Surface-Enhanced Infrared Absorption and Density Functional Theory

Article pubs.acs.org/JPCC

Surface-Enhanced Infrared Absorption and Density Functional Theory Study of Dihydroxybenzene Isomer Adsorption on Silver Nanostructures Donald A. Perry,* Taylor M. Razer, Katherine M. Primm, TsungYen Chen, Jenna B. Shamburger, Jon W. Golden, Aaron R. Owen, Adam S. Price, Reece L. Borchers, and William R. Parker Department of Chemistry, University of Central Arkansas, Conway, Arkansas 72035, United States ABSTRACT: A combination of surface-enhanced infrared absorption (SEIRA) and density functional theory (DFT) was used to study the adsorption of the dihydroxybenzene (DH) isomers on silver nanostructures (SNSs). No evidence was observed for the oxidation of any of the DH isomers during adsorption on SNSs. It was found that the SNSs weakened intermolecular hydrogen bonding in thin DH layers adsorbed on SNSs versus the bulk powders with the effect being more pronounced for para-dihydroxybenzene (PDH) than ortho-dihydroxybenzene (ODH) and meta-dihydroxybenzene (MDH). DFT simulations of the infrared spectra of ODH and MDH dimers were good reproductions of the SEIRA spectra and infrared spectra of ODH/MDH powder, but DFT dimer infrared simulations were less effective at modeling the PDH results due to the large variations of hydrogen bonding between PDH films formed on SNSs versus PDH powder. Hydrogen-bonding effects were observed between acetone and both PDH and ODH in thin layers adsorbed on SNSs, and C−H···O hydrogen bonding was also seen between n-heptane and PDH and ODH in adsorption experiments on SNSs. These hydrogen-bonding effects were not detected between MDH and either acetone or n-heptane due to differences in resonance effects between MDH versus ODH and PDH.

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INTRODUCTION Phenolic compounds and their quinone oxidation products are integral to many biochemical, environmental, and industrial processes.1,2 Some quinones are important coenzymes in cells and play a role in activities such as respiration and photosynthesis.2 In the Unites States, 5 phenols are controlled under the Clean Air Act, and 11 phenols are monitored under the National Primary Drinking Water Regulation.3,4 Dihydroxybenzene (DH) isomers are some of the most versatile phenolic compounds. Catechol, or ortho-dihydroxybenzene (ODH), occurs naturally in some fruits and vegetables and is used extensively to make different pesticides and fragrances.5 Resorcinol, or meta-dihydroxybenzene (MDH), is a common disinfectant and antiseptic.6 Hydroquinone, or para-dihydroxybenzene (PDH), is an aqueous reducing agent with uses such as in photographic development.7 All of the DH isomers are important intermediates in synthetic chemistry.5−7 Most recently, even click chemistry has been demonstrated, where quinones that were photochemically oxidized from phenols bind with surface adsorbed thiols.8 DH isomers and their quinone oxidation products have been characterized with Raman spectroscopies including surfaceenhanced Raman spectroscopy (SERS).9−14 A number of infrared studies also exist involving the DH isomers.15−19 SERS results suggest that at least for hydroquinone there is a π interaction with silver or gold nanostructures, but it is not clear if there is any oxidation of the DH isomers in the monolayer during adsorption onto the metal nanostructures.13,14 SERS is known to be a short-range effect whereby vibrational © XXXX American Chemical Society

enhancement is confined predominantly to the monolayer. Surface-enhanced infrared absorption (SEIRA) has more of a long-range effect whereby vibrational spectra of adsorbates can be enhanced in several layers in proximity to metal nanostructures.20−24 It is a surprise that no SEIRA manuscripts exist for the DH isomers, especially when considering that hydroquinone is a common metal reducing agent used to form metal nanostructures in solution that are suitable for SERS or SEIRA.25,26 However, there has been a number of recent SEIRA studies involving other phenols.22,23,27−29 Two separate groups found clear evidence of oxidation of the nitrophenol isomers in the monolayer during adsorption onto metal nanostructures.23,28 In other work, no evidence was found in SEIRA studies for oxidation of aminophenol isomers adsorbed onto metal nanostructures.23 Another study used a combination of SERS and SEIRA to characterize fulvic acid micelles that contain phenol components.30 The DH isomers have been the subject of numerous studies involving various diffraction methods since the 1920s.31−36 ODH is known to form a hydrogen-bonded helical structure in the solid phase under standard state conditions.31,32 At ambient pressure, MDH forms a room temperature α and a hightemperature β phase with the most stable α phase having a hydrogen-bonded, orthorhombic structure where adjacent MDH molecules are out-of-plane from each other.33−36 PDH Received: December 10, 2012 Revised: February 25, 2013

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dx.doi.org/10.1021/jp3121462 | J. Phys. Chem. C XXXX, XXX, XXX−XXX

The Journal of Physical Chemistry C

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matrix solution was evaporated on the DH film consisting of 10 mg of α-cyano-4-hydroxycinnamic with a drop of 0.1% trifluoroacetic diluted to 1 mL with a 50/50 acetonitrile/ water solution. DFT calculations involving simulation of vibrational spectra and calculation of binding energies used the Gaussian 2009 suite at the B3LYP level of theory.39−41 A cc-PVTZ basis set was used for all calculations. All vibrational frequencies were scaled by a factor of 0.9651 to account for anharmonicity and other issues inherent to approximations made in the calculation. Further binding energies were also determined using DFT at the B3PW91 and MPW1PW91 levels of theory.39

can form α, β, and γ phases where the β and γ phases spontaneously convert to the α phase under standard conditions, and the α form is known to form a hydrogenbonded, double-helix structure.37 Here a combination of SEIRA and density functional theory (DFT) calculations was used to study the adsorption of the DH isomers on silver nanostructures (SNSs) using a number of deposition solvents with different polarities. SEIRA was used to monitor DH film growth from the monolayer through several multilayers. DFT calculations were used to model vibrational spectra and obtain binding energies of the DH isomers to silver ions and solvents. When considering the importance of phenolic and quinone compounds in a plethora of biological, synthetic, and environmental scenarios in conjunction with the rise of research and accompanying applications involving metal nanoparticles, it is apparent why it is necessary to study the interaction of the DH isomers with SNSs.

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RESULTS AND DISCUSSION Figure 1 has the DFT-optimized structures of the dimers for each DH isomer. Infrared spectra obtained from these simulations will be used to interpret experimental results. Figure 2a,b shows the SEIRA spectra of 5 and 400 μL exposures of a 50 ppm solution of ODH/CH2Cl2, respectively, deposited on a 7 nm silver film formed on BaF2. It is important to remember in SEIRA experiments that a 25 μL exposure of a 50 ppm solution results in approximately a monolayer of adsorbate after solvent evaporation. Hence, the 5 μL exposure of a 50 ppm solution of ODH in Figure 2a represent a submonolayer coverage of adsorbed ODH, and the 400 μL exposure is indicative of a multilayer. Figure 2c. is the ATRFTIR of ODH powder. DFT infrared simulations of one gasphase ODH molecule, the ODH dimer from Figure 1b, and the ODH dimer from Figure 1c are found in Figure 2d−f, respectively. Columns 1 and 2 of Table 1 are the band frequencies for the 5 and 400 μL exposures from the SEIRA spectra in Figure 2. Band frequencies for the ATR-FTIR of OHA powder are in column 3 of Table 1. In columns 4, 5, and 6 of Table 1 are the DFT-derived band frequencies from one ODH molecule, ODH dimer 1, and dimer 2. Corresponding mode assignments are in column 7. Figure 3 displays the OH stretch region of all infrared (SEIRA) results presented in this study. From bottom to top, the red spectra in Figure 3 are the SEIRA spectra obtained from 5 and 400 μL deposits of ODH/CH2Cl2 on SNSs, followed by the ATR-FTIR of ODH powder. The black spectra are SEIRA spectra of 25 and 200 μL exposures of a 50 ppm solution of MDH/CH2Cl2, followed by the ATR-FTIR of MDH powder. In the blue spectra are SEIRA spectra of 5 and 400 μL exposures of 50 ppm of PDH/CH2Cl2, followed by the ATRFTIR of PDH powder. The SEIRA spectra for thin ODH layers in Figure 2a,b resemble the ATR-FTIR of ODH powder in Figure 2c. A few weak bands appear in the multilayer ODH SEIRA spectrum that are not present in the submonolayer SEIRA spectrum. There are also a few minor shifts in band frequency and some small changes in relative band intensities between the two SEIRA spectra in Figure 2 that are attributed to a stronger interaction of ODH molecules in the monolayer with the SNSs. Similar differences exist between the SEIRA spectra and the ATR-FTIR spectrum of OHA powder that can also be ascribed to minor changes in the ODH layers induced by SNSs. In Figure 3, in the SEIRA spectra for ODH the OH stretch of the hydroxyl group with a hydrogen not participating in intramolecular hydrogen bonding is at 3442 cm−1, and the stretching frequency of the OH with a hydrogen atom participating in hydrogen bonding with the adjacent oxygen is at 3319 cm−1. SEIRA spectra of thin ODH films formed on

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EXPERIMENTAL SECTION All DH isomers were purchased from Aldrich. Solvents used in solution preparation were HPLC or Optima grade (Thermo Fisher or Aldrich). We purchased 25 × 4 mm polished CaF2 or BaF2 windows from International Crystal Laboratories. 99.99% pure silver wire for silver film preparation was from Myron Toback. SEIRA and attenuated total reflectance Fourier transform infrared spectroscopy (ATR-FTIR) spectra were obtained on a Thermo-Nicolet IR100 FTIR spectrometer. All infrared spectra were conducted in transmission mode (2−4 cm−1 resolution and 16 scans were averaged). ATR-FTIR spectra were collected with a Thermo Foundation Series Performer ATR attachment with a diamond ATR crystal (single-bounce design). SNSs were grown on CaF2 or BaF2 windows by thermal evaporation of 7 nm of silver, as measured with an Infinicon quartz crystal microbalance in a home-built vacuum chamber with a base pressure better than 1 × 10−6 Torr. BaF2 crystals were polished with a 0.3 μm alumina/methanol suspension, rinsed and sonicated in methanol, and dried in air. CaF2 crystals were polished with a silica suspension rinsed and sonicated in methanol and dried in air. A monolayer exposure of each DH isomer was prepared for SEIRA and SERS studies by pipetting 25 μL of a 50 ppm solution onto a silver film and allowing the solvent to evaporate. This is approximately a monolayer assuming an experimentally determined average spot size of four cm2 for all solvents resulting in reasonably uniform layer of ∼250 ng/cm2. The monolayer approximation here accounts for the number of DH molecules in the given area on a flat surface and does not specifically account for surface roughness. Typically SEIRA enhancement factors observed from DH adsorbates on these silver films were about a factor of 50 as compared with thick DH films deposited on bare CaF2 windows. Scanning electron microscopy and atomic force microscopy has been previously employed to investigate these silver films, which are typically rounded particles with an average particle size of 50−100 nm.38 Matrix-assisted laser desorption ionization−time-of-flight (MALDI-TOF) mass spectrometry was also utilized to look for any type of oxidation products in thin DH layers deposited on SNPs using a Waters MALDI Micro MX. To prepare a sample for MALDI-TOF analysis first in individual experiments, 10 μL of a 1000 ppm solution of each DH isomer in acetone was deposited on a 7 nm silver film formed on CaF2 and the solvent was allowed to evaporate. Then, the MALDI B



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Figure 2. (a) SEIRA spectrum of 5 μL of 50 ppm of ODH/CH2Cl2. (b) SEIRA spectrum of 50 ppm of 400 μL of ODH/CH2Cl2. (c) ATRFTIR of ODH powder. (d) DFT infrared simulation of a gas-phase ODH molecule. (e) DFT infrared simulation of the ODH dimer from Figure 1c. (f) DFT infrared simulation of the ODH dimer from Figure 1b. Panels c−f are downscaled to compare with the SEIRA spectra in panels a and b.

Examples exist in the literature where DFT dimer simulations have been shown to do reasonably well at reproducing infrared spectra where there exist strong intermolecular attractions between adjacent molecules.42,43 An inspection of the DFT results in Figure 2d−f demonstrates that the ODH dimer simulations in Figure 2e,f do a better job of reproducing the SEIRA/ATR-FTIR spectra of ODH than the infrared simulation extracted from one ODH molecule in Figure 2d. This is most apparent when comparing bands in the OH bend region between 1100 and 1250 cm−1. In fact, in Table 1, an inspection of the ODH ATR-FTIR band frequencies versus the frequencies derived from the simulation of one gas-phase ODH reveals that there are not even enough bands in the gas-phase ODH infrared simulation to account for all of the ODH ATRFTIR peaks. Note in the optimized ODH dimers structures in Figure 1c,d that adjacent ODH molecules are tilted with respect to each other. Tilted dimer geometries such as those seen for ODH are entirely consistent with the solid-state ODH helical structure derived from diffraction experiments.31,32 What this tilted geometry means in the resulting infrared simulation is that many normal modes are duplicated in each molecule of an ODH dimer and shifted by a few wavenumbers because the two ODH molecules in the dimer are not in geometrically equivalent positions. This can be seen in Table 1, whereby there are two frequencies assigned for each fundamental type of normal mode for the ODH dimers 1 and 2. Of course, a doublet will not be observed in the experimental spectra for each normal mode because many of these doublet bands will not be well-resolved. However, the doublet frequency shift for similar normal modes in adjacent molecules of the dimer does account for a number of the bands in the SEIRA/ATR-FTIR ODH spectra. An example in Table 1 is two bands in the

Figure 1. DFT-optimized structures of (a) PDH dimer, (b) ODH dimer 1, (c) ODH dimer 2, and (d) MDH dimer.

SNSs have OH stretch bands at 3448 and 3325 cm−1. A shift to higher value of six wavenumbers in the OH stretching bands going from the ATR-FTIR to adsorption on SNSs suggest that the SNSs are weakening hydrogen bonding between adjacent ODH molecules to a small degree. Weakening of intermolecular hydrogen bonding in ODH systems is likely limited by strong intramolecular hydrogen bonding between OH groups in ortho positions. Overall similarities between the ATR-FTIR and SEIRA spectra of ODH imply that there is little ionization of the hydroxyls in ODH layers adsorbed on SNSs. Additionally, MALDI-TOF experiments only showed that masses consist of unoxidized ODH (the same was true for MDH and PDH). Ionization of phenols by SNSs is not unprecedented and has been observed for nitrophenol isomers, as alluded to in the Introduction.23,28 C



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Table 1. ODH Band Frequencies in the Finger Print Region and Mode Assignments SEIRA low coverage

SEIRA high coverage

ATR

DFT ODH

721

721

721

727

744

742

739

754

770

769

770

821

852

827

849

835

866

850 868

922

916

1032

1014 1039

859 869 916 937 1018 1039

1097

1095

1092

1073

1150

1150

1150

1130

1167

1167

1164

1139

1190

1190

1183

1167

1244

1236

1229

1255 1273

1255 1277

1254 1279

1255

1342

1344

1363

1363 1383

1362 1383

1350

1470

1471

1469

1458

1494 1510 1525

1490 1512 1525

1491

1494

1601

1601

1597

1592

1618

1618

1620

1597

897 935 1017

1316

SEIRA spectra in column 2 at 1255 and 1277 cm−1, which appear at 1255 and 1266 cm−1 in the infrared DFT simulation of ODH dimer 1 in column 5 and are assigned as a ring deformation/CO stretch mode. Mode assignments are consistent with previous infrared studies of ODH.16−18 Figure 4 from bottom to top has SEIRA spectra of 5 and 400 μL of 50 ppm of a PDH/CH2Cl2 solution (base substrate 7 nm Ag/BaF2), the ATR-FTIR of PHA powder, the DFT infrared simulation of one PDH, and the DFT infrared simulation of the PDH dimer from Figure 1a. Table 2 has vibrational frequencies in the fingerprint region from Figure 4. In columns 1 and 2 are band frequencies for SEIRA spectra of 5 and 400 μL exposures of a 50 ppm solution of PDH/CH2Cl2, column 3 has the band frequencies for the ATR/FTIR of PDH powder, column 4 has band frequencies from gas-phase hydroquinone obtained from the literature,42 column 5 has band frequencies from the infrared DFT simulation of one PDH, column 6 has band frequencies from the infrared DFT simulation of the PDH dimer in Figure 1a, and column 7 has the corresponding mode assignments.

DFT ODH 1 dimer

DFT ODH 2 dimer

728 729 747 755 818 835 840 895 899 935 938 1021 1023 1074 1075 1138 1140 1142 1165 1181 1221 1231 1255 1266 1313 1319 1332 1355 1454 1458 1494 1496

731 732 755 757 825 833 838 902 906 932 946 1017 1019 1073 1079 1138 1142 1158 1169 1183 1222 1238 1254 1255 1316 1334 1348 1355 1458 1459 1494 1497

1587 1591 1594 1594 1598

1588 1593 1599 1604

modes CH bend ring breath CH bend ring def. CH bend CH bend CC str. ring def. OH bend CH/OH bend CH/OH bend CH/OH bend ring def. OH bend ring def. CO str. ring def. OH bend ring def. OH bend ring def. OH bend ring def. OH bend

ring def. OH bend ring def. OH bend

In Figure 4, the OH bend/CO stretch region from about 1100−1300 cm−1 is quite different when comparing the SEIRA spectra for PDH adsorbed in SNSs to the ATR-FTIR of PDH powder. The low-coverage SEIRA spectra have only two strong bands in this region at 1290 and 1190 cm−1 attributed to CO stretch modes. Recent work performed on germanium shows that ionized hydroquinone has a pair of doublets at 1290 and 1190 cm−1 that are similar to the CO stretch bands observed here at 1213 and 1240 cm−1.45 However, in Figure 3, the PDH SEIRA spectra have two bands at 3375 and 3324 cm−1 attributed to the symmetric and asymmetric OH stretch modes, respectively. This is much different from the ATRFTIR spectrum of PDH powder that has one broad band at 3205 cm−1 and the gas-phase infrared spectrum of PDH that has one band at 3630 cm−1. The presence of two OH stretch bands in the SEIRA spectra of PDH adsorbed on SNSs immediately confirms that PDH has not ionized during the adsorption process. Differences among the SEIRA, ATR-FTIR, and gas-phase spectra for PDH can be attributed to different degrees of hydrogen-bonding in each system. For the FTIRATR of PDH, the broad cluster of unresolved OH bend/CO D



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Figure 4. (a) SEIRA spectrum of 5 μL of 50 ppm of PDH/CH2Cl2, (b) SEIRA spectrum of 50 ppm of 400 μL of PDH/CH2Cl2, (c) ATRFTIR of PDH powder, (d) DFT infrared simulation of a gas-phase PDH molecule, and (e) DFT infrared simulation of the PDH dimer from Figure 1c. (f) DFT infrared simulation of the PDH dimer from Figure 1a. Panels c−e are downscaled to compare to the SEIRA spectra in panels a and b.

Figure 3. OH stretch region from bottom to top: (BLUE) SEIRA spectrum of 5 and 400 μL of 50 ppm of ODH/CH2Cl2, ATR-FTIR of ODH powder; (BLACK) SEIRA spectrum of 25 and 200 μL of 50 ppm of MDH/CH2Cl2, ATR-FTIR of MDH powder; and (RED) SEIRA spectrum of 5 and 400 μL of 50 ppm of PDH/CH2Cl2, ATRFTIR of PDH powder. All ATR-FTIR spectra have been downscaled.

predicts much stronger CH/OH bend modes at 1142 and 144 cm−1 than is actually observed in the ATR-FTIR spectrum. No doubt this is due to dissimilarities between the simple dimer simulation and the more complex hydrogen-bonding network in PDH powder. In Figure 5a−d from top to bottom are SEIRA spectra of 25, 50, 100, and 200 μL of MDH/CH2Cl2 deposited on a 7 nm silver/CaF2 film. Figure 5e is the ATR-FTIR of MDH powder, and Figure 5f is the DFT simulation of the MDH dimer found in Figure 1d. Table 3 contains information pertaining to Figure 5. Column 1 of Table 3 has the band frequencies for the SEIRA spectra of MDH, and column 2 has the vibrational frequencies from the ATR-FTIR of MDH powder. The band frequencies for a DFT infrared simulation of a gas-phase MDH is in column 3, and columns 4 and 5 have the band frequencies from the DFT infrared simulation of the MDH dimer and mode assignments, respectively. Mode assignments are determined in conjunction with the DFT calculations and reference to the literature.16,19 Results for the MDH experiments mimic in many ways what was observed for ODH. In general, in Figure 5, the SEIRA spectra for MDH are similar to the ATR-FTIR of MDH powder spare some of the OH bend modes. This suggests that there is little oxidation of MDH during adsorption on SNSs. It was also found that the MDH dimer DFT infrared simulation was a better match to the MDH SEIRA/ATR-FTIR data than the DFT infrared simulation of a single MDH molecule. Hence the DFT infrared spectrum of a gas-phase MDH is not included in Figure 5, but the band frequencies from that simulation are included in Table 3 for reference. As can be seen in Figure 1b, the optimized DFT structure is twisted (just as was the case for ODH and PDH), which is consistent with the out-of-plane structure observed for MDH powder in diffraction work.33−36 A

stretch modes from 1100 to 1300 cm−1 and the broad OH band centered at 3205 cm−1 are indicative of significant hydrogenbonding. For the SEIRA spectra, the lack of overlapping OH bends from 1100 to 1300 cm−1 and two resolvable OH stretching modes at 3375 and 3324 cm−1, which are blueshifted with respect to the ATR band at 3205 cm−1, imply less hydrogen bonding in the thin PDH films adsorbed on SNSs versus PDH powder. Gas-phase PDH has only one resolvable OH stretch mode at 3630 cm−1 (assigned as the OH asymmetric stretch), which represents a complete absence of hydrogen bonding.44 PDH in the gas phase has only one OH stretching mode because the symmetry of a lone PDH molecule is such that the transition moments of the symmetric OH stretches cancel each other. Once PDH is adsorbed onto SNSs, this symmetry is broken and both the symmetric and asymmetric OH stretch modes are observed. SEIRA results in Figure 4 bear many similarities to recent infrared reflectance studies involving PDH adsorption on germanium, although those results suggested that there was some PDH oxidation on germanium.45 Also presented in Figure 4 are infrared DFT simulations of a lone PDH and the PDH dimer. These infrared simulations do not match the SEIRA data for PDH because of the large variations in hydrogen-bonding observed in the experiments. However, simulations of the PDH dimer do a reasonable job of reproducing the ATR-FTIR of PDH powder. The one significant difference between the ATR-FTIR PDH spectrum and the DFT infrared PDH dimer simulation is that the dimer E



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Table 2. PDH Vibrational Frequencies in the Finger Print Region and Mode Assignments SEIRA low coverage

SEIRA high coverage

738 756 816 829

739 756 811 829

ATR 756 810 825

1012

1012

1008

1101

1099

1097

gas 758 822

DFT PDH

DFT PDH dimer

740 778 810 835

738 742 814 820 992 994 1078 1082 1142 1144 1148 1150 1163 1195 1212 1226 1244 1249

992 1086

1078

1120

1140

1164

1145

1186

1158

1213

1213

1205

1174

1226

1240

1240

1238 1259

1246

1246

1294 1313 1346 1367 1470 1491

1320 1348 1366 1458 1471

1321 1334 1354 1460 1468

1514

1514

1514

1532

mode ring def. ring def. CO str. CC/CH bend CH bend ring def. CH/OH bend CH/OH bend CH/OH bend CH/OH bend CO str. CO str.

1312

1313

ring def. OH bend

1326

1321

ring def. OH bend ring def. OH bend ring def. OH bend

1450

1439

1510

1499

1314 1323 1333 1441 1442 1496 1501 1587 1592

1532

1589

ring def. OH bend ring def. OH bend ring def. OH bend

DFT infrared MDH dimer simulation lead to a series of doublets that are shifted in frequency, just like the other DH isomers, and attributed to similar normal modes residing on each molecule in the MDH dimer. Inconsistencies between the experimental and theoretical results in Figure 5 can be found in the OH bend region from 1300 to 1400 cm−1. In Table 3, there are three bands in the 1300−1400 cm−1 region in the MDH SEIRA spectra but only two bands in the 1300−1400 cm−1 for both the ATR-FTIR of MDH powder and MDH dimer DFT infrared simulation. There are also some differences in the CH/ OH bend modes below 1220 cm−1 between the SEIRA and ATR-FTIR spectra of MDH. In all likelihood, these dissimilarities in SEIRA versus ATR-FTIR spectra of MDH are attributed to a subtle rearrangement of the MDH layers induced by the SNSs. This subtle rearrangement is further supported by the fact that for the SEIRA spectra of MDH adsorbed on SNSs in Figure 3 there is a broad band centered at ∼3300 cm−1 due to extensive hydrogen bonding between adjacent MDH molecules in the thin films. This broad band is centered at 3175 cm−1 in the ATR-FTIR of MDH. A shift from 3300 to 3175 cm−1 going from the powder form of MDH to thin MDH layers on SNSs shows that the SNSs weaken the hydrogen-bonding interaction between adjacent MDH molecules. In Figure 6, from bottom to top, gas-phase infrared spectrum of acetone, SEIRA spectra (substrate is 7 nm of silver evaporated on CaF2) of 25 and 400 μL of a 50 ppm solution of PDH/acetone, 50 μL exposure of a 1000 ppm PDH acetone solution, and 50 μL of a 1000 ppm solution of ODH/acetone are shown. A quick inspection of these data will reveal that

Figure 5. From bottom to top: (a−d) SEIRA spectrum of 25, 50, 100, and 200 μL of 50 ppm of MDH/CH2Cl2, (e) ATR-FTIR of MDH powder, and (f) DFT infrared simulation of the MDH dimer from Figure 1d.

F



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Table 3. MDH Band Frequencies in the Finger Print Region and Mode Assignments SEIRA 1080 1147 1167 1204 1217

1271 1298

ATR

DFT MDH

DFT MDH dimer

1110 1128 1144 1167

1061 1132 1138 1158 1186

1061 1062 1132 1136 1141 1159 1177 1184 1281 1288 1305 1308 1330 1347

1254 1283 1296 1308

1286 1305

1342 1368 1385 1463 1489

1377 1489

1466

1506

1506

1484

1535 1570 (broad) 1605 1622

1330

1606 1622

1465 1466 1485 1486 1586 1589 1602 1606

1590 1602

mode

CH/OH bend

ring def. CO str. ring def. ring def. OH bend ring def. OH bend ring def. OH bend ring def. OH bend ring def. OH bend ring def. OH bend

vacuum to eliminate acetone from the films. In the hydroquinone spectra, observe a substantial increase in acetone incorporation into the PHA films with increasing PDH/acetone exposure. (Note that the 50 μL/1000 ppm PDH/acetone spectrum is downscaled by a factor of 6.) Increased acetone intercalation into ODH films with cumulative ODH/acetone exposure was also observed. Note in Figure 6 that PDH bands in the lower coverage SEIRA spectra in Figure 4 in the range 1400−1600 cm−1 were completely masked by acetone after the highest acetone exposure. In the bottom of Figure 7 is the gas-phase spectrum of nheptane, followed by SEIRA spectra (substrate: 7 nm of silver grown on CaF2) of 25 and 400 μL exposures of a 50 ppm solution of ODH/n-heptane and 400 μL of a 50 ppm solution of PDH/n-heptane. In Figure 7, for the low-coverage ODH SEIRA spectrum (25 μL), the most intense n-heptane band can already be discerned at 1468 cm−1. For the 400 μL exposure of ODH in Figure 6, the 1468 cm−1 has now grown in intensity and n-heptane bands below 1400 cm−1 are now present. The 400 μL SEIRA spectrum for PDH is similar to that for ODH. Hydroxyl bend modes can still be observed in the range 1200− 1300 cm−1 for both PDH and ODH 400 μL exposures in Figure 7. N-heptane is seen to only intercalate into the ODH or PDH films in small increments with increasing exposure. No peak shifts or broadening are observed with the n-heptane bands as a function of increasing exposure, and n-heptane peak positions/fwhm in the thin films were similar to gas-phase nheptane. Results in Figure 6 are consistent with hydrogen-bonding between ODH/PDH and acetone, and results in Figure 7 are indicative of C−H···O hydrogen bonding between ODH/PDH and n-heptane. Recently a number of studies have documented C−H···O hydrogen bonding in thin films grown on metal interfaces.46−49 The ODH and PDH films adsorbed on SNSs in Figures 5 and 6 are only a few layers thick, so it is improbable that acetone or n-heptane could simply get trapped in the films.

Figure 6. From bottom to top: gas-phase infrared spectrum of acetone (downscaled), followed by SEIRA spectra of: 25 and 400 μL of 50 ppm of PDH/acetone, 50 μL of 1000 ppm of PDH/acetone (downscaled by a factor of 6), and 50 μL of 1000 ppm of ODH/ acetone (downscaled by a factor of 2).

there is a significant amount of acetone incorporated into the PDH and ODH layers deposited on SNSs. Gas-phase acetone has a cluster of bands just above 1200 cm−1 and bands at 1365, 1435, and 1738 cm−1. The same acetone bands appear in the ODH and PDH films at the same frequencies and full width at half-maximum (fwhm) as gas-phase acetone. Acetone bands remain constant in the ODH and PDH films for hours, and it was necessary to pump the ODH and PDH films in a high G



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Figure 7. From bottom to top: gas-phase infrared spectrum of nheptane (downscaled), followed by SEIRA spectra of: 25 and 400 μL of 50 ppm of ODH/n-heptane and 400 μL of 50 ppm of PDH/nheptane.

As another test, in four separate experiments test ODH and PDH powders were saturated individually with acetone and nheptane on glass. After 20 min there was no evidence of any of the solvent left in the ATR-FTIR spectra of the ODH/PDH powders. In contrast, n-heptane and acetone were stable in the thin PDH and ODH films on SNSs for hours. It appears that the presence of the SNS is polarizing thin layers of the PDH and ODH in such a way that the attractive interaction between PDH/ODH and either n-heptane or acetone is increased. This polarizing effect would not be observed in MDH because a −meta species cannot resonantly distribute charge residing on the hydroxyl groups, as happens with PDH and ODH. Hence, intermolecular attractions between MDH and either acetone or n-heptane are weaker than that observed for the other DH isomers. Hydrogen-bonding effects in DH films using CH2Cl2 or methanol as the deposition solvents were not observed. Figure 8a,b contains the DFT/B3LYP optimized structures for PDH/acetone and ODH/acetone, and Figure 8c,d has the DFT/B3LYP optimized structures for PDH/ethane and ODH/ ethane, where ethane serves as a simpler model versus a calculation involving n-heptane. The binding energies presented in the next few sentences in parentheses are from the different DFT levels of theory calculated with a cc-pvtz basis set: (B3LYP/B3PW91/MPW1PW91). From these DFT calculations, the binding energies between PDH and acetone are (−32.89/−29.60/32.91) kJ/mol and ODH and acetone are (−36.60/−33.44/−36.98) kJ/mol. The binding energies between PDH and ethane are (−3.13/−1.17/2.77) kJ/mol and ODH and ethane are (−3.92/−1.84/−3.93) kJ/mol. In the calculations, the B3PW91 level of theory consistently gave binding energies a few kilojoules per mole weaker than the other theories. However, being that all calculations were within a few kilojoules per mole for each level of theory, this lends support that these calculations provide a good model of the different hydrogen-bonding interactions presented here.

Figure 8. DFT optimized structures of (a) acetone/PDH, (b) acetone/ODH, (c) ethane/PDH, (d) ethane/ODH.

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CONCLUSIONS A combination of SEIRA and DFT calculations is employed to study the adsorption of the DH isomers on SNSs from the monolayer into the multilayer. No evidence of any oxidation of the DH isomers as a result of adsorption onto SNSs is found. Red shifts in OH stretching mode frequencies in combination with changes in the fingerprint region demonstrate that adsorption of the DH isomers onto SNSs weakens the intermolecular hydrogen-bonding networks typically discernible in the ATR-FTIR spectra of the DH powders. DFT simulations of the infrared spectra of ODH and MDH dimers did a reasonable job of reproducing the hydrogen-bonding interactions in both the SEIRA and ATR-FTIR spectra of ODH and MDH. For PDH, there were obvious variations in the DFT infrared simulations of the PDH dimer against both the PDH SEIRA and ATR-FTIR powder spectra due to wide variations in hydrogen-bonding effects, as observed going from experimental gas phase, to SEIRA, to bulk powder spectra. Hydrogen bonding was also seen between acetone and PDH/ ODH thin films adsorbed on SNSs, and C−H···O hydrogenH



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bonding was also documented between n-heptane and PDH/ ODH thin films in SEIRA experiments. These hydrogenbonding effects were never observed in MDH layers deposited on SNSs due to differences in resonance effects with −meta species.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: (501)450-5937. 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 fruitful discussions.

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Surface-Enhanced Infrared Absorption and Density Functional Theory

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