Electronic Transitions of PB+, R-PF+, and C6H7 Isomers
A R T I C L E S
2
2
˜
single excitation of this mode is forbidden for the B1rX B1
transition; therefore, the energy of ∼205 cm-1 must correspond
observed absorptions at 532 and 326 nm. Therefore, the E and
G systems (Figure 4) are assigned to R-HF (Table 1). The
absorptions at 532 and 326 nm grow in intensity at the expense
of c-C6H7 upon UV irradiation. The absorption of UV photons
by c-C6H7 leads not only to the ring-opening reaction but also
to the photoisomerization to R-HF. The latter process is feasible,
as the calculated barrier of ∼219 kJ/mol11 for the isomerization
of PB+ to R-PF+ should be similar to the neutral one, and it
can be overcome by the electronic excitation of c-C6H7. The
to 2ν23. The frequency difference of the ν23 mode in the ground
and the B B1 states is quite large and reflects a geometry change
2
˜
along this coordinate upon electronic excitation. The profile of
the absorption at ∼310 nm is dominated by the long progression
of the 205 cm-1 overtone bands and its combination with the
∼796 cm-1 mode. The frequency of the latter is close to the
calculated, totally symmetric ν11 (871 cm-1) mode.
+
strongest E and G systems are observed in the reaction of H3
UV irradiation of c-C6H7 with a mpHg lamp using a
λ g 305 nm cutoff filter leads to a decrease of the visible and
UV systems of this species and simultaneously increases the
strong absorption at ∼282 nm and weaker ones in the 310-340
nm and visible ranges (Figure 3). It is well established that
1,3-CHD32,33 and its cation,34 which are structurally similar to
c-C6H7, undergo a photoinduced ring-opening reaction leading
to 1,3,5-hexatriene or its cation, respectively. In the case of
neutral 1,3-CHD, the ring-opening takes place in a fraction of
a picosecond, and the product relaxes to the more stable
(tZt-hexatriene) form in several tenths of a picosecond.32 The
open-chain structure, produced from 1,3-CHD or 1,3-CHD+ by
the UV-induced ring-opening reaction, absorbs at shorter
wavelength than the parent species.
It is expected that a similar process takes place in the case of
UV irradiation of c-C6H7, and the strong band system with onset
at 282 nm is assigned to the open-chain isomer. CAS(5,6)/
6-311G(d,p) calculations predict two electronic transitions for
the two neutral, open-chain isomers E- and Z-HT at ∼2.8 and
4.2 eV; the latter transition lies close to the observed M system
(4.39 eV). By analogy to the photoisomerization of 1,3-CHD
to tZt-hexatriene, the M system is assigned to E-HT. The relative
absorption of E-HT (at ∼282 nm) with respect to the 310 nm
system of c-C6H7 varies with the experimental conditions. The
with benzene in a neon matrix. The G band is stronger than M
in this case, which means that the isomerization to R-HF
dominates over the ring-opening reaction.
Apart from the discussed L, M, D, E, and G systems in the
330-340 nm range, there is a weak F system present which
cannot be attributed to c-C6H7, E-HT, or R-HF species, because
only one transition is predicted for these species in this range.
Therefore, the F system is due to another isomer of C6H7. CAS
calculations predict a transition around 300 nm for methyl-
hydrogenated fulvene (MHF) and slightly lower in energy for
ꢀ-hydrogenated fulvene (275 nm). The F lies close to the
literature value for methylcyclopentadienyl radical (336 nm).35,36
Also the vibrational pattern of the F system is similar. Therefore,
absorption F, with the onset at 336 nm, is assigned to MHF.
The excitation of the ν14 mode and its overtones is observed
and agrees well with data reported earlier.35,36
6. Conclusions
+
The presented investigations on C6H7 trapped in a neon
matrix provide a direct spectroscopic characterization of two
structural isomers of this cation, which have been postulated in
the earlier gas-phase reactivity studies.13-18 We confirm that
one of these isomers is protonated benzene, and its electronic
spectrum in a neon matrix agrees well with the one obtained in
an earlier photodissociation experiment.9 The second, less
reactive isomer of C6H7+ reported in earlier gas-phase studies14
is identified as R-protonated fulvene and is characterized
spectroscopically for the first time. The electronic transitions
+
weakest M absorption was observed in the experiment of H3
with benzene trapped in the matrix. In this case, E-HT was
present as a result of the ring-opening process of the excited
c-C6H7 produced in the exothermic (117 kJ/mol) reaction of
C6H6 with H generated by neutralization of H3+. The M
absorption of E-HT was also observed following deposition of
C6H7+ without an electron scavenger. In such experiments, E-HT
was formed by the neutralization of collisionally produced
E-HT+ from PB+ in a neon matrix.
1
1
+
1
1
+
˜
˜
˜
˜
A B2rX A1 of PB and A A′rX A′ of R-PF lie in the UV
range, and they overlap. This fact will hinder their future gas-
phase spectroscopic observation, as both isomers are produced
concomitantly from a number of precursors.
Though the CAS calculations predict the electronic transition
of E-HT also in the visible range (∼438 nm), near the weak E
band system with onset at 532 nm, which grows in intensity
upon UV irradiation of the matrix, the system E cannot be
assigned to E-HT because its relative intensity with respect to
absorption at ∼282 nm varies with the experimental conditions.
The intensity of the E system correlates well with the G system,
with onset at 326 nm.
Besides cyclohexadienyl radical, three other isomers of C6H7
are identified following UV induced neutralization of C6H7
and characterized spectroscopically for the first time: R-HF,
MHF, and E-HT. Photoinduced isomerization of c-C6H7 to
E-HT and R-HF was observed.
+
Acknowledgment. This work has been supported by the Swiss
National Science Foundation (project no. 200020-124349/1).
After deposition of C6H7+, the absorption of R-PF+ was
detected. Thus, neutral R-hydrogenated fulvene (R-HF) should
also be present in the matrix as a result of neutralization of the
corresponding ion. CAS calculations predict two transitions at
459 and 298 nm for this neutral, which are quite close to the
Supporting Information Available: Cartesian coordinates and
energies of the cationic and neutral C6H7 isomers shown in
Figure 5; complete ref 30. This material is available free of
JA106470X
(32) Kuthirummal, N.; Rudakov, F. M.; Evans, C. L.; Weber, P. M.
J. Chem. Phys. 2006, 125, 133307.
(35) Dimauro, L. F.; Heaven, M.; Miller, T. A. Chem. Phys. Lett. 1986,
124, 489–492.
(33) Schonborn, J. B.; Sielk, J.; Hartke, B. J. Phys. Chem. A 2010, 114,
4036–4044.
(36) Yu, L.; Cullin, D. W.; Williamson, J. M.; Miller, T. A. J. Chem. Phys.
1991, 95, 804–812.
(34) Shida, T.; Kato, T.; Nosaka, Y. J. Phys. Chem. 1977, 81, 1095–1103.
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