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\title{Nitrile vibrations as reporters of field-induced phase |
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transitions in 4-cyano-4'-pentylbiphenyl} |
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\author{James M. Marr} |
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\author{J. Daniel Gezelter} |
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\email{gezelter@nd.edu} |
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\affiliation[University of Notre Dame]{251 Nieuwland Science Hall\\ |
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Department of Chemistry and Biochemistry\\ |
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University of Notre Dame\\ |
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Notre Dame, Indiana 46556} |
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\date{\today} |
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\begin{document} |
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\maketitle |
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\begin{doublespace} |
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\begin{abstract} |
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4-cyano-4'-pentylbiphenyl (5CB) is a liquid-crystal-forming compound |
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with a terminal nitrile group aligned with the long axis of the |
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molecule. Simulations of condensed-phase 5CB were carried out both |
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with and without applied electric fields to provide an understanding |
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of the the Stark shift of the terminal nitrile group. A |
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field-induced isotropic-nematic phase transition was observed in the |
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simulations, and the effects of this transition on the distribution |
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of nitrile frequencies were computed. Classical bond displacement |
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correlation functions exhibit a $\sim 40 \mathrm{~cm}^{-1}$ red |
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shift of a portion of the main nitrile peak, and this shift was |
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observed only when the fields were large enough to induce |
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orientational ordering of the bulk phase. Our simulations appear to |
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indicate that phase-induced changes to the local surroundings are a |
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larger contribution to the change in the nitrile spectrum than |
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direct field contributions. |
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\end{abstract} |
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\newpage |
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\section{Introduction} |
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|
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Nitrile groups can serve as very precise electric field reporters via |
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their distinctive Raman and IR signatures.\cite{Boxer:2009xw} The |
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triple bond between the nitrogen and the carbon atom is very sensitive |
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to local field changes and has been observed to have a direct impact |
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on the peak position within the spectrum. The Stark shift in the |
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spectrum can be quantified and mapped into a field value that is |
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impinging upon the nitrile bond. This has been used extensively in |
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biological systems like proteins and |
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enzymes.\cite{Tucker:2004qq,Webb:2008kn} |
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|
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The response of nitrile groups to electric fields has now been |
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investigated for a number of small molecules,\cite{Andrews:2000qv} as |
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well as in biochemical settings, where nitrile groups can act as |
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minimally invasive probes of structure and |
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dynamics.\cite{Lindquist:2009fk,Fafarman:2010dq} The vibrational Stark |
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effect has also been used to study the effects of electric fields on |
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nitrile-containing self-assembled monolayers at metallic |
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interfaces.\cite{Oklejas:2002uq,Schkolnik:2012ty} |
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|
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Recently 4-cyano-4'-pentylbiphenyl (5CB), a liquid crystalline |
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molecule with a terminal nitrile group, has seen renewed interest as |
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one way to impart order on the surfactant interfaces of |
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nanodroplets,\cite{Moreno-Razo:2012rz} or to drive surface-ordering |
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that can be used to promote particular kinds of |
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self-assembly.\cite{PhysRevLett.111.227801} The nitrile group in 5CB |
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is a particularly interesting case for studying electric field |
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effects, as 5CB exhibits an isotropic to nematic phase transition that |
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can be triggered by the application of an external field near room |
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temperature.\cite{Gray:1973ca,Hatta:1991ee} This presents the |
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possiblity that the field-induced changes in the local environment |
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could have dramatic effects on the vibrations of this particular CN |
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bond. Although the infrared spectroscopy of 5CB has been |
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well-investigated, particularly as a measure of the kinetics of the |
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phase transition,\cite{Leyte:1997zl} the 5CB nitrile group has not yet |
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seen the detailed theoretical treatment that biologically-relevant |
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small molecules have |
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received.\cite{Lindquist:2008bh,Lindquist:2008qf,Oh:2008fk,Choi:2008cr,Waegele:2010ve} |
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|
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The fundamental characteristic of liquid crystal mesophases is that |
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they maintain some degree of orientational order while translational |
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order is limited or absent. This orientational order produces a |
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complex direction-dependent response to external perturbations like |
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electric fields and mechanical distortions. The anisotropy of the |
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macroscopic phases originates in the anisotropy of the constituent |
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molecules, which typically have highly non-spherical structures with a |
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significant degree of internal rigidity. In nematic phases, rod-like |
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molecules are orientationally ordered with isotropic distributions of |
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molecular centers of mass. For example, 5CB has a solid to nematic |
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phase transition at 18C and a nematic to isotropic transition at |
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35C.\cite{Gray:1973ca} |
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|
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In smectic phases, the molecules arrange themselves into layers with |
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their long (symmetry) axis normal ($S_{A}$) or tilted ($S_{C}$) with |
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respect to the layer planes. The behavior of the $S_{A}$ phase can be |
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partially explained with models mainly based on geometric factors and |
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van der Waals interactions. The Gay-Berne potential, in particular, |
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has been widely used in the liquid crystal community to describe this |
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anisotropic phase |
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behavior.~\cite{Gay:1981yu,Berne72,Kushick:1976xy,Luckhurst90,Cleaver:1996rt} |
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However, these simple models are insufficient to describe liquid |
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crystal phases which exhibit more complex polymorphic nature. |
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Molecules which form $S_{A}$ phases can exhibit a wide variety of |
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subphases like monolayers ($S_{A1}$), uniform bilayers ($S_{A2}$), |
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partial bilayers ($S_{\tilde A}$) as well as interdigitated bilayers |
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($S_{A_{d}}$), and often have a terminal cyano or nitro group. In |
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particular, lyotropic liquid crystals (those exhibiting liquid crystal |
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phase transition as a function of water concentration), often have |
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polar head groups or zwitterionic charge separated groups that result |
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in strong dipolar interactions,\cite{Collings:1997rz} and terminal cyano |
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groups (like the one in 5CB) can induce permanent longitudinal |
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dipoles.\cite{Levelut:1981eu} |
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Macroscopic electric fields applied using electrodes on opposing sides |
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of a sample of 5CB have demonstrated the phase change of the molecule |
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as a function of electric field.\cite{Lim:2006xq} These previous |
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studies have shown the nitrile group serves as an excellent indicator |
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of the molecular orientation within the applied field. Lee {\it et |
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al.}~showed a 180 degree change in field direction could be probed |
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with the nitrile peak intensity as it changed along with molecular |
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alignment in the field.\cite{Lee:2006qd,Leyte:1997zl} |
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|
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While these macroscopic fields work well at indicating the bulk |
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response, the atomic scale response has not been studied. With the |
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advent of nano-electrodes and coupling them with atomic force |
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microscopy, control of electric fields applied across nanometer |
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distances is now possible.\cite{citation1} While macroscopic fields |
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are insufficient to cause a Stark effect without dielectric breakdown |
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of the material, small fields across nanometer-sized gaps may be of |
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sufficient strength. For a gap of 5 nm between a lower electrode |
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having a nanoelectrode placed near it via an atomic force microscope, |
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a potential of 1 V applied across the electrodes is equivalent to a |
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field of 2x10\textsuperscript{8} $\frac{V}{M}$. This field is |
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certainly strong enough to cause the isotropic-nematic phase change |
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and as well as Stark tuning of the nitrile bond. This should be |
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readily visible experimentally through Raman or IR spectroscopy. |
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In the sections that follow, we outline a series of coarse-grained |
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classical molecular dynamics simulations of 5CB that were done in the |
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presence of static electric fields. These simulations were then |
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coupled with both {\it ab intio} calculations of CN-deformations and |
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classical bond-length correlation functions to predict spectral |
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shifts. These predictions made should be easily varifiable with |
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scanning electrochemical microscopy experiments. |
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\section{Computational Details} |
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The force field used for 5CB was taken from Guo {\it et |
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al.}\cite{Zhang:2011hh} However, for most of the simulations, each |
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of the phenyl rings was treated as a rigid body to allow for larger |
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time steps and very long simulation times. The geometries of the |
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rigid bodies were taken from equilibrium bond distances and angles. |
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Although the phenyl rings were held rigid, bonds, bends, torsions and |
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inversion centers that involved atoms in these substructures (but with |
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connectivity to the rest of the molecule) were still included in the |
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potential and force calculations. |
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Periodic simulations cells containing 270 molecules in random |
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orientations were constructed and were locked at experimental |
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densities. Electrostatic interactions were computed using damped |
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shifted force (DSF) electrostatics.\cite{Fennell:2006zl} The molecules |
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were equilibrated for 1~ns at a temperature of 300K. Simulations with |
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applied fields were carried out in the microcanonical (NVE) ensemble |
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with an energy corresponding to the average energy from the canonical |
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(NVT) equilibration runs. Typical applied-field runs were more than |
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60ns in length. |
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Static electric fields with magnitudes similar to what would be |
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available in an experimental setup were applied to the different |
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simulations. With an assumed electrode seperation of 5 nm and an |
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electrostatic potential that is limited by the voltage required to |
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split water (1.23V), the maximum realistic field that could be applied |
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is $\sim 0.024$ V/\AA. Three field environments were investigated: |
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(1) no field applied, (2) partial field = 0.01 V/\AA\ , and (3) full |
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field = 0.024 V/\AA\ . |
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After the systems had come to equilibrium under the applied fields, |
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additional simulations were carried out with a flexible (Morse) |
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nitrile bond, |
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\begin{displaymath} |
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V(r_\ce{CN}) = D_e \left(1 - e^{-\beta (r_\ce{CN}-r_e)}\right)^2 |
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\end{displaymath} |
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where $r_e= 1.157437$ \AA , $D_e = 212.95 \mathrm{~kca~l} / |
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\mathrm{mol}^{-1}$ and $\beta = 2.67566 $\AA~$^{-1}$. These |
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parameters correspond to a vibrational frequency of $2375 |
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\mathrm{~cm}^{-1}$, a bit higher than the experimental frequency. The |
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flexible nitrile moiety required simulation time steps of 1~fs, so the |
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additional flexibility was introducuced only after the rigid systems |
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had come to equilibrium under the applied fields. Whenever time |
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correlation functions were computed from the flexible simulations, |
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statistically-independent configurations were sampled from the last ns |
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of the induced-field runs. These configurations were then |
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equilibrated with the flexible nitrile moiety for 100 ps, and time |
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correlation functions were computed using data sampled from an |
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additional 200 ps of run time carried out in the microcanonical |
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ensemble. |
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\section{Field-induced Nematic Ordering} |
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In order to characterize the orientational ordering of the system, the |
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primary quantity of interest is the nematic (orientational) order |
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parameter. This was determined using the tensor |
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\begin{equation} |
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Q_{\alpha \beta} = \frac{1}{2N} \sum_{i=1}^{N} \left(3 \hat{e}_{i |
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\alpha} \hat{e}_{i \beta} - \delta_{\alpha \beta} \right) |
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\end{equation} |
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where $\alpha, \beta = x, y, z$, and $\hat{e}_i$ is the molecular |
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end-to-end unit vector for molecule $i$. The nematic order parameter |
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$S$ is the largest eigenvalue of $Q_{\alpha \beta}$, and the |
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corresponding eigenvector defines the director axis for the phase. |
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$S$ takes on values close to 1 in highly ordered (smectic A) phases, |
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but falls to zero for isotropic fluids. Note that the nitrogen and |
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the terminal chain atom were used to define the vectors for each |
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molecule, so the typical order parameters are lower than if one |
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defined a vector using only the rigid core of the molecule. In |
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nematic phases, typical values for $S$ are close to 0.5. |
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|
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The field-induced phase transition can be clearly seen over the course |
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of a 60 ns equilibration runs in figure \ref{fig:orderParameter}. All |
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three of the systems started in a random (isotropic) packing, with |
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order parameters near 0.2. Over the course 10 ns, the full field |
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causes an alignment of the molecules (due primarily to the interaction |
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of the nitrile group dipole with the electric field). Once this |
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system started exhibiting nematic ordering, the orientational order |
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parameter became stable for the remaining 50 ns of simulation time. |
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It is possible that the partial-field simulation is meta-stable and |
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given enough time, it would eventually find a nematic-ordered phase, |
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but the partial-field simulation was stable as an isotropic phase for |
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the full duration of a 60 ns simulation. |
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\begin{figure} |
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\includegraphics[trim = 5mm 10mm 3mm 10mm, clip, width=4in]{P2} |
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\caption{Evolution of the orientational order parameter for the |
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no-field, partial field, and full field simulations over the |
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course of 60 ns. Each simulation was started from a |
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statistically-independent isotropic configuration.} |
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\label{fig:orderParameter} |
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\end{figure} |
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|
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The field-induced isotropic-nematic transition can be visualized in |
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figure \ref{fig:Cigars}, where each molecule has been represented |
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using and ellipsoids aligned along the long-axis of each molecule. |
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Both the zero field and partial field simulations appear isotropic, |
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while the full field simulations has been orientationally ordered |
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\begin{figure} |
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\includegraphics[width=7in]{Elip_3} |
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\caption{Ellipsoid reprsentation of 5CB at three different field |
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strengths, Zero Field (Left), Partial Field (Middle), and Full |
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Field (Right) Each image was created from the final configuration |
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of each 60 ns equilibration run.} |
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\label{fig:Cigars} |
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\end{figure} |
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|
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\section{Sampling the CN bond frequency} |
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|
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The primary quantity of interest is the distribution of vibrational |
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frequencies exhibited by the 5CB nitrile bond under the different |
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electric fields. Three distinct methods for mapping classical |
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simulations onto vibrational spectra were brought to bear on these |
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simulations: |
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\begin{enumerate} |
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\item Isolated 5CB molecules and their immediate surroundings were |
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extracted from the simulations, their nitrile bonds were stretched |
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and single-point {\em ab initio} calculations were used to obtain |
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Morse-oscillator fits for the local vibrational motion along that |
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bond. |
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\item The potential - frequency maps developed by Cho {\it et |
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al.}~\cite{Oh:2008fk} for nitrile moieties in water were |
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investigated. This method involves mapping the electrostatic |
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potential around the bond to the vibrational frequency, and is |
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similar in approach to field-frequency maps that were pioneered by |
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work done by Skinner {\it et al.}\cite{XXXX} |
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\item Classical bond-length autocorrelation functions were Fourier |
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transformed to directly obtain the vibrational spectrum from |
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molecular dynamics simulations. |
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\end{enumerate} |
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\subsection{CN frequencies from isolated clusters} |
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|
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For quantum calculation of the nitrile bond frequency, Gaussian 09 was |
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used. A single 5CB molecule was selected for the center of the |
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cluster. For effects from molecules located near the chosen nitrile |
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group, parts of molecules nearest to the nitrile group were |
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included. For the body not including the tail, any atom within 6~\AA |
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of the midpoint of the nitrile group was included. For the tail |
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structure, the whole tail was included if a tail atom was within 4~\AA |
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of the midpoint. If the tail did not include any atoms from the ring |
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|
|
structure, it was considered a propane molecule and included as |
327 |
|
|
such. Once the clusters were generated, input files were created that |
328 |
jmarr |
4008 |
stretched the nitrile bond along its axis from 0.87 to 1.52~\AA at |
329 |
gezelter |
4007 |
increments of 0.05~\AA. This generated 13 single point energies to be |
330 |
|
|
calculated. The Gaussian files were run with B3LYP/6-311++G(d,p) with |
331 |
jmarr |
4008 |
no other keywords for the zero field simulation. Simulations with |
332 |
|
|
fields applied included the keyword ''Field=Z+5'' to match the |
333 |
|
|
external field applied in molecular dynamic runs. Once completed, the central nitrile bond frequency |
334 |
gezelter |
4007 |
was calculated with a Morse fit. Using this fit and the solved energy |
335 |
jmarr |
4018 |
levels for a Morse oscillator, the frequency was found. Each set of |
336 |
jmarr |
4020 |
frequencies were then convolved together with a lorezian lineshape |
337 |
jmarr |
4018 |
function over each value. The width value used was 1.5. For the zero |
338 |
|
|
field spectrum, 67 frequencies were used and for the full field, 59 |
339 |
|
|
frequencies were used. |
340 |
gezelter |
4007 |
|
341 |
gezelter |
4029 |
\subsection{CN frequencies from potential-frequency maps} |
342 |
|
|
Before Gaussian silumations were carried out, it was attempt to apply |
343 |
|
|
the method developed by Cho {\it et al.}~\cite{Oh:2008fk} This method involves the fitting |
344 |
|
|
of multiple parameters to Gaussian calculated freuencies to find a |
345 |
|
|
correlation between the potential around the bond and the |
346 |
|
|
frequency. This is very similar to work done by Skinner {\it et al.}~with |
347 |
|
|
water models like SPC/E. The general method is to find the shift in |
348 |
|
|
the peak position through, |
349 |
|
|
\begin{equation} |
350 |
|
|
\delta\tilde{\nu} =\sum^{n}_{a=1} l_{a}\phi^{water}_{a} |
351 |
|
|
\end{equation} |
352 |
|
|
where $l_{a}$ are the fitting parameters and $\phi^{water}_{a}$ is the |
353 |
|
|
potential from the surrounding water cluster. This $\phi^{water}_{a}$ |
354 |
|
|
takes the form, |
355 |
|
|
\begin{equation} |
356 |
|
|
\phi_{a} = \frac{1}{4\pi \epsilon_{0}} \sum_{m} \sum_{j} |
357 |
|
|
\frac{C^{H_{2}O}_{j \left(m \right) }}{r_{aj \left(m\right)}} |
358 |
|
|
\end{equation} |
359 |
|
|
where $C^{H_{2}O}_{j \left(m \right) }$ indicates the partial charge |
360 |
|
|
on the $j$th site of the $m$th water molecule and $r_{aj \left(m\right)}$ |
361 |
|
|
is the distance between the site $a$ of the nitrile molecule and the $j$th |
362 |
|
|
site of the $m$th water molecule. However, since these simulations |
363 |
|
|
are done under the presence of external fields and in the |
364 |
|
|
absence of water, the equations need a correction factor for the shift |
365 |
|
|
caused by the external field. The equation is also reworked to use |
366 |
|
|
electric field site data instead of partial charges from surrounding |
367 |
|
|
atoms. So by modifing the original |
368 |
|
|
$\phi^{water}_{a}$ to $\phi^{5CB}_{a}$ we get, |
369 |
|
|
\begin{equation} |
370 |
|
|
\phi^{5CB}_{a} = \frac{1}{4\pi \epsilon_{0}} \left( \vec{E}\bullet |
371 |
|
|
\left(\vec{r}_{a}-\vec{r}_{CN}\right) \right) + \phi^{5CB}_{0} |
372 |
|
|
\end{equation} |
373 |
|
|
where $\vec{E}$ is the electric field at each atom, $\left( \vec{r}_{a} - |
374 |
|
|
\vec{r}_{CN} \right)$ is the vector between the nitrile bond and the |
375 |
|
|
cooridinates described by Cho around the bond and $\phi^{5CB}_{0}$ is |
376 |
|
|
the correction factor for the system of parameters. After these |
377 |
|
|
changes, the correction factor was found for multiple values of an |
378 |
|
|
external field being applied. However, the factor was no linear and |
379 |
|
|
was overly large due to the fitting parameters being so small. |
380 |
|
|
|
381 |
|
|
|
382 |
|
|
\subsection{CN frequencies from bond length autocorrelation functions} |
383 |
|
|
|
384 |
gezelter |
4007 |
Classical nitrile bond frequencies were found by replacing the rigid |
385 |
jmarr |
4008 |
cyanide bond with a flexible Morse oscillator bond |
386 |
|
|
($r_0= 1.157437$ \AA , $D_0 = 212.95$ and |
387 |
|
|
$\beta = 2.67566$) . Once replaced, the |
388 |
gezelter |
4007 |
systems were allowed to re-equilibrate in NVT for 100 ps. After |
389 |
|
|
re-equilibration, the system was run in NVE for 20 ps with a snapshot |
390 |
|
|
spacing of 1 fs. These snapshot were then used in bond correlation |
391 |
|
|
calculation to find the decay structure of the bond in time using the |
392 |
|
|
average bond displacement in time, |
393 |
|
|
\begin{equation} |
394 |
|
|
C(t) = \langle \left(r(t) - r_0 \right) \cdot \left(r(0) - r_0 \right) \rangle |
395 |
|
|
\end{equation} |
396 |
|
|
% |
397 |
|
|
where $r_0$ is the equilibrium bond distance and $r(t)$ is the |
398 |
|
|
instantaneous bond displacement at time $t$. Once calculated, |
399 |
|
|
smoothing was applied by adding an exponential decay on top of the |
400 |
jmarr |
4023 |
decay with a $\tau$ of 6000. Further smoothing |
401 |
gezelter |
4007 |
was applied by padding 20,000 zeros on each side of the symmetric |
402 |
|
|
data. This was done five times by allowing the systems to run 1 ns |
403 |
|
|
with a rigid bond followed by an equilibrium run with the bond |
404 |
jmarr |
4023 |
switched back to a Morse oscillator and a short production run of 20 ps. |
405 |
gezelter |
4007 |
|
406 |
|
|
|
407 |
jmarr |
4017 |
This change in phase was followed by two courses of further |
408 |
jmarr |
4019 |
analysis. First was the replacement of the static nitrile bond with a |
409 |
jmarr |
4017 |
morse oscillator bond. This was then simulated for a period of time |
410 |
jmarr |
4022 |
and a classical spetrum was calculated. Second, ab intio calcualtions |
411 |
jmarr |
4023 |
were performed to investigate if the phase change caused any change |
412 |
|
|
spectrum through quantum effects. |
413 |
jmarr |
4017 |
|
414 |
jmarr |
4019 |
The classical nitrile spectrum can be seen in Figure 2. Most noticably |
415 |
|
|
is the position of the two peaks. Obviously the experimental peak |
416 |
|
|
position is near 2226 cm\textsuperscript{-1}. However, in this case |
417 |
|
|
the peak position is shifted to the blue at a position of 2375 |
418 |
|
|
cm\textsuperscript{-1}. This shift is due solely to the choice of |
419 |
jmarr |
4022 |
oscillator strength in the Morse oscillator parameters. While this |
420 |
jmarr |
4019 |
shift makes the two spectra differ, it does not affect the ability to |
421 |
jmarr |
4022 |
qualitatively compare peak changes to possible experimental changes. |
422 |
jmarr |
4019 |
With this important fact out of the way, differences between the two |
423 |
|
|
states are subtle but are very much present. The first and |
424 |
|
|
most notable is the apperance for a strong band near 2300 |
425 |
jmarr |
4020 |
cm\textsuperscript{-1}. |
426 |
jmarr |
4013 |
\begin{figure} |
427 |
|
|
\includegraphics[width=3.25in]{2Spectra} |
428 |
jmarr |
4017 |
\caption{The classically calculated nitrile bond spetrum for no |
429 |
|
|
external field application (black) and full external field |
430 |
|
|
application (red)} |
431 |
jmarr |
4013 |
\label{fig:twoSpectra} |
432 |
|
|
\end{figure} |
433 |
jmarr |
4020 |
|
434 |
|
|
|
435 |
jmarr |
4023 |
Due to this, Gaussian calculations were performed in lieu of this |
436 |
|
|
method. A set of snapshots for the zero and full field simualtions, |
437 |
|
|
they were first investigated for any dependence on the local, with |
438 |
|
|
external field included, electric field. This was to see if a linear |
439 |
|
|
or non-linear relationship between the two could be utilized for |
440 |
|
|
generating spectra. This was done in part because of previous studies |
441 |
|
|
showing the frequency dependence of nitrile bonds to the electric |
442 |
|
|
fields generated locally between solvating water. It was seen that |
443 |
|
|
little to no dependence could be directly shown. This data is not |
444 |
|
|
shown. |
445 |
|
|
|
446 |
jmarr |
4020 |
Since no explicit dependence was observed between the calculated |
447 |
|
|
frequency and the electric field, it was not a viable route for the |
448 |
|
|
calculation of a nitrile spectrum. Instead, the frequencies were taken |
449 |
jmarr |
4024 |
and convolved together with a lorentzian line shape applied around the |
450 |
|
|
frequency value. These spectra are seen below in Figure |
451 |
jmarr |
4020 |
4. While the spectrum without a field is lower in intensity and is |
452 |
jmarr |
4024 |
almost bimodel in distrobution, the external field spectrum is much |
453 |
jmarr |
4023 |
more unimodel. This tighter clustering has the affect of increasing the |
454 |
jmarr |
4020 |
intensity around 2226 cm\textsuperscript{-1} where the peak is |
455 |
jmarr |
4023 |
centered. The external field also has fewer frequencies of higher |
456 |
|
|
energy in the spectrum. Unlike the the zero field, where some frequencies |
457 |
|
|
reach as high as 2280 cm\textsuperscript{-1}. |
458 |
jmarr |
4013 |
\begin{figure} |
459 |
jmarr |
4018 |
\includegraphics[width=3.25in]{Convolved} |
460 |
jmarr |
4020 |
\caption{Lorentzian convolved Gaussian frequencies of the zero field |
461 |
|
|
system (black) and the full field system (red)} |
462 |
jmarr |
4018 |
\label{fig:Con} |
463 |
|
|
\end{figure} |
464 |
gezelter |
4007 |
\section{Discussion} |
465 |
jmarr |
4024 |
Interestingly, the field that is needed to switch the phase of 5CB |
466 |
|
|
macroscopically is larger than 1 V. However, in this case, only a |
467 |
|
|
voltage of 1.2 V was need to induce a phase change. This is impart due |
468 |
|
|
to the short distance of 5 nm the field is being applied across. At such a small |
469 |
|
|
distance, the field is much larger than the macroscopic and thus |
470 |
|
|
easily induces a field dependent phase change. However, this field |
471 |
|
|
will not cause a breakdown of the 5CB since electrochemistry studies |
472 |
|
|
have shown that it can be used in the presence of fields as high as |
473 |
|
|
500 V macroscopically. This large of a field near the surface of the |
474 |
|
|
elctrode would cause breakdown of 5CB if it could happen. |
475 |
|
|
|
476 |
jmarr |
4020 |
The absence of any electric field dependency of the freuquency with |
477 |
jmarr |
4025 |
the Gaussian simulations is interesting. A large base of research has been |
478 |
jmarr |
4024 |
built upon the known tuning of the nitrile bond as the local field |
479 |
|
|
changes. This difference may be due to the absence of water or a |
480 |
|
|
molecule that induces a large internal field. Liquid water is known to have a very high internal field which |
481 |
jmarr |
4020 |
is much larger than the internal fields of neat 5CB. Even though the |
482 |
jmarr |
4024 |
application of Gaussian simulations followed by mapping it to |
483 |
jmarr |
4020 |
some classical parameter is easy and straight forward, this system |
484 |
|
|
illistrates how that 'go to' method can break down. |
485 |
gezelter |
4007 |
|
486 |
jmarr |
4020 |
While this makes the application of nitrile Stark effects in |
487 |
jmarr |
4024 |
simulations without water harder, these data show |
488 |
jmarr |
4021 |
that it is not a deal breaker. The classically calculated nitrile |
489 |
|
|
spectrum shows changes in the spectra that will be easily seen through |
490 |
|
|
experimental routes. It indicates a shifted peak lower in energy |
491 |
jmarr |
4024 |
should arise. This peak is a few wavenumbers from the leading edge of |
492 |
|
|
the larger peak and almost 75 wavenumbers from the center. This |
493 |
|
|
seperation between the two peaks means experimental results will show |
494 |
|
|
an easily resolved peak. |
495 |
jmarr |
4021 |
|
496 |
jmarr |
4024 |
The Gaussian derived spectra do indicate an applied field |
497 |
jmarr |
4023 |
and subsiquent phase change does cause a narrowing of freuency |
498 |
jmarr |
4025 |
distrobution. With narrowing, it would indicate an increased |
499 |
|
|
homogeneous distrobution of the local field near the nitrile. |
500 |
gezelter |
4007 |
\section{Conclusions} |
501 |
jmarr |
4024 |
Field dependent changes |
502 |
gezelter |
4007 |
\newpage |
503 |
|
|
|
504 |
|
|
\bibliography{5CB} |
505 |
|
|
|
506 |
|
|
\end{doublespace} |
507 |
|
|
\end{document} |