THE JOURiNAL OF
SICAL CHEMISTRY Registered in U.S.Patent Office 0 Copyright, 1980, by the American Chemical Society
FEBRUARY 7,1980
VOLUME 84, NUMBER 3
Ultraviolet Spectrum and Reaction Kinetics of the Formyl Radicalt C. J. Hochanadel," T. J. !Sworskl, andl P. J. Ogren' Cf'JemlstryDivision, Oak RUge National Laboratory, Oak RMge, Tennessee 37830 [Recelved June 18, 1979) Publication costs assisted by the U.S. Department of Energy
A transient ultraviolet absorption attributed to the formyl radical, HCO, was produced by the flash photolysis of water vapor in the presence of CO. For CO saturated with H20 at a total pressure of 1atm, the formation of HCO i3 complete within -60 CLS, after which it undergoes second-order decay, with kle(230 nm) = (1.5 f 0.2) X lo7cm s-l. During the relatively slow formation reaction (H + CO (+MI HCO),the competing reaction €1 + HCO H2 + CO is significant. By employing the HzO + CHI system as an actinometer to determine
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-
the amount of H20 dissociated by the flash (discussed elsewhere) and by computer modeling the observed formation and decay of HCO as a function of CO pressure, the following absolute rate constanta were obtained: k,(H+CO(+M)-HCO) =: (3.6 f 0.9) X lo7 M-2s-l for M = CO, (5.8 f 0.6) X lo7 M-2 s-l for M = CHI, and (3.8 f 0.4) X lo7 M-2 s-l for M = H2, k4(HCO+HCO+products) = (1.4 f 0.3) X 1O'O M-' s-l, and k,(H+HCO-products) = (6.9 A: 1.7) X 1O1O IM-' s-l. The molar extinction coeffecient e(HC0, 230 nm) = 941 f 171 R4-l cm-l.
Introduction The formyl radical, HCO, is an important intermediate in several kinetic systems of current interest, especially combustion kinetics. There is an obvious need for accurate rate constants of elementary reactions of this radical. In our studies, the radical was produced by combination of H with CO. The principal results to be reported here are its ultraviolet absorption spectrum and rate constants for its formation, bimolecular decay, and reaction with H atoms. The spectrum of HCO has been the subject of several detailed experimental and theoretical studies. In the optical spectrum the weak red bands that appear in absorption between 450 and 900 nm were first discovered in the flash photolysis of various aldeh~des.l-~ The much stronger ultraviolet band system was jfirst observed in emissions between 230 arid 410 nm and i8 usually refierred Research sponsored by the Office of Basic Energy Sciences of the U. S. Department of Energy under Contract W-7405-eng-26with the Union Carbide Corporation. Visiting scientist from Earlham College, Richmond, Ind. 47374.
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0022-3854/80/2084-0231$01 .OO/O
to as the hydrocarbon flame bands! Later, the absorption counterpart of the flame bands was observed between 210 and 260 nm by using a low-temperature matrix-isolation technique.6 The 000-000transition was estimated to be at 259 nm. Theoretical calculations of Bruna et aleeshow a vertical excitation of 5.43 eV (228.3 nm) that is consistent with the observed intensity maximum.6 Third-order rate constants for the combination of H with CO, k,(H+CO(+M)-*HCO), have been measured by two groups by use of Lyman-a absorption to observe the H atoms that were generated either by pulse radiolysis7p8or flash photolysis.8 The values reported at 298 K were (4.01 0.6) x 107 M-Z8-1 and (2.6 0.4) X lo7 M-2 s-l for M = H2 and Ar, respectively,71sand 2.9 X lo7,2.2 X lo7, 2.5 X lo7, 2.2 X lo7, and '1.7 X lo7 M-2 s-l for M = H2,Ar, Kr, He, and Ne, respe~tively.~The reaction was in the! third-order range at least up to 1 atm p r e s ~ u r e . ~ ~ ~ Reilly et al.'O studied reactions of HCO radicals, which they produced by photolyzing formaldehyde with a pulsed ultraviolet laser. A novel method of time-resolved intracavity dye laser spectroscopy was employed to follow the reactions of HCQ by absorption in the 614.4-nm bands.
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0 1980 American Chemical Society
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232 The Journal of Physical Chemistty, Vol. 84, No. 3, 1980
Hochanadel, Sworski, and Ogren
TABLE I: Reactions in H,O-CO Systems
rate constant
ref
reaction hu (1)H,OH t OH (2)OHtCO-+HtCO,
h , = (9.2
(3)H t C 0 M " C O (a) (M = CO) ( b ) (M = CH,, K O ) (c) (M = H,) (4) HCO t HCO products ( 5 ) H t HCO products
k,,= ( 3 . 6 i 0.9) x 1 0 7 M z 2 - ~ ' 1 k,b = (5.8 f 0.6) X 10' M-' s-' k,, = (3.8i 0.4) x l o 7 M - 2 s-l k , = (1.4f 0.3)X 10" M - I s - ' k , = (6.9 i 1 . 7 ) X IO'OM-' s-'
a a a a
k,, = (3.3 t 0.2) x 109 M-2
k a b = (9.0 i: 2.0) X l o l o M - Z S - l k , , = (5.5 i: 0.3) X M-2 s-l ksd = (2.9 i 0.2) X l o 9 M-, S-'
22 26 23,24 23-25
k , , = (1.7 i 0.6) X 10" M-'s-' k7b = (9.5 f 3.5) X 10" M-, S-' k , = (1.4 i 0.2) X l o 9 M-' s-l
27 28 31-33
k,, = (6.2 i: 2.3) x 1 O ' O M-, s-' h9b (3.4 i 1.3) X I O " M-'S-' k , , = 3.0 X 1O'O M-I s-' k , , = (5.1 i: 1.1)X 10, M-'s-l k , , = (4.2 i 0.4) x 10, M - I s-' k , , = (2.6 0.3) x 1 0 ' M - ' s-' k , , = 4.6 X 10'O M-l s"
34 34 38 21 21 35 36, 37
k , = (3.1 i 0.6) X 10"
s-'
16
k , , = (1.2 i 0.3) x 10" M-' s-' k , , = (5.6 i 1.5) x 10,' Mu] s-'
21 21
-+
-+
i:
1.0) x 107 M-,
s-l
20
a
M
(6)HtH 4 H 2 (a) (M = CO) (b) (M = H,O) (c) (M = CH,) (d) (M = H,) ( 7 ) H t O H 3 H,O (a) (M = CO, H,) (b) (M = H,O, CH,) ( 8 ) OH t OH H,O t 0 -+
8-1
lo9
M
( 9 ) OH t OHH20, (a) (M= co, H,) (b) (M = H,O, CH,) (10) OH t HCO H,O t CO (11)OH t H, -+ H,O t H (12) OH t CH, + H,O + CH, (13) H t CH, H, t CH, (14) CH, t HCO CH, t CO -+
-+
-+
(15) CH, t CH,
M --+
C,H,
M
(16) CH, t H --- CH, (17) CH, t OH -+ products a
_+
M-I
The italicized values were determined in this study.
They reported values for k,(HCO+HCO) of 10-10~2f0~6 cm3 molecule-'^-^ and for k5(H+HCO) of 1049.26f0.3 cm3 molecule-l s-l. Nadtochenko et al.ll produced HCO by the flash photolysis of formaldehyde, and also employed intracavity laser spectroscopy to follow the reactions of HCO. They reported a ratio of rate constants, IZ5/k4 = 6.7 f 2.7. In another paper, Nadtochenko et al.13 refer to a measurement12 of k4 based upon the flash photolysis of acetaldehyde. The value for k4 was 3 f 1.3 X cm3 molecule-l s-l, and, from the ratio k 5 / k 4 = 6.7, k5 = 2 f 0.7 X cm3 molecule-' s-l. They also found both k4 and IZ5 to be independent of pressure above 10 torr.
Experimental Section Materials. The CO was Matheson purity grade (>39.99%, Matheson Co.) which flowed through a silica tube furnace at a dull red heat in order to eliminate any O2 and decompose any carbonyl. The CHI was Matheson Co. ultra-high-purity grade (>99.97%) which was further purified by using a Matheson Co. Hydrox purifier which is claimed to reduce the O2to 0.1 ppm. The H2 was Matheson Co. ultra-high-purity grade (>99.999%) with a stated O2 content of
