G. Tunç, B. Canımkurbey, B. Dedeog˘lu et al.
Journal of Molecular Structure 1240 (2021) 130545
tem [33]. The concepts of global chemical hardness and softness,
which are a measure of charge transfer, contributing to the devel-
opment of hard-soft acids and bases (HSAB) theory qualitatively,
provide an insight into the stability of molecular systems. Calcu-
bon atoms, which are almost close to zero electrostatic potential.
Besides, –COOH moiety carbonyl group atoms; C12 atom (0.0297
a.u.) for PN1-a and C28 atom (0.0151 a.u.) for PN2 atom are those
with high positive value compared to other carbon atoms due to
the electronegative oxygen atom drawing the electron density over
itself. A similar state having positive potential is also valid for the
all hydrogen atoms on both structures, the most positive ones were
determined for the -OH group H15 atom (0.0334 a.u.) for PN1-a
monomer and H2 atom (0.0213 a.u.) for PN2 monomer. The cen-
tre of nucleophilic attacks to PN structures may be regions with
positive potential values (blue-coded on the map) and particu-
larly where of hydrogen atoms with the highest positive value.
Prominent active sites on the MEP and contour map confirm the
presence of intermolecular O-HꢀO hydrogen bond interaction that
binds the two monomer structures of PN1-a and PN2.
lated χ, η and S values
are 5.878 eV, 2.507 eV and 0.398 eV−1
for the PN1-a monomer, 5.193 eV, 1.921 eV and 0.520 eV−1 for
the PN2 monomer, respectively (Table S.7). PN2 monomer has a
higher global chemical softness value than PN1-a monomer struc-
ture, which indicates that it has a soft molecular structure. Soft
molecules can be more reactive and more polarized; therefore, in-
tramolecular charge transfer can be directed much more easily. The
fact that PN2 has a narrower HOMO-LUMO gap compared to PN1-a
supports this result. The other parameter commonly used in chem-
ical reactivity analysis is the global electrophilicity index (ω) de-
scriptor that determines the energy change of an electrophile when
it is saturated with electrons and classifies electrophilic structure
quantitatively at a relative scale [35,72].
When the reactivity analyses for PN structures are evaluated to-
gether (Fukui functions and MEP analysis), there is a difference
which might result from the fact that the condensed Fukui func-
tions are highly dependent on the population analysis scheme and
the basis set selection [74]. Moreover, Fukui functions take into ac-
count soft-soft interactions in molecular systems, while MEP de-
scribes hard-hard interactions [75].
The molecular structure depending on whether the value of ω
less than 0.8 eV is, between 0.8 to 1.5 eV and greater than 1.5 eV,
can be evaluated as marginal, medium and strong electrophile, re-
spectively [73]. The calculated ω values for PN monomers (6.891
eV for PN1-a, 7.015 eV for PN2) show that both can be consid-
ered as strong electrophiles. However, PN2 monomer with a lower
η (1.921 eV), higher μ (-5.193 eV) and ω (7.015 eV) values shows
a better electrophilic character than PN1-a monomer.
3.7. NLO analysis
In order to determine the local field reactivity and site selec-
Total static dipole moment (μtot ), mean polarizability (< α >),
the anisotropic of the polarizability (ꢀα) and total first-order po-
larizability (βtot ) values of the PN1-a and PN2 monomer struc-
tures were calculated with the Eqs. (5-8) and tabulated in Table
S.9. For PN1-a ve PN2 structures, μtot values were 2.183 Debye
and 3.072 Debye, while <α> and ꢀα -components independent
and dependent on the direction of the applied electric field- val-
ues were obtained as 18.215×10−24 esu, 39.867×10−24 esu and
15.685×10−24 esu, 52.809×10−24 esu, respectively. It was deter-
mined that PN2 monomer exhibits higher polarizability capability
than PN1-a monomer. A similar situation is observed in βtot val-
ues and calculated as 3030.6×10−33 for PN1-a and 13394.75×10−33
esu for PN2. For the prediction of the potential use as NLO mate-
rial of molecular structures; βtot values are particularly known to
be an important parameter and compared with the urea molecule
which is considered as a reference. At the same theoretical level,
the βtot value for urea was calculated as 782.99×10−33 esu and
tivity of PN structures, Fukui functions ( fk+
,
fk−, fk0) and dual de-
scriptor values have been calculated (Table S.8). This analysis con-
tributed to the individual investigation of the reactivity tendencies
of atoms and provided the prominent reactivity sites to be deter-
mined by considering the entire molecular structure. Calculated
,
fk− and fk0 values indicate probable regions for nucleophilic,
electrophilic and radical attack in molecular structures. According
to the results obtained for the structure of PN1-a monomer; higher
fk+ values are for N13, C3, N14, O16, C8 atoms, higher fk− val-
ues are for N14, N13, C11, C8, C6 atoms and higher fk0 values are
N13, N14, C3, C8, C11 atoms. In the case of PN2 monomer struc-
ture, the highest fk+; fk− and fk0 values have been obtained for N3,
C7, N13, C16, C19; C24, C6, C19, N3, C14 and C24, C6, N3, C7, C19
atoms, respectively. It is also possible to specify the regions prone
to electrophilic and nucleophilic attacks by considering the signs of
dual descriptors. The graphical representation of dual descriptors is
given in Fig. S.11 The dual descriptors with a positive sign (ꢀfk>0)
indicate the favourable site for the nucleophilic attack, whilst those
with a negative sign ꢀfk<0 may point out the potential site for the
electrophilic attack. It can be concluded that the carboxylic acid
group region may be favoured for nucleophilic attack in the PN1-a
monomer, while has a partial potential for both nucleophilic and
electrophilic attacks in the PN-2 monomer.
fk+
the βtot
/
ratio was determined as 3.8 for the PN1-a structure
(urea)
and 17.1 for the PN2 structure. The fact that the PN2 structure
has a narrow energy gap (ꢀE=3.84 eV), a higher chemical softness
value (S=0.52 eV−1) than PN1-a, and therefore a higher potential
of intramolecular charge transfer also supports this possibility with
its high βtot value. In the design of technological products, ma-
terials that exhibit NLO properties are remarkable significance at
telecommunication, military and scientific environmental monitor-
ing, data storage in information technologies, sensor design, laser
and on a wide scale optoelectronic applications etc. areas [76,77].
The obtained results for PN skeletal structures which consist of
phthalonitrile moieties with strong π-electron charge density and
–COOH with the electron-withdrawing, show that they may have
the potential to be used as NLO material in such applications.
Another tool to get information about reactivity of molecules
is to examine electrostatic potential maps which show charge dis-
tribution within the molecule with different colour codes accord-
ing to the electrostatic potential values. The obtained MEP and 2D
contour maps for PN structures are illustrated in Fig. 5. The re-
gions having negative electrostatic potential on the molecular sur-
face of PN1-a monomer are around the N13 with the value of -
0.0180 a.u., N14 with the value of -0.0183 a.u., O16 with the value
of -0.0089 a.u. and O15 with the value of -0.0015 a.u. atoms. In
the structure of PN2, negative electrostatic potential is around the
atoms of N3, N13, O1 and O2 with values of -0.0252, -0.0237, -
0.0157 and -0.0048 a.u. MEP analysis points out that reactivity
fields prone to electrophilic attacks for both PN structures may
be around these atoms. The negative electrostatic potential value
has not been observed at the position of PN monomer structures
carbon atoms (0.0039-0.0297 a.u. for PN1-a, 0.0020-0.0151 a.u. for
PN2), the green coded areas of the map indicate the region of car-
3.8. Thermodynamic properties
Characteristic thermodynamic parameters such as heat capac-
ity, Cp0,m, entropy, Sm0 , and enthalpy change, ꢀHm0 , for the PN1-a
and PN2 monomer structures were examined at temperature val-
ues ranging from 100K to 800K and at 1 atm pressure with on the
basis of vibrational analysis at with B3LYP/6-311++G(d,p) method.
The results showing the variation of these parameters with tem-
perature are given in Table S.9 As can be seen in Table S.9, it is
9