Resonance Raman investigation on the interaction of styrene and 4-methyl styrene oligomers on sulphated titanium oxide

Article abstract of DOI:10.1016/S1386-1425(99)00128-6

In order to understand the nature of the interaction that gives rise to the yellow-orange colour observed when styrene or 4-methyl styrene are put in contact with sulphated TiO₂, the resonance Raman spectra of such systems, including deuterated styrene (ring-deuterated d₅ and perdeuterated d₈) and allylbenzene were investigated. In all cases a substantial enhancement of the ring v(CC) stretching mode was observed. A charge transfer process involving a transition from the ring π-electrons to the empty d-π orbitals of titanium was ascribed responsible for the absorption in the visible. Two types of resonance Raman spectra were observed depending on the excitation wavelength, which can be explained by the presence of two kinds of oligomers, saturated and unsaturated, on the surface of the oxide with the former giving rise to a Raman enhancement at a higher excitation energy.

Full text of DOI:10.1016/S1386-1425(99)00128-6

Spectrochimica Acta Part A 56 (1999) 145–155
www.elsevier.nl/locate/saa
A resonance Raman investigation on the interaction of
styrene and 4-methyl styrene oligomers on sulphated
titanium oxide
L.K. Noda, O. Sala *
Instituto de Qu ´ı mica, Uni6ersidade de S a˜ o Paulo, Caixa Postal 26077, 05599 970 Sao Paulo, Brazil
Received 20 November 1998; accepted 24 May 1999
Abstract
In order to understand the nature of the interaction that gives rise to the yellow–orange colour observed when
styrene or 4-methyl styrene are put in contact with sulphated TiO , the resonance Raman spectra of such systems,
2
including deuterated styrene (ring-deuterated d and perdeuterated d ) and allylbenzene were investigated. In all cases
5
8
a substantial enhancement of the ring w(CC) stretching mode was observed. A charge transfer process involving a
transition from the ring p-electrons to the empty d-p orbitals of titanium was ascribed responsible for the absorption
in the visible. Two types of resonance Raman spectra were observed depending on the excitation wavelength, which
can be explained by the presence of two kinds of oligomers, saturated and unsaturated, on the surface of the oxide
with the former giving rise to a Raman enhancement at a higher excitation energy. © 2000 Elsevier Science B.V. All
rights reserved.
Keywords: Resonance Raman; Titanium oxide; Styrene; 4-Methyl styrene
1
. Introduction
proved to be very valuable in providing informa-
tion on the characterization of the catalyst [3,4].
Additionally, the straightforward observation of
low frequency vibrations of molecules on oxide
surfaces is an advantage of Raman spectroscopy
over infrared absorption since in the IR the low
frequency region is hidden by strong metal oxide
absorption. So, both techniques contribute to a
better understanding of catalytic processes. If
there is an enhancement mechanism for the Ra-
man signal, for instance by the resonance Raman
effect, the observation of spectra of molecules
adsorbed on surfaces is largely favoured [5].
Infrared spectroscopy has been a very useful
tool in the characterization of compounds formed
in catalytic reactions or adsorbed on the catalyst
surfaces [1,2]. On the other hand, Raman spec-
troscopy is not such a widespread technique for
this application, due to its low sensitivity, but has
*
Corresponding author. Tel.: +55-11-8183853; fax: +55-
1
1
1-8155579.
E-mail address: oswsala@quim.iq.usp.br (O. Sala)
386-1425/00/$ - see front matter © 2000 Elsevier Science B.V. All rights reserved.
PII: S1386-1425(99)00128-6

1
46
L.K. Noda, O. Sala / Spectrochimica Acta Part A 56 (2000) 145–155
Some sulphated metal oxides, such as Fe 0 ,
400°C, for 3 h. The temperature was then lowered
2
3
−
2
TiO and ZrO , are described as superacids [6–8],
to 100°C and the system evacuated at 10
for 10 min.
Torr
2
2
catalysing several reactions, for example the iso-
merization and alkylation of hydrocarbons even
at room temperature, while conventional catalysts
are active only at high temperatures. In particular,
After the sample was cooled to room tempera-
ture, in vacuum, styrene or 4-methyl styrene was
introduced with a syringe through a septum. After
the sample turned coloured all volatile materials
were vacuum eliminated and the tube sealed.
The synthesis of the ring-deuterated styrene
2
−
the polymerization of styrene by TiO –SO4 was
2
investigated by IR spectroscopy by Sheppard et
al. [9,10], who observed the formation of styrene
oligomers on the surface of the catalyst at room
temperature. An interesting aspect of that system
is the formation of a yellow to orange colour over
the titanium oxide surface that is not present in
the isolated oligomer. The authors claim that the
initial polymerization over the catalyst surface
leads to the formation of C H (R)CꢀTi* (R=
(styrene-d ) was performed in steps: (i) totally
5
deuterated benzene (Aldrich) was brominated us-
ing bromine and iron filings as catalyst. The ob-
tained product was distilled at 20 Torr and its
purity checked by gas chromatography; (ii) the
deuterated bromobenzene was then reacted with
magnesium ribbon in dried ethyl ether in a dry
atmosphere. After that a solution of acetaldehyde
(Riedel-de H a¨ en, purified by vacuum distillation)
in dry ethyl ether was added. The resulting mix-
ture was poured to crushed ice mixed with sul-
phuric acid. After solvent removal, the product
(ring-deuterated phenyl ethanol) was purified by
fractional distillation at 10 Torr; (iii) the obtained
compound was dehydrated using anhydrous
cupric sulphate as catalyst at 120°C [11]. The
ring-deuterated styrene was distilled at 34–36°C
at 20 Torr as soon as it was formed, directly from
6
5
CH or polymer chain). An extended conjugation
3
involving the phenyl ring and the CꢀTi moiety
would promote a decrease in the electronic transi-
tion energy and consequently lead to absorption
in the visible or near UV, thus explaining the
yellow to orange colour. If this is really the case
one should expect a Raman resonance enhance-
ment of the w(CꢀTi) vibrational mode associated
with the extended chromophore.
In the present paper the resonance Raman spec-
tra of styrene and 4-methyl styrene over sulphated
TiO are investigated aiming to identify the chro-
mophore responsible for the observed colour on
the catalyst surface.
Deuterated styrenes and allylbenzene were also
studied in order to facilitate the understanding of
the interaction of this class of compounds with
TiO2.
the mixture. Humidity was removed with 4 A
,
2
molecular sieves.
The Raman spectra of the compounds on TiO2
were obtained with a spinning tube, to minimize
the heating effect caused by the laser beam, in a
Jobin-Yvon U-1000 spectrometer provided with a
RCA C31034-A02 photomultiplier tube. The
−
1
spectral resolution was ca 6 cm , and the 514.5,
+
4
88.0, 476.5 and 457.9 nm lines of an Ar laser
+
2. Experimental
and 406.7 nm line of a Kr laser (Innova 90
Coherent Radiation) were employed to excite the
spectra. The laser power was kept at ca. 3 mW. A
Renishaw Raman System 3000 equipped with an
Olympus microscope (BTH2) and a CCD detector
was also employed, using the lines at 457.9, 514.5
The sulphate-doped anatase, obtained from ti-
tanyl sulphate hydrolysis, was kindly donated by
2
−1
Dr J.L Zotin, from Cenpes/Petrobras (180 m g
specific surface area).
Styrene and 4-methyl styrene (Aldrich) were
vacuum distilled after several freeze–pump–thaw
+
nm (Ar laser, Omnichrome), 632.8 nm (He–Ne
laser, Spectra Physics) and 782.0 nm (GaAs–Al-
doped, Renishaw laser). The high sensitivity of
this instrument allowed the spectra to be obtained
with low laser power (ca. 70 mW) without the
need of a spinning cell.
cycles. Styrene-d (Aldrich), 98% purity, was used
8
as received.
The sulphated TiO2 was activated by slowly
heating the sample in air, in a glass tube, up to

L.K. Noda, O. Sala / Spectrochimica Acta Part A 56 (2000) 145–155
147
The optical spectra, in the 400–700 nm range,
were obtained with a diffuse reflectance accessory
in the Bomem DA3 interferometric instrument
with the sample in the same glass tube used for
the Raman spectra.
For styrene the optical absorption spectra (Fig.
1) present a strong broad band with maximum at
ca. 515 nm and a wing at the blue side (orange
sample) and at ca. 450 nm (yellow sample) with a
shoulder at ca. 515 nm. For 4MeSt (Fig. 1) the
bands are red shifted in comparison to styrene,
with a peak at ca. 545 nm with a wing at the blue
side (red sample) and at ca. 470 and 530 nm
3. Results and discussion
(
yellow–orange sample).
The Raman spectra of styrene (monomer and
When styrene is introduced into the reaction
polymer) and 4MeSt (monomer, oligomer and
tube containing TiO , two coloured regions are
−1
2
polymer) in the 400–1700 cm region are shown
observed: one orange and the other yellow. In the
case of 4-methyl styrene (4MeSt), a reddish colour
and a yellow–orange colour are observed.
−1
in Fig. 2. The strong band at ca. 1630 cm
observed in the monomer spectra and assigned to
vinyl w(CꢀC), is not present in the spectra of the
polymers. For the oligomer (extracted from
−
1
4
MeSt/TiO ) a weak band at 1644 cm
is
2
observed.
3.1. Styrene on TiO2
The Raman spectra of coloured samples of
styrene on sulphated TiO2 (Fig. 3) excited at
32.8, 514.5 and 457.9 nm present striking
6
differences.
Excited at 632.8 nm the spectrum shows strong
−
1
bands from TiO (anatase) at 515 and 635 cm
2
as well as bands that resemble the styrene ones
with small but significant changes in the 1550–
−
1
1
cm
650 cm region. Strong bands at 1600 and 1585
−
1
−1
show up and the 1629 cm
band, styrene
−
1
vinyl w(CꢀC), shifts now to 1647 cm
(probably
due to the same vinyl w(CꢀC) vibration but from
an unsaturated styrene oligomer formed on the
TiO surface).
2
With excitation at 514.5 nm a marked feature is
the absence of the oxide bands clearly indicating
the resonance of the oligomer bands. The band at
−
1
1
585 cm
is noticeably enhanced relatively to
−
1
the band at 1000 cm , while the bands at 1600
and 1647 cm
−
1
are no longer observed.
Excitation at 457.9 nm also gives rise to inten-
−
1
sified bands in the 1600 cm
region, but now
two bands are seen, the strongest one at 1600
−
1
−1
cm
and a shoulder at 1585 cm . The TiO2
bands are not observed.
Fig. 1. Optical diffuse reflectance spectra of: (top) styrene on
The absence of TiO Raman bands with excita-
TiO : (a) orange and (b) yellow samples; (bottom) 4MeSt on
2
2
TiO : (a) red and b) orange samples.
tion at 514.5 and 457.9 nm can be explained by
2

1
48
L.K. Noda, O. Sala / Spectrochimica Acta Part A 56 (2000) 145–155
Fig. 2. Raman spectra of: styrene: (a) monomer and (b) polymer; 4MeSt: (c) monomer (d) oligomer and (e) polymer.
−
1
the huge enhancement of the oligomers bands due
to the resonance Raman effect that is not present
for the TiO2.
The frequencies and assignment of the Raman
spectra of styrene (monomer, polymer and on
cm
(w(CꢀC) of unsaturated oligomeric species)
is also observed. Other bands with positions and
intensities similar to the 4MeSt oligomer ones are
seen (Fig. 2). These features match those observed
in the spectrum of styrene on TiO2 excited at
632.8 nm, corresponding to a normal Raman
spectrum of the oligomers.
TiO ) are presented in Table 1, but for styrene on
2
TiO the assignment of the bands in the 1600
2
−
1
cm
region will be discussed later.
Excitation at 632.8 nm shows that the band at
−
1
1
cm
600 cm
is greatly intensified and the 1610
−
1
3.2. 4MeSt on TiO2
one disappears, while the oxide bands show
up very weakly. In this case a pre-resonance Ra-
man effect is present as to be expected from the
broad electronic absorption band with the maxi-
mum in the green region (Fig. 1).
On TiO the Raman spectra of 4MeSt (Fig. 4)
excited at 782.0, 632.8, 514.5 and 457.9 nm show
a similar behaviour.
2
Excitation at 782.0 nm allows the observation
At 514.5 nm the same behaviour is observed,
although a greater enhancement of the oligomers
bands can be inferred from the higher signal to
noise ratio.
of the strong Raman bands from TiO (400, 515
2
−
1
and 635 cm ). Bands of 4MeSt show up at 1610
and 1600 cm , and a very weak band at 1645
−
1

L.K. Noda, O. Sala / Spectrochimica Acta Part A 56 (2000) 145–155
149
Fig. 3. Raman spectra of styrene on TiO2 at the indicated exciting radiations. * Bands of TiO2 (anatase).
Table 1
−
1
Raman shift (cm ) and assignment of the most characteristic bands of styrene: monomer, polymer and on sulphated TiO , at the
2
a
indicated exciting lines (nm)
Monomer
Polymer
Assignmentb
Wilson numbering
Styrene on TiO2
4
57.9
514.5
632.8
1
1
–
1
–
1
–
–
1
1
1
9
7
–
–
629 vs
599 s
–
Vinyl w(CꢀC)
Ring w(CC)
Ring w(CC)
Ring w(CC)
l (CH )
Vinyl l(ꢀCH2)
l(CH2)
–
Ring w(CꢁX)
Ring i(CꢁH)
Ring i(CꢁH)
Trig. breathing
Ring breathing
TiO2 anatase
TiO2 anatase
–
–
–
–
1647 m
1600 s
1585 m
–
1453 w
–
1601 m
–
1583 w
1450 w
–
8a
8a
8b
–
–
–
1600 s
1585 sh
1560 m
–
1585 s
1560 w
–
–
1364 w
–
–
573 w
411 w
s
3
–
–
–
1364 w
1230 w
–
1185 w
1027 w
1000 w
728 w
–
1370 w
–
–
201 m
180 w
031 w
97 vs
72 m
1199 w
1182 w
1031 m
1001 vs
798 w
–
7a
9a
–
12
1
1195 w
1181 w
1029 m
1000 s
–
1180 w
1000 w
–
–
–
–
–
635 vs
515 vs
–
–
a
ls=symmetric deformation; i=in plane bending; k=out of plane bending; sh=shoulder.
From Refs. [13–16].
b

1
50
L.K. Noda, O. Sala / Spectrochimica Acta Part A 56 (2000) 145–155
Fig. 4. Raman spectra of 4MeSt on TiO2 at the indicated exciting radiations. * Bands of TiO2 (anatase).
Excitation at 457.9 nm shows. as for the 782.0
based on the obtained results.
−
1
nm excitation, two bands in the 1600 cm
re-
−
1
gion: one at 1610 cm , strongly intensified and a
3.3. Possibilities of interaction of styrene and
4MeSt with TiO2
−
1
shoulder at 1600 cm . Bands at 1557 and 728
−
1
cm
are also observed.
A red shift of the resonance Raman enhance-
Three hypotheses can be considered to explain
the nature of the interaction between styrene or
ment for 4MeSt on TiO relative to styrene on
2
TiO is noticed, as expected from the observed
4MeSt and TiO which gives rise to the observed
2
2
optical spectra (Fig. 1).
colour: (1) formation of a p-complex between the
compound and the titanium ion through the vinyl
group of the monomer or oligomer bearing a
double bond; (2) interaction of the titanium with
the ring p-electrons; (3) a conjugative effect be-
tween the aromatic ring and the CꢀTi group of an
intermediate species, as proposed by Davanzo et
al. [10]. The first and the second hypothesis will be
discussed together.
From the detailed spectra of 4MeSt on TiO in
2
−
1
the 1600 cm , shown in Fig. 5, it is evident an
enhancement pattern of the bands in this region.
It can be seen that the relative intensities of the
−
1
bands at 1600 and 1610 cm
on going from 514.5 to 413.1 nm.
The frequencies and assignments for the 4MeSt
Raman spectra (monomer and polymer) are pre-
sented in Table 2 together with the assignment for
the Raman bands of 4MeSt on TiO2.
change gradually
The first hypothesis is based on previous results
2
−
of the polymerization of ethylene on TiO –SO4
2
Since the most intensified Raman bands are in
[9]. The authors observed a shift of the w(CꢀC)
−
1
−1 −1
the 1600 cm
region, the correct assignment of
from 1623 cm
(ethylene vapour) to 1605 cm
,
these bands is mandatory in order to identify the
chromophore. Different interactions of styrene
due to the formation of a p-complex between the
titanium ion and the p-electrons of the ethylene
double bond. So, in the case of styrene on the
and 4MeSt with the TiO will be considered below
2

L.K. Noda, O. Sala / Spectrochimica Acta Part A 56 (2000) 145–155
151
−
1
TiO surface the intensified band at ca. 1600
The Raman spectra in the 1450–1650 cm
2
−
1
cm
should be assigned to the w(CꢀC) of the
region of the deuterated compounds, neat and on
TiO , at several exciting radiations, are shown in
Fig. 6 and the frequencies and assignments are
presented in Table 3. The practically identical
spectra of the deuterated compounds on TiO for
all exciting radiations show that the vinylic
w(CꢀC) mode is not observed, i.e. the ring stretch-
ing modes are the enhanced ones, evidenciating
that the charge transfer transition, responsible for
the colour, involves an orbital mainly localized on
the aromatic ring.
−
1
vinyl group shifted by 30 cm
with the monomer.
when compared
2
According to the second hypothesis the chro-
mophore should be localized in the aromatic ring,
due to the interaction of the p-electrons with the
2
−
1
titanium ion, and so the band at ca. 1600 cm
should be assigned to a ring stretching mode.
To decide which of these two hypotheses is
most likely to occur, the Raman spectra of deuter-
ated styrene (styrene-d and styrene-d ) on TiO
5
8
2
were obtained. Since in neat styrene-d5 and
styrene-d the vinyl w(CꢀC) stretching is found at
From the resonance Raman spectra of styrene
and 4MeSt on TiO , the third hypothesis (the
8
2
−
1
1
629 and 1574 cm
[12], respectively, a different
conjugation of the phenyl ring with the CꢀTi
bound [10]) seems to be unlikely, as there was no
noticeable enhancement of a band in the low
frequency region which could be assigned to a
CꢀTi vibrational mode. Furthermore, a sample of
allylbenzene (ƒ-CH ꢁCHꢀCH ) on TiO was pre-
pattern of the enhanced bands of these two com-
pounds should be expected if the chromophore
involves the vinylic group. In this case, styrene-d5
on TiO would present an intensified band at ca.
2
−
1
1
600 cm , while for styrene-d8 on TiO2 the
2
2
2
−
1
enhanced band would be lower than 1574 cm
.
pared to test this third hypothesis. This com-
pound is not expected to present any conjugative
effect between the ring p-electrons and the eventu-
ally formed CꢀTi moiety.
On the other hand, if the interaction involves the
aromatic ring, the spectra of both deuterated
compounds on TiO would be the same.
2
In fact, the Raman spectra of allylbenzene on
TiO exhibit an enhancement pattern similar to
2
that of styrene on TiO , i.e. intensification of the
2
−
1
bands in the 1600 cm
region with a change in
their relative intensities going from green to blue
excitation. The electronic absorption spectrum is
also practically identical to that of styrene on
TiO , with absorptions at ca. 450 and 510 nm,
2
originating the same yellow to orange colours.
Thus, based on the obtained Raman and elec-
tronic absorption spectra, the interaction of allyl-
benzene with TiO should be of the same type
2
found for styrene or 4MeSt.
Trans-substituted ethylenes have a characteris-
tic hydrogen wagging vibration which absorbs in
−
1
the IR at 980–965 cm [15]; with one aryl group
as substituent, for instance (C H )HCꢀCH(ƒ), a
2
5
−
1
characteristic Raman band at 1652 cm
is
present [16]. Similarly, the Raman bands at ca.
−
1
1
645 cm , observed away from the resonance
region (632.8 nm for styrene/TiO and 782.0 nm
2
for 4MeSt/TiO ) can be assigned to unsaturated
2
oligomers on the oxide surface. The IR spectra
Fig. 5. Detailed Raman spectra of 4MeSt on TiO in the
2
−
1
1
470–1640 cm
region at the indicated exciting radiations.
show the presence of ꢁCH and ꢁCH groups [10]
3
2

1
52
L.K. Noda, O. Sala / Spectrochimica Acta Part A 56 (2000) 145–155
Table 2
−
1
Raman shift (cm ) and assignment of the most characteristic bands of 4MeSt: monomer, polymer and on sulphated TiO , at the
2
a
indicated exciting lines (nm)
Monomer
Polymer
Assignmentb
Wilson numbering
TiO /4MeSt
2
4
–
57.9
514.5
632.8
782.0
1
1
–
1
1
1
1
–
1
–
1
1
–
–
–
–
–
1
–
1
8
–
8
7
6
-
630 vs
610 s
–
Vinyl w(C=C)
Ring w(CC)
–
–
8a
–
8b
19a
–
–
–
–
–
–
–
–
–
–
–
–
7a
–
9a
1
–
11
12
6b
–
–
–
–
–
–
–
–
1645 w
1610 s
1600 sh
–
1513 w
1449 w
–
–
1377 w
–
–
–
1609 vs
–
1578 w
–
1446 m
–
–
1375 m
–
1320 wb
–
–
–
–
–
1610 vs
1600 sh
1557 s
1513 wm
1600 vs
–
1512 w
1458 w
–
–
–
–
1600 vs
–
1513 w
–
–
–
1375 w
–
–
–
1276 sh
1265 w
–
–
571 w
513 w
450 vw
404 m
Ring w(CC)
Ring w(CC)
l (CH )
–
a
3
Vinyl l(=CH2)
–
1404 wm
1398 wm
–
1353 sh
1326 wm
–
–
379 w
l (CH )
s
3
–
–
320 m
299 wm
–
–
Vinyl i(=CH−)
–
–
–
–
–
1275 sh
1265 wm
–
–
–
–
1191 m
1177 sh
–
–
–
1266 vw
–
–
–
1245 w
1233 w
1215 wm
1209 m
–
–
–
204 s
1197 s
–
1180 s
825 s
w(CꢁX)
–
–
1192 m
1181 sh
827 vw
–
–
–
1194 w
1182 wm
821 vw
804 w
–
180 s
25 w
Ring i(CꢁH)
Ring breathing
–
Ring k(CꢁH)
Ring deform.
Ring h(CꢁCꢁC)
TiO2 anatase
TiO2 anatase
TiO2 anatase
TiO2 anatase
1185 m
–
824 w
794 w
–
809 w
10 w
13 w
43 w
806 s
–
640 m
–
–
–
–
–
–
–
–
–
–
–
728 m
–
–
–
–
–
–
–
–
635 vw
515 vw
400 vw
145 w
635 s
515 s
400 s
145 vs
–
–
–
b
From Refs. [14–17].
a
l =antisymmetric deformation; l =symmetric deformation; i=in plane bending; k=out of plane bending.
a
s
(
characteristic of hydrocarbon chain) and of a
470 nm), suggesting that in both cases more
than one species is present on the surface.
2. To simplify, only the case of 4MeSt will be
considered in the following. The most en-
hanced Raman band at 514.5 nm excitation is
−
1
band at 960 cm (trans CH wag), indicating that
the oligomer has an internal, instead of a terminal
CꢀC bond, as in (R)HCꢀCH(ƒ).
−
1
the 1600 cm
assigned to a ring stretching
mode (8a) of an unsaturated oligomeric spe-
4
. Conclusions
cies, specifically a ring conjugated to a CꢀC
−
1
bond. This mode is down shifted by 10 cm
relative to the monomer, indicating a signifi-
From the above considerations the following
points can be highlighted:
1
cant interaction with TiO . It is then proposed
. Two bands are observed in the optical spec-
trum of styrene on TiO (515 and 450 nm) and
in the spectrum of 4MeSt on TiO (530–545,
2
a charge transfer from the ring p-electrons to
the empty d-p orbitals of the titanium ion,
2
2

L.K. Noda, O. Sala / Spectrochimica Acta Part A 56 (2000) 145–155
153
which gives rise to an electronic absorption
band at 530–545 nm.
weaker interaction for these species with the
titanium ion when compared with the interac-
tion for the unsaturated oligomers. The second
band, as mentioned above, derives from the
residual enhancement 530 nm transition.
3
. The enhanced Raman bands at 457.9 nm exci-
−
1
tation are 1610 cm
cm
(very strong) and 1600
−
1
(weak). The first one has its origin from
the 470 nm transition and is assigned to a ring
mode of the saturated oligomeric chain; this
frequency is practically the same observed for
the neat monomer and polymer, suggesting a
4. The Raman spectrum excited at 413.6 nm
−
1
shows only the band at 1610 cm , in this
region, because the resonance is solely with the
transition at 470 nm.
−
1
Fig. 6. Raman spectra of styrene-d and styrene-d in the 1400–1700 cm
region. (a) neat compounds; compounds on TiO at the
2
5
8
exciting radiations: (b) 457.9 nm (c) 488.0 nm and (d) 514.5 nm.

1
54
L.K. Noda, O. Sala / Spectrochimica Acta Part A 56 (2000) 145–155
Table 3
−
1
−1
Raman shift (cm ) of styrene and deuterated styrene (d and d ): neat and on sulphated TiO , in the 1350–1650 cm region, at
5
8
2
the indicated exciting lines (nm)
Styrene
Assignment
Wilson numbering
Styrene on TiO2
5
14.5
488.0
457.9
Undeuterated
1
1
629 vs
599 s
Vinyl w(CꢀC)
Ring w(CC)
–
8a
–
–
1
–
–
–
1600 s
1585 sh
1560 m
–
585 s
1585 s
1560 m
1508 w
1
1
573 w
495 w
Ring w(CC)
Ring w(CC)
8b
19a
1560 w
1512 w
d5
1
1
1
629 s
575 sh
564 m
Vinyl w(CꢀC)
2×789
Ring w(CC)
–
–
8a
–
8b
–
–
–
–
–
–
–
–
–
1575 s
1552 s
1535 m
1516 w
1556 s
–
1515 w
1555 s
–
1515 w
1
539 w
Ring wCC
d8
1
1
574 s
563sh
Vinyl w(CꢀC)
Ring w(CC)
–
8a
–
–
8b
–
–
–
–
–
–
1572 s
1552 s
–
1533 m
1517w
1557 s
1558 s
1
1
554 w
536 wm
958+594
Ring w(CC)
–
–
–
–
–
–
From the above considerations it seems that
two kinds of interaction are present: one giving
these separated oligomers [19] it was found that in
case (i) saturated oligomers prevail, while in (ii)
unsaturated species (containing a CꢀC group di-
rectly bound to the ring) predominate.
−
1
origin to the Raman band at 1600 cm
other to the band at 1610 cm . For benzene and
and the
−
1
toluene on TiO [18], where there is no formation
2
The overall results obtained from the resonance
of oligomeric species, a single electronic absorp-
tion band is observed, suggesting only one kind of
interaction.
Raman spectra of 4MeSt/TiO , gel permeation
2
chromatography and vibrational spectra of the
separated oligomers led to the following final
conclusions: the interaction that gives rise to the
electronic absorption in the visible region is due
to charge transfer transitions involving the d-p
orbitals of titanium ion and the ring-p electrons of
the oligomers; for saturated long oligomers and
unsaturated short oligomers these transitions are
in the blue and green regions, respectively.
Regarding the different colours in the samples
(
4MeSt on TiO , for instance) two experiments
2
were performed: (i) the monomer was introduced
very slowly in the reaction tube containing the
catalyst and a yellow–orange coloured sample
was obtained (strongest absorption in the blue);
(
ii) the monomer was introduced quickly and the
red sample was formed (strongest absorption in
the green). Results from gel permeation chro-
matography of the oligomers separated from the
coloured samples indicate a high amount of long
oligomers in (i), while short oligomers predomi-
nate in (ii) [19]. From the vibrational spectra of
Preliminary results from gel permeation chro-
matography and vibrational spectroscopy of
styrene oligomers separated from the surface of
sulphated TiO are similar to those of 4MeSt, so
2
leading to the same explanation.

L.K. Noda, O. Sala / Spectrochimica Acta Part A 56 (2000) 145–155
155
[
7] M. Hino, K. Arata, J. Chem. Soc. Chem. Commun. 18
1980) 851.
Acknowledgements
(
[
[
8] M. Hino, K. Arata, Chem. Lett. 10 (1979) 1259.
9] F. Al-Mashta, N. Sheppard, C.U. Davanzo, Mater.
Chem. Phys. 13 (1985) 315.
The authors wish to acknowledge the Funda c¸ a˜ o
de Amparo a` Pesquisa do Estado de S a˜ o Paulo
(
FAPESP) for financial support. L.K.N. is in-
[10] C.U. Davanzo, N. Sheppard, F. Al-Mashta, Spectrochim.
Acta 41A (1985) 263.
debted to FAPESP and Conselho Nacional de
Pesquisas (CNPq) for fellowship grants.
[
[
[
[
11] R.V. Hoffman, R.D. Bishop, P.M. Fitch, R. Hardenstein,
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(
1976) 466.
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