Table 3 Energy of HOMO and LUMO orbitals in ground and singlet
3365-II. Mass spectra (electron impact, 70 eV) and recon-
excited states
structed total ion chromatograms were obtained by automatic
scanning in the mass range m/z 30–350 at 2.2 scans sϪ1.
E/eV
TBBS
CBS
MBT (thiol)
MBT (thione)
Isotachophoresis (ITP) analyses of the samples were per-
formed on an analyzer ZKI-02 (LABECO Co., Spisˇska Nová
Ves, Slovakia) using a joint-column technique. The diameter of
the pre-separation column was 0.8 mm, and 0.3 mm for the
analytical column. Both columns were equipped with a con-
ductivity detector. For chloride ion determination the electro-
lyte system used was: solvent, H2O; leading anion, 10 mmol LϪ1
nitrate; counter ion, Cd2ϩ β-alanin; additive, 0.1% w/v methyl
hydroxyethyl cellulose (MHEC), pH 3.7; terminating anion,
10 mmol LϪ1 tartrate, pH 7. Cl2 and HClO were reduced in
every sample with potassium iodide prior to ITP analysis.
HOMOS0
LUMOS0
HOMOS1
LUMOS1
Ϫ7.10
1.75
Ϫ7.14
1.78
Ϫ7.12
1.73
Ϫ7.16
1.75
Ϫ8.73
1.59
Ϫ8.94
1.62
Ϫ8.14
1.44
Ϫ8.21
1.49
calculations of the frontier molecular orbitals (Table 3). Thus,
MBT forms too unstable an exciplex, but it can be decomposed
directly by photolysis. The reaction intermediates formed in
this way could produce both Cl ions as well as CCl3 radicals.
Carbon trichloride radicals could abstract the hydrogen atom
from MBT, forming chloroform and lowering the amount of
C2Cl6.
Ϫ
ؒ
Experimental set-up
All the experiments were carried out with the set-up described
previously,15 consisting of a photochemical reactor equipped
with a glass (SIMAX) insertion cell and an ultrasound emitter
(25 kHz, 16 W cmϪ2). The UV light source was a mercury
125 W lamp from Tesla Holesˇovice, Czech Republic. Photo-
and sono-photolyses were performed with 100 mL of 2 × 10Ϫ2
M solutions of the appropriate electron donor in CCl4. Water
(50 mL) was added to the reaction cell (upper layer) to trap
the evolved hydrogen chloride. The samples were thoroughly
de-aerated by continuous bubbling of nitrogen through the
solution before and during the irradiations. Progress of the
reaction was monitored by withdrawal of aliquots from
the water or CCl4 layers in the cell, which were analyzed by
ITP (ClϪ and carboxylic acids) or UV–VIS (donors). The time
dependence of C2Cl6 formation was examined discontinuously:
for each reaction time an experiment was performed with a
fresh solution of the electron donor in carbon tetrachloride;
immediately after the irradiation was stopped, the mixture was
concentrated and analyzed for the content of C2Cl6 in carbon
tetrachloride and ClϪ in water. All the experiments, with and
without sonication, were performed under identical conditions.
The observed negative ultrasound effect could be explained
also assuming that ultrasound can increase the rate of return
of the excited states to the fundamental state by population
of vibrational states induced by ultrasound. This could also
explain the observed greater effect of ultrasound in the experi-
ments using TBBS and CBS as the electron donors because they
contain more flexible groups than MBT. Unfortunately we do
not possess the necessary instruments to prove or disprove this
assumption. The change of flexible SNR group for a more rigid
group like phenyl would not give a clear answer to this question
because the electronic effect of the substituent would be also
very different.
In conclusion, sonication of a photolytic reaction implying
unstable transient species such as excimers or exciplexes or ion
pairs can result in a loss of efficiency, or even in the complete
absence of any effect. Beyond the theoretical interest of such a
conclusion, the practical consequences can be of some impor-
tance. Photolysis, with or without a catalyst, has been studied
by several groups mentioned in the Introduction in attempts
to decompose pollutants resistant to conventional cleaning
methods. Basic research is obviously performed on simple
systems containing, most of the time, only one component.
Real effluents are more complex, and can contain substances
liable to depress the cleavage efficiency of the actual targets. In
further studies environmental chemists should exercise care in
their choice of a model system, otherwise they risk finding that
the proposed applications remain unfruitful.
Theoretical computation
The geometries of the studied molecules were optimized using
the AM1 method. Energies of molecular orbitals were com-
puted using Gaussian 98 ab initio HF/6-31ϩG** for ground
states and CI/6-31ϩG** for excited states.
Experimental
Acknowledgements
Materials
This work has been carried out under the auspices of the
European COST Chemistry Network, Project D10/0008. The
authors wish to thank the Ministry of Education of the Slovak
Republic for the financial support, and DUSLO Co. (Slovakia)
for a generous gift of benzothiazole compounds. We would like
to express our gratitude also to the referees of this paper for
their interesting comments.
TBBS, CBS, and MBT were re-crystallized from ethanol or
acetonitrile. Carbon tetrachloride (Lichrosolv grade, Merck
Co.) was used without further purification. Redistilled water
was used throughout this study.
Analytical methods
UV–VIS spectra of the samples were measured at room
temperature in CCl4 solution using a Diode Array Spectro-
photometer HP 8452 A. HRGC analyses of the samples were
performed on a Hewlett Packard (Palo Alto, CA, USA) 5890a
Series II gas chromatograph equipped with split-splitless
injector (300 ЊC, splitting ratio 1 : 30) and flame ionization
detector operated at 300 ЊC. The columns used were CP-SIL 5
CB (Chrompack) cross-linked fused-silica capillary columns
(10 m × 0.32 mm id) coated with 0.25 µm-thick polydimethyl-
siloxane. The oven temperature was programmed from 40 to
320 ЊC at 20 ЊC minϪ1. Helium (Tatragas, 99.995%) was used as
the carrier (inlet pressure 50 kPa). Air and hydrogen flow rates
were 300 and 30 mL minϪ1, respectively. The injection volume
was 0.5 µL, with n-tetradecane or p-cresol as internal standards.
The chromatograms were processed with an HP Chemstation
References
1 N. J. Bunce, in Handbook of Organic Photochemistry, ed. W. M.
Horspool, CRC Press, Boca Raton, Fl, 1995, pp. 1142–1228.
2 N. J. Bunce, J. Org. Chem., 1982, 47, 1948–1955.
3 N. J. Bunce and J. C. Gallacher, J. Org. Chem., 1982, 47, 1955–1958.
4 R. S. Davidson, J. W. Goodin and J. E. Pratt, Tetrahedron Lett.,
1982, 23, 225–228.
5 P. K. Freeman and N. Ramnath, J. Org. Chem., 1991, 56, 3646–3651;
P. K. Freeman, G. E. Clapp and B. K. Stevenson, Tetrahedron
Lett., 1991, 32, 5705–5708; D. Reyman, A. Pardo, J. M. L. Poyato
and J. G. Rodriguez, J. Photochem. Photobiol., A, 1996, 98, 39–44.
6 H. Shimamori and T. Okuda, J. Phys. Chem., 1994, 98, 2576–2581.
7 A. Gáplovský, J. Donovalova, P. Hrncˇiar and P. Hrdlovicˇ,
J. Photochem. Photobiol., A, 1989, 49, 339–346.
8 M. Nowakowska and K. Szczubilaka, J. Photochem. Photobiol.,
A, 1995, 91, 81–85.
J. Chem. Soc., Perkin Trans. 2, 2002, 652–656
655