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2009, 113, 9445–9449 Published on Web 05/06/2009
Molecule-Surface Orientational Averaging in Surface Enhanced Raman Optical Activity Spectroscopy Benjamin G. Janesko* and Gustavo E. Scuseria Department of Chemistry, Rice UniVersity, Houston, Texas, 77005 ReceiVed: March 20, 2009; ReVised Manuscript ReceiVed: April 23, 2009
Surface enhanced Raman optical activity spectroscopy (SEROA) of chiral molecules has proved much more challenging than conventional surface enhanced Raman spectroscopy (SERS). To further understand this, we calculated the “chemical effects” of molecule-metal binding in SEROA. The SEROA peaks’ signs and magnitudes are very sensitive to molecule-metal orientation; thus, SEROA from different molecules may cancel in ensemble measurements. We suggest that future SEROA experiments use ordered monolayers of chiral molecules to minimize orientational effects. Detection and characterization of molecular chirality plays a crucial role in modern bio- and nanotechnology. Many biomolecules are chiral, and their chirality plays an essential role in their structure and interactions. Chiral spectroscopic methods provide a route to determining the structure and dynamics of large proteins in their native environments.1 Additionally, highthroughput methods for determining absolute configuration and enantiomeric excess are important for the manufacture of chiral pharmaceuticals.2 Vibrational Raman optical activity spectroscopy (ROA) provides an incisive probe of the structure and dynamics of chiral (bio)molecules, of arbitrary size, in aqueous solution.1 ROA experiments measure small differences in the intensity of Raman scattering from chiral molecules in right- vs left-hand circularly polarized incident light, and/or a small circularly polarized component in the Raman scattered light.3-5 ROA is exquisitely sensitive to molecular conformation.1 The magnitudes and even signs of ROA peaks often depend strongly on conformation6-10 and solvation.11,12 ROA’s conformational sensitivity has been exploited to determine protein secondary and tertiary structure,13 to characterize the dynamics of partially unfolded proteins,14 and even to characterize intact virus capsids in solution.1 However, such sensitivity can also present a significant experimental challenge. Because of this, new ROA methods are typically tested on small, rigid chiral molecules like R-pinene where conformational effects are minimized.15-17 Despite its promise, widespread application of ROA has been hindered by its relatively weak signal. ROA intensities are typically 3-4 orders of magnitude weaker than the parent Raman intensities, due to ROA’s reliance on higher-order electric quadrupole and magnetic dipole scattering to generate the circular intensity difference.3 Vibrational Raman scattering can be enhanced by several orders of magnitude when the incident and scattered light interacts with surface plasmon states of a nearby nanostructured noble metal surface.18,19 This surface enhanced Raman spectroscopy (SERS) can provide chemical information with sen* To whom correspondence should be addressed. E-mail: bjanesko@ rice.edu.
10.1021/jp9025514 CCC: $40.75
sitivity down to the single-molecule level.20-22 SERS has matured into a valuable tool for molecular detection23-25 and characterization26 at the nanoscale. SERS enhancements are typically modeled as a sum of two effects: electromagnetic effects of the nanoparticle surface plasmon and chemical effects arising from molecule-metal bonding.19 Resonance Raman enhancements from molecule-metal charge transfer excited states can form a significant part of the chemical enhancement.27-31 Several investigators have attempted to use surface enhancement to improve ROA’s weak signal. Theoretical treatments of this surface enhanced Raman optical activity (SEROA) spectroscopy date back almost three decades.32-38 Unfortunately, experimental realizations of SEROA have proved challenging. Reports from the 1990s were plagued by large signal-to-noise ratios.39,40 More recent studies treat very large systems, often include resonance Raman enhancement, and yield very complicated spectra.41-45 To our knowledge, a definitive experiment showing mirror-image SEROA spectra from opposite enantiomers has not yet been presented. This is particularly troubling in light of ROA’s known sensitivity to experimental artifacts.46,47 It also raises an interesting fundamental question: Why are SEROA experiments so much more difficult than SERS experiments? Answers to this question must lie in the differences between the two techniques. One difference is that ROA requires circular polarization of the incident and/or scattered light. Reflection from metal surfaces can change circularly polarized light to elliptical polarization, degrading the ROA signal. Efrima concluded that typical model SERS substrates can still maintain sufficient circular polarization for SEROA.33 However, Etchegoin and co-workers recently noted that hot spots at nanostructured metal surfaces, which may be responsible for much of the total SERS enhancement,19,22 are particularly effective at destroying circular polarization.35 Another fundamental difference is that Raman scattering is typically dominated by electric dipole effects, while ROA requires higher-order magnetic dipole and/or electric quadrupole effects.3 Our semiclassical model of SEROA electromagnetic enhancements predicts that SEROA depends critically on the 2009 American Chemical Society
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J. Phys. Chem. C, Vol. 113, No. 22, 2009
Letters
Figure 1. Six stable Au2-CHFClBr conformations, modeling (S)-(+)-CHFClBr at an Au surface. The relative energies of conformations 1-6, including zero-point energies, are 0.2, 0.9, 0.0, 4.0, 3.7, and 4.3 kcal/mol. The largest Au2-CHFClBr binding energy (conformation 3), including zero-point and counterpoise corrections, is 15.4 kcal/mol.
substrate’s electric quadrupole response.36 Our results suggest that substrates dominated by an electric dipole response will be poor for SEROA, while substrates with a large quadrupole response may give large SEROA signals. Here, we focus on a third fundamental difference between surface enhanced Raman and ROA. ROA signals are signed, and (as discussed above) are very sensitive to conformation. Surface enhanced vibrational spectroscopies typically sample molecules in a range of fairly strongly perturbing local environments, arising from different molecule-metal orientations. Ensemble averaging over these environments often makes experimental SERS spectra broader and more complicated than the corresponding unenhanced Raman spectra.48-50 We argue that the effects of ensemble averaging will be even more acute for SEROA, particularly if the local environments are sufficiently strongly perturbing to change the sign of the SEROA peaks. For example, if a particular ROA peak has a positive sign in some molecule-surface conformations and a negative sign in others, the positive and negative signals will tend to cancel in an ensemble measurement. This mechanism, which is not present for (positive-definite) SERS signals, will tend to make SEROA of orientationally averaged molecules relatively weaker and more complicated than in the absence of orientational averaging. In this work, we demonstrate that these effects occur in very simplified model systems. We calculate the chemical effects on SEROA for small chiral molecules weakly bound to small metal clusters. The magnitudes and signs of the calculated SEROA peaks are strongly conformation dependent. While averages over the small number of molecule-surface conformations generated here still give reasonable SEROA intensities, the “scrambling” effects of multiple orientations are clear. We note that a recent publication by Jensen also investigated chemical effects in SEROA but did not treat multiple adsorption geometries.38 Our model SEROA spectra are generated as follows. We calculate the nonresonant Raman and ROA spectra of moleculemetal complexes in the gas phase, without a model of the substrate’s electromagnetic enhancements. We model a chiral molecule’s interaction with a nanostructured noble metal surface as binding to an Au2 cluster. Despite their obvious limitations, such small cluster models are regularly used to model chemical effects in SERS.51-57 These cluster models only need to capture the surface’s perturbations to the molecular vibrational spectrum, not the details of the molecule-surface interaction itself. Moreover, experimental Raman spectra of the silver salts of
phthalimide51 and salicylate55 reproduce many of the chemical effects seen in phthalimide and salicylate SERS, indicating that a single Ag atom provides a reasonable model for binding to the SERS substrate in these systems. Au2 is the smallest “electronic magic number” gold cluster. It is less reactive than other small gold clusters,58 and thus, we expect it to be a fairly reasonable model for noncovalent bonding to an Au surface despite its low valence. Finally, we note that this small cluster model is consistent with our present focus on simple model systems. Our calculations use a development version of the Gaussian electronic structure program,59 which includes gauge invariant (London) atomic orbitals60,61 and analytic third derivatives for calculating ROA.62-66 Raman and ROA spectra are computed using density functional theory (DFT) with the PBEh global hybrid functional,67-69 the LANL2DZ basis set and effective core potential on Au,70,71 and the 6-311++G(2d,2p) basis set on other atoms. (Other basis sets and functionals give qualitatively similar results.) Representative Au2-molecule geometries are generated by optimizing ∼1000 initial conformations with the UFF force field,72 reoptimizing the ∼20 resulting local minima with DFT, and calculating the Raman and ROA spectra of the resulting unique local minima with DFT. DFT calculations use the harmonic approximation for vibrational frequencies and Raman activities, very tight thresholds for SCF and geometry convergence, and a large numerical integration grid (300 radial, 770 angular points) for the energy and CPHF calculations. All species are assumed to be closed-shell singlets. The frequency calculations confirm that all reported structures are local minima on the geometric potential energy surface. IR + IL (IR - IL) denotes the calculated backscattered sum (difference) of Raman scattering in right- vs left-hand circularly polarized incident light, analytically averaged over all lab-molecule orientations and calculated at an incident wavelength of 532 nm. Spectra are plotted as the calculated Raman scattering activities, including a uniform Lorentzian broadening of width 10 cm-1. We begin with SEROA of (S)-(+)-CHFClBr.11,36,37,73 Figure 1 shows the six stable conformations we found for the Au2-CHFClBr complex modeling CHFClBr at an Au surface. The highest- and lowest-energy conformations differ by 2000 cm-1 are rescaled by 1/4. Spectra are shifted along the ordinate for clarity, and vertical lines denote spectral regions omitted from the plots.
Figure 3. Six stable Au2-CHF(OH)CH3 conformations modeling CHF(OH)CH3 at an Au surface. The relative energies of conformations 1-6, including zero-point energies, are 0.0, 0.2, 0.8, 4.5, 3.5, and 3.7 kcal/mol. The largest Au2-CHF(OH)CH3 binding energy (conformation 1), including zero-point and counterpoise corrections, is 13.6 kcal/mol.
ing over this small set of conformations still yields a nonnegligible ROA spectrum, certain peaks (such as the ∼3180 C-H stretch) are notably weaker in the averaged spectrum. We expect that the additional molecule-substrate conformations available on more realistic substrates will further complicate the SEROA spectra and reduce the SEROA intensity. Raman and ROA intensities in the complexes are generally somewhat larger than those in the isolated molecule. It is interesting to consider whether such chemical enhancement also occurs in experimental SEROA spectra. As a further illustration, we investigate SEROA of CHF(OH)CH3. Figure 3 shows the six stable conformations we found for the Au2-CHF(OH)CH3 complex, modeling CHF(OH)CH3 at an Au surface. The highest- and lowest-energy
conformations differ by 2000 cm-1 are rescaled by 1/8. Other details are as in Figure 2.
that all of these effects may produce additional “scrambling” and degradation of the SEROA from orientationally averaged molecules. For example, our semiclassical model predicts that substrate electromagnetic enhancements can change the sign of SEROA peaks. Our calculations,36 and those of Bourˇ,37 showed that the electromagnetic enhancements from different model substrates yielded different signs for the SEROA peaks of CHFClBr. Local variations in the substrate electromagnetic response, arising from different local environments, may change the sign of SEROA peaks and contribute to the effects discussed here. The calculated spectra in Figures 2 and 4 suggest that ensemble-averaged SEROA will be significantly degraded if the analyte molecules experience a wide range of local environments due to different molecule-surface adsorption geometries. This result appears consistent with the experimental SEROA spectra reported to date. The pentapeptide SEROA reported in ref 41 was significantly more structured than the SERS, consistent with peaks from multiple adsorption geometries. Similar conclusions hold for the proline SEROA in ref 45 and the controversial45 SEROA report of ref 74. References 42-44 reported resonance enhanced, surface enhanced ROA from myoglobin and cytochrome c. The resonance Raman and ROA signals were dominated by the heme chromophores, which may have been “insulated” by the surrounding protein from the worst moleculesurface orientational effects. Despite this, the surface enhanced resonance ROA spectra in ref 43 differed significantly from the conventional resonance ROA, while the conventional and surface enhanced resonance Raman spectra were quite similar. This is consistent with our assertion that SEROA is more sensitive than SERS to molecule-surface interaction effects.
These results suggest a new strategy for SEROA experiments. SERS spectra are often optimized at monolayer surface coverage, where an ordered monolayer of analyte molecules minimizes molecule-surface orientational averaging and inhomogeneous broadening. Our calculations suggest that this problem is more acute for SEROA. We propose that ordered monolayers of chiral molecules, covalently attached to a (quadrupolar36) SERS substrate, may provide a route to definitive smallmolecule SEROA. Acknowledgment. This work was supported by the National Science Foundation (CHE-0807194) and the Welch Foundation (C-0036). We thank Naomi J. Halas and Laurence A. Nafie for useful discussions and James R. Cheeseman for invaluable technical assistance. References and Notes (1) Barron, L. D.; Hecht, L.; McColl, I. H.; Blanch, E. W. Mol. Phys. 2004, 102, 731. (2) Schmitt, U.; Branch, S. K.; Holzgrabe, U. J. Sep. Sci. 2002, 25, 959. (3) Barron, L. D. Molecular Light Scattering and Optical ActiVity, 2nd ed.; Cambridge University Press: 2004. (4) Hecht, L.; Nafie, L. A. Mol. Phys. 1991, 72, 441. (5) Nafie, L. A. Annu. ReV. Phys. Chem. 1997, 48, 357. (6) Pecul, M.; Rizzo, A.; Leszczynski, J. J. Phys. Chem. A 2002, 106, 11008. (7) Kapita´n, J.; Baumruk, V.; Kopecky´, V., Jr.; Bourˇ, P. J. Phys. Chem. A 2006, 110, 4689. (8) Herrmann, C.; Ruud, K.; Reiher, M. Chem. Phys. 2008, 343, 200. (9) Haesler, J.; Schindelholz, I.; Riguet, E.; Bochet, C. G.; Hug, W. Nature 2007, 446, 526. (10) Mukhopadhyay, P.; Zuber, G.; Beratan, D. N. Biophys. J. 2008, 95, 5574.
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