Infrared spectroscopy of gas phase C
3
H
5
+
: The allyl and 2-propenyl cations
Gary E. Douberly, Allen M. Ricks, Paul v. R. Schleyer, and Michael A. Duncan
a͒
Department of Chemistry, University of Georgia, Athens, Georgia 30602-2556, USA
͑Received 15 November 2007; accepted 4 December 2007; published online 10 January 2008͒
C
3
H
5
+
cations are probed with infrared photodissociation spectroscopy in the 800–3500 cm
−1
region using the method of rare gas tagging. The ions and their complexes with Ar or N
2
are
produced in a pulsed electric discharge supersonic expansion cluster source. Two structural isomers
are characterized, namely, the allyl ͑CH
2
CHCH
2
+
͒ and 2-propenyl ͑CH
3
CCH
2
+
͒ cations. The
infrared spectrum of the allyl cation confirms previous theoretical and condensed phase studies of
the C
2
v
charge delocalized, resonance-stabilized structure. The 2-propenyl cation spectrum is
consistent with a C
s
symmetry structure having a nearly linear CCC backbone and a
hyperconjugatively stabilizing methyl group. © 2008 American Institute of Physics.
͓DOI: 10.1063/1.2828553͔
The importance of carbocations as transient reaction in-
termediates is well recognized in mechanistic organic
chemistry.
1,2
Carbocations are also known to play a signifi-
cant role in both interstellar chemistry
3
and terrestrial com-
bustion processes.
4
Carbocations isolated in condensed phase
superacid media have been characterized by NMR ͑Ref. 5͒
and IR ͑Ref. 6͒ spectroscopy, providing valuable insight into
the putative structures. For some of the smaller carbocations,
gas phase reaction and collision studies implicate multiple
isomers, which have been predicted by theory to lie close in
energy.
7
C
3
H
5
+
is a well-known example for which isomers
have been proposed since the earliest work.
6,8–11
The two
main structures, namely, the 2-propenyl cation and the
resonance-stabilized allyl cation, have been extensively in-
vestigated with theory.
11–16
Houle and Beauchamp measured
the photoelectron spectrum of the C
3
H
5
allyl radical, which
exhibited a 420 cm
−1
progression assigned to the symmetric
CCC bending mode of the allyl cation.
17
More recently a
436 cm
−1
progression was observed in a higher resolution
pulsed field ionization photoelectron spectrum and also as-
signed to the allyl cation.
18
No other gas phase spectroscopy
of C
3
H
5
+
has been reported. We now report the infrared spec-
trum of C
3
H
5
+
produced from various precursors in an elec-
tric discharge, demonstrating the existence of both the allyl
and 2-propenyl cations.
The allyl cation provides a simple model for investigat-
ing electron delocalization in
-bonded conjugated systems
and has been studied extensively at various levels of
theory.
11–16
The ab initio covalent bond order of the allyl
cation has been determined to be 1.44,
11
which is interpreted
as each CC bond consisting of one
bond and one-half of a
bond. The CCC bond angle is 117°, and the charge is
delocalized over the entire molecular framework, with a to-
pological analysis of the electron density showing 78% of
the charge localized on the two CH
2
groups.
13
In comparison
to the allyl cation, the 2-propenyl cation is predicted to be
8 kcal /mol higher in energy, and it is separated from allyl by
an 18 kcal /mol barrier.
11
The central carbon atom in the
2-propenyl cation is sp hybridized, and the CCC bond angle
is nearly 180°.
12
Like the allyl cation, much of the charge in
the 2-propenyl cation is delocalized to the more electroposi-
tive hydrogen atoms.
15
Carbocations with the C
3
H
5
+
stoichiometry have been
observed many times in mass spectrometers;
7,9–11,19–22
collision
10
and reaction
11
studies support the presence of both
structural isomers. Various branching ratios of 2-propenyl
and allyl were observed, depending on both the precursors
and whether the ions were produced by electron impact or
proton transfer reactions. The study of C
3
H
5
+
in superacid
media remained elusive until Buzek et al. generated the cat-
ion from the cyclopropyl bromide precursor in a cryogenic
SbF
5
matrix.
6
With this precursor, polymerization reactions
were avoided, and the infrared spectrum was obtained and
assigned to the allyl cation based on comparisons to ab initio
harmonic frequency calculations.
6
We report here the infra-
red photodissociation spectrum of isolated C
3
H
5
+
in the
800–3500 cm
−1
region along with ab initio harmonic fre-
quency calculations of both the allyl and 2-propenyl cations.
Comparisons to the predicted spectra indicate that both ions
are present, with the relative abundance depending on the
employed precursor.
Infrared photodissociation ͑IRPD͒ spectra are obtained
with the rare gas tagging technique described previously.
23–27
Given the low binding energies and minor perturbation to the
ab initio predicted C
3
H
5
+
spectrum, Ar and N
2
provide suit-
able leaving groups for probing C
3
H
5
+
with IRPD.
28
We gen-
erate C
3
H
5
+
and its complexes with Ar or N
2
in a pulsed
electric discharge supersonic expansion
29
of either Ar or N
2
seeded with ethylene ͑ϳ1%͒ or with the ambient vapor pres-
sure of either allyl bromide or cyclopropyl bromide. Colli-
sional cooling in the expansion results in ions with tempera-
tures near 50 K.
29
The cations from the discharge cluster
source are pulsed into a specially designed reflectron time-
of-flight mass spectrometer. Pulsed deflection plates located
in the first flight tube deflect all ions except for the
C
3
H
5
+
uAr,N
2
complexes, which are subsequently excited
a͒
Electronic mail: maduncan@uga.edu.
THE JOURNAL OF CHEMICAL PHYSICS 128, 021102 ͑2008͒
0021-9606/2008/128͑2͒/021102/4/$23.00 © 2008 American Institute of Physics128, 021102-1