Vibronic spectrum of the jet-cooled 2,6-dimethylbenzyl radical in a corona excitation

Article abstract of DOI:10.1021/jp0350215

A jet-cooled 2,6-dimethylbenzyl radical was generated for the first time from a precursor 2,6-dimethylbenzyl chloride with a large amount of inert carrier gas He in a corona-excited supersonic expansion using a pinhole type glass nozzle. The visible fluorescence emanating from the jet was recorded with a long path monochromator to observe the vibronically resolved jet-cooled emission spectrum. The spectrum was analyzed to obtain accurate electronic transition and active vibrational mode frequencies in the ground electronic state by comparison with those from an ab initio calculation.

Full text of DOI:10.1021/jp0350215

6554
J. Phys. Chem. A 2003, 107, 6554-6557
Vibronic Spectrum of the Jet-Cooled 2,6-Dimethylbenzyl Radical in a Corona Excitation
Sang Kuk Lee,* Byoung Uk Ahn, and Sang Kyu Lee
Department of Chemistry and Chemistry Institute for Functional Materials, Pusan National UniVersity,
Pusan 609-735, Republic of Korea
ReceiVed: April 16, 2003; In Final Form: July 1, 2003
A jet-cooled 2,6-dimethylbenzyl radical was generated for the first time from a precursor 2,6-dimethylbenzyl
chloride with a large amount of inert carrier gas He in a corona-excited supersonic expansion using a pinhole
type glass nozzle. The visible fluorescence emanating from the jet was recorded with a long path monochromator
to observe the vibronically resolved jet-cooled emission spectrum. The spectrum was analyzed to obtain
accurate electronic transition and active vibrational mode frequencies in the ground electronic state by
comparison with those from an ab initio calculation.
Introduction
In this paper, we report for the first time the observation of
the dimethyl-substituted benzyl radical, the 2,6-dimethylbenzyl
radical, which has been generated from 2,6-dimethylbenzyl
chloride in a corona-excited supersonic expansion using a
pinhole-type glass nozzle, from which the vibrational modes in
the ground electronic state have been assigned by comparison
with those from both an ab initio calculation and the precursor.
Although the benzyl radical has received much attention as
a prototype of aromatic radicals, alkyl-substituted benzyl radicals
have been less studied as large aromatic radicals.1,2 The earlier
works of the xylyl radical in the visible region were reported
3
4
5,6
by Schuler et al. and by Walker and Barrow. Bindley et al.
made tentative assignments of the vibronic bands from an
analysis of the emission spectra of xylyl radicals produced by
an electric discharge of the corresponding xylenes.
Selco and Carrick have presented an analysis of the low-
resolution vibronic emission spectra of xylyl radicals generated
in a corona-excited supersonic expansion, in which many
Experimental Section
7
The 2,6-dimethylbenzyl radical has been generated and
vibronically excited using a technique of a corona-excited
supersonic expansion, in which the experimental apparatus is
2
2
8
very similar to those described elsewhere. Briefly, it consists
of a pinhole type glass nozzle coupled with a high-voltage dc
discharge, a portable six-way cross type Pyrex expansion
chamber, and a spectrometer for observation of the vibronic
emission spectrum.
vibrational modes have been identified. Charlton and Thrush
have obtained the laser-induced fluorescence spectra of alkyl-
substituted benzyl radicals and have measured the lifetime in
the excited vibronic states. Lee and colleagues9-12 have extended
the assignments of vibrational modes of xylyl radicals from the
vibronic emission spectra. Recently, the controversial assign-
ments of p-xylyl radicals have been resolved by an analysis of
the vibronic bands from high-resolution emission spectra. The
torsional barrier of the internal methyl group has been calculated
for the xylyl radicals by Lin and Miller13 from an analysis of
the laser-induced fluorescence excitation and dispersed emission
spectra.
The radical was generated and vibronically excited in a jet
by a corona discharge from the gas mixture of a large amount
of inert carrier gas He and 2,6-dimethylbenzyl chloride, which
was synthesized in an organic laboratory. The concentration of
the precursor in the carrier gas of 2 atm was adjusted for the
maximum emission intensity monitored from the strongest band
and believed to be about 1% in the gas mixture. The gas mixture
was expanded through the 0.4 mm diameter of pinhole type
glass nozzle made in this laboratory according to the method
12
The supersonic free jet expansion has been used as a powerful
spectroscopic technique for the observation of molecular spectra
2
3
1
4
described previously. The sharpened long tungsten rod acting
as an anode was connected to the high-voltage dc power supply
in the negative polarity, in which the axial discharging current
was 5 mA at a 1500 V dc potential and stabilized using a 150
kΩ current-limiting ballast resister.
in the gas phase. The spectral simplification and stabilization
associated with the expansion of inert buffer gas cannot usually
be obtained in any other way. The combination of the supersonic
expansion technique with emission spectroscopy has had an
enormous impact on the repertoire of spectroscopic studies of
molecular species. Of the emission sources developed so far
for these purposes, the only one providing enough continuous
15,16
In the original nozzle developed by Engelking,
the coronal
electrode sits just behind the nozzle opening on the high-pressure
side, which leads to excitation before expansion, causing the
problem of nozzle clogging when heavy organic compounds
are employed as a precursor. Thus, we have made the metal tip
of the anode extend through the nozzle to reduce clogging of
the nozzle throat by decomposition products and soot deposits
of 2,6-dimethylbenzyl chloride. Although the metal tip was
exposed less than 1.0 mm outside the end of the nozzle capillary,
this significantly improved the clogging problem by partially
allowing excitation to occur after expansion. The modified
photon intensity for high-resolution studies of weak transition
is the pinhole type glass nozzle,1
5,16
which has been widely
employed for the observation of the vibronic emission spectra
of transient molecules.17 This has also been applied to the
observation of the vibronic emission spectra of many jet-cooled
benzyl-type radicals in the gas phase.1
8-21
*
Corresponding author. Fax: +82-51-516-7421. E-mail: sklee@
pusan.ac.kr.
1
0.1021/jp0350215 CCC: $25.00 © 2003 American Chemical Society
Published on Web 08/01/2003

Mode Assignments of the 2,6-Dimethylbenzyl Radical
J. Phys. Chem. A, Vol. 107, No. 34, 2003 6555
The weak visible emission from the benzyl-type radicals is
2
believed to arise from transitions to the 1 B2 ground state from
2
2
25
the close-lying 2 B2 and 1 A2 excited states. Two excited
electronic states can be mixed by vibronic coupling, and ring
substitution is expected to affect the energies of the excited
electronic states differently. Most of the benzyl-type radicals
2
have the lowest excited electronic state of 1 A2 except for the
2
26,27
p-cyanobenzyl radical, which has that of 2 B2.
In methyl-substituted benzyl radicals, the electronic interac-
tion between the methyl group and the aromatic ring is
undoubtedly of the second-order compared to the interaction
between the methylene group and the ring, because the molecule
has a planar structure with seven delocalized π electrons, in
which the CH2 group contributes an electron. Thus, the
electronic structure of the 2,6-dimethylbenzyl radical should
exhibit a close relation to that of the benzyl radical and one
may be able to closely relate the two lowest lying electronic
states of the 2,6-dimethylbenzyl radical to the parental benzyl
Figure 1. Portion of the vibronic emission spectrum of the jet cooled
2
,6-dimethylbenzyl radical in the D
assignment. The weak bands around 19 400 cm are from the well-
known swan system of the C which is one of the main fragments in
1 0
f D transition with the
-
1
2
the corona discharge of precursor.
2
2
2
B2(D2) and 1 A2(D1) states.
Engelking type nozzle has been applied to the generation of
many benzyl-type radicals.
The Pyrex expansion chamber of six-way cross type was
made of thick-walled Pyrex tubes (Chemglass CG-138-02) of
Figure 1 shows a portion of the vibronic emission spectrum
of the 2,6-dimethylbenzyl radical in the D1 f D0 transition, in
which many of the strong bands are observed with an excellent
-
1
S/N in the region of 19 000-21 500 cm . The vibrational
5
0 mm in diameter. The chamber was evacuated by a 800 L/min
mechanical vacuum pump, resulting in the pressure range 0.5-
.0 Torr during continuous expansion with 2.0 atm of backing
-
1
structure of the radical appears in the region within 2000 cm
from the origin band because the emission spectrum observed
with an Engelking type pinhole nozzle is similar to the dispersed
fluorescence spectrum obtained by exciting the origin band of
the electronic transition. Thus, the spacing of the vibronic bands
from the origin band represents the vibrational mode frequencies
in the ground electronic state.
1
pressure. A cathode made of a long copper rod was positioned
to be parallel to the jet direction under the expansion chamber
to avoid arching noise reaching to the spectrometer.
A bright blue-green jet was the evidence of the presence of
the 2,6-dimethylbenzyl radicals in the expansion. The light
emanating from the downstream jet area 5 mm away from the
nozzle opening was collimated by a quartz lens (f ) 5.0 cm)
placed inside the expansion chamber and focused onto the slit
of the monochromator (Jobin Yvon U1000) containing 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 a resolution
The spectrum consists of the strong origin band of the D1 f
-1
D0 transition at 21 164 cm (in air), followed to lower energies
by a series of vibronic bands. The origin band of the radical
does not exhibit the strongest intensity in the emission spectrum.
The intensity of the origin band of the benzyl radical at 22 000
-
1
0
1
0
1
cm is smaller than those of the 6a and 6b bands as a result
2
2
of the vibronic coupling between the 2 B and 1 A electronic
2
2
1
-
1
of about 2 cm in the visible region. The spectral region from
states, which is similar to that of the benzyl radical. However,
the absence of bands with observable intensity to the blue of
the origin confirms the origin band, resulting from the efficient
vibrational cooling at the excited electronic state, which is an
important characteristic of an Engelking-type pinhole nozzle.
-
1
-1
1
8 000 to 22 000 cm was scanned at the step of 1.0 cm
over 2 h to obtain the final spectrum shown in Figure 1. The
wavenumber of the spectrum was calibrated using the He atomic
lines24 observed at the same spectral region as the 2,6-
dimethylbenzyl radical and is believed to be accurate within
Because the 2,6-dimethylbenzyl radical belongs to the C2V
point group, the bands observed in this study should exhibit
the a- or b-type bandshape, depending on the vibrational modes.
Other types of benzyl radicals with the same symmetry show
distinguishable band shapes in the emission spectra. The
bandshapes of p-xylyl radical have cleared the controversial
assignments of the vibronic bands observed in the emission
spectra. Also, the band shapes of many heavy molecules such
as benzyl-type radicals have been analyzed to determine the
direction of the transition dipole moment by simulating the
-
1
(
1.0 cm .
Because the 2,6-dimethylbenzyl radical has many vibrational
modes and the assignments have not been completed, ab initio
calculations on the D0 state have been carried out to assist the
assignment of the vibronic structure of the spectrum. The
calculations were executed with a personal computer equipped
with an Intel Pentium 600 MHz processor and 128 MB RAM,
and with the standard methods included in the GAUSSIAN 98
program for windows package. The geometry optimization and
vibrational frequency calculations were performed at the UHF
level and the 6-31g* basis set was employed in all calculations.
30
rotational contour of the bands observed. In this resolution,
however, the large amplitude of internal rotation of the two
methyl groups in the dimethylbenzyl radical has blotted out the
band shape of the vibronic bands observed, as shown in Figure
Results and Discussion
2
.
It is apparent that a well-controlled corona discharge of
substituted toluenes seeded in a large amount of inert buffer
gas predominantly produces corresponding benzyl-type radicals
in the excited electronic state. The metastable He atom produced
in a corona excitation transfers the excess energy to the
substituted toluenes through a collisional process, generating
corresponding benzyl-type radicals by the extraction of the
chloride atom from the methylene chloride group.
Thus, the vibronic bands observed were assigned with the
help of the known vibrational frequencies of 1,2,3-trimethyl-
benzene as well as those from an ab initio calculation. From
the comparison between two molecules, it seems clear that the
vibrational structures in the ground electronic state of both
molecules are subjected to isodynamic approximation, which
is the correspondence of the vibrational mode frequencies and

6556 J. Phys. Chem. A, Vol. 107, No. 34, 2003
Lee et al.
observed at 615 cm- with stronger intensity. The observations
1
are in good agreement with the calculations (476.2 and 532.3
-
1
cm ).
The band at -1024 cm- from the origin has been assigned
to mode 18a of the C-CH3 in-plane bending mode because
the observed intensity and frequency matches with those of the
calculation. Modes 18a and 18b are also degenerate in benzene
and are fairly insensitive to substitution. The counterpart mode
1
1
1
8b has not been observed because the mode is not active in
,2,3-trisubstituted benzenes. Calculation shows that the fre-
Figure 2. Observed band shape of the strongest band at 20 616 cm-1.
The band shows about 5 cm of bandwidth (fwhh) due to the internal
rotation of two methyl groups.
quency of the 18b component is higher than that of 18a in m-
and p-substitution, whereas in o-substitution the case is reversed.
-
1
-
1
The strong band at -1220 cm has been assigned to the
mode 20a of the C-CH3 stretching vibration due to the
TABLE 1: List of the Vibronic Bands Observed and Their
Assignments
-1
coincidences with that of the precursor (1248 cm ) and the
-
1
calculation (1249.9 cm ). In this mode the precursor shows a
very strong intensity in Raman spectrum.
positiona
intensityb
spacing from origin
assignments
0
0
21 164
20 712
20 616
20 140
19 944
8
3
100
18
0
452
548
1024
1220
origin band (0 )
One of the most important modes, mode 1 of ring breathing
0
-1
6a
6b
has been calculated to locate at -674 cm from the origin band.
1
0
However, in this experiment, this mode was not observed
because the two methyl groups have insignificant contribution
to the amplitude of vibration.
1
0
1
18a
0
17
13
1
-
1
a
Measured in air (cm-1). b Normalized with respect to the strongest
The several bands at 19400 cm with moderate intensity
3
3
are from the well-known swan system A Πg-X′ Πu of C2
band.
which is one of the main fragments in the decomposition of
hydrocarbons. With increasing discharging voltage, the in-
TABLE 2: Vibrational Frequencies (cm-1) of the
29
2
,6-Dimethylbenzyl Radicala
tensity of C2 bands increases in the spectrum whereas the
intensity of the dimethylbenzyl radical decreases significantly.
Thus, the optimization of the discharging condition is crucial
in the observation of the vibronic emission spectrum.
ab initioc
precursor
d
this work UHF/6-31g* 1,2,3-trimethylbenzene symmetry
(D (D (S
modeb
origin
)
(C2V)
0
0
)
0
)
21164
452
548
1024
1220
In our previous experiment, we have observed several low-
frequency sequence bands regularly positioned in the vicinity
6
6
1
1
a
b
8a
3
476.2
532.3
1090.3
1249.9
485
539
1095
1193
a
a
b
2
1
1
1
8
of every strong vibronic band. The origin of the sequence
bands are believed to belong to the combination bands associated
with the excited vibrational state in the upper electronic state.
a
1
a
Measured in air. Reference 31. c Multiplied by a scaling factor
b
3
0
1
0
1
d
Cossart-Magos and Cossart have assigned the 16a 11 , 18b ,
0 1 1
of 0.95. Reference 28.
1
1
1
1 , and 10b for the bands observed at -11.3, +7.1, -27.2,
1 1
-
1
and -54.8 cm , respectively, from the origin band of the
p-fluorobenzyl radical in the D1 f D0 transition. But we could
not observe these bands with any noticeable intensity because
of the limited sensitivity of the spectrometer and weak fluo-
rescence.
transition intensity between the molecules with similar struc-
tures. This has already been applied to the vibronic assignments
of many benzyl-type radicals.19-21 The bands observed in this
study are listed in Table 1, together with the assignments.
It has generally been accepted that the calculation using the
GAUSSIAN 98 program at the UHF level with a the 6-31g*
basis set predicts the vibrational mode frequencies within (10%
of the experimental values. The calculation was carried out as
described in the Experimental Section. From the calculation for
the 2,6-dimethylbenzyl radical, a total of 54 vibrational mode
frequencies have been obtained, of which 18, 8, 11, and 17
vibrational modes belong to the A1, A2, B1, and B2 symmetries
in the C2V point group, respectively. The calculated values were
multiplied by a scaling factor of 0.95 to match the observed
values, as in the case of the 2,6-difluorobenzyl radical. Table 2
lists the observed and calculated vibrational mode frequencies
Acknowledgment. This work was financially supported by
the Korea Research Foundation Grant. KRF-2001-002-D00141.
References and Notes
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(
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(
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Mode Assignments of the 2,6-Dimethylbenzyl Radical
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