J . Phys. Chem. 1990, 94, 5860-5865
5860
of C O is too small for this mechanism to be important in stratospheric chemistry. Acknowledgment. The authors are grateful to several colleagues, especially Stanley Sander, Randall Friedl, Y. L. Yung,
Tony Cox, Carleton Howard, and Mario Molina, for helpful comments and suggestions. Randall Friedl kindly supplied us with a sample of CI20. This research was carried out by the Jet Propulsion Laboratory, California Institute of Technology, under contract with the National Aeronautics Administration.
Formyl Chloride: UV Absorption Cross Sections and Rate Constants for the Reactions with Ci and OH H. C.Libuda, F. Zabel,* E. H. Fink, and K. H. Becker Physikalische Chemie, Fachbereich 9, Bergische Uniuersitat-Gesamthochschule Wuppertal, Gaussstrasse 20, 5600 Wuppertal 1, FRG (Received: December 1. 1989; In Final Form: March 8, 1990)
Spectroscopic and kinetic properties of gaseous formyl chloride (CHCIO) were studied in large reaction chambers. The near-UV absorption spectrum was investigated with a spectral resolution of 0.7 nm. The UV spectrum is similar to that of formaldehyde but shifted to lower wavelengths by about 45 nm. The absorption maximum lies between 250 and 260 nm, cm2 at 260 nm. The reactions of CHCIO with OH, C1, and H,O with an absorption cross section of (6.0 1.2) X were studied by relative rate methods. For the rate constant of the reaction CHCIO + OH products, an upper limit of cm3/s was determined at room temperature. The reaction CHClO + CI products was studied at atmospheric 13.2 X pressure as a function of temperature between 266 and 322 K. In this temperature range the rate constant is represented by the Arrhenius expression 1.39 X lo-" exp(-7.2 f 1.4 kJ.mol-'/RT) cm3/s. From these data, a lower limit of about 1 month is derived for the tropospheric lifetime of CHClO with respect to photolysis and reactions with OH and C1. Actually, the tropospheric lifetime of CHCIO may be limited by hydrolysis as only an upper limit of 5 X cm3/s could be determined for the rate constant of the gas-phase reaction of CHClO with water vapor at 298 K, which corresponds to a lower limit of its lifetime of about 2 h at typical tropospheric conditions.
*
Introduction Formyl chloride (CHCIO) was first identified by Hisatsune and Heicklen' via its gas-phase infrared absorption spectrum. It is an intermediate in the atmospheric degradation of several important chlorinated hydrocarbons, e.g., CH3CI, CH2C12, and C2HC13.'" To date, it has not been detected in ambient air. In the laboratory, formyl chloride readily undergoes thermal decomposition on the walls of reaction chambers, giving C O and HCI as products. Although CHCIO has often been detected in situ as a reaction intermediate in laboratory chemical systems by its infrared absorption spectrum,]-* data on its kinetic behavior are very sparse. The rate constant for its reaction with CI atoms has been reported to be fairly low at room t e m p e r a t ~ r e . ~The ,~ rate constants for the reaction with O H radicals and for photolysis are not known. Recently, the near-UV spectrum of CHCIO was investigated at modest resolution by using a 1.5-m spectrograph.IO The vibrational structure was analyzed between 269 and 314 nm, and the absorption was assigned to the ALA" X'A' transition. UV absorption coefficients have not been published.
-
( I ) Hisatsune, 1. C.; Heicklen. J. Con. J . Spectrosc. 1973, 18, 77. (2) Gay, B. W.; Hanst, P. L.; Bufalini, J. J.; Noonan, R. C. Enuiron. Sci. Techno/. 1976, IO, 58. ( 3 ) Tuazon, E. C.; Atkinson, R.; Aschmann, S. M.; Goodman, M. A,; Winer, A. M. fnr. J. Chem. Kinet. 1988, 20, 241. (4) Niki, H.; Maker. P. D.; Savage, C. M.; Breitenbach, L. P. fnr.J . Chem. Kiner. 1980, 12, 1001. ( 5 ) Niki, H.; Maker, P. D.; Savage, C. M.; Breitenbach, L. P.; Martinez, R. 1.; Herron, J. T. J . Phys. Chem. 1982, 86, 1858. (6) Sanhueza, E.;Heicklen, J. J . Phys. Chem. 1975, 79, 7. (7) Tuazon, E. C.; Atkinson, R.;Winer, A. M.; Pitts, Jr., J. N. Arch. Enoiron. Contam. Toxicol. 1984, 13, 691. (8) Blume, C . W.; Hisatsune, 1. C.; Heicklen, J. fnr. J . Chem. Kiner. 1976,
8. 235. (9) Niki, H.; Maker, P. D.; Savage, C. M.; Breitenbach, L. P. fnr.J . Chem. Kiner. 1980, 12, 915. (IO) Judge, R. H.; Moule, D. C. J . Mol. Specrrosc. 1985, 113. 302.
0022-3654/90/2094-5860$02.50/0
- -
To provide a better data base for calculations of the kinetic and photochemical behavior of formyl chloride in the atmosphere, the near-UV absorption coefficients of CHCIO have been determined and the reactions of CHCIO with OH and H 2 0 have been investigated at room temperature and atmospheric pressure and also with CI atoms in the temperature range 266-322 K.
Experimental Section Two reaction chambers have been used for the present experiments: (i) A cylindrical temperature controlled (from -30 to +50 "C) reaction vessel made from Duran glass of 420-L volume (60-cm i.d., length 150 cm) with a built-in White mirror system for long-path IR absorption measurements, usually operated at an optical path length of 50.4 m. IR spectra were analyzed with a Fourier transform spectrometer (Nicolet 7 199) at a spectral resolution of 1 cm-I. This apparatus has been described previously in more detail.'' Recently, a UV absorption path with a single reflection on a flat mirror (optical path length 3 m) was installed in this chamber, thus allowing the simultaneous recording of both UV and IR spectra. UV absorption spectra were measured by using a deuterium lamp in combination with a 22-cm spectrometer (SPEX) and a diode array detector (PAR 1412). A spectral resolution of about 0.4 nm was achieved with a grating of 1200 lines/" covering a spectral range of 70 nm on the diode array detector. The typical exposure time for a single spectrum was 1 s. (ii) A cylindrical Duran glass vessel of 480-L volume (45-cm i.d., length 300 cm) designed for measurements at room temperature. This vessel was equipped with a built-in White mirror system for long-path UV absorption, usually operated at an optical path length of 52.6 m. The same lamps and detection system were ( I 1) Barnes, I.; Becker, K. H.; Fink, E. H.; Reimer, A.; Zabel, F.; Niki,
H.I n t . J . Chem. Kinet. 1983, 15. 631.
0 1990 American Chemical Society
The Journal of Physical Chemistry, Vol. 94, No. 15, 1990 5861
Formyl Chloride Reactions with CI and O H used with this chamber as described under i above. A spectral resolution of 0.7 nm was achieved with this system. Formyl chloride was prepared by flowing formic acid vapor through phosphorous pentachloride powder packed in a U-tube (1-cm i.d.) as described by Takeo and Matsumura.12 Formyl chloride is formed according to the reaction HCOOH
+ PCls
-
POC13
+ CHCIO + HCI
Residual HCOOH and the product POCl, were trapped at 195 K; CHClO and part of the product HCI were collected in a trap at 77 K. HCI was removed by fractionated distillation, and the residual CHCIO could be stored at 195 K for several days. The boiling and the melting point at 1 bar are at 259 f 4 and 168 K, respectively, as measured with a NiCr/Ni thermocouple imersed in the CHCIO sample. During its transfer to the reaction chamber and the preparation of the reaction mixture, a considerable proportion of CHClO decomposed, with CO and HCI as the only detectable products. Once the reaction mixture was ready for the experiment, a mixing fan was switched on, and the first-order rate constant for the loss of CHClO at the walls was determined. It ranged from 6 X to 4 X lo4 s-l depending on temperature. The UV spectrum of CHClO was measured in the 480-L chamber at a total pressure of 1 bar with a spectral resolution of 0.7 nm. The wavelength scale was calibrated by using atomic emission lines from a low-pressure mercury lamp. For the determination of the absolute absorption coefficients, a mixture of CHC10, C12, and N2 was photolyzed at room temperature in the 420-L chamber. UV and IR absorptions of the reaction mixture were simultaneously monitored. With the photolysis lamps off, CHCIO slowly disappeared by thermal decomposition on the walls, releasing HCI and CO to the gas volume. With the photolysis lamps on, the residual CHCIO was rapidly converted to CO and HCI by the reaction sequence
-
C12 CI
+ hv
net:
-
+ CHClO
COCl (+M) CHClO
-
2C1
(1)
+ COCl
(2)
+ C1 (+M) HCI + CO
(3)
HCl
CO
(4)
The conversion was >90% within the integration time for the first IR spectrum with lights on (18 s) and was completed during sampling the second spectrum. The concentration changes of CHCIO in the period from immediately before to about 1 min after starting the photolysis were equated to those of the products HCI and CO formed in the same time interval, according to reaction 4. With the photolysis lamps on, [HCl] then remained constant but [CO] slowly decreased with COC12 and C 0 2 building up as products, according to the mechanism"
+ C1 (+M) COCl + 02 (+M) COCl + Cl2 CO
2C(O)Cl02
-
--
-
COCl (+M)
C(O)C102 (+M)
COCl,
2c02
+ CI
+ 2CI + 0 2
(5) (6) (7)
(8)
For this reason, small corrections of 2-3% were made in the carbon balance for the measured amounts of the secondary products COC12 and C 0 2 . Effective 1R absorption coefficients of HCI, CO, COCl2, and C 0 2 were determined according to Lambert-Beer's law, In (lo/l) = eel. Mixtures for the calibration experiments were prepared both by successively adding CO and HCI to the diluent N2 with gas-tight syringes and by diluting more concentrated mixtures of CO and HCI in N2. Good agreement was obtained for t in both cases, indicating that adsorption of CO and HCl on the chamber walls was negligible. However, strong deviations from Lam(12) Takeo, H.; Matsumura, C. J . Chem. Phys. 1976,64, 4536. (13) Ohta, T. Bull. Chem. SOC.Jpn. 1983.56, 869.
bert-Beer's law were observed for HC1 and CO at high concentrations ((2-30) X 1015cm-)) due to saturation of the absorption lines. From the product yields of HCI and CO in the C1-catalyzed conversion of CHCIO to HCI and CO, the absolute concentration of CHCIO was derived and both the UV and IR absorption coefficients of CHCIO were determined. The rate of the reaction of CHCIO with O H radicals CHCIO O H products (9)
-
+
was investigated at 298 K and a total pressure of 800 mbar by a relative rate technique using a H 0 2 N 0 2 N O mixture as a "dark" O H source14 and n-butane as the reference substance. H 0 2 N 0 2was prepared by exposing liquid H 2 0 2to NO2 as described in ref 14. CHC10/n-butane/02/N2/H02N02 mixtures with a total pressure of 800 mbar were prepared in the reaction chamber, and the wall loss of CHClO was measured by FTIR absorption. The reaction was started by expanding a NO/N2 mixture that was at a total pressure of IO00 mbar into the reaction vessel, thus adding an excess of NO (-250 ppmv) to the reaction system. Then OH radicals were formed by reactions 10 and 11. HO2N02 s H02 NO2 (10)
+
+
HO2
+ NO
OH
+ NO2
(1 1) During the production of O H radicals, the concentrations of CHCIO and of n-butane were measured as a function of time by FTIR absorption and gas chromatography (Porasil C), respectively. Typically, concentrations were measured every minute for a total reaction time of 12 min. As both CHCIO and n-butane are exposed to the same concentrations of OH radicals, kinetic analysis yields the relationship
-=( k9
kl2
+
d[CHClO]/dt - R d[n-butane]/dt
1
with k 1 2being the rate constant for the reaction n-C4Hlo + O H n-CdH9 H2O
+
(12) and R being the first-order wall loss rate of CHCIO as measured in the same experiment before the addition of NO. With kI2taken from the literature,15 kg can be derived from the measured ratio +
k 9 / k12*
The rate constants for the reaction of CHCIO with Cl atoms were determined by a similar relative rate technique at a total pressure of 1000 mbar in the temperature range 266-322 K. Mixtures of CHCIO, CH4, C12,and N2 were photolyzed for about 15 min, with chlorine as a photolytic source of CI atoms and CH4 as the reference substance. Typically, the concentrations of CHClO (by FTIR absorption) and the reference substance CH4 (by gas chromatography and/or IR absorption) were measured every minute. The measurement of CH4 concentrations by either gas chromatography or by the absorption of its R( 11) line of v3 at 3131 cm-I gave data of comparable quality and identical results. At 321 K, which is the highest temperature used in the present work, the gas chromatographic values showed considerable scatter and, therefore, the evaluation was based solely on IR absorption. As both CHCIO and CH4 are exposed to the same concentrations of CI atoms throughout the reaction time, the same expression as above is obtained:
-=( k2
d[CHCIO]/dt - R
k13
d[ChI/dt
-
1
with k13 being the rate constant for the reaction CH4 CI CH3 + HCI
+
(13)
(14) Barnes, I.; Bastian, V.; Becker, K. H.; Fink, E.H.;Zabel, F. Amos. Enoiron. 1982, 16, 545.
(15) Atkinson, R.; Baulch, D. L.; Cox, R. A.; Hampson,Jr., R. F.; Kerr, J. A.; Trw, J. J . Phys. Chem. Ref. Data 1989, 18, 881. (16) Unpublished data of G. K. Moortgat and W . Schneider, cited in ref 15.
(17) Moule, D. C.; Foo, P.D.J . Chem. Phys. 1971, 55, 1262.
Libuda et al.
The Journal of Physical Chemistry, Vol. 94, No. 15, 1990
5862
TABLE XI: Absorption Cross Sections of Formyl Chloride at the Band Maxima (298 K, 1013 mbar of N,, Spectral Resolution 0.7 nm) 6-
5-
"5
236.1 241.5 247.3 25 1.4 253.7 256.1 258.2 260.2 263.5 265.7 267.9 269.1 270.2 27 I .4 273.8 276.3 277.7 278.9
L-
R
'P 3 -
-
i
i-
0-
I /
,
I
2LC
4
1
,
1
200
260
I 300
320
il n m l
Figure 1. U V absorption spectrum of formyl chloride at 298 K in 1 bar
of N2; spectral resolution 0.7 nm. TABLE I: Determination of the Absolute UV Absorption Cross Sections at 260.2 nm for Formyl Chloride Based on Stoichiometric Product Yields for the CI-Atom-Catalyzed Reaction"
CHClO 2 HCI
run 1
2 3 4
+ CO
A[CHCIO]
A[CHCIO]
from A[CO]," IO'6 cm-3
from A[HCI], 10l6 cm-3
a(260.2 nm),
2.76 1.15 1.48 0.4 I
2.72 1.10 1.51 0.44
5.4 6.8 6.0 5.8 mean 6.0 i 1.2 (2a)
1
280.2 282.7 285.3 286.8 288.0 289.4 292.2 294.9 296.7 298.1 299.5 302.3 305.2 308.1 309.3 311.1 314.1 316.7 3 18.7
3.8 4.9 5.6 5.4 6.0 5.6 5.8 6.0 5. I 5.3 5.2 3.9 3.5 4.0 4. I 3.4 2.4 2.1
CHCIO
cm2
'Spectral resolution 0.4 nm. blncluding small corrections due to COCI, and C 0 2 formation. and R being the first-order wall loss rate of CHCIO. As k 1 3is well-known in an extended temperature regime,I5k2 can be derived from the measured ratio k 2 / k , 3 . In some experiments, liquid water was added to a CHC10/N2 mixture by injection with a syringe at 293 K and a total pressure of 1018 mbar. The small water droplets present on the chamber walls after injection vaporized within 2-3 min, as was indicated by both visible inspection and water vapor IR absorption measurements. The CHClO concentration was measured by FTIR absorption before the addition of H20, during its evaporation, and for a certain time after evaporation was completed.
Results and Discussion UV Spectrum of CHCIO. The near-UV spectrum of formyl chloride at a spectral resolution of 0.7 nm is shown in Figure 1. Figure 1 is constructed from three individual spectra covering the wavelength regions 230-250, 250-297, and 292-320 nm, respectively, which were obtained in the 480-L chamber from mixtures with different CHCIO concentrations. The absorbance (log (loll))did not exceed 0.2 in any part of the spectrum. Deviations from Lambert-Beer's law became apparent at absorbances greater than about 0.3. The absorption cross section at 260.2 nm was determined in the 420-L chamber in four experiments, based on the stoichiometric conversion of CHCIO to C O and HCI. Within experimental uncertainties, evaluation of the IR absorbances of the products C O and HCI gave the same cm2 at results (see Table I), a(260.2 nm) = (6.0 f 1.2) X a resolution of 0.4 nm, based on a = In ( l o / I ) / c d . To assign absolute absorption cross sections to the spectrum in Figure 1 , which was taken at a resolution of 0.7 nm, it was assumed that there was no difference of a(260.2 nm) at a resolution of 0.7 and 0.4 nm. This assumption seems to be justified since, different from the absorption bands at h > 265 nm, the 260-nm band did not change its appearance when the resolution changed from 0.4 to 0.7 nm. In Table 11, the absorption cross sections of CHCIO at the absorption maxima are summarized. The band maxima of the present work are in good agreement with the band heads reported in the higher resolution work of Judge and Moule,Io the average deviation between both sets of data being 0.12 nm. The
2.4 2.3 1.64 1.04 0.86 0.97 0.8 1 0.46 0.32 0.22 0.25 0.172 0.080 0.027 0.021 0.020 0.013 0.008 0.007
v2
' ptot = 0 53 mbar
i;
- a m
v1 = 29335 vz = 1783 3 v3 :1306 5 v4 : 7386 v5 = 4576 v6 = 931 9
cm-l cm-1 cm-' cm-1 cm4 cm-l
ICH stretchl IC0 strdchl ICH M I ICCI stretch1 lCCl bend1 but-of-plane bend1
T i 298K
I1 vL v3
I
Figure 2. IR absorption spectrum of formyl chloride at 298 K; total pressure 0.53 mbar; spectral resolution 1 cm-I; vibrational assignments according to ref I ; frequencies of Q branches or band centers from the present work.
absorption cross sections become less accurate above 305 nm due to very small absorbances and corresponding uncertainties in the base line, with a total error at 320 nm of about 40%. The shape of the CHCIO spectrum at low resolution is very similar to that of ozone. As, in addition, the absorption maxima for CHCIO and O3 are at nearly the same wavelength, small UV absorptions of CHCIO are easily overlooked if it is produced in the reaction of ozone with chloroethylenes. The structure and shape of the CHCIO spectrum are similar to those of the spectrum of CH2O but shifted to shorter wavelengths by about 45 nm. In Table 111, the present spectrum of CHCIO is compared with the near-UV spectra of C H 2 0and CCI2O. The spectral properties of CHCIO are intermediate between those of C H 2 0 and CCI2O with respect to the degree of fine structure, the position of the absorption maximum, and the oscillator strength. While the structure of the UV spectrum does not change very much for CC120 when the resolution is changed from 0.5 to 0.02 nm,I8 the CHCIO absorption bands above 265 nm became distinctively sharper in the present work, when the resolution was only slightly improved from 0.7 (480-Lchamber) to 0.4 nm (420-L chamber). IR Spectrum of CHCIO.The IR spectrum of CHCIO has been described in detail by Hisatsune and Heicklen,' who could identify all six fundamentals. The IR spectrum of CHCIO between 5 0 0 and 4100 cm-' as monitored in the present work with 1-cm-l spectral resolution at 298 K and a total pressure of 0.53 mbar ( 1 8) Schneider, W.; Moortgat,
G. Private communication, 1989.
The Journal of Physical Chemistry, Vol. 94, No. 15, I990 5863
Formyl Chloride Reactions with C1 and O H TABLE III: Near-UVSpeftn of CH*O,CHCIO,and CCI2O
spectral resolution, nm av over IO-nm intervals av over 0.5-nmintervals
CH2O CHClO CCI,O
0.7
av over 0.5-nmintervals
~ m .
nm 310 304 260 232
4hGlUL 10-lo cm2 4.6 6.9 6.0 f 1.2 13.6
appearance of spectrum strongly structured at 0.5 nm resolution
ref 15, 16
structured at 0.7-nmresolution; see Figure little struct at 0.5-nmresolution
this work
1
17. 18
TABLE IV: Temwrature Dewndence of k,
run
T, K
k2/k13
k2," 1O-I) cm3/s
1 2 3 4
265.8 265.8 265.8 281.8 282.0 282.0 301.2 301.8 303.2 321 .O 321.2 321.2 321.3
9.18 9.17 9.88 8.45 8.05 8.64 7.64 1.54 6.84 6.9 1 6.61 6.89 6.26
5.21 5.20 5.61 6.47 6.18 6.63 8.06 8.02 7.43 9.70 9.30 9.70 8.82
5 6 7 8 9
IO 11 12 13
"With k13 = 1.1
X
0
- 0.2 - 0.1
-0 - 0 E -
+
-
0 CHLIgc) x
IO-" exp(-1400/T) from ref 15.
- 0.e
-
(19) Starcke. J.; Zabel. F.;Elsen, L.; Nelsen, W.;Barnes, 1.; Becker, K. H.Fifth European Symposium on Physicc4hemical Behavior of Atmospheric
Pollutants, Varese (Italy), Sept 25-28, 1989; to be published; Restelli, G., Ed.
\
CHCIOIIRJ
m
is shown in Figure 2. The absorption lines of the decomposition products HCI and CO and of the impurities H 2 0 and COz are very weak due to the low total pressure and have not been subtracted. The Q-branch positions of the fundamentals of CHClO from the present work, included in Figure 2, are in agreement with the values of ref 1 within 0.5 cm-I. The following relative Q-branch intensities were determined from 21 spectra in 1 bar of N2: t ( ~ z ) / t ( ~ 4 ) 1.40 f 0.1; c ( v ~ ) / c ( Y=~ )0.113 f 0.007; c(vs)/e(v4) = 0.16 f 0.03; t(v6)/c(.4) = 0.048 f 0.007. These ratios are different from those in Figure 2 due to the higher total pressure. In run 4 from Table I, the absorption of the 4 vibration was weak enough to obey Lambert-Beer's law, and from the measured HCI and CO yields the absolute absorption coefficient of the Q-branch at 931.9 cm-' was determined to be 5.3 X cm2. With this value and the above intensity ratios the following Q-branch absorption coefficients c were derived (t = In (Io/l)/cd, in units of cmz): 92 (vz); 7.4 (v3);66 (v4); 10.5 (4. The value for uZ is in surprisingly good agreement with the value 1.08 X IO-'* cm2 derived by Sanhueza and Heicklen6 from a fairly complex chemical system (Cl-catalyzed oxidation of CH2CIz),thus giving additional confidence in their kinetic analysis. Rate Constant for the Reaction of CHCIO with CI. In the presence of CI atoms, the concentrations of CHCIO and the reference compound CHI decayed exponentially with time, after a short induction time. Typical concentration-time profiles at 266 K are shown in Figure 3. The rate constants k2 as obtained from these plots are summarized in: Table IV and presented in Figure 4 as a function of temperature. k2 obeys the Arrhenius expression k2 = 1.39 X lo-" exp(-7.2 f 1.4 kT.mol-'/RT) cm3/s in the temperature range 266-322 K, with k2 = (7.6 f 0.8) X cm3/s at 298 K (2u error). These results are in excellent agreement with the rate constant of Niki et al.? who measured a value of 7.8 X cm3/s at 298 K relative to the reaction products, and with a value of Sanhueza and CH3CI CI Heicklen: which was determined at 305 K relative to the reaction CH2Clz CI products (1.23 X cm3/s from ref 6 vs 0.82 X cm3/s from the above Arrhenius expression). Reaction 2 is considerably slower than other reactions RCHO + CI products at room temperature, e.g., for R = CH3 (7.6 X lo-" cm3/sls), R = H (7.3 X IO-" cm3/sI5), R = CH2Cl (1.1 X IO-" cm3/sI9), R = CHCIZ(1.3 X 10-" cm3/sI9), and R = CCI3 (0.90 X IO-" cm3/sI9). CHCIO fits into this series; obviously the
+
lights'on
CHL I I R )
k z l k j j = 9 88
\
CHCIO + CI-products
- 1.i
\
- 1.1 0
200
LOO
800
600
1wO
1200
t Is1
Figure 3. Concentration-time profiles during the photolysis of a CHCIO/CH,/C12/N2 mixture (for run 3 from Table IV); determination of rate constant ratio k 2 / k I 3from changes of the slopes for CHClO and CH,.
- -lZL k2:
:
30
1 3 9 ~ 1 0 - ~ ~ e x p I - 7k2J 0mol"/RT)
32
34
1 IT
36
38
40
L~o-~K-~I
Figure 4. Arrhenius plot for k2: 0, present work (values from Table IV); V,
Niki et al.;9 0,Sanhueza and Heicklem6
stronger electron-drawing effect of R = CI as compared to R = CH3, H, CHzCl, CHCI2, and CC13 further reduces the rate constant for H atom abstraction by Cl atoms. Rate Constant for the Reaction of CHClO with OH. In CHC10/n-butane/O2/HO2NO2/N2 mixtures, the concentration of n-butane decreased by 10-30% immediately after the addition of NO due to reaction with OH radicals, but no measurable consumption of CHCIO occurred in the same time interval. From this result, an upper limit of k9 5 3.2 X lo-" cm3/s could be established for the rate constant of reaction 9 at room temperature (299.2 K). This upper limit is lower by a factor of 20 than a value of k9 = 6.1 X cm3/s estimated by Tuazon et aL3 from structurereactivity relationships. If the same Arrhenius preexponential factors are assumed for the temperature dependence of the reactions of OH with CHCIO and with CHzO (1.1 X 10-"
5864 The Journal of Physical Chemistry, Vol. 94, No, 15, 1990 TABLE V: Rate Constants for Reactions of Substituted Formaldehvdes with OH ( k d and CI ( k n ) at Room Temperature
R-CHO C2H5-CHOa CHj-CHO H-CHO CH2CI-CHO CCI3-CHO CI-CHO
kOH,
kch
10-12 cm3/s 19.6 14 IOb 3.2 1.7‘ 2.3
C ~ ~ / San~activation ~ ) , energy of 18.8 kJ/mol can be derived for reaction 9 from the above upper limit of k9 5 3.2 X cm3/s. A rough estimate of k9 may be obtained in the following way: In Table V, rate constants of reactions of the type RCHO + CI products and RCHO + O H products are collected for R = C2H5, CH3, H, CH,CI, CC13, and CI. An average of 5.4 is obtained for the known ratios of these rate constants. Applying this value to R = CI, k9 = 1.4 X IO-” cm3/s is derived from the value of k 2 measured in the present work. Rate Constant for the Reaction of CHCIO with H 2 0 . When 1.0 mL of liquid water was added to a CHCIO/N2 mixture in the 420-L reaction chamber, no measurable change of the firstorder loss rate constant of CHCIO due to wall reaction occurred during and after the evaporation of the water. From the corresponding gas-phase water concentration of 8.0 X 10l6~ m - an ~, upper limit of k I 4< 5 X cm3/s was derived at 296 K for reaction 14: CHCIO
-
-
+ H20
products
(14)
This can be compared to the homogeneous gas-phase reaction of COC12 with H 2 0 : C0Cl2
+ H20
products
(15)
which has been measured between 533 and 619 K,26 showing an activation energy of 59.4 kJ/mol. Extrapolation of the data from ref 26 gives a value of k l S= 6 X cm3/s at 298 K. Possible products of these hydrolysis reactions are CHCIO + H20 COCl2 + H20
-+
-
HC(0)OH
+ HCI, AH0,298 = -58 kJ/mol (14’)
(CIC(0)OH
+ HCI)
-
+
C02 2HCI, = -24 kJ/mol (15’)
by using heats of formation from ref 15 except for CHCIO, where AH01298 -172 kJ/mol is derived from group additivity rules.27 (20) This aldehyde is not as conclusive an example as the other aldehydes in this table because abstraction of H atoms from both the C2H, and the CHO group may significantly contribute to the reactivity of this molecule. In this case a linear relationship between the kCl/koHratio is less probable as the ka/koH ratio is on the order of 100 for aliphatic hydrocarbons?1 In the other aldehydes included in this table reactivity toward OH and CI is strongly dominated by abstraction of an H atom from the CHO group. For CH3CH0 it has been shown that reaction with CI atoms proceeds by H atom abstraction from the CHO group by more than 99%?2 and a similar behavior may be expected for CH2CICHO as well as for the correspondingreactions with OH radicals. The fact that C2HsCH0 fits well into this table may indicate that the major reaction path is H atom abstraction from the CHO group as for the other aldehvdes. (21) Wallin&n, T. J.; Skewes, L. M.; Siegl, W. 0.; Wu, C. H.; Japar, S. M. Int. J . Chem. Kinef. 1988, 20, 867. (22) Niki, H.; Maker, P. D.;Savage, C. M.; Breitenbach, L. P. J. Phys. Chem. 198589, 588. (23) Atkinson, R. Chem. Reu. 1986, 86, 69. (24) Nielsen, 0.J.; Sidebottom,H. W.; Nelson, L.; Tracy, J. J. Submitted to Int. J. Chem. Kinet. (25) D6W. S.; Khachatryan, L. A.; BCrces T. Ber. Bunsen-Ges. Phys. Chem. 1989, 93, 847. (26) Butler, R.; Snelson, A. J. Air Pollut. Confrol Assoc. 1979, 29, 833. ~~~
~
~~
TABLE VI: Atmospheric Lifetime of CHCIO with Respect to Pbotolvsis and Reactions with OH, C1. and H,O atmospheric reaction krurwuu. J remarks lifetime CHCIO + O H 45 days 1O5 cm-3 24 50 days CHCIO + hu July 1 7.6 X cm3/s [CI] = 3 X 14 years CHCIO CI IO3 cm-) 25 CHCIO H 2 0 2 hours i o i 7cm-’
+ +
“Reference 20. bThat is, 5 X and 36.5 X cm3/s per (equivalent) H atom for koH and kc,, respectively. CAverageof refs 24 and 25.
-
Libuda et al.
As both reactions are appreciably exothermic, it is difficult to draw any conclusions on kI4 from these figures. Atmospheric Implications. The degradation of most organic compounds in the atmosphere is initiated mainly by reaction with O H radicals. With an average tropospheric O H concentration of 8 X lo5 cm-3,28the first-order loss rate constant of CHClO is calculated to be 52.6 X s-l from the upper limit for k9 of the present work. This corresponds to a tropospheric lifetime of CHCIO with respect to reaction with OH of 1 4 5 days. In the upper troposphere reaction with CI atoms may become important. With [CI] = 1 X IO3 cm-329and T = 220 K for the upper troposphere, a first-order rate constant of 2.7 X s-I is derived with the Arrhenius expression for k2 of the present work, which corresponds to a lifetime of CHClO of 100 years with respect to reaction with CI atoms. The combination of a typical tropospheric water content of 10 mbar with the upper limit for k14 leads to a lower limit of 2 h for the tropospheric lifetime of CHClO with respect to the reaction with water vapor. The UV absorption coefficients from the present work can be used to estimate the photolysis frequency of CHCIO in the atmosphere. Assuming a quantum yield for photodissociation of C#J = 1, with the solar fluxes at 40’ N, July 1, noon time from ref 30, a photolysis frequency of J = (1.8 f 0.5) X S-I is derived. Allowing for 4 being smaller than 1, this figure results in a lower limit of 50 days for the photolytic lifetime of CHCIO under these conditions. The above estimates are summarized in Table VI. It is obvious from these data that the rate of the reaction of CHCIO with water vapor has to be known more precisely in order to estimate a reliable tropospheric lifetime. As the water vapor content cannot be increased to more than -20 mbar at room temperature, experiments at elevated temperatures are necessary in order to drastically reduce the upper limit for k14 of the present work. This will be a difficult task due the relatively fast heterogeneous processes of CHCIO on chamber walls. With respect to photolysis and reactions with O H and CI, a lower limit of 26 days is derived for the atmospheric lifetime of CHCIO. If the source strengths of CHCIO were known, the concentration of CHCIO in the troposphere could be estimated by using this figure for the lifetime. The strongest sources of CHCIO in the troposphere arise probably from the oxidation of CH3CI, C2HCI3,and CH2CI2. With the data of Singh et aL3I for the tropospheric concentrations of these compounds close to the ground, the rate constants for their reactions with O H radicals from ref 15, and a stationary O H concentration of 8 X IO5 ~ 1 3 1 ~ ~ : ~ a source strength of IO3 molecules CHC10/cm3.s is derived. When this figure for the source strength is equated to the lower limit of the total rate of the loss processes from Table VI, except of the reaction with water vapor, a stationary concentration for CHCIO in the lower troposphere of 100 pptv results. Similarly,
-
-
~~~~~
(27) Benson,S. W. Thermochemical Kinetics, 2nd ed.;Wiley: New York, 1976. (28) (a) Jeong, K. M.; Kaufman, F. Geophys. Res. k r t . 1979,6,757. (b) Singh, H. B. Geophys. Res. Lett. 1977, 4, 453. (29) Singh, H. B.; Kasting, J. F. J. Atmos. Chem. 1988, 7, 261. (30) Finlayson-Pitts,B. J.; Pitts, Jr., J. N. Atmospheric Chemistry; Wiley: New York, 1986. (31) Singh, H.B.; Salas, L. J.; Stiles, R. E. J. Geophys. Res. 1983, 88, 3684. Singh, H. B.; Salas, L. J.; Shigeishi, H.;Scribner, E. Report, Atmos. Sa.Lab., Stanford Res. Inst., Menlo Park, CA, 1978.
J . Phys. Chem. 1990,94, 5865-5870 a mixing ratio of >50 pptv is derived for the upper troposphere, with CH$I as the most important source compound and photolysis and the reaction with O H radicals as the major loss processes. As the rates of other possible loss processes of CHCIO, like the homogeneous reaction with water vapor and the reactions on aerosols and in water droplets are not known, formyl chloride should be considered, at the present time, as a possible ubiquitous
5865
trace gas in the troposphere at the 10-100 pptv level. Acknowledgment. Financial support of this work by the Bundesminister fur Forschung and Technologie (BMFT) and the Umweltbundesamt (UBA) is gratefully acknowledged. Registry No. CHCIO, 2565-30-2;Cl, 22537-15-1; OH,3352-57-6.
Effect of Solvent and Pressure on the Reactivity of Photoproduced M(CO), Transients, As Revealed by the Observed Quantum Yields for the Photosubstitution of M(CO)6 (M = Cr, Mo, W) Stefan Wieland" and Rudi van Eldik*Jb Institute for Physical and Theoretical Chemistry, University of Frankfurt, Niederurseler Hang, 6000 Frankfurt 50, Federal Republic of Germany, and the Institute for Inorganic Chemistry, University of Witten/Herdecke, Stockumer Strasse 10, 5810 Witten, Federal Republic of Germany (Received: December 4, 1989) The effect of solvent and pressure up to 200 MPa on the quantum yield for photosubstitution of CO was studied for the hexacarbonyls of chromium, molybdenum, and tungsten. The employed solvents include different n-alkanes (ranging from n-pentane to n-dodecane), the very weakly coordinating solvent perfluorohexane, and acetonitrile. The M(CO)&olvent) intermediates were trapped with piperidine, pyridine, and acetonitrile. The quantum yields for CO substitution in Cr(C0)6 at ambient conditions vary between 0.47 0.02 and 0.73 f 0.01, depending on the employed solvent and ligand. For the photosubstitution of M(CO)6 by pyridine in n-heptane, the quantum yield decreases along the series Mo > W > Cr (0.93, 0.79, and 0.72, respectively) at ambient conditions. For all the investigated systems the quantum yields decrease with increasing pressure, and the corresponding volumes of activation (AP+,(,-+)) vary between +5 and +14 cm3 mol-l. Different models are discussed to account for the experimental data.
*
Introduction Pressure effects on reaction rates and quantum yields constitute important phenomena and help to complete our comprehension of chemical kinetics. The estimation of volumes of activation has become an important tool in the elucidation of the mechanism of thermal and photochemical reactions of inorganic and organometallic complexes in s ~ l u t i o n . ~ -The ~ photosubstitution reactions resulting from the ligand field (LF) excitations of dS e and d6 Werner-type transition-metal complexes were the first for which systematic studies on the effect of pressure were conducted. It was possible to estimate volumes of activation for photochemical reaction steps of rhodium (HI) ammine complexes by combining quantum yield and luminescence lifetime In general, the experimentally determined volume of activation has to be considered as a composite of contributions originating from the displacement of atoms in the transition state and from changes in electrostriction in forming the transition state. The latter contribution plays an important role when net charge creation or neutralization is involved, and can complicate the interpretation of the observed pressure effect. We have therefore undertaken a series of high-pressure studies on the photochemical reactions of metal complexes of the type M(CO)6, M(CO)5L, and M( I ) (a) University of Frankfurt. (b) University of Witten/Herdecke. (2)van Eldik, R. Comments Inorg. Chem. 1986,5, 135. (3)van Eldik, R.,Ed. Inorganic High Pressure Chemistry: Kinetics and Mechanisms; Elsevier: Amsterdam, 1986. (4)Ford, P.C. Chapter 6 in ref 3. (5) Merbach, A. E. Pure Appl. Chem. 1987,59, 161. (6)Kotowski, M.;van Eldik, R. Coord. Chem. Reu. 1989,93, 19. (7)van Eldik, R.;Asano, T.; le Noble, W. J. Chem. Rev. 1989,89,549. (8) Weber, W.; van Eldik, R.; Kelm, H.; Diknedetto, J.; Ducommun, Y.; Offen, H.; Ford, P. C. Inorg. Chem. 1983, 22, 623. (9)Weber, W.; DiBenedetto, J.; Offen, H.; van Eldik, R.; Ford, P. C. Inorg. Chem. 1984,23, 2033. (10)Skibsted, L. H.;Weber, W.; van Eldik, R.; Kelm, H.; Ford, P. C. Inorg. Chem. 1983,22, 541. (11) Wieland, S.; DiBenedetto, J.; van Eldik, R.; Ford, P. C. Inorg. Chem. 1986,25,4893.
0022-3654J9012094-5865sO2.50 JO
(CO)4LL (M = Cr, Mo, W; L = N- or P-donor ligand).12-ls In these complexes the central metal is in the zero oxidation state, and the leaving and entering ligands are neutral, in order to minimize possible solvational contributions to the observed pressure effects. Thus the intrinsic contribution to the volume of activation ( A P ) should dominate. The present study deals with the effect of solvent and pressure on the photosubstitution of CO in M(CO)6 (M = Cr, Mo, W) by various ligands (piperidine, pyridine, and solvent). Many studies have been devoted to the photolysis of Cr(CO)6 in the gas phase and the reaction of the produced Cr(CO)5 fragment with C0.1b2' These studies indicate that Cr(CO)5 has a square-pyramidal structure and that the reactions of such a "naked" complex can only be investigated in the gas phase. Much interest exists in the photochemical behavior of Cr(C0)6 and other hexacarbonyls in solution. In the presence of even very weak coordinating solvents the participation of M(CO)s(solvent) transients must be considered.22-35 In this respect it is important to note that coordinating (1 2) Wieland, S. Doctoral Dissertation, University of Frankfurt, 1988. (13)Wieland, S.;van Eldik. R.; Crane, D. R.; Ford, P. C. Inorg. Chem. 1989,28, 3663. (14)Wieland, S.;van Eldik, R. J. Chem Soc., Chem. Commun. 1989,367. (1 5) Wieland, S.; Bal Reddy, K.; van Eldik, R. Organometallics, in press. (16)M e r , T. A.; Church, S. P.; Ouderkirk. A. J.; Weitz, E. J. Am. Chem. SOC.1985,107, 1432. (17)Fletcher, T.R.;Rosenfeld, R. N. J. Am. Chem. Soc. 1985,107,2203. (18)Seder, T.A.; Church, S.P.; Weitz, E. J . Am. Chem. Soc. 1986,108, 4721. (19)Weitz, E. J . Phys. Chem. 1987,91,3945. (20)Breckenridge, W. H.; Stewart, G. M. J . Am. Chem. Soc. 1986,108, 364. (21)Tyndall, G. W.; Jackson, R. L. J . Chem. Phys. 1988,89, 1364. (22)Kelly, J. M.; Bent, D. V.; Hermann, H.; Schulte-Frohlinde, D.; von Gustorf, E. K. J. Organomet. Chem. 1974,69,259. (23)Flamigni, L. Radiat. Phys. Chem. 1979,13, 133. (24)Turner, J. J.; Burdett, J. K.; Perutz, R. N.; Poliakoff, M. Pure Appl. Chem. 1977,49,271. (25)Bonneau, R.;Kelly, J. M. J . Am. Chem. Soc. 1980,102, 1220. (26)Kelly, J. M.;Long, C.; Bonneau, R. J . Phys. Chem. 1983.87,3344. (27)Welch, J. A.; Peters, K. S.; Vaida, V. J. Phys. Chem. 1982.86, 1941. (28)Simon, J. D.; Peters, K. S. Chem. Phys. Lett. 1983,98,53.
0 1990 American Chemical Society
