Evidence for Charge Transfer and Impact of ... - ACS Publications

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Evidence for Charge Transfer and Impact of Solvent Polar Properties on Aminobenzonitrile Adsorption on Silver Nanostructures Donald A. Perry,* James S. Cordova, Elizabeth M. Schiefer, Tsung-Yen Chen, and Taylor M. Razer Department of Chemistry, University of Central Arkansas, Conway, Arkansas 72035, United States

Alexandru S. Biris Applied Science Department, UALR Nanotechnology Center, University of Arkansas, Little Rock, Arkansas 72204, United States S Supporting Information *

ABSTRACT: Aminobenzonitrile adsorption on silver nanoparticles (SNPs) was studied with SERS, SEIRA, and DFT. It was found that 4-aminobenzonitrile (4ABN) and 2aminobenzonitirle (2ABN) could resonantly distribute an amino group charge so that the cyano group could strongly interact with the SNPs in the mono/multilayer. Cyano stretching frequency shifts in SEIRA spectra showed that a 4ABN multilayer interacted strongly with the SNPs when deposited using an alkane solvent with nonpolar bonds. 2ABN could interact with the SNPs even when deposition occurred using a solvent with polar bonds because intramolecular hydrogen-bonding in 2ABN limited solvation. 3Aminobenzonitrile (3ABN) never interacted strongly with the SNPs in the multilayer because it could not resonantly distribute the amino group charge. 2ABN also formed C−H hydrogen-bonds with n-heptane, which could interact with both the amino and cyano groups of 2ABN. Charge transfer between adsorbed ABN isomers and the SNPs is evidenced in SERS spectra by the presence of nontotally symmetric vibrational bands that do not appear intense in Raman spectra of the ABN powders. It is anticipated that this work will affect applications where interactions of benzonitrile compounds with metal nanoparticles is important.

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INTRODUCTION In 1962, Lippert et al. reported on the dual fluorescence of 4N,N-dimethylaminobenzonitrile (DMABN).1 A red-shifted band in the emission spectra similar to DMABN has since been observed in many structurally similar molecules,2 thus stimulating much interest in the properties of substituted benzonitriles such as the aminobenzonitriles over the past 50 years.3−7 Aminobenzonitriles are important in the synthesis of dyes8 and the use of aminobenzonitriles in polymers ranging from supramolecular chemistry9 to epoxys.10,11 There is also great attention in the use of aminobenzonitrile isomers as intermediates in the synthesis of compounds to combat infectious agents such as prions,12 methicillin-resistant Staphylococcus aureus (MRSA),13,14 and Mycobacterium tuberculosis.15 Considering the rise in the use of metal nanoparticles such as silver or gold as antimicrobial agents16 and other medical applications,17 it is becoming increasingly important to understand in more detail how aminobenzonitrile isomers and other architecturally comparable moieties interact with metal nanoparticles under standard laboratory and physiological conditions as well as in the environment. An advantage of studying the interaction of an adsorbate such as an aminobenzonitrile with silver nanoparticles (SNPs) is the amplified vibrational signal from the adsorbate in Raman and infrared spectroscopies induced by local surface plasmons associated with the SNPs. Prior work has shown that rough © 2012 American Chemical Society

silver films formed by evaporating a few nanometers of silver can enhance the Raman signal of an adsorbate monolayer by ×105 or more, termed surface-enhanced Raman spectroscopy (SERS), while the infrared signal of a monolayer and even the multilayer of an adsorbate can be enhanced by ×50 or more (surface-enhanced infrared absorption, or SEIRA).18−26 Previous Raman experiments in silver sol showed that benzonitrile27 and aminobenzonitrile28 adsorbed to the SNPs with the benzene ring lying flat through a CN π interaction. However, more recent studies involving benzonitrile have suggested that on a Au(111) electrode surface the CN π interaction is dominant at negative potentials but that benzonitrile takes a more vertical orientation via interaction with the SNPs through the nitrogen as the surface becomes more positive.29,30 Early SERS studies have suggested that when 4-cyanobenzoic acid adsorbs to the silver surface as a benzoate ion, the CN π interaction is still important.28 Later density functional theory (DFT) simulations have shown that silver interaction with the cyano group of the 4-cyanobenzoate ion does little to alter the Raman spectrum compared to that of a sole benzoate interaction with silver.23 Received: September 2, 2011 Revised: January 27, 2012 Published: January 30, 2012 4584

dx.doi.org/10.1021/jp208489w | J. Phys. Chem. C 2012, 116, 4584−4593

The Journal of Physical Chemistry C

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RESULTS Figure 1 contains from bottom to top SEIRA spectra of 5, 15, 25, 50, 100, 200, 400, and 800 μL exposures of 4ABN deposited

In this work, a combination of DFT, SERS, and SEIRA were used to investigate the adsorption of 2-aminobenzonitrile (2ABN), 3-aminobenzonitrile (3ABN), and 4-aminobenzonitrile (4ABN) on SNPs using a range of solvents with different polar properties. SERS was used to characterize the monolayer, SEIRA was used to document the multilayer exposures, and DFT calculations were used to simulate vibrational spectra. It is believed that this research is necessary to understand how aminobenzonitriles interact with metal nanoparticles in many synthetic, biological, nanotechnological, and environmental applications.

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EXPERIMENTAL SECTION

The aminobenzonitrile (ABN) isomers were purchased from Aldrich and silver wire (99.9% pure) was acquired from Myron Toback. Dichloromethane and n-heptane used as solvents for the preparation of solutions were HPLC grade from Thermo Fisher Scientific. Polished CaF2 windows (25 × 4 mm) were purchased from International Crystal Laboratories. SEIRA spectra were obtained in transmission mode (2−4 cm−1 resolution, and 16 scans were averaged) with a ThermoNicolet IR100 FTIR spectrometer. Attenuated total reflectance Fourier transform infrared (ATR FTIR) spectra were collected on a Thermo Foundation Series Performer ATR Attachment with a diamond ATR crystal (single-bounce design) on an IR100 spectrometer. SERS and Raman spectra were taken in a back scattering geometry with a Horiba Jobin Yvon HR00UV Raman spectrometer at 18 mW with a 632.81 nm laser line. SEIRA and SERS substrates were prepared by vacuum deposition of silver on CaF2 substrates in a house-built vacuum chamber with a base pressure of 1 × 10−6 Torr equipped with an Inficon quartz crystal microbalance. CaF2 windows were polished with a Buehler Mastermet 2 colloidal silica suspension, rinsed and sonicated in nanopure water, and air-dried before usage. It has been previously shown that a seven nanometer silver film is optimal for SEIRA, and these silver films have been fully characterized with UV−vis/NIR, AFM, and SEM.18,19 Henceforth, the phrase “a silver film” refers to a seven nanometer silver film evaporated on a CaF2 substrate. An approximate monolayer ABN coverage on a silver film was prepared by pipeting 25 μL of a 50 ppm ABN solution and allowing the solvent to evaporate. An average spot size of 4 cm2 for 25 μL of a 50 ppm solution of all solvents on a silver film was determined, which resulted in reasonably uniform ABN films of about 200 ng/cm2. Density functional theory (DFT) calculations were performed using the Gaussian09 suite31 at the B3LYP32,33 level of theory to simulate infrared and Raman spectra. A LANL2DZ basis set was used for calculations of each ABN isomer and every ABN isomer complexed with a silver ion(s). All reported frequencies obtained from calculations that used a LANL2DZ basis set calculations are scaled by 0.9612.34 For each ABN isomer, separate calculations were performed as follows: one silver ion in proximity to the cyano group, one silver ion close to the amino group, and two silver ions associated with both the cyano and amino groups. Calculations were also performed for each ABN isomer with a 6-311G(d,p) basis set and scaled by 0.9668.34 A cc-PVTZ basis set was used to model the interaction of a 2ABN molecule with ethane, as will be discussed later.

Figure 1. From bottom to top, SEIRA spectra of 5, 15, 25, 50, 100, 200, 400, and 800 μL exposures of a 50 ppm 4ABN/dichloromethane solution, two 25 μL aliquot exposures of a 200 ppm 4ABN/ dichloromethane solution, a thorough rinse with dichloromethane (second spectrum from top), and the ATR FTIR of 4ABN powder (top spectrum).

with a 50 ppm solution of 4ABN in dichloromethane followed by two 25 μL exposures of a 200 ppm solution of 4ABN/ dichloromethane. The second SEIRA spectrum from the top shows the same substrate used for the 4ABN depositions in Figure 1 after a copious rinse with dichloromethane, and the top spectrum is the ATR FTIR of 4ABN powder. Figure 2 contains SEIRA spectra from bottom to top following 5, 25, 50, 100, 200, 400, and 800 μL exposures of a 50 ppm 4ABN/nheptane solution, and Figure 3 is a magnification of the CN stretch region of the same experiments outlined in Figure 2. The bottom spectrum in Figure 4 is the ATR FTIR of 3ABN powder, and the other spectra in Figure 4 are SEIRA multilayer exposures of 3ABN deposited from dichloromethane (middle) and n-heptane (top). Figure 5 shows SEIRA spectra from bottom to top following 5, 15, 25, 50, 100, 200, 400, and 800 μL exposures of a 50 ppm solution of 2ABN in dichloromethane followed by the ATR FTIR of 2ABN powder. In the bottom spectrum of Figure 6 is the ATR FTIR of 2ABN powder followed by SEIRA spectra from bottom to top of 5, 15, 25, 50, 100, 200, and 800 μL exposures of a 50 ppm solution of 2ABN in n-heptane. In Figure 7 are SERS spectra of monolayer exposures of 3ABN (bottom), 4ABN (second from bottom), and 2ABN (third from top) deposited from acetone. Raman spectra of 4ABN powder (third from bottom), 2ABN powder 4585



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Figure 4. ATR FTIR of 3ABN powder (bottom). SEIRA spectrum of a multilayer exposure of 3ABN/dichloromethane (middle spectrum). SEIRA spectrum of a multilayer exposure of 3ABN/n-heptane (top).

Figure 2. From bottom to top, SEIRA spectra of 5, 15, 25, 50, 100, 200, 400, and 800 μL exposures of a 50 ppm 4ABN/n-heptane solution.

Figure 3. Magnification of the cyano stretch region from Figure 2.

(second from top), and DFT simulations of gas phase 4ABN (middle) and 2ABN (top) are also in Figure 7. Figure 8 shows the optimized structures from the DFT simulation of each ABN isomer with a silver ion in close proximity with the cyano group. Tables 1−3 are for 4ABN, 3ABN, and 2ABN, respectively. Each table contains mode assignments and associated vibrational frequencies from the DFT calculation of the appropriate ABN isomer in columns 1 and 2 using a 6-311G(d,p) basis set. Columns 3 and 4 contain the DFT/LANL2DZ basis set mode assignments and vibrational frequencies of the ABN isomer complexed with a silver ion in proximity to the cyano group (see Figure 8). In columns 5, 6, 7, and 8 are the vibrational frequencies from the ATR FTIR of the ABN powder, the SEIRA spectra of the ABN isomer deposited from dichloromethane, the SEIRA spectra of the ABN deposited from nheptane, and the SERS spectrum of the ABN deposited from acetone, respectively. In Tables 1 and 3, there is also a column 9 containing the Raman vibrational frequencies for 4ABN and 2ABN powder. The Raman spectrum of 3ABN powder exhibits a significant amount of fluorescence and is not presented here.

Figure 5. From bottom to top, SEIRA spectra of 5, 15, 25, 50, 100, 200, 400, and 800 μL exposures of a 50 ppm 2ABN/dichloromethane solution. ATR FTIR of 2ABN (top).

Table 4 contains select results from four separate DFT/ B3LYP/LANL2DZ calculations involving 4ABN. Columns 1, 2, 3, and 4 have the labels, basic structure, calculated −NH2 wag frequencies, and silver ion/nitrogen (of the amino group) distance in Å for gas phase 4ABN, 4ABN with a silver ion near the amino group, silver ions close to the amino and cyano groups of 4ABN, and 4ABN with a silver ion adjacent to the cyano group.

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DISCUSSION The impact of the polar properties of the deposition solvent is apparent when comparing SEIRA spectra of the adsorbed 4586



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Figure 6. ATR FTIR of 2ABN (bottom) followed by, from bottom to top, 5, 15, 25, 50, 100, 200, and 800 μL exposures of a 50 ppm 2ABN/ n-heptane solution.

4ABN multilayers in Figure 1 (dichloromethane solvent) versus Figure 2 (n-heptane solvent). In Figure 1, as the exposure of 4ABN is increased, SEIRA spectra bands grow that are consistent with the ATR FTIR of 4ABN powder. Even the CN stretching band for adsorbed 4ABN in Figure 1 is within a few wavenumbers of the ATR FTIR of 4ABN powder (see Table 1), and the frequency of the CN stretching mode does not change with 4ABN coverage. The SEIRA spectra in Figure 1 are most sensitive to 4ABN in the multilayer, suggesting that the dichloromethane solvates the 4ABN during the adsorption process, thus limiting the interaction of 4ABN with the underlying SNPs. Nearly identical SEIRA spectra were obtained using other solvents with polar bonds including acetone, methanol, ether, and carbon tetrachloride. Evidence for a SNP−cyano interaction is the presence of ring deformation/ metal−cyano stretch bands at 422 and 705 cm−1 in the SERS of 4ABN in Figure 7 (Table 1). The circumstances are much different in Figure 2 when 4ABN is adsorbed onto SNPs using n-heptane as the solvent (a solvent containing only nonpolar bonds). n-Heptane was found to be a good alkane solvent in previous experiments because its evaporation rate is slow enough to allow ample time for adsorbate equilibration with the SNPs.25 Figure 3, which is a magnification of the CN stretching region of Figure 2, reveals a dramatic shift in the maximum peak position with coverage. At the lowest exposure of 5 μL of 4ABN/n-heptane, the CN stretch is at 2221 cm−1. As the coverage of 4ABN is increased, the CN stretching maximum shifts to lower frequency and reaches 2211 cm−1 by an 800 μL exposure of 4ABN/n-heptane.

Figure 7. From bottom to top, SERS spectra of 3ABN and 4ABN, Raman of 4ABN powder, DFT simulation of gas phase 4ABN (upscaled), SERS of 2ABN, Raman of 4ABN powder, and a DFT simulation of gas phase 2ABN (upscaled).

This occurs because n-heptane with nonpolar bonds solvates the 4ABN to a lesser extent than solvents such as acetone or dichloromethane with more polar bonds. As more 4ABN molecules enter the multilayer using the n-heptane deposition solvent, there is less interaction of each 4ABN molecule on average with the SNPs. In this situation, the cyano group of the 4ABNs interacts less with the SNPs, and the CN stretching mode begins to shift to a lower frequency and approach that expected for bulk 4ABN. A shift to higher frequency of the CN stretch mode in the SEIRA spectra using a deposition solvent with nonpolar bonds in Figures 2 and 3 (as compared to bulk 4ABN) is entirely consistent with an interaction between the lone electron pair of the nitrogen of the cyano group as is commonly observed in cyano transition metal conplexes.28 This occurs readily in a 4ABN molecule because the amino group of 4ABN can resonantly share its lone pair electron charge with the cyano group. Since the amino group has resonantly distributed charge throughout the 4ABN molecule, it is less likely that the amino group of 4ABN will interact to a large degree with the SNPs in the multilayer. In the SEIRA spectra of Figures 1−3, no 4587



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Additionally, a band appears at about 1398 cm−1 in the SEIRA spectra of 4ABN deposited from n-heptane in Figure 2 assigned to either the C−NH 2 stretching or a ring deformation/NH2 bend mode of 4ABN (see Table 1 and the Supporting Information containing pictures of some important 4ABN normal modes). This band appears at about 1367 cm−1 in the ATR FTIR of 4ABN (broad, weak band) and in 4ABN layers deposited from a polar solvent (Figure 1). DFT simulations involving a silver ion in proximity to the cyano group of 4ABN predict a shift to a higher frequency of the C− NH2 stretching mode of 4ABN induced by the silver ion interaction. Using a LANL2DZ basis set, the C−NH2 stretch is predicted to be at 1297 cm−1 for gas phase 4ABN and at 1330 cm−1 with a silver ion close in proximity to the cyano group of 4ABN. Assuming the band at 1398 cm−1 is a ring deformation/ NH2 bend mode, for a LANL2DZ basis set, that mode is at 1299 cm−1 for gas phase 4ABN and 1344 cm−1 for 4ABN with a silver ion close to the cyano group. This demonstrates that both of these modes are quite susceptible to frequency shifts induced by the metal ions or possibly intramolecular attractive forces. We note that the calculations predict that the only band for 4ABN of large intensity in the 1300−1400 cm−1 range is the C−NH2 stretch mode. It is difficult to determine in the monolayer whether 4ABN is standing upright and adsorbing to the SNPs through the cyano group or if 4ABN adsorbs flat such that both the amino and cyano groups interact directly with the SNPs. Some observations in the SERS spectrum of a 4ABN monolayer adsorbed on SNPs in Figure 7 support a predominantly lone pair interaction involving the nitrogen of the cyano group of

Figure 8. DFT optimized structures of 2ABN, 3ABN, and 4ABN each with a silver ion in proximity to the cyano group.

evidence was found in the multilayer for a strong amino group interaction with the SNPs. Table 1. 4ABN Vibrational Assignments 4ABN modes

ring def. ring OP bend ring CC IP stretch CH bend CH bend ring def. NH2 rock CH/NH bend CH bend CH bend/C−CN str. ring def. ring def. C−NH2 str. ring def./ NH bend ring def. ring def./NH bend ring def./NH2 scis. NH2 scis. CN str.

4ABN freq. 6-311G (d,p)

644 709 821 927 934 992 1037 1116 1159 1193 1280 1313 1286

4ABN/Ag+ modes

4ABN/Ag+ freq. LANL2DZ

SEIRA CH2Cl2

ATR

SEIRA heptane

SERS acetone

Raman powder

ring def./Ag−NC str. ring OP bend ring OP bend

451 469 544

422 494 553

ring def./Ag−NC str. CH bend

710 840

705 844

CH/CC bend

968 973

NH2 bend

1011

885 955 (broad) 1033

CH bend ring def.

1177 1215

1132 1169 1207

1134 1165 1211

1174

1179 1232

1177 1210

ring def.

1305

1309

1313

1306

1269

1317

C−NH2 str. or ring def./NH bend

1367 (broad) ∼1450

1367

1398

1403

1458

1456

651 695 845

1421

ring def.

1330 1344 1442

1496 1548

ring def. ring def./NH bend

1497 1523

1506 ∼1560

1500 ∼1560

1518 ∼1560

1515

1511

1597

ring def./NH bend

1599

1593

1597

1603

1607

1608

1614 2251

ring def. NH2 scis CN str.

1638 2250

1614 2202

1616 2200

1622 2221 (shifts)

2228

1635 2213

4588

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Table 2. 3ABN Vibrational Assignments 3ABN freq. 6-311 (d,p)

3ABN/Ag+ mode

NH2 bend

532

ring def./Ag−NC str.

CH bend CC bend CH bend CC str. CH/NH2 bend CH/NH2 bend CH bend CH bend ring def. C−NH2 str. ring def. NH2 bend

863 919 947 975 1051 1096 1141 1158 1275 1295

CH bend ring def. NH2 ring def. NH2 ring def. NH2 ring def. NH2 NH2 scis. CN str.

1310 1433 1474 1580 1590 1613 2251

3ABN mode

bend bend bend scis.

3ABN/Ag+ freq. LANL2DZ

ATR

SEIRA CH2Cl2

SEIRA heptane

SERS acetone

CH bend CH bend

492 522 869 903

540 880 925

CH bend NH bend

993 1027

995 1036

CH bend/C−CN Str. CH bend CH bend C−NH2 str. CH bend C−NH2 str. ring def. NH2 bend ring def. NH2 bend ring def. NH2 bend ring def. ring def. ring def. ring def. NH2 scis. CN str.

1156 1178 1298 1320 1352 1426 1426 1468 1550 1591 1635 2111

1065 1151 1163 1288 1317 1441 1471 1485 1574 1591 1635 2224

1157 1170 1294 1325 1393

1159 1171 1294 1325 1393

1452 1491 1581 1601 1624 2228

1452 1491 1583 1601 1626 2226

1275 1333 1399 1460

1607 2235

When considering the adsorption of 3ABN, the lone pair charge on the amino group cannot be resonantly distributed throughout the 3ABN molecule, as is the case for 4ABN. In the SERS spectrum for a monolayer of 3ABN in Figure 7 (and in Table 2), the cyano stretching maximum is at 2235 cm−1, while the cyano stretch in the ATR FTIR of 3ABN powder is at 2224 cm−1. This demonstrates that the cyano group of 3ABN interacts with the SNPs in the monolayer. There is also a band at 540 cm−1 in the SERS spectrum of 3ABN in Figure 7, which could be from a SNP−cyano stretch mode (see Table 2). However, in Figure 4, the SEIRA spectra of a multilayer 3ABN deposited from dichloromethane and n-heptane are virtually identical to each other, and both SEIRA spectra are quite similar to the ATR FTIR of 3ABN powder. In Table 2, the cyano stretching frequencies for 3ABN powder and 3ABN deposited from dichloromethane and n-heptane are all within 4 cm−1, indicating limited interaction of the 3ABN with the SNPs in the multilayer. One obvious difference between the ATR FTIR of 3ABN powder and the SEIRA spectra of a 3ABN multilayer in Figure 4 is the appearance of a band in the SEIRA spectra at 1393 cm −1 tentatively assigned as a ring deformation/NH2 bend mode, which is supported by the 3ABN/Ag+ DFT simulation showing a band at 1352 cm−1 (Table 2). In Figure 5, the SEIRA spectra of 2ABN multilayers deposited from dichloromethane are almost identical to that of the ATR FTIR of 2ABN powder except for the position of the CN stretch mode. The CN stretching frequency has a maximum of 2199 cm−1 for 2ABN powder, but the CN stretching frequency for 2ABN deposited onto SNPs from dichloromethane is at 2214 cm−1. A strong interaction of the cyano nitrogen with the SNPS based on a shift of the cyano stretch to higher frequency is to be expected because 2ABN, like 4ABN, can resonantly distribute the lone pair charge of the amino group throughout the molecule. For 4ABN in Figure 1, it has been suggested that the lack of a shift in the 4ABN cyano stretching frequency upon adsorption onto SNPs was due to the more complete solvation of 4ABN by a solvent with polar

4ABN. First, in the SERS spectra of adsorbed 4ABN, the vibrational frequency of the cyano group stretch of 4ABN in the monolayer has a maximum at 2228 cm−1 (2213 cm−1 for 4ABN powder), an even higher vibrational frequency than that found in SEIRA spectra of a 4ABN multilayer (2211 cm−1) in Figures 2 and 3. Second, there is a band appearing at 1033 cm−1 in the SERS spectrum of 4ABN, which is consistent with the rocking motion of a free amino group.37 These results imply a strong interaction of only the cyano group of 4ABN with the SNPs. Additionally, in Figure 7, an in-plane CC stretch mode in the Raman spectrum of 4ABN powder at 845 cm−1 appears significantly less intense and is replaced by a broad band centered at 955 cm−1 in the SERS spectrum of 4ABN. Previous authors have assigned this broad band to an NH2 rocking vibration in SERS experiments on silver sol.28 Current DFT calculations involving a 4ABN/Ag+ complex (see Table 1) suggest that these bands could be due to strong CC out of plane or CH bend modes. Either mode assignment might be ascribed to a flatter orientation of adsorbed 4ABN. Finally, the disappearance of the NH2 scissor mode in the SERS spectrum of adsorbed 4ABN (the NH2 scissor mode is at 1635 cm−1 in the Raman spectrum of 4ABN powder) might also indicate that the amino group as well as the cyano group of 4ABN interacts with the SNPs in the monolayer. In Figure 1, the second SEIRA spectrum from the top is a 4ABN multilayer that has been rinsed thoroughly with the dichloromethane solvent. No 4ABN can be detected in the SEIRA spectrum after the rinse. A similar situation was observed in SERS experiments with 4ABN/dicholoromethane and in all 4ABN SEIRA and SERS experiments using different solvents including n-heptane. The same held true for 3ABN and 2ABN. A common approach in these types of experiments is to rinse the substrate with solvent after adsorbate deposition to ensure only monolayer adsorption when studying adsorbates that have a functional group such as a carboxylate or sulfur that interacts strongly with the metal nanoparticles.35,36 It is clear here that the cyano−metal interaction is not strong enough for this method to work when studying any of the ABN isomers. 4589



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Table 3. 2ABN Vibrational Assignments 2ABN modes

2ABN freq. 6-311G (d,p)

NH2 bend

419

CC bend

570

ring CC IP stretch

715

Ag+/2ABN modes

Ag+/2ABN freq. LANL2DZ

SEIRA CH2Cl2

SEIRA heptane

SERS acetone

Raman powder

ring def./Ag−NC str. CH/NH2 bend ring def. ring breath

478

477

466

525 555 711

537 564 651

558

ring def.

835

826

CH bend NH2 bend ring def./NH2 bend CH/NH2 bend CH bend

966 991 1029

917 992 1037

728

ring def. CH bend CH/NH2 bend CH/NH2 bend

835 926 1010 1040

CH/NH2 bend CH bend and C−CN str.

1121 1148 1176

C−CN str.

1223

C−CN str.

1192

C−NH2 str. ring def. NH2 bend

1249 1301

ring def./C−NH2 str.

1309

C−NH2 str. ring def./NH2 bend ring def./NH2 bend

851

1440

n-heptane CH bend ring def. NH2 bend

ring def.

1474

n-heptane CH bend ring def.

ring def.

1545

ring def. NH2 scis. NH2 scis. CN str.

1591 1613 2243

ring def.

ATR

1024

1028 1138 1163 1186

1250 1313

1136 1151 1165 1179 weak 1198 1225 1259 1308

1361

1333

1128 1174

1161

1163

1203 1234 1265 1318

1230 1263 1317

1264 1305

1367 1394

1365

1374

1029

shoulder ∼1160

1186 1262 1326

1390 1430

1448

1458

1420 1462 1475

1451

1487

1493

1493

1515

1493

ring def.

1488 1469 1540

1562

1570

1576

1570

ring def. NH2 scis. NH2 scis. CN str.

1594 1639 2159

1600 1616 2199

1605 1633 2214

1568 weak 1606 1630 2214

1604

1608

2231

2210

cyano stretching mode is again at 2214 cm−1, the same frequency as 2ABN adsorption in dichloromethane. This supports the assertion that intramolecular hydrogen-bonding limits solvation effects during 2ABN adsorption. Note that the SERS spectrum of 2ABN in Figure 7 has a band assigned to a SNP−cyano stretch mode at 477 cm−1 that further confirms a SNP−cyano interaction (Table 3). Bands in the SEIRA spectra of 2ABN deposited from nheptane in Figure 6 at 1390 and 1475 cm−1 are indicative of nheptane incorporation into the 2ABN multilayer. Since it has already been shown that the cyano group of 2ABN interacts with the SNPs, it is likely that the n-heptane forms weak C−H hydrogen bonds with the lone pair of the amino group in 2ABN. Similar C−H hydrogen bonding has been demonstrated between the carboxylate group of 4-fluorbenzoate ions and nheptane in the multilayer in previous work.25,26 It is likely that the close proximity of the amino and cyano groups in 2ABN will allow for one n-heptane to form C−H hydrogen bonds with both substituent groups simultaneously, thus explaining the n-heptane intercalation into the 2ABN multilayer. A formal definition of a hydrogen bond according to Pimentel and McClellan is a bond that sterically involves a hydrogen atom that is already bonded to another atom.38,39 Along these lines,

Table 4. 4ABN−NH2 Wag Calculations

bonds. However, 2ABN, unlike 4ABN, is capable of intramolecular hydrogen-bonding between the cyano and amino groups. Intramolecular hydrogen-bonding is likely to dominate any solvation effect from dichloromethane, thus the cyano group of 2ABN can have a more intense interaction with the SNPs when deposited using a solvent with polar bonds. When 2ABN is deposited onto SNPs from n-heptane (a solvent with nonpolar bonds) as seen in the SEIRA spectra in Figure 6, the 4590



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an n-heptane molecule interacting with the amino and cyano groups of 2ABN would fit this definition of a hydrogen bond. As a model of this type of interaction, Figure 9 shows the

probably weaker than those of 2ABN and 4ABN. In fact, there is a nontotally symmetric CH out of plane bend mode in the DFT simulation of 3ABN at 947 cm−1 in Table 3 that contributes to the broad set of 3ABN SERS bands from 900 to 1000 cm−1. Unfortunately, fluorescence inhibited the obtainment of a Raman spectrum of 3ABN powder, making it more difficult to explore the idea of 3ABN−SNP charge transfer. There is also some recent evidence in the literature that suggests that the broad bands in the SERS spectra of the ABN isomers in Figure 7 around 900−1000 cm−1 might have a contribution from a normal mode containing a strong −NH2 wag component.41,42 To explore this hypothesis, DFT simulations are performed using B3LYP/LANL2DZ on gas phase 4ABN, 4ABN with a silver ion close to the amino group (Ag1), 4ABN with silver ions in proximity to both the amino and cyano groups (Ag2), and 4ABN with a silver ion near the cyano group (Ag3). Table 4 summarizes the outcomes of these calculations. The DFT calculated vibrational frequency of the −NH2 wag mode from the gas phase 4ABN simulation is 401 cm−1, for Ag1 the frequency of the −NH2 wag mode shifts to 1082 cm−1, for Ag2 the frequency shifts back closer to gas phase 4ABN at 539 cm−1, and the frequency for Ag3 is 544 cm−1. These results suggest that placing a silver ion close to the amino group of 4ABN blue shifts the −NH2 wag modes by 681 cm−1 in qualitative agreement to that demonstrated by prior researchers performing similar DFT calculations.42 Normally, the amino group of 4ABN has approximate sp2 hybridization in gas phase 4ABN. However, the presence of a silver ion close to the amino group of 4ABN changes the hybridization of the amino group to more sp3 character, which leads to the blue shift of the −NH2 wag mode. Through the Ag2 calculation, it is shown that the addition of a second silver ion close to the cyano group draws charge toward the cyano group, thus weakening the amino group interaction with the adjacent silver ion and red shifting the −NH2 wag mode back to 539 cm−1. Weakening of the amino/silver ion interaction is demonstrated by the fact that the silver ion-nitrogen (of the amino group) distance of 2.25 Å in Ag1 increases to 3.37 Å in Ag2 when a silver ion now also neighbors the cyano group. In fact, in Ag3 when there is a silver ion interaction with only a cyano group, the −NH2 wag mode is at 544 cm−1 and is shifted by only 5 cm−1 versus Ag2 when there are silver ions near both the amino and cyano groups. This further confirms that the amino/silver ion interaction is weak in the presence of a cyano/silver ion interaction. Such facts raise doubts on the premise that there is a significant amino group interaction during 4ABN adsorption on SNPs and to the possible contribution of −NH2 wag modes to the broad bands around 900−1000 cm−1 in SERS spectra of adsorbed monolayers of 4ABN isomers in Figure 7.

Figure 9. DFT optimized structure demonstrating C−H hydrogenbonding between 2ABN and ethane.

optimized structure resulting from a DFT/B3LYP/cc-PVTZ simulation of an ethane molecule in proximity to both the amino and cyano groups of 2ABN. The binding energy of the complex in Figure 9 was −2.71 kJ/mol. It is known in SERS experiments that the strongest bands typically arise from totally symmetric vibrational modes (analogous to unenhanced Raman spectroscopy) if the adsorbate is oriented perpendicular to the surface, considering only electromagnetic enhancement.40 However, it has been demonstrated that nontotally symmetric bands can appear in SERS spectra when there is a strong degree of charge transfer between SNPs and the adsorbate.40 On the basis of this knowledge, there is significant evidence for charge transfer between the ABN isomers and the SNPs that explain differences between the SERS spectra of the ABN isomers and the corresponding Raman spectra of their powders. First, in the SERS spectra of all three ABN isomers, there are the bands associated with an Ag−cyano stretch (see Tables 1−4) and shifts in the cyano stretch frequency versus the corresponding Raman powder spectra. These bands establish an interaction between the SNPs and the adsorbates while suggesting an upright orientation. Previously, it was stated that a strong band in the Raman spectrum of 4ABN powder at 845 cm−1 was less intense in the SERS spectrum of 4ABN and was replaced by a broad band at 955 cm−1. In Table 1, the band at 845 cm−1 is identified as a ring breathing mode (A1 symmetry) in the DFT simulation of 4ABN at 821 cm−1, and a likely contribution to the broad band at 955 cm−1 is an out of plane C−H bend mode (B2 symmetry) centered at 934 cm−1 in the 4ABN DFT calculation. The appearance of such an intense SERS band for 4ABN, a molecule of C2v symmetry, associated with a nontotally symmetric vibrational mode of B2 symmetry (such modes would not be active in conventional Raman spectra), is an indication of charge transfer between 4ABN and the SNPs. A similar situation holds for 2ABN. Disappearance in the SERS spectrum of the ring breathing mode observed in the Raman spectrum of 2ABN at 728 cm−1 and the appearance of a broad set of bands in the 900−1000 cm−1 associated with C−H out of plane bend modes (see Table 3) are strong evidence for 2ABN−SNP charge transfer. For 3ABN, there is also a broad set of bands in the SERS spectrum around 900−1000 cm−1 that implies a degree of 3ABN−SNP charge transfer even though it has been shown that the 3ABN interaction with the SNPs is

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SUMMARY SERS, SEIRA, and DFT calculations were used to investigate the adsorption of ABN isomers on SNPs as a function of the polar properties of the deposition solvent. Dissimilarities in the adsorption properties and thus vibrational adsorption spectra of the individual ABN isomers are attributed in part to differences in the resonance, charge transfer, and intramolecular hydrogenbonding properties of the isomers. It was shown that 2ABN and 4ABN, which both can resonantly distribute charge on the amino group throughout the molecule, could interact strongly with the SNPs in both the monolayer and multilayer. 4ABN would only interact strongly with the SNPs when deposition occurred using an alkane solvent with nonpolar bonds. 2ABN, 4591



Article

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however, interacted strongly with the SNPs regardless of the polar properties of the deposition solvent because intramolecular hydrogen-bonding between the amino and cyano group of 2ABN limited solvation effects. 3ABN was never observed to interact strongly with the SNPs because the lone pair charge on the amino group could not be resonantly distributed throughout the molecule. The appearance of broad bands associated with nontotally symmetric C−H bend modes in the range 900−1000 cm−1 in the SERS spectra of the ABN isomers provides the evidence for ABN−SNP charge transfer. There is also evidence that might suggest that these broad bands around 900−1000 cm−1 in the SERS spectra of the adsorbed aminobenzonitrile isomers have a contribution from −NH2 wag modes that are blue-shifted because of a change in hybridization of the amino group induced by interaction with the SNPs. C−H hydrogen-bonding was also seen between 2ABN and n-heptane because one n-heptane molecule can interact with both the amino and cyano groups of one 2ABN molecule. It is believed that this work will be important in a range of synthetic, physiological, and environmental applications where the interaction of benzonitrile analogues with metal nanoparticles is important.

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ASSOCIATED CONTENT

* Supporting Information S

Key normal vibrational modes for 4ABN including DFT calculated frequencies, IR absorbance, and Raman intensities. 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]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The National Science Foundation grant number CHE-1008096 funded this research. We acknowledge Dr. Patrick Desrochers, Dr. Jerry Manion, and Dr. K.C. Weaver for many discussions regarding the chemistry of aminobenzonitrile isomers.

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REFERENCES

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