A guided-ion-beam mass spectrometer is used to study the reactions of Co+ + CD4 and CoCH2+ + D2 and thereby experimentally probe the potential energy surface for activation of methane by Co+. The results obtained are compared to recent theoretical results and agree with the conclusion that dehydrogenation of methane by Co+ is hindered by a tight four-center transition state complex. The major discrepancy observed between experiment and theory is in the height of this barrier, which theory predicts is 96-109 kJ/mol versus our experimental result of 34 ± 8 kJ/mol. The endothermicities of all reactions are measured and allow the determination of £>o(Co+-CD) = 422 ± 37 kJ/mol and Dq(Co+-C) = 347 ± 29 kJ/mol. We also find extensive hydrogen scrambling in the CoCH2+ + D2 reaction, a result that is interpreted by using phase space theory to help understand how various features on the potential energy surface control branching ratios among the various channels observed.
The potential energy surface for the
CoC2H6
+ system is probed by using
a guided-ion beam tandem mass
spectrometer in conjunction with a dc discharge/flow tube ion source.
Bimolecular reactions of Co+ +
C2H6, Co+(C2H4) + H2 (D2), and
CoCH2
+ + CH4 (CD4)
are examined, as well as threshold collisional activation of
Co+(C2H6)
and Co+(C2H4) with Xe.
The results allow details of the
[CoC2H6]+ potential energy
surface (PES) to be elucidated.
Key features of the PES include barrier heights of 1.04 ± 0.11
eV (0.29 ± 0.09 eV in excess of the endothermicity)
for the reaction of Co+ with ethane to form
CoCH2
+ + CH4 and of 0.32 ±
0.12 eV for the exothermic reaction to
form Co+(C2H4) +
H2. From the collisional activation studies, we
determine 0 K bond dissociation energies for
Co+−C2H4 and
Co+−C2H6 complexes of 1.86 ±
0.07 and 1.04 ± 0.05 eV, respectively, in good agreement
with
literature values.
D
0(DCo+−C2H4)
= 1.13 ± 0.16 eV is also determined although it is somewhat more
speculative.
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