Vibrational and Theoretical Studies of the Structure and Adsorption

Article pubs.acs.org/JPCA

Vibrational and Theoretical Studies of the Structure and Adsorption Mode of m‑Nitrophenyl α‑Guanidinomethylphosphonic Acid Analogues on Silver Surfaces Edyta Proniewicz,*,† Ewa Pięta,† Andrzej Kudelski,‡ Natalia Piergies,† Dominika Skołuba,† Younkyoo Kim,§ and Leonard M. Proniewicz† †

Faculty of Chemistry, Jagiellonian University, ul. Ingardena 3, 30-060 Krakow, Poland Faculty of Chemistry, University of Warsaw, ul. L. Pasteura 1, 02-093 Warsaw, Poland § Department of Chemistry, Hankuk University of Foreign Studies, Yongin, Kyunggi-Do, 449-791, Korea ‡

S Supporting Information *

ABSTRACT: This work presents Fourier transform Raman (FT-Raman), Fourier transform absorption infrared (FT-IR), and surface-enhanced Raman scattering (SERS) spectroscopic investigations of three m-nitrophenyl α-guanidinomethylphonic acids, including m-NO2PhG(cHex)P, m-NO2PhG(Morf)P, and m-NO2PhG(An)P, adsorbed onto colloidal and roughened silver surfaces. The SERS spectra were deconvoluted to determine the overlapped bands from which the specific molecular orientation can be deducted. The vibrational wavenumbers are calculated through density functional theory (DFT) at the B3LYP/6-31+ +G** level with the Gaussian 03, Raint, GaussSum 0.8, and GAR2PED software packages. The experimental and calculated vibrational bands are compared to those from SERS for the investigated compounds adsorbed on colloidal and roughened silver surfaces. The geometry of these molecules on the SERS-active silver surfaces is deduced from the observed changes in both the intensity and width of the Raman bands in the spectra of the bound species relative to the free species.

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INTRODUCTION Over the last few decades, there has been increasing interest in the applications of phosphorus analogues of amino acids composed of at least one CP(O)(OH)2 group because phosphonates are chemically and enzymatically stable,1 are highly water-soluble, and are sparingly soluble in organic solvents.1,2 In addition, the functional groups of these compounds characterize a wide range of applications from medicine to agriculture.3,4 For example, analogues of these compounds have been used as enzyme inhibitors (i.e., cholesterol, angiotensin, HIV protease, phenylalanine ammonia−lyase, and the parasite that causes malaria (Plasmodium falciparum)), antibacterial agents, antibiotics, neuroactive compounds, andmost promisinglyanticancer agents.1,5−8 The analogues of amino acids can penetrate cancer cells much more easily than normal cells.2 Phosphonates are also known to be substrates for the treatment of osteoporosis.1 They are commonly employed to control pests as insecticides, herbicides © XXXX American Chemical Society

(i.e., against cress (Lepidium sativum) and cucumber (Cucumis sativus)), and growth regulators for plants.2,8,9 α-Guanidinophosphonic acids additionally contain the guanidine group,  NHC(NH)NH2. This structural modification is thought to be responsible for neuroactive properties and for fungicidal and herbicidal activities.5,9 Another modification with nitrobenzene (a compound highly soluble in organic solvents, such as alcohol or benzene, and in lipids, but slightly soluble in water) demonstrates interesting properties.10,11 Although many nitro-substituted aromatics are classified as carcinogenic, mutagenic, and toxic (generally, these effects are more severe with the ortho- and para-isomers than with the meta-isomer),12 these compounds are also widely used as pharmaceuticals, food additives, antimicrobial agents, and Received: March 27, 2013 Revised: May 17, 2013

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dx.doi.org/10.1021/jp403045g | J. Phys. Chem. A XXXX, XXX, XXX−XXX

The Journal of Physical Chemistry A

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Table 1. Molecular Structures of the Investigated Compounds

pesticides.13 Consequently, the nitro group is crucial in the action of certain drugs. Although the mechanism of the action of many nitro-substituted drugs is not well-known, these drugs play an essential role in the treatment of many diseases because the nitro group is characterized by strong electron withdrawing ability, which forms electrophilic sites inside the molecule. In interactions in living systems, these electron deficient regions can react with different types of intra- and extracellular biological nucleophiles, such as amino acids, proteins, and enzymes.14 This interaction can occur through various mechanisms, such as nucleophilic addition or displacement, an exchange of electrons involved in reduction and oxidation reactions, or through complexation without formation of a covalent bond. The results can be harmful for the organism as a whole, but often this toxicity is selective and poisons the bacteria or cancer cells. Thus, nitro-substituted aromatics play an essential role in chemotherapy. It should be emphasized that there are many drugs (i.e., antibiotics, antineoplastics, antiparasitics, and tranquilizers), insecticides, fungicides, and herbicides based on nitro-substituted aromatic rings.14 In this paper, we have studied the adsorption mechanism of three m-nitrophenyl α-guanidinomethylphosphonic acid analogues deposited on the surface of colloidal and roughened silver substrates because silver exhibits a very large enhancement factor over a wide range of excitation wavelengths and is an active catalyst for a number of important reactions. The compounds studied are m-(NO 2 )PhCH(NHC( NH)NH-cHex)PO 3 H 2 ([N′-cyclohexylguanidino-(3nitrophenyl)methyl]phosphonic acid, m-NO2PhG(cHex)P), m-(NO 2 )PhCH(NHC(NH)NH-4N-Morf)PO 3 H 2 ([(morpholine-4-carboxamidino)-(3-nitrophenyl)methyl]phosphonic acid, m-NO2PhG(Morf)P), and m-(NO2)Ph CH(NHC(NH)NHA)PO3H2 ([N′-(phenylamino) guanidine-(3-nitrophenyl)methyl]phosphonic acid, m-NO2PhG-

(An)P) (where Ph denotes phenyl, nBut is n-butane, cHex is cyclohexane, Morf is morfoline, and An is aniline). Table 1 shows the molecular structures of the investigated compounds. The orientation of the surface species on these substrates, the surface bonds, and the changes in the adsorption mode because of the structural modifications of the α-guanidino fragment are analyzed in detail on the basis of the surface-enhanced Raman scattering (SERS) spectra. The SERS technique is a simple and rapid method to probe different types of supramolecular architectures and study adsorption phenomena at the molecular level.15−20 On a metal surface/molecule interface, the molecule has fragments that directly interact with this surface. These fragments usually determine the adsorption behavior of molecules on the given metal surface. Therefore, analysis of the SERS signal (enhancement, width, and wavenumber) produced from the functional groups of the molecules is important to understand the possible ways in which a molecule can interact with the surrounding medium. Hence, on the basis of the so-called surface selection rules, the molecular properties, and the ability of molecules to form ordered monolayers on metal surfaces, the molecules investigated here provide, through their molecular geometries, a unique and unprecedented opportunity to probe a molecule−metal interface at the molecular level and to obtain specific information about the molecular conformational changes that occur at that interface with structural modifications.18−22 Thus, we provide missing structural information related to the chemisorption of these molecules on silver surfaces. The application of the above-mentioned surface selection rules to a number of the Raman-active vibrations allowed us to deduce the orientation of the aromatic ring present in the structure of the investigated α-guanidinophosphonate acids immobilized on the SERS-active silver substrates. For example, for benzene and substituted benzenes, Weaver and co-workers B



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have postulated that two characteristic ring modes, ν12 and ν18a, play a significant role in the prediction of the surface geometry.23 Weaver et al. used the significant redshift in the wavenumbers of these two bands (20−30 cm−1) and the broadening of the width of these bands as proof of the flat orientation of the benzene ring relative to the surface of a gold electrode. Creighton has classified the characteristic vibrations of the aromatic ring into four symmetry groupsA1, A2, B1, B2and has shown that the higher enhancement of the A2 group symmetry modes (ν10a, ν16a, and ν17a) is associated with horizontal orientation of the aromatic ring on the metal surface, whereas vertical orientation of the aromatic ring relative to the metal surface is demonstrated by strong modes with B2 group symmetry (ν6b, ν3, ν4, ν14, ν19b, ν8b, and ν10b).24 This phenomenon arises from the component of the polarizability tensor that is normal to the surface; if the benzene ring is flat relative to the metal surface, the component for the A2 modes is larger than that for the B2 modes. The situation is reversed for the vertical adsorption geometry. In contrast, on the basis of the propensity rules for the electromagnetic mechanism of SERS, the strongest SERS bands are from the ν12 and ν2 modes of the aromatic ring approximately perpendicular to the metal surface.25 To properly understand the SERS spectra, a reliable assignment of all of the vibrational bands was essential. Therefore, we performed the definitive band assignments needed to generate the vibrational spectra useful for a structural analysis primarily based on DFT (density functional theory) calculations at the B3LYP/6-31++G** level with Gaussian 03 software. Our aim was to produce an extensive table of Raman spectra that could make the structural determination of these molecules a rapid and accurate process.

The SERS spectra were collected three times with a InVia spectrometer (Renishaw) equipped with an air-cooled chargecoupled device (CCD) detector. The spectral resolution was 4 cm−1. The 785.0 nm line of a diode laser was used as the excitation source. The laser out power was 40 mW. All the SERS spectra were recorded within 1 h of adding the sample to the sol solution. The obtained spectra were almost identical except for small differences (up to 5%) in some band intensities. No spectral changes that could be associated with sample decomposition were observed during these measurements. Roughened Silver Substrate. Roughened silver substrates were prepared for measurements in accordance with the standard procedure.26 The SERS spectra for the compounds immobilized on the as-prepared roughened silver substrates were recorded with a Labram HR800 (Horiba, Jobin Yvon) Raman spectrometer with a 600 grooves/mm holographic grating, an Olympus BX40 microscope with a 50× longdistance objective, and a 1024 × 256 pixel, Peltier-cooled CCD detector. A HeNe laser at 632.8 nm was used as the radiation source. The measurements were performed on 10−4 M samples with a laser power of 1 mW (∼104 W/cm2). No spectral changes associated with sample decomposition or desorption were observed. Theoretical Analysis. The Gaussian 03 software package27 at the Academic Computer Center “Cyfronet” in Krakow was used to obtain optimized structures, vibrational wavenumbers, and Raman and IR intensities for the examined compounds. All calculations were performed with DFT at the B3LYP/6-311+ +G** level of theory.18 The PCM (polarizable continuum model) was used as a first-order approximation of the crystal surroundings for all of the investigated molecules in this study. The PCM and water, used as the model solvent, yielded calculated spectra that effectively assisted in band assignments of the experimental spectra. This procedure is commonly used in calculations of similar compounds and obtains reliable results.19−22,28,29 In the optimization, no imaginary wavenumbers were obtained, which showed that the calculated structures corresponded to energy minima on the potential energy surface for nuclear motion. The GaussSum 0.8 freeware program was utilized to check the outputs and to generate the theoretical spectra.30 The theoretical Raman intensities IiR were obtained from the Raman scattering activities (Si) calculated with Gaussian 03 with the following expression:31

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EXPERIMENTAL AND THEORETICAL METHODS Synthesis of m-Nitrophenyl α-Guanidinomethylphosphonic Acid Analogues. [N′-Cyclohexylguanidino-(3nitrophenyl)methyl]phosphonic acid (m-NO2PhG(cHex)P), [(morpholine-4-carboxamidino)-(3-nitrophenyl)methyl]phosphonic acid (m-NO2PhG(Morf)P), and [N′-(phenylamino) guanidine-(3-nitrophenyl)methyl]phosphonic acid (mNO2PhG(An)P) were synthesized in accordance with a previously described procedure.5 The purity and chemical structures of all the compounds were determined by 1H, 13C, and 31P NMR and CONP elemental analyses. FT-Raman Measurements. FT-Raman measurements were performed for samples placed on a glass plate. The FTRaman spectra were recorded on a Nicolet spectrometer (model NXR 9650, Thermo Scientific) with a liquid-nitrogencooled germanium detector. Typically, 1000 scans were collected with a resolution of 4 cm−1. Excitation at 1064 nm was from a continuum-wave Nd3+:YAG laser. The output power was maintained at 200 mW. SERS Measurements. Colloidal Silver. AgNO3 was purchased from Sigma (Poland) and was used without further purification. A 10 μL aliquot of 10−4 M sample solution was added to a silver nitrate solution (5 × 10−4 M). Addition of organic ligand to the AgNO3 solution increased the photoreduction rate and the stability. The resulting Ag+−sample complex was irradiated with a 514.1 nm laser line until the solution turned yellow-brown, which indicated the presence of photoreduced colloidal nanoparticles. These nanoparticles were confirmed by the UV−vis excitation spectrum.

Ii R = 10−12(ν0 − νi)4 νi−1Si

where ν0 is the excitation wavenumber (9398.5 cm−1 for a Nd:YAG laser) and νi is the wavenumber of normal mode i calculated with DFT. The calculated frequencies were scaled with the scaling factor 0.989. The PEDs (potential energy distributions) of the normal modes in terms of the natural internal coordinates were obtained with the GAR2PED freeware program, which was incorporated through a visualization script.32 The theoretical spectra were plotted in the condensed phase with a 50%/50% Gaussian/Lorentzian band shape with a fwhm (full width at half-maximum) of 10 cm−1. Both the wavenumbers and the intensities of the experimental and theoretical spectra were in good agreement. Deconvolution Procedure. Deconvolution of the SERS spectra of the investigated compounds was conducted with a GRAMS/AI program (Galactic Industries Co., Salem, NH). A C



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Figure 1. Experimental (black solid lines) and theoretical (red dashed lines) FT-Raman spectra of m-NO2PhG(cHex)P, m-NO2PhG(Morf)P, and mNO2PhG(An)P from 3500 to 400 cm−1.

however, the contribution of some weak bands from specific side-chains, such as N′-cyclohexylguanidine, morpholine-4carboxamidine, or guanidino-N′-phenylamine, could not be excluded. m-NO2Ph Vibrations. Examples of the characteristic mNO2Ph vibrations are the spectral features at 3089−3041 (ν2[ν(CH)] according to the Willson numbering scheme33), approximately 1584 (ν8b [ν(ring)]), approximately 1531 (νas(NO2)), approximately 1350 (νs(NO2)), approximately 1207 (ν7 [ρipb(CH)]), approximately 1027 (ν18a [ρoopb(CH)]), and approximately 1002 (ν12[ring breathing]), approximately 800 (δ(NO2) + ρoopb(CH)), and approximately 632 cm−1 (ν6b [ρipb(CCC)]).33,34 The noticeable spectral shift to higher wavenumbers for the ν8a mode of m-NO2Ph compared to that of phenyl reflected the redistribution of the π-electrons caused by the electron acceptor character of the nitro group. In addition, the relatively weak Raman scattering at approximately 1262, 1193, 1173, 1150, 1100, 1057, 931, and 844 cm−1 could be attributed to the m-NO2Ph modes. −NHPh Vibrations. For m-NO2PhG(An)P, the characteristic aromatic ring vibrations of NH2Ph (aniline, An) overlapped with those from m-NO2Ph (see Table S1, Supporting Information). However, three distinct Raman bands at 3340, 1604, and 996 cm−1 (Figure 1, bottom trace) and absorptions at 3339, 1496, and 1162 cm−1 (Figure S1, bottom trace, Supporting Information) could be assigned without difficulty to aniline.35 Side-Chain Vibrations. The previous investigations36 and the present DFT calculations (Table S1, Supporting Information) showed that the relatively low intensity bands of m-NO2Ph are influenced by the vibrations of the guanidine (1173 and 1150 cm−1 [ρb(CNH) + νs(CN), δ(NH)] and

50/50% Lorentzian/Gaussian band shape was assumed and was fixed for all the bands.

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RESULTS AND DISCUSSION FT-Raman, FT-IR, and DFT Studies. Figure 1 shows the experimental (black solid line) and theoretical (red dashed line) FT-Raman spectra of the three m-nitrophenyl α-guanidinomethylphosphonic acid analogues, m-NO2PhG(cHex)P, mNO2PhG(Morf)P, and m-NO2PhG(An)P, in the solid state in the spectral range between 3700 and 400 cm−1. The corresponding infrared absorption spectra (FT-IR) are presented in Figure S1 (Supporting Information). As is evident from these figures, the predictions of the theoretical calculations, the wavenumbers and intensities, are in good agreement with the experimental values. Table S1 (Supporting Information) contains a comprehensive summary of the experimental and theoretical wavenumbers for these compounds and the calculated PED (%) of the vibrational bands. Figure 1 shows that the majority of the respective Raman wavenumbers and intensities appear to be quite similar for the investigated molecules, except for those in the 2950−2800 cm−1 range. In this wavenumber range, the C−H stretches of the −CH2− moieties of the morfoline (m-NO2PhG(Morf)P) and cyclohexane (m-NO2PhG(cHex)P) rings exhibit strong scattering, whereas the −Csg−H (Csg is the carbon atom between the aromatic ring and the phosphorus atom) oscillations in m-NO2PhG(An)P are the relatively weak band at 2966 cm−1. In addition, the −CH2− vibrations manifest at approximately 1443 cm−1 in the m-NO2PhG(Morf)P and mNO2PhG(cHex)P FT-Raman spectra. In general, the Raman spectra of all the investigated compounds primarily contain a group of bands that are common to the m-NO2Ph oscillations; D



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1100, 1057, and 931 cm−1 [ν(NC)]), cyclohexane (1262 and 1193 cm−1 [ρw/ρt/ρr(CC(H2)C)] and 1057 cm−1 [ρw/ ρt(CC(H2)C) + ν(CC)]), and morfoline (1169, 1144, 1111, 1093, 932, and 850 cm−1 (see Table S1, Supporting Information, for detailed band assignments)) groups. The bands from the morfoline and guanidine moieties, in contrast to the FT-Raman spectra, show strong absorbance in the FT-IR spectra (Figure S1, Supporting Information). Several other medium and strong intensity bands (at 1680, 1635, 908, 686, and 561 cm−1 for m-NO2PhG(cHex)P; at 1680, 1640, 1562, 1447, 1116, 1016, 989, 941, 925, 821, 686, 568, and 444 cm−1 for m-NO2PhG(Morf)P; and at 1674, 1629, 935, 919, 732, 685, and 550 cm−1 for m-NO2PhG(An)P) attributed to the vibrations of the side-chain groups are also observed in the FT-IR spectra. From these vibrations, according to Aliaga and co-workers37 and Faria and co-workers38 (experimental and theoretical calculations for arginine), the CN antisymmetric and symmetric stretching vibrations coupled with the deformations of NH [ν(CN)+δ(NH)] and the deformations of the NC(N)N moiety are expected to absorb at approximately 1678 and 1638 cm−1 and 717 and 573 cm−1, respectively. Our calculations, with DFT/B3LYP at the 6311++G(d,p) level of theory, agreed with those previously published for Arg (Table S1, Supporting Information). These calculations also showed that in the high-wavenumber range (3450−3300 cm−1) the guanidine ν(NH) modes appear. Phosphonate Group Vibrations. The presence of the phosphonate group is mainly manifested in the vibrational spectra at 3245−3200, approximately 1173, 989, and 850 cm−1 (Figure 1 and Figure 1S, Supporting Information).26,29 The 3250−3100 cm−1 bands are because of the OH stretching vibrations [ν(OH)], whereas the spectral features at 1173, 989, and 850 cm−1 are assigned to the ν(PO), ρb(POH), and ν(PO) modes, respectively. SERS Studies. Silver Sol. Figure 2 compares the SERS spectra of m-NO2PhG(cHex)P, m-NO2PhG(Morf)P, and mNO2PhG(An)P in an aqueous solution of a silver sol. From this comparison, it is clear that the characteristic Raman bands from the nitrophenyl ring vibrations dominate the m-NO2PhG(cHex)P and m-NO2PhG(Morf)P spectra, whereas the aniline modes are much larger than the bands from the other molecular fragments of m-NO2PhG(An)P. This result is not surprising because it is known that the adsorption mode of the adsorbate and the strength of competitive adsorption/ interaction of the functional groups of the adsorbate strongly depend on the properties and the electrodynamical behaviors of the adsorbed molecules. In contrast, the molecular properties and the electrodynamical behavior are conditioned by the pH of the solution (through the formation of anionic, zwitterionic, or cationic species) and by the charge of the metal surfaces. Thus, because (1) the probability of a large contribution from binuclear molecules, azobenzene, and hydrazobenzene in alkaline and neutral solutions39 and (2) the pK values of most substituted anilines and nitrobenzenes are less than 3,40 it was preferable to perform measurements in acidic solutions, in which m-NO2PhG(cHex)P and m-NO2PhG(Morf)P are neutral but m-NO2PhG(An)P is present as the anilinium cation. Additionally, in most cases, it was observed that the aromatic nitro molecules are subjected to photoreduction (NO2 → NH2) on the silver surface.41 Therefore, the use of πdonating substituents in the meta-position, such as −NO2, that favor ring protonation through stabilization of the ringprotonated base by donation,42 seemed reasonable, especially

Figure 2. SERS spectra of m-NO2PhG(cHex)P, m-NO2PhG(Morf)P, and m-NO2PhG(An)P adsorbed on a colloidal silver surface.

because the m-nitrobenzene derivatives are less biologically aggressive than the para- and ortho-isomers. Indeed, the nitro group bands at 1527 and 1349 cm−1 for all the investigated m-nitrophenyl α-guanidinomethylphonic acids in the silver sol are the bands most influenced by the surface effects. In particular, the relative intensities of both the aforementioned SERS signals decrease for m-NO2PhG(cHex) P and m-NO2PhG(An)P, whereas, for m-NO2PhG(Morf)P, the former band disappears. The 1346 cm−1 band for mNO2PhG(Morf)P (Figure 2, middle trace) is the strongest among the investigated compounds. However, the asymmetry in the shape of this band could explain this bigger enhancement; this asymmetry indicates that the spectral feature at 1327 cm−1 (the lower-wavenumber shoulder at 1346 cm−1) because of the ν(CN) mode (Figure, 2, inset D) overlaps with 1346 cm−1 and contributes to its intensity. Thus, the decrease of the general relative intensity at 1527 and 1349 cm−1 compared to the FT-Raman spectra could imply that photoreduction occurs at the −NO2 group.41 However, only a slight broadening (Δfwhm = 4−6 cm−1) and shift in wavenumbers by ∼1−3 cm−1 (Table 2 lists the wavenumbers and the full width at halfmaximum of the m-nitrophenyl Raman and SERS bands) is observed for these bands, and these effects indicate that the −NO2 group of all the m-nitrophenyl α-guanidinomethylphonic acids weakly interacts with the silver nanoparticles; however, the group interacts with different strengths because of the distance effects. Because the relative intensities of the 1349 cm−1 bands for m-NO2PhG(cHex)P and m-NO2PhG(An)P are 20 and 40%, respectively, which are lower than that for mNO2PhG(Morf)P, it seemed that the nitro group of mE



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Table 2. Wavenumbers and Full Width at Half-Maximum of the FT-Raman and SERS Bands of m-NO2PhG(cHex)P, mNO2PhG(Morf)P, and m-NO2PhG(An)P Adsorbed on Colloidal and Roughened Silver Surfaces m-NO2PhG(cHex)P FT-Raman mode δ(NO2) + ρoopb(CH) δ(NH) δ(NH) ν10a ν17b ν12 ν18a ν3 νs(NO2) ν(NO) δ(CC)R δ(CH2) δ(CH2) δ(CH2) νas(NO2) ν8b ν8a δ(NH) + ν(CN)

−1

SERS Ag colloid −1

ν (cm )

−1

fwhm (cm )

820 933 1002 1026 1304 1350

7 12 7 10 4 14

1432 1441 1449 1530 1583

4 9 7 10 9

SERS Ag roughened substrate −1

ν (cm )

fwhm (cm )

ν (cm−1)

fwhm (cm−1)

778 797 840

23 31 29

801

14

1000 1034

11 20

1349 1333

16 19

996 1039 1304 1347

12 18 13 15

1390 1417 1435 1455 1551 1576 1592 1628

32 22 19 27 36 26 42 38

1423 1442 1460 1527 1579 1592 1615 m-NO2PhG(Morf)P

FT-Raman

19 20 17 18 15 25 21 SERS Ag colloid

mode

ν (cm−1)

fwhm (cm−1)

ν6b ρw(CNO2) ρw(CN) + ρw(CNO2) δ(NO2) + ρoopb(CH) δ(NH) ν10a ν17b ν12 ν18a ν13 ν7a ν3 ν(CN) νs(NO2) ν19a ν(CC)R ν(CC)ϕ + δ(CH2)R ν(CC)ϕ + δ(CH2)R ν(NO) νas(NO2) ν8b ν8a δ(NH) + ν(CN) ν(CN)

621

9

754 799

10 12

821 933 1000 1022

12 15 6 9

1308

6

1349

SERS Ag roughened substrate

ν (cm−1)

fwhm (cm−1)

ν (cm−1)

fwhm (cm−1)

724 762 792 812 834

18 19 22 21 31

721

20

1000 1041

10 18

999

14

12

1327 1346

24 18

1343 1351

18 16

1444

11

1434 1451

17 36

1528 1582

10 10

1615

10

1578 1594 1616 1660 m-NO2PhG(An)P

19 27 20 19

1390 1434 1451 1496 1544 1581 1609 1637 1664

32 20 26 21 41 35 35 41 20

FT-Raman

SERS Ag colloid


mode

ν (cm−1)

fwhm (cm−1)

ν (cm−1)

fwhm (cm−1)

ν (cm−1)

fwhm (cm−1)

ν6b δ(CN) δ(NH) δ(CC)ϕ ρw(CN) + δ(NH) δ/ρr(CN) ν10a δ(CC(H)C)ϕ

619

6

12

9

19 19 40 18 12 22 20 23

611

816

614 729 774 792 811 833 859 874

832 859 876

17 22 27

F



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Table 2. continued m-NO2PhG(An)P FT-Raman

SERS Ag colloid


mode

ν (cm−1)

fwhm (cm−1)

ν (cm−1)

fwhm (cm−1)

ν (cm−1)

fwhm (cm−1)

ν12 NHPh ν12 m-NO2Ph ν18a ν13 δ(NH) + ν(CN) νs(NO2) ν7a ν3 ν19a νas(NO2) ν8b m-NO2Ph ν8a m-NO2Ph ν8a NHPh δ(NH) + ν(CN)

996 1002 1025 1107

6 6 7 7

996 1003

14 8

989 994

12 10

1347

9

35

9

1530 1582

9 9

1604 1620

11 8

15 13 17 14 10 22 18 22 27

1201

1305

1152 1349 1209 1314 1468 1529 1575 1593 1612

1523 1573 1590 1602 1621

17 22 22 30 38

surface charge for the silver under open circuit conditions and the absence of coordination ability for the −NH2+− group. This interaction might take place if deprotonation of −NH2+− occurs at the interface. The contributions from the SERS bands of the guanidine, cyclohexane, or morfoline fragments are almost negligible in Figure 2 because these bands are weak. For instance, the broad, relatively weak band with a maximum centered at approximately 1442 cm−1 in the SERS spectra of m-NO2PhG(cHex)P and m-NO2PhG(Morf)P in the silver sol shows two (1460 and 1423 cm−1; Figure 2, inset B) and one (1451 cm−1; Figure 2, inset E), respectively, sub-bands because of the CH2 deformations that overlap with the aromatic ring modes (1442 cm−1, inset B; 1434 cm−1, inset E). The spectral features at 1162, 840, 778, and 568 cm−1 (Figure 2, insets A and C) could be assigned to the NH deformation of the guanidine moiety. Likewise, the CNH unit of the guanidine group gives the 1660 cm−1 SERS signal in the m-NO2PhG(Morf)P SERS spectrum alone (Figure 2, middle trace). Thus, it could be stated that the aforementioned fragments, which are somewhat close to the colloidal silver surface, assist in the interaction with this surface. However, the CN bond of m-NO2PhG(Morf)P is directed toward the silver surface. This situation for the N′cyclohexylguanidine, morpholine-4-carboxamidine, or guanidino-N′-phenylamine fragments and the arrangement of the mNO2Ph fragment cause the phosphonate group to be directed out of the surface and prevent participation of the phosophonate group in the adsorption on the silver nanoparticles. Indeed, there is no spectral evidence for such interactions. Roughened Silver Substrate. Figure 3 shows the SERS spectra of the investigated m-nitrophenyl α-guanidinomethylphonic acids immobilized on the roughened silver substrate in an aqueous solution. These spectra differ from the corresponding Raman and SERS spectra of those molecules adsorbed on the colloidal silver surface. This result was expected because, in the SERS technique, the roughness of the surface, especially unevenness such as needle or nanostructure clusters, is a source of additional strong electromagnetic fields. These fields directly act on the molecules adsorbed on the metal surface and increase the intensity of the entire electromagnetic field related to the adsorbed molecule. Thus, the structure of the monolayer of the adsorbed species and the strength of the competitive

NO2PhG(Morf)P is located closer to the colloidal silver surface than that of m-NO2PhG(cHex)P, and the nitro group of mNO2PhG(An)P is somewhat distant from this surface. In addition, because the approximately 1349 cm−1 SERS is attributed to the coupled ν(NO2) + ρs(CNO2) + ν(CN) modes (Table 1), this result suggests that the surface interaction induces redistribution of the π-electron system around not only the nitro group but also the aromatic ring near the substitution site.43 Therefore, in the SERS spectra presented in Figure 2, several well-known spectral features because of the phenyl ring are enhanced. These features include the approximately 1584, 1488, 1216, 1162, 1034, and 1000 cm−1 bands (see Table 2 for detailed band wavenumbers). From these bands, the 1000 cm−1 is the strongest band in the SERS spectra and is more intense than that of the corresponding FT-Raman spectra (based on the ratio of the relative intensities of the phenyl bands). This phenomenon, the neglected shift in wavenumber, and the slight broadening of this band (Table 2) imply that, for all the investigated compounds, the m-nitrophenyl ring, which is near the surface of the silver nanoparticles, adopts a vertical orientation relative to the surface. It is also noteworthy that, for m-NO2PhG(An)P, the 1000 cm−1 band is the widest (fwhm = 16 cm−1) among the spectra of the m-nitrophenyl α-guanidinomethylphonic acids and exhibits two overlapped maxima (at 1003 and 996 cm−1) (Figure 2, inset G). The former and weaker sub-band is from the breathing vibrations of the m-NO2Ph ring, whereas the stronger 996 cm−1 spectral feature is attributed to −NHPh. These results indicate that, in the interaction of m-NO2PhG(An)P with the colloidal sliver surface, aniline, which is parallel to the substrate, plays a more important role than nitrobenzene. This statement is supported by the appearance of the moderately weak 1593, 1468, and 774 cm−1 (Figure 2, inset F) and prominent 1209 and 1152 cm−1 SERS signals of the aniline ring. Two of these bands (1152 and 774 cm−1) are contributed by the oscillations of the −NH− moiety. In addition, the ν(CN) mode of −NHPh influences the 1152 cm−1 SERS signal. Therefore, it could be concluded that the −NHPh ring is oriented such that the C−N bond is tilted toward the surface of the silver nanoparticles and the free electron pair can interact with the silver surface. Direct interaction of the −NH2+− group of the anilinium cation with the silver surface is debatable because of the positive G



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approximately 1435 cm−1; Figure 3, insets A and C), ν18a (at approximately 1039 cm−1), ν12 (at approximately 996 cm−1), and ρb(CNH) + νs(CN) + δ(NH) (at approximately 1146 cm−1) modes (see Table 2 for the precise band wavenumbers and the fwhm). Of these bands, the ν12 is much less intense in the m-NO2PhG(cHex)P SERS spectrum (Figure 3, top trace) than in the m-NO2PhG(Morf)P and m-NO2PhG(An)P SERS spectra. In contrast, the ν19b mode is stronger for mNO2PhG(cHex)P (Figure 3, inset A) than that for mNO2PhG(Morf)P (Figure 3, inset C) and is much stronger than that for m-NO2PhG(An)P. These two modes are indicative of the π overlap of the benzene and the silver surface (either ring−surface π donation and/or surface−ring π* back-donation) and provide direct evidence for the orientation of the benzene ring on the silver surface.44 Hence, given that (1) approximately comparable relative FT-Raman (Figure 1) intensities are observed for the ν12 band for all the mnitrophenyl α-guanidinomethylphonic acids investigated here, (2) the strongest SERS bands correspond to totally symmetric A1 ring modes (i.e., ν12, ν18a, and ν8a) for the aromatic ring perpendicular to a metal surface, and (3) the B2 symmetry modes (i.e., ν19b) are more intense if the aromatic ring is lying on this surface, it can be inferred that the m-NO2Ph ring of mNO2PhG(cHex)P is approximately horizontal relative to the roughened silver surface, whereas that of m-NO2PhG(Morf)P is “standing up” on this surface. In contrast, the m-NO2Ph and  NHPh planes of m-NO2PhG(An)P appear to be tilted toward the silver substrate. This result is supported by both a decrease in wavenumbers (Δν = 5 and 7 cm−1, respectively) and band broadening (Δfwhm = 4 and 6 cm−1, respectively) (Table 2) of the ν12 spectral feature between the FT-Raman spectra (Figure 1) and SERS (Figure 3, inset F) spectra on the roughened silver substrate. Second, the SERS signals from the nitro group (approximately 1544 and 1348 cm−1) show larger decreases in relative intensities for m-NO2PhG(cHex)P and m-NO2PhG(An)P than for m-NO2PhG(Morf)P. In consequence, these spectroscopic results indicate either that the −NO2 moiety attached to the benzene ring is in weak contact with the roughened silver surface or it is consumed through photoreduction. Third, some other specific differences among the spectra presented in Figure 3 can be clearly observed. For instance, the bands at 1455, 1390, 1268, and 1184 cm−1 in the SERS spectra of m-NO2PhG(cHex)P and m-NO2PhG(Morf)P are characteristic of interactions of cyclohexane and morfoline with the roughened silver surface. Furthermore, as for the colloidal silver surface, no evidence for formation of the phosphonate group···Ag complex is observed.

Figure 3. SERS spectra of m-NO2PhG(cHex)P, m-NO2PhG(Morf)P, and m-NO2PhG(An)P immobilized on a roughened silver substrate.

adsorption/interaction of the functional groups of the adsorbate strongly depend on the roughness of the metal surface. Therefore, we suggest that the differences between the SERS spectra of the two silver substrates are caused by conformational differences in the attached molecules that follow the differences in the geometry of the silver and by differences between the charges of the colloidal and roughened substrate surfaces. On the basis of the above assumption and upon careful inspection of Figure 3, several points of interest should be mentioned. First, the deconvolution of the intense, very broad spectral feature (53−96 cm−1; Table 2) at approximately 1580 cm−1 (Figure 3, insets B, D, and E) shows that this band consists of several overlapped bands from the NH deformations coupled with the CN stretches (at 1628, 1637, and 1621 cm−1 for m-NO2PhG(cHex)P, m-NO2PhG(Morf)P, and m-NO2PhG(An)P, respectively), the m-NO2Ph ring vibrations (at 1592 and 1576 cm−1, at 1609 and 1581 cm−1, and at 1590 and 1575 cm−1 for m-NO2PhG(cHex)P, mNO2PhG(Morf)P, and m-NO2PhG(An)P, respectively), and νas(NO2) (at 1551, 1544, and 1523 cm−1 for m-NO2PhG(cHex)P, m-NO2PhG(Morf)P, and m-NO2PhG(An)P, respectively). In addition, for m-NO2PhG(Morf)P and m-NO2PhG(An)P, the ν(CN) and aniline ring modes, respectively, are calculated (Figure 3, insets D and E). The enhancement of all the aforementioned modes suggests that m-nitrobenzene, aniline, and guanidine are adsorbed in the electrochemical environment. Similar to the silver sol, for m-NO2PhG(Morf)P, the guanidine CN bond is directed toward the roughened silver substrate. The above statements are supported by the appearance of νas(NO2) (at approximately 1347 cm−1), ν19b (at

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CONCLUSIONS In this paper, using SERS, we discussed the structures and adsorption modes of three m-nitrophenyl α-guanidinomethylphonic acids (m-NO2PhG(cHex)P, m-NO2PhG(Morf)P, and m-NO2PhG(An)P) immobilized on colloidal and roughened silver substrates. The analysis of the adsorption geometry of these molecules was performed on the basis of the observed changes in the enhancement, width, and wavenumber of the corresponding SERS and FT-Raman bands. To reliably assign bands and draw correct conclusions, the complex SERS spectra were deconvoluted and the FT-Raman spectra for the nonadsorbed molecules were measured and were calculated with DFT at the B3LYP/6-311++G** level of theory. H



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Figure 4. A proposed manner of binding to the colloidal and roughened silver substrates of the investigated compounds.

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We showed that, on the colloidal silver substrate, the mnitrophenyl ring of all the m-nitrophenyl α-guanidinomethylphonic acids accepts an orientation perpendicular to the metal surface, whereas the phosphonate group is distant from the surface and does not influence the adsorption process. In addition, the NO2 nitro group of m-NO2PhG(Morf)P is closer to the surface of the silver nanoparticles than that of mNO2PhG(cHex)P and is much closer than that of mNO2PhG(An)P, which is directed out of the surface. The CN bond of m-NO2PhG(Morf)P is turned toward the colloidal substrate. The CN bond of the NHPh ring, that is more or less vertical with respect to the silver nanoparticle surface, in m-NO2PhG(An)P adopts a tilted orientation that favors interaction between the silver and free electron pair. We demonstrated that m-NO2PhG(cHex)P, m-NO2PhG(Morf)P, and m-NO2PhG(An)P primarily adsorb on the roughened silver surface through the m-nitrobenzene, and mNO2PhG(An)P also adsorbs through the aniline moiety. In contrast, the guanidine, cyclohexane, and morfoline fragments only assist in the adsorption process. For example, the mNO2Ph ring of m-NO2PhG(cHex)P accepts an approximately horizontal orientation on the roughened silver surface, whereas, for m-NO2PhG(Morf)P and m-NO2PhG(An)P, the ring adopts a perpendicular position. For the colloidal silver surface, the phosphonate group is farther from the metal substrate, and the CN bond of m-NO2PhG(Morf)P is directed toward the roughened silver surface. A proposed manner of binding to the colloidal and roughened silver substrates of the investigated compounds is given in Figure 4.

ASSOCIATED CONTENT

S Supporting Information *

Description of the FT-IR measurements. Figure S1 showing experimental (black solid lines) and theoretical (red dashed lines) FT-IR spectra of m-NO2PhG(cHex)P, m-NO2PhG(Morf)P, and m-NO2PhG(An)P from 3500 to 400 cm−1. Table S1 showing calculated and experimental wavenumbers and the potential energy distribution for m-NO2PhG(cHex)P, m-NO2PhG(Morf)P, and m-NO2PhG(An)P. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: +48-12-6632077. Fax: +48-12-634-0515. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors gratefully acknowledge Prof. P. Kafarski and Prof. A Mucha, the Department of Bioorganic Chemistry, Faculty of Chemistry, Wroclaw Technical University, for donating the samples. The authors also gratefully acknowledge the Academic Computer Center “Cyfronet” in Krakow for computational facilities. This work was supported by the National Research Center of the Ministry of Science and Higher Education (Grant No. N N204 544339 to E.P.) and by the Faculty of Chemistry of Jagiellonian University (K/DSC/000972 to L.M.P.). Y.K. gratefully acknowledges HUFS for financial support.

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Vibrational and Theoretical Studies of the Structure and Adsorption

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