J. Phys. Chem. 1983, 87,3722-3724
3722
A Fourier Transform Infrared Study of the Kinetics and Mechanism for the Reaction CI CHsHgCH3
+
H. Nlkl,'
P. S. Maker, C. M. Savage, and L. P. Brekenbach
Research and Engineering Staff, Ford Motor Company, Dearborn, Michigan 48 12 1 (Received: December 2 1, 1982)
Gaseous and aerosol products formed in the C1-atom initiated reaction of CH3HgCH3were studied by the FT IR method in the photolysis of C12-CH3HgCH3mixtures in the presence and absence of O2 at -300 K. The results are consistent with the predominant occurrence of the displacement reaction C1+ CH3HgCH3 CH3HgC1 + CH, as the primary process. The relative rate constant for the C1-atom reactions of CH3HgCH3and n-C4Hio = 1.25 f 0.06 (u) by the competitive decay method. was determined to be k(C1+CH3HgCH3)/k(C1+n-C4Hlo) cm3molecule-' s-l gives This value combined with the literature value of k(Cl+n-C4Hlo)= (2.20 f 0.13) X cm3 molecule-' s-'. k(C1+CH3HgCH3)= (2.75 f 0.30) x
-
Introduction Although the photolysis and pyrolysis of CH3HgCH3 have been the subject of numerous studies'V2 no quantitative experimental data are as yet available for the gasphase bimolecular reactions involving this compound. During the course of our systematic FT IR studies of chemical reactions related to atmospheric chemistry, attempts were made to determine the kinetics and mechanism for the reactions of CH3HgCH3with species such as 03,HO radicals, and C1 atoms. An exploratory study showed that while the 03-CH3HgCH3reaction is extremely slow, i.e., 300 nm was negligibly slow as compared with those of C12-CH3HgCH3 mixtures. The C1-atom initiated reaction of CH3HgCH3 in N2 diluent exhibited chain nature as evidenced by approximately 100-fold retardation in the CH3HgCH3decay rate upon addition of O2 (>lo torr). Figure 2 illustrates typical spectral data obtained from the photolysis of C12-CH3HgCH3mixtures in the torr range in N2 diluent. Namely, Figure 2A corresponds to an absorbancespectrum in the frequency region of 600-3200 cm-' after less than 1s irradiation of a mixture initially containing CH3HgCH3 (0.52 torr), C12(1.5 torr), and N2 (100 torr). This spectrum contains 0.22 torr of unreacted CH3HgCH3,and the remaining spectral features are largely due to the formation of CH3C1(0.223 torr) as shown in Figure 2B. The minor products included CHI (29), CC12H2( l l ) , HC1 (7), CH20 (4),and CHClO (3) in mtorr units. Thus, in terms of material balance, these products can account for only one of the two CH3 groups of CH3HgCH3reacted and the product(s) corresponding to the other CH, group and Hg consumed could not be uniquely identified in these runs. The missing product was found to occur in the form of aerosols. Light scattering by the aerosol formation could be readily detected by visual inspection. In addition, the residual spectrum, Figure 2C, which was derived from Figure 2A by removing the spectral contributions of all the (7)T.Shimanouchi, J. Phys. Chem. Ref. Data, 3, 269 (1974). ( 6 ) T.Charnley and H. A. Skinner, J. Chem. SOC.,1921 (1951).
0022-3654103/2O07-3722$OI.50/O 0 1983 American Chemical Soclety
Kinetics and Mechanism for the Reaction Ci
+ CH,HgCH,
The Journal of Physical Chemistry, Vol. 87, No. 79, 1983
CH3 Spectrum
CH,Hg
servation can be qualitatively explained by the frequency dependent light scattering and absorption of the incident IR signals by aerosol particles. The broad peaks at 702, 1213, and 2943 cm-’ in this residual spectrum are probably due to the aerosol CH3 group. These results were found to be essentially independent of the diluent N2 pressures ranging from 100 to 700 torr. Furthermore, the addition of O2 (>lo torr) to the above mixture did not alter either the deficiency in the material balance or the residual spectrum, although the major carbon-containing products were displaced by those attributable to the self-reaction of CH300 radicals, Le., CH20, CH,OH, and CH300H.g Several tentative conclusions concerning the mechanism for the reaction C1+ CH3HgCH3can be drawn from the above results. First, the major product CH3C1is a clear indication of the formation of CH, and its subsequent reaction with C12, Le., CH, + C1, CH3C1 Cl. The regeneration of C1 atoms by this reaction sustained the chain consumption of CH3HgCH3in Nz diluent, and was suppressed by the competing reaction CH3 + Oz CH,OO in the presence of added 02.A plausible mechanism for the formation of CH, is the displacement reaction, reaction 1. The expected accompanyingproduct CH,HgCl has low volatilitf and can account for the formation of aerosols containing the missing CH,Hg group. An alternative H-abstraction mechanism, reaction 2, is not compatible with the observed low yield of HCl. Also, it is doubtful C1+ CH3HgCH3 CH3HgCH2+ HC1 AH N -5 kcal/mol (2)
-
I
3000
2000 I/
x
1000
(cm-‘)
3723
+
-
-+
Figwe 1. Absorbance specimn of CH,HgCH, in the frequency region of 500-3200 cm-’. [CH,HgCH,] = 2.5 torr in 700 ton of air; L = 100 cm; and log (I,,/Z) = 0.0676 for the peak at 558 cm-‘ as indicated by the asterisk. The band notation v and y refer to stretching and wagging modes.
Flgure 2. Photolysis of a mixture contained CH,HgCH, (0.52 ton) and CIS (1.5 torr) in 100 torr of N,: (A) product spectrum after
