Nitrile Vibrations as Reporters of Field-Induced Phase Transitions in 4

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Nitrile Vibrations as Reporters of Field-Induced Phase Transitions in 4‑Cyano-4′-pentylbiphenyl (5CB) James M. Marr and J. Daniel Gezelter* Department of Chemistry and Biochemistry, University of Notre Dame, 251 Nieuwland Science Hall, Notre Dame, Indiana 46556, United States

ABSTRACT: 4-Cyano-4′-pentylbiphenyl (5CB) is a liquid crystal forming compound with a terminal nitrile group aligned with the long axis of the molecule. Simulations of condensed-phase 5CB were carried out both with and without applied electric fields to provide an understanding of the Stark shift of the terminal nitrile group. A field-induced isotropic−nematic phase transition was observed in the simulations, and the effects of this transition on the distribution of nitrile frequencies were computed. Classical bond displacement correlation functions exhibit a ∼2.3 cm−1 red-shift of a portion of the main nitrile peak, and this shift was observed only when the fields were large enough to induce orientational ordering of the bulk phase. Distributions of frequencies obtained via cluster-based fits to quantum mechanical energies of nitrile bond deformations exhibit a similar ∼2.7 cm−1 red-shift. Joint spatial-angular distribution functions indicate that phase-induced anticaging of the nitrile bond is contributing to the change in the nitrile spectrum.

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kinetics of the phase transition,13 the 5CB nitrile group has not yet seen the detailed theoretical treatment that biologically relevant small molecules have received.14−19 The fundamental characteristic of liquid crystal mesophases is that they maintain some degree of orientational order while translational order is limited or absent. This orientational order produces a complex direction-dependent response to external perturbations like electric fields and mechanical distortions. The anisotropy of the macroscopic phases originates in the anisotropy of the constituent molecules, which typically have highly nonspherical structures with a significant degree of internal rigidity. In nematic phases, rod-like molecules are orientationally ordered with isotropic distributions of molecular centers of mass. For example, 5CB has a solid to nematic phase transition at 18C and a nematic to isotropic transition at 35C.11 In smectic phases, the molecules arrange themselves into layers with their long (symmetry) axis normal (SA) or tilted (SC) with respect to the layer planes. The behavior of the SA

INTRODUCTION Because the triple bond between nitrogen and carbon is sensitive to local electric fields, nitrile groups can report on field strengths via their distinctive Raman and IR signatures.1 The response of nitrile groups to electric fields has now been investigated for a number of small molecules,2 as well as in biochemical settings, where nitrile groups can act as minimally invasive probes of structure and dynamics.3−6 The vibrational Stark effect has also been used to study the effects of electric fields on nitrile-containing self-assembled monolayers at metallic interfaces.7,8 Recently, 4-cyano-4′-pentylbiphenyl (5CB), a liquid crystalline molecule with a terminal nitrile group, has seen renewed interest as one way to impart order on the surfactant interfaces of nanodroplets,9 or to drive surface ordering that can be used to promote particular kinds of self-assembly.10 The nitrile group in 5CB is a particularly interesting case for studying electric field effects, as 5CB exhibits an isotropic to nematic phase transition that can be triggered by the application of an external field near room temperature.11,12 This presents the possibility that the field-induced changes in the local environment could have dramatic effects on the vibrations of this particular nitrile bond. Although the infrared spectroscopy of 5CB has been well-investigated, particularly as a measure of the © XXXX American Chemical Society

Special Issue: James L. Skinner Festschrift Received: April 1, 2014 Revised: May 9, 2014

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dx.doi.org/10.1021/jp503235s | J. Phys. Chem. B XXXX, XXX, XXX−XXX

The Journal of Physical Chemistry B

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these substructures (but with connectivity to the rest of the molecule) were still included in the potential and force calculations. Periodic simulation cells containing 270 molecules in random orientations were constructed and were locked at experimental densities. Electrostatic interactions were computed using damped shifted force (DSF) electrostatics.32 The molecules were equilibrated for 1 ns at a temperature of 300 K. Simulations with applied fields were carried out in the microcanonical (NVE) ensemble with an energy corresponding to the average energy from the canonical (NVT) equilibration runs. Typical applied-field equilibration runs were more than 60 ns in length. Static electric fields with magnitudes similar to what would be available in an experimental setup were applied to the different simulations. With an assumed electrode separation of 5 nm and an electrostatic potential that is limited by the voltage required to split water (1.23 V), the maximum realistic field that could be applied is ∼0.024 V/Å. Three field environments were investigated: (1) no field applied, (2) partial field = 0.01 V/Å, and (3) full field = 0.024 V/Å. After the systems had come to equilibrium under the applied fields, additional simulations were carried out with a flexible (Morse) nitrile bond

phase can be explained with models based solely on geometric factors and van der Waals interactions. The Gay−Berne potential, in particular, has been widely used in the liquid crystal community to describe this anisotropic phase behavior.20−24 However, these simple models are insufficient to describe liquid crystal phases which exhibit more complex polymorphic nature. Molecules which form SA phases can exhibit a wide variety of subphases like monolayers (SA1), uniform bilayers (SA2), partial bilayers (S Ã ), as well as interdigitated bilayers (SAd), and often have a terminal cyano or nitro group. In particular, lyotropic liquid crystals (those exhibiting liquid crystal phase transitions as a function of water concentration) often have polar head groups or zwitterionic charge separated groups that result in strong dipolar interactions,25 and terminal cyano groups (like the one in 5CB) can induce permanent longitudinal dipoles.26 Modeling of the phase behavior of these molecules either requires additional dipolar interactions27 or a unified-atom approach utilizing point charges on the sites that contribute to the dipole moment.28 Macroscopic electric fields applied using electrodes on opposing sides of a sample of 5CB have demonstrated the phase change of the molecule as a function of electric field.29 These previous studies have shown the nitrile group serves as an excellent indicator of the molecular orientation within the applied field. Lee et al. showed a 180° change in field direction could be probed with the nitrile peak intensity as it changed along with molecular alignment in the field.13,30 While these macroscopic fields work well at indicating the bulk response, the response at a molecular scale has not been studied. With the advent of nanoelectrodes and the ability to couple these electrodes to atomic force microscopy, control of electric fields applied across nanometer distances is now possible.31 In special cases where the macroscopic fields are insufficient to cause an observable Stark effect without dielectric breakdown of the material, small potentials across nanometersized gaps may have sufficient strength. For a gap of 5 nm between a lower electrode having a nanoelectrode placed near it via an atomic force microscope, a potential of 1 V applied across the electrodes is equivalent to a field of 2 × 108 V/m. This field is certainly strong enough to cause the isotropic− nematic phase change and an observable Stark tuning of the nitrile bond. We expect that this would be readily visible experimentally through Raman or IR spectroscopy. In the sections that follow, we outline a series of coarsegrained (united atom) classical molecular dynamics simulations of 5CB that were done in the presence of static electric fields. These simulations were then coupled with both ab intio calculations of CN deformations and classical bond-length correlation functions to predict spectral shifts. These predictions should be verifiable via scanning electrochemical microscopy.

V (rCN) = De(1 − e−β(rCN − re))2

(1)

where re = 1.157 Å (the fixed CN bond length from the force field of Guo et al.28), De = 212.95 kcal mol−1 (the average bond energy for CN triple bonds), and β = 2.526 Å−1. These parameters correspond to a vibrational frequency of ∼2226 cm−1, which is very close to the frequency of the nitrile peak in the vibrational spectrum of neat 5CB. The flexible nitrile moiety required simulation time steps of 1 fs, so the additional flexibility was introduced only after the rigid systems had come to equilibrium under the applied fields. Whenever time correlation functions were computed from the flexible simulations, statistically independent configurations (separated in time by 10 ns) were sampled from the last 110 ns of the induced-field runs. These configurations were then equilibrated with the flexible nitrile moiety for 100 ps, and time correlation functions were computed using data sampled from an additional 20 ps of run time carried out in the microcanonical ensemble.

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FIELD-INDUCED NEMATIC ORDERING In order to characterize the orientational ordering of the system, the primary quantity of interest is the nematic (orientational) order parameter. This was determined using the tensor Q αβ =

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COMPUTATIONAL DETAILS The force field used to model 5CB was a united-atom model that was parameterized by Guo et al.28 However, for most of the simulations, both of the phenyl rings and the nitrile bond were treated as rigid bodies to allow for larger time steps and longer simulation times. The geometries of the rigid bodies were taken from equilibrium bond distances and angles. Although the individual phenyl rings were held rigid, bonds, bends, torsions, and inversion centers that involved atoms in

1 2N

N

∑ (3uî αuî β − δαβ) i=1

(2)

where α, β = x, y, z and ûi is the molecular end-to-end unit vector for molecule i. The nematic order parameter S is the largest eigenvalue of Qαβ, and the corresponding eigenvector defines the director axis for the phase. S takes on values close to 1 in highly ordered (smectic A) phases but falls to much smaller values (0 → 0.3) for isotropic fluids. Note that the nitrogen and the terminal chain atom were used to define the vector for each molecule, so the typical order parameters are lower than if one defined a vector using only the rigid core of B



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Figure 1. Evolution of the orientational order parameters for the no-field, partial-field, and full-field simulations over the course of 60 ns. Each simulation was started from a statistically independent isotropic configuration. On the right are ellipsoids representing the final configurations at three different field strengths: zero field (bottom), partial field (middle), and full field (top).

obtain Morse-oscillator fits for the local vibrational motion along that bond. (2) The empirical frequency correlation maps developed by Choi et al.16,17 for nitrile moieties in water were utilized by adding an electric field contribution to the local electrostatic potential. (3) Classical bond-length autocorrelation functions were Fourier transformed to directly obtain the vibrational spectrum from molecular dynamics simulations. CN Frequencies from Isolated Clusters. The size of the condensed-phase liquid crystal system prevented direct computation of the complete library of nitrile bond frequencies using ab initio methods. In order to sample the nitrile frequencies present in the condensed phase, individual molecules were selected randomly to serve as the center of a local (gas phase) cluster. To include steric, electrostatic, and other effects from molecules located near the targeted nitrile group, portions of other molecules nearest to the nitrile group were included in the quantum mechanical calculations. Steric interactions are generally shorter ranged than electrostatic interactions, so portions of surrounding molecules that cause electrostatic perturbations to the central nitrile (e.g., the biphenyl core and nitrile moieties) must be included if they fall anywhere near the CN bond. Portions of these molecules that interact primarily via dispersion and steric repulsion (e.g., the alkyl tails) can be truncated at a shorter distance. The surrounding solvent molecules were therefore divided into “body” (the two phenyl rings and the nitrile bond) and “tail” (the alkyl chain). Any molecule which had a body atom within 6 Å of the midpoint of the target nitrile bond had its own molecular body (the 4-cyano-biphenyl moiety) included in the configuration. Likewise, the entire alkyl tail was included if any tail atom was within 4 Å of the target nitrile bond. If tail atoms (but no body atoms) were included within these distances, only the tail was included as a capped propane molecule. Inferred hydrogen atom locations were added to the cluster geometries, and the nitrile bond was stretched from 0.87 to 1.52 Å at increments of 0.05 Å. The stretching was carried out by displacing the nitrogen atom position along the CN bond vector. This generated 13 configurations per gas phase cluster. Single-point energies were computed using the B3LYP

the molecule. In nematic phases, typical values for S are close to 0.5. The field-induced phase transition can be clearly seen over the course of a 60 ns equilibration run in Figure 1. All three of the systems started in a random (isotropic) packing, with order parameters near 0.2. Over the course of 10 ns, the full field causes an alignment of the molecules (due primarily to the interaction of the nitrile group dipole with the electric field). Once this system began exhibiting nematic ordering, the orientational order parameter became stable for the remaining 150 ns of simulation time. It is possible that the partial-field simulation is metastable and, given enough time, would eventually find a nematic-ordered phase, but the partial-field simulation was stable as an isotropic phase for the full duration of the 60 ns simulation. Ellipsoidal renderings of the final configurations of the runs show that the full field (0.024 V/Å) experienced an isotropic−nematic phase transition and was ordered with a director axis that is parallel to the direction of the applied field.

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SAMPLING THE CN BOND FREQUENCY

The vibrational frequency of the nitrile bond in 5CB depends on features of the local solvent environment of the individual molecules as well as the bond’s orientation relative to the applied field. The primary quantity of interest for interpreting the condensed-phase spectrum of this vibration is the distribution of frequencies exhibited by the 5CB nitrile bond under the different electric fields. There have been a number of elegant techniques for obtaining vibrational line shapes from classical simulations, including a perturbation theory approach,18 the use of an optimized QM/MM approach coupled with the fluctuating frequency approximation,15 and empirical frequency correlation maps.16,17 Three distinct (and comparatively primitive) methods for mapping classical simulations onto vibrational spectra were brought to bear on the simulations in this work: (1) Isolated 5CB molecules and their immediate surroundings were extracted from the simulations. These nitrile bonds were stretched by displacing the nitrogen along the CN bond vector with the carbon atom remaining stationary. Single-point ab initio calculations were used to C



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Figure 2. Cluster calculations were performed on randomly sampled 5CB molecules (shown in red) from the full-field and no-field simulations. Surrounding molecular bodies were included if any body atoms were within 6 Å of the target nitrile bond, and tails were included if they were within 4 Å. Included portions of these molecules are shown in green. The CN bond on the target molecule was stretched and compressed, and the resulting single point energies were fit to Morse oscillators to obtain a distribution of frequencies.

functional33,34 and the 6-311++G(d,p) basis set. For the cluster configurations that had been generated from molecular dynamics running under applied fields, the density functional calculations had a field of 5 × 10−4 atomic units (Eh/(ea0)) applied in the +z direction in order to match the molecular dynamics simulations. The energies for the stretched/compressed nitrile bond in each of the clusters were used to fit Morse potentials, and the frequencies were obtained from the 0 → 1 transition for the energy levels for this potential.35 To obtain a spectrum, each of the frequencies was convoluted with a Lorentzian line shape with a width of 1.5 cm−1. This line width corresponds to a vibrational lifetime of ∼3.5 ps, which is within the reported ranges (∼1−5 ps) for CN stretching vibrational lifetimes in other molecules.19,36,37 Available computing resources limited the sampling to 100 clusters for both the no-field and full-field spectra. Comparisons of the quantum mechanical spectra to the classical spectra are shown in Figure 4. The mean frequencies obtained from the distributions give a field-induced red-shift of 2.68 cm−1. CN Frequencies from Potential-Frequency Maps. One approach which has been used to successfully analyze the spectrum of nitrile and thiocyanate probes in aqueous environments was developed by Choi et al.16,17 This method involves finding a multiparameter fit that maps between the local electrostatic potential at selected sites surrounding the nitrile bond and the vibrational frequency of that bond obtained from more expensive ab initio methods. This approach is similar in character to the field-frequency maps developed by the Skinner group for OH stretches in liquid water.38,39 To use the potential-frequency maps, the local electrostatic potential, ϕa, is computed at 20 sites (a = 1 → 20) that surround the nitrile bond

Figure 3. Observed cluster frequencies have no apparent correlation with the electric field felt at the centroid of the nitrile bond. Upper panel: vibrational frequencies plotted against the component of the field parallel to the CN bond. Middle panel: vibrational frequencies plotted against the magnitude of the field components perpendicular to the CN bond. Lower panel: vibrational frequencies plotted against the total field magnitude. 20

δν ̃ =

∑ laϕa a=1

ϕa =

1 4πε0

∑ j

qj |raj|

(4)

The simulations of 5CB were carried out in the presence of externally applied uniform electric fields. Although uniform fields exert forces on charge sites, they only contribute to the potential if one defines a reference point that can serve as an origin. One simple modification to the potential at each of the probe sites is to use the centroid of the CN bond as the origin for that site

(3)

Here qj is the partial charge on atom j (residing on a different molecule) and raj is the distance between site a and atom j. The original map was parameterized in liquid water and comprises a set of parameters, la, that predict the shift in nitrile peak frequency D



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C(t ) = ⟨δr(t ) ·δr(0))⟩

where δr(t) = r(t) − r0 is the deviation from the equilibrium bond distance at time t. Because the other atomic sites have very small partial charges, this correlation function is an approximation to the dipole autocorrelation function for the molecule, which would be particularly relevant to computing the IR spectrum. Eleven statistically independent correlation functions were obtained by allowing the systems to run 10 ns with rigid CN bonds followed by 120 ps equilibration and data collection using the flexible CN bonds. This process was repeated 11 times, and the total sampling time, from sample preparation to final configurations, exceeded 160 ns for each of the field strengths investigated. The correlation functions were filtered using exponential apodization functions,40 f(t) = e−|t|/c, with a time constant, c = 3.5 ps, and were Fourier transformed to yield a spectrum

Figure 4. Spectra of nitrile frequency shifts for the no-field (black) and full-field (red) simulations. Upper panel: frequency shifts obtained from ab initio cluster calculations. Lower panel: classical bond-length autocorrelation spectra for the flexible nitrile measured relative to the natural frequency for the flexible bond. The dashed lines indicate the mean frequencies for each of the distributions. The cluster calculations exhibit a 2.68 cm−1 field-induced red-shift, while the classical correlation functions predict a red-shift of 2.29 cm−1.

ϕa′ = ϕa +

1 ⃗ E ·( ra⃗ − rCN ⃗ ) 4πε0

(6)

∞

I(ω) =

∫−∞ C(t )f (t )e−iωt dt

(7)

This time constant was chosen to match the Lorentzian linewidth that was used for computing the quantum mechanical spectra, and falls within the range of reported lifetimes for CN vibrations in other nitrile-containing molecules. The sampleaveraged classical nitrile spectrum can be seen in Figure 4. The Morse oscillator parameters listed above yield a natural frequency of 2226 cm−1 (close to the experimental value). To compare peaks from the classical and quantum mechanical approaches, both are displayed on an axis centered on the experimental nitrile frequency. The classical approach includes both intramolecular and electrostatic interactions, and thus, it implicitly couples CN vibrations to other vibrations within the molecule as well as to nitrile vibrations on other nearby molecules. The classical frequency spectrum is significantly broader because of this coupling. The ab initio cluster approach exercises only the targeted nitrile bond, with no additional coupling to other degrees of freedom. As a result, the quantum calculations are quite narrowly peaked around the experimental nitrile frequency. Although the spectra are quite noisy, the main effect seen in both distributions is a moderate shift to the red (2.29 cm−1 classical and 2.68 cm−1 quantum) after the electrostatic field had induced the nematic phase transition.

(5)

where E⃗ is the uniform electric field and (ra⃗ − rC⃗ N) is the displacement between the coordinates described by Choi et al.16,17 and the CN bond centroid. ϕa′ then contains an effective potential contributed by the uniform field in addition to the local potential contributions from other molecules. The sites {ra⃗ } and weights {la} developed by Choi et al.16,17 are quite symmetric around the CN centroid, and even at large uniform field values, we observed nearly complete cancellation of the potential contributions from the uniform field. The frequency shifts were computed for 4000 configurations sampled every 1 ps after the systems had equilibrated. The potential frequency map produces a small blue-shift of 0.34 cm−1, and the frequency shifts are quite narrowly distributed. However, the parameters for the potential frequency maps were derived for nitrile bonds in aqueous solutions, where the magnitudes of the local fields and electrostatic potentials are much larger than they would be in neat 5CB. We note that in 5CB there does not appear to be a particularly strong correlation between the electric field strengths observed at the nitrile centroid and the calculated vibrational frequencies. In Figure 3, we show the calculated frequencies plotted against the field magnitude as well as the parallel and perpendicular components of that field. CN Frequencies from Bond Length Autocorrelation Functions. The distribution of nitrile vibrational frequencies can also be found using classical time correlation functions. This was done by replacing the rigid CN bond with a flexible Morse oscillator described in eq 1. Since the systems were perturbed by the addition of a flexible high-frequency bond, they were allowed to re-equilibrate in the canonical (NVT) ensemble for 100 ps with 1 fs time steps. After equilibration, each configuration was run in the microcanonical (NVE) ensemble for 20 ps. Configurations sampled every fs were then used to compute bond-length autocorrelation functions

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DISCUSSION Our simulations show that the united-atom model can reproduce the field-induced nematic ordering of the 4-cyano4′-pentylbiphenyl. Because we are simulating a very small electrode separation (5 nm), a voltage drop as low as 1.2 V was sufficient to induce the phase change. This potential is significantly smaller than 100 V that was used with a 5 μm

Figure 5. Definitions of the angles between two nitrile bonds. E



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Figure 6. Contours of the angle-dependent pair distribution functions for nitrile bonds on 5CB in the no-field (upper panel) and full-field (lower panel) simulations. Dark areas signify regions of enhanced density, while light areas signify depletion relative to the bulk density.

Figure 7. Contours of the angle-dependent pair distribution function, g(r, cos θ), for finding any other atom at a distance and angular deviation from the center of a nitrile bond. The top edge of each contour plot corresponds to local density along the direction of the nitrogen in the CN bond, while the bottom is in the direction of the carbon atom. Bottom panel: g(z) data taken by following the C → N vector for each nitrile bond shows that the field-induced phase transition reduces the population of atoms that are directly in line with the nitrogen motion.

The primary structural effect of the field-induced phase transition is apparent in Figure 6. The nematic ordering transfers population from the perpendicular (cos ω ≈ 0) and antialigned (cos ω ≈ −1) to the nitrile-aligned peak near cos ω ≈ 1, leaving most other features undisturbed. This change is visible in the simulations as an increased population of aligned nitrile bonds in the first solvation shell. Although it is certainly possible that the coupling between closely spaced nitrile pairs is responsible for some of the redshift, that is not the only structural change that is taking place. The second two-dimensional pair distribution function, g(r, cos θ), shows that nematic ordering also transfers population that is directly in line with the nitrile bond (see Figure 7) to the sides of the molecule, thereby freeing steric blockage which can directly influence the nitrile vibration. This is confirmed by observing the one-dimensional g(z) obtained by following the C → N vector for each nitrile bond and observing the local density (ρ(z)/ρ) of other atoms at a distance z along this direction. The full-field simulation shows a significant drop in the first peak of g(z), indicating that the nematic ordering has moved density away from the region that is directly in line with the nitrogen side of the CN bond.

gap to study the electrochemiluminescence of rubrene in neat 5CB,41 and suggests that, by using electrodes separated by a nanometer-scale gap, it will be relatively straightforward to observe the nitrile Stark shift in 5CB. Both the classical correlation function and the isolated cluster approaches to estimating the IR spectrum show a nitrile shift of ∼2.5 cm−1 to the red of the unperturbed vibrational line. To understand the origin of this shift, a more complete picture of the spatial ordering around the nitrile bonds is required. We have computed the angle-dependent pair distribution functions g (r , cos ω) =

1 ⟨∑ ∑ δ(r − rij)δ(cos ωij − cos ω)⟩ ρN i j (8)

g (r , cos θ ) =

1 ⟨∑ ∑ δ(r − rij)δ(cos θi − cos θ)⟩ ρN i j (9)

which provide information about the joint spatial and angular correlations present in the system. The angles ω and θ are defined by vectors along the CN axis of each nitrile bond (see Figure 5). F



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Electrode/SAM/Solution Interfaces. Int. J. Mol. Sci. 2012, 13, 7466− 7482. (9) Moreno-Razo, J. A.; Sambriski, E. J.; Abbott, N. L.; HernandezOrtiz, J. P.; de Pablo, J. J. Liquid-crystal-mediated self-assembly at nanodroplet interfaces. Nature 2012, 485, 86−89. (10) Whitmer, J. K.; Wang, X.; Mondiot, F.; Miller, D. S.; Abbott, N. L.; de Pablo, J. J. Nematic-Field-Driven Positioning of Particles in Liquid Crystal Droplets. Phys. Rev. Lett. 2013, 111, 227801. (11) Gray, G.; Harrison, K.; Nash, J. New family of nematic liquid crystals for displays. Electron. Lett. 1973, 9, 130−131. (12) Hatta, A.; Amano, H.; Suëtaka, W. Electric field response of 5CB liquid crystal molecules in the electrode boundary region as probed by modulation infrared {ATR} spectroscopy. Vib. Spectrosc. 1991, 1, 371−376. (13) Leyte, J. C.; Woerkom, P. C. M. V. FT-IR TRS of Liquid Crystalline 5CB in AC, DC, and AC + DC Electric Fields. Appl. Spectrosc. 1997, 51, 1711−1714. (14) Lindquist, B. A.; Haws, R. T.; Corcelli, S. A. Optimized Quantum Mechanics/Molecular Mechanics Strategies for Nitrile Vibrational Probes: Acetonitrile and para-Tolunitrile in Water and Tetrahydrofuran. J. Phys. Chem. B 2008, 112, 13991−14001. (15) Lindquist, B. A.; Corcelli, S. A. Nitrile Groups as Vibrational Probes: Calculations of the CN Infrared Absorption Line Shape of Acetonitrile in Water and Tetrahydrofuran. J. Phys. Chem. B 2008, 112, 6301−6303. (16) Oh, K.-I.; Choi, J.-H.; Lee, J.-H.; Han, J.-B.; Lee, H.; Cho, M. Nitrile and thiocyanate IR probes: molecular dynamics simulation studies. J. Chem. Phys. 2008, 128, 154504. (17) Choi, J.-H.; Oh, K.-I.; Lee, H.; Lee, C.; Cho, M. Nitrile and thiocyanate IR probes: Quantum chemistry calculation studies and multivariate least-square fitting analysis. J. Chem. Phys. 2008, 128, 134506. (18) Morales, C. M.; Thompson, W. H. Simulations of Infrared Spectra of Nanoconfined Liquids: Acetonitrile Confined in Nanoscale, Hydrophilic Silica Pores. J. Phys. Chem. A 2009, 113, 1922−1933. (19) Waegele, M. M.; Gai, F. Computational Modeling of the Nitrile Stretching Vibration of 5-Cyanoindole in Water. J. Phys. Chem. Lett. 2010, 1, 781−786. (20) Gay, J. G.; Berne, B. J. Modification of the overlap potential to mimic a linear site-site potential. J. Chem. Phys. 1981, 74, 3316−3319. (21) Berne, B. J.; Pechukas, P. Gaussian Model Potentials for Molecular Interactions. J. Chem. Phys. 1972, 56, 4213−4216. (22) Kushick, J.; Berne, B. J. Computer simulation of anisotropic molecular fluids. J. Chem. Phys. 1976, 64, 1362−1367. (23) Luckhurst, G. R.; Stephens, R. A.; Phippen, R. W. Computer simulation studies of anisotropic systems. XIX. Mesophases formed by the Gay-Berne model mesogen. Liq. Cryst. 1990, 8, 451−464. (24) Cleaver, D. J.; Care, C. M.; Allen, M. P.; Neal, M. P. Extension and generalization of the Gay-Berne potential. Phys. Rev. E 1996, 54, 559−567. (25) Collings, P.; Hird, M. Introduction to Liquid Crystals: Chemistry and Physics; Liquid Crystals Book Series; Taylor & Francis: Philadelphia, PA, 1997. (26) Levelut, A. M.; Tarento, R. J.; Hardouin, F.; Achard, M. F.; Sigaud, G. Number of SA phases. Phys. Rev. A 1981, 24, 2180−2186. (27) Bose, T. K.; Saha, J. Origin of tilted-phase generation in systems of ellipsoidal molecules with dipolar interactions. Phys. Rev. E 2012, 86, 050701. (28) Zhang, J.; Su, J.; Guo, H. An Atomistic Simulation for 4-Cyano4′-pentylbiphenyl and Its Homologue with a Reoptimized Force Field. J. Phys. Chem. B 2011, 115, 2214−2227. (29) Lim, J. K.; Kwon, O.; Kang, D. S.; Joo, S.-W. Raman spectroscopy study and density functional theory calculations of the nematic liquid crystal 4-n-pentyl-4′-cyanobiphenyl under an electric field. Chem. Phys. Lett. 2006, 423, 178−182. (30) Lee, L. M.; Kwon, H. J.; Kang, J. H.; Nuzzo, R. G.; Schweizer, K. S. Anchoring and electro-optical dynamics of thin liquid crystalline films in a polyimide cell: Experiment and theory. J. Chem. Phys. 2006, 125, 024705.

We are suggesting an anticaging mechanism herethe nematic ordering provides additional space directly in line with the nitrile vibration, and since the oscillator is fairly anharmonic, this freedom provides a fraction of the nitrile bonds with a significant red-shift. The cause of this shift does not appear to be related to the alignment of those nitrile bonds with the field but rather to the change in local steric environment that is brought about by the isotropic−nematic transition. We have compared configurations for many of the clusters that exhibited the lowest frequencies (between 2190 and 2215 cm−1) and have observed some similar structural features. The lowest frequencies appear to come from configurations which have nearly empty pockets directly opposite the nitrogen atom from the nitrile carbon. However, because we do not have a particularly large cluster population to interrogate, this is certainly not quantitative confirmation of this effect. The prediction of a small red-shift of the nitrile peak in 5CB in response to a field-induced nematic ordering is the primary result of this work, and although the proposed anticaging mechanism is somewhat speculative, this work provides some impetus for further theory and experiments.

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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 authors thank Steven Corcelli and Zac Schultz for helpful comments and suggestions. Support for this project was provided by the National Science Foundation under grant CHE-0848243. Computational time was provided by the Center for Research Computing (CRC) at the University of Notre Dame.

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REFERENCES

(1) Boxer, S. G. Stark Realities. J. Phys. Chem. B 2009, 113, 2972− 2983. (2) Andrews, S. S.; Boxer, S. G. Vibrational Stark Effects of Nitriles I. Methods and Experimental Results. J. Phys. Chem. A 2000, 104, 11853−11863. (3) Tucker, M. J.; Getahun, Z.; Nanda, V.; DeGrado, W. F.; Gai, F. A New Method for Determining the Local Environment and Orientation of Individual Side Chains of Membrane-Binding Peptides. J. Am. Chem. Soc. 2004, 126, 5078−5079. (4) Webb, L. J.; Boxer, S. G. Electrostatic Fields Near the Active Site of Human Aldose Reductase: 1. New Inhibitors and Vibrational Stark Effect Measurements. Biochemistry 2008, 47, 1588−1598. (5) Lindquist, B. A.; Furse, K. E.; Corcelli, S. A. Nitrile groups as vibrational probes of biomolecular structure and dynamics: an overview. Phys. Chem. Chem. Phys. 2009, 11, 8119−8132. (6) Fafarman, A. T.; Sigala, P. A.; Herschlag, D.; Boxer, S. G. Decomposition of Vibrational Shifts of Nitriles into Electrostatic and Hydrogen-Bonding Effects. J. Am. Chem. Soc. 2010, 132, 12811− 12813. (7) Oklejas, V.; Sjostrom, C.; Harris, J. M. SERS Detection of the Vibrational Stark Effect from Nitrile-Terminated SAMs to Probe Electric Fields in the Diffuse Double-Layer. J. Am. Chem. Soc. 2002, 124, 2408−2409. (8) Schkolnik, G.; Salewski, J.; Millo, D.; Zebger, I.; Franzen, S.; Hildebrandt, P. Vibrational Stark Effect of the Electric-Field Reporter 4-Mercaptobenzonitrile as a Tool for Investigating Electrostatics at G



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(31) Kranz, C. Recent advancements in nanoelectrodes and nanopipettes used in combined scanning electrochemical microscopy techniques. Analyst 2014, 139, 336−352. (32) Fennell, C. J.; Gezelter, J. D. Is the Ewald summation still necessary? Pairwise alternatives to the accepted standard for longrange electrostatics. J. Chem. Phys. 2006, 124, 234104. (33) Becke, A. D. Density-functional thermochemistry. III. The role of exact exchange. J. Chem. Phys. 1993, 98, 5648−5652. (34) Lee, C.; Yang, W.; Parr, R. G. Development of the Colle-Salvetti correlation-energy formula into a functional of the electron density. Phys. Rev. B 1988, 37, 785−789. (35) Morse, P. M. Diatomic Molecules According to the Wave Mechanics. II. Vibrational Levels. Phys. Rev. 1929, 34, 57−64. (36) Ghosh, A.; Remorino, A.; Tucker, M. J.; Hochstrasser, R. M. 2D IR photon echo spectroscopy reveals hydrogen bond dynamics of aromatic nitriles. Chem. Phys. Lett. 2009, 469, 325−330. (37) Ha, J.-H.; Lee, K.-K.; Park, K.-H.; Choi, J.-H.; Jeon, S.-J.; Cho, M. Integrated and dispersed photon echo studies of nitrile stretching vibration of 4-cyanophenol in methanol. J. Chem. Phys. 2009, 130, 204509. (38) Corcelli, S. A.; Lawrence, C. P.; Skinner, J. L. Combined electronic structure/molecular dynamics approach for ultrafast infrared spectroscopy of dilute HOD in liquid H2O and D2O. J. Chem. Phys. 2004, 120, 8107−8117. (39) Auer, B.; Kumar, R.; Schmidt, J. R.; Skinner, J. L. Hydrogen bonding and Raman, IR, and 2D-IR spectroscopy of dilute HOD in liquid D2O. Proc. Natl. Acad. Sci. U.S.A. 2007, 104, 14215−14220. (40) Filler, A. S. Apodization and Interpolation in Fourier-Transform Spectroscopy. J. Opt. Soc. Am. 1964, 54, 762−764. (41) Kojima, H.; Takagi, Y.; Teramoto, H. Electrochemiluminescence in liquid crystals. Electrochim. Acta 1988, 33, 1789−1793.

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Nitrile Vibrations as Reporters of Field-Induced Phase Transitions in 4

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