17174 J. Phys. Chem., Vol. 100, No. 43, 1996
Jarvis et al.
to the formation of the collision complex, this should occur to
a greater extent with the larger PFCs and not with C2F4.
Similarly, the stability of the complexed cation is expected to
be greater if fluorine atoms are attached directly to the positively
charged carbon atom, C+ (i.e., in the R position), and to a lesser
extent if one or more CF3 groups are attached to C+ (i.e., the
fluorine atoms are in the â position). In other words, these
two effects both predict that C2F4 should react with these ions
with a greater efficiency than observed. For the three saturated
PFCs studied, H3O+, NO+, and O2+ are all unreactive, and H2O+
and N2O+ are also unreactive with C2F6. All the other ions
react with the three saturated PFCs with a rate coefficient at or
close to the collisional value, the only exception being the
reaction of N2O+ with C3F8.
of PFCs, for example, by determining suitable ions for detection
of specific PFCs.
Acknowledgment. Financial support from the Chemical and
Biological Defence Establishment (Ministry of Defence), Porton
Down, and the EC (CEC Contract CHRX-CT94-0485) for this
project is gratefully acknowledged. One of the authors (G.K.J.)
thanks NERC for providing a research studentship. We also
thank Dr. Yong Liu who helped in taking some preliminary
measurements. Our thanks also extend to Professor J. M. Dyke
(Chemistry Department, University of Southampton, U.K.) for
communicating to us his recently measured valence photoelec-
tron spectrum of C3F6.
References and Notes
Perhaps the most important conclusions from this work
concern the mechanism of how the ion-molecule reactions
proceed. For many of the reactions ion products are observed
that could result from a long-range electron jump, but for the
same reactions some of the products can only result from an
intimate collision. As mentioned earlier, it may be argued that
the two reaction processes are not incompatible, i.e., a short-
range intimate interaction can compete with a long-range
interaction. Thus, different product channels could result from
different encounters (e.g., different trajectories, steric effects,
etc.). This may indeed be the case for some of the reactions
we have studied, and we have no evidence to support or reject
this suggestion. However, a long-range charge transfer requires
favorable Franck-Condon factors connecting the ground state
of the neutral molecule to an appropriate ionic state. Further-
more, good Franck-Condon factors connecting the ground state
reactant ion to its neutral ground state may also be required. If
these criteria are met, then we suggest that the long-range
interactions are likely to dominate. However, for many of the
reactions studied, these criteria are not met. In addition, many
of the observed products can only be formed following charge
transfer and bond formation within an interacting complex. Only
for a small number of reactions studied (e.g., N+ + C2F6) does
it appear that there can be direct competition between short-
range and long-range processes. This raises the question of why
does competition occur for such reactions when the long-range
process is expected to dominate. A possible solution is that
some other physical property of the reacting system (other than
poor Franck-Condon factors) inhibits formal charge transfer,
and a short-range process leads to all of the observed products.
We therefore believe that long-range charge transfer does not
take place for most of the reactions studied and that short-range
processes dominate. We note that the large polarizabilites
associated with the molecules in this study may promote the
formation of the collision complex.30
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JP960071D