Spectroscopic evidence of α-methylbenzyl radical in the gas phase

Article abstract of DOI:10.1016/j.cplett.2008.10.008

We report the observation of the spectroscopic evidence of the α-methylbenzyl radical in a corona excited supersonic expansion using a pinhole-type glass nozzle for the first time. The precursors, toluene, ethylbenzene, and isopropylbenzene, seeded in a l

Full text of DOI:10.1016/j.cplett.2008.10.008

Chemical Physics Letters 465 (2008) 193–196
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Chemical Physics Letters
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Spectroscopic evidence of a-methylbenzyl radical in the gas phase
Gi Woo Lee, Hyeon Geun Ahn, Tae Kyu Kim, Sang Kuk Lee *
Department of Chemistry, The Chemistry Institute of Functional Materials, Pusan National University, Pusan 609-735, Republic of Korea
a r t i c l e i n f o
a b s t r a c t
Article history:
We report the observation of the spectroscopic evidence of the a-methylbenzyl radical in a corona
Received 14 June 2008
In final form 6 October 2008
Available online 8 October 2008
excited supersonic expansion using a pinhole-type glass nozzle for the first time. The precursors, toluene,
ethylbenzene, and isopropylbenzene, seeded in a large amount of inert carrier gas helium, were electri-
cally discharged to produce benzyl-type radicals as a result of the breaking off of a C–H or a C–C bond
from the alkyl chain. The vibronic emission spectra, obtained in the visible region from the precursors,
were compared to identify the species generated in the corona discharge of the precursors, from which
we found the spectroscopic evidence of the
a-methylbenzyl radical.
Ó 2008 Elsevier B.V. All rights reserved.
1
. Introduction
of jet-cooled benzyl-type radicals in the gas phase. Although the
mechanism for generation and excitation of benzyl-type radicals
in a corona discharge is not exactly understood, the analysis of
such vibronic spectra observed provides spectroscopic evidence
for the identification the transient species produced.
The benzyl radical, a prototypical aromatic free radical, has at-
tracted much attention from spectroscopists. Earlier work on ben-
zyl radicals in the visible region have been reported by Schuler
et al. [1] and Walker and Barrow [2]. Bindley et al. [3,4] made
the first assignments of the vibronic bands from an analysis of
the emission spectra of the xylyl radicals produced by an electric
discharge of xylenes.
Charlton and Thrush [5] have applied the technique of laser-in-
duced fluorescence (LIF) to the observation of the vibronic emis-
sion spectra of benzyl-type radicals as well as to measurements
of the lifetime in the excited vibronic states. Recently, Lee group
In this study, we report the first spectroscopic evidence of the
a-methylbenzyl radical, which has been believed to play an impor-
tant role as a reaction intermediate in the chain reaction of alkyl-
benzenes. From a comparison of the vibronic emission spectra
obtained from the different precursors in the corona discharge,
the formation of the
. Experimental
The corona discharge of precursors toluene, ethylbenzene, and
a-methylbenzyl radical could be confirmed.
2
[
6–12] extended the assignments of the vibrational modes of
benzyl-type radicals by employing the technique of corona-excited
supersonic expansion, from which the vibronic emission spectra of
many benzyl-type radicals having substituents in the benzene ring
have been observed with better resolution and S/N.
isopropylbenzene was carried out using the CESE technique, of
which the experimental apparatus is similar to those described
elsewhere [16].
The technique of corona excited supersonic expansion (CESE)
has been recognized as a powerful spectroscopic technique for
observation of vibronic emission spectra of jet-cooled transient
species [13]. The spectral simplification and molecular stabiliza-
tion associated with the expansion of an inert carrier gas cannot
be achieved in any other way. The combination of supersonic
expansion with the emission spectroscopy has greatly expanded
the repertoire of spectroscopic studies of transient molecular spe-
cies. Of the emission sources utilizing this combination, the one
providing enough continuous photon intensity for high resolution
studies of the weak transition is the pinhole-type glass nozzle
Electronically excited but jet-cooled benzyl-type radicals were
produced from the corona discharge of the precursors (Aldrich, re-
agent grade), which were seeded in a large amount of carrier gas
helium. The concentration of the precursor in 3 bar of carrier gas
was adjusted by opening a by-pass valve for the maximum emis-
sion intensity monitored from the strongest band, and was be-
lieved to be less than 1% in the mixture. The gas mixture was
expanded through the 0.3 mm diameter of the pinhole-type glass
nozzle fabricated in this laboratory according to the method de-
scribed previously [17]. A sharpened long tungsten rod, acting as
an anode, was connected to a high-voltage dc power supply in
the negative polarity, in which the axial discharging current was
[
14,15], which has been widely employed to observe the vibronic
emission spectra of transient species. Accordingly, this nozzle has
been applied to the observation of the vibronic emission spectra
5
mA at a 1500 V dc potential and stabilized using a 150 kX cur-
rent-limiting ballast resister. The Pyrex expansion chamber was
evacuated by a 800 l/min mechanical rotary vacuum pump, result-
ing in a pressure of 2.0 mbar during continuous expansion, with 3.0
bar of backing pressure.
*
Corresponding author. Fax: +82 51 516 7421.
E-mail address: sklee@pusan.ac.kr (S.K. Lee).
0
009-2614/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved.
doi:10.1016/j.cplett.2008.10.008

1
94
G.W. Lee et al. / Chemical Physics Letters 465 (2008) 193–196
A weak blue-green colored jet provides the evidence for the for-
mation of the benzyl-type radicals in the D ? D transition. The
In substituted benzyl-type radicals, the electronic interaction
1
0
between the substituents and the benzene ring is undoubtedly of
the second-order compared with that between the methylene
group and the benzene ring, since the molecule has a planar struc-
light emanating from the downstream jet area 5 mm from the noz-
zle opening was collimated using a quartz lens (d = 3.8 cm,
f = 5.0 cm) placed inside the chamber and focused onto the slit of
the monochromator (Jobin Yvon U1000, path length = 2.0 m) con-
taining two 1800 lines/mm gratings, and detected with a cooled
photomultiplier tube (Hamamatsu R649) and a photon counting
system. During the scans, the slits were set to 0.100 mm, providing
ture with 7 delocalized
p electrons, in which the methylene group
contributes an electron. Thus, the electronic structure of substi-
tuted benzyl-type radicals should exhibit a close relationship to
that of the benzyl radical, and one might be able to closely relate
the two lowest-lying electronic states of such benzyl-type radicals
ꢀ
1
2
2
an effective resolution of about 2 cm in the visible region. The
to the parental benzyl 2 B
2
ðD
2
Þ and 1 A
2
ðD Þ states.
1
ꢀ
1
spectral region from 18000 to 22000 cm
was scanned in
The weak visible emission from benzyl-type radicals is believed
to arise from transitions to the 1 B
close-lying D and D excited electronic states [19], in which the
ꢀ1
2
2
.0 cm
increment over 2 h to record the spectra shown in
2
ðD
0
Þ ground state from the
Fig. 1. The wavenumber of the spectrum was calibrated using the
He atomic lines [18] observed in the same spectral region as the
benzyl-type radicals, and is believed to be accurate within
2
1
two excited states could be mixed by vibronic coupling [20], and
ring substitution is also expected to affect the energies of the ex-
ꢀ1
±1.0 cm
.
cited states differently. Most of the benzyl-type radicals have the
2
lowest excited electronic state of 1 A
2
[21]. The rapid collisional
relaxation from the D
tion to the D state, thereby rendering observable only the transi-
tions from the D to D state.
2 1
and D states transfers most of the popula-
3
. Results and discussion
1
It has been demonstrated that a well-controlled corona dis-
1
0
Fig. 1 shows a portion of the vibronic emission spectra observed
from the transient species formed in the corona discharge of the
precursors, (a) toluene, (b) ethylbenzene, and (c) isopropylbenzene
in the visible region, with a large amount of inert carrier gas he-
lium, in which spectrum 1(a) is in exact agreement with the vib-
ronic emission spectra of the benzyl radical reported previously
charge of substituted toluene seeded in a large amount of inert car-
rier gas predominantly produces corresponding benzyl-type
radicals in the excited vibronic state. Although the mechanism
for generation and excitation of benzyl-type radicals was not
exactly established, it has been suggested that the metastable He
atom produced in a corona discharge transfers its excess energy
to the precursor through a collisional process, which generates cor-
responding benzyl-type radicals by extracting a hydrogen atom
from the methyl group rather than the benzene ring. The molecular
radicals produced undergo collisional relaxation to lose vibrational
energy in the excited electronic states, and eventually emit a fluo-
rescence corresponding to the energy difference between the two
vibronic states.
0
0
[
22,23]. The strong 6b and 6a bands and the weak origin band
1 1
are the characteristics of the benzyl radical, because the corona
discharge of toluene produces only the product, benzyl radical,
by breaking off of the C–H bond of the methyl group, as illustrated
in Fig. 2a. Spectrum 1(b) observed from the corona discharge of
ethylbenzene shows the strong vibronic structure of the benzyl
ꢀ1
radical as well as the weak band at 21778 cm , which we assume
α-methylbenzyl radical
0
1
6
b
0
1
1
(c)
*
(b)
0
1
0
1
6
b
6a
0
1
8b
0
1
2
1
(a)
0
0
0
19500
20500
21500
22500
-
1
Wavenumber (cm )
1 0
Fig. 1. A portion of the visible vibronic emission spectra in the D ? D transition observed from the corona discharge, using a pinhole-type glass nozzle in a CESE scheme, of
the precursors, (a) toluene, (b) ethylbenzene, and (c) isopropylbenzene, seeded in a large amount of inert carrier gas helium. The peak marked by an asterisk in spectrum (c)
indicates the He atomic transition.

G.W. Lee et al. / Chemical Physics Letters 465 (2008) 193–196
195
Table 2
Vibrational frequencies (cmꢀ1) of the
a
-methylbenzyl radical.a
a
b
Position
Spacing
Ab initiob B3LYP/6-311g Ethylbenzenec
Moded
(
D
0
)
0
)
0
(D (S )
21778
21166
21000
0
612
778
Origin
6b
1
623
785
624
771
a
ꢀ1
Measured in vacuum (cm ).
Not scaled.
Ref. [26].
b
c
d
Ref. [27].
at 21346, 21486, and 21700 cmꢀ1, shifted by 656, 516, and
ꢀ1
3
02 cm from benzyl radical, for the o-, m-, and p-isomers, respec-
tively [24]. With substitution in the benzyl radical, the emission
spectra observed with a pinhole-type nozzle shows a very strong
origin band intensity for the benzyl-type radicals, because highly
efficient vibronic relaxation increases the population in the vibra-
c
1
tionless D state. Also, the similarity in molecular structure in the
two electronic states contributes to the increasing origin band
intensity, owing to the large Franck–Condon integral. Thus the
ꢀ
1
observation of the strong vibronic band at 21778 cm strongly
indicates the origin band of the -methylbenzyl radical, which is
the first observation in any spectral region. Another vibronic band
a
ꢀ
1
ꢀ1
of mode 6b was identified at 21166 cm , shift by 612 cm from
the origin band which is the well known mode of C–C–C angle
deformation. The splitting between the modes 6a and 6b, degener-
Fig. 2. Products obtained from decomposition of the precursors in a CESE scheme.
(
a) toluene[1] produces benzyl radical[4] only. Whereas (b) ethylbenzene[2]
produces the benzyl radical[4] as the major product and the -methylbenzyl
radical[5] as the minor product, (c) isopropylbenzene[3] produces the -methyl-
-dimethylbenzyl radical[6] as the
a
a
ꢀ1
ate in benzene at 606 cm increases with increasing mass of the
benzyl radical[5] as the major product and the a,a
substituents. Another important vibrational mode in benzyl-type
radicals, mode 1 of ring breathing vibration was assigned to the
minor product.
ꢀ1
ꢀ1
band at 21000 cm , a shift of 774 cm because the frequency
of this mode should be less sensitive to substitution. The ab initio
calculation shows very good agreement with the observation as
well as that of ethylbenzene as shown in Table 2.
is the evidence of the
corona discharge of ethylbenzene by the breaking off of the C–
CH bond. Since the C–C bond (D = 345 kJ/mol) is significantly
weaker than the C–H bond (D = 411 kJ/mol), the -methylbenzyl
a-methylbenzyl radical generated from the
3
0
0
a
In spectrum 1(c), we observed many bands of very strong inten-
radical can be produced as the minor product as shown in
Fig. 2b. In order to confirm this assumption, we chose isopropyl-
benzene as the precursor, because this precursor can produce the
sity belonging to the C
the combustion process of hydrocarbons [25]. Nevertheless, it is
strongly believed that production of the -dimethylbenzyl radi-
2
species, which is thought to be abundant in
a,a
a
-methylbenzyl radical as the major product by the breaking off
cal may not be possible in the combustion process of isopropylben-
zene. The radicals that are formed easily undergo a process of
decomposition into small fragments by a further process.
of the C–CH bond, and can produce the -dimethylbenzyl radi-
cal as the minor product by the breaking off of the C–H bond in the
3
a,a
corona discharge, as shown in Fig. 2c. Thus, spectrum 1(c) shows
In summary, several relevant precursors were seeded in a large
amount of carrier gas helium to produce benzyl-type radicals in a
CESE using a pinhole-type glass nozzle. The visible vibronic emis-
sion spectra observed from the different precursors were
compared to identify the new aromatic radicals, among which
the greatly increasing intensity of the
the same wavenumber observed from ethylbenzene.
0
The origin band of the benzyl radical in the D ? D electronic
a-methylbenzyl radical at
1
ꢀ1
transition was observed at 22002 cm with weak intensity be-
cause of the electronically forbidden by electric dipole selection
rule [22]. The substitution in the benzene ring shows the origin
band shifted to the red region, as listed in Table 1. The xylyl radi-
the a-methylbenzyl radical was detected for the first time.
Acknowledgement
1 0
cals show their origin band of the D ? D electronic transition
This work was supported by a Korea Research Foundation Grant
funded by the Korea Government (MOEHRD) (KRF-2007-314-
C00145 and R14-2003-033-01002-0).
Table 1
The origin band in the D
? D
0
transition of benzyl-type radicals.a
Origin band
1
Molecules
Shift
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ꢀ
1
(cm )d
ꢀ1
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