| 114 |
|
phase transition,\cite{Leyte:1997zl} the 5CB nitrile group has not yet |
| 115 |
|
seen the detailed theoretical treatment that biologically-relevant |
| 116 |
|
small molecules have |
| 117 |
< |
received.\cite{Lindquist:2008bh,Lindquist:2008qf,Oh:2008fk,Choi:2008cr,Waegele:2010ve} |
| 117 |
> |
received.\cite{Lindquist:2008bh,Lindquist:2008qf,Oh:2008fk,Choi:2008cr,Morales:2009fp,Waegele:2010ve} |
| 118 |
|
|
| 119 |
|
The fundamental characteristic of liquid crystal mesophases is that |
| 120 |
|
they maintain some degree of orientational order while translational |
| 136 |
|
van der Waals interactions. The Gay-Berne potential, in particular, |
| 137 |
|
has been widely used in the liquid crystal community to describe this |
| 138 |
|
anisotropic phase |
| 139 |
< |
behavior.~\cite{Gay:1981yu,Berne72,Kushick:1976xy,Luckhurst90,Cleaver:1996rt} |
| 139 |
> |
behavior.~\cite{Gay:1981yu,Berne:1972pb,Kushick:1976xy,Luckhurst:1990fy,Cleaver:1996rt} |
| 140 |
|
However, these simple models are insufficient to describe liquid |
| 141 |
|
crystal phases which exhibit more complex polymorphic nature. |
| 142 |
|
Molecules which form $S_{A}$ phases can exhibit a wide variety of |
| 296 |
|
primary quantity of interest for interpreting the condensed phase |
| 297 |
|
spectrum of this vibration is the distribution of frequencies |
| 298 |
|
exhibited by the 5CB nitrile bond under the different electric fields. |
| 299 |
< |
Three distinct methods for mapping classical simulations onto |
| 300 |
< |
vibrational spectra were brought to bear on these simulations: |
| 299 |
> |
There have been a number of elegant techniques for obtaining |
| 300 |
> |
vibrational lineshapes from classical simulations, including a |
| 301 |
> |
perturbation theory approach,\cite{Morales:2009fp} the use of an |
| 302 |
> |
optimized QM/MM approach coupled with the fluctuating frequency |
| 303 |
> |
approximation,\cite{Lindquist:2008qf} and empirical frequency |
| 304 |
> |
correlation maps.\cite{Oh:2008fk} Three distinct (and somewhat |
| 305 |
> |
primitive) methods for mapping classical simulations onto vibrational |
| 306 |
> |
spectra were brought to bear on the simulations here: |
| 307 |
|
\begin{enumerate} |
| 308 |
|
\item Isolated 5CB molecules and their immediate surroundings were |
| 309 |
|
extracted from the simulations. These nitrile bonds were stretched |
| 310 |
|
and single-point {\em ab initio} calculations were used to obtain |
| 311 |
|
Morse-oscillator fits for the local vibrational motion along that |
| 312 |
|
bond. |
| 313 |
< |
\item The potential - frequency maps developed by Cho {\it et |
| 314 |
< |
al.}~\cite{Oh:2008fk} for nitrile moieties in water were |
| 313 |
> |
\item The empirical frequency correlation maps developed by Cho {\it |
| 314 |
> |
et al.}~\cite{Oh:2008fk} for nitrile moieties in water were |
| 315 |
|
investigated. This method involves mapping the electrostatic |
| 316 |
|
potential around the bond to the vibrational frequency, and is |
| 317 |
< |
similar in approach to field-frequency maps that were pioneered by |
| 318 |
< |
Skinner {\it et al.}\cite{XXXX} |
| 317 |
> |
similar in approach to field-frequency maps for OH stretches that |
| 318 |
> |
were pioneered by the Skinner |
| 319 |
> |
group.\cite{Corcelli:2004ai,Auer:2007dp} |
| 320 |
|
\item Classical bond-length autocorrelation functions were Fourier |
| 321 |
|
transformed to directly obtain the vibrational spectrum from |
| 322 |
|
molecular dynamics simulations. |
| 384 |
|
local electrostatic potential at selected sites surrounding the |
| 385 |
|
nitrile bond and the vibrational frequency of that bond obtained from |
| 386 |
|
more expensive {\it ab initio} methods. This approach is similar in |
| 387 |
< |
character to the field-frequency maps developed by Skinner {\it et |
| 388 |
< |
al.} for OH stretches in liquid water.\cite{XXXX} |
| 387 |
> |
character to the field-frequency maps developed by the Skinner group |
| 388 |
> |
for OH stretches in liquid water.\cite{Corcelli:2004ai,Auer:2007dp} |
| 389 |
|
|
| 390 |
|
To use the potential-frequency maps, the local electrostatic |
| 391 |
|
potential, $\phi_a$, is computed at 20 sites ($a = 1 \rightarrow 20$) |
| 457 |
|
investigated. |
| 458 |
|
|
| 459 |
|
The correlation functions were filtered using exponential apodization |
| 460 |
< |
functions,\cite{FILLER:1964yg} $f(t) = e^{-c |t|}$, with a time |
| 460 |
> |
functions,\cite{FILLER:1964yg} $f(t) = e^{-|t|/c}$, with a time |
| 461 |
|
constant, $c =$ 6 ps, and were Fourier transformed to yield a |
| 462 |
|
spectrum, |
| 463 |
|
\begin{equation} |
| 490 |
|
|
| 491 |
|
\section{Discussion} |
| 492 |
|
|
| 493 |
+ |
It is clear that united-atom simulations can reproduce the |
| 494 |
+ |
field-induced nematic ordering of the 4-cyano-4'-pentylbiphenyl. |
| 495 |
+ |
Because we are simulating a small electrode separation (5nm), a |
| 496 |
+ |
voltage drop as low as 1.2 V was sufficient to induce the phase |
| 497 |
+ |
change. This potential is significantly lower than the 500V that is |
| 498 |
+ |
known to cause dielectric breakdown in 5CB.\cite{XXX} |
| 499 |
|
|
| 500 |
< |
Observation of Field-induced nematic ordering |
| 500 |
> |
|
| 501 |
> |
|
| 502 |
|
Ordering corresponds to shift of a portion of the nitrile spectrum to |
| 503 |
|
the red. |
| 504 |
|
At the same time, the system exhibits an increase in aligned and anti-a |
