Photodissociation of C-H and C-O bonds of p-methoxytoluene and p-methoxybenzyl alcohol in solution

Article abstract of DOI:10.1063/1.475232

The photodissociation of p-methoxytoluene and p-methoxybenzyl alcohol at 266 nm in n-heptane solution is studied by nanosecond fluorescence and absorption spectroscopy. The formation of a p-methoxybenzyl radical is identified by its fluorescence which is

Full text of DOI:10.1063/1.475232

Photodissociation of C–H and C–O bonds of p -methoxytoluene and p -methoxybenzyl
alcohol in solution
M. Fujiwara and K. Toyomi
Citation: The Journal of Chemical Physics 107, 9354 (1997); doi: 10.1063/1.475232
View online: http://dx.doi.org/10.1063/1.475232
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Photodissociation of C–H and C–O bonds of p-methoxytoluene
and p-methoxybenzyl alcohol in solution
M. Fujiwara and K. Toyomi
Department of Chemistry, Faculty of Science, Hiroshima University, Kagamiyama,
Higashi-Hiroshima 739, Japan
͑Received 7 July 1997; accepted 8 September 1997͒
The photodissociation of p-methoxytoluene and p-methoxybenzyl alcohol at 266 nm in n-heptane
solution is studied by nanosecond fluorescence and absorption spectroscopy. The formation of a
p-methoxybenzyl radical is identified by its fluorescence which is induced by excitation at 308 nm.
The yields of the radical are of the order of ϳ10Ϫ3 for dissociation of p-methoxytoluene and
p-methoxybenzyl alcohol. The growth rate of 1.5ϫ108 sϪ1 for the radical is equal to the decay rate
of (1.5Ϯ0.3)ϫ108 sϪ1 for the precursor fluorescence in dissociation of p-methoxytoluene, whereas
the growth rate of Ͼ1.0ϫ109 sϪ1 for the radical is much faster than the decay rate of (1.8Ϯ0.3)
ϫ108 sϪ1 for the precursor fluorescence in dissociation of p-methoxybenzyl alcohol. The formation
of the radical depends linearly on the photolysis pulse fluence for dissociation of p-methoxytoluene
and p-methoxybenzyl alcohol. The data show existence of two distinct dissociation channels.
p-Methoxytoluene dissociates from thermally equilibrated levels of the S1 state after vibrational
relaxation, whereas p-methoxybenzyl alcohol dissociates from vibrationally excited levels of the S1
state in competition with vibrational relaxation. The difference of these channels is explained on a
model of electronic coupling between the precursor and product states in the geometry where the
C–H and C–O bonds are stretched in a plane perpendicular to the benzene rings. For
p-methoxytoluene, the S1 state does not correlate adiabatically to the ground state of the C–H bond
fission products, so intersystem crossing or internal conversion precedes dissociation. For
*
p-methoxybenzyl alcohol, avoided crossing between the ␲␲ ͑benzene͒ configuration and the
*
np(O)␴ (C–O) repulsive configuration results in the adiabatic potential-energy surface which
evolves to the ground state of the C–O bond fission products allowing rapid dissociation. © 1997
American Institute of Physics. ͓S0021-9606͑97͒02646-9͔
I. INTRODUCTION
fluorescence and absorption experiments are performed. Ex-
citation of these molecules at 266 nm produces
a
p-methoxybenzyl radical. Its fluorescence is observed by ex-
citation at 308 nm. The quantum yield and rate for the dis-
sociation are measured. The number of photons necessary for
the dissociation is determined. Two dissociation channels are
identified. p-Methoxytoluene dissociates from thermally
equilibrated levels of the S1 state after vibrational relaxation,
whereas p-methoxybenzyl alcohol dissociates from vibra-
tionally excited levels of the S1 state in competition with
vibrational relaxation. The difference of the mechanisms is
explained in terms of electronic coupling between the pre-
cursor and product states.
It is well known that photoemission in the electronically
excited states of organic molecules occurs from their vibra-
tional ground levels in the condensed phase. The rate con-
stants of 100 –109 sϪ1 for emission are much smaller than
those of ϳ1011 sϪ1 for vibrational relaxation, and emission
cannot compete with vibrational relaxation in vibrationally
excited levels.
For photodissociation of organic molecules, toluene,1–4
and benzyl alcohol5 have been reported to yield a benzyl
radical from highly vibrationally excited levels of the S0
states by excitation to the S3 states at 193 nm in the gas
phase. The dissociation rates of (1.9Ϯ0.2)ϫ106 sϪ1 for
toluene4 and (1.7Ϯ0.2)ϫ106 sϪ1 for benzyl alcohol5 have
been measured along with the benzyl yield of 0.75 for
toluene.4 From these values, the rate constants of ϳ106 sϪ1
for dissociation are deduced.
How about photodissociation in the condensed phase? If
the rate constants for dissociation are of the order of
ϳ106 sϪ1 in vibrationally excited levels of the S1 states, dis-
sociation may occur from the vibrational ground levels, not
from vibrationally excited levels, of the S1 states by ultra-
violet excitation. Whether this is the case or not is a problem
that needs experimental investigation.
II. EXPERIMENT AND SIMULATION CALCULATION
p-Methoxytoluene
͑Tokyo
Chemical,
Ͼ99%͒,
p-methoxybenzyl alcohol ͑Kanto Chemical, Ͼ98%͒, and
p-methoxybenzyl chloride ͑Tokyo Chemical͒ were distilled.
Naphthalene ͑Nacalai Tesque͒ was recrystallized from etha-
nol. n-Heptane ͑Dojindo, spectrosol͒ was used as received.
The sample solutions, containing p-methoxytoluene (2.0
ϫ10Ϫ3 mol dmϪ3), p-methoxybenzyl alcohol (2.5ϫ10Ϫ3
mol dmϪ3), and p-methoxybenzyl chloride (ϳ10Ϫ3
mol dmϪ3) with n-heptane, were degassed by freeze–pump–
thaw cycles. The molar extinction coefficients were mea-
sured with an absorption spectrophotometer ͑Hitachi U-
3210͒.
The present paper deals with the photodissociation of
p-methoxytoluene and p-methoxybenzyl alcohol in
n-heptane solution at room temperature. The nanosecond
9354
J. Chem. Phys. 107 (22), 8 December 1997
0021-9606/97/107(22)/9354/7/$10.00
© 1997 American Institute of Physics
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9355
M. Fujiwara and K. Toyomi: Photodissociation of p-methoxytoluene
For the two-pulse fluorescence measurement, a pulse of
the fourth harmonic ͑266 nm, Ͻ0.5 ns rms jitter͒ of a
Nd:YAG laser ͑Quanta-Ray GCR-11͒ was used as the pho-
tolysis light. The fluorescence was induced by a second pulse
͑308 nm͒ from a XeCl excimer laser ͑Lumonics 500͒. The
laser power was attenuated with ND filters ͑Sigma FNDU-
50C02-10, -20, -50͒ and measured with a thermopile monitor
͑Ophir 03A-P, DGX͒. The delay time was adjusted with a
digital delay generator ͑EG&G PAR 9650, Ͻ0.1 ns rms jit-
ter͒. The laser beams were focused on a 10ϫ10 mm quartz
cell coaxially. The fluorescence was collected into a mono-
chromator ͑Ritsu MC-10N͒ and detected with a photomulti-
plier ͑Hamamatsu R636, 2.0 ns rise͒. A part of the 308 nm
beam was directed on a PIN photodiode ͑Hamamatsu S1722-
02, 5.8 ns rise͒. The output signals from both the photomul-
tiplier and photodiode were simultaneously fed to separate
channels of a digital oscilloscope ͑Tektronix 2440, 500 MHz
sampling, Ͻ0.1 ns rms jitter͒, which was interfaced to a per-
sonal computer ͑NEC PC-9801UV͒ for data storage.
best forms are Gaussian functions with widths ͑FWHM͒ of
4.5 and 4.0 ns, respectively. Rdetect(t) is the response func-
tion of the detection system including the photomultiplier
and digital oscilloscope. It is assumed to have a Gaussian
form with a width of 3.5 ns. Fgrow(t) is the population func-
tion of the ground-state radical, and Fdecay(t) is the depopu-
lation function of the excited-state radical, which is propor-
tional to the decay of the radical fluorescence.
Fgrow t͒ϭ1Ϫexp Ϫkgrowt͒,
͑2͒
͑3͒
͑
͑
Fdecay t͒ϭexp Ϫkdecayt͒,
͑
͑
kgrow is the growth rate constant of the ground-state radical
that is to be determined, and kdecay is the decay rate constant
of the excited-state radical which is fixed to be 7.2
ϫ106 sϪ1. The calculation is made with 0.2 ns increment.
III. RESULTS
A. Identification of radical
For the transient absorption measurement, the photolysis
light was the 266 nm laser pulse. The white light was pro-
vided by a Xe arc lamp ͑Ushio UXL-500D-O͒. The laser and
lamp beams were collimated on the cell at right angles with
each other. The detection apparatus for the white light was
the same as for the fluorescence. All experiments were run at
room temperature ͑293 K͒.
In order to identify the fragment radical formed from
photodissociation of p-methoxytoluene and p-methoxy-
benzyl alcohol, the two-pulse fluorescence and transient ab-
sorption spectra are measured.
The two-pulse fluorescence spectra, shown in Fig. 1, are
observed by excitation with the 308 nm pulse after photoly-
sis of p-methoxytoluene and p-methoxybenzyl alcohol with
the 266 nm pulse. Excitation with the 308 nm pulse alone
does not induce the fluorescence. The spectra exhibit a broad
band at 510 nm, which is assigned to the p-methoxybenzyl
radical and is similar to the band reported previously.6 The
fluorescence lifetimes, determined from photolysis of
p-methoxytoluene and p-methoxybenzyl alcohol, are of a
value of 141Ϯ12 ns and agree with the previous lifetime.6
The fluorescence band disappears at a 100 ␮s delay from the
266 to the 308 nm pulse. The result shows that C–H bond
fission in p-methoxytoluene and C–O bond scission in p-
methoxybenzyl alcohol occur to form the p-methoxybenzyl
radical.
The time profile of the radical fluorescence can be repro-
duced using convolution by the following expression:
*
t͒ I
S t͒ϭ
͑
F
t͒
͑
͖
pump
͓
͑
͕
grow
*
*
detect
ϫIprobe t͒ Fdecay t͒ R
t͒.
͑
͑1͒
͑
͔
͑
*
The symbol means convolution. Ipump(t) and Iprobe(t) are
the intensity time profiles of the two excitation pulses. Their
The transient absorption spectra are recorded by excita-
tion of p-methoxytoluene and p-methoxybenzyl alcohol with
the 266 nm pulse. However, the band assignable to the
p-methoxybenzyl radical is obscured by broad bands in the
300–470 nm region, which are assigned to the T1 states of
p-methoxytoluene and p-methoxybenzyl alcohol by com-
parison with the band of the T1 state of benzene.7 The life-
times of the T1 states are determined to be 3.0Ϯ0.6 ␮s for
p-methoxytoluene and 2.9Ϯ0.6 ␮s for p-methoxybenzyl al-
cohol.
The two-pulse fluorescence and transient absorption
measurements
are
performed
by
photolysis
of
p-methoxybenzyl chloride with the 266 nm pulse. This mol-
ecule has been reported to yield the p-methoxybenzyl
radical.6,8 The same fluorescence band ͑with the same life-
time͒ as in Fig. 1 is observed by excitation with the 308 nm
pulse. An absorption band is recorded at 288 nm, which is
associated with the p-methoxybenzyl radical.6,8,9 The fluo-
rescence and absorption bands disappear at a 100 ␮s delay.
FIG. 1. Two-pulse fluorescence spectra of a p-methoxybenzyl radical ob-
served by excitation with a 308 nm pulse at 1 ␮s after excitation of ͑a͒
p-methoxytoluene and ͑b͒ p-methoxybenzyl alcohol with a 266 nm pulse.
Upper curves are generated with both 266 and 308 nm pulses, while lower
ones with only a 308 nm pulse.
J. Chem. Phys., Vol. 107, No. 22, 8 December 1997
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9356
M. Fujiwara and K. Toyomi: Photodissociation of p-methoxytoluene
FIG. 2. Time evolution of fluorescence of ͑a͒ p-methoxytoluene and ͑b͒
p-methoxybenzyl alcohol recorded by excitation with a 266 nm pulse. ͑c͒
Time evolution of a 266 nm pulse. Monitoring wavelength: ͑a͒ and ͑b͒ 300
nm.
From the observation it is confirmed that the
p-methoxybenzyl radical is formed from dissociation of
p-methoxytoluene and p-methoxybenzyl alcohol.
FIG. 3. Observed and calculated fluorescence time profiles of
a
p-methoxybenzyl radical at various delay times from a 266 to a 308 nm
pulse. Fluorescence is induced by excitation with a 308 nm pulse after
excitation of p-methoxytoluene at 0 ns with a 266 nm pulse. Monitoring
wavelength is set at 510 nm. Delay times of a 308 nm pulse: ͑a͒ Ϫ11.8; ͑b͒
Ϫ4.8; ͑c͒ Ϫ1.8; ͑d͒ Ϫ0.6; ͑e͒ 0.8; ͑f͒ 2.6; ͑g͒ 3.4; ͑h͒ 7.4; ͑i͒ 11.2; ͑j͒ 17.8;
͑k͒ 23.4; ͑l͒ 28.6 ns.
B. Radical yields
The quantum yields of the p-methoxybenzyl radical
from dissociation of p-methoxytoluene, p-methoxybenzyl al-
cohol, and p-methoxybenzyl chloride are estimated as fol-
lows: Firstly, the ratio of the p-methoxybenzyl yields from
dissociation of p-methoxytoluene, p-methoxybenzyl alcohol,
and p-methoxybenzyl chloride is measured by comparing its
fluorescence intensities by excitation with the 308 nm pulse
after photolysis with the 266 nm pulse. Secondly, the
p-methoxybenzyl yield from dissociation of p-methoxy-
benzyl chloride is measured by comparison with the naph-
thalene triplet one as the standard from its absorbance by
excitation with the 266 nm pulse. The extinction coefficients
states can be determined as those of the fluorescence from
the S1 states. The fluorescence time profiles of
p-methoxytoluene and p-methoxybenzyl alcohol, observed
by excitation with the 266 nm pulse, are shown in Fig. 2.
Thermally equilibrated levels of the S1 states are found to
decay with rates of (1.5Ϯ0.3)ϫ108 sϪ1 for p-methoxy-
toluene and (1.8Ϯ0.3)ϫ108 sϪ1 for p-methoxybenzyl alco-
hol.
For the ͑ground-state͒ p-methoxybenzyl radical, its
growth rates are evaluated from the dependence of its fluo-
rescence time profile on the delay from the 266 nm ͑photoly-
sis͒ to the 308 nm ͑probe͒ pulse. The jitter of the 266 nm
pulse is limited to be Ͻ0.5 ns, while the delay of the 308 nm
pulse is calibrated by the photodiode signal. Unknown fluo-
rescence, observed by alternative excitation with either the
266 or 308 nm pulse, is subtracted from fluorescence, ob-
tained by simultaneous excitation with both the 266 and 308
nm pulses, after calibration of the delay.
9
used are 1.1ϫ104 dm3 molϪ1 cmϪ1 ͑288 nm͒ for the
p-methoxybenzyl radical and 1.4ϫ104 dm3 molϪ1 cmϪ1
10
͑415 nm͒ for naphthalene triplet. The intersystem crossing
yield for naphthalene is 0.68.10 In this way, an estimate for
the p-methoxybenzyl yields of ϳ1.3ϫ10Ϫ3, ϳ2.9ϫ10Ϫ3
,
and 0.38 is made from dissociation of p-methoxytoluene,
p-methoxybenzyl alcohol, and p-methoxybenzyl chloride,
respectively.
The fluorescence time profiles of the p-methoxybenzyl
radical at various delays of the 308 nm pulse are shown in
Fig. 3 for dissociation of p-methoxytoluene. It is noted that
the intensity and delay of the fluorescence change with the
delay of the 308 nm pulse. The fluorescence intensity begins
to grow at a Ϫ1.8 ns delay of the 308 nm pulse ͓Fig. 3͑c͔͒,
and reaches a plateau at a 17.8 ns delay of this pulse ͓Fig.
3͑j͔͒. The intensity growth is accompanied with the delay of
C. Rates of S1 decay and radical growth
To obtain knowledge of excited states that lead to disso-
ciation, the growth rates of the p-methoxybenzyl radical are
measured along with the decay rates of the S1 states of
p-methoxytoluene and p-methoxybenzyl alcohol.
For p-methoxytoluene and p-methoxybenzyl alcohol,
the decay rates of thermally equilibrated levels of the S1
J. Chem. Phys., Vol. 107, No. 22, 8 December 1997
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9357
M. Fujiwara and K. Toyomi: Photodissociation of p-methoxytoluene
FIG. 5. Logarithmic plots of a fluorescence intensity of a p-methoxybenzyl
radical versus ͑a͒ and ͑c͒ 266 and ͑b͒ and ͑d͒ 308 nm pulse fluences. Fluo-
rescence is induced by excitation with a 308 nm pulse at 1 ␮s after excita-
tion of ͑a͒ and ͑b͒ p-methoxytoluene and ͑c͒ and ͑d͒ p-methoxybenzyl al-
cohol with a 266 nm pulse. Monitoring wavelength: 510 nm.
FIG. 4. Observed and calculated fluorescence time profiles of
a
p-methoxybenzyl radical at various delay times from a 266 to a 308 nm
pulse. Fluorescence is induced by excitation with a 308 nm pulse after
excitation of p-methoxybenzyl alcohol at 0 ns with a 266 nm pulse. Moni-
toring wavelength is set at 510 nm. Delay times of a 308 nm pulse: ͑a͒
Ϫ13.2; ͑b͒ Ϫ6.8; ͑c͒ Ϫ3.8; ͑d͒ Ϫ2.6; ͑e͒ Ϫ0.8; ͑f͒ 1.2; ͑g͒ 2.1; ͑h͒ 3.9; ͑i͒
4.9; ͑j͒ 6.5; ͑k͒ 9.5; ͑l͒ 12.7 ns.
This leads to the explanation that the dissociation channel of
the C–O bond occurs from vibrationally excited levels of the
S1 state for p-methoxybenzyl alcohol.
The analysis of the fluorescence time profiles assumes
that one dissociation channel with one growth rate of the p
methoxybenzyl radical exists for each dissociation process of
p-methoxytoluene and p-methoxybenzyl alcohol. Nonethe-
less, the observed and calculated time profiles agree to a
great extent, when the fluctuations of the 266 and 308 nm
pulse fluences are taken into account. This means that the
major dissociation channel is identified, but this does not
preclude a possibility that minor dissociation channels exist.
the fluorescence itself. After a 17.8 ns delay of the 308 nm
pulse, the fluorescence intensity does not change but the
fluorescence delays following the 308 nm pulse. To simulate
the fluorescence time profiles in Fig. 3, the growth rate of
1.5ϫ108 sϪ1 for the p-methoxybenzyl radical can be satis-
factorily used, which is equal to the decay rate for the fluo-
rescence of p-methoxytoluene. This shows that the dissocia-
tion channel of the C–H bond occurs from thermally
equilibrated levels of the S1 state for p-methoxytoluene.
The fluorescence time profiles of the p-methoxybenzyl
radical at various delays of the 308 nm pulse are shown in
Fig. 4 for dissociation of p-methoxybenzyl alcohol. The
fluorescence intensity begins to grow at a Ϫ6.8 ns delay of
the 308 nm pulse ͓Fig. 4͑b͔͒, and the intensity growth is
completed at a 4.9 ns delay of this pulse ͓Fig. 4͑i͔͒. The
fluorescence does not delay during its intensity growth. After
a 4.9 ns delay of the 308 nm pulse, the fluorescence delays
following the 308 nm pulse without change of its intensity.
Simulation with the growth rate of 1.8ϫ108 sϪ1 for the
p-methoxybenzyl radical, the value equal to the decay rate
for the fluorescence of p-methoxybenzyl alcohol, does not
reproduce the experimental results. Instead, the growth rate
of Ͼ1.0ϫ109 sϪ1 is found to be necessary to simulate the
fluorescence time profiles in Fig. 4. It is clear that the growth
rate for the p-methoxybenzyl radical is much faster than the
decay rate for the fluorescence of p-methoxybenzyl alcohol.
D. One- or two-photon photochemistry
The number of photons required for excitation of
p-methoxytoluene and p-methoxybenzyl alcohol to their dis-
sociative states is determined from the dependence of the
fluorescence intensity of the p-methoxybenzyl radical on the
fluences of the 266 nm ͑photolysis͒ and 308 nm ͑probe͒
pulses.
The logarithmic plots of the fluorescence intensity of the
p-methoxybenzyl radical versus the 266 nm pulse fluence are
shown in Figs. 5͑a͒ and 5͑c͒ for dissociation of p-methoxy-
toluene and p-methoxybenzyl alcohol. Observed points are
linearly fitted with slopes of 1.08Ϯ0.24 for dissociation of
p-methoxytoluene and 0.94Ϯ0.23 for dissociation of
p-methoxybenzyl alcohol. It is evident that the dissociation
occurs by 266 nm one-photon excitation.
The p-methoxybenzyl fluorescence intensity is plotted as
a function of the 308 nm pulse fluence in Figs. 5͑b͒ and 5͑d͒
for dissociation of p-methoxytoluene and p-methoxybenzyl
alcohol. Linear fitting results in slopes of 1.06Ϯ0.21 for dis-
J. Chem. Phys., Vol. 107, No. 22, 8 December 1997
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9358
M. Fujiwara and K. Toyomi: Photodissociation of p-methoxytoluene
(37 600 cmϪ1) populates the S1 states of p-methoxytoluene
(␯0ϭ35 200 cmϪ1
)
and p-methoxybenzyl alcohol (␯
0
ϭ35 700 cmϪ1) with excess vibrational energies. Clearly,
the energies that the molecules possess are sufficient to frag-
ment them into the radical.
B. Adiabatic potential-energy surfaces
For p-methoxytoluene, a channel leading to dissociation
is shown to occur with the yield of ϳ1.3ϫ10Ϫ3 from ther-
mally equilibrated levels of the S1 state ͑Fig. 3͒. Vibrational
relaxation is a dominant energy dissipation mechanism from
the Franck–Condon state, and dissociation is allowed to oc-
cur after vibrational relaxation. For p-methoxybenzyl alco-
hol, a channel leading to dissociation is found to occur with
the yield of ϳ2.9ϫ10Ϫ3 from vibrationally excited levels of
the S1 state ͑Fig. 4͒. An energy relaxation mechanism from
vibrationally excited levels of the S1 state consists of two
channels: One that crosses to a potential-energy surface lead-
ing to dissociation, and a second that relaxes vibrationally to
thermally equilibrated levels of the S1 state.
FIG. 6. Absorption spectra of ͑a͒ p-methoxytoluene and ͑b͒ p-methoxy-
benzyl alcohol.
The difference of the dissociation processes between
p-methoxytoluene and p-methoxybenzyl alcohol may be un-
derstood by considering the symmetry of the potential-
energy surfaces along fission of the C–H and C–O
bonds.13–17 The most favorable geometry of stretch of the
C–H and C–O bonds that leads to the planar structure of the
benzyl radical is a one in which it occurs in a plane perpen-
dicular to the benzene rings of toluene and benzyl alcohol
͑see Figs. 7 and 8͒. Since the energy of this geometry is
comparable to the energies of other distorted geometries in
which the C–H and C–O bonds are stretched, the assump-
tion is made that the reaction coordinates of fission of the
C–H and C–O bonds lie in the symmetry plane perpendicu-
lar to the benzene rings of p-methoxytoluene and
p-methoxybenzyl alcohol.
sociation of p-methoxytoluene and 0.97Ϯ0.06 for dissocia-
tion of p-methoxybenzyl alcohol. It is shown that one 308
nm photon is required to populate the fluorescent state of the
radical. A possibility is excluded that two-photon absorption
occurs in the duration of the 308 nm pulse for formation of
the radical and population of its fluorescent state.
IV. DISCUSSION
A. Excitation wavelength and bond dissociation
energies
In Fig.
6 are shown the absorption spectra of
p-methoxytoluene and p-methoxybenzyl alcohol. Weak
bands appear with maxima at 36 000 cmϪ1͑2.0ϫ103 dm3
molϪ1 cmϪ1͒ for p-methoxytoluene and 36 500 cmϪ1͑1.2
ϫ103 dm3 molϪ1 cmϪ1͒ for p-methoxybenzyl alcohol, and
correspond to the transitions to the S1(1B2u) state in benzene
For p-methoxytoluene, photoexcitation promotes the
1
*
molecule to the S1͓ B2u ,␲␲ (benzene)͔ state which is of
A symmetry relative to the symmetry plane perpendicular to
Љ
the benzene ring. The ground (2B2) and excited (2A2) states
which are allowed due to vibronic coupling with the
of the p-methoxybenzyl radical possess A and A symme-
Ј Љ
vibration in the D6h symmetry.11 The 0→0
tries, respectively. In the conventional classification of the
C2v point group to which the benzyl radical belongs,18,19 the
plane of the benzene ring of the p-methoxybenzyl radical is
represented as the xz plane and that plane perpendicular to
the benzene ring which is the symmetry plane for the present
model is represented as the yz plane with the primary sym-
metry axis being the z axis. The 2B2 and 2A2 state labels are
the symmetry representations for the C2v point group, while
the A and A labels are the symmetry representations with
Ϫ1
520 cm
e
2g
transitions are seen at 35 200 cmϪ1 for p-methoxytoluene
and 35 700 cmϪ1 for p-methoxybenzyl alcohol. The 0→n
Ϫ1
transitions (nϭ1,2,3) in the 920 cm
a
vibration are also
1g
seen for p-methoxytoluene and p-methoxybenzyl alcohol.
By absorption of one 266 nm photon (37 600 cmϪ1),
p-methoxytoluene and p-methoxybenzyl alcohol are excited
to the vibrational levels of S1 states that are above the vibra-
tional ground levels by 2400 and 1900 cmϪ1, respectively.
The dissociation of the C–H bond of p-methoxytoluene
and the C–O bond of p-methoxybenzyl alcohol is shown to
occur by one-photon excitation at 266 nm ͓Figs. 5͑a͒ and
5͑c͔͒. The bond dissociation energies are estimated to be
30 800 cmϪ1 for CH3OC6H4CH2–H and 28 500 cmϪ1 for
CH3OC6H4CH2–OH from those for C6H5CH2–H
(30 800 cmϪ1), CH3CH2–H (35 000 cmϪ1), and CH3CH2–
OH (32 700 cmϪ1).12 Excitation with one 266 nm photon
Ј
Љ
respect to the symmetry plane perpendicular to the benzene
ring. The symmetry of the ground-state (2S) H atom is A .
Ј
Thus, the S (A ) potential-energy surface of the precursor
Љ
1
does not correlate adiabatically to the ground (2B ϩ2S,A )
Ј
2
state of the products, but does correlate to the excited (2A2
ϩ2S,A ) state of the products. If the S state were to par-
Љ
1
ticipate in C–H bond fission, a symmetry-imposed energy
barrier must be overcome. As a result, C–H bond fis-
J. Chem. Phys., Vol. 107, No. 22, 8 December 1997
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9359
M. Fujiwara and K. Toyomi: Photodissociation of p-methoxytoluene
FIG. 8.
A state correlation diagram for C–O bond fission of
FIG. 7.
A state correlation diagram for C–H bond fission of
p-methoxybenzyl alcohol. States of p-methoxybenzyl alcohol and
a
p-methoxytoluene. States of p-methoxytoluene and a p-methoxybenzyl
radical are represented with corresponding ones of benzene (D6h) and a
benzyl radical (C2v), respectively. Symmetry representations relative to a
symmetry plane are noted for singlet ͑solid lines͒ and triplet ͑broken lines͒
states in parentheses. A benzene ring is perpendicular to a symmetry plane,
on which a C–H bond lies. RϭOCH3.
p-methoxybenzyl radical are represented with corresponding ones of ben-
zene (D6h) and a benzyl radical (C2v), respectively. Symmetry representa-
tions relative to a symmetry plane are noted for singlet ͑solid lines͒ and
triplet ͑broken lines͒ states in parentheses. A benzene ring is perpendicular
to a symmetry plane, on which C–O and O–H bonds lie. RϭOCH3.
*
tween the ␲␲ (benzene)(A ) electronic configuration and
Љ
sion cannot occur adiabatically from the S1 surface. It is
*
allowed to proceed via intersystem crossing to vibrationally
the np͑O͒␴ ͑C–O͒(A ) repulsive electronic configuration at
Љ
excited levels of the T1(3B1u) state ͑of A symmetry͒. Al-
the geometry of the stretched C–O bond. It results in the A
Ј
Љ
ternatively, it may proceed via internal conversion to vibra-
potential-energy surface which evolves adiabatically from
*
␲␲ (benzene) character in the Franck–Condon region to
tionally excited levels of the S state ͑of A symmetry͒. In-
Ј
0
*
tersystem crossing or internal conversion cannot compete
effectively with vibrational relaxation in the S1 surface, so
C–H bond cleavage follows vibrational relaxation. This ac-
counts for the experimental observable that the dissociation
rate is determined by the decay rate of thermally equilibrated
levels of the S1 state. In addition, since dissociation occurs
actually from vibrationally excited levels of the T1 or S0
state, the low dissociation yield of ϳ1.3ϫ10Ϫ3 results from
competition with vibrational relaxation of which the rate is
expected to be ϳ1011 sϪ1 in the T1 or S0 state.
np͑O͒␴ ͑C–O͒ character beyond the barrier along the reac-
tion coordinate of C–O bond fission. Rapid C–O bond fis-
sion can proceed directly on the resulting A adiabatic
Љ
potential-energy surface from the S1(1B2u) state of the pre-
cursor to the ground (2B2ϩ2⌸) state of the products. In this
way, the presence of an out-of-plane p orbital on the O atom
allows an adiabatic reaction pathway consistent with rapid
C–O bond fission. In addition, since there exists an exit
channel barrier on the A surface, statistical partitioning of
Љ
excess vibrational energy is required for crossing over the
barrier to C–O bond fission. This predicts that dissociation
proceeds from vibrationally excited levels of the S1 state
after intramolecular vibrational redistribution. The low dis-
sociation yield of ϳ2.9ϫ10Ϫ3 is accounted for by competi-
tion with vibrational relaxation whose rate is expected to be
ϳ1011 sϪ1 in the S1 state.
In contrast, for dissociation of p-methoxybenzyl alcohol,
1
*
the S1 B2u ,␲␲ (benzene) state of the precursor possesses
͓
͔
A symmetry relative to the symmetry plane perpendicular to
Љ
*
Љ
the benzene ring. The dissociative np͑O͒␴ ͑C–O͒ state of
͓
͔
alkyl alcohol (␯0ϳ55 000 cmϪ1) is of A symmetry with
respect to this symmetry plane when the O–H bond lies in
this symmetry plane. The symmetry of the ground-state (2⌸)
The present model requires that the H atom of
p-methoxytoluene and the O–H bond of p-methoxybenzyl
OH radical is A ϩA . Thus, avoided crossing occurs be-
Ј
Љ
J. Chem. Phys., Vol. 107, No. 22, 8 December 1997
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9360
M. Fujiwara and K. Toyomi: Photodissociation of p-methoxytoluene
alcohol lie in the symmetry plane perpendicular to the ben-
zene rings ͑Figs. 7 and 8͒. The energy barriers to torsion of
the methyl group for the p-fluorotoluene are 4.8 cmϪ1 in the
S0 state and 33.7 cmϪ1 in the S1 state.20 The energy differ-
ence from the trans to the gauche conformation with respect
to the C–O bond for ethyl alcohol is 41.2Ϯ5.0 cmϪ1 in the
S0 state.21 Therefore, the H atom of p-methoxytoluene and
the O–H bond of p-methoxybenzyl alcohol will be restricted
in the symmetry plane perpendicular to the benzene rings
with high probabilities, though the explanation does not con-
sider hyperconjugation involving the benzene rings.
the latter for dissociation of p-methoxybenzyl alcohol. The
formation of the radical depends linearly on the photolysis
pulse fluence for the dissociation of p-methoxytoluene and
p-methoxybenzyl alcohol. The observation shows existence
of two dissociation channels. p-Methoxytoluene dissociates
from thermally equilibrated levels of the S1 state after vibra-
tional relaxation, whereas p-methoxybenzyl alcohol dissoci-
ates from vibrationally excited levels of the S1 state in com-
petition with vibrational relaxation. The difference of these
channels is explained on a model of electronic coupling
between the precursor and product states. For p-methoxy-
toluene, the S1 state does not correlate adiabatically to the
ground state of the C–H bond fission products, so intersys-
tem crossing or internal conversion precedes dissociation.
For p-methoxybenzyl alcohol, the adiabatic potential energy
surface, evolving from the S1 state to the ground state of the
C–O bond fission products, allows rapid dissociation along
it.
C. Other dissociation mechanisms
The photodissociation pathway of p-methoxytoluene in
the liquid phase may be similar to that of toluene,1–4 benzyl
alcohol,5 and benzyl halides22 in the gas phase. In the liquid
phase, the fission of the C–H bond occurs ultimately from
vibrationally excited levels of the T1 or S0 state after inter-
system crossing or internal conversion. The actual dissocia-
tion rate from vibrationally excited levels of the T1 or S0
state is not measured. It is limited by the decay rate of ther-
mally equilibrated levels of the S1 state. The dissociation is a
minor channel ͑ϳ10Ϫ3 yield͒, since the rapid vibrational re-
1 N. Ikeda, N. Nakashima, and K. Yoshihara, J. Chem. Phys. 82, 5285
͑1985͒.
2 K. Tsukiyama and R. Bersohn, J. Chem. Phys. 86, 745 ͑1987͒.
3 Y. Kajii, K. Obi, I. Tanaka, N. Ikeda, N. Nakashima, and K. Yoshihara, J.
Chem. Phys. 86, 6115 ͑1987͒.
laxation process (ϳ1011 Ϫ1) exists. In the gas phase, the
s
4 U. Brand, H. Hippler, L. Lindemann, and J. Troe, J. Phys. Chem. 94, 6305
͑1990͒.
fission of the C–H, C–O, and C–X bonds ͑XvCl, Br, I͒
occurs from highly vibrationally excited levels of the S0
states by internal conversion after excitation to the S3 states
at 193 nm. The measured dissociation rates (ϳ106 sϪ1) are
predicted by the statistical unimolecular dissociation theory
on the assumption that the electronic energies in the photo-
excited states are distributed over the vibrational modes. The
dissociation is a dominant energy dissipation process ͑0.75
yield͒ from the excited state for toluene.
The photodissociation mechanism of p-methoxybenzyl
alcohol in the liquid phase seems to be different from that of
1- and 2-͑halomethyl͒naphthalenes in the liquid phase.23,24
For p-methoxybenzyl alcohol, the fission of the C–O bond is
5 P. K. Chowdhury, J. Phys. Chem. 98, 13112 ͑1994͒.
6 K. Tokumura, T. Ozaki, M. Udagawa, and M. Itoh, J. Phys. Chem. 93,
161 ͑1989͒.
7 N. Nakashima, M. Sumitani, I. Ohmine, and K. Yoshihara, J. Chem. Phys.
72, 2226 ͑1980͒.
8 K. Tokumura, T. Ozaki, H. Nosaka, Y. Saigusa, and M. Itoh, J. Am.
Chem. Soc. 113, 4974 ͑1991͒.
9 R. F. C. Claridge and H. Fischer, J. Phys. Chem. 87, 1960 ͑1983͒.
10 I. Carmichael and G. L. Hug, in CRC Handbook of Organic Photochem-
istry, edited by J. C. Scaiano ͑CRC, Boca Raton, Florida, 1989͒, Vol. 1,
Chap. 16.
11 J. Petruska, J. Chem. Phys. 34, 1120 ͑1961͒.
12 D. Griller and J. M. Kanabus-Kaminska, in CRC Handbook of Organic
Photochemistry, edited by J. C. Scaiano ͑CRC, Boca Raton, Florida,
1989͒, Vol. 2, Chap. 17.
13 L. Salem, J. Am. Chem. Soc. 96, 3486 ͑1974͒.
14 N. J. Turro, W. E. Farneth, and A. Devaquet, J. Am. Chem. Soc. 98, 7425
͑1976͒.
*
characterized by adiabatic crossing to the np͑O͒␴ ͑C–O͒
potential-energy surface. This is because the lowest excited
15 S. S. Hunnicutt, L. D. Waits, and J. A. Guest, J. Phys. Chem. 95, 562
͑1991͒.
*
state of alkyl alcohol is the np͑O͒␴ ͑C–O͒ one, and because
*
the np͑O͒→␴ ͑C–O͒ transition is of energy of
16 M. D. Person, P. W. Kash, S. A. Schofield, and L. J. Butler, J. Chem.
Phys. 95, 3843 ͑1991͒.
ϳ55 000 cmϪ1. For 1- and 2-͑halomethyl͒naphthalenes, the
fission of the C–X bonds ͑XvCl, Br͒, occurring after exci-
tation to the S2 states at 266 or 299 nm, is distinguished by
intersystem crossing to upper triplet states which are them-
17 M. D. Person, P. W. Kash, and L. J. Butler, J. Chem. Phys. 97, 355
͑1992͒.
18 J. E. Rice, N. C. Handy, and P. J. Knowles, J. Chem. Soc. Faraday Trans.
II 83, 1643 ͑1987͒.
*
selves or cross to dissociative ␴␴ triplet states.
19 F. Negri, G. Orlandi, F. Zerbetto, and M. Z. Zgierski, J. Chem. Phys. 93,
600 ͑1990͒.
20 K. Okuyama, N. Mikami, and M. Ito, J. Phys. Chem. 89, 5617 ͑1985͒.
21 P. K. Kakar and C. R. Quade, J. Chem. Phys. 72, 4300 ͑1980͒.
22 A. Freedman, S. C. Yang, M. Kawasaki, and R. Bersohn, J. Chem. Phys.
72, 1028 ͑1980͒.
V. CONCLUSIONS
Excitation of p-methoxytoluene and p-methoxybenzyl
alcohol at 266 nm produces a p-methoxybenzyl radical in
n-heptane solution. The growth rate of the radical is equal to
the decay rate of the precursor fluorescence for dissociation
of p-methoxytoluene, whereas the former is much faster than
23 D. F. Kelley, S. V. Milton, D. Huppert, and P. M. Rentzepis, J. Phys.
Chem. 87, 1842 ͑1983͒.
24 E. F. Hilinski, D. Huppert, D. F. Kelley, S. V. Milton, and P. M. Rentz-
epis, J. Am. Chem. Soc. 106, 1951 ͑1984͒.
J. Chem. Phys., Vol. 107, No. 22, 8 December 1997
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