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\title{Nitrile vibrations as reporters of field-induced phase |
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transitions in 4-cyano-4'-pentylbiphenyl (5CB)} |
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\author{James M. Marr} |
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University of Notre Dame\\ |
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Notre Dame, Indiana 46556} |
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|
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\date{\today} |
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|
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\begin{document} |
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|
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\maketitle |
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|
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\begin{doublespace} |
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\begin{tocentry} |
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%\includegraphics[width=9cm]{Elip_3} |
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\includegraphics[width=9cm]{Figure2} |
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\end{tocentry} |
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|
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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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isotropic-nematic phase transition was observed in the simulations, |
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and the effects of this transition on the distribution of nitrile |
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frequencies were computed. Classical bond displacement correlation |
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< |
functions exhibit a $\sim~10~\mathrm{cm}^{-1}$ red shift of a |
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functions exhibit a $\sim~3~\mathrm{cm}^{-1}$ red shift of a |
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portion of the main nitrile peak, and this shift was observed only |
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when the fields were large enough to induce orientational ordering |
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of the bulk phase. Joint spatial-angular distribution functions |
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levels for this potential.\cite{Morse:1929xy} To obtain a spectrum, |
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each of the frequencies was convoluted with a Lorentzian lineshape |
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with a width of 1.5 $\mathrm{cm}^{-1}$. Available computing resources |
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limited the sampling to 67 clusters for the zero-field spectrum, and |
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59 for the full field. Comparisons of the quantum mechanical spectrum |
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to the classical are shown in figure \ref{fig:spectra}. |
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limited the sampling to 100 clusters for both the zero-field and full |
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field soectra. Comparisons of the quantum mechanical spectrum to the |
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classical are shown in figure \ref{fig:spectra}. |
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|
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\begin{figure} |
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\includegraphics[width=\linewidth]{Figure3} |
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contributions from the external field. This reparameterization is |
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outside the scope of the current work, but would make a useful |
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addition to the potential-frequency map approach. |
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|
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We note that in 5CB there does not appear to be a particularly strong |
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correlation between the electric field observed at the nitrile |
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centroid and the calculated vibrational frequency. In |
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Fig. \ref{fig:fieldMap} we show the calculated frequencies plotted |
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against the field magnitude and the parallel and perpendicular |
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components of the field. |
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|
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\begin{figure} |
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\includegraphics[width=\linewidth]{Figure7} |
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\caption{The observed cluster frequencies have no apparent |
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correlation with the electric field felt at the centroid of the |
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nitrile bond. Lower panel: vibrational frequencies plotted |
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against the total field magnitude. Middle panel: mapped to the |
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component of the field parallel to the CN bond. Upper panel: |
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mapped to the magnitude of the field perpendicular to the CN |
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bond.} |
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\label{fig:fieldMap} |
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\end{figure} |
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|
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|
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\subsection{CN frequencies from bond length autocorrelation functions} |
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|
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The distribution of nitrile vibrational frequencies can also be found |
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the quantum calculations are quite narrowly peaked around the |
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experimental nitrile frequency. Although the spectra are quite noisy, |
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the main effect seen in both the classical and quantum frequency |
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distributions is a moderate shift $\sim 10~\mathrm{cm}^{-1}$ to the |
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distributions is a moderate shift $\sim 3~\mathrm{cm}^{-1}$ to the |
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red when the full electrostatic field had induced the nematic phase |
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transition. |
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|
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|
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Both the classical correlation function and the isolated cluster |
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approaches to estimating the IR spectrum show that a population of |
| 521 |
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nitrile stretches shift by $\sim~10~\mathrm{cm}^{-1}$ to the red of |
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nitrile stretches shift by $\sim~3~\mathrm{cm}^{-1}$ to the red of |
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|
the unperturbed vibrational line. To understand the origin of this |
| 523 |
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shift, a more complete picture of the spatial ordering around the |
| 524 |
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nitrile bonds is required. We have computed the angle-dependent pair |
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|
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\bibliography{5CB} |
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|
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\end{doublespace} |
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\end{document} |
