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Notre Dame, Indiana 46556} |
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\keywords{} |
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\keywords{Nitrile vibrational frequency, field-induced shift, 5CB, |
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phase transition} |
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\begin{document} |
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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 $\sim~3~\mathrm{cm}^{-1}$ red shift of a portion |
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functions exhibit a $\sim~2.3~\mathrm{cm}^{-1}$ red shift of a portion |
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of the main nitrile peak, and this shift was observed only when the |
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fields were large enough to induce orientational ordering of the |
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bulk phase. Distributions of frequencies obtained via cluster-based |
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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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\label{eq:morse} |
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\end{equation} |
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where $r_e= 1.157$ \AA (the fixed CN bond length from the force field |
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of Guo {\it et al.}\cite{Zhang:2011hh}), $D_e = 212.95 \mathrm{~kcal~} |
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/ \mathrm{mol}^{-1}$ (the average bond energy for CN triple bonds) and |
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where $r_e= 1.157$ \AA\ (the fixed CN bond length from the force field |
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of Guo {\it et al.}\cite{Zhang:2011hh}), |
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$D_e = 212.95$ kcal~mol$^{-1}$ (the average bond energy |
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for CN triple bonds) and |
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$\beta = 2.526 $\AA~$^{-1}$. These parameters correspond to a |
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vibrational frequency of $\approx 2226 \mathrm{~cm}^{-1}$, which is |
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very close to the frequency of the nitrile peak in the vibrational |
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Inferred hydrogen atom locations were added to the cluster geometries, |
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and the nitrile bond was stretched from 0.87 to 1.52~\AA\ at |
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increments of 0.05~\AA. This generated 13 configurations per gas phase |
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cluster. Single-point energies were computed using the B3LYP |
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functional~\cite{Becke:1993kq,Lee:1988qf} and the 6-311++G(d,p) basis |
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set. For the cluster configurations that had been generated from |
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molecular dynamics running under applied fields, the density |
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functional calculations had a field of $5 \times 10^{-4}$ atomic units |
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($E_h / (e a_0)$) applied in the $+z$ direction in order to match the |
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molecular dynamics simulations. |
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increments of 0.05~\AA. The stretching was carried out by displacing |
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the nitrogen atom position along the CN bond vector. This generated |
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13 configurations per gas phase cluster. Single-point energies were |
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computed using the B3LYP functional~\cite{Becke:1993kq,Lee:1988qf} and |
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the 6-311++G(d,p) basis set. For the cluster configurations that had |
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been generated from molecular dynamics running under applied fields, |
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the density functional calculations had a field of $5 \times 10^{-4}$ |
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atomic units ($E_h / (e a_0)$) applied in the $+z$ direction in order |
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to match the molecular dynamics simulations. |
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The energies for the stretched / compressed nitrile bond in each of |
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the clusters were used to fit Morse potentials, and the frequencies |
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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 |
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nitrile stretches shift by $\sim~3~\mathrm{cm}^{-1}$ to the red of |
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nitrile stretches shift by $\sim~2.5~\mathrm{cm}^{-1}$ to the red of |
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the unperturbed vibrational line. To understand the origin of this |
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shift, a more complete picture of the spatial ordering around the |
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nitrile bonds is required. We have computed the angle-dependent pair |
