My PhD dissertation (“candidate”) was devoted
mostly to the prediction of thermal stability of some liquid crystals (nematic
Schiff bases), or, more precisely, the temperature of mesophase destruction
(the temperature at which a liquid crystal becomes a regular liquid). The
results were published in [1-5]. Several other tasks appeared within this work.
Approximately 50 compounds (Schiff bases) were
investigated, e.g., those shown below:
First, we computed NMR spectra of some of
these molecules, and built a correlation between these computed NMR chemical
shifts and experiment:
The computations were performed using the GIAO
B3LYP/6-311G(D,P) method. B3LYP/6-311++G(2DF,PD) yielded somewhat better
agreement with experiment but this level of theory was too expensive for us.
The GIAO and CSGT algorithms yielded almost identical results (the difference
was much smaller than other sources of error).
For molecule 1 (see above) the
crystallography data is available. We did the following: the NMR chemical
shifts for this molecule were computed not only with the optimized structure,
but also with the structure taken from the crystallographic data (the bond
lengths C..H were taken from the computation, since the crystallography does
not provide the coordinates of H atoms in molecule). The agreement
(correlation) between the computed and experimental chemical shifts appears to
be much better in the first case; it means that the optimized structure (which
corresponds to the gas phase) is closer to the real structure in liquid state
than the crystal structure (in other words, the computational error is much smaller
than the impact of the packing forces in crystal).
We found that the NMR spectra can be used for
getting some information about the structure of the investigated molecules. At
the same time, we found that the structural parameters of these molecules are
much less important for our tasks than anticipated by previous researchers.
The molecular polarizability of a number of Schiff
bases was computed. For some of these molecules the experimental polarizability
data is available, and we found a good correlation between the computed and
experimental components of the polarizability tenzor:
We also found a relatively high correlation
between the computed polarizability anisotropy of the molecules and
experimental polarizability anisotropy of substituent for homologues:
Then we plotted a correlation between the
anisotropy of molecular polarizability of our Schiff bases (computed with DFT)
and the experimental temperature of liquid crystal phase destruction
(nematic-isotropic phase transition):
This “correlation” is quite low. Anyway, it
can be stated that the anisotropy of molecular polarizability of nematic liquid
crystals can be used for predicting their thermal stability. It seems evident
that the low degree of correlation is not caused by the computational error
(the picture above proves that this error is much smaller).
References:
1. G. A. Zhurko, V. V. Aleksandriiskii,
V. A. Burmistrov. Conformational state of benzilidene aniline derivatives from
ab initio calculation and NMR spectroscopy data// Journal of Structural
Chemistry (J STRUCT CHEM+) 47(4):622-628 · July 2006 with 16 Reads DOI:
10.1007/s10947-006-0348-y
2. G. A. Zhurko, V. V. Aleksandriiskii,
M. K. Islyaikin, V. A. Burmistrov. Structure and molecular polarizability of
mesogenic Shiff bases from quantum chemical calculation data// Journal of Structural Chemistry (J STRUCT
CHEM+) 48(3):440-446 · May 2007 with 19 Reads DOI: 10.1007/s10947-007-0066-0
3. Zhurko
G.A., Alexandriyskiy V.V., Burmistrov V.A. Conformational state of benzilidene
derivatives by NMR 13C
data and quantum chemistry computations//IV Russian conference “New
achievements of NMR in structural investigations.” Kazan, 4-4 April 2005, p.64.
4.
Structure and stability of liquid crystals – proton donor H-complexes in
solutions by 13C
NMR spectroscopy and semi-empirical AM1 method data // Alexandriiskii V. V.,
Burmistrov V. A., Isliyakin M. K., Zhurko G. A. // Intern. Symp. and Summer
School “Nuclear Magnetic Resonance in Condensed Matter”, 3rd Meeting “NMR in
Heterogeneous System”. Saint Petersburg, Petrodvorets, Russia, 9-13 July 2006, p.50.
5.. In Russian: Zhurko G. A. Ispolzovanie
kvantovokhimicheskih raschetov dlya izucheniya molecularnoy structuri nekotorikh
nematicheskih osnovaniy Shiffa // Zhurko G.A., Alexandriyskiy V. V., Burmistrov
V. A. // Zhidkie kristally i ih prakticheskoe ispolzovanie. 2005. N1. c 13-22
English:
Zhurko G. A. The use of quantum chemistry computations for investigation of the
molecular structure of some nematic Schiff bases // Zhurko G.A., Alexandriyskiy
V. V., Burmistrov V. A. // Liquid crystals and their practical use. 2005, N1,
p.13-22.
