Dissociations of Gas-Phase CHClF and CHCl2
J. Phys. Chem., Vol. 100, No. 1, 1996 231
f CF+, and the formation of CF+ will show a kN value
simulating that for the elimination of HCl. That loss of Cl•
from CClF•+ is facile is documented by the NR mass spectrum
of the latter ion, which shows abundant CF+(49.6%), survivor
CClF•+ (14.6%), Cl+ (29.2%), and weak CCl+ (6.3%) and F+
(0.3%).
The formations of CHCl and CCl• due to loss of F• and
elimination of HF, respectively, show only small kN values
attesting to inefficient neutral dissociations by these channels.
This is consistent with the very low fractions of reionized F+
and HF•+ for which the kN values could not be measured because
of low ion intensities. Note that the formation of CCl by
consecutive dissociations, CHClF• f CClF + H•, CClF f CCl
+ F•, requires 307 + 488 ) 795 kJ mol-1, which hampers its
occurrence in neutral dissociations.
fraction increases with increasing ∆IE, the internal energies in
the reionized CHClF+ will also shift to higher values and affect
the ion dissociation kinetics. However, the internal energy
deposition upon collisional reionization may depend on the
internal energy of the intermediate neutral species,36 which
would further complicate the analysis. Although the present
kinetic analysis does not yield any details of the internal energy
distribution in CHClF•, the relative insensitivity of the neutral
dissociations toward the ∆IE clearly indicates that the internal
energy distribution in CHClF• is broad and lacks pronounced
minima or maxima.
Conclusions
The results obtained from the combination of variable-time
neutralization-reionization mass spectrometry and ab initio
calculations allow us to make the following conclusions.
Fractions of stable CHClF• and CHCl2• radicals are formed by
vertical reduction of their respective cations. The dissociations
of the intermediate radicals are distinguished from those of the
reionized cations by the variable-time measurements. Loss of
chlorine atom is a major unimolecular dissociation of CHClF•
and CHCl2• that occurs competitively on the microsecond time
scale. It is concluded that CHClF• radicals possessing >333
kJ mol-1 internal energy can serve as an efficient source of
chlorine atoms and contribute to ozone depleting reactions.
Thermochemical data (Table 6) indicate that threshold
energies for the losses from CHClF• of H• and Cl• are
comparable to the activation barrier for HF elimination. Kineti-
cally, the direct bond cleavages should have steeper k(E) curves
than does the elimination, which requires a tighter transition
state (Chart 1). Hence, the direct bond cleavages are predicted
to outcompete the HF elimination in dissociating CHClF•.
The thermochemical data (Tables 4 and 6) further predict
the elimination of HCl to be the most favorable ion dissociation
of CHClF+. In accord with the dissociation energetics, the ki
values for elimination of HCl from CHClF+ are uniformly
greater than those for the more endothermic loss of Cl• (Table
3). The branching ratio for HCl elimination and Cl• loss,
ki(HCl)/ki(Cl), shows a minimum for neutralization with di-n-
butylamine. This indicates that neutralization with the latter
reagent, followed by reionization, produces the largest fraction
of high-energy CHClF+ ions. Although this is consistent with
the overall greater extent of dissociation following neutralization
with di-n-butylamine (Table 1), the rate parameters for dis-
sociations of di-n-butylamine-neutralized CHClF• radicals are
comparable to those from neutralizations with the other targets.
The fractions of competitive eliminations of HCl and HF from
reionized CHClF+ depend on the neutralization energetics (Table
3). In particular, the rate parameter for the elimination of HF
from CHClF+, which has a higher activation energy, increases
with the neutralization exoergicity.
Acknowledgment. Financial support by the National Science
Foundation (Grant CHE-9412774) and the University of Wash-
ington Royalty Research Fund is gratefully acknowledged.
Acknowledgments are also made to the donors of the Petroleum
Research Fund, administered by the American Chemical Society,
for support of this work. The ab initio computations were
conducted by using the resources of the Cornell Theory Center,
which receives major funding from the National Science
Foundation and New York State with additional support from
the Advanced Research Projects Agency, the National Center
for Research Resources at the National Institutes of Health, IBM
Corporation, and members of the Corporate Research Institute.
References and Notes
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