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Chemical Physics 298 (2004) 87–95
www.elsevier.com/locate/chemphys
Infrared absorption, Raman, and SERS investigationsin conjunction with theoretical simulations on a
phenothiazine derivative
M. Bolboaca a, T. Iliescu a,*, W. Kiefer b,1
a Department of Physics, Babes-Bolyai University, M. Kogalniceanu 1, 3400 Cluj-Napoca, Romaniab Institut f€ur Physikalishe Chemie, Universit€at W€urzburg, Am Hubland, D-97074 W€urzburg, Germany
Received 5 September 2003; accepted 7 November 2003
Abstract
The vibrational characterization of the most stable conformer of 10-isopentyl-10H-phenothiazine-5,5-dioxide (10-I-10H-P-5,
5-D) was performed by means of infrared absorption, Raman and surface-enhanced Raman spectroscopy (SERS). Hartree–Fock
and density functional theory calculations were carried out to find the optimised structures and the computed vibrational wave-
numbers of the title compound. The comparison of SER spectra obtained only in activated silver colloid with the corresponding
Raman spectrum reveals small shifts and changes in the relative intensities proving the partial chemisorption of the molecules on the
silver surface. The electromagnetic mechanism represents the main mechanism of the overall SERS enhancement. The changes
observed in the SER spectra at different pH values were explained by considering the reorientation of the adsorbed molecule with
respect to the metal surface.
� 2003 Elsevier B.V. All rights reserved.
1. Introduction
Chemotherapeutic agents are usually designated and
used according to their most predominant pharmaco-
logical activity. There are very few drugs, however with
a single specific function. Several studies have demon-strated the potential role of the phenothiazine and its
derivatives as anti-tumor [1], anti-viral [2,3] and anti-
plasmid agents [4,5]. All chemical compounds possessing
moderate to powerful anti-microbial properties have
been grouped together under the common term ‘‘non-
antibiotics��. Several groups of workers have repeatedly
reported on the existence of moderate to powerful anti-
microbial property in a variety of non-antibiotic com-pounds, particularly the phenothiazines [6–10]. A new
series of phenothiazine derivatives found to be impor-
tant intermediates in the metabolism of phenothiazine
drugs have been prepared [11,12] and the schematic
* Corresponding author. Tel.: +40-264-405300; fax: +40-264-191906.
E-mail addresses: ilitra@phys.ubbcluj.ro (T. Iliescu), wolfgang.kie-
fer@mail.uni-wuerzburg.de (W. Kiefer).1 Tel.: +49-931-8886330; fax: +49-931-8886332.
0301-0104/$ - see front matter � 2003 Elsevier B.V. All rights reserved.
doi:10.1016/j.chemphys.2003.11.007
structure of 10-isopentyl-10H-phenothiazine-5,5-dioxide
(10-I-10H-P-5,5-D) with the labelling of the atoms is
illustrated in Fig. 1.
Raman spectroscopy is proven to be a powerful tool
to provide information about the structure and inter-
acting mechanisms of biologically active molecules. TheRaman spectra of phenothiazine and its radical cation
were reported by Pan and Phillips [13], while Hester and
Williams [14] have reported the resonance Raman
spectra of phenothiazine, 10-methyl-phenothiazine and
their radical cations. In some cases the weak intensity of
the Raman scattered light and the interference of the
fluorescence reduces the application field of conven-
tional Raman spectroscopy. Surface-enhanced Ramanspectroscopy (SERS) offers a possibility to overcome
these disadvantages; it allows the detection of very low
sample concentrations [15,16]. The origin of the en-
hancement of Raman scattering cross-section at rough
surfaces has been an active field of research. The general
consensus states that the observed enhancement is
the result of contributions from two mechanisms: an
electromagnetic enhancement and a chemical effect[15,17,18].
Fig. 1. Schematic structure of the 10-isopentyl-10H-phenothiazine-5,5-
dioxide compound with the labeling of the atoms.
88 M. Bolboaca et al. / Chemical Physics 298 (2004) 87–95
In the present paper, we report a fairly detailed ex-
perimental and theoretical investigation of the 10-I-10H-
P-5,5-D derivative. The first part of the study presents
the vibrational analysis of the most stable conformer of
the title compound performed by means of infrared
absorption and Raman spectroscopy in combination
with theoretical (HF and DFT) simulations, while in the
second part of the study the SER spectra at different pHvalues are reported and analysed in order to elucidate
the adsorption behaviour of the molecules on colloidal
silver particles and to find out the pH influence.
Fig. 2. Optimised geometries of six most probable conformers of
10-isopentyl-10H-phenothiazine-5,5-dioxide. The differences between
the energy of the most stable conformer and the energy of the other
conformers, obtained at the BPW91/6-31G* level of theory, are indi-
cated in parenthesis.
2. Experimental
All starting materials involved in substrate and sam-ple preparation were purchased from commercial sour-
ces as analytical pure reagents.
A sodium citrate silver colloid, prepared according to
the standard procedure of Lee and Meisel [19], was
employed as SERS substrate. The resultant colloid was
yellowish gray with an absorption maximum at 407 nm.
Small amounts of 10-I-10H-P-5,5-D 10�1 M ethanol
solution were added to 3 ml silver colloid. NaCl solution(10�2 M) was also added (10:1) for producing a stabil-
isation of the colloidal dispersion that yields to a con-
siderable enhancement of the SER signal [20]. The final
concentration of the sample was �2.5 � 10�4 M.
The UV–Visible absorption spectra were recorded
with a Perkin–Elmer Lambda 19 UV–VIS–NIR spec-
trometer with a scan speed of 240 nm/min.
The FT-Raman spectrum of the polycrystallinesample was recorded using a Bruker IFS 120HR spec-
trometer with an integrated FRA 106 Raman module
and a resolution of 2 cm�1. Radiation of 1064 nm from
a Nd-YAG laser was employed for excitation. A Ge
detector, cooled with liquid nitrogen, was used. The
infrared spectrum in KBr pellets was recorded with a
Bruker IFS 25 spectrometer and a resolution of 2 cm�1.
The SER spectra of the sample on silver colloid,
collected in the back-scattering geometry, were recorded
with a Spex 1404 double spectrometer using 514.5 nm
and 300 mW output of a Spectra Physics argon ion laser.
The detection of the Raman signal was carried out with
a Photometrics model 9000 CCD camera. The spectralresolution was 2 cm�1.
Theoretical calculations of the structures and vibra-
tional wavenumbers of the investigated compound were
performed using the Gaussian 98 program package [21].
Density functional theory (DFT) calculations were car-
ried out with Becke�s 1988 exchange functional [22] and
the Perdew–Wang 91 gradient corrected correlation
functional (BPW91) [23] and Becke�s three-parameterhybrid method using the Lee–Yang–Parr correlation
M. Bolboaca et al. / Chemical Physics 298 (2004) 87–95 89
functional (B3LYP) [24]. Ab initio calculations were
also performed at the restricted Hartree–Fock (RHF)
level of theory. The 6-31G* Pople split-valence polari-
zation basis set was used in the geometry optimisation
and normal modes calculations at all theoretical levels.At the optimised structures of the examined species no
imaginary frequency modes were obtained, proving that
a local minimum on the potential energy surface was
found.
3. Results and discussion
3.1. Vibrational analysis
By looking at the geometry of the 10-I-10H-P-5,5-D
molecule one can observe that it allows for several
Table 1
Selected calculated bond lengths (pm) and angles (�) of 10-isopentyl-10H-phe
the phenothiazine
10-I-10H-P-5,5-D
Calculateda Calculatedb
Bond lengths (pm)
C–Saverage 178.440 178.441
C–Naverage 140.995 140.995
C1–C2 141.594 141.594
C2–C3 139.814 139.814
C3–C4 140.531 131.031
C4–C5 139.714 139.715
C5–C6 140.133 140.133
C6–C1 141.773 141.773
C2–H2 109.135 109.135
C3–H3 109.360 109.360
C4–H4 109.240 109.240
C5–H5 109.266 109.266
S–O1 148.638 148.638
S–O2 148.852 148.852
C7–N 147.780 147.780
Angles (degree)
Dihedral angle 145.814 145.788
C6–S–C60 98.191 98.191
C1–N–C10 120.639 120.639
C1–C2–C3 120.716 120.717
C2–C3–C4 121.280 121.280
C3–C4–C5 119.015 119.015
C4–C5–C6 119.720 119.719
C5–C6–C1 122.267 122.267
C6–C1–C2 116.958 116.958
C1–C2–H2 120.139 120.139
C2–C3–H3 118.409 118.786
C3–C4–H4 120.604 120.604
C4–C5–H5 121.729 121.730
C6–S–O1 110.035 110.035
C6–S–O2 108.564 108.564
C60–S–O1 110.076 110.076
C60–S–O2 108.428 108.429
aRHF/6-31G*.bBPW91/6-31G*.c B3LYP/6-31G*.dRef. [21].
conformers due to the flexibility of the isopropyl group.
The optimised geometries of the six most probable
conformers calculated at the BPW91/6-31G* level of
theory are illustrated in Fig. 2. Analytical harmonic vi-
brational modes have also been calculated to ensure thatthe optimised structures correspond to minima on the
potential energy surface. The total energy of the most
stable conformer, which was found to be the conformer
1, including zero point corrections is )1261.834149Hartree. The differences between the energy of the most
stable conformer and the energy of the other relevant
conformers, obtained at this theoretical level, are also
indicated in Fig. 2. The experimental and theoreticalinvestigations were further carried out for the conformer
1, which will be denoted as 10-I-10H-P-5,5-D.
According to the X-ray diffraction investigations [25]
the phenothiazine molecule is folded about the N–S axis
nothiazine-5,5-dioxide derivative compared to the experimental data of
Phenothiazine
Calculatedc Experimentald
177.489 177
140.698 140.6
140.847 138.5
139.165 139
139.867 136.7
139.026 136.7
139.508 139.1
140.938 139.7
108.403 98
108.663 105
108.541 98
108.550 93
142.756
147.402
147.342
145.253 153.30
98.571 99.60
120.475 121.50
120.658 119.8
121.297 120.5
119.016 119.4
119.662 119.7
122.299 119.2
117.025 119.5
120.126 118.5
118.823 115.8
120.580 117
121.683 122.8
109.955
108.662
109.994
108.526
3400 3200 3000 2800 1600 1400 1200 1000 800 600 400
b
a
Wavenumber / cm-1
Fig. 3. FT-Raman (a) and infrared (b) spectra of 10-isopentyl-10H-
phenothiazine-5,5-dioxide derivative.
90 M. Bolboaca et al. / Chemical Physics 298 (2004) 87–95
with the two planes containing the phenyl rings having a
dihedral angle of 158.5�. It was reported [13] that the
amount of folding increases for larger substituents on
10-substituted derivatives, chlorpromazine having a di-
hedral angle of 139.4�.Table 1 contains selected optimised structural pa-
rameters of 10-I-10H-P-5,5-D derivative calculated
by various methods together with the available X-ray
values of the ground state of the phenothiazine [25]. As
one can see the theoretical dihedral angle between the
two phenyl rings is smaller with respect to the dihedral
angle of the phenothiazine and agrees with previous
findings [13]. The calculated bond lengths and bondangles are in good agreement with the reported param-
eters [25], the B3LYP method giving the best results. At
this level of calculation the differences between the the-
oretical and experimental values of the structural pa-
rameters that involve the S and N atoms are mainly due
to the presence of the substituents.
FT-Raman and infrared spectra of the phenothiazine
derivative 10-I-10H-P-5,5-D in the range from 3400 to400 cm�1 are presented in Fig. 3. The observed bands as
well as the vibrational assignment performed with the
help of the results obtained from theoretical simulations
and the work of Pan and Phillips [13] are summarised in
Table 2.
The neglect of anharmonicity effects and the incom-
plete incorporation of electron correlation in the ab
initio theoretical treatment lead to harmonic vibrationalwavenumbers larger than the fundamentals experimen-
tally observed [26]. Having in view that Hartree–Fock
calculations overestimate relatively uniform vibrational
wavenumbers because of improper dissociation behav-
iour, the predicted wavenumber values have to be scaled
with scaling factors to adjust the observed experimental
values [27]. Thus, the RHF calculated vibrational
wavenumbers presented in Table 2 have been uniformlyscaled by 0.8953 according to the work of Scott and
Radom [27]. Even after scaling, in comparison to the
experiment, the RHF wavenumbers are overestimated in
the high wavenumber region, but are comparable to the
experimental values in the low wavenumber region.
In agreement with previous studies [27,28] the vi-
brational wavenumbers calculated using the B3LYP
functional are also much larger than those calculatedwith the BPW91 method compared to the experi-
mental values (see Table 2). Thus, according to the
work of Rauhut and Pulay [29] a scaling factor of
0.963 has been uniformly applied to the B3LYP cal-
culated wavenumbers from Table 2. The observed
disagreement between the theory and experiment
could be a consequence of the anharmonicity and of
the general tendency of the quantum chemical meth-ods to overestimate the force constants at the exact
equilibrium geometry [29]. However, as can be seen
from Table 2 the theoretical results reproduce well the
experimental data and allow the assignment of the
vibrational modes.
By analysing Fig. 3 and Table 2 one can remark that
the bands given by the CH stretching vibration of the
phenyl ring and isopentyl group dominate the highwavenumber region (3200–2800 cm�1) of the infrared
and Raman spectra of the 10-I-10H-P-5,5-D compound.
The stretching vibrations of the phenyl rings give rise to
bands present in the range between 1610 and 1575 cm�1
of both spectra. The strong Raman band at 1324 cm�1
(calc. 1307 cm�1) and its corresponding infrared band at
1319 cm�1 were also attributed to the CC stretching
vibrations of the ring. The phenyl ring breathing vi-bration gives rise to the medium intense infrared and
Raman bands at 1049 and 1051 cm�1 (calc. 1043 cm�1),
respectively. The bands that occur at 1011 (calc. 1009
cm�1), 879 (calc. 874 cm�1), and in the spectral range
between 675 and 600 cm�1 of both spectra are due to the
in-plane deformation vibrations of the phenyl rings,
while the out-of-plane deformation vibrations appear at
430 (calc. 441 cm�1), 407 (calc. 395 cm�1) and in the580–570 cm�1 spectral region of both infrared and Ra-
man spectra of the phenothiazine derivative.
The medium intense Raman band at 1248 cm�1 (calc.
1245 cm�1) and its corresponding infrared band at 1250
cm�1 were attributed to the symmetric CNC stretching
vibration, while the weak Raman band at 1217 cm�1
(calc. 1217 cm�1) and the medium intense infrared band
at 1218 cm�1 were assigned to the asymmetric CNCstretching vibration. The bands given by the CSC
stretching vibration appear around 1080 cm�1 (calc.
1051 cm�1) in the infrared and Raman spectra of the 10-
I-10H-P-5,5-D derivative. The ring chair deformation
vibrations give rise to medium intense infrared and
Raman bands at 718 (calc. 721 cm�1) and 553 cm�1
Table 2
Assignment of the theoretical wavenumber values (cm�1) to the experimental bands of the 10-isopentyl-10H-phenothiazine-5,5-dioxide derivative
10-I-10H-P-5,5-D Vibrational assignment
Experimental Theoretical
IR Raman Calculateda Calculatedb Calculatedc
167w 174 169 170 C1NC10 , C6SC60 twist
187m 194 197 195 CCC skel def
268m 266 273 270 CH def. (CH3)
307w 296 290 290 Ring chair def. + O1SO2 def
336m 335 334 333 C1NC10 , C6SC60 twist
407m 407m 399 395 395 Out-of-plane Ph ring def. + O1SO2 wag +
418m 418sh 431 429 443 C10;9;11 def
432m 430sh 450 441 443 Out-of-plane Ph ring def
455m 457vw 460 455 453 C7;8;9 def. + CH def. (CH2, CH3)
515m 514vw 549 538 540 O1SO2 bend
552s 553w 569 548 550 Ph ring chair def
571s 569m 572 555 556 Out-of-plane Ph ring def
580s 578sh 592 594 590
605m 605vw 603 607 604 In-plane Ph ring def
621m 622vw 664 662 659
670w 672m 725 698 702
718ms 718sh 736 721 721 Ring chair def
730m 750 739 739 CH wag (ring) + CH def. (CH2)
751s 761sh 768 751 748
760s 766w 772 762 758
832w 831m 801 822 810 C10;9;11 stretch. + CH twist (Ph ring)
855m 852w 864 833 839
880m 879vw 878 874 876 C3;4;5, C30 ;40 ;50 bend
943m 947vw 937 940 929 CH twist (Ph ring) + CH def (CH3)
950w 958w 960 947 950
982w 983w 979 973 963
1012m 1011w 1006 1009 1001 C1;2;3, C3;4;5, C10 ;20 ;30 , C30 ;40 ;50 bend
1040sh 1038m 1019 1026 1014 C7;8 stretch. + O1SO2 stretch
1049m 1051m 1025 1043 1030 Ph ring breathing
1084s 1075w 1055 1051 1045 C6SC60 stretch. + NC7 stretch
1126m 1112 1125 1113 CH def. (CH2, CH3) + C10;9;8 stretch
1141s 1136m 1134 1138 1127 CH bend (Ph ring)
1167s 1169m 1166 1169 1155
1184m 1195 1174 1161
1218m 1217w 1213 1217 1208 C1NC10 as. stretch. + CH bend (Ph ring)
1250m 1248m 1251 1245 1234 C1NC10 s. stretch. + CH bend (Ph ring)
1287s 1281m 1285 1281 1273 CH def. (CH2) + CH rock + O1SO2 stretch
1319sh 1324s 1306 1307 1295 CCC stretch. (Ph ring)
1338sh 1334sh 1336 1333 1304
1352sh 1351m 1348 1352 1326 CH def. (CH2, CH3) + CH rock (Ph ring)
1370sh 1372sh 1370 1367 1355
1379s 1380w 1396 1383 1377
1451sh 1456m 1460 1465 1451 C6;1, C60 ;10 stretch
1465s 1462m 1477 1483 1470
1483sh 1483sh 1487 1486 1473 CH def. (CH2, CH3)
1489m 1494 1496 1482
1576s 1576m 1593 1580 1569 Ph ring stretch
1592s 1591m 1606 1597 1584
1608sh 1607s 1618 1614 1599
2869m 2869s 2883 2963 2892 CH stretch. (CH, CH2, CH3)
2897sh 2896m 2921 2999 2926
2927m 2927m 2970 3071 2936
2855m 2957m 3016 3078 2977
3040vw 3039m 3035 3095 3029 CH stretch. (Ph ring)
3070s 3049 3124 3074
3080vw 3088s 3052 3140 3093
Abbreviations: Ph¼ phenyl, w-weak, m-medium, s-strong, sh-shoulder, stretch.¼ stretching, bend¼bending, twist¼ twisting, wag¼wagging,
rock¼ rocking, def¼ deformation.
Calculated with: aRHF/6-31G*,b BPW91/6-31G*,c B3LYP/6-31G*.
M. Bolboaca et al. / Chemical Physics 298 (2004) 87–95 91
1800 1600 1400 1200 1000 800 600 400 200
1005
881
836
1462
1576
1250
1281
1321
408
340
730
674
336
735
1051
1283
124814
65
1579
1607 24
2
1324
408
1049
672
b
a
Ram
an in
tens
ity
Wavenumber / cm-1
Fig. 4. FT-Raman (a) and SER (b) spectra of 10-isopentyl-10H-phe-
nothiazine-5,5-dioxide compound.
92 M. Bolboaca et al. / Chemical Physics 298 (2004) 87–95
(calc. 548 cm�1) and the weak Raman band at 307 cm�1
(calc. 290 cm�1). Other bands given by the CNC and
CSC out-of-plane deformation vibrations appear in the
Raman spectrum of the phenothiazine derivative at 336
(calc. 334 cm�1) and 167 cm�1 (calc. 169 cm�1).The OSO stretching vibrations give rise to the me-
dium intense Raman bands at 1281 (calc. 1281 cm�1)
and 1038 cm�1 (calc. 1026 cm�1) and their corre-
sponding infrared bands at 1287 and 1040 cm�1, re-
spectively. The bands observed at 514 (calc. 538 cm�1),
407 (calc. 395 cm�1) and 307 cm�1 (calc. 290 cm�1) were
attributed to the deformation vibrations of the OSO
group. The other bands present in the infrared andRaman spectra of the 10-I-10H-P-5,5-D are mostly due
to the vibrations of the isopentyl group.
3.2. Adsorption on the silver surface
In Fig. 4 the normal Raman spectrum of polycrys-
talline 10-I-10H-P-5,5-D compound is compared with
the SER spectrum of the molecules adsorbed on silvercolloid. SERS enhancements were detected only for
molecules adsorbed on activated hydrosols, obtained by
the co-adsorption of the chloride anions. The assign-
ment of the normal vibrational modes of the phenothi-
azine derivative to the SERS bands at different pH
values is summarised in Table 3.
It is well known [15–18] that there are two possibili-
ties of molecule adsorption on the metal surface: phys-isorption and chemisorption. When the molecules are
physisorbed on the metal surface, the SER spectra are
very similar to those of the free molecules, the electro-
magnetic mechanism being the main mechanism of the
Raman enhancement [15]. In the case of chemisorption,
a new metal–molecule SERS complex is formed that
leads to dramatical changes of the position and relative
intensities of the SERS bands relative to their corre-sponding Raman bands. In this case, the charge-transfer
(CT) effect is the dominant mechanism of the Raman
enhancement [17,18].
By looking at the geometry of the molecule one can
assume that it may bind to the silver surface either
through the p orbitals of the phenyl rings or through the
lone pair electrons of the oxygen atoms, the nitrogen–
metal interaction being sterically hindered. From Fig. 4and Table 3 can be observed that the SERS bands
present shifts never exceeding 5 cm�1 compared to their
corresponding Raman bands and changes in the relative
intensities, while their band-widths are almost unaf-
fected. Therefore, we suppose that the molecules are
adsorbed on the silver surface through the oxygen atom,
otherwise shifts larger than 10 cm�1 and a broadening of
the bands should occur [30].The metal–molecule interaction is further demon-
strated (Fig. 5) by the presence in the low wavenumber
region of the SER spectra of some bands mainly at-
tributed to the Ag-adsorbate vibrations [31,32]. As can
be seen from Fig. 5 on passing from acidic to alkaline
environment the intensity of the band observed at 240
cm�1 in the SER spectrum at pH 1 and assigned to the
Ag–Cl stretching vibration [31] decreases, while the in-
tensity of the band at 221 cm�1, evidenced as a shoulder
at pH 1 and given by the Ag–O stretching vibration [32],
increases. The presence of the Ag–O stretching band atall pH values further supports the assumption that the
molecules are partially chemisorbed on the silver surface
through the lone pair electrons of the oxygen atom.
The UV–Visible absorption spectra of the colloid and
mixture of the colloid and 10-I-10H-P-5,5-D, before and
after addition of NaCl, were recorded and are presented
in Fig. 6. The absorption spectrum of the silver colloid
shows an intense band at about 407 nm given by thesmall particle plasma resonance. When two metallic
spheres approach each other, by ageing of the colloidal
particles or by effect of adsorption of ligand, this band
remains at the original single sphere wavelength, while
another resonance develops at longer wavelengths and
thus a secondary peak occurs in the 500–800 nm spectral
region [33]. This new broad band is alternatively at-
tributed either to the coagulation of silver particles inthe sol in the presence of the adsorbed molecules [34] or
to a CT band due to the molecule–metal interaction [35].
From Fig. 6(a) and (b) one can see that the band around
407 nm becomes broader and is shifted to longer
wavelengths only by 2 cm�1 after sample addition in the
silver hydrosol, while the secondary plasmon resonance
peak does not occur. The addition of NaCl causes a
further shift to longer wavelengths of the absorptionband, while no new peak in the 500–800 nm spectral
range appears. This behaviour clearly indicates the main
contribution of the electromagnetic mechanism to the
SERS enhancement.
Table 3
Assignment of the normal vibrational modes of 10-isopentyl-10H-phenothiazine-5,5-dioxide derivative to the SERS bands at different pH values
Raman SERS Vibrational assignment
pH 1 pH 6 pH 14
221sh 221sh 221m Ag–O stretch
240s 240s 240sh Ag–Cl� stretch
307w 309w 306m 312m Ring chair def. + O1SO2 def
336m 336w 340m 343w C1NC10 , C6SC60 twist
407m 401m 408m 412m Out-of-plane Ph ring def. + O1SO2 wag +
418sh 412m C10;9;11 def
553w 555sh 555m 555mw Ring chair def
569m 565m 571m 573m Out-of-plane Ph ring def
622vw 618m In-plane Ph ring def
672m 674m 674m 676m
718sh 709m Ring chair def
730m 734m 735m 737ms CH wag (Ph ring) + CH def. (CH2)
831m 841vw 836m C10;9;11 stretch. + CH twist (Ph ring)
879vw 881m 881m 884m C3;4;5, C30 ;40 ;50 bend
1011w 1007w 1005m 1007ms C1;2;3, C3;4;5, C10 ;20 :30 , C30 ;40 ;50 bend
1051m 1050s 1049s 1051s Ph ring breathing
1075w 1080w 1079m 1087m C6SC60 stretch. + NC7 stretch
1136m 1139w 1140m 1143m CH bend (Ph ring) + C10;9;8 stretch
1169m 1170m 1170m 1170m
1217w 1219w 1221w C1NC10 a. stretch. + CH bend (Ph ring) + CH def. (CH2)
1248m 1243w 1250w 1254vw C1NC10 s. stretch. + CH bend (Ph ring)
1281m 1279w 1283sh 1299sh CH def. (CH2) + CH rock (Ph ring) +
1293w O1SO2 stretch
1324s 1321ms 1321s 1323s CCC stretch. (Ph ring)
1351m 1368ms CH def. (CH2, CH3) + CH rock (Ph ring)
1372sh 1395ms
1380w 1404s
1456m 1454m 1440s C6;1, C60 ;10 stretch
1462m 1465vw 1451sh
1576m 1582s 1579m 1579ms Ph ring stretch
1591m 1596sh
1607s 1608s 1607s 1609s
2869s 2854sh 2866sh 2878s CH stretch. (CH, CH2, CH3)
2896m 2876sh 2896sh 2903s
2927m 2920sh 2938s 2934vs
2957m 2937s 2970sh 2965sh
3039m 3036sh CH stretch. (Ph ring)
3070s 3073ms 3071s 3065ms
3088s
Abbreviations: Ph¼ phenyl, w-weak, m-medium, s-strong, sh-shoulder, stretch.¼ stretching, bend¼bending, twist¼ twisting, wag¼wagging,
rock¼ rocking, def¼ deformation.
M. Bolboaca et al. / Chemical Physics 298 (2004) 87–95 93
It is known [36,37] that the changes evidenced in the
SER spectra recorded at different pH values are either
due to a change in the chemical structure of the adsor-
bate or to a reorientation of adsorbates with respect to
the metal surface. By analysing Fig. 7 and Table 3 one
can see that no new bands occur in the SER spectra at
different pH values relative to the Raman spectrum, andtherefore we assume that the differences between the
spectra are given by an orientational change of the
molecule relative to the silver surface.
From the enhancement of relevant bands following
the surface selection rules [38,39] one can predict the
orientation of the adsorbed molecules with respect to the
metal surface. According to these rules a vibrational
mode with its normal mode component perpendicular tothe surface will be more enhanced than a parallel one.
Furthermore, the CH stretching vibrations were re-
ported to the unambiguous probes for adsorbate ori-
entations [40].
By comparing the SER spectra of 10-I-10H-P-5,5-D
derivative depicted in Fig. 8 noticeable changes can be
observed on passing from acidic to alkaline environ-
ment. Some bands are enhanced only in the SER spectraat pH values below 6, while others appear only in the
SER spectra recorded in alkaline environment, and
therefore only the SER spectra at pH values of 1 and 14
will be discussed.
Thus, in the SER spectrum recorded at pH value of 1
the band at 567 cm�1, assigned to the out-of-plane de-
formation vibration of the phenyl ring, appears en-
hanced. The band at 709 cm�1 attributed to the ringchair deformation vibration is also enhanced at this pH
380 330 280 230 180
240
221
pH=12
pH=14
pH=3
pH=6
pH=9
pH=1
Ram
an in
tens
ity
Wavenumber / cm-1
Fig. 5. pH dependence of the metal-adsorbate stretching mode from
the SER spectra of the 10-isopentyl-10H-phenothiazine-5,5-dioxide
compound.
350 450 550 650 750 850 950 1050
412
407
409
514 nm
(b)
(c)
(a)
Abs
orpt
ion
Wavelength / nm
Fig. 6. Absorption spectra of silver colloid (a), with 10�1 M 10-iso-
pentyl-10H-phenothiazine-5,5-dioxide (b), with 10�1 M 10-isopentyl-
10H-phenothiazine-5,5-dioxide and 10�2 M NaCl (c).
1780 1580 1380 1180 980 780 580 380 180
555
1404
709
567
676
408
1368
1170
1007
884
733
1440
pH=12
pH=14
pH=3
pH=6
pH=9
pH=1
Ram
an in
tens
ity
Wavenumber / cm-1
Fig. 7. SER spectra of 10-isopentyl-10H-phenothiazine-5,5-dioxide
compound at different pH values as indicated.
Fig. 8. Schematic model for the adsorption geometry of 10-isopentyl-
10H-phenothiazine-5,5-dioxide on a colloidal silver surface at pH< 6
(a) and pH> 6 (b).
94 M. Bolboaca et al. / Chemical Physics 298 (2004) 87–95
value. In the spectral range between 1404 and 1365 cm�1
two bands due to the CH deformation vibrations of the
CH2 and CH3 groups are enhanced in the SER spectrum
at pH 1. At this pH value the bands due to the in-plane
deformation vibration of the phenyl ring are not present
or are only weakly enhanced. Having in view all these
considerations we assume that in acidic environment
(pH< 6) the molecule adopts a tilted orientation on the
silver surface (see Fig. 8(a)).
By looking at the SER spectrum at pH 14 one can
observe that in contrast to the spectra at acidic pH, atthis pH value the bands at 884 and 1007 cm�1 due to the
in-plane deformation vibration of the phenyl ring are
enhanced. The band at 1440 cm�1 attributed to the C6C1
and C60C10 stretching vibrations appears also enhanced
at this pH value. Furthermore, the bands at 1221 and
1254 cm�1 attributed to the CNC stretching vibration
are also enhanced in the SER spectrum recorded at pH
14. At pH values above 6 the bands attributed to the CHstretching vibrations of the CH2 and CH3 groups are
M. Bolboaca et al. / Chemical Physics 298 (2004) 87–95 95
more enhanced compared to those obtained at acidic
pH. By taking into account the behaviour of these bands
we assume that in alkaline environment (pH> 6) the 10-
I-10H-P-5,5-D molecule is adsorbed on the metal sur-
face in such a way that the phenyl rings have an uprightorientation with respect to the surface (see Fig. 8(b)). At
all pH values the adsorption on the metal surface is
maintained through the oxygen atom.
4. Conclusion
Analytical (infrared and Raman spectroscopy) and
theoretical (HF and DFT calculations) investigations on
the most stable conformer of 10-isopentyl-10H-pheno-thiazine-5,5-dioxide (10-I-10H-P-5,5-D) derivative have
been performed. The SER spectra of the sample in ac-
tivated silver colloids were recorded and compared to
the corresponding Raman spectrum. The small shifts of
the SERS bands (Dm6 5 cm�1) relative to the corre-
sponding Raman bands and the presence of the Ag-
molecule stretching band at all pH values allow us to
conclude that the molecules are chemisorbed on themetal surface, the electromagnetic mechanism being
the main mechanism of the Raman enhancement. The
changes observed in the SER spectra at different pH
values were explained by considering the reorienta-
tion of the adsorbed molecule with respect to the silver
surface.
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