I. Ajaj et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 150 (2015) 575–585
581
To explain substituent effects on electronic absorption spectra
of the investigated pyridones, the mmax of the unsubstituted com-
pound 1 in used solvents was taken as the reference. The LFER
results indicate complex influences of both solvent and substituent
effect on UV–Vis absorption maxima of both forms (Tables 5 and
S16). The correlation results (Table 5), classified in the eight sets
of solvents, and three groups (columns) of substituent-dependent
correlation results, reflect the complex and balanced interplay of
solvent and substituent electronic effects on tautomeric absorption
maxima shifts. Similar results were found for compounds in 6-PY
form (Table S16).
In the two sets of protic solvents and three set of aprotic
solvents, except DMAc (first column; Table 5), negative correlation
slopes were obtained. Somewhat higher sensitivity of mmax to sub-
stituent effect was found for protic solvents. In the third (AcN, THF)
and fourth set of solvents (Chl, Anisol), and DMAc negative solva-
tochromism was noticed with respect to substitutent effects.
Somewhat higher values and similar trend of correlation slope
were found for second column, except DMSO, indicating the signif-
icance of the solvent dipolarity/polarizabilty and hydrogen-
bonding ability to higher stabilization of excited states. Results
presented in the third column indicate high sensitivity of mmax to
substituent effect in aprotic solvents (AcN, THF, Chl and Anisol).
In the second set of protic solvents contribution of aliphatic alcohol
residue plays a significant role in the stabilization of ground state
of compounds 6, 10 and 14.
Generally, results of LFER study show better stabilization of
both forms in the excited state in protic solvents. Lower sensitivi-
ties of absorption frequencies to substituent effects in aprotic sol-
vents can be explained by the effect of high relative permittivity of
surrounding medium which causes that the energies necessary to
bring about charge separation in the ground and excited state are
relatively similar, which gives rise to a lower susceptibility to elec-
tronic substituent effects. Dipolar aprotic solvents behave as poor
anion solvators, while they usually better stabilize larger and more
dispersible positive charges. The electronic systems of the investi-
gated pyridones, considering their non-planarity, could be more
susceptible to the substituent influence. Study of the transmission
The observed q values indicate different susceptibilities of the
SCS to substituent effects. It can be noticed from Tables 6 and
S19 that correlations are of good to high quality which means that
the SCS values reflect electronic substituent effects. It is apparent
0
that chemical shifts of C1 show an increased susceptibility and
normal substituent effect. Reverse substituent effect was observed
at C2 for para-substituents, as well as for C3 and C4 atoms (Table 6).
The negative sign of reaction constant,
i.e. the value of SCS decreases although the electron-withdrawing
ability of the substituents, measured by , increases. The reverse
substituent effect at C2 (for para-substituents), C3 and C4 atoms
can be attributed to localized -polarization [50], which predomi-
q, means reverse behavior,
r
p
nates over the extended
p
-polarization (Table 6) in the compounds
0
in 2-PY form. Correlations for C1 carbon are slightly improved
when electrophilic substituent constants r
+ are used (second line;
Table 6), which indicates that contribution of extended resonance
interaction, i.e. more intensive interaction of electron-donating
0
substituent with electrophilic C1 carbon, is operative within
phenyl ring. The SCS of C6 showed nonlinear (parabolic)
dependence with respect to substituent effect. Similar results were
found for compounds in 6-PY form (Table S19), except normal
substituent effect found for C2 carbon.
To measure separate contributions of the polar (inductive/field)
and resonance effects of substituent, the regression analysis
according to DSP Eq. (4) with rI and rR constants has been per-
formed, and the results are given in Tables 7 and S20. The DSP
equation does not provide significant improvement in fits when
compared to the results of SSP Eq. (1). The positive
q
I and
q
R values
0
have been obtained for the C1 , C2 (3-sub. comps.), C3 and C5 atoms,
while the negative values have been found for C2 (comps. 1, 2, 3, 6
and 10) and C4 confirming that reverse substituent effect is opera-
tive at these carbons. All the
s values are higher than 1, except for
C5 carbon, which means that resonance substituent effect predom-
inates over field effect, and the most pronounced is at C4. The res-
onance interaction significantly depends on spatial arrangement of
the molecules, i.e. the values of torsional angle h, and thus, the
0
resonance substituent effect is most effectively transmitted to C1
and C4 carbons, i.e. C4 is para-position of the carbon of pyridone
ring with respect to N-substituted phenyl ring.
of substituent electronic effects through defined
of investigated compounds showed contribution of the transmis-
sion of electronic effect through isolated -electronic unit and
p-resonance units
Comparative analysis of the structural effect of the substituent
at C6 position: OH group in compounds in 2-PY form and methyl
group in a series of N(1)-(4-substituted phenyl)-3-cyano-4,6-dime
thyl-2-pyridones [51] could give some additional information on
structural effect, the mode and extent of transmission of sub-
stituent effect.
p
overall conjugated system, and their ratio depend on substitution
pattern under consideration.
LFER analysis of NMR data
On the basis of the sign of the constants
compounds (Table 7) and from the literature data for
q for investigated
A comprehensive analysis of the 13C NMR chemical shifts has
been performed to get a better insight into transmission mode of
substituent effect. The experimental and calculated 13C NMR
chemical shifts of the corresponding carbon atoms are given in
Tables S4 and S5. The differentiation in 13C NMR chemical shifts is
less than 8 ppm. The general conclusion derived from the data in
Tables S4 and S5 indicates that all substituents from the
N(1)-phenyl ring influence SCS values of the carbon atoms of inter-
N(1)-(4-substituted
phenyl)-3-cyano-4,6-dimethyl-2-pyridones
0
[51], similar behavior was found for C1 and C5, while positive
and significantly lower proportionality constants for C2 and C3
atoms were obtained [51]. The highest influence of substituent
0
effects is observed at C1 carbon for investigate compounds in
2-PY form and also highest and somewhat lower values were
found for N(1)-(4-substituted phenyl)-3-cyano-4,6-dimethyl-2-pyr
idones. Higher values of correlation coefficients for C3 and C5 car-
bons indicate the significance of electron-donating capability of
hydroxyl group at C6 carbon, in comparison to low hyperconjuga-
tive character of methyl group.
0
est (C1 , C2-C6) via their electronic effects. The effective transmission
of substituent effects, i.e. differences in SCS values, is affected by the
conformational (geometry) change of the investigated molecules
which stems from an out-of-plane rotation of the N(1)-phenyl ring,
i.e. defined by the torsion angle h values (Fig. 1). Optimized
geometries were calculated by the use of B3LYP functional with
6-311G(d,p) basis set (Tables S17 and S18) [25].
DFT, TD-DFT and Bader’s analysis. Evaluation of electronic transition
and charge density change
The analysis of the substituent effect on the SCS of the carbon
atoms of interest, performed by the use of LFER’s Eq. (3) (i.e. SSP)
An additional analysis of solvent and substituent effects on
absorption frequencies, tautomeric equilibria and conformational
changes of the studied compounds, necessitated quantum-
chemical calculations, i.e. geometry optimization and charge
with
r
or
r
+ substituent constants have been applied, and the cor-
0
relation results obtained for C1 , C2–C6 carbons are presented in
Tables 6 and S19.