Evidence of Deprotonation of Aromatic Acids and Amides Adsorbed

Article pubs.acs.org/Langmuir

Evidence of Deprotonation of Aromatic Acids and Amides Adsorbed on Silver Colloids by Surface-Enhanced Raman Scattering José L. Castro, Maria R. Lopez-Ramirez, Juan F. Arenas, Juan Soto, and Juan C. Otero* Department of Physical Chemistry, Faculty of Science, University of Málaga, E-29071-Málaga, Spain S Supporting Information *

ABSTRACT: The surface-enhanced raman scattering (SERS) of benzoic acid/benzamide and salicylic acid/salicylamide on silver colloids show important wavenumber shifts with respect to the Raman spectrum of the band assigned to mode 1;νring when adsorbed on the metal surface (ca. +50 cm−1). In the case of the acids, this shift is originated by the deprotonation of the carboxylic group in agreement with the well-known fact that aromatic acids are adsorbed on silver as carboxylates. However, the main conclusion of this work is that a similar behavior is found for the respective amides that do not behave as acids in water solution. The here studied aromatic amides are adsorbed as azanions on silver nanoparticles even at pH 7 and link to the metal through the nitrogen and oxygen atoms of the ionized carboxamide group. This is a very surprising result given that amides are not significantly ionized even at pH 13−14. The deprotonation of these amides is not determined exclusively by the pH, but it is mainly caused by the strong affinity of the anionic species to the metal. Therefore, the SERS must be cautiously used as a universal pH sensor if the adsorption occurs through the ionizable group. In order to support this conclusion, theoretical DFT force field calculations have been carried out, confirming that deprotonated benzamide and salicylamide interact with the metallic surface.

■

INTRODUCTION There is nowadays a general consensus that the huge intensification of the Raman signal observed in SERS (surface-enhanced raman scattering) is mainly originated by the giant electromagnetic enhancement induced by localized surface plasmon resonances (LSPR) of nanometric noble metal particles or clusters.1−4 Due to its unique capabilities, SERS has been widely used to study the adsorption of many organic molecules on some metal colloids, mainly Ag, Cu, and Au. Its high sensitivity allows an accurate structural study of many molecules at very low bulk concentrations. This property has been used to obtain very detailed information of adsorbates such as the mechanism and geometry of the adsorption or which functional groups are in close contact with the metal. However, and in spite of the elapsed time from the SERS discovery, the detailed interpretation of each particular spectrum remains a challenge, given that different surface selection rules have been proposed in order to account for the selective enhancement of the SERS bands. The most popular rules are derived from the so-called electromagnetic enhancement mechanism (EM) by studying the strengthening or weakening of specific Raman/SERS bands in order to decide the molecular orientation and geometrical configuration of the adsorbed species on the metal surface.5,6 Much more complex selection rules are involved in another mechanism related to resonance effects produced by photoinduced charge transfer (CT) processes involving the metal−adsorbate system.7,8 However, the analysis of the shifts observed between the Raman and SERS wavenumbers is not dependent on the SERS enhancement mechanism, providing very detailed information about the adsorption of molecules on metals.9 This work is just © 2012 American Chemical Society

focused on discussing these observed vibrational wavenumber shifts of aromatic carboxylic acids and amides upon adsorption on silver nanoparticles. The initial aim of this work was to analyze the SERS spectrum of 2-hydroxybenzamide (salicylamide, SM) on silver sols. This compound, as other derivatives of salicylic acid, belongs to the nonsteroidal anti-inflammatory category of drugs (NSAID).10 Salicylamide is not as effective as acetylsalicylic acid, but it is still used as an analgesic, antipyretic, and antirheumatic drug as well as a fungicide,11 and its monosodium salt is also used for cancer immunotherapies.12,13 Difficulties found in the preliminary SERS analysis have forced us to go down in molecular complexity and to start with the analysis of the Raman spectra of simpler molecules such as benzamide (BM) and benzoic (BA) and salicylic (SA) acids. The main conclusion of this work is that the SERS of these systems correspond to the respective deprotonated species at neutral pH values. Although this is a very well-known behavior in the case of aromatic carboxylic acids, amides show no significant deprotonation in water even under strong basic conditions. This conclusion has been obtained thanks to the detailed molecular information provided by SERS together with the very favorable features of the band assigned to vibration 1;νring. Special Issue: Colloidal Nanoplasmonics Received: November 29, 2011 Revised: January 20, 2012 Published: January 23, 2012 8926

dx.doi.org/10.1021/la204702w | Langmuir 2012, 28, 8926−8932

Langmuir

■

Article

EXPERIMENTAL SECTION

Samples and Instrumentation. Colloidal silver solutions have been prepared in deionized and triply distilled water according to the method described by Creighton et al.,14 which basically consists of reducing a solution of 10−3 M AgNO3 with an excess of NaBH4. In this way, one volume of 10−3 M AgNO3 is added dropwise to three volumes of 2 × 10−3 M NaBH4 that has been previously cooled to a temperature of between 0 and 5 °C. After stirring the mixture for a while to homogenize it, it is allowed to rest at room temperature for approximately 90 min. The result is a silver colloidal solution of yellow color, with a maximum in its absorption spectrum at 390 nm and that is stable for several months. Adsorbate is then added to the colloid as an aqueous solution to obtain the desired concentration, giving final pH of 5/6 and 7 in the cases of the acids and the amides, respectively. A change in the color of the system, from the initial yellowish to a final blue-greenish, is observed when the adsorbate is added to the colloid. Raman spectra were recorded with a Jobin-Yvon U-1000 double monocromator spectrometer fitted with a cooled Hamamatsu R943-02 photomultiplier, using the 514.5 nm exciting line from a Spectra Physics 2020 Ar+ gas laser. A constant slit width was used that allowed a spectral resolution of 4 cm−1. The laser power reaching the sample was always 60 mW. In the case of liquid samples, a quartz cell with an 1 cm path length was used, while a glass capillary was used for the microcrystalline solids. The measurement of the wavenumbers was done with the help of the laser plasma lines as standards, whereby an accuracy of ±1 cm−1 was obtained under the operating conditions employed. Computational Details. All calculations were performed with the GAUSSIAN 09 program package.15 The optimized geometry and force field of all the molecules were calculated at B3LYP/LanL2DZ level of theory. The density functional calculation using Becke’s three parameter hybrid functional combined with the Lee−Yang−Parr correlation function (B3LYP)16 was chosen among the DFT methods due to its good performance in molecular structure and force field determinations.17 The geometrical optimizations of the studied systems yield planar structures with Cs, C2h, or C2v symmetries shown in Figures 4 and 7.

Figure 1. (a) Raman spectrum of solid benzamide and (b) SERS spectrum of a 5 × 10−4 M solution on silver colloid at pH 7.

Table 1. Vibrational Wavenumbers (cm−1) and Proposed Assignments for the Raman and SERS Spectra of Benzamidea Raman solid

■

1692 1640 1610 1576 1504 1458 1416

RESULTS AND DISCUSSION Raman and SERS Spectra of Benzamide (BM) and Benzoic Acid (BA). Figure 1a and b shows the Raman spectrum of solid BM and the SERS of a 5 × 10−4 M solution of this compound on silver sol at pH 7, respectively. It is not possible to obtain the Raman of the aqueous solution due to the low solubility of BM in water, even under basic conditions. Table 1 summarizes the proposed assignments for both spectra. There are no complete studies on the SERS of this molecule, but the bands recorded at 1004, 1030, 1424, and 1602 cm−1 correspond to the Raman bands observed at 1008, 1032, 1416, and 1610 cm−1 in the Raman of the solid and they are assigned to the vibrational modes 12;δring, 18a;δ(CH), amide III, and 8a;νring, respectively.18,19 Additionally, two significant SERS bands recorded at 1209 and 830 cm−1 undergo important blueshifts in SERS. The broad SERS band recorded at 1209 cm−1 seems to keep up a correspondence to the Raman band seen at 1154 cm−1. This band is probably equivalent to that recorded at 1217 cm−1 in the SERS of pyrazinamide, which was assigned to ν(C−CONH2) stretching mode.20 The SERS band at 830 cm−1 corresponds with the Raman band observed at 777 cm−1, and it is assigned to mode 1;νring, a ring stretching fundamental including contributions of the ν(C−CONH2) and δ(CCN) internal coordinates.19 The shift undergone by the mode 1;νring in BM looks like that observed in the SERS spectrum of benzoic acid (BA), a molecule studied at length in SERS.21−23 Figure 2 shows the Raman spectra of solid BA (Figure 2a) and the 1 M aqueous

1183 1154 1129 1032 1008 852 810 777 621 591

SERS (5 × 10‑4 M, pH 7)

1602 1498 1452 1424 1396 1308 1209 1133 1030 1004 928

830

459 403

a

assignments amide I amide II 8a; νring 8b; νring 19a; νring 19b; νring amide III δ(CNH) ? 3; δ(CH) 9a; δ(CH) ν(C−CONH2) 9b; δ(CH) 18a; δ(CH) 12; δring 17b; γ(CH) 10a; γ(CH) 11; γ(CH) 1; νring 6b; δring 6a; δring δ(CCN) 16b; γring

ν, stretching; δ, in-plane bending; γ, out-of-plane bending.

solution at pH 14 (Figure 2b), and Figure 2c shows in turn the SERS spectrum of a 10−4 M silver colloid solution at pH 7. The SERS of BA is significantly different from the Raman spectrum of the crystalline solid but it is very similar to the Raman spectrum of benzoate in solution (Figure 2b). This fact 8927


Langmuir

Article

Figure 3. Azanion formation by deprotonation of the tauromeric carboxamide (I) and carboximidic (II) forms of benzamide. Bond delocalization in the ionized benzoic acid (III).

Figure 2. Raman spectra of (a) solid benzoic acid, (b) 1 M aqueous at pH 14, and (c) SERS spectrum of a 10−4 M solution of benzoic on silver colloid at pH 7.

indicates that BA is adsorbed ionized on the surface as the respective benzoate anion. Additionally, the νs(OCO) band recorded at 1390 cm−1 in the SERS spectrum (Figure 2c) is red-shifted and noticeably broadened upon surface adsorption, what confirms that the carboxylate group interacts directly with the metal. In the Raman of solid BA the band recorded at 795 cm−1 (Figure 2a) and assigned to mode 1;νring, undergoes a blue-shift of +49 cm−1 after carboxylic ionization (Figure 2b) or +50 cm−1 upon adsorption on silver, given that it is observed at 845 cm−1 in SERS (Figure 2c). A similar behavior of this mode can be seen in the spectra of BM (Figure 1). A blue-shift of the same order of magnitude (+53 cm−1) is observed for vibration 1 in the SERS spectrum of the amide with respect to the Raman of the solid (Figure 1a and b), suggesting that this molecule is adsorbed also as its azanion on the metal surface, that is, with the carboxamide group partially deprotonated. This possibility was pointed out in the previously reported SERS of pyrazinamide where mode 1 shows a very similar behavior.20 The stability of the deprotonated amide could be explained by the resonant structures proposed in Figure 3 (I and II), allowing for a bond delocalization very similar to that of carboxylate (Figure 3, III).24 The excess of negative charge in the nitrogen and oxygen atoms of the ionized forms would facilitate the bonding with the metal as occurs in carboxylate group. By carrying out theoretical calculations of the geometries and vibrational wavenumbers of BA and BM, we shall be able to predict which fundamentals must shift in each chemical species. Figure 4 shows the optimized geometries of different systems modeling the molecular environment in the solid (as centrosymmetric dimers), in solution (as sodium salt with two water molecules in the case of BA−) and in SERS (as the respective anions forming surface complexes with Ag+) as well

Figure 4. Experimental and B3LYP/LanL2DZ wavenumbers (cm−1) of mode 1;νring calculated for different structures modeling the molecular environment of (a) benzoic acid (BA) and (b) benzamide (BM) in solid phase, aqueous solution and SERS, respectively.

as the experimental and theoretical wavenumbers calculated for mode 1;νring for both benzoic acid and benzamide. The agreement between experimental and theoretical wavenumbers of mode 1;νring of BA is very good in spite of the very simple systems assumed to model the intermolecular interactions in the condensed media we are studying. Nevertheless, the experimental and calculated wavenumbers of mode 1 are very similar in the case of the solid (795 and 792 cm−1, respectively), in the solution at pH 7 (844 and 820 cm−1), and in SERS conditions (845 and 827 cm−1), with the wavenumber shift of this mode being predicted to amount to +35 cm−1 in SERS, in agreement with the observed value of +50 cm−1. Therefore, it can be confirmed that the important shift of mode 1;νring observed in the SERS of BA is due to the ionization of the carboxylic group. As in the case of BA, the agreement between the theoretical and experimental results in the case of the BM is very good. The +60 cm−1 calculated blue-shift of mode 1;νring for the BM−-Ag+ azanion−silver complex with respect to the dimer species agrees with the experimental value (+60 cm−1) and supports the hypothesis of deprotonation of the carboxamide group when this molecule is adsorbed on the metal. Additionally, the optimized geometry of the BM−−Ag + complex shows that both N and O atoms of the ionized amide interact with the metal surface. On the other hand, the ionization originates bond delocalization along the π-structure of the functional group (Figure 3) and a change in the force 8928


Langmuir

Article

Table 2. Vibrational Wavenumbers (cm−1) and Proposed Assignments for the Raman and SERS Spectra of Salicylamidea

constants mainly associated to the C−N, CO, and C− CONH2 bonds. This restructured force field modifies the coupling between these internal coordinates in specific vibrations, being the cause of shifting their respective wavenumbers as is the case of vibration 1. The optimized bond lengths confirm these changes between the isolated molecule and the silver complex, giving rise to a Δr(C−N) of −0.0075 Å, Δr(CO) of +0.0252 Å, and Δr(C−CONH2) of −0.0089 Å (see Figure 3). In agreement with these results, one can see in the Raman spectra of Figure 1 that the ν(C−N) vibration (amide III) undergoes a blue-shift of +8 cm−1 after adsorption (from 1416 to 1424 cm−1); the ν(CO) vibration (amide I) is observed at 1692 cm−1 in Figure 1a but unfortunately is missing in the SERS; and ν(C−CONH2) is recorded at 1154 cm−1 in the solid and at 1209 cm−1 on silver sol. The calculated wavenumbers obtained of the latter vibrational mode are 1180 cm−1 for the dimer species and 1233 cm−1 for the silver complex. Therefore, a blue-shift of +53 cm−1 is predicted in agreement with the experimental value of +55 cm−1. Raman and SERS Spectra of Salicylamide (SM) and Salicylic Acid (SA). SM is very slightly soluble in pure water so preventing the record of its Raman spectrum. Figure 5a and b

Raman solid 1681 1626 1594 1498 1454 1436 1368 1308 1250 1171 1147 1130 1086 1042 946 850 792 750

565 521 455 423 401

Raman aq. sol. (1 M, pH 14)

SERS (5 × 10‑4 M, pH 7) 1642 (sh) 1618

1600 1554 1468

1490 1436 (sh) 1404 1360 1314 1248

1386 1320 1258 1165 1149 1130 1086 1040

1200 1088 1042 950 860

862 741

798 739

611 563

571 445

430 (sh) 393

397

assignments amide I 8a; νring 8b; νring or amide II 19b; νring 19a; νring amide III δ(C−OH) ν(C−O) 14; νring 15; δ(CH) ν(C−CONH2) ρ(NH2) 9b; δ(CH) 18b; δ(CH) 10a; γ(CH) 12; δring 10b; γ(CH) 1; νring 11; γ(CH) δ(OCN) 6a; δring 6b; δring 16a; γring δ(C−CO-NH2) 16b; γring

ν, stretching; δ, in-plane bending; γ, out-of-plane bending; ρ, rocking; sh, shoulder. a

as well as on the force field calculations carried out in the present work and the characteristic wavenumbers of the carboxamide group.31,32 The significant bands we observe in the record of Figure 5a at 750, 1042, 1147, 1250, 1308, 1436, and 1626 cm−1 are assigned to vibrations 1;νring, 18a;δ(CH), ν(C−CONH2) with participation of ν(C−OH), 14;νring, ν(C− OH) in turn coupled with ν(C−CONH2), amide III, and 8a;νring, respectively. The amide I and amide II bands (this latter overlapped with the 8b;νring band) are registered very weakly at 1681 and 1594 cm−1, respectively. The ionization of the ortho-phenolic group (Figure 5b) produces significant changes in the relative intensities and wavenumbers of some Raman bands. Modes 8a, 8b, and 19b undergo important redshifts (−26, −40, and −30 cm−1, respectively) which are in the same direction than those observed when the 2-hydroxybenzoate anion (2-salicylate anion) transforms into 2-oxybenzoate anion.26,27 The amide III vibration is also red-shifted (−50 cm−1),33 whereas the amide I is missing and the vibration ν(C− O) shifts toward the blue (+12 cm−1). Opposite to the observed behavior when carboxylic acids are deprotonated, the ionization of the hydroxyl group does not change noticeably the wavenumber of mode 1 which is observed at 750 and 741 cm−1 in the Raman of the solid and the aqueous solution at pH 14, respectively. This result is reproduced by the force field calculations. Ionization of the hydroxyl group does not affect significantly the wavenumber of mode 1, remaining unchanged or even red-shifted. An illustrative example is the phenol molecule for which identical value of 813 cm−1 is predicted for mode 1 in the neutral and the anionic forms.

Figure 5. Raman spectra of (a) solid salicylamide, (b) 1 M aqueous solution at pH 14, and (c) SERS spectrum of a 5 × 10−4 M solution on silver colloid at pH 7.

shows the respective Raman spectra of solid crystalline SM and of 1 M aqueous solution at pH 14, where the hydroxyl group should be deprotonated (pKa = 8,8925). Figure 5c shows in turn the SERS spectrum of a 5 × 10−4 M silver colloid solution at pH 7. The assignment of the Raman spectrum of salicylamide (Table 2) is based on those previously proposed for BM19 and for other related molecules such as salicylic acid,26,27 pyrocatechol,28 phthalic acid,29 and 2-hydroxybenzonitrile,30 8929


Langmuir

Article

fundamental. The SERS of Figure 6d is dominated by the very strong intensity of the band assigned to this vibration 1 recorded at 811 cm−1 and therefore, showing a blue-shift of +37 cm−1 with respect to the neutral molecule. A similar behavior can been seen in the SERS recorded by Goulet and Aroca on silver island films,34 but the spectrum reported by Wang et al. show large discrepancies, including a set of bands of unknown origin.35 Figure 7 shows the experimental and the theoretical wavenumbers of mode 1;νring of SA and SM. The B3LYP/

The comparison between the spectra in Figure 5 seems to indicate that the SERS spectrum of SM could be correlated to either the Raman of neutral amide (Figure 5a) or its oxyanion (Figure 5b). Nevertheless, the comparative study of this SERS with those of the previously analyzed molecules suggests that SM is again adsorbed on silver nanoparticles with the carboxamide group partially deprotonated. The SERS spectrum of SM (Figure 5c) is dominated by three bands at 1618, 1404, and 1248 cm−1 that correspond to vibrations 8a;νring, amide III, and 14;νring, respectively. Next in order of intensity are a band at 1314 cm−1 assigned to ν(C−OH) with participation of ν(C− CONH2) and two bands at 1042 and 798 cm−1 assigned to 18b;δ(CH) and 1;νring modes, respectively. This last vibration 1 is recorded at 750 cm−1 in the Raman of solid SM and, therefore, shifting +48 cm−1 in SERS in a similar way to that observed in BA and BM; moreover, the absence of the rocking mode ρ(NH2) in the SERS of BM or SM, that should be registered at some 1130 cm−1,18,30 seems also to confirm that the carboxamide group is deprotonated. The behavior of SM in SERS looks like that of salicylic acid (SA) (Figure 6), which is adsorbed on silver nanoparticles as

Figure 7. Experimental and B3LYP/LanL2DZ wavenumbers (cm−1) of mode 1;νring calculated for different structures modeling the molecular environment of (a) salicylic (SA) acid and (b) salicylamide (SM) in solid phase, aqueous solution and SERS, respectively.

LanL2DZ calculated values correspond to different molecular models whose optimized geometries are also shown in Figure 7: the SA or SM dimers related to the solid phase; the respective anions (and dianion in the case of salicylate SA2−) linked to two water molecules and a sodium cation modeling a neutral water (or basic at pH 14) solution, and finally, the SA−Ag+ or BM−-Ag+ surface complexes related to the SERS environment. The agreement between experimental and theoretical wavenumbers is very good; the SERS shifts predicted for this mode 1;νring with respect to the solid have been estimated to be +41 and +61 cm−1 for SA and SM, respectively while the experimental shifts amount to +37 and +48 cm−1. It is to be stressed that the calculated wavenumber for this fundamental in the complexes formed between the two neutral amides and silver, BM0−Ag+ and SM0−Ag+, or the dianion of salicylamide with two silver cations SM2‑−Ag22+, do not reproduce the observed blue-shifts. The respective values are 774, 736, or 755 cm−1, what discards the participation of these species in SERS. Therefore, the important shift toward the blue observed in vibration 1;νring of the studied amides when absorbed on silver nanoparticles is a conclusive proof of the adsorption of the respective azanions. This behavior has been also observed in the case of pyrazinamide20 and could be reasonably generalized to all the aromatic molecules that possess the carboxamide group. Finally, in previous works, we have proposed a propensity rule to detect the involvement of the CT enhancement mechanism in a particular SERS based on the relative intensity of the band assigned to mode 8a. We have concluded that the main feature of the SERS-CT spectra of six-membered aromatic

Figure 6. Raman spectra of (a) solid salicylic acid, 1 M aqueous solution at (b) pH 7 and (c) pH 14, and (d) SERS spectrum of a 10−3 M solution on silver colloid at pH 7.

hydroxybenzoate anion with the ortho-phenolic group not ionized.26 In the Raman of neutral SA (solid or aqueous solution), the mode 1;νring is recorded at ca. 774 cm−1 (Figure 6a and ref 27, respectively); it is up-shifted to 815 cm−1 in the aqueous solution of hydroxybenzoate (Figure 6b) and observed at 810 cm−1 in the Raman of the solution at pH 14 which should contain the oxybenzoate anion (Figure 6c). Once again, the ionization of the carboxylic group shifts mode 1;νring some +41 cm−1, whereas the subsequent deprotonation of the orthophenolic does not change noticeably the wavenumber of this 8930


Langmuir

Article

(3) Aroca, R. Surface-enhanced vibrational spectroscopy; Wiley: Chichester, 2006. (4) Le Ru, E. C.; Etchegoin, P. G. Principles of surface enhanced Raman spectroscopy and related plasmonic effects; Elsevier: Amsterdam, 2009. (5) Moskovits, M.; DiLella, D. P.; Mainard, K. J. Langmuir 1988, 4, 67−76. (6) Creighton, J. A. The selection rules for surface-enhanced Raman spectroscopy. In Spectroscopy of surfaces; Clark, R. J. H. Hester, R. E., Eds.; Wiley: Chichester, 1988; pp 37−89. (7) Avila, F.; Fernandez, D. J.; Arenas, J. F.; Otero, J. C.; Soto, J. Chem. Commun. 2011, 47, 4210−4212. (8) Otto, A.; Mrozek, I.; Grabhorn, H.; Akemann, W. J. Phys.: Condens. Matter 1992, 4, 1143−1212. (9) Soto, J.; Fernández, D. J.; Centeno, S. P.; López Tocón, I.; Otero, J. C. Langmuir 2002, 18, 3100−3104. (10) Murray, W. J. J. Clin. Pharmacol. 1967, 7, 150−155. (11) Krogsgaard-Lars, P. A Textbook of Drug Design and Development; T. Liljefors: Amsterdam, 1996. (12) Ohno, T. U.S. Pat. Appl. Publ. 2004, 28, 663. (13) Ohno, T. U.S. Pat. Appl. Publ. 2005, 28, 901. (14) Creighton, J. A.; Blatchford, C. G.; Albrecht, M. G. J. Chem. Soc., Faraday Trans. II 1979, 75, 790−798. (15) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Scalmani, G.; Barone, V.; Mennucci, B.; Petersson, G. A.; Nakatsuji, H.; Caricato, M.; Li, X.; Hratchian, H. P.; Izmaylov, A. F.; Bloino, J.; Zheng, G.; Sonnenberg, J. L.; Hada, M.; Ehara, M.; Toyota, K.; Fukuda, R.; Hasegawa, J.; Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai, H.; Vreven, T.; Montgomery, Jr., J. A.; Peralta, J. E.; Ogliaro, F.; Bearpark, M.; Heyd, J. J.; Brothers, E.; Kudin, K. N.; Staroverov, V. N.; Kobayashi, R.; Normand, J.; Raghavachari, K.; Rendell, A.; Burant, J. C.; Iyengar, S. S.; Tomasi, J.; Cossi, M.; Rega, N.; Millam, N. J.; Klene, M.; Knox, J. E.; Cross, J. B.; Bakken, V.; Adamo, C.; Jaramillo, J.; Gomperts, R.; Stratmann, R. E.; Yazyev, O.; Austin, A. J.; Cammi, R.; Pomelli, C.; Ochterski, J. W.; Martin, R. L.; Morokuma, K.; Zakrzewski, V. G.; Voth, G. A.; Salvador, P.; Dannenberg, J. J.; Dapprich, S.; Daniels, A. D.; Farkas, Ö .; Foresman, J. B.; Ortiz, J. V.; Cioslowski, J.; Fox, D. J. Gaussian 09, Revision A.1; Gaussian, Inc.: Wallingford CT, 2009. (16) Lee, C.; Yang, W.; Parr, R. G. Phys. Rev. B 1988, 36, 785−789. (17) Bauschlicher, C. W. Chem. Phys. Lett. 1995, 246, 40−44. (18) Parellada, R.; Arenas, J. F. An. Quim. 1970, 66, 365−374. (19) Palomar, J.; De Paz, J. L. G.; Catalán, J. Chem. Phys. 1999, 246, 167−208. (20) Arenas, J. F.; Castro, J. L.; Otero, J. C.; Marcos, J. I. J. Raman Spectrosc. 1992, 23, 249−252. (21) Pagannone, M.; Fornari, B.; Mattei, G. Spectrochim. Acta, Part A 1987, 43, 621−625. (22) Kwon, Y. J.; Son, D. H.; Ahn, S. J.; Kim, M. S.; Kim, K. J. J. Phys. Chem. 1994, 98, 8481−8487. (23) Zhao, X.; Fang, Y. J. Mol. Struct. 2006, 789, 157−161. (24) Chiu, F. C. K.; Lo, C. M. Y. J. Am. Soc. Mass Spectrom. 2000, 11, 1061−1064. (25) Agreen, A. Acta Chem. Scand. 1955, 9, 49−56. (26) Castro, J. L.; Arenas, J. F.; Lopez-Ramirez, M. R.; Peláez, D.; Otero, J. C. J. Colloid Interface Sci. 2009, 332, 130−135. (27) Humbert, B.; Alnot, M.; Quilés, F. Spectrochim. Acta, Part A 1998, 54, 465−476. (28) Lewandowsky, W.; Baranska, H. Appl. Spectrosc. 1987, 41, 976− 980. (29) Wilson, H. W. Spectrochim. Acta 1974, 30A, 2141−2152. (30) Joo, S. W.; Han, S. W.; Han, H. S.; Kim, K. J. Raman Spectrosc. 2000, 31, 145−150. (31) Roeges, N. P. G. Guide to the Complete Interpretation Of Infrared Spectra of Organic Structures; Wiley: Chichester, 1994. (32) Binev, Y. I.; Georgieva, M. K.; Daskalova, L. I. Spectrochim. Acta, Part A 2004, 60A, 2601−2010. (33) Velcheva, E. A.; Stamboliyska, B. A. J. Mol. Struct. 2008, 875, 264−271.

molecules is the strong relative enhancement of the SERS band corresponding to this mode 8a recorded at some 1600 cm−1.36−38 In the SERS spectra of BA, SA, and SM, a significant relative enhancement of the 8a vibration with respect to the Raman can be seen, what points out the participation of this enhancement mechanism. On the contrary, this cannot be drawn in the case of BM given that the 8a band is the strongest one in both the Raman and the SERS spectra, what prevents to derive any conclusion.

■

CONCLUSIONS By combining the analysis of the Raman and SERS spectra of benzoic and salicylic acids, benzamide, and salicylamide with the results of the quantum mechanical calculations, we are able to establish that these aromatic amides adsorb ionized on the surface of the silver colloid through their respective azanions and that this behavior is probably a general one for other aromatic amides. This conclusion is derived from the noticeable shifts toward the blue that are observed in the characteristic band corresponding to vibration 1;νring upon adsorption and implies that no direct relationship between ionization state of the adsorbate and pH can be established if the ionizable group is directly involved in the adsorption, as is the case of the here studied carboxylic acids and amides. Moreover, this conclusion precludes the universal use of SERS spectroscopy as pH sensor but is not relevant in molecular systems where different functional groups are involved in the ionization and adsorption processes, respectively. This is just the case for the bifunctional p-mercaptobenzoic acid used by the Halas group to demonstrate the ability of SERS to be used as an optical nanoscale pH-meter.39 Finally, it has to be stressed that deprotonated amides can be only observed under very extreme experimental conditions,24 and therefore, this kind of result can be useful to improve the understanding of adsorption processes relevant in electrochemistry or heterogeneous catalysis and, of course, to correctly analyze the SERS spectra.

■

ASSOCIATED CONTENT

S Supporting Information *

Four tables with the B3LYP/LanL2DZ calculated wavenumbers for different structures of BA, BM, SA, and SM modeling the molecular environment in solid phase, aqueous solution, and SERS. This material is available free of charge via the Internet at http://pubs.acs.org.

■

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS This research has been supported by the Spanish MICINN (Project Number CTQ2009-08549) and Junta de Andaluciá (Project Numbers FQM-5156 and FQM-6778). The authors thank SCAI and Rafael Larrosa of University of Málaga for computational facilities and support.

■

REFERENCES

(1) Willets, K. A.; Van Duyne, R. P. Annu. Rev. Phys. Chem. 2007, 58, 267−297. (2) Moskovits, M. Rev. Mod. Phys. 1985, 57, 783−826. 8931


Langmuir

Article

(34) Goulet, P. J. G.; Aroca, R. Can. J. Chem. 2004, 82, 987−997. (35) Wang, Y.; Li, Y.-S.; Zhang, Z.; An, D. Spectrochim. Acta, Part A 2003, 59A, 589−594. (36) Centeno, S. P.; López-Tocón, I.; Arenas, J. F.; Soto, J.; Otero, J. C. J. Phys. Chem. B 2006, 110, 14916−14922. (37) Lopez-Ramirez, M. R.; Ruano, C .; Castro, J. L.; Arenas, J. F.; Soto, J.; Otero, J. C. J. Phys. Chem. C 2010, 114, 7666−7672. (38) Arenas, J. F.; López-Tocón, I.; Castro, J. L.; Centeno, S. P.; López-Ramírez, M. R.; Otero, J. C. J. Raman Spectrosc. 2005, 36, 515− 521. (39) Bishnoi, S.; Levin, C. S.; Gheith, M.; Johnson, B.; Rozell, C.; Johnson, D. H.; Halas, N. J. Nano Lett. 2006, 6, 1687−1692.

8932


Evidence of Deprotonation of Aromatic Acids and Amides Adsorbed

Recommend Documents