Vinylidene (H2C=C) is shown to be the largest photodecomposition channel in the direct photolysis of both C2H2 and C2H4. The chemistry of H2C=C as it relates to planetary atmospheres is discussed. The vinyl radical (C2H3), important in the acetylene chemistry cycle, has been directly observed spectroscopically and the kinetics of several key reactions of this species measured

Fahr, Askar

Herron, John

Laufer, Allan H.

English

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N90-26766-
A LABORATORY INVESTIGATION OF THE PRODUCTION AND PROPERTIES OF
MOLECULAR AND RADICAL SPECIES PERTINENT TO PLANETARY ATMOSPHERES
ASKAR FAHR, JOHN HERRON* and ALLAN H. LAUFER**
* Askar Fahr and John Herron: Chemical Kinetics Division National Institute
of Standards and Technology, Gaithersburg, Maryland 20899
** Allan H. Laufer: Office of Basic Energy Sciences U. S. Department of Energy,
Washington, D. C. 20545
ABSTRACT
Vinyl idene(H2C=C) is shown to be the largest photodecomposition channel
in the direct photolysis of both C2H 2 and C2H 4. The chemistry of H2C=C as
it relates to planetary atmospheres is discussed. The vinyl radical (C2H3),
important in the acetylene chemistry cycle, has been directly observed
spectroscopically and the kinetics of several key reactions of this species
measured.
INTRODUCTION
An understanding of the chemistry of planetary atmospheres is critically
dependent upon knowledge of the kinetic parameters of a suite of elementary
chemical reactions. In the outer planets and several of their satellites, where
carbon-hydrogen chemistry is paramount, the important vacuum ultraviolet photo-
chemically induced processes and reactions involve small hydrocarbon molecules
and radicals. Photochemical processes of the major planetary hydrocarbon, CH4,
have been known quantitatively for several decades. The carbon containing free
radicals formed in the CH 4 photolysis, CH2 and CH3 corresponding to H2 and H
formation respectively, lead, through self-reactions, to C2H 2 and C2H41.
CH2 + CH 2 = C2H 2 + H2 (or 2H) (i)
CH2 + CH 3 = C2H 4 + H (2)
In this paper, we review the quantum yield of formation and identity of the free
radicals produced in the vacuum ultraviolet photolysis of the two unsaturated
hydrocarbons, C2H 2 and C2H 4, and kinetic processes of several previously non-
investigated species.
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EXPERIMENTAL
The experimental apparatus used in the work to be described has been
discussed in several papers from this laboratory 2, so will be described only
briefly here. A photolysis flash of about 5us, full width at half-maximum
intensity, has an output maximized in the vacuum ultraviolet region, the spectral
region where the energy of the incident solar flux is sufficient to dissociate
small hydrocarbon molecules and where the molecules of interest have their
maximum optical absorption. Following the initial flash, a Garton-type analysis
lamp of 1 us pulse width probed the photolysis mixture. The photolysis cells,
constructed of either LiF or Suprasil, permitted photolysis to 105 nm or 155 nm,
respectively. The output of the analysis flash, essentially continuous from 120
through 185nm, was focused through LiF optics onto the entrance slit of a 2 m
Eagle vacuum spectrograph whose dispersion was 2.77A/mm. The temporal profile
of all species was monitored directly through well-characterized absorptions.
RESULTS
Vin_,l idene Formation
Vinylidene (H2C=C) was initially observed, in its electronically excited
triplet state at 137 and 151 nm, following the photolysis of C2H 2 in the vacuum
ultraviolet 3. The species has a long lifetime toward isomerization to acetylene
and, therefore, may be of importance as an intermediate in planetary hydrocarbon
systems if its quantum yield of formation fromreasonable precursors is large.
Acetylene is known to form both C2H and C2 from photolysis in the vacuum
ultraviolet with estimated primary quantum yields of 0.3 and 0.1 at 147 nm,
respectively 4. We find that H2C=C is formed with a quantum yield of 0.4 making
it the single largest dissociative channel in the acetylene photodecomposition.
Previously, the formation of unmixed H2 or D2 from the low intensity photo-
lysis of C2H 4 and its deuterated isotopes provided evidence for production of
vinylidene radicals, i.e.,
CH2CD 2 = CH2C + D2 and CD2C + H2 (3)
In our direct determination, we find a quantum yield equal to 0'75 +/- 0.2 for
triplet vinylidene formation from ethylene. Though not directly observed, there
228
is evidence that the remainder proceeds through vinyl (C2H3) radical formation.
The work has been described in greater detail earlier 5.
V!n_,l idene Chemistry
The long-lived vinylidene moiety we observe is the electronically excited
triplet that is calculated to be about 40-50 kcal/mol above the ground singlet
state. The singlet, by comparison, has an ephemeral existence with a lifetime
of the order of 10 -12 seconds with respect to isomerization to acetylene. We
have looked specifically for a chemical reaction, i.e., hydrogen abstraction,
by triplet vinylidene with H2, CH4 and C2H4. Under conditions of our experiments,
that is room temperature, we found no evidence for direct chemical reaction
although the results with ethylene were less clear. Removal of the H2C:C in the
absence of chemical reaction, presumably by quenching to the ground electronic
state by added Ar, H2, CO and N2, occurred with reasonable rate constants. The
slow rate observed for reaction agrees with predicted calculated values based
upon a simple BEBO model6.
Vin_,l Radical Absorption Spectrum
The vinyl (C2H 3) radical has been implicated in the acetylene chemical
cycle. Although several studies have utilized mass spectrometric detection for
analysis, there has not been an observable, intense ultraviolet or visible
spectral signature for this species, an omission that has probably limited
kinetic studies of this species.
Using flash photolysis of selected precursors in conjunction with vacuum
ultraviolet absorption spectroscopy we have observed two relatively intense
transient absorption features that we attribute to the vinyl radical (Figure I).
The features at 164.71 and 168.33 nm are assigned on the basis of precursor,
kinetic data, and the spacing between the bands. The optimum precursors were
Sn(C2H3) 4 and Hg(C2H3) 2. The photolysis of either resulted in I)the observed
spectrum, 2) produced the expected dimerization product, 1,3-butadiene, that
was observed by gas chromatographic analysis and 3) a spacing between the
transitions of 1306 cm-I in agreement with both calculations and measurements
of the vinyl ion spectrum. The spectrum was sufficiently intense, with extinc-
tion coefficients in excess of II00 atm-lcm "I that it could serve as a probe
for kinetic studies. The work has been described in greater detail elsewhere 7.
229
¢q
FIGURE CAPTIONS
Figure i. Densitometer trace of the observed absorption bands of the
C2H 3 radical. The identified emission lines of AI III are from the
source.
230
Vin_l Radical Chemistr X
The initial reaction of vinyl radicals studied using its optical absorption
was its reaction with molecular 02 . This reaction had been examined earlier with
alternate techniques 8 and confirmation of the kinetic parameters using the C2H3
vacuum ultraviolet absorption served, in part, to confirm the assignment of the
carrier to the vinyl radical. A major loss mechanism in any system containing
free radicals is the self-reaction of the two radicals; in this case to dispropor-
tionate forming C2H2 and C2H4 or to combine yielding 1,3-butadiene. The optical
measurement of the radical concentration with time following its production by
flash photolysis resulted in a rate constant of l.OxlO'lOcm3molec'Is -I. A ratio
of 4.7 was found for the combination/disproportionation ratio as ascertained
through gas chromatographic analysis of the products.
Future efforts in our studies of vinyl radical chemistry will be to examine
other rapid radical-radical reactions that are conceptually of importance in
planetary atmospheres; e.g., its reaction with CH3 radicals. The reactions will
also be investigated over a wide range of temperature to permit the direct use of
the kinetic information in atmospheric models and to permit improved mechanistic
understanding, on a molecular level, of this important class of reactions.
REFERENCES
1. A. H. Laufer, Rev. Chem. Intermediates, 4, 225 (1981)
2. A. Fahr and A. H. Laufer, J. Phys. Chem._ 92,7229 (1989) and references therein
3. A. H. Laufer, J. Chem. Phys., 73, 49 (1980T-
4. H. Okabe, J. Chem. Phys., 78, "I_J12 (1983)
5. A. Fahr and A. H. Laufer, _'7. Photochem., 34, 261 (1986)
6. A. H. Laufer and Y. L . Yung, J. Phys. Che-m. 87, 181 (1983)
7. A. Fahr and A. H. Laufer, J. Phys. Chem., 92,-'7229 (1989)
8. I. R. Slagle, J.Y. Park, M.C. Heaven, D. G'u-tman, J. Am. Chem. Soc.,
106,4356(1984); H. Krueger and E. Weitz, J. Chem. Phys., 88, 1608(1988)
231


A laboratory investigation of the production and properties of molecular and radical species pertinent to planetary atmospheres

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