J . Phys. Chem. 1986,90, 6842-6848
6842
core of the high Rydberg state (RH+...e-) may have an unpaired electron distribution similar to that of the radical cations, the C-H bond scission at the chain end is again expected. The observed selective formation of 1-alkyl radical from the crystalline neat linear alkane8 is thus quite reasonable from the view of the unpaired electron distribution in the linear alkane radical cations observed in SF6 and in other halocarbon mat rice^.^ It is well-known that only the fully extended chains exist in the crystal of linear alkanes.17 So, there may be no possibility of forming the primary cations with the gauche conformation in the neat crystals. The above-mentioned observations and their interpretation are strongly supported by the present work, which gives clear evidence for the correlation of the unpaired electron density and the site selection of deprotonation from the linear alkane radical cations with the extended conformations. Concluding Remarks The present work clearly indicates that the linear alkane radical cations with the extended structure undergo selective deprotonation from the chain end, as is expected from the unpaired electron distribution when the extended conformer is exclusively formed ~
~
~~
(17) Kanesaka, I.; Synder, R. G.;Straws, H. L. J . Chem. Phys. 1986.84, 395 and references cited therein.
and the population of the gauche conformer is negligible even at elevated temperature, at which the deprotonation reaction proceeds. Formation of the 2-alkyl radical is expected from the gauche conformer, in which the unpaired electron density is higher in the in-plane C-H bond at the C2 atom than that in the other end. The observation of the 2-alkyl radical formation in CFC12CF2C1,in which the gauche conformer is often stabilized, may be ascribable to the higher rate of deprotonation from the gauche form because of the higher unpaired electron density in the in-plane C-H bond at C2 than that at C I in the corresponding extended conformer. This idea has been suggested in our earlier paper,2 in which we did not use the terminology of the gauche conformer but what is meant is the same. The citation of this description by Lund et al. as a R structure in their discussion is a misunderstanding. The present work not only gives strong support for our interpretation for the selective formation of the 1-alkyl radical in the radiolysis of neat crystalline linear alkanes8 but also may play a crucial role in clarifying the confusion resulting from arguments reported by Lund and his co-workersgJOon the structure and reactions of the alkane radical cations. Registry No. n-C4Hlo,106-97-8; n-C5HI2,109-66-0; n-C6HI4,11054-3; n-C7H16, 142-82-5; n-C4Hlo+, 34479-72-6; n-CSH,,+, 34479-74-8; fl-C6Hl4+, 34478-20-1; n-C7H],+, 34480-77-8.
Chemlhrmhrescent Reactions In Photodissociated Cyanogen-Oxygen Mixtures G. Black,* L. E. Jusinski, M.-R. Taherian, T. G . Slanger, and D. L. Huestis Department of Molecular Physics, Chemical Physics Laboratory, SRI International, Menlo Park, California 94025 (Received: June 13, 1986; In Final Form: September 12, 1986)
F2 (1 58 nm) and ArF (1 93 nm) excimer radiation have been used to photodissociate C2N2and produce chemiluminescence in C2N2-02 mixtures. In addition, LIF measurements have followed the temporal behavior of the CN and NCO radicals. The behavior of CN is understood in terms of its reactions with O2and 0. The NCO decay is controlled by radical or atom intermediates. This study was focused on the NO y-band chemiluminescence, and evidence is presented to support the view that it arises from energy transfer from N2(A) to N O and the N2(A) is made by the reaction N(2D) + NCO N2(A) CO. An estimate of =25% is obtained for the N2(A) yield of this reaction.
-
Introduction Study of chemiluminescence in the cyanogen-oxygen system dates back over 50 years.' Most of the more recent work2J has been concerned with mechanisms exciting the CN(A and B) emissions. However, the C N O2system has great potential as a source of electronically excited nitrogen species. Basco4 and Schmatjko and Wolfrums have demonstrated that two of the principal early reactions in the CN-O2 afterglow system are
+
CN(X)
--co + +
+0 2
o ( 3 ~+)CN
NCO
0
N(*D)
= 94%
a
= 80%
(1) (2)
+
X lo-" and (2.5 f 0.2) X lo-'' cm3 molecule-' s-l being obtained for the u = 0 level of CN. The consequence of these two reactions proceeding rapidly and with high yields is that the production rates of both N C O and N(2D) are high, raising the interesting possibility of their interaction:
N(2D) + N C O
-
N2
-CN+NO
+ CO
(3a) (3b)
Reaction 3a is enormously exothermic, AH being 233 kcal/mol ( > l o eV), so that even production of such energetic species as
N2(A3Z,+) and CO(A'II) is easily accommodated. N2(A) is an Thus, a copious source of N(2D) is in principle available, to the extent that CN can be efficiently generated. Reactions 1 and energetic species which might find application as the energy donor in a chemical laser if an efficient chemical source can be identified. 2 are both fast, with rate coefficients of 7.6 X and 1.7 X lo-'' cm3 molecule-' s-I, respectively. The most recent ~ o r k ~ , ~In fact, reaction 3a can produce all N2 states up to the dissociation on reaction 1 suggests that it is even faster-values of (2.0 f 0.2) limit. Whether reaction 3a is the dominant one, and the products are excited, needs to be determined. Lacking information on the potential surfaces, angular momentum constraints are relatively ( 1 ) Harteck, P.; Kopsch, U. J . Phys. Chem. 1931, 12, 327. ineffective in predicting the electronic distribution of the products. (2) Setser, D. W.; Thrush, B. A. Proc. R. SOC.London, A 1965,288,256. This contrasts with reaction 2 in which the doublet channel is (3) Setser, D. W.; Thrush, B. A. Proc. R. SOC.London, A 1965,288, 275. (4) Basco, N. Proc. R. Soc. London, A 1965, 283, 302. favored by the stability of the doublet N C O reaction intermediate. (5) Schmatjko, K. J.; Wolfrum, J. Ber. Bunsenges. Phys. Chem. 1978,82, In the cyanogen-oxygen system, there are a number of chemical 419. pathways that can produce NO. For example, in addition to the (6) Li, X.; Sayah, N.; Jackson, W. M. J . Chem. Phys. 1984, 81, 833. (7) Lichtin, D. A,; Lin, M. C. Chem. Phys. 1985, 96, 473. 6% yield from reaction 1, NO will also be generated by 0022-3654/86/2090-6842$01.50/0
0 1986 American Chemical Society
The Journal of Physical Chemistry, Vol. 90, No. 26, 1986 6843
Photodissociated Cyanogen-Oxygen Mixtures N(2D)
+ O2
-
and
NCO
NO
+ O(’D or 3P)
- + co - +
+o(3~)
NO
(4) 11
(5) 10
In the presence of this NO, the N2(A) will manifest itself by the very efficient energy-transfer process NZ(A)
+ NO
N2
NO(A)
9
(6)
8
with subsequent emission of the N O y-bands. This emission has been reported b e f ~ r e and ~ , ~attributed to energy transfer from N2(A) or CO(a311) formed by CN + NO N2 + C O (7)
r
IC
+
or NCO
+ N(4S)
-
N2
+ CO
4
(8)
Although information has been obtained on all the chemiluminescence features of the CN-02 system, the focus of this study is on the N O y-bands and an attempt to determine the contributions of the various reactions that might produce it. In contrast to previous studies which used steady-state measurements in afterglows, this work employed photodissociation of C2N2to generate C N radicals and initiate the chemistry. Our initial measurements used the Fz (158 nm) emission from an excimer laser. In addition to dissociating C2N2(a = 1.6 X cm2), this also generates atomic oxygen by Oz photodissociation (u = 8 X lo-’* cm2). However, it was found that these same emissions were also produced when A r F (193 nm) was used to dissociate C2Nz( u = cm2).8 In this case reaction 1 provides atomic oxygen. Using 193 nm, we have measured the temporal profiles of the N O y-bands for various O2pressures and also for several N O pressures at fixed O2 (and C2N2) pressure. In addition to these chemiluminescence measurements, LIF has been used to probe the kinetic behavior of the C N and N C O radicals and further elucidate the chemiluminescence mechanisms. Experimental Section Most of the apparatus has been described previously? In most cases, the intensity of the excimer photolysis beam in the center of the photolysis cell was increased by using a MgF2 lens. (Focal lengths from 25 cm to 1 m were used.) Typical energies transmitted through the cell were 0.5-2 mJ of 158-nm radiation and 10-80 mJ of 193-nm radiation. For the LIF studies, a Quanta-Ray Nd:YAG dye laser system provided 5-10 mJ in the region of the (0,O)CN(B-X) transition at 388 nm (using Exciton LD 390). This same dye solution was also used to monitor NCO, pumping the A22+(001)-X211i(OOO) transition at 398 nm and detecting the A2Z+(OOl)-X211,(OOl) emission a t 431 nm. For some of the N C O measurements the dye solution was changed, and the A22+(000)-X211,(000) transition at 440 nm was used for both excitation and observation. The lasers were operated at 10 Hz and were adjusted to overlap spatially along the length of the photolysis cell. For determining the kinetics of the C N and NCO radicals, a variable delay could be introduced between the two laser pulses. Baffles with l-cmdiameter holes were situated in the side arms of the cell to reduce scattered light. The cell was also equipped with MKS Baratron pressure gauges and was evacuated with a small rotary pump which gave -20 s for the residence time of the gas in the cell. Some measurements were also made with the residence time reduced to 2 2 s to confirm that the results were not affected by any accumulation of photolysis products. The detection system, situated perpendicular to the laser beams, consisted of a 1/4-m monochromator equipped with an RCA C31034 photomultiplier. Its output passed to a boxcar averager and then to a chart recorder. The gases used were supplied by Matheson Gas Products and were used without further purifi(E) Lu,R.;Halpern, J. B.; Jackson, W. M. J . Phys. Chem. 1984,88,3419. (9) Black, G. J . Chem. Phys. 1984,80, 1103.
5
3
2 1
I 2
I
0
1
I 4
I
3
5
0, ( 1 0 1 ~molec cm-3)
Figure 1. CN(u”=O) decay rate vs. O2 concentration. C2N2 (lo1> molecules cm”) in Ar (3.2 X 10’’ molecules ~ m - ~ F2 ) . laser (158 nm)
photodissociation. cation. The gases passed through flowmeters and mixed before entering the photolysis cell. For measuring the temporal profiles of the chemiluminescent emissions, the boxcar was replaced with an MCA ( N D 100) interfaced to a DEC VAX 111750 which was used for data storage and analysis. Most of the N O y-band measurements were made with the monochromator on the strongest y-band, the (0,l) band at 237 nm. Results and Discussion LZF Measurements. The most recent measurements6v7of the rate coefficient for reaction 1 give values a factor of 2-3 higher than a number of earlier studies. To determine whether the occurrence of reaction 2 or other complicating chemistry (e.g., C N N(% or 2D) C N2) might be contributing to the higher recent values, the decay rate of the C N radicals was measured for both high and low concentrations of added 02.The measurements at low O2are shown in Figure l . The slope of the line gives a rate coefficient k l of 2.0 x IO-’’ cm3 molecule-’ s-’ for reaction 1, for v” = 0, and was, within experimental error, the same value obtained at larger O2 additions (up to 2.6 X l O I 5 molecules ~ m - and ~ ) in the other recent ~tudies.6,~ At the low O2 additions, since the C N concentrations produced by photodissociation were comparable to the O2 addition, the good single-exponential decays of C N provide further evidence that the values of k l and k2 are very similar since the depletion of O2 and its replacement by 0 does not lead to a significant change in the observed decay. A few measurements of CN removal by N O confirmed that the reaction was slow, as expected from the recent cm3 molecule-’ s-’ for vrr measurementLoof (1.6 f 0.3) X = 0. It appears that, despite the extensive chemiluminescent phenomena, the behavior of C N radicals is explained by reactions 1 and 2. The behavior of the NCO was much more complicated. In this case the decay of NCO was not controlled by either C,N2 or 02. With N O addition, the fast ( k = 3.3 X lo-” cm3 molecule-’ s - l ) l 1,12 removal reaction NCO N O NzO CO (9)
+
-+
+
-+
+
(IO) Li, X.;Sayah, N.; Jackson, W. M. J . Chem. Phys. 1985, 83, 616. (11) Cookson, J. L.; Hancock, G.; McKendrick, K. G. Ber. Bunsenges. Phys. Chem. 1985,89, 335. (12) Perry, R. A. J . Chem. Phys. 1985, 82, 5485.
6844
The Journal of Physical Chemistry, Vol. 90, No. 26, 1986
Black et ai.
TABLE I: Reaction Scheme for N,(A) Generation and NO y-Band Emission in Photodissociated C2N2-02 Mixtures reaction AH(298 K), rate coefficient, no. reaction kcal/mol cm3 molecule-' s-I
-+
CN wall CN 0 2 NCO 0 C N + 0 N('D) + CO CN + N 2" +C CO CN NO 2" NCO wall NCO 0 NO CO N, CO NCO N NCO + N(2D) N2(A) + CO NCO NO N2O CO NCO C C N CO N(2D) wall N(2D) + O2 NO + 0 N(2D) 0 N 0 N(2D) + N2 N + N2 N CO N(2D) + CO N(2D) + NO N2 + 0 N(2D) + CN N2 C N2(A) wall Nz(A) 0 2 N2 + 0 + 0 Nz(A) 0 N2 + 0 N2(A) + NO N 2 NO(A) y-bands N2 + N2(A) N2(A) + N2(A) N2(A) CO N2 + CO N2 + N N2(A) N C NO- CN 0 c + 02' co + 0 N NO N2 0 C N 0 2 4 CO NO CN 0 N + CO C + C2N2 --+ C2N + CN N(2D) C2N2 C2N N 2 N2(A) C2N2 -* N2 CN CN C2N N CN C N C2N + 0 --* CN + CO 0 wall N wall C2N wall C wall
1
2
-
+
+
-7 -22 -46 -152"
-+ + -+ + + - + -+ - + -- + - + -+ + -- + +
+
7 5 8 3a 9
+ + +
-101 -176" -89" -65 -130
+
4
-86 -55 -5 5 -55 -130 -101 -22 -142 -142 -142 -142 -142 -29 -137 -7 5 -108 -77 -8 -109 -8 -38 -115
-+
6 10
+ + + +
1
2
+
-
4
+
11
+
+
---
12
+
+ + +
-
+
+ + +
"Reactions with sufficient exothermicity to product N2(A).
units of
9OOb 2.2 x 1.4 X 1 .o x 1.4 X 900b 1.7 X 3.3 x 1.0 x 3.3 x 3.3 x 9OOb 7.0 X 1.8 X 1.7 x 2.5 X 6.3 X 1.0 x 900b 3.0 X 3.0 X 8.0 x 3.0 X
source measured 6, 7 5 19 I O , 20 estimate 21 21 estimate 11, 12 estimate estimate 22 23 24 24 24 estimate estimate 25, 26 21 28 28, 29 16, 30 31 32 32 33
10-11 IO-" 10-10
lo-" lo-" 10-11 10-'0 10-11 lo-"
10-l2 10-14
IO-" 10-10 lo-" 10-1'
4.0 X
5.0 x 1.1 x 3.3 x 2.7 X 1.4 X 3.0 X 3.0 X 7.0 X 4.0 X 2.0 x 2.0 x 900b 9OOb 9OOb 9OOb
10-14 10-10
10-11 IO-"
5
5 19 estimate 16 estimate estimate estimate estimate estimate estimate
lo-" lo-" IO-" lo-" 10-11
s-l.
CN(A-X) A v = 3
i-rn-CN(A-X) A v = 2
1-
(6.3)
(3.0)
2nd Order CN(8-X) NO(A-XI
m
(0.51
(0.0)
NCO(A-X)
I
CN(B-XI
I Av= 1 Av=-1 Av=O
8500
8000
7500
7000
6500
6000
5500
5000
4500
4000
3500
3000
2500
2000
WAVELENGTH ( X i
Figure 2. Emission spectrum produced by 158-nm radiation of a mixture of C2Nz ( 10Ismolecules cm-3) and O2 (lok5molecules cm") in helium (2.7 X lo1' molecules cm-'). NO y-band emission region (2000-3000 A) enhanced by addition of NO (9 X 1014molecules cm-'). Spectrum uncorrected for spectral response.
was seen and would decrease the NCO lifetime to that of the CN precursor. However, in the absence of NO, the decay of N C O was found to be determined by the excimer intensity, and the decay rate appeared to be roughly proportional to the initial NCO signal.
Since the decay was exponential, removal by an N C O + N C O reaction cannot be dominant. What seems most likely is that removal of NCO by reaction with 0 (reaction 5 ) is an important loss mechanism. This reaction has been postulated in a number
The Journal of Physical Chemistry, Vol. 90, No. 26, 1986 6845
Photodissociated Cyanogen-Oxygen Mixtures
A
B 1
8.00Et12 I
I
8.00E+12 I
,
c VI
7
-E
c w
4.00€+12
I-
z
n
z
? ?-
0.00E+00 0.0
2.OE-4
4.OE-4 6.OE-4 TIME (SI
8.OE-4
0.0
1.OE-3
2.OE-4
4.OE-4 6.OE-4 TIME (SI
8.OE-4
1.OE-3
1.50Et13 c
m
E
t-
0
2 7.50Et12 w
7.50€+12
I-
z 0
z
$ ?-
0.00E+00 0.0
1
0.ooE +oo 2.OE-4
4.OE-4 6.OE-4 TIME (SI
8.OE-4
1.OE-3
0.0
2.OE-4
4.OE-4 6.OE-4 TIME (SI
8.OE-4
1.OE-3
2,OE-4
4.OE-4 6.OE-4
8.OE-4
1.OE-3
0.0
2.OE-4
4.OE-4 6.OE-4 8.OE-4
1.OE-3
2.00€+13
c
IVI
m
-6 > c, v)
$
1.00€+13
!-
z
Q
z
?
?-
0.00E+OO 0.0
TIME (s)
TIME (s)
Figure 3. Intensity-time profiles of NO y-band emission for three O2concentrations: -, model; 0 , normalized experimental points. Experimental ~ , O2concentrationsas shown. In addition to these inputs, model conditions were [He] = 3.5 X 10’’ molecules cm-), [C2N2]= 10l5molecules ~ m -and calculations used initial [CN] = lOI4 molecules cm-). (A) N(2D) + NCO as only N2(A) source; (B) N(4S) + NCO as only N2(A) source.
of earlier studies13-15and should be fast. The modeling studies (see later), using the kinetic scheme shown in Table I, confirmed that the major loss of N C O would be by reaction with 0 under a variety of conditions (although this depends on an estimated rate coefficient of 1.7 X lo-” cm3 molecule-l s-l for reaction 5). It can be seen that reaction 5 produces NO so that, if reactions 9 and 5 have similar rate coefficients, then the observed singleexponential decay may be explained. It was clear, however, that this study could not give a good value of the rate coefficient for reaction 5 or assess the role of other NCO removal processes (like reaction 3), so further attempts to characterize NCO chemistry by LIF measurements in this system were abandoned. NO ?Band Emission. Figure 2 shows the emission spectrum produced by 158-nm radiation of a C2N2-0, mixture. The strongest chemiluminescent features are the CN(A) and CN(B) emissions. (CN(A) up to u’= 5 can be made directly at 158 nm.) A very similar spectrum was obtained with 193-nm radiation. (In (13) Basco, N. Proc. R. SOC.London, A 1965, 283, 302. (14) Morrow, T.; McGrath, W. D. Tram. Faraday SOC.1969, 62, 642. (15) Hand, C. W. J . Am. Chem. SOC.1970, 92, 1825.
this case CN(A) cannot be made directly, and the LIF studies revealed CN(X) produced in only the u” = 0 and 1 levels.) In addition, emissions at 466,440, and 329 nm are thought to arise from the triatomic radicals CCN, NCO, and NCN, respectively. Between 200 and 300 nm lies the weakest chemiluminescent feature, the NO y-bands. In other unpublished work in this laboratory, CO 4+ emission from CO(A1rI) has also been measured between 130 and 180 nm, but its intensity relative to the other features is not shown. This N O y-band emission is mainly from u’ = 0 which must closely represent initial production since vibrational relaxation of NO(A) is small under our experimental conditions. The (1,O) band a t 215 nm can be seen which, when corrected for spectral response, indicates that 580% of the emission comes for u’ = 0. A small (milliTorr) addition of N O enhances the emission without changing its spectral distribution. The spectral distribution of the emission is strong evidence that the species transferring its energy to NO is the N2(A32,+) which is knownI6 to produce >8Wo of the NO(A2Z+) in u’= 0. The barely (16) Clark, W. G.; Setser, D. W. J . Phys. Chem. 1980, 84, 2225.
6846 The Journal of Physical Chemistry, Vol. 90, No. 26, 1986
Black et al.
a
6
1.50E+ 13
1
-,
n
n
-
r
E
>
t-
t-
V,
7.50E+12
z w
w t-
z
t-
D
0
W
%o
z
a
P
0.00E+00
0.0
--
-
7.50€+12
z
z
+
D
-6
->
+
1.50€+14
1.OE-4
2.OE-4 3.OE-4 TIME ( 5 )
0.00E+00 0.0
4,OE-4 5.OE-4
2.OE-4 3.OE-4 TIME ( 5 )
NO = 9.9 x 1014 molec cm-3
,
n
4,OE-4 5.OE-4
-
4.00E+13
2 .OO E+ 14
1.OE-4
NO = 9.9 x 1014 moiec cm-3
-6 > t-
FJ 1.00Et14
2.00E+ 13
z w
t-
z
+
0
z
+
O.OOE+OO
O.OOE+OO
0.0
--
1.OE-4
2.OE-4 3.OE-4 TIME ( 5 )
4.OE-4
5.OE-4
0.0
1.OE-4
2.OE-4 3.OE-4 TIME ( 5 )
4.OE-4 5.OE-4
I
1.00E+14 I
NO = 1.76x 1015 molec c ~ n - ~
Lo
PI
-6 > t-
FJ 5.00€+13
z
W
t
z
0
z a
cp+
0.00 E+00 0.0
1.OE-4
2.OE-4 3.OE-4 TIME (s)
4.OE-4
5.OE-4
0.0
1.OE-4
2.OE-4 3.OE-4 TIME (SI
4.OE-4 5.OE-4
Figure 4. Intensity-time profiles of NO y-band emission for three NO concentrations: -, model; 0 , normalized experimental points. Experimental conditions were [He]= 3.5 X lot7molecules [CzNz]= l0l5molecules cm-j, [O,] = 3.6 X 1014molecules cm-), and NO concentrations as shown. In addition to these inputs, model calculations used initial [CN] = 1014molecules cm-'. (A) N(2D) + NCO as only N,(A) source; (B) N(%) + NCO as only Nz(A) source. discernible feature on the long-wavelength shoulder of the (0,5) y-band (but more apparent in other spectra) may be the (0,6) NO @-band,which would indicate a small contribution by energy transfer from CO(a311)'7which could also be produced by reaction 3a. (In contrast to the substantial yield of NO (B211,) in thermal collision^'^ of CO(a311) with NO, more recent workl8 at higher energies, under single-collision conditions in a beam, shows that the energy transfer favors NO(A2Z+) production and discusses this difference.) Temporal profiles of the y-band emission (using 193-nm photolysis) were recorded in cyanogen-oxygen and cyanogen-oxygen-nitric oxide mixtures. Of the three reactions sufficiently exothermic to produce N,(A) (reactions 3a, 7, and 8), reaction 7 can be dismissed in the cyanogen-xygen system since it is slow and since the NO y-band yield is enhanced by O2addition to a cyanogen-NO mixture rather (17) Taylor, G. W.; Setser, D. W. J. Chem. Phys. 1973, 58, 4840. (18) Ottinger, Ch.; Simonis, J.; Setser, D. W. Ber. Bumengrs. Phys. Chem. 1978, 82, 655,
than suppressed, as it would be if reaction 7 were the N2(A) source. It view of our inability to understand the NCO kinetics observed in our LIF studies and since N(*D) and N(%) were not measured, it was decided to proceed with a computer model of the chemistry to attempt to determine the roles of reactions 3a and 8 in generating N,(A) and producing the measured temporal profiles of NO y-band emission. The computer simulation was carried out with the aid of the OLCHEM chemical kinetics program developed by Whitten (Science Applications Incorporated) and adapted by Baldwin (now at Intel) and Golden (SRI's Chemical Physics Laboratory). Table I shows the total reaction scheme. When possible, measured rate coefficients were used, but a number of rate coefficients were estimated. (For example, for the important N(,D) + CN reaction, the measured ratelg for N(4S) + C N was used.) For some reactions, previous estimates of rate coefficients were used (like ref 20 for the reactions NCO 0 and NCO + N). Diffusive losses
+
(19) Whyte, A. R.; Phillips, L. F. Chem. Phys. Lett. 1983, 98, 590.
Photodissociated Cyanogen-Oxygen Mixtures
The Journal of Physical Chemistry, Vol. 90, No. 26, 1986 6841
10-1 play an important part. The temporal profile experiments used H e ( N 10 Torr) as the buffer gas. The measured C N diffusive loss rate with this buffer gas was =go0 s-’ (to compare with -450 s-I, with Ar (10 Torr) as the buffer gas, shown in Figure 1). Lacking measurements, the same diffusive loss rate was used for all the transient species. Much of the chemistry shown in Table 5 10-2 I plays a very minor role-it was included to suggest ways of -m making the C N ( A and B) and triatomic chemiluminescent .emissions which may be the subject of future study. The temporal .-C o w behavior of the CO 4+ particularly its variation with e .oxygen concentration, was very similar to that of the NO y-bands. w Reaction 3a can produce either CO 4+ emission or N,(A), and ?2 the similar temporal behaviors provide good evidence for reaction 9 10-3 Lu 3a as the important reaction. (Reaction 8 is not sufficiently > exothermic to produce CO 4+.) Figure 3 shows experimental y-band profiles for three O2 9 0 concentrations and the computed profiles with either reaction 3a Io or reaction 8 as the N,(A) source. Clearly, reaction 3a provides a 2 10-4 the better fit. Variation of the temporal profiles as a function of added N O is similar for the two reactions, as Figure 4 shows. Further evidence to support reaction 3a as the dominant reaction was found in measurements of the absolute y-band yield. For these measurements the F2laser was used on a mixture of C2N2 and O2 (lOI5 molecules ~ m - in ~ )He (2.7 X 10” molecules ~ m - ~ ) and the intensity of the CN(A+X) (2,O) band was used as an 10-5 internal actinometer-previous work in this laboratory35estab1013 1014 1015 10’6 lished a CN(A)d=2 quantum yield of 4 0 % a t 158 nm. The INITIAL [CN] (molec ~ r n - ~ ) monochromator-photomultiplier combination was calibrated with Figure 5. Model calculations of time-integrated yields of N,(A) and NO standard lamps, and the sensitivity at the (0,l) NO y-band at 237 y-bands as a function of initial C N concentration. [C2N2] = I O l 5 nm was found to be a factor of 2.2 larger than the sensitivity a t molecules ~ m - [O,] ~, = molecules cm-). the CN(A-X) (2,O) band a t 787 nm. The CN(A)d=2emission intensity was found to be linear in laser energy, and its intensity as the only N,(A) source, for a C2N2-0, ( l O I 5 molecules ~ m - ~ ) was not enhanced by 0, addition so that any chemiluminescence mixture. This graph shows a predicted y-band yield of e 5 X was small compared to the direct production. Based on specfor [CN] = 1015 molecules ~ m - ~This . yield would rise to troscopic information on Franck-Condon factors and transition =2.4 X lo-, if the C2N2were completely bleached. The initial moments, the (0,l) y-band corresponds to -30% of the y-band production of 0 by 0,photodissociation at 158 nm reduces the emission from u’ = 0 and the (2,O) CN(A-X) band is -37% of y-band yield in the bleached situation (from 2.4 X lo-, to 1.9 X the u’ = 2 emission of CN(A). A factor of 2 was applied to the lo-, for initial [O] = 6 X 1014molecules ~ m - ~ Hence, ). reaction intensity of the CN(A),,,, emission to allow for quenching by C2N2 3a producing N,(A) with 2.25% yield can account for the yield ( k = 7.3 X lo-” cm3 molecule-’ s-1)34and 0,( k not known for observations (assuming that the rate coefficient is the 1 X u’ = 2 but assumed to be 3 X 10-l’ cm3 molecule-‘ s-l as meacm3 molecule-’ s-l used in the model). Calculations for [C2N2] ~ u r e for d ~ u’~ = 0 and l). With 1-2 mJ of 158-nm radiation = [O,] = 1015molecules cm-3 using reaction 8 as the only N,(A) focused into the cell, the peak C N densities produced were essource give a y-band yield of 1.5 X lW3 for [CN] = 10” molecules ~, to contimated as (5-10) X lOI4 molecules ~ m - corresponding if the C2N2were completely ~ m - ~This . value rises to 4.4 X siderable depletion of the initial C2N2concentration, and y-band bleached. Hence, for reaction 8 to be the N2(A) source, not only quantum yields (i.e., y-band photons per absorbed photon) as high would the C2N2have to be completely bleached but the N2(A) were observed (with a factor of 3 estimated unceras 5 x yield would have to be 100%. The situation is made even more tainty). Figure 5 shows the calculated yields, with reaction 3a difficult since the initial production of 0 by O2photodissociation reduces the y-band yield in the bleached situation (from 4.4 X for initial [O] = 6 X 1014 molecules ~ m - ~ ) . to 3.3 X Furthermore, the efficiency of reaction 6 may not be the 100% (20) Lam, L.; Dugan, C. H.; Sadowski, C. M. J . Chem. Phys. 1978,69, 2877. assumed in this model. Also, if the energy pooling loss of N,(A) (21) Lifshitz, A.; Frenklach, M. Int. J . Chem. Kinet. 1980, 12, 159. (reaction 10 in Table I) has a rate coefficient as high as 2 X lo4 (22) Black, G.; Slanger, T. G.; St. John, G. A.; Young, R. A. J . Chem. cm3 molecule-’ s-l (ref 37). this would reduce the y-band yield Phys. 1969, 51, 116. in the bleached situation. (Without bleaching, the effect is 510% (23) Davenport, J. A.; Slanger, T. G.; Black, G. J . Geophys. Res. 1976, on the y-band yield because of the fast removal of N,(A) by 81, 12. (24) Schofield, K. J . Phys. Chem. ReJ Data 1979,8, 723. C2N2). Even with a higher rate coefficient for reaction 8, it seems (25) Piper, L. G.; Caledonia, G. E.; Kennealy, J. P. J. Chem. Phys. 1981, unlikely that reaction 8 could account for all the y-band yield 74, 2888. (although a contribution cannot be excluded). (26) Ianuzzi, M. P.; Kaufman, F. J. Phys. Chem. 1981, 85, 2163. In fact, some yield might be anticipated from results3* on the (27) Piper, L. G.; Caledonia, G. E.; Kennealy, J. P. J. Chem. Phys. 1981, 75, 2847. yield of isoelectronic reaction of N(4S) N3 in which an ~ 2 0 % (28) Hill, R. M.; Gutcheck, R. A.; Huestis, D. L.; Mukherjee, D.; Lorents, N2(B3ng)is formed. This emits first positive radiation to produce D. C. Report MP 74-39; Stanford Research Institute: Menlo Park, CA, July N2(A). In the present experiment, if the sole source of N,(A) 1974. were via N2(B) (and both reactions 3a and 8 are sufficiently (29) Nadler, I.; Rotem, A.; Rosenwaks, S. Chem. Phys. 1982, 69, 375. (30) Slanger, T. G.; Wood,B. J.; Black, G. J . Photochem. 1973, 2, 63. exothermic to produce N2(B)), 1+ emission would have been (31) Dum, 0. J.; Young, R. A. Int. J. Chem. Kinet. 1976, 8, 161. detected in the visible and IR regions (despite the presence of the (32) Braun, W.; Bass, A. M.; Davis, D. D.; Simmons, J. D. Proc. R . Soc. CN(A+X) emission in this region). London, A 1969, 312,417. Additional evidence against an important contribution from (33) Baulch, D. L.; Drysdale, D. D.; Home, D. G.; Lloyd, A. C. Evaluated Kinetic Data for High Temperature Reactions; Butterworths; London, 1973; reaction 8 was found in the effect of N O on the y-band yield. In
-
~~
+
VOl. 2. (34) Taherian, M.-R.; Slanger, T. G., unpublished results. (35) Taherian, M.-R.; Slanger, T. G. J . Chem. Phys. 1984, 81, 3814. (36) Taherian, M.-R.; Slanger, T. G. J. Chem. Phys. 1985, 82, 2511.
(37) Hayes, G. N.; Oskam, H. J. J . Chem. Phys. 1973, 59, 1507. (38) David, S.J.; Coombe, R. D. J . Phys. Chem. 1985, 89, 5206.
J. Phys. Chem. 1986, 90,6848-6853
6848
Figure 4, the experimental points were simply normalized to the computed curves to look for differences in shapes. However, the relative yields of y-bands are available from the experiments and show that small additions of N O enhance the y-band emission, as expected from reaction 6. The experiments also show that the enhancement (relative to no added NO) persists even at the highest ) . is consistent with N O addition (1.76 X lOI5 molecules ~ m - ~ This reaction 3a as the N2(A) source but not reaction 8. This arises because additional NO introduces a large additional loss of N(%), whereas N(2D) is already being removed by C2N2and therefore the effect of the additional removal by N O is mitigated. Thus, although none of the evidence is conclusive, it all points to reaction 3a as the important source of N2(A) and hence N O y-band chemiluminescence in the photodissociated cyanogenoxygen system. It is interesting to note that the N O y-bands are a very weak feature of the chemiluminescence produced in the photodissociated C2N& system. Clearly, CN(A and B) and the triatomic emitters NCO, NCN, and C C N are more efficiently produced. In fact, from Figure 1 and the relative monochromator-photomultiplier sensitivity,it appears that the most intense chemiluminescent feature, CN(B) emission, has a quantum efficiency of 20.1. It is interesting to speculate that, since a major loss of N(2D) is
-
assumed to be reaction with C2N2, if the reaction proceeds by N(2D) + C2N2
Nz
+ C2N
AH = -109 kcal/mol (1 1)
there is sufficient exothermicity to produce the observed C2N(A-+X) emission at 466 nm. Furthermore, C2N will be very reactive and its reaction with the 0 atoms present AH = -115 kcal/mol (12) C2N 0 CO C N
+
-+
+
is sufficiently exothermic to produce the CN(A) and CN(B) that are observed. It is this same reaction that was invoked39to explain the C N emissions observed when C2N2is added to partially titrated active nitrogen (with NO giving oxygen atoms). Clearly, measurements of the type described in this paper for the y-bands and additional modeling studies would help to determine the chemiluminescent mechanisms for the other emissions.
Acknowledgment. This work was supported by Contract No. F29601-84-C-0099 with the Air Force Weapons Laboratory. R@tV NO. C2N2,460-19-5; 0 2 , 7782-44-7; C N , 2074-87-5; NCO, 22400-26-6; 0, 17778-80-2; N2, 7727-37-9; N O , 10102-43-9; CO, 630-
08-0. (39) Safrany, D. R.; Jaster, W. J . Phys. Chem. 1968, 72, 3305.
Thermal Decompogition of Energetic Materials. 17. A Relatlonship of Molecular Compodtion to H M O Formation: Bicycio and Spiro Tetranitramlnes T. B. Brit]* and Y. Oyumi Department of Chemistry, University of Delaware, Newark, Delaware 19716 (Received: July 14, 1986)
The IR-active gas products from the rapid thermal decomposition of six cyclic nitramines, five of which contain bicyclic and spiro ring systems, were quantified. Four of the compounds are structural isomers CsHloNsOs. Combining these results with 11 nitramines from our previous studies led to the discovery of a relationship between the H / N 0 2 ratio of the parent molecule and the amount of HONO released in the initial 0.2 s of fast decomposition. Further analysis suggests that HONO forms largely from the adventitious encounter of H' and NO2' rather than by a specific, well-defined reaction, that CH2 units may be more effective H' donors than CH3units in the nitramines studied to date, that by sterically crowding together the NO2 and CH2 units, the production of HONO might be enhanced, and that HONO is an effective source of NO via its secondary reactions.
Introduction Compounds containing the nitramine functional group, >NNO, can be important ingredients in chemical propellants. Recent studies suggest that the formation and further reactions of H' radicals play a prominent role in the thermal decomposition of these energetic The decomposition of octahydro1,3,5,7-tetranitro-1,3,5,7-tetrazacene (HMX) in the solid phase exhibits a primary kinetic isotope effect consistent with the rate-controlling step being homolysis of the C-H bond.' HONO was proposed to be the product stemming from the abstraction of H' from -CH2- by NO;.' Theoretical calculations of the bond energies of simple gas-phase nitramines along the reaction coordinate suggest that the thermal decomposition can be driven by autocatalytic-generated H atoms attacking the nitramine fragmentG2These studies cast light on some of the elementary reactions that may take place during what is, undeniably, a very complicated physicochemical process. The formation of HONO from a thermally decomposing nitramine has been proposed and discussed at some length in recent (1) Shackelford, S. A.; Coolidge, M. B.; Goshgarian, B. B.; Loving, B. A,; Rogers, R. N.; Janney, J. L.; Ebinger, M. H . J. Phys. Chem. 1984,89, 31 18. (2) Melius, C. F.; Binkley, J. S . Symp. (Int.) Combust., Z l s r , 1986, in press.
years by others3-' Hindering a more full understanding has been the fact that the experimental evidence for H O N O consisted of its identification as a minor product from the very-low-pressure pyrolysis of dimethylnitramine (DMNA)8 and indirect evidence from mass spectral studies on larger nitra~nines.~-'~ This void has been filled by the development of fast pyrolysis methods (3) Shaw, R.; Walker, F. E. J . Phys. Chem. 1977, 81, 2572. (4) Schroeder, M. A. ARBRL-MR-03370, Ballistic Research Laboratory, Aberdeen, MD, 1984; CPIA Publ. 1979,11(308), 17; Proc. 18th JANNAF Combustion Mtg, CPIA Publ. 1981, II(347). 395. (5) Goshgarian, B. B. Report AFRPL-TR-78-76, Air Force Rocket Propulsion Laboratory, Edwards AFB, CA, 1978. (6) Brill, T. B.; Reese, C. 0. J . Phys. Chem. 1980, 84, 1376. (7) Fifer, R. A. Prog. Astronaut. Aeronaut. 1984, 90, 171. (8) McMillan, D. F.; Barker, J. R.; Lewis, K. E.; Trevor, P. L.; Golden, D. M. Final Report on SRI Project PYU 5787, Stanford Research International, June, 1979. (9)Bulusu, S.; Axenrod, T.; Milne, G. W. A. Org. Mass Spectrom. 1970, 3, 13. (10) Stals, J.; Buchanan, A. S.; Barraclough, C. G. Trans. Faraday SOC. 1971, 67, 1756. (1 1) Vouros, P.; Petersen, B. A.; Karger, B. L.; Harris, H. Anal. Chem. 1977,49, 1039. (12) Bradley, J. N.; Butler, A. K.; Capey, W. D.; Gilbert, J. R. J . Chem. Soc., Faraday Trans. I , 1977, 73, 1789. (13) Farber, M.; Srivastava, R. D. CPIA Publ. 1979, (308). 59. (14) Farber, M.; Srivastava, R. D. Chem. Phys. Leu. 1979, 64, 307.
0022-3654/86/2090-6848$01.50/00 1986 American Chemical Society
