J. Phys. Chem. 1985,89, 2914-2916
2914
Disproportionation to Combination Ratios of Alkdxy Radicals with Nltric Oxidet Paul Morabito and Julian Heicklen* Department of Chemistry and Center for Air Environment Studies, The Pennsylvania State University, University Park, Pennsylvania 16802 (Received: December 27, 1984)
-
-
The disproportionation to combination ratios were measured at 175 OC for the reactions RO + l5NO ROI5NO (2a) and RO + I5NO RCHO + H15N0 (2b), with the following alkoxy radicals: C2H50,n-C3H70,?&&go, and i-C4H90. The alkoxy radical was generated by the thermal decomposition of the corresponding alkyl nitrite in the presence of I5NO. The rate of the corresponding isotopically enriched alkyl nitiite was measured by mass spectrometry while the aldehyde rate was determined by gas chromatography. The results obtained for kZb/k2were 0.22 f 0.02,0.26 0.03,0.29 0.05, and 0.33 A 0.03, respectively, for C ~ H S On-C3H70, , n-C4H90,and i-C4H90. With these values of kZb/k2we were able to determine the primary quantum yield of the photolysis of the corresponding alkyl nitrites at 366 nm to be, respectively, 0.32 f 0.04, 0.44 f 0.06, 0.19 0.04, and 0.19 0.02.
*
*
*
*
Introduction
TABLE I: Literature Valuea for the Disproportionation to Total
The pyrolysis or photolysis of alkyl nitrites (RONO) gives NO plus alkoxy radicals
Reaction Ratio ( k s J k 2 )for the Reactions of Alkoxy Radicals with NO
+ -
RONO RO + NO The products of reaction 1 can then react with each other by combination (reaction 2a) or disproportionation (reaction 2b) RONO (2a) RO NO R'CHO + H N O (2b)
alkoxy radical CH,O CH30 CH30
The values for the disproportionation to total reaction ratios, kZb/k2,have been determined for several alkoxy radicals, where k2 kZa kZb. These values are listed in Table I. A popular technique employed in determining these ratios involved pyrolyzing the alkyl peroxide in the presence of NO.' Unfortunately, unwanted side reactions and wall effects are common in proxide pyrolysis, which makes kinetic interpretation d i f f i c ~ l t . ~Another ,~ method used as a source of alkoxy radicals has been the photolysis of alkyl nitrites. In this case the products of reaction 2a cannot be measured. Values of kZb/kZhave been computed by assuming a quantum yield of photodecomposition of 1.0. The results obtained by the two technique differ. A more direct approach which was employed in this study involves pyrolyzing the appropriate alkyl nitrite in the presence of lSNO. The rate of recombination product can be directly determined by mass spectrometry while the disproportionation product can be measured by gas chromatography. The relative rate of the two processis yields kZa/kzbfrom which kZb/k2can then be directly calculated. Using the alkyl nitrite as a thermal source of alkoxy radicals eliminates unwanted effects which occur in alkyl peroxide pyrolysis. When both processes are monitored directly, if any unknown side reactions did occur, its effects should be canceled out by the relative rate measurement. In this study we obtain values of kZb/k2for CZHSO, n-C3H70, n-C4H90, and i-C.+HgO radicals. N o earlier reports exist for n-C4H90 and i-C4H90,and only one other determination exists for n-C3H70 radicals.
C2HSO
A or hu
CZH50 C2H50
C2H5O i-C3H70 i-C3H70 i-C3p70 i-C3H70 i-C3H70 i-C3H70 i-C3H70 n-C3H70 sec-C4Hg0 sec-C4H90
+
Experimental Section
Mixtures of R O N O ( R = C2H5, n-C3H7, n-C4H9, i-C4H9), 15N0,and N 2 were pyrolyzed at 175 OC in a 329-mL cylindrical Pyrex reaction vessel equipped with quartz windows. The reaction vessel was enclosed in an aluminum furnace resistively heated by an Omega 1522 proportional temperature controller. Continuous product analysis was performed by allowing the contents of the cell to bleed through a small pinhole into an Extranuclear Type I1 quadrupole mass spectrometer. The reagents were mixed directly in the reaction cell, after which continuous recording of the mass spectrum in the range of m/e 35-85 was 'CAES Report No. 729-84.
0022-3654/85/2089-2914$01.50/0
temp,. O C ref 25 Wiebe and Heicklen" 174 Arden et al.' 25 Gray and PrattIz 95 Arden et al.' 135 Arden et a1.l 25 East et al.I3 25 Rebbert14 200 Livermore and Phillip~'~ 26 Ludwig and McMillanI6 77 Ludwig and McMillanI6 16 M~Millan'~ 0.17 71 McMillanI7 0.18 104-1 49 Hughes and Phillips'* 0.15 26-180 Ludwig and M ~ M i l l a n ~ ~ 0.13 121-159 Yee Quee and Thynne2" 0.29-0.31 100-150 East and Phillipsz' 0.21 100-140 Walker and Phillipsz2 0.17 140 East and Phillipsz3 k2blkZ
0.145 0.33 0.12 0.23 0.24 >0.17 -0.23 0.2s 0.15 0.20 0.14
obtained with a scan time of 30 s. Mass spectrometric determinations were performed with 40-eV electrons by measuring the product ion peak (CH2015NO;m l e 61) relative to the m l e 40 peak from a known amount of argon which was added initially with reagents. Measurements relative to the argon peak reduced errors from instrumental and other periodic fluctuations. Calibrations were obtained for each nitrite in which the calibration factors for C H 2 0 N 0 (m/e 60) and CH2015N0(m/e 61) fragments were assuhed to be identical. After the combination rate was measured, the aldehyde was analyzed on a Gow Mac gas chromatograph by allowing the contents of the cell to expand into a gas sample loop. A 6 ft by 1/4 in. diameter Teflon column containing GP 20% SP-2100/0.1% Carbowax 1500 on Supelcoport, run at 23 "C and a helium flow rate of 68 mL/min, was used. The aldehydes were analyzed by a thermistor detector. The nitrites were prepared as before and stored over mercury and Drierite in the dark a t -196 0C.4-7 Ninety-nine percent acetaldehyde, propionaldehyde, butyraldehyde, and isobutyr(1) E. A. Arden, L. Phillips, and R. Shaw, J . Chem. Soc., 5126 (1964). (2) G. R. McMillan, T. Kumari, and D. L. Snyder, 'Chemical Reactions in Urban Atmospheres", C. S . Tuesday, Ed., Elsevier, Amsterdam, 1971, p 7T
-I.
(3) S . Zabarnick, Ph.D. Thesis, Penn State University, 1984. (4) S. Zabarnick and J. Heicklen, Int. J . Chem. Kinet., in press (part I). ( 5 ) S . Zabarnick and J. Heicklen, In?. J . Chem. Kinef.,in press (part 11). (6) P. Morabito and J. Heicklen, unpublished work at Penn State University, 1983. (7) S . Zabarnick and J. Heicklen, fnt. J. Chem. Kine?.,in press (part 111).
0 1985 American Chemical Society
The Journal of Physical Chemistry, Vol. 89, No. 13, 1985 2915
Alkoxy Radicals with Nitric Oxide
TABLE II: Values for the Disproportionationto Total Reaction Ratio ( k u / k l ) for the Reactions of RO with lSNO at 175 “C“ RO C2H50
n-C,H,O
n-C4H90
i-C4H90
[NOIT,torr
d(RO”NO)/dt
1.99 2.03 10.39 10.49 11.00 1.89 2.07 10.26 13.87 10.10 10.36 10.61 10.84 24.92 27.90 10.52 10.66 11.65 25.51
0.020 0.020 0.027 0.026 0.026 0.021 0.020 0.029 0.028 0.014 0.014 0.015 0.016 0.017 0.016 0.016 0.018 0.018 0.021
d(R’CHO)/dr
f 0.002 f 0.001 f 0.001 f 0.002 f 0.002 f 0.003 f 0.001 f 0.001 & 0.001 f 0.002 f 0.003 f 0.003 f 0.003 f 0.001 f 0.001 f 0.001 f 0.002 f 0.001 f 0.002
k2b/k2
0.0062 f 0.0003 0.0065 f 0.0003 0.0082 f 0.0004 0.0081 f 0.0004 0.0083 f 0.0004 0.0084 f 0.0004 0.0082 f 0.0004 0.0110 k 0.0009 0.0109 f 0.0009 0.0064 0.0003 0.0065 & 0.0003 0.0068 f 0.0003 0.0069 f 0.0003 0.0078 f 0.0004 0.0080 f 0.0004 0.0093 f 0.0005 0.0096 f 0.0005 0.0095 f 0.0005 0.0121 f 0.0009
0.22 0.22 0.21 0.22 0.22 0.26 0.26 0.25 0.26 0.30 0.29 0.29 0.28 0.29 0.30 0.34 0.32 0.32 0.34
*
f 0.02 f 0.01 f 0.02 f 0.02 f 0.02 f 0.04 f 0.02
f 0.02 f 0.03 f 0.04 f 0.06 f 0.06 f 0.05 f 0.02 f 0.02 f 0.03 f 0.04 f 0.02 f 0.04
* [RONO] = 4-6 torr; [Ar] = 4-5 torr; N2 added to give a total pressure of 150 torr; [15NO]/[NO]T= 0.887.
/
C, H,O” NO L
1501 125
.
I 100
400
300 REACTION T I M E , 6ec
500
Figure 1. Plot of ROI5NOpressure vs. reaction time at 175 OC. For the C2H50N0system [C2H50NO]= 5.28 torr and [NOIT = 10.39 torr. For the n-C4H90N0system [n-C4H90NO]= 5.06 torr and [NOIT =
i 0
1
100
I
300
200
400
KM
REACTION T I M E , oec
Figure 2. Plot of ROI5NOpressure vs. reaction time at 175 O C . For the n-C,H,ONO system [n-C,H,ONO] = 5.68 torr and [NOIT= 13.87 torr. For the i-C4H90N0system [i-C4H90NO]= 5.1 1 torr and [NOIT =
10.01 torr.
11.65 torr.
aldehyde were obtained from Aldrich and used without purification. 15N0 was obtained from MSD Isotopes and distilled trap-to-trap from -186 to -196 “C. I5NO purity was analyzed by mass spectrometry and found to be 88.7% in lSN. Extra dry grade N2 was obtained from the Linde Co. and used without further purification.
raised by a factor of -2.5. In all cases the rates of the isotopically labeled nitrite and aldehyde increased with increasing 15N0 pressure.
RHults The thermal decomposition of alkyl nitrites in the presence of lSNOproduces the corresponding 15N-labelednitrite and aldehyde. Initially all experiments were performed with 5 torr of RONO and 10 torr of total NO pressure, [NO],, which was 88.7% rich in ISNO. In each of the experiments sufficient N 2 was present to eliminate “hot” radical effects and to ensure that the recombination of R O N O was in the high-pressure limit.e7 Figures 1 and 2 illustrate the RO15N0 growth as a function of time. In all cases the growth was linear over the time range of the study. The slopes of each plot determined from a leastsquare analysis reveal the rates of reaction 2a which are listed in Table 11. After sufficient reaction took place, the reaction mixture was temperature quenched to 23 O C and the aldehyde measured. The rates of aldehyde formation are also listed in Table 11. Subsequent experiments were done by changing the pressure of 15N0. For the C2H50N0and n-C3H70N0 experiments the lSNO pressure was lowered by a factor of 5 whereas in the nC , H 9 0 N 0 and i-C4H90N0experiments the l5NO pressure was
Discussion The mechanism of the pyrolysis of RONO, NO, and N 2 mixtures is RONO
RO
-
-
RO
-
RO
+ ISNO-,~
RO
+
-
+ NO
-
--
+ NO 0
1
(1) 5
~
0
RCHO
+ HISNO (2b(15))
RONO
(W14))
RCHO
+ HNO
decomposition or isomerization
(2b(14))
(3)
where the rate coefficients k2a(15) = kZa(14) = kza and k2+,( 15) = k2b(14) = k2b,if the small kinetic isotope effect is ignored. The steady-state rate law based on the above mechanism is R(RO’5NO] R(aldehyde1
k h [”NO] k2b [NOIT
(a)
where R(X]is the rate of formation of product X and [NO], is the total N O concentration. Once k Z a / k Zisb calculated, kzb/kz can be determined from
2916
“=(k+l)
J. Phys. Chem. 1985,89, 2916-2918
-1
k2 At 175 “C, the thermal decomposition of alkyl nitrites proves to be a convenient source of alkoxy radicals. Batt has shown previously that the decompositions of the alkyl nitrites are independent of the alkyl group and typically have the same Arrhenius parameters.* However, the decomposition (and in some cases the isomerization) of the alkoxy radical does depend on the alkyl group, and some would be expected to decompose at 175 OC. Batt has shown that straight-chain alkoxy radicals can decompose primarily by two pathways RO
+H CH2O + R RCHO
-+
-+
(3a) (3b)
If reaction 3a occurs to an appreciable extent, our aldehyde rate measurement would be higher than expected and cause considerable problems with the analysis. Fortunately, previous studies have shown reaction 3b to be the sole fate of straight-chain R O decomposition reactions a t 175 O C . * Thus, all the aldehyde produced in these systems is produced from reaction 2b and not from the decomposition of the alkoxy radical. This is confirmed experimentally by the fact that the computed values for kZb/k2 are independent of N O pressure. Also, the isomerization reactions that can occur with the n-C4H90 radical would not be expected to produce n-C3H7CH0.6*9 From an inspection of the results in Table I1 the values of kzb/kz in all cases are independent of [NO],. This would be expected since we are only interested in the relative rates of both processes. For the C 2 H 5 0and n-C3H70systems we were able to measure both rates at relatively low [NOIT. However, for the i-C4H90 and n-C4H90 systems we were unable to measure these rates at low [NOIT and therefore had to start a t = l o torr of [NO],, because of the fast decomposition (or isomerization) of these radicals relative to the R O ISNO reaction. We can conclude that k2,/k2 = 0.22 f 0.02,0.26 f 0.03,0.29 f 0.05, and 0.33 f 0.03, respectively, for C z H 5 0 , n-C3H70, n-C4H90, and i-C4H90. The uncertainties represent estimated
+
(8) L. Batt, Znt. J . Chem. Kinet., 11, 977 (1979), and references therein. (9) A. C. Baldwin and D. M. Golden, Chem. Phys. Lerr. 60,108 (1978).
TABLE III: Primary Quantum Yields of RONO at 366 nm RONO k2blk2‘ 6ikdk2 41 rep C2HsONO 0.22 f 0.02 0.070 f 0.006 0.32 f 0.04 4 n-CpHTONO 0.26 f 0.03 0.115 f 0.010 0.44 f 0.06 5 n-CdH90NO 0.29 f 0.05 0.054 f 0.005 0.19 f 0.04 6 i-C4H90N0 0.33 f 0.03 0.064 f 0.005 0.19 f 0.02 7 “This work. *For absolute errors. Furthermore, an apparent trend exists between k2,/k2and the molecular complexity. As the molecular complexity increases, the value of k2,/k2 increases. It is not clear why this occurs. The values are independent of temperature, so the effect is not caused by an activation energy. Our k2,/k2values for C 2 H S 0and n-C3H70agree with the literature values presented in Table I within the experimental uncertainty. Finally, since the determination of the primary quantum yield of the photolysis of alkyl nitrites has been hampered by inherent problems of the techniques employed,I0 the values for k2,/k2 determined in this study can be coupled with ratios of kZb&/k2 determined in other studiese7 to calculate the primary quantum yield, c#J~. The calculated values of are presented in Table 111.
Acknowledgment. This work was supported by the Center for Air Environment Studies at Penn State University for which we are grateful. Registry No. C2HSON0, 109-95-5; n-C3H70N0, 543-67-9; nC4H90N0,544-16-1; i-C4H90N0, 542-56-3; C2H50,2154-50-9; nC3H70,16499-18-6; n-CdH90, 19062-98-7; i-C4H90,26397-34-2; 15N0, 15917-77-8. (10) P. Morabito and J. Heicklen, Int. J . Chem. Kinet.,in press. (11) H. A. Wiebe and J. Heicklen, J. Am. Chem. SOC.,91, 1085 (1969). (12) P. Gray and M. W. Pratt, J . Chem. SOC.,3403 (1958). (13) R. L. East, J. R. Gilbert, and L. Phillips, J . Chem. SOC.A , 1673 (1968). (14) R. E. Rebbert, J . Phys. Chem., 67, 1923 (1963). (15) R. A. Livermore and L.Phillips, J . Chem. SOC.E , 640 (1966). (16) B. E. Ludwig and G. R. McMillan, J . Am. Chem. SOC.,91, 1085 (1969). (17) G. R. McMillan, J . Am. Chem. Soc., 83, 3018 (1961). (18) G. A. Hughes and L. Phillips, J. Chem. SOC.A, 894 (1967). (19) B. E. Ludwig and G. R. McMillan, J . Phys. Chem., 71,672 (1967). (20) M. J. Yee Quee and J. Thynne, Tram. Faraday SOC.,64, 1296 ( 1968). (21) R. L. East and L. Phillips, J . Chem. SOC.A , 331 (1970). (22) R. F. Walker and L. Phillips, J . Chem. SOC.A , 2103 (1968). (23) R. L. East and L. Phillips, J . Chem. SOC.A , 1939 (1967).
Polarizatlon of CN(B2z+-X2z+) Emission Produced in Collision of Ar(3P,,,) with BrCN Takashi Nagata, Tamotsu Kondow,* Kozo Kuchitsu, Department of Chemistry, Faculty of Science, The University of Tokyo, Bunkyo-ku, Tokyo 113. Japan
Kiyohiko Tabayashi, Shigeru Ohshima, and Kosuke Shobatake Institute for Molecular Science, Myodaiji, Okazaki 444, Japan (Received: January 2, 1985)
Rotational alignment in the CN(B22+)fragment was observed by measuring polarization of the CN(Bz2+-X28+) emission from BrCN excited by Ar(3Po,z)impact. The degree of polarization with respect to the beam axis was 0.023 & 0.004 at a collision energy of 1.33 eV and decreased with the impact energy. This observation indicates that the production of CN(B) is caused primarily by energy transfer from Ar(3P,,2) to BrCN.
Introduction The CN(BZz+-XzE+) emission produced by Vacuum-UV photodissociation or electron-impact dissociation
hu or c
-
BrCN [BrCN]* Br + CN(B22+) (1) has been studied extensively. This emission is found to be polarized
with respect to the direction of the electric vector, c, of the photons or the direction Of the momentum vector, k, of the impinging electrons.’-“ This polarization originates from the fact that the (1) G. A. Chamberlain and J. P. Simons, J . Chem. SOC.,Faroday Trans. 2, 71, 2043 (1975).
0022-3654/85/2089-2916$01.50/00 1985 American Chemical Society
