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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