| 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} |
| 495 |
> |
Because we are simulating what is in effect a small electrode |
| 496 |
> |
separation (5nm), a voltage drop as low as 1.2 V was sufficient to |
| 497 |
> |
induce the phase change. This potential is significantly lower than |
| 498 |
> |
the 500V that is known to cause dielectric breakdown in 5CB.\cite{XXX} |
| 499 |
|
|
| 500 |
+ |
Both the classical correlation function and the isolated cluster |
| 501 |
+ |
approaches to estimating the field-induced changes to the IR spectrum |
| 502 |
+ |
show an increase in the population of nitrile stretches that appear at |
| 503 |
+ |
a shift of $\sim 40 \mathrm{cm}^{-1}$ to the red of the unperturbed |
| 504 |
+ |
vibrational line. The cause of this shift does not appear to be |
| 505 |
+ |
related to the alignment of those nitrile bonds with the field, but |
| 506 |
+ |
rather to the change in local ordering that is brought about by the |
| 507 |
+ |
isotropic-nematic transition. |
| 508 |
|
|
| 509 |
|
|
| 510 |
|
Ordering corresponds to shift of a portion of the nitrile spectrum to |
| 511 |
< |
the red. |
| 512 |
< |
At the same time, the system exhibits an increase in aligned and anti-a |
| 505 |
< |
|
| 506 |
< |
|
| 507 |
< |
|
| 508 |
< |
Since no explicit dependence was observed between the calculated |
| 509 |
< |
frequency and the electric field, it was not a viable route for the |
| 510 |
< |
calculation of a nitrile spectrum. Instead, the frequencies were taken |
| 511 |
< |
and convolved together with a lorentzian line shape applied around the |
| 512 |
< |
frequency value. These spectra are seen below in Figure 4. While the |
| 513 |
< |
spectrum without a field is lower in intensity and is almost bimodel |
| 514 |
< |
in distrobution, the external field spectrum is much more |
| 515 |
< |
unimodel. This tighter clustering has the affect of increasing the |
| 516 |
< |
intensity around 2226 cm\textsuperscript{-1} where the peak is |
| 517 |
< |
centered. The external field also has fewer frequencies of higher |
| 518 |
< |
energy in the spectrum. Unlike the the zero field, where some |
| 519 |
< |
frequencies reach as high as 2280 cm\textsuperscript{-1}. |
| 511 |
> |
the red. At the same time, the system exhibits an increase in aligned |
| 512 |
> |
and anti-a |
| 513 |
|
|
| 521 |
– |
Interestingly, the field that is needed to switch the phase of 5CB |
| 522 |
– |
macroscopically is larger than 1 V. However, in this case, only a |
| 523 |
– |
voltage of 1.2 V was need to induce a phase change. This is impart due |
| 524 |
– |
to the short distance of 5 nm the field is being applied across. At |
| 525 |
– |
such a small distance, the field is much larger than the macroscopic |
| 526 |
– |
and thus easily induces a field dependent phase change. However, this |
| 527 |
– |
field will not cause a breakdown of the 5CB since electrochemistry |
| 528 |
– |
studies have shown that it can be used in the presence of fields as |
| 529 |
– |
high as 500 V macroscopically. This large of a field near the surface |
| 530 |
– |
of the elctrode would cause breakdown of 5CB if it could happen. |
| 514 |
|
|
| 532 |
– |
The absence of any electric field dependency of the freuquency with |
| 533 |
– |
the Gaussian simulations is interesting. A large base of research has been |
| 534 |
– |
built upon the known tuning of the nitrile bond as the local field |
| 535 |
– |
changes. This difference may be due to the absence of water or a |
| 536 |
– |
molecule that induces a large internal field. Liquid water is known to have a very high internal field which |
| 537 |
– |
is much larger than the internal fields of neat 5CB. Even though the |
| 538 |
– |
application of Gaussian simulations followed by mapping it to |
| 539 |
– |
some classical parameter is easy and straight forward, this system |
| 540 |
– |
illistrates how that 'go to' method can break down. |
| 515 |
|
|
| 516 |
|
While this makes the application of nitrile Stark effects in |
| 517 |
|
simulations without water harder, these data show |
