| 493 |
|
|
| 494 |
|
Our simulations show that the united-atom model can reproduce the |
| 495 |
|
field-induced nematic ordering of the 4-cyano-4'-pentylbiphenyl. |
| 496 |
< |
Because we are simulating what is in effect a small electrode |
| 497 |
< |
separation (5nm), a voltage drop as low as 1.2 V was sufficient to |
| 498 |
< |
induce the phase change. This potential is significantly lower than |
| 499 |
< |
the 500V that is known to cause dielectric breakdown in 5CB.\cite{XXX} |
| 496 |
> |
Because we are simulating a very small electrode separation (5nm), a |
| 497 |
> |
voltage drop as low as 1.2 V was sufficient to induce the phase |
| 498 |
> |
change. This potential is significantly lower than the 500V that is |
| 499 |
> |
known to cause dielectric breakdown in 5CB.\cite{XXX} |
| 500 |
|
|
| 501 |
|
Both the classical correlation function and the isolated cluster |
| 502 |
|
approaches to estimating the field-induced changes to the IR spectrum |
| 503 |
|
show an increase in the population of nitrile stretches that appear at |
| 504 |
|
a shift of $\sim 40 \mathrm{cm}^{-1}$ to the red of the unperturbed |
| 505 |
< |
vibrational line. The cause of this shift does not appear to be |
| 506 |
< |
related to the alignment of those nitrile bonds with the field, but |
| 507 |
< |
rather to the change in local environment that is brought about by the |
| 508 |
< |
isotropic-nematic transition. |
| 509 |
< |
|
| 510 |
< |
The angle-dependent pair distribution functions, |
| 505 |
> |
vibrational line. To understand the origin of this shift, a more |
| 506 |
> |
complete picture of the spatial ordering around the nitrile bonds is |
| 507 |
> |
required. The angle-dependent pair distribution functions, |
| 508 |
|
\begin{align} |
| 509 |
|
g(r, \cos \omega) = & \frac{1}{\rho N} \left< \sum_{i} |
| 510 |
|
\sum_{j} \delta \left(r - r_{ij}\right) \delta\left(\cos \omega_{ij} - |
| 525 |
|
\end{figure} |
| 526 |
|
|
| 527 |
|
In figure \ref{fig:gofromega}, one of the structural effects of the |
| 528 |
< |
field-induced phase transition is clear. The nematic ordering |
| 529 |
< |
transfers population from the perpendicular or unaligned region in the |
| 530 |
< |
center of the plot to the nitrile-alinged peak near $\cos\omega = |
| 531 |
< |
1$. Most other features are undisturbed. The major change visible is |
| 532 |
< |
the increased population of aligned nitrile bonds in the first |
| 533 |
< |
solvation shells. |
| 537 |
< |
|
| 528 |
> |
field-induced phase transition is clear. The nematic ordering |
| 529 |
> |
transfers population from the perpendicular or unaligned region ($\cos |
| 530 |
> |
\omega \approx 0$) to the nitrile-alinged peak near $\cos\omega = 1$, |
| 531 |
> |
leaving most other features are undisturbed. This change is visible |
| 532 |
> |
in the simulations as an increased population of aligned nitrile bonds |
| 533 |
> |
in the first solvation shell. |
| 534 |
|
\begin{figure} |
| 535 |
|
\includegraphics[width=\linewidth]{Figure4} |
| 536 |
|
\caption{Contours of the angle-dependent pair distribution functions |
| 540 |
|
the bulk density.} |
| 541 |
|
\label{fig:gofromega} |
| 542 |
|
\end{figure} |
| 547 |
– |
|
| 543 |
|
Although it is possible that the coupling between closely-spaced |
| 544 |
|
nitrile pairs is responsible for some of the red-shift, that is not |
| 545 |
< |
the complete picture. The other two dimensional pair distribution |
| 546 |
< |
function, $g(r,\cos\theta)$, shows that nematic ordering also |
| 547 |
< |
transfers population that is directly in line with the nitrile bond |
| 548 |
< |
(see figure \ref{fig:gofrtheta}) to the sides of the molecule, thereby |
| 549 |
< |
freeing steric blockage that directly blocks the nitrile vibratio |
| 545 |
> |
the only structural change that is taking place. The other two |
| 546 |
> |
dimensional pair distribution function, $g(r,\cos\theta)$, shows that |
| 547 |
> |
nematic ordering also transfers population that is directly in line |
| 548 |
> |
with the nitrile bond (see figure \ref{fig:gofrtheta}) to the sides of |
| 549 |
> |
the molecule, thereby freeing steric blockage that is more directly |
| 550 |
> |
influencing the nitrile vibration. |
| 551 |
|
\begin{figure} |
| 552 |
< |
|
| 557 |
< |
\includegraphics[width=\linewidth]{Figure5} |
| 552 |
> |
\includegraphics[width=\linewidth]{Figure6} |
| 553 |
|
\caption{Contours of the angle-dependent pair distribution function, |
| 554 |
|
$g(r,\cos \theta)$, for finding any atom at a distance and angular |
| 555 |
< |
deviation from the nitrile bond centroid. The right side of each |
| 556 |
< |
plot corresponds to local density directly the direction of |
| 557 |
< |
nitrile bond. Increased density at $\cos\theta = 1$ corresponds |
| 558 |
< |
to steric hindrance of the nitrile bond.} |
| 559 |
< |
\label{fig:gofromega} |
| 555 |
> |
deviation from the centrile of a nitrile bond. The top of each |
| 556 |
> |
contour plot corresponds to local density along the direction of |
| 557 |
> |
the nitrogen in the CN bond, while the bottom is in the direction |
| 558 |
> |
of the carbon atom. $g(z)$ data taken by following the |
| 559 |
> |
\ce{C -> N} vector for each nitrile bond (bottom panel) shows |
| 560 |
> |
that the field-induced phase transition reduces the population |
| 561 |
> |
atoms that are directly in line with the vibrational motion.} |
| 562 |
> |
\label{fig:gofrtheta} |
| 563 |
|
\end{figure} |
| 564 |
|
|
| 567 |
– |
.At the same time, the system exhibits an increase in aligned |
| 568 |
– |
and anti-a |
| 565 |
|
|
| 566 |
|
|
| 567 |
|
|
| 568 |
+ |
The cause of this shift does not appear to be |
| 569 |
+ |
related to the alignment of those nitrile bonds with the field, but |
| 570 |
+ |
rather to the change in local environment that is brought about by the |
| 571 |
+ |
isotropic-nematic transition. |
| 572 |
|
|
| 573 |
|
|
| 574 |
– |
|
| 575 |
– |
|
| 574 |
|
While this makes the application of nitrile Stark effects in |
| 575 |
|
simulations without water harder, these data show |
| 576 |
|
that it is not a deal breaker. The classically calculated nitrile |
