T H E
J O U R N A L OF
PHYSICAL CHEMISTRY Registered in U.S. Patent Ofice
@ Copyright, 1973, by the American Chemical Society
VOLUME 77, NUMBER 21 OCTOBER 11, 1973
HF and DF Infrared Chemiluminescence and Energy Partitioning from the Reactions of Fluorine Atoms with c6-clO Cycloalkanes and Propane46 K. C. Kim and
D. W. Setser"
Department of Chemistry, Kansas State University, Manhattan, Kansas 66506 (Received April 24, 1973)
The infrared emission from the H F and DF products from the reactions of F atoms with cyclohexane, cyclohexane-dlz, cycloheptane, cyclooctane and cyclodecane has been studied, The initial product H F relative vibrational populations, Nl:N$N3:N4, are: C6Hl2 = 0.28:0.56:0.18:trace, C6D12 = 0.08:0.32:0.43:0.17, C7H14 = 0.25:0.54:0.20:0.01, CsH16 = 0.27:0.50:0.20:0.02, and CloHzo = 0.26:0.54:0.20:0.01. The mean fractional conversion of the available energy to vibrational energy of HF by these reactions (50%) is lower than for reactions with typical primary C-H bonds (60%). In order to provide reference data for reaction with a noncyclic secondary C-H bond, H F and DF overtone emission CD3CHZCD3 was studied. The relative vibrational populations are Nz:N3 = 0.62:0.38 for H F from F and Nz:N3:N4 = 0.37:0.41:0.23 for DF. A very small HF, u' = 4 population was observed from the cyclic alkanes but not from CD3CHzCD3, which is consistent with the difference in bond energies. Although the steady-state H F rotational populations are partially relaxed, an estimate of the initial rotational distributions suggests that -7% of the available energy was partitioned as H F rotational energy for the reaction of fluorine atoms with cyclic alkanes.
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Introduction In preceding papers,lJ the results of the H F infrared chemiluminescence from the F atom abstraction reactions with polyatoinic hydride molecules were reported. Relative rate constants for formation of H F in individual vibrational quantum states and information about the general nature of energy partitioning patterns were obtained. The importance of thermochemistry, differing mass combinations, variable bond types, substituent effects, and other factors have been discussed. We report here the H F infrared chemiluminescence arising from the reaction of F atoms with secondary C-H bonds of c6-(& cyclic alkanes3 and CD3CH2CD3. The results from CH3CDzCH3 and previously published1 data from reactions with other primary C-H bonds are used for reference. The bond energy of the secondary C-H bonds is -6 kcal mol-I lower than the bond energies of primary C-H bonds, and this change could affect the energy partitioning. If the claimls2,4 that these F atom abstraction reactions populate vibrational-rotational levels up to the thermochemical limit is valid, then the lowered C-H bond energy should be evident from the highest observed H F level. Furthermore, a comparison between the secondary C-H bonds of cyclic and aliphatic alkanes should illus-
trate the consequences of the cyclic radical reorganization energy3 and possibly other factors upon the H F vibrational and rotational populations. Reactions of F atoms with polyatomic deuteride molecules have not previously been studied by the chemiluminescence technique. Although the DF fundamental spectrum falls in the region where the sensitivity of the lead sulfide detector is declining very rapidly, the enhanced sensitivity in the overtone region compensates somewhat for the smaller Einstein coefficients, relative to HF, and HR. Since the the slower reaction rates, relative to F spacing of the vibration-rotational energy levels for DF are smaller than for HF, the DF results from F c-CgD12, CD4, and CD3CHzCD3 provide a check for the relative rate constants and the energy partitioning patterns of the F c - C ~ H ICHI, ~ , and CHsCDzCH3 reactions. The experiments were carried out in a fast flow apparatus1,2 with the walls of the reaction vessel cooled to liquid nitrogen temperature.5 The absence of vibrational relaxation was tested by varying reagent flow rate, background pressure, and F atom source. For some conditions, the degree of rotational relaxation could be altered and tentative estimates can be placed on the fractional conversion of the total energy into H F rotational energy.
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K. C. Kim and D. W. Setser
Experimental Section Apparatus and Techniques. All cyclic alkanes were purchased from Columbia Organic Chemicals Co. Cyclic C6D12 was obtained from Stohler Isotope Chemicals with the specified isotopic purity of 99.5%. CD3CHzCD3 was prepared from CD3COCD3 according to the standard LiAlH4 reduction followed by chlorination of the resulting 2-propanol-d6 and hydrolysis of the Grignard reagent. Mass spectral analysis showed greater than 98% isotopic and chemical purity. CD4 and CH3CD2CH3 were obtained from Merck Laboratory Chemicals. The main features of the apparatus have been described previous1y.l The cold-walled (77°K) reactor was pumped through a liquid nitrogen trap by a 6-in. diffusion pump and a 500-l./min mechanical pump. The systems of gas inlets, pressure measuring devices, the optics, monochromator, and PbS detector were functionally the same as that used in the previous w0rk.l The detector signal was amplified by a PAR Model HR-8 lock-in amplifier and PAR Model BZ-1 chopper, which was operated a t 600 Hz. Fluorine atoms were produced by microwave discharge of SFF, or CF4 in a quartz tube. The discharged CF4 or SF6 was mixed with reagent cia a concentric mixing arrangement; the central quartz tube (0.8 mm i d . ) was 1 cm longer than the outer tube. A typical flow for SF6 or CF4 was -4 pmol/sec. Typical flows of reagents were 2-6 lmol/sec. In addition to concentric mixing, a nozzle geometry in which the F atom flow was crossed with the flow of the substrate was tried. For the latter geometry, the tips of the two nozzles were -4 cm apart. The reagents were introduced to the vacuum line from a reagent flask after thorough degassing. The vapor pressure of the cyclic alkanes a t room temperature was used as the back pressure and the flow was monitored with a Gilmont flowmeter. The operating pressure, measured a t the bottom of the reactor under stabilized flow conditions, was 1-2 x Torr for typical experimental conditions. The actual density of the gas mixture in the mixing and emission zone undoubtedly is higher than indicated by the static pressure measurement. The entire optical system was purged continuously before and during the experiment with dry nitrogen to minimize the absorption by atmospheric water. Emission from vibrationally excited HF,,J was recorded in the fundamental (0.4-mm slit) and overtone (2.0-mm slit) spectral region for most substrates. Typical sets of H F and DF spectra are shown in Figure 1 ( F c-CgH16) and Figure 2 (F C-CgD12). Data Treatment Population analyses were made by computer simulation2 of the observed spectra. The transition probabilities were calculated by the methods of Heaps and HerzbergGa and Herman-WallisGb for the first term in the dipole moment expansion, with the spectroscopic constants of Mann, et al 7 The detector response in the frequency range of interest was calibrated with a Barnes Engineering black body source. The line positions were calculated using the Herzberg expression. These positions were confirmed subsequently with the more accurate Dunham expression. I t is necessary to use the latter for populations involving J levels higher than those observed in this work. An approximate set of initial populations ( N , I ) and the detector response are entered into the program which calculates line positions and intensities which then are combined according to a triangular peak shape. The rotational populations of each vibrational level are normalized. Thus the relative vibrational popu-
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The Journal of Physical Chemistry, Vol. 77, No. 21, 1973
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