Assessing the Role of Water on the Electronic Structure and

Article pubs.acs.org/crystal

Assessing the Role of Water on the Electronic Structure and Vibrational Spectra of Monohydrated L‑Aspartic Acid Crystals A. M. Silva,† S. N. Costa,† B. P. Silva,† V. N. Freire,† U. L. Fulco,‡ E. L. Albuquerque,‡ E. W. S. Caetano,§,* and F. F. Maia, Jr.∥ †

Departamento de Física, Universidade Federal do Ceará, Centro de Ciencias, Caixa Postal 6030, Campus do Pici, 60455-760 Fortaleza, Ceará, Brazil ‡ Departamento de Biofísica e Farmacologia, Universidade Federal do Rio Grande do Norte, 59072-970 Natal, Rio Grande do Norte, Brazil § Instituto de Educaçaõ , Ciêencia e Tecnologia do Cearâ, 60040-531 Fortaleza, Ceará, Brazil ∥ Universidade Federal Rural do Semi-Á rido, UFERSA, Campus Angicos, 59515-000 Angicos, Rio Grande do Norte, Brazil S Supporting Information *

ABSTRACT: In this work, we compare the calculated electronic and vibrational properties (infrared and Raman spectra) of anhydrous and monohydrated L-aspartic acid crystals to unveil the role of water in the later. This is accomplished through density functional theory (DFT) simulations within the Tkatchenko and Scheffler dispersion corrected generalized gradient approximation (GGA+TS). Both crystals are shown to have indirect band gaps and the simulations predict that water has a small effect on the Kohn− Sham band gap value (60 meV). Δ-sol corrected gaps, in contrast, exhibited a larger difference between the anhydrous and monohydrated crystals (main gap 0.30 eV larger for the latter). The conduction bands of the monohydrated species are much flatter because of structural changes produced by the presence of water, which leads to very large electron effective masses. However, the hole effective mass along the stacking direction of water molecules in the GGA+TS optimized monohydrated crystal is smaller than in other directions, suggesting that water stacking can improve on hole transport in similar bioorganic systems. These effects highlight the complex role of water on the carrier transport properties in monohydrated L-aspartic acid crystals, which is in contrast with the general belief that water simply increases the electrical conductance in molecular crystals. Finally, the calculated infrared and Raman spectra of the monohydrated phase exhibit molecular water vibrational signatures as well as water related peak shifts of as much as 100 cm−1 in comparison to those of the anhydrous structure. Remarkable water related Raman intensity peaks were obtained for the monohydrated crystal in the wavenumber ranges between 600 and 1000 cm−1 and between 2350 and 3450 cm−1, while for the infrared spectrum, a set of water absorption bands can be clearly identified in the 1550−1750 and 2800−3400 cm−1 wavenumber intervals.

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INTRODUCTION

The understanding of charge transfer processes at the biomolecular level has been the subject of intense research in the past few years, particularly for proteins and DNA. The role of tightly bound water molecules cannot be disregarded in the development of protein-based solid-state electronic conductors,11 being known that water has a relevant contribution to the charge transport properties in proteins.12 Simulations of interprotein electron transfer across a water interface13 show the existence of a structured water-mediated charge transport regime with an anomalous weak distance decay at relatively close protein−protein contact distances, and a bulk watermediated regime at large distances, suggesting that water may

The last two decades have witnessed the development of many diverse applications demanding materials with specific electronic properties and advances for the integration of nanotechnology to biological systems.1−3 Organic materials are very promising due to their simpler fabrication processes, lower cost, flexibility, and ability to be easily incorporated into other materials.4−7 On the other hand, they also tend to degrade with time and to react with water or other substances in undesirable ways, which must be prevented with the use of adequate containers. Charge transport in organic crystals is more complex than in typical semiconductors. Organic molecules in a crystal tend to form localized electronic states in which electrons move from one molecule to the next through quantum tunneling, which can be (possibly) controlled by external tuning of the electronic structure of the system.8−10 © 2013 American Chemical Society

Received: July 5, 2013 Revised: October 13, 2013 Published: October 28, 2013 4844

dx.doi.org/10.1021/cg401016v | Cryst. Growth Des. 2013, 13, 4844−4851

Crystal Growth & Design

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aspartic acid crystals obtained from first principle calculations carried out within the density functional theory formalism using the methodology of Tkatchenko and Scheffler33 to take into account dispersion effects. The unit cell data for the inputs was taken from X-ray diffraction data published by Derissen et al. for the anhydrous phase, and Umadevi et al. for the monohydrated crystal.34 Anhydrous L-aspartic acid crystals are monoclinic with space group P21 and Z = 2, while the monohydrated crystal are orthorhombic with space group P212121 and Z = 4. Their unit cells are shown in Figure 1a and 1b, and their unit cell parameters are given in Table 1.

be a strong tunneling mediator when occupying a sterically constrained space between redox cofactors. In the case of DNA double helices, it is well-known that the electronic conductance is very sensitive to the type of electrical contact, mechanical stress, type of solvation, hydration, temperature, molecular shape and molecular orientation,14 which has led to contradictory reports indicating metallic, semiconductor or insulating characteristics for DNA strands.15 Theoretical modeling, however, suggests that the increase of relative humidity around the phosphate group and DNA bases reduces the voltage gap and increases nonlinearly the maximum of conductance.16 Crystallographic data seems to point the formation of ordered water structures near B-DNA molecules which could be possibly involved in the conductance process.17 DFT calculations for four base pairs long B-DNA 5′-GAAT-3′ sequences18 revealed that water molecules clustered around distinct DNA regions produce electronic states in the π−π* gap, which are associated with the gap between the HOMO orbital of guanine and the LUMO orbital of thymine. Water states are very close to counterion (Na and Mg) states, implying the possibility of charge transport via hopping between water and counterions. Indeed, theoretical simulations have revealed that the DNA direct gap can be doped by water states15 and, in the case of A-DNA oligonucleotides, electronic wave function localization, induced either by electrical dipole and/or by the electrostatic disorder originating from the first few water solvation layers, drastically suppresses elastic conductance, leading to the suggestion that electron transport through DNA is a consequence of sequence-specific shortrange tunneling across a few bases together with diffusive/ inelastic processes.19 One can approach the problem of explaining humidity effects on the electronic conductivity by considering anhydrous and hydrated crystals of simple target organic/biological systems, such as DNA nucleobases20 and amino acids.21,22 The theoretical description of charge transport in organic crystals generally follows one of two modeling schemes: incoherent hopping and coherent band transport, depending on the complex interplay of several energy parameters.23 Effective masses calculated within the band structure framework, for example, are in good agreement with experimental values for the electronic transport in molecular field-effect transistors,24 and parameters obtained from density functional theory for organic crystals have been successfully used to provide an ab initio description of charge transport in organic semiconductors.25 The adhesion of amino acids on inorganic surfaces was investigated aiming the use of amino acid films in the development of optoelectronic devices and biosensors.26,27 A proposal to use amino acids and proteins on semiconductor surfaces to create new biodevices was reported,28,29 and biomolecules integrated with nanostructures were advanced as well.30 Crystals of alanine, leucine, isoleucine, and glycine were pointed to behave as wide gap semiconductors, while valine and cysteine are insulators.31,32 In a recent paper,22 density functional calculations for the structural and optoelectronic properties of L-aspartic acid anhydrous crystals and experimental optical absorption measurements resulted in band gap and effective mass estimates indicating that they are wide band gap semiconductors if an electric field is applied along a direction parallel to the molecular planes of L-aspartic acid.22 In this work, we present a comparison between the electronic and vibrational properties of anhydrous and monohydrated

Figure 1. (a) Unit cell of the anhydrous aspartic acid crystal, (b) unit cell of the monohydrated L-aspartic acid crystal, and (c) a representation of the water molecule stacking (water columns) along the a-axis of the monohydrated L-aspartic acid crystal. 4845



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however, we have some remarkable differences for the a and c lattice parameters, the first being smaller than the experimental value by about 0.1 Å (−2.3%) and latter being larger than the experimental value by 0.3 Å (2.5%). One can suspect that they are caused by the lack of many-body dispersion effects in the dispersion correction scheme we adopted. For example, the dispersion energy beyond the pairwise approximation has been recently shown to significantly affect the structures and energies of different polymorphs of glycine39,40 and other molecular crystals.41−43 Because of these differences observed between the optimized and experimental unit cells for the monohydrated crystal, we also have performed DFT calculations using the experimental lattice parameters and internal atomic coordinates to obtain and compare its electronic structure with that of the optimized case. It is known that Kohn−Sham band gaps are inaccurate, severely underestimating solid band gaps by up to 100%.44 Thus, aiming to improve our estimates, we have employed the Δ-sol technique proposed by Chan and Ceder,45 which extended the Delta self-consistent-field (ΔSCF) approach for molecules to crystalline structures. In their approach, total energy differences and electronic dielectric screening are taken into account to achieve much better gap estimates (they obtained a 70% reduction of mean absolute errors in comparison with Kohn−Sham gaps for over 100 compounds) at low computational cost from single point calculations using one of a set of well tested exchange-correlation functionals, including the GGA-PBE we adopt here.

Table 1. Structural Parameters of Anhydrous and Monohydrated L-Aspartic Acid Crystals: Experimental Data and DFT-GGA+TS Simulation Results anhydrous a (Å) b (Å) c (Å) V (Å3) β (deg)

monohydrated

exp.

GGA+TS

exp.

GGA+TS

7.617 6.982 5.142 269.438 99.840

7.646 6.959 5.118 268.880 99.091

5.587 9.822 11.813 648.244

5.714 9.725 11.521 640.165

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MATERIALS AND METHODS Structurally, the L-aspartic acid molecules in the anhydrous phase have their four carbon atoms contained in the same plane. The same planarity, however, is absent in the monohydrated crystal. A network of hydrogen bonds is essential to stabilize both crystals. Following the atomic labeling presented in Figure 1a and 1b, one can see that the anhydrous crystal has four distinct hydrogen bonds, denoted as η11 (H1−O1), η42 (H4−O2), η52 (H5−O2), and η63 (H6−O3). For the monohydrated L-aspartic acid unit cell, there are six distinct hydrogen bonds, three between L-aspartic acid molecules (η11, η43, η52, the first and the third reminiscent of the anhydrous structure) and three between a water molecule and an aspartic acid molecule (φ11, φ21, and φ6w). One can note from Figure 1c that the water molecules form stacks along the direction of the a axis, which can enhance hole transport along the Γ → Z direction, as we will show later. Geometry optimization was performed using a generalized gradient approximation (GGA) for the exchange-correlation functional including dispersion corrections. Afterward, the Kohn−Sham electronic bands and carrier effective masses were evaluated at some selected points in the Brillouin zone. A more detailed account of the results for the anhydrous phase was already published by the authors,22 so we focus here mainly on the description of the monohydrated crystal simulation results, contrasting them with our previously published data for the anhydrous case. Following the geometry optimization, we have calculated the vibrational properties for both crystals, obtaining their infrared and Raman spectral characteristics. The CASTEP code was used in all computer simulations.35 Internal atomic coordinates and unit cell lattice parameters were optimized in order to minimize the total energy. The GGA Perdew−Burke−Ernzerhof (PBE)36 exchange-correlation functional was chosen, supplemented by the dispersion correction scheme of Tkatchenko and Scheffler,33 which allows one to avoid the use of high-level quantum methods to include van der Waals interactions. Norm-conserving pseudopotentials37 replace the atomic core electrons with valence configurations C-2s22p2, N-2s22p3, and O-2s22p4. A Monkhorst-Pack38 3 × 4 × 5 (4 × 3 × 2) sampling was employed to evaluate integrals in the reciprocal space for the anhydrous (monohydrated) unit cell, ensuring a well converged electronic structure. The anhydrous lattice parameters obtained after the calculations are in good agreement with the experimental data, as one can see from Table 1. The calculated unit cell volume for the anhydrous (monohydrated) crystal is only 0.20% (1.2%) smaller than the X-ray diffraction measurements; a zwitterionic character for the charge distribution in the L-aspartic acid molecules was obtained (not discussed here, but see ref 22. for the anhydrous case). In the case of the monohydrated crystal,

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RESULTS AND DISCUSSION As the calculated GGA+TS lattice parameters for the monohydrated crystal are not very close to the experimental values, we have investigated how this difference affects the Kohn−Sham electronic band structure. The band curves near the main band gap for the two cases (experimental and optimized geometries) can be seen in Figure 2. As one can see,

Figure 2. Kohn−Sham band structure and density of states of the monohydrated L-aspartic acid crystal for the GGA+TS optimized geometry (black) and experimental lattice parameters (red).

the band gap values and band shapes are remarkably distinct, with the experimental structure having an indirect gap of only 3.57 eV, while the GGA+TS optimized structure exhibits an indirect gap about 1 eV larger, of 4.59 eV. However, some band features are the same for both crystalline geometries: the valence band maximum is at the Γ point and the conduction band minimum is near the Y point. In the valence band, the band curvatures along the Γ−T, Γ−Y, Γ−R, and Γ−S lines are larger for the experimental structure than for the optimized 4846



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indirect transitions between the Γ point at the uppermost valence band and the T and Y points at the lowest conduction band, respectively. As we discussed in the previous paragraph, the 4.59 eV indirect main gap for the GGA+TS optimized monohydrated crystal increases to 5.53 eV when one uses the Δ-sol approach. Structural and electronic changes caused by the water molecules do not significantly alter the value of the Kohn−Sham main band gap of the monohydrated phase in comparison with the anhydrous phase (an increase of only 60 meV), but produce a more noticeable change in the Δ-sol corrected gaps (increase of 0.30 eV of monohydrated phase with respect to the anhydrous phase) and remarkable differences in the pattern of valence and conduction bands below and above the band gap. For instance, the monohydrated crystal has its uppermost valence bands originating mainly from the carboxyl group, while the anhydrous crystal has a strong contribution of the COOH lateral chain. The monohydrated crystal has a set of valence bands near −0.5 eV mainly due to the COOH lateral chain, which does not occur for the anhydrous crystal. These changes are mostly because of the structural modifications promoted by the insertion of water molecules in the monohydrated phase, as the water molecules themselves do contribute to the electronic valence bands only for levels between −1.0 eV and −0.7 eV. The top of the valence band for the anhydrous crystal occurs at the B point, while for the monohydrated crystal the maximum of the valence band is at Γ. The bottom of the conduction band of the monohydrated structure has two distinct sets of two very flat and close band curves originating almost exclusively from electronic states at the COOH lateral chain while (see density of states plots at the right side of Figure 3), in the anhydrous case, one can also observe two sets of not so flat and not so close band curves, their dispersion being originated from the overlap of carboxyl and COOH lateral chain orbitals through the η11 hydrogen bond. Indeed, in the monohydrated crystal, the η11 hydrogen bond seems to be weakened because of the interaction of the O1 atom with two neighbor water molecules through hydrogen bonds φ11 and φ21 (see Figure 1b). On a per atom basis, there is a very strong contribution from O 2p levels to the valence and conduction bands near the main band gap, and a set of conduction bands with a small degree of H 1s character, with more pronounced contributions being observed in the monohydrated crystal because of the presence of water. The mobility of charge carriers in the L-aspartic acid crystals can be estimated through their effective masses, which are obtained from the band curvature at selected points of interest in reciprocal space.22 Looking now to Figure 4, we have obtained anhydrous crystal DFT-GGA+TS estimates for electron effective masses (in units of the free electron mass mo) ranging from 2.35 mo along the Γ−B line up to 11.6 mo, along Γ−E (not shown in Figure 4, see ref 22), while hole effective masses vary from 1.40 mo (B−Γ) up to 7.91 mo (B−α1, not shown in Figure 4, see ref 22), thus having a very pronounced anisotropy and heavier electrons (in comparison with holes). The results suggest that L-aspartic anhydrous crystals tend to behave as semiconductors if an electric field is applied in a direction parallel to the L-aspartic acid molecular planes. The monohydrated crystal, on the other hand, has much larger electron effective masses in comparison with the anhydrous case (see Figure 4, middle), ranging from 7.52 mo (electron, Γ−S) up to 23.8 mo (electron, Γ−Z) in the GGA +TS optimized unit cell, and from 5.76 mo (electron, Γ−Y) to

one, while for the Γ−X and Γ−U lines they are smaller. For the two lowest conduction bands, the band shapes along the Z−Γ− T−Y−Γ−X−U path are very similar, but some curvature differences can be noted along U−Γ−R−S−Γ. The total bandwidth for the four lowest conduction bands is larger for the optimized crystal in comparison with the experimental case. Conduction bands with higher energies tend to have larger curvatures in the experimental structure than those corresponding to the optimized crystal. The Δ-sol corrected band gaps for the experimental and GGA+TS optimized systems are 5.30 and 5.53 eV, respectively, which decreases the difference between the pure Kohn−Sham gaps from 1 eV down to 0.23 eV. Theoretical consistency, however, obliges us to consider the optimized GGA+TS crystal in future discussions and comparisons with the anhydrous crystal and especially for the calculation of the infrared and Raman spectra, as these rely on the quality of geometry optimization to avoid imaginary frequencies. The top part of Figure 3 shows the electronic band structure for the anhydrous L-aspartic acid crystal. A detailed discussion

Figure 3. Kohn−Sham band structure and density of states of (a) anhydrous and (b) monohydrated L-aspartic crystals near the main band gap. The most important valence to conduction band transitions are indicated by arrows. The density of states show the contributions of the carboxyl (solid black), COOH lateral (solid red), and H2O groups (dashed blue).

of its main features can be found in ref 22. Suffice to say that it has a main band gap (indirect) of 4.53 eV and a direct band gap of about 4.85 eV, the main gap increasing to 5.23 eV after applying the Δ-sol correction. One can compare the band curves for the anhydrous with the monohydrated phase (see Figure 3, bottom). The latter has its two smallest band gaps with very close energy values: 4.59 and 4.60 eV involving 4847



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(IA1599, IA1638, IA3280, IA3368) and Raman (RA681, RA726, RA3280, RA3321) modes are available as Supporting Information. The infrared absorption intensities were evaluated from the dynamical matrix and Born effective charges, the later approximated within the linear response formalism, while the Raman cross sections, on the other hand, were numerically calculated from the spatial derivatives of the macroscopic polarization along eigenvectors of each Raman active normal mode, also employing the linear response formalism.46,47 The anhydrous (monohydrated) phase has 93 (225) normal modes with A and B (A, B1, B2, and B3) symmetry. Infrared and Raman spectroscopy are very useful tools to investigate hydrogen bonds in molecular systems, as the characteristics of the A−H stretching vibration bands in A− H···B hydrogen bonds are modified. In particular, the diversity of hydrogen bonds observed in molecular crystals gives rise to vibrational signatures which can be related to the particular way in which intermolecular interactions occur in the lattice. For example, the investigation of the infrared spectra of isotopically diluted crystals revealed hydrogen/deuterium (H/D) selforganization effects.48 The coupling of intermolecular excitons to intramolecular vibrations and the oscillator strength for emission processes in crystals can be very different from those of an isolated molecule.49 Hydrated crystals, on the other hand, are more likely to display hydrogen bonding patterns and electronic structure changes related to their presence in comparison with the isolated molecules and anhydrous crystals can modify the vibrational spectra by altering the effective strength of an A−H covalent bond, increasing its stretching elastic constant and shifting the corresponding normal-mode frequency to higher values. If one compares the IR spectra of anhydrous and monohydrated L-aspartic acid crystals a series of water vibrational signatures can be pointed out. For example, in the wavenumber range between 1550 and 1750 cm−1, the anhydrous phase has three pronounced peaks at 1563, 1620, and 1656 cm−1, which are assigned, respectively, to NH3 wagging (γ NH3), NH deformation (δ NH), and NH2 scissoring (β NH2). On the other hand, the monohydrated crystal has the δ NH band slightly shifted down to 1616 cm−1, and two peaks related to the scissors movement of H2O at 1599 and 1638 cm−1, also depicted in the unit cell picture just below the infrared spectra curves at the left side of Figure 5. In the wavenumber range between 2800 and 3400 cm−1, two infrared peaks due to NH2 wagging modes are prominent at 2909 and 3000 cm−1 for the anhydrous crystal spectrum, while for the monohydrated case NH, NH2 and NH stretching bands occur at 2976, 3082, and 3125 cm−1, respectively, shifted up with respect to the anhydrous crystal corresponding frequencies probably due to the increase of the effective elastic constant of ammonium covalent bonds which also form a hydrogen bond (ϕ6w) with neighbor water molecules. Water stretching normal modes at 3280 and 3368 cm−1 can be seen as well (see middle unit cell picture at the right side of Figure 5). The Raman spectra of the monohydrated L-aspartic acid crystal has water related intensity peaks in the wavenumber range between 600 and 1000 cm−1, also between 2350 and 3450 cm−1. For the anhydrous crystal, the Raman spectra in those regions exhibit the following features: a COO deformation band near 760 cm−1 and a CH deformation band near 930 cm−1, two NH stretching bands near 2910 and 2990 cm−1, respectively, and one CH stretching band near 3060 cm−1. In the lowest wavenumber range for the monohydrated

Figure 4. DFT-estimated GGA+TS effective masses (in free electron masses, mo) of the (a) anhydrous, (b) GGA+TS optimized monohydrated L-aspartic acid crystals, and (c) GGA+TS experimental monohydrated L-aspartic acid crystals.

24.9 mo (electron, Γ−Z) in the experimental unit cell (Figure 4, bottom), indicating an insulating behavior for these carriers. Hole effective masses, however, are somewhat smaller for the GGA+TS optimized crystal, with the largest effective mass being 13.5 mo along the Γ → S direction. The Γ → Z hole effective mass, which corresponds to the direction of the water stacking in the monohydrated phase, is the smallest one, being 4.8 mo. For the experimental parameters, one can see most electron effective masses smaller than hole masses for almost all directions, except along Γ−Z, and a high degree of hole mass isotropy, with values ranging between 8.50 to 10.3 mo. Taking only the GGA+TS optimized cell into account, one may conclude that the ordered stacking of water molecules can help to favor hole transport along its length. The experimental unit cell results, on the other hand, are more favorable to electron transport along Γ−Y and hole transport along Γ−S and Γ−T. Some selected regions of the calculated infrared (IR) and Raman spectra of anhydrous and monohydrated L-aspartic acid crystals are shown in Figure 5. Animation files for the IR 4848



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Figure 5. Main water vibrational signatures in the infrared (two top panels) and Raman (two bottom panels) spectra of anhydrous A and monohydrated M L-aspartic acid crystals. Red vertical lines indicate the vibrational normal modes and respective intensities. Depictions of the most important normal modes involving water are also presented, with their assignments and frequencies (im cm−1) in the top and bottom of the unit cells, respectively. γ, δ, β, ν, and ζ stand for wagging, deformation, scissoring, stretching and rocking motions, in this order. νs means a symmetric stretching of bonds.

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CONCLUDING REMARKS In conclusion, we have performed a comparison between the electronic band structures, infrared and Raman spectra of anhydrous and monohydrated L-aspartic acid crystals, aiming to investigate the impact of water on the electronic transport and vibrational spectra of these materials. The anhydrous L-aspartic crystal has a Kohn−Sham main gap of 4.53 eV and the monohydrated crystal has a Kohn−Sham main gap of 4.59 eV for the GGA+TS optimized unit cell geometries. As these values are prone to exhibit significant error in comparison with experimental measurements, we have obtained improved band gaps using the Δ-sol approach, which increased these figures to 5.23 (anhydrous) and 5.53 eV (monohydrated), respectively. The insertion of water molecules significantly changes the valence and conduction bands near the main band gap and band curvature-effective mass analysis revealed that the wet (monohydrated) GGA+TS optimized L-aspartic acid crystals seem to be an n-type insulator and a p-type semiconductor for hole transport along the direction of water stacking, a result to be tested experimentally yet. It turned also very clear that

crystal, we have two bands related to CH deformations at 620 and 830 cm−1, with the band at 830 cm−1 corresponding to the 930 cm−1 band of the anhydrous case. On the other hand, the COO band of the anhydrous crystal is practically absent in the monohydrated structure. Besides, for the latter, a NH deformation band appears at about 960 cm−1, and two small Raman peaks related to water rocking and wagging occur at 682 and 726 cm−1, respectively (see unit cell picture at the leftbottom of Figure 5). In the largest wavenumber range, an OH stretching band occurs in the Raman curve for the monohydrated crystal near 2440 cm−1, and a CH2 symmetric stretch band appears near 3000 cm−1. NH stretching modes produce high intensity Raman lines between 3120 and 3130 cm−1, about 100 cm−1 above the corresponding lines for the anhydrous crystal. Finally, a set of bond stretching normal modes related to water produce a band centered at approximately 3300 cm−1 with most intense Raman peaks at 3280 cm−1 and 3321 cm−1 (see unit cell picture at the rightbottom of Figure 5). 4849



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molecular water stackings can contribute significantly to improve directional hole transport in hydrated amino acid crystals and similar ordered bioorganic structures, while at the same time preventing electron transport due to very large electron effective masses. Our results suggest that water can have a complex role on the carrier transport properties of biomolecular crystals, which is in contrast with the general belief that the inclusion of water in materials simply increases their electrical conductance. At the same time, as the molecular geometry of the monohydrated crystal changes in comparison with the anhydrous case, we have observed a series of characteristic infrared absorption and Raman intensity peaks which can be explained by changes in the local electronic structure related with the formation of hydrogen bonds with the water molecules, especially the ϕ6w bond involving the NH3 group. These hydrogen bonds contribute to shift a series of peaks above 2600 cm−1 to higher wavenumber values. At the same time, water molecule bands appear in the infrared spectra at nearly 1600, 1640, 3270, and 3370 cm−1, and in the Raman spectra near 700 and 3300 cm−1. A δ-CH normal mode at 930 cm−1 for the anhydrous crystal, on the other hand, is shifted down to 830 cm−1. We hope this work will stimulate experimental efforts to unveil the rich role of crystallized water in the structural, optoelectronic and vibrational properties of others hydrated biomolecular systems in the solid state.

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ASSOCIATED CONTENT

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AUTHOR INFORMATION

S Supporting Information *

Animation files for the visualization of some infrared (IA1599, IA1638, IA3280, and IA3368) and Raman (RA681, RA726, RA3280, RA3321) normal modes. This information is available free of charge via the Internet at http://pubs.acs.org/. Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work received financial support from the Brazilian Research Agencies CAPES (PROCAD and Rede NanoBioTec), CNPq (INCT-Nano(Bio)Simes) and FAPERN/CNPq (Pronex). E.W.S.C. received financial support from CNPq projects 304283/2010-0 and 474734/2011-0.

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Assessing the Role of Water on the Electronic Structure and

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