E. Wierzbicka et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 130 (2014) 488–493
489
¼ 92:75ꢁ
of the HB system are complicated due to large anharmonicity and
a ¼ 7:73 Å b ¼ 6:443 Å c ¼ 3:749 Å
a
b ¼ 109:15ꢁ
c
¼ 95:95ꢁ
proton delocalization [14]. The commonly used models of the cyclic
dimer, describing the shape of the HB band in IR spectroscopy,
assumed that it is the result of: (a) strong anharmonic coupling of
the OAH stretching vibration with the stretching vibration of HB
[15–18], (b) the Fermi resonance [19], appearing between the
OAH stretching vibration and the first overtone of the OAH bending
vibration [20], and (c) Davydov coupling [18,21,22] occurring
between a pair of isoenergetic hydrogen bonds in the cyclic dimer.
The calculations which are presented in this paper, also take into
account the coupling between the vibrations of OAH bonds in the
crystal, which is a factor that strongly influences the HB properties.
Such interactions are very important considering the fact that very
often the hydrogen bonds are most important for holding together
organic crystals. Understanding this is extremely important for the
development of modeling, starting from crystals of small size to
large structures of macromolecules [23]. The progress of computa-
tional methods leads to increasing compatibility of the HB models
with real systems.
Car and Parrinello are the creators of new kind molecular
dynamics (CPMD) [24], where the main idea is the best use of
quantum-mechanical time-scale separation of fast electrons and
slow nuclei motions. This is achieved by converting the quan-
tum-classical problem in a purely classical (in two separable time
scales), on expense of losing information about the physical time in
the quantum network [24–26]. The great advantage of the CPMD
method is the possibility of quantum chemical description of the
system, with effects on the HB factors, such as the crystal period-
icity and thermal fluctuations [27,28], which are not included in
other computational methods, such as DFT [29,30]. In the present
paper, the results of the CPMD calculations for the terephthalic
acid crystal and DFT calculations for its cyclic dimer have been
compared. Also we performed study of the isotopic substitution
effect, on the spectra. CPMD is recently used to simulate IR spectra
of hydrogen-bonded crystals [31,32].
Computational methods
Calculations performed using the CPMD package were aimed
to determine the optimal geometry of the structure of the crystal
with four different isotopic substitutions. Consequently, we calcu-
lated the appropriate trajectories and dipole moments. The
obtained results enabled us to simulate of vibrations of the sys-
tem, leading to theoretical power spectra and infrared spectra.
As a model system four terephthalic acid molecules were used,
which corresponds to 72 atoms. They form two hydrogen-bonded
dimers I and II as shown in Fig. 1. Inside the unit cell are two
pairs of hydrogen bonds. In the two other model systems, hydro-
gen atoms in the carbonyl group were substituted by deuterium
atoms forming isotopomer structures. In the first model system
positions I-H1, H6, H10, H60 and the second I-H1, H60, II-H1, H60
were substituted.
All dynamic calculations were performed using the BLYP func-
tional [35]. To describe the electronic structure of the core we used
Goedecker pseudopotentials [36], while for the valence electrons
Bloch plane wave basis set [37] with the parameter CUTOFF 120
Ry. Optimization of geometry was done with an accuracy 10ꢂ6
,
what permitted to achieve the trajectories. Dipole moment was
calculated at tenth iteration of the optimized geometry. In order
to determine the trajectories, 220,000 iterations of trajectories
were carried for each of the system, with time step equal 2 a.u.
(0.049 fs). Such large number of steps were required because it
was necessary to omit first 70,000 steps in the calculated trajecto-
ries, to stabilize the system. In addition, the geometry optimization
was performed for the isolated dimer using DFT method, with
functional B3LYP and 6-31++G** basis set.
Terephthalic acid is an aromatic organic compound, which is
involved in controlling the processes of some chemical reactions
in organisms, such as being an antioxidant for protection against
damage of internal organs, and myocardial infarction. It is used
widely in industry for the production of high tensile strength poly-
ester fibers. It occurs in nature in the form of white crystals. Its
molecular weight is 166.13 g/mol, and density 1.522 g/cm3. Its
melting point is 573 K, and at temperature 675 K is sublimed. It
is weakly soluble in water and in organic liquids. It crystallizes in
the triclinic system. The unit cell which belongs to the space group
P1, contains one molecule of the acid [33,34]. Terephthalic acid has
two polymorphic forms. Our study concerns one of them because it
is more stable as a result of its higher density and second form
tended to become rarer on storage [34]. Lattice parameters of the
unit cell of the studied crystal acid are:
Experimental section
Infrared spectra were measured for the hydrogen form of tere-
phthalic acid, and for deuterated crystals with deuterium atoms
replacing hydrogen atoms in the carboxylic groups. This substitu-
tion has been obtained by heating a suspension of terephthalic
acid crystals in tetrahydrofuran with the addition of heavy water,
followed by vacuum evaporation. Due to low solubility of tere-
phthalic acid, efficiency of the isotopic exchange process was
low. Only partially isotopically substituted acid was obtained.
ATR technique was used for the spectroscopic measurements,
which were done on Thermo Scientific Nicolet IR200
spectrometer.
Fig. 1. Model system of terephthalic acid crystal.
Wierzbicka, Ewa
