Surface enhanced Raman spectroscopic (SERS) behavior of substituted propenoic acids used in heterogeneous catalytic asymmetric hydrogenation

Article abstract of DOI:10.1002/jrs.4741

The strength and geometry of adsorption of substituted propenoic acids on silver surface were studied by means of surface enhanced Raman spectroscopy (SERS) using silver sol. Based on their SERS behavior, two classes of phenylpropenoic acids studied were distinguished. The first class of propenoic acids (atropic acid, (E)-2,3-diphenylpropenoic acid, (E)-2-(2-methoxyphenyl)-3-phenylpropenoic acid, (E)-2,3-di-(4-methoxyphenyl)phenylpropenoic acid and (E)-2-(2-methoxyphenyl)-3-(4-fluorophenyl)propenoic acid) has shown strong charge transfer (CT) effect. We suggest bidentate carboxyl bonded species based on the SERS enhanced bands of νCOO^- around 1394 cm^-1 and νC-C of the -C-COO^- moiety at 951 cm^-1. In these series the plane of the α-phenyl group (γCH out-of-plane vibrations at 850-700 cm^-1) is almost parallel to the silver surface, while the β-phenyl group is in tilted position depending on the type and the position of substituent(s) showing strong SERS enhanced bands of νCC + βCH (in-plane mode) at 1075 cm^-1, νCC (ring breathing mode, in-plane) at 1000 cm^-1 and γCCC (out-of-plane mode) around 401 cm^-1. The other class of propenoic acids (cinnamic acid, (E)-2-phenyl-3-(4-methoxyphenyl)propenoic acid) has shown weak electromagnetic (EM) enhancement (CC bands is enhanced in cinnamic acid). In this case no significant carboxyl enhancement was observed, so we suggest that adsorbed species lie parallel to the surface. The two types of adsorption can be related to the dissociation ability of the carboxylic group. In the first case the carboxylic H dissociates, while in the second case it does not, as indicated also by the characteristic νCO band at 1686 cm^-1 in the FT-Raman spectra of methanolic solutions.

Full text of DOI:10.1002/jrs.4741

Research article
Received: 4 April 2015
Revised: 18 May 2015
Accepted: 18 May 2015
Published online in Wiley Online Library: 23 June 2015
(wileyonlinelibrary.com) DOI 10.1002/jrs.4741
Surface enhanced Raman spectroscopic (SERS)
behavior of substituted propenoic acids used
in heterogeneous catalytic asymmetric
hydrogenation
Tamás Firkala,a Emília Tálas,b* Sándor Kristyán,b György Szöllősi,c
Eszter Drotár,b János Minkb and Judith Mihályb
The strength and geometry of adsorption of substituted propenoic acids on silver surface were studied by means of surface en-
hanced Raman spectroscopy (SERS) using silver sol. Based on their SERS behavior, two classes of phenylpropenoic acids studied
were distinguished. The first class of propenoic acids (atropic acid, (E)-2,3-diphenylpropenoic acid, (E)-2-(2-methoxyphenyl)-3-
phenylpropenoic acid, (E)-2,3-di-(4-methoxyphenyl)phenylpropenoic acid and (E)-2-(2-methoxyphenyl)-3-(4-fluorophenyl)propenoic
acid) has shown strong charge transfer (CT) effect. We suggest bidentate carboxyl bonded species based on the SERS enhanced
bands of νCOOꢀ around 1394 cmꢀ1 and νC―C of the ―C―COOꢀ moiety at 951 cmꢀ1. In these series the plane of the α-phenyl group
(γCH out-of-plane vibrations at 850–700 cmꢀ1) is almost parallel to the silver surface, while the β-phenyl group is in tilted po-
sition depending on the type and the position of substituent(s) showing strong SERS enhanced bands of νCC + βCH (in-plane
mode) at 1075 cmꢀ1, νCC (ring breathing mode, in-plane) at 1000 cmꢀ1 and γCCC (out-of-plane mode) around 401 cmꢀ1. The
other class of propenoic acids (cinnamic acid, (E)-2-phenyl-3-(4-methoxyphenyl)propenoic acid) has shown weak electromagnetic
(EM) enhancement (C¼C bands is enhanced in cinnamic acid). In this case no significant carboxyl enhancement was observed,
so we suggest that adsorbed species lie parallel to the surface. The two types of adsorption can be related to the dissociation
ability of the carboxylic group. In the first case the carboxylic H dissociates, while in the second case it does not, as indicated
also by the characteristic νC¼O band at 1686 cmꢀ1 in the FT-Raman spectra of methanolic solutions. Copyright © 2015 John
Wiley & Sons, Ltd.
Keywords: SERS; 4-hydroxy benzoic acid; phenylpropenoic acids; adsorption geometry; enantioselective hydrogenation
be reactivated for further use.[22] Optically pure carboxylic acids
Introduction
and their substituted derivatives are important chiral building
blocks.[23] A possible way to synthesize them is the asymmetric hy-
Analogously to the spectra obtained by other vibration techniques,
the surface enhanced Raman spectrum can be regarded as a finger-
print of the studied compound.[1,2] More, information can also be
obtained about the orientation of adsorbed molecules,[3–7] the
changes in their chemical bonds[8,9] or the formation of new chem-
ical bonds between the adsorbed molecule and the metal
surface.[10–12] SERS can be applied on roughened metal surfaces
and on metal colloids. Ag, Au and Cu colloids show not only en-
hanced Raman scattering but also catalytic activity; therefore, they
are applicable for in situ investigation of catalytic reactions allowing
the detection of both, surface adsorbed reagents and products. De-
spite the huge number of SERS studies in literature, there are rela-
tively few reports in relation to heterogeneous catalytic
reactions.[13–20]
The enantiomeric pairs of biologically active compounds may
have different pharmacological effects;[21] consequently, the pro-
duction of enantiopure materials is essential in pharmaceutical, ag-
rochemical and cosmetic industry. Catalytic asymmetric processes
are convenient methods to synthesize optically pure chiral com-
pounds. The development of a heterogeneous catalytic process
would be extremely beneficial, because the heterogeneous cata-
lysts can be separated easily from the reaction mixture and can
drogenation of the appropriate prochiral unsaturated carboxylic
acids, a reaction which can be successfully carried out over sup-
ported Pd catalyst modified with cinchona alkaloid chiral
templates.[24,25] Diphenylpropenoic acids (α-phenylcinnamic acid
derivatives) are frequently used as starting materials. Substituents
on both aromatic rings of α-phenylcinnamic acid have been found
to influence significantly on the enantiomeric excess (ee (%)=
100 × ([S]ꢀ [R])/ [S] + [R]; where [S] and [R] are the concentration
*
Correspondence to: Emília Tálas, Institute of Materials and Environmental
Chemistry, Research Centre for Natural Sciences, Hungarian Academy of
Sciences, H-1519 Budapest, P.O. Box 286, Hungary.
E-mail: talas.emilia@ttk.mta.hu
a Department of Inorganic and Analytical Chemistry, Budapest University of
Technology and Economics, Szt. Gellért tér 4, Budapest H-1111, Hungary
b Institute of Materials and Environmental Chemistry, Research Centre for Natural
Sciences, Hungarian Academy of Sciences, Magyar Tudósok körútja 2, Budapest
H-1117, Hungary
c
MTA-SZTE Stereochemistry Research Group, Dóm tér 8, Szeged H-6720, Hungary
J. Raman Spectrosc. 2015, 46, 1102–1109
Copyright © 2015 John Wiley & Sons, Ltd.

SERS behavior of substituted propenoic acids
of the saturated acid enantiomers) and reaction rate.[26–29] Al-
though the geometry and strength of the adsorption of these com-
pounds are supposed to play an important role in the asymmetric
reaction, there are only a few studies on this topic.[30] SERS studies
of the chiral templates and the prochiral molecules can provide
important new information about the heterogeneous catalytic
asymmetric hydrogenations. SERS behavior of cinchonidine chiral
template on polycrystalline platinum prepared by electrodeposi-
tion of ultrathin platinum films onto roughened gold surface has
already been reported,[4,5,9] but there are no SERS data on the
hydrogenated molecules. To the best of our knowledge, this is
the first SERS study on phenylpropenoic acids.
Our aim was to get information about the orientation of the
adsorbed molecules relative to the silver surface of (E)-2,3
diphenylpropenoic acids (labeled below as Acid1–5) and to find
correlation between the adsorbed structure and catalytic perfor-
mance. Atropic acid (Acid6), cinnamic acid (Acid7), i.e. propenoic
acids with only one phenyl group, and reference material 4-hydroxy
benzoic acid (HBA) were also studied. Carboxylic acids investigated
with SERS in this study and their abbreviations are shown in Fig. 1.
Ag nanoparticles were prepared by using Lee and Meisel’s
method[35] often used in SERS investigations to synthesize silver
substrates.[36–38] Briefly, 0.09 g AgNO3 was dissolved in 500 ml[3]
double distilled water, the solution was boiled and 10 ml 1%
trisodium–citrate aqueous solution was added dropwise into the
boiling solution accompanied by vigorous stirring. Boiling was con-
tinued for 10 more minutes. Finally, a greenish slightly grey solution
was received. The silver sol was used after a week of ageing time.
The silver colloid solution was characterized by UV–Visible spectros-
copy and by transmission electron microscopy (TEM). UV–Visible
absorption spectra of silver colloids were recorded in the
800–300 nm region with a Unicam UV 500 spectrophotometer
using cells with 1 mm path length at room temperature. TEM stud-
ies of the silver colloid were carried out in a FEI Morgagni 268D type
TEM (100 kV accelerating voltage, W-filament).
Spectroscopic measurements of carboxylic acids
Raman and SERS spectra were recorded with the Bio-Rad (Digilab)
dedicated FT-Raman spectrometer equipped with a Spectra-
Physics Nd-YAG-laser (1064 nm) and high sensitivity liquid-N2
cooled Ge detector. The laser power used was about 400 mW at
the samples. The resolution of the Raman instrument was ca.
4 cmꢀ1, and a backscattered geometry was used. For each spec-
trum 256 individual spectra were averaged.
Experimental
Materials
Atropic acid was prepared as described previously[31]. NaNO3
(Reanal), HBA (Merck) and cinnamic acid (Sigma-Aldrich) were used
as received. The substituted 2,3-diphenylpropenoic acids were pre-
pared by Perkin condensation according to the Feiser method[32–34]
using the corresponding aromatic aldehydes and arylacetic acids
purchased from Fluka and Aldrich.[27,28]
Raman spectra of the carboxylic acids were recorded in both,
solid state and 1–4 w% methanol solution. For SERS measurements
10ꢀ3 M methanol solution of the carboxylic acids was used, and fi-
nal concentration of carboxylic acids in colloidal solution was
2.4 × 10ꢀ4 M. In case of HBA the aqueous solution was also studied.
For the SERS sample preparation 200 μl solution of carboxylic acid
Figure 1. Structures and the abbreviations of the carboxylic acids investigated by SERS.
J. Raman Spectrosc. 2015, 46, 1102–1109
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F. Tamás et al.
was added to 600 μl silver colloid, then 40 μl of water solution of
NaNO3 (5× 10ꢀ1 M) was added into the mixture, similarly to the
method of Sanchez et al.[10] SERS spectra were recorded after
5 min standing time.
Computational method
We have performed a goal-oriented computer simulation to esti-
mate equilibrium geometries, proton affinities and partial charges
of our probe molecules using the Spartan’06 package[39] for ab
initio (density functional (DFT)) and molecular mechanics (MM) cal-
culations. In detail, the probe molecules were relaxed using MM
(MMFF94 force field) to pre-equilibrate, and thereafter the more re-
liable theoretical level B3LYP/6-31 +G(d,p) calculation was per-
formed to locate more accurate equilibrium geometries and to
estimate proton affinities and partial charges. Moreover, an addi-
tional frequency calculation on each molecular system has con-
firmed that the optimized geometries correspond to minima on
the potential energy surface, not reported here for the sake of brev-
ity. Proton affinities have served a comparison among our mole-
cules in relation to the subject of this work, particularly the
reaction R-COOꢀ + H+ → R-COOH, wherein the negative (exotherm)
value means that the anion attracts the proton. Ground state of the
two relaxed entities (anion and acid) was calculated, as well as be-
cause this calculation has served an approximate ranking only,
there were no additional calculations like entropy, thermodynamics
or zero point energy estimations, only energy calculation.
Results and discussion
The feasibility of SERS measurements
Adding the solution of probe molecules to the silver colloid did not
induce the aggregation of the Ag nanoparticles and SERS spectra
could not be recorded. Well-detectable SERS spectra were obtained
only by adding NaNO3 solution which partially aggregated the silver
sol (scattering at 220 cmꢀ1). This observation is in accord with that of
Sanchez et al.[10] on caffeic acid and other hydroxyl-carboxylic acids.
UV–Visible spectra of silver colloid with and without NaNO3 solution
are depicted in Fig. 2A (line b and a, respectively). The decrease of the
peak about 430 nm parallel with the increase of new shoulder at
720 nm indicated the partial aggregation of the colloid. TEM images
(Fig. 2B–C) have confirmed the presence of partial aggregation.
Figure 2. Influence of the addition of NaNO3 solution to the silver colloid.
A: UV–Visible spectra of silver colloid. a—in the absence of NaNO3; b—in
the presence of NaNO3. B: TEM image of silver colloid in the absence of
NaNO3. C: TEM image of silver colloid in the presence of NaNO3.
is expected around 1600 cmꢀ1 which overlaps with the aromatic
ring νCC vibrations and is usually weak in the Raman spectrum. A
weak feature near 1250cmꢀ1 can be ascribed to the minor fraction
of monodentate ―C(¼O)―O―Ag coordination. The characteristic
aromatic ring vibrations of AgHBA aq at 1607 (strong), 857 (strong),
~630 (weak) and ~400 (weak) cmꢀ1 and their relative intensities
are very similar to those obtained for HBA solid Raman spectrum
(1611, 841, 639 and 390 cmꢀ1, respectively). Consequently, there
are no silver-aromatic ring interactions in this case.
However, big differences can be observed in the SERS spectrum
of water (AgHBA aq) and methanol (AgHBA met) solutions. The
strong SERS band at 1232 cmꢀ1 can be interpreted as monodentate
carboxylate group: ―C(¼O)―O―Ag.[43] A weak feature near
1370 cmꢀ1 indicates that a minor fraction of HBA may coordinate
to the silver surface as bidentate ligand. The slightly shifted and en-
hanced bands recorded at 1585, 806, 632 and 419 cmꢀ1 are
assigned to ring vibrations as ring stretching, νCC (8a), out-of-plane
CH deformation, γCH (11), in-plane ring deformation, δCCC (6b) and
out-of-plane ring deformation, γCCC (16a), respectively. (These
Raman spectroscopic behavior of the 4-hydroxy-benzoic acid
reference material
Figure 3 shows the Raman spectrum of solid HBA, HBA in methanol
solution, as well as SERS spectra of aqueous and methanol solution
of HBA nominated as HBA solid, HBA met, AgHBA aq, AgHBA met,
respectively. Aqueous solution of hydroxyl-benzoic acids is often
used in SERS studies,[40] but here, the methanol as a solvent was
also chosen for sample preparation, because the solubility of the
phenylpropenoic acids in water is much lower than in alcohols.
In accord with the literature data,[41,42] no significant wavenum-
ber shift in the SERS spectrum of the AgHBA aq could be observed
comparing to the Raman spectrum of HBA solid. This fact indicates
the presence of π-bonding between the benzene ring and the Ag
surface. The appearance of a well-developed νsCOOꢀ band at
1378 cmꢀ1 in the SERS spectrum of AgHBA aq shows that the ma-
jority of molecules is attached to the Ag surface through the
―COOꢀ as bidentate ligand. The antisymmetric ―COOꢀ stretch
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SERS behavior of substituted propenoic acids
Figure 3. Raman spectroscopic behavior of 4-hydroxy benzoic acid (HBA) under different conditions. HBA solid: FT-Raman of HBA in solid state; HBA met: FT-
Raman of HBA in methanol solution; AgHBA met: SERS spectrum of HBA dissolved in methanol on Ag colloid; AgHBA aq: SERS spectrum of HBA dissolved in
water on Ag colloid.
notations for benzene ring vibrations are taken from Ref.[44,45]). Be-
cause the umbrella mode of CH deformation (11) is downshifted to
806 cmꢀ1 and the ring skeletal deformations (δCCC and γCCC) are
well enhanced, it can be concluded that the benzene ring is also co-
ordinated to the Ag surface and positioned close to parallel orienta-
tion. This can be described by a geometry wherein the carboxyl is
rotated out from the molecular plane. Furthermore, the strong co-
ordinative binding between the carboxyl group oxygen and the sil-
ver surface involves charge transfer (CT) which is part of the surface
enhancement mechanism.[42]
first series of propenoic acids are Acid1–2 and Acid4–6 (series I). Ra-
man and SERS spectra of these probe molecules are presented in
Fig. 4, while the SERS band assignments are summarized in Table 1.
Two bands (around 1394 cmꢀ1 and at 951 cmꢀ1) exhibit strong
enhancement in the SERS spectra. The former band can be
assigned to symmetric ―COOꢀ stretching of bidentate carbonyl
group while the lower wavenumber band at 951 cmꢀ1 is the en-
hanced C―C stretching mode of C―COOꢀ group. This band is
missing from the SERS spectrum of AgHBA (Fig. 3) when the carbox-
ylic group is directly bonded to the aromatic ring.
The stronger relative intensity of the 951 cmꢀ1 band with respect
to the νsCOOꢀ indicates stronger adsorption of probe molecule to
the Ag surface. Based on the SERS spectra, the following order of
relative intensity can be established: Acid6 > Acid2>
Acid1> Acid5 > Acid4. The lower acidity of Acid4 as compared to
Raman spectroscopic behavior of unsaturated carboxylic acids
Based on their SERS behavior, two classes of the studied
phenylpropenoic acids were distinguished. The members of the
Figure 4. Raman spectroscopic behavior of Acid1–2 and Acid4–6 (Series I). A: Raman spectra in solid form; B: Raman spectra in methanol solution; C: SERS
spectra of acids dissolved in methanol on a Ag colloid.
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F. Tamás et al.
Table 1. Band assignment for the SERS spectra of HBA, series I and series 2
HBA met
Acid1
Acid2
Acid4
Acid5
Acid6
Acid3
Acid7
Band assignment
1693 w
νCOOH (νC¼O)
1631 s
1632 s
1623 w
1602 s
1624 s
1624 w
1603 m
1634 w
νC¼C (8a)
1585 m
1596 vs
1597 s
1598 vs
1588 s
1599 w
νCC (ring) (8b)
νCC (ring) (19a)
1473 m, br
1398 s, br
1391 vs, br
1394 m, br
1393 m, br
1398 vs
νsCOO―Ag (bidentate)
δCH (3)
1262 w
1232 s
νCOO―Ag (monodentate)
νCCX (7a)
βCH (15)
1213 w
1182 w
1075 s
1000 s
951 m-s
928 m
830 w
1216 w
1170 w
1037 vs
1211 m
1183 w
1076 m
1000 s, sh
951 m
1216 w
1176 w
1081 vs
1004 s, sh
951 s
1175 s
1176 m
1077 m
νCCX (18a)
1003 w, sh
Ph ring (ring breathing mode, 1)
νC―COO (νC―C)
γCH (17a)
γCH (11)
γCH (11)
951 s-m
923 m
839 w
951 w-m
810 m
835 w
779 w
734 w
806 m
810 w
795 w
800 w, br
746 m
738 w
763 w
δCCX (12)
632 vs
644 s, br
δCCC (6a)
418 vvs
401 vs
403 vs
403 w-m
401 vs
γCCC (16a)
Notation of benzene ring vibrations are taken from Ref.[44] and Ref.[45]
the Acid5 is in line with this order. Acid6 has the highest adsorption
strength: it contains only one aromatic ring without any methoxy
substituents on the phenyl ring. The adsorption strength of Acid2
and Acid1 might be determined by the steric effect of the methoxy
substituent in orto position on the α-phenyl moiety. Accordingly,
the latter substituent must have a decisive effect on the adsorption
strength through the carboxylic acid group, which could be the or-
igin of its significant influence on the enantioselectivities obtained
in the asymmetric hydrogenations of these acids.
see that no new band appears and the electromagnetic (EM) en-
hancement is weak. Also, a very small EM enhancement can be ob-
served in case of Acid7. The ―COOH group is not involved in the
adsorption on the Ag surface; no bands belonging to COOꢀ moiety
are present in the SERS spectra, only the νC¼C (1634cmꢀ1), δCH
(1252 cmꢀ1) vibrations are enhanced.[46] These observations sug-
gest that the C¼C bond is positioned parallel to the Ag surface.
Consequently, the adsorbed probe molecule as a whole lies parallel
to the metal surface.
Based on the experience with reference molecule HBA, we sug-
gest an EM enhancement of the α-phenyl group (except Acid2 con-
taining fluorine), because only a slight shift of wavenumbers
compared to corresponding solid Raman spectra could be ob-
served. The plane of the α-phenyl group must be almost parallel
to the silver surface indicated by the enhanced γCH out-of-plane vi-
brations at 850–700 cmꢀ1. The β-phenyl groups, however, are in-
volved in a strong CT, as indicated by the strongly enhanced
bands around 1075 cmꢀ1 (νCC+ βCH (14), in-plane), 1000 cmꢀ1
(νCC (1, ring breathing mode), in-plane) and 400 cmꢀ1 (γCCC
(17b), out-of-plane), coming from the conjugation between the car-
boxylate, the double bond and the β-phenyl group. The relative in-
tensity of out-of-plane and in-plane bands is indicative for the
tilting position of β-phenyl group. Based on band intensity ratio
of 400 cmꢀ1 versus 1075 cmꢀ1, the following trend of β-phenyl
group tilting from the perpendicular to Ag surface can established:
Acid4 > Acid6> Acid1> Acid5. Interestingly, in case of F-
substituted phenylpropenoic acid (Acid2), because of the existence
of trans-fluorinated ring, these two bands are downshifted to
1037 cmꢀ1 and possible below 350 cmꢀ1, respectively. The en-
hanced vibration bands around 850–700 and 650 cmꢀ1, however,
can be assigned to α-phenyl ring vibrations participating in CT, sim-
ilarly to the reference molecule, AgHBA met.
In order to support the SERS behavior of the two types of
propenoic acids, molecular modeling calculations were performed.
Figure 6 shows the calculated Mulliken charges and equilibrium ge-
ometries of the studied propenoic acids.
The calculated energies and equilibrium geometries of Acid1 and
Acid5 are similar to the literature data[47] (Acid1 is named as 3a and
Acid5 is named as 1a in Fig. 1 of Ref.[47]). Although the protonation
(R-COOH+ H+ → R-C(OH)2 ) of α-substituted cinnamic acids in
+
superacidic media was studied in that work,[47] the neutral mole-
cules can be compared to our case. As shown in Fig. 6, no direct cor-
relation between the types of SERS behavior and the calculated
equilibrium geometries can be observed indicating that the ad-
sorption is responsible for the change in the geometry of these
molecules. Polarization of α- and β-phenyl rings also does not have
influence on the types of SERS behavior and seems independent
from the partial charge on the carbonyl oxygen atoms. On the other
hand, the appearance of the different types of adsorption could be
related to the different dissociation ability of the carboxylic group in
methanol solution (compare Fig. 4B to Fig. 5B). In the first class of
probe molecules (series I) the carboxylic H dissociates (indicated
by the lack of νC¼O band in the Raman spectra) in methanol solu-
tion of the investigated propenoic acids. Conversely, in the second
class of probe molecules (series II) the carboxylic H exists in
un-dissociated form as indicated by the characteristic νC¼O band
at 1686 cmꢀ1 in the Raman spectra of methanol solution. This ob-
servation is supported partly by the computational estimation of
the acidity of phenylpropenoic acids. Calculated proton affinities
(Table 2) show the most negative value in case of Acid7 (series II)
Acid3 and Acid7 belong to the second series (series II) of our
probe molecules; their Raman and SERS spectra are presented in
Fig. 5.
Comparing the SERS spectrum of Acid3 dissolved in methanol
(Fig. 5C) to the Raman spectrum of solid Acid3 (Fig. 5A), one can
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SERS behavior of substituted propenoic acids
Figure 5. Raman spectroscopic behavior of Acid3 and Acid7 (Series II). A: Raman spectra in solid form; B: Raman spectra in methanol solution; C: SERS spectra
of acids dissolved in methanol on a Ag colloid.
Figure 6. Mulliken charges and equilibrium geometries of the studied propenoic acids calculated at B3LYP/6-31 + G(d,p)[39] level of theory.
which is in accord to the spectroscopic results observed. However,
the second most negative value is the case of Acid4 (series I),
while the third negative value is the case of Acid 3 (series II).
The difference in the dissociation ability is obviously caused by
the substituents on the β-phenyl ring combined with the steric
constrain of the α-phenyl moiety. According to the literature,[27]
Acid3 (pKa = 7.23, named as pMeOFFS in Ref.[27]) is weaker acid
than Acid5 (pKa = 7.00, named as a-FFS in Ref.[27]). As can be seen
from Table 2, the polarization of the olefin double bond (C8–C7, i.
e. the difference between the Mulliken charges on C8 and C7 car-
bon atoms) is a peaking negative value in case of Acid7 (series II),
which can be correlated to the observation that C¼C was signifi-
cantly enhanced in this probe molecule (SERS enhancement of
C¼C at 1634 cmꢀ1).
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F. Tamás et al.
Table 2. Calculated proton affinities and partial charges. (See the num-
bering for C7 and C8 on Fig. 1, marked only on Acid1)
as compared to Acid 5, because of this steric effect, which guides
the adsorption of the molecule in presence of the modifier.
No
Proton
affinity,
kcal/mol
Mulliken charges
H8
C7
C8
C8–C7 difference
Summary
Acid1
Acid2
Acid3
Acid4
Acid5
Acid6
Acid7
ꢀ354.0
ꢀ352.5
ꢀ354.3
ꢀ355.4
ꢀ352.6
ꢀ353.6
ꢀ356.2
+0.34
+0.34
+0.32
+0.32
+0.32
+0.32
+0.32
ꢀ0.12
ꢀ0.11
ꢀ0.11
ꢀ0.12
ꢀ0.12
ꢀ0.24
ꢀ0.08
ꢀ0.06
ꢀ0.06
ꢀ0.07
ꢀ0.06
ꢀ0.05
+0.03
ꢀ0.19
0.06
0.05
Based on their SERS behavior, the phenylpropenoic acids selected
for this work were divided into two classes. The members in the first
group (Acid1–2 and 4–6) show a strong charge transfer (CT) effect
and the plane of the α-phenyl group is almost parallel to the silver
surface, while the β-phenyl group is in tilted position depending on
the type and the position of substituent(s). Contrary, a weak electro-
magnetic (EM) enhancement is characteristic for the members of
the second group (Acid3 and 7), where the C¼C bands are en-
hanced and no significant carboxyl enhancement is observed. Con-
sequently, the adsorbed species lie parallel to the surface. The two
types of adsorption can be related to the dissociation ability of the
carboxylic group. Although no direct correlations were obtained
between the adsorption strength and modes derived from the SERS
measurements and the enantiomeric excess measured in the hy-
drogenation of these acids over Pd catalyst modified by
cinchonidine, the results have shown clear multiple effects, which
have influence on the enantiodiscrimination in these asymmetric
heterogeneous processes.
0.04
0.05
0.07
0.27
ꢀ0.11
Heterogeneous catalytic asymmetric hydrogenation of the
unsaturated carboxylic acids in relation to our SERS
experiments
It is necessary to mention that the surface and electronic properties
of silver nanoparticles differ from that of the real catalyst (Pd/
Al2O3). Furthermore, the strongly adsorbing cinchona alkaloid chiral
template and the hydrogenated molecule exist together on the sur-
face in the asymmetric hydrogenation. In spite of these deviations
between the SERS and the real reaction conditions, the order of
strength of ―COOH adsorption by the SERS experiments and the
proton affinity of the five 2,3-diphenylpropenoic acid derivatives cor-
relates well, these results may be useful to interpret the
enantioselective hydrogenations of these acids over Pd catalyst
modified by cinchonidine.[28,48] Although, as shown in Fig. 7, no clear
correlation between the proton affinity or the ―COOH adsorption
strength and the enantiomeric excess (ee) was obtained, the lower
acidity of Acid3 and Acid4 as compared to Acid 5 could explain the
enantioselectivities obtained in the reactions of these acids. Weaker
acids will result in the formation of carboxylates of stronger basic
character, which are able to interact stronger with the protonated
cinchona alkaloid modifier, resulting an increase in enantiomeric ex-
cess. On the other hand, results obtained in reactions of Acid1 and
Acid2 could be attributed to the steric effect of the methoxy group
in orto position. Although the proton affinity and the adsorption
strength of these acids are the highest, higher ee’s can be obtained
Acknowledgement
The authors would like to express their thanks to Beáta Hermán for
the preparation of substituted diphenylpropenoic acids.
References
[1] L. Kocsis, E. Horváth, J. Kristóf, R. L. Frost, Á. Rédey, J. Mink,
J. Chromatogr. A 1999, 845, 197.
[2] E. Horváth, Gy. Kátai, E. Tyihák, J. Kristóf, Á. Rédey, Chromatographia
2000, 51, 297.
[3] H. I. S. Nogueira, S. M. O. Quintal, Spectrochim. Acta A 2000, 56, 959.
[4] W. Chu, R. J. LeBlanc, C. T. Williams, Catal. Commun. 2002, 3, 547.
[5] J. LeBlanc, W. Chu, C. T. Williams, J. Mol. Catal. A: Chem. 2004, 212, 277.
[6] S. Thomas, S. Venkateswaran, S. Kapoor, R. D’Cunha, T. Mukherjee,
Spectrochim. Acta A 2004, 60, 25.
[7] N. Huynh, S. E. Bottle, T. Notoya, D. P. Schweinsberg, Corros. Sci. 2002;
44, 2583.
[8] B. Pergolese, M. Muniz-Miranda, A. Bigotto, Vib. Spectrosc. 2008; 48, 202.
[9] R. J. LeBlanc, C. T. Williams, J. Mol. Catal. A: Chem. 2004; 220, 207.
[10] S. Sanchez-Cortes, J. V. Garcia-Ramos, J. Colloid Interface Sci. 2000, 231,
98.
[11] A. Kudelski, Surf. Sci. 2009, 603, 1328.
[12] B. Pergolese, M. Muniz-Miranda, A. Bigotto, J. Phys. Chem. B 2004,
108, 5698.
[13] M. Muniz-Miranda, B. Pergolese, A. Bigotto, Chem. Phys. Lett. 2006,
423, 35.
[14] B. Pergolese, M. Muniz-Miranda, A. Bigotto, Vib. Spectrosc. 2008, 48, 107.
[15] B. Pergolese, M. Muniz-Miranda, A. Bigotto, Chem. Phys. Lett. 2007,
438, 290.
[16] P. Petersen, W. Krasser, Appl. Surf. Sci. 1996, 103, 91.
[17] C. T. Williams, A. A. Tolia, H. Y. H. Chan, C. G. Takoudis, M. J. Weavery,
J. Catal. 1996, 163, 63.
[18] A. Kudelski, M. Janik-Czachor, M. Varga, M. Dolata, J. Bukowska,
Á. Molnár, A. Szummer, Appl. Catal. A: General. 1999, 181, 123.
[19] M. Janik-Czachor, A. Kudelski, M. Dolata, M. Varga, A. Szummer,
J. Bukowska, Á. Molnár, Mat. Sci. Eng. A 1999, 267, 227.
[20] L. Baia, L. Diamandescu, L. Barbu-Tudoran, A. Peter, G. Melinte,
V. Danciu, M. Baia, J. Alloys Compd. 2011, 509, 2672.
[21] Chirality in Drug Research, (Eds: E. Francotte, W. Lindner), Wiley-VCH,
Weinheim, 2006.
Figure 7. Enantiomeric excesses (ee) obtained in the hydrogenation of the
studied (E)-2,3-diphenylpropenoic acids over Pd catalyst modified by
cinchonidine (for reaction conditions see Ref[28]),at 295 K (dark columns), at
273 K (light columns).
[22] Chiral Reactions in Heterogeneous Catalysis, (Eds: G. Jannes, V. Dubois),
Plenum Press, New York, 1995.
wileyonlinelibrary.com/journal/jrs
Copyright © 2015 John Wiley & Sons, Ltd.
J. Raman Spectrosc. 2015, 46, 1102–1109

SERS behavior of substituted propenoic acids
[23] M. Breuer, K. Ditrich, T. Habicher, B. Hauer, M. Kessler, R. Stürmer,
T. Zelinski, Angew. Chem. Int. Ed. 2004, 43, 788.
[24] H. U. Blaser, C. Malan, B. Pugin, F. Spindler, H. Steiner, M. Studer, Adv.
Synth. Catal. 2003, 345, 103.
[25] H. U. Blaser, B. Pugin, F. Spindler, J. Mol. Catal. A: Chem. 2005, 231, 1.
[26] T. Sugimura, J. Watanabe, T. Okuyama, Y. Nitta, Tetrahedron: Asymmetry
2005, 16, 1573.
[27] Gy. Szöllősi, B. Hermán, K. Felföldi, F. Fülöp, M. Bartók, J. Mol. Catal. A:
Chem. 2008, 290, 54.
[28] Gy. Szöllősi, B. Hermán, K. Felföldi, F. Fülöp, M. Bartók, Adv. Synt. Catal.
2008, 350, 2804.
[35] P. C. Lee, D. Meisel, J. Phys. Chem. 1982, 86, 3391.
[36] S. E. J. Bell, N. M. S. Sirimuthu, J. Phys. Chem. A 2005, 109, 7405.
[37] L. P. Ding, Y. Fang, Appl. Surf. Sci. 2007, 253, 4450.
[38] J. Guichetau, S. Christesen, D. Emge, A. Tripathi, J. Raman Spectrosc.
2010, 41, 1342.
[39] W.J. Hehre, W.W. Huang, P.E. Klunzinger, B.J. Deppmeier, A.J. Driessen,
Spartan’06. Wavefunction, Inc., 18401Von Karman Ave., Suite 370,
Irvine, CA 92612, 2006.
[40] D. Wu, Y. Fang, J. Colloid Interface Sci. 2003, 265, 234.
[41] D. Wu, Y. Fang Y. Spectrochim. Acta Part A 2004, 60, 1845.
[42] J. L. Castro, J. F. Arenas, M. R. López-Ramírez, D. Peláez, J. C. Otero,
J. Colloid Interface Sci. 2009, 332, 130.
[43] J. S. Suh, J. Kim, J. Raman Spectr. 1998, 29, 143.
[44] G. Varsányi, Vibrational Spectra of Benzene Derivates, Akadémiai Kiadó,
Budapest, Hungary, 1969.
[29] Gy. Szöllősi, B. Hermán, E. Szabados, F. Fülöp, M. Bartók, J. Mol. Catal. A:
Chem. 2010, 333, 28.
[30] F. Meemken, N. Maeda, K. Hungerbühler, A. Baiker, ACS Catal. 2012,
2, 464.
[31] Gy. Szöllősi, K. Balázsik, M. Bartók, Appl Catal A: Gen. 2007, 319, 193.
[32] L. Feiser, Experiments in Organic Chemistry, 3rd ed. D.C. Health and Co,
Boston, 1955, p. 182.
[33] K. Felföldi, M. Sutyinszky, N. Nagy, I. Pálinkó, Synth. Commun. 2000,
30, 1543.
[45] N.P.G. Roeges, A Guide to the Complete Interpretation of Infrared Spectra
of Organic Structures, John Wiley & Sons, Chichester, UK, 1994.
[46] J. L. Castro, M. R. Lopez-Ramirez, I. L. Tocon, J. C. Otero, J. Raman Spectr.
2002, 33, 455.
[47] I. Pálinkó, A. Burrichter, G. Rasul, B. Török, G. K. S. Prakash, G. A. Olah,
[34] B. Halton, A. I. Maidmen, D. L. Officer, J. M. Warnes, Aust. J. Chem. 1984,
37, 2119.
J. Chem. Soc. Perkin Trans. 2. 1998, 379.
[48] S. Kristyan, S. Gőbölös, Comput. Theor. Chem. 2011, 975, 24.
J. Raman Spectrosc. 2015, 46, 1102–1109
Copyright © 2015 John Wiley & Sons, Ltd.
wileyonlinelibrary.com/journal/jrs