High resolution electronic absorption spectra of aniline, anilino and silicon dioxide.

Article abstract of DOI:10.1016/S1386-1425(03)00061-1

High resolution electronic absorption spectra of aniline and anilino free radical have been recorded in the vapor phase at room temperature by flash photolysis technique, and subsequent reactions have been investigated by kinetic spectroscopy. It was poss

Full text of DOI:10.1016/S1386-1425(03)00061-1

Spectrochimica Acta Part A 59 (2003) 2785Á2789
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www.elsevier.com/locate/saa
High resolution electronic absorption spectra of aniline,
anilino and silicon dioxide
a,
Fuat Bayrakceken *, Ipek S. Karaaslan
b
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a Department of Electronics Engineering, Yeditepe University, Kayisdagˇi, Istanbul 81120, Turkey
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b Department of Physics, Yeditepe University, Kayisdagˇi, Istanbul 81120, Turkey
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Received 7 August 2002; received in revised form 24 January 2003; accepted 29 January 2003
Abstract
High resolution electronic absorption spectra of aniline and anilino free radical have been recorded in the vapor
phase at room temperature by flash photolysis technique, and subsequent reactions have been investigated by kinetic
spectroscopy. It was possible to follow the kinetics of the anilino radical’s decay which occurred predominantly by
bimolecular recombination. Decay parameters of anilino free radical were measured from the absorption bands.
# 2003 Elsevier Science B.V. All rights reserved.
Keywords: Aniline; Anilino; Flash photolysis; Silicon dioxide; Bimolecular decay
1. Introduction
although practically most of the energy is lost by a
photophysical process, a definite action of exciting
energy results in neutral radical or ion-radical
formation. The photolytic method is often a good
way of producing specific aromatic radicals as one
can irradiate with light of energy capable of
breaking only the required bond. The lifetime of
most aromatic radicals is of the order of milli-
seconds in solution and microseconds in the gas or
vapor phase, and high speed time resolution
studies have to be made for them. Aromatic
radicals are of growing significance in environ-
mental chemistry due to their toxic nature and
their suspected carcinogenic properties. Their use
in the manufacturing of dyes, cosmetics, medicines
and rubber increases the possibility of release into
the environment with the risk being exacerbated
further by the fact that some aromatic amines can
Radicals can be prepared by either oxidation or
reduction of aromatic compounds. Such transfor-
mations may be carried out by a variety of
methods: Photolysis, flash photolysis, radiolysis,
chemical oxidation or reduction, pyrolysis, elec-
trolysis, and electrical discharge. The first three
methods are the most commonly used. Absorption
of light by an aromatic molecule results in the
formation of an unstable excited state. This excited
state becomes deactivated by either a photophysi-
cal process or a photochemical process, dissocia-
tion into free radicals or ionization. In most cases,
* Corresponding author. Tel.: ꢀ
216-578-0400.
E-mail address: bayrakf@yeditepe.edu.tr (F. Bayrakceken).
/
90-216-578-0433; fax:ꢀ90-
/
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1386-1425/03/$ - see front matter # 2003 Elsevier Science B.V. All rights reserved.
doi:10.1016/S1386-1425(03)00061-1

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be released from the degradation of pesticides.
Therefore there is a clear need for a suitable
sensing strategy to be designed. Aniline and its
derivatives have been widely used for a variety of
applications from photochemistry to medicine as
antibacterial, anticancer, herbicidal, and fungici-
dal agents.
Microwave studies of the gas phase and X-ray
diffraction of the solid state have shown the
nitrogen environment in aniline to be non-planar.
In fact, the nitrogen is out of the plane of the
phenyl ring and the nitrogen itself is pyramidal.
The structure has been difficult to reproduce
theoretically, but the energy barriers to internal
motional processes are in close agreement with the
experimentally measured values. The rotation and
inversion barriers for aniline have been measured
experimentally to be 23 and 6 kJ/mol, respectively
[13,14].
The radicals produced being trapped in inert
rigid media and thus prevented from reacting. The
matrices most commonly used were mixtures of
ethanol, isopentane, and ether (EPA), and methyl
cyclohexane and isopentane (MP). By flash photo-
lytic method momentarily high concentrations of
radicals were produced photo-chemically with a
short-duration, high energy light flash. A quite
simple way of differentiating between radical and
triplet state absorptions occuring in flash photo-
lysis and laser flash photolysis is to compare the
spectra observed in the presence and absence of
air. Since the rate of quenching triplet states by
paramagnetic oxygen is so fast, therefore no triplet
state absorption is seen in the presence of air,
where as both radical and triplet state absorptions
are seen in the absence of air. This method
depends on the fact that in general, aromatic
radicals react rather slowly with oxygen but it is
only suitable when the presence of air does not
interfere with photolytic formation of triplet states
and radicals. The usual small shifts in going from
2. Experimental
2.1. Flash photolysis apparatus
This employed two air or argon filled photolysis
lamps in series, with a flash duration of 2 ms, at a
discharge energy of 845 J. Spectra were recorded
on a Hilger medium quartz spectrograph, slitwidth
0.025 mm. The Ilford, HP-3 plates used were
developed in Ilford PQ universal developer.
2.2. Plate photometry
gasÁ
/
liquidÁsolid phases occur for aromatic radi-
/
cals, for example, the wavelength maximum of the
anilino free radical in the vapor phase, liquid
paraffin and methyl-pentane glass (77 K) are:
300.8, 300.7 and 308.8 nm, respectively. Our
apparatus has a high light output with short flash
duration time which enabled kinetic as well as
spectroscopic investigations to be carried out for
the short lived, (i.e. microseconds), free radicals.
The radicals are decaying rapidly by dimerisation,
disproportionation or reaction with solvent. In
such cases, special steps have to be taken in order
The spectra were photometered on a JoyceÁ
/
Loeble double beam recording microdensitometer
model MKIIB. Calibrated optical densities on the
photographic plate were obtained with a seven
step filter (Hilger, F-1273). Pressure measure-
ments: low pressures of parent molecules (0.5
mTorrÁ3 Torr) were flashed after mixing with
/
excess of oxygen free nitrogen, (at pressures up to
700 Torr), to maintain isothermal conditions.
Pressures were measured with a McLoad gauge
(up to 0.1 Torr), and above 50 Torr, with a
mercury manometer. Face to face collision of
two anilino radicals produce dianilino molecule
which is in the steady state.
The lifetime of the anilino free radical was
measured from the decay of the radical at 300.8
nm, which is the maximum location of the
absorption band of the radical.
to observe their absorptions [1Á10]. Phenolic
/
compounds can also pose serious problems in
environmental solutions and are major interferents
in the direct electrochemical detection of aniline.
This is primarily due to the formation of polimeric
species on the electrode surface upon oxidation of
the phenolic compound [11,12].

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Fig. 1. Flash photolysis of aniline, (7.9ꢁ
radical and its decay in the vapor phase. Photolysis energyꢃ
/
10ꢂ2 Torr) and N2 (500 Torr), showing the absorption spectrum of aniline, anilino free
720 J.
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Fig. 2. (a) Background substracted absorption spectrum of the anilino radical in the vapor phase; (b) microdensitometer trace of the
anilino radical in the vapor phase; (c) microdensitometer background trace.

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3. Results and discussion
In this paper, the high resolution electronic
absorption spectra of aniline, anilino radical and
its decay by time is recorded photographically in
the vapor phase at room temperature. The win-
dows of the reaction cell were spectroscopically
pure silicon dioxide therefore high resolution
absorption spectrum of silicon dioxide is also
recorded in the ultraviolet region. The flash
photolysis of aniline vapor at 25 8C and at 0.79
mm mercury pressure in the presence of oxygen
free nitrogen 500 mm mercury resulted in the
appearance of a strong band at 300.8 nm and two
weak bands at 303 and 305 nm. Weak bands were
rather diffuse and their relative intensities was
lower than the main band at 300.8 nm. Fig. 1
shows the high resolution absorption spectra of
aniline, anilino free radical in the vapor phase at
room temperature and high resolution absorption
spectra of silicon dioxide in the solid state. Fig. 1
illustrates the complete decay of anilino free
radical. The first strip written ‘before’ was re-
corded by using only the spectral flash alone.
Before these were recorded by using photolysis
flash and spectral flash with delay times from 2 to
200 ms, immediately after the photolysis flash
within 1 ms experimental error. The calibration of
the spectrograph was checked with a mercury light
source. Second order decay was observed for
anilino radical and the rate constant was found
Fig. 3. (a) Decay of the anilino radical at 300.8 nm; (b) second
order plot for the decay of the anilino radical at 300.8 nm.
to do any investigation at wavelengths shorter
than 210 nm. Therefore the absorption spectrum
of the radical searched in the 210Á780 nm region.
/
The lifetime of the anilino free radical is about 100
ms. The primary photochemical process (side chain
fission) is as follows:
to be 81 000ꢁ
/
1/optical densityꢁ1/s. Fig. 2(a)
/
illustrates the background substracted absorption
spectrum of the anilino radical in the vapor phase;
(b) microdensitometer trace of the anilino radical;
(c) microdensitometer background trace (micro-
densitometer trace of the empty cell). Fig. 3(a)
illustrates the decay of the anilino radical at 300.8
nm; and (b) second order plot for the decay of the
anilino radical at 300.8 nm. The appearance and
decay of the transient at 300.8 nm were similar to
that in the benzyl radical at 305.3 nm [15].
The transitions are due to the excitation of
valence electrons therefore there is no likelihood of
another band occuring at wavelengths longer than
780 nm, but there is always a possibility that other
absorptions may occur in the vacuum ultraviolet.
Our flash photolysis apparatus does not permit us
C6H5NH2 ꢀUV-photons 0 C6H5NH+ ꢀH+
The rate constant for the recombination of
anilino has been presented in terms of its extinc-
tion coefficient at 300.8 nm in the vapor phase.
Unfortunately, the extinction coefficient was un-
known and end-product analysis was undertaken
by the method of gas chromatography in an
attempt to measure this quantity and the corre-
sponding rate constant in absolute units. The
aniline vapor in the reaction cell in different
pressures was flashed, (energy of the flash lamp
was 1125 J/pulse), a hundred times in the presence
of a large excess of oxygen free nitrogen. The
photoproduct was dissolved in an appropriate

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[3] H.E. Birkett, J.C. Cherryman, A.M. Chippendale, P.
Hazendonk, R.K. Harris, J. Mol. Struct. 602Á603 (2002)
59.
solvent and most of the solvent was then evapo-
rated in order to concentrate the photoproduct for
gas chromatographic measurements. No signifi-
cant change in the absorption was observed in the
aniline spectrum itself, therefore rate of the decay
/
[4] L.T. Bozic, Vibr. Spectrosc. 28 (2002) 235.
[5] A.N. Taha, N. True, J. Phys. Chem. A 104 (2000) 2985.
[6] N.G. Vasiilev, V.S. Dimitrov, J. Mol. Struct. 522 (2000)
37.
was given in terms of 1/optical densityꢁ1/s,
/
[7] G.L. Estiu, J. Mol. Struct. (Theochem.) 401 (1997) 157.
[8] R. Nakagaki, I. Aoyama, K. Chimizu, M. Akagi, J. Phys.
Org. Chem. 6 (1993) 261.
instead of absolute units. In view of these un-
certainities it would be most interesting to extend
these measurements by use of more sensitive gas
chromatographic equipment in order that the vital
product analysis should not be so close to the
limits of detectability.
[9] K. Sawada, T. Kanda, Y. Naganuma, T. Suzuki, J. Chem.
Soc. Dalton Trans. (1992) 2955.
[10] F. Bayrakceken, M. J. Pure Appl. Sci. 5 (2) (1972)
¸
177.
[11] J.C. Merriman, D.H.J. Anthony, J.A. Kraft, R.J. Wilk-
inson, Chemosphere 23 (1991) 1605.
[12] T. Berjerano, C. Forgacs, E. Gileadi, J. Electroanal. Chem.
27 (1970) 469.
References
[13] D.G. Lister, J.K. Tyler, J. Chem. Soc. Chem. Comm.
(1996) 152.
¨
[1] F. Bayrakceken, S. Aktas, M. Toptan, A. .Unlugedik,
¸
Spectrochim. Acta Part A 59 (2003) 135.
¸
¨
[14] M. Fukuyo, K. Hirotsu, T. Higuchi, Acta Crystallogr. B
38 (1982) 640.
[15] F. Bayrakceken, Chem. Phys. Lett. 74 (2) (1980) 298.
¸
[2] E.H. Seymour, N.S. Lawrence, E.L. Beckett, J. Davis,
Talanta 57 (2002) 233.