it can be noted that the rate constant for D2CN + H at 298 K is larger than that for the abstraction reaction C2H3 + H ( k = 1.5 X IO-’’ cm-3 s-1 ) 20 while it seems somewhat smaller than that for the addition reaction CH, + H at its high-pressure limit ( k = 4.7 X
+ DCN
(2a)
D2 + H C N (or H N C )
Channel 2b can only occur via complex formation, while channel 2a can also occur in a direct abstraction mechanism. The ratio of the signals at m l e = 3 and m / e = 27 was measured for various H atom concentrations and injector positions. Signal at both masses was detected and the ratio a,/ab= 5 f 3 determined. This estimate assumes that detection sensitivity for H D is similar to that for Hzand that any H N C formed in channel 2b is rapidly converted to HCN. It appears that the reaction proceeds at least partially via complex formation. The radical H D C N was not detected, and it is concluded that H / D exchange is slow relative to channels 2a and 2b.
Discussion The work described in this report represents one of only two studies on the gas-phase kinetics of methyleneamidogen (HzCN). The errors associated with the measured rate constants are somewhat larger than usual, and it is worth considering the source of these uncertainties.
+
+
+
10-10
cm3
s-l),21
‘4s mentioned in the Introduction, the formation and reaction of the H,CN radical can be shown to be important in models of (17) Marston, G.; Stief, L. Res. Chem. Intermed. 1989, I 2 , 161. (18) Howard, M. J . ; Smith, I . W. M. Chem. Phys. Lerf. 1980, 69, 40. (19) Messing, 1.; Filseth, S. V.; Sadowski. C. M.;Carrington, T. J . Chem, Phys. 1981, 74, 3874. (20) Keil, D. G.; Lynch, K . P.; Cowfer, J . A,: Michael, J. V . int. J . Chem. Kinet. 1916, 8, 825. (21) Brouard. M.: Macpherson, M. T.: Pilling, M. J. J . Phys. Chem. 1989, 93, 4047.
Reactions of H2CN and D2CN with N and H
The Journal of Physical Chemistry, Vol. 94, No. 12, 1990 4951
the atmosphere of Titan,' the chemistry of 0-rich circumstellar clouds3 and interstellar cloud^,^ and laboratory studies of active nitrogen/hydrocarbon reactions.>' One common feature of these apparently diverse systems is that the reaction that has been proposed to account for the formation of H C N is N C H 3 HCN + H, (or 2H). Since it is now known1' that the dominant channel yields H2CN + H, all of these models must be modified to include formation and reaction of H2CN. In all these systems, the direct source of HCN is one of the reactions of H2CN reported in this study. In the atmosphere of Titan and in circumstellar or interstellar clouds, the predominance of H over N makes it clear H C N + H 2 will be more imthat the reaction H H2CN portant. In laboratory studies of active nitrogen/hydrocarbon reactions, the important reaction sequence is
+
-
+
N N
+ CH3
-
+
+ H2CN
H2CN
+H
(3c)
HCN
+ NH
(1)
+
due to the very high concentrations of atomic nitrogen. This sequence, or related ones involving hydrocarbon radicals larger than CH3, thus provides a quantitative explanation for the discrepancy between [ N ] as determined by N O titrationZ2and by the HCN yield,23the former value being twice the latter.23 Since two N's are consumed in the fomation of one HCN, the H C N yield method underestimates the true [N] by a factor of 2. However, it has been observed that addition of N atoms, either in the same discharge24used for N or in a separate discharge25 downstream, increases the H C N yield to the point where the NO titration method and the HCN yield method give identical results for the N atom concentration. This is readily explained by the fact that addition of sufficient H replaces the N atom reaction ( I ) by the H atom reaction (2). Then the reaction sequence becomes
studied here thus provide an understanding of two aspects of HCN formation in active nitrogen/hydrocarbon reactions which have hitherto been obscure. The H2CN radical is also implicated in models for H C N formation on Jupiter and H atom recombination in the atmosphere of Titan. The mechanism of H C N formation on Jupiter is different from those discussed above involving the N CH3 reaction. On Jupiter the coupled photochemistry of ammonia and acetylene has been proposed by Kaye and Strobe12 to lead to the reaction NH2 + C2H3 C2HSN (11)
+
+
Depending on which of the four possible isomers of C2HSN is considered and on the wavelength of the dissociating radiation, photolysis of C2H5Ncan lead directly to H C N C2HsN + hv H C N CH3 H (12)
+
+
or to the methyleneamidogen radical C2HsN + hv H2CN
+ CH3
+
(13) In the latter case, subsequent reaction of H2CN with H (reaction 2) yields H C N . The recombination of H on Titan is probably catalyzed by reaction with diacetylene (C4H2) or acetylene (C*H2).' +
H H
+ HX
+ H2X
-+ M
+
H2X
HX
H2
(15)
where X = C4H2 or C2H2and H X = C4H3or C2H3. Yung et a1.I have proposed that H C N can also participate in this process via the reactions H H
+ HCN
+ H2CN
M
H2CN H C N H2
.-c
+
(16)
and there is now a 1 : 1 correspondence between N consumed and HCN produced. The kinetics and mechanism of the two reactions
(2) The above examples demonstrate that the methyleneamidogen radical, H2CN, is an important intermediate in a wide variety of complex systems. Further work on the production, detection, and reactivity of H2CN would clearly add to our understanding of several laboratory and atmospheric processes.
(22) Kistiakowsky, G. B.; Volpi, G.G. J. Chem. Phys. 1957, 27, 1141. Westenberg, A. A.; de Haas, N. J. Chem. Phys. 1964, 40, 3087. (23) Verbeke, G. J.: Winkler. C. A. J. Phys. Chem. 1960, 64, 319. (24) Herron, J. T.J . Phys. Chem. 1965, 69, 2736. (25) Safrany. D. R.: Jaster, W. J. Phys. Chem. 1968, 72, 518.
Acknowledgment. This work was supported by the NASA Planetary Atmospheres Program. George Marston thanks the National Academy of Sciences for the award of a Research Associate. Fred L. Nesbitt acknowledges support under NASA Grant NSG-5 173 to the Catholic University of America.
N H
+ CH3
+
+ HzCN
H2CN
+
HCN
+H
+ H2
(3c) (2)
+
