ˇ
´
62
M.P. Rancic et al. / Journal of Molecular Structure 1049 (2013) 59–68
Table 3
Correlation results of the SCS values for C , Cb and C7 carbons for series 1 with the DSP
a
(Reynolds) equation.
Atom
qF
qR
R
F
sd
k = qR/qF
C
Cb
C7
–1.65 0.85
8.15 1.75
6.35 1.52
–2.41 0.80
17.03 1.65
24.76 1.43
0.827
0.977
0.990
9
84
193
0.55
2.09
0.99
1.46
2.09
3.90
a
3.2. LFER analysis applied to atomic charges of series 1
In many cases good correlations between the NMR chemical
shifts and the computational atomic charges have been observed
[35]. Therefore, we also probed the ability of the calculated NBO
atomic charges to predict the observed effects of the substituents
Fig. 2. Cross-correlation of experimental vs. calculated chemical shifts.
on the C , Cb and C7 atoms. Computational NBO charges of the min-
a
be explained by predominance of the phenyl ring anisotropy and/
or neighboring effects over the substituent electronic effects. Dif-
ferences in SCS values (Table 1) of TZDs are also affected by the
geometry arising from an out-of-plane rotation of the substituted
imum energy conformations of the TZD molecules (series 1) are
presented in Table 4. From the calculated atomic NBO charges, it
can be concluded that electron-donating substituents cause
increasing of atomic charge at Cb and C7 atoms and the opposite
phenyl ring for the torsion angles h [17]. Correlations for C and
a
is true for C . The electron-accepting substituents follow analogous
a
Cb carbons are not improved when electrophilic substituent con-
stants rþp is used, which indicates that contribution of extended
resonance interaction is of lesser importance in transmitting over-
all substituent electronic effects.
trend.
Satisfactory correlations of the NBO charges with substituent
constants rp/
rþp were obtained and the trends (normal/reverse)
of the substituent effects are similar with those observed with
the SCS values (Tables 5 and 6). Additionally, similar effect on
atomic charges, i.e. reverse substituent effect on NBO charges
To measure separate contributions of the polar (inductive/field)
and resonance effects of substituent X, the regression analysis
according to DSP Eq. (2) with rF and r0R substituent constants
has been performed and the results are given in Table 3.
change was also observed for C atom.
a
This similarity supports the idea that the substituent electronic
effects influence both the atomic charges and the chemical shifts.
The changes in NBO atomic charges provide an estimation of the
substituent-sensitive polarization of the TZD moiety. Good correla-
tions between the atomic charges and SCS values (Table 7) are,
The positive qF and qR values have been obtained for the C7 and
Cb atoms, while the negative values have been found for C con-
a
firming that reverse substituent effect is operative at this carbon.
All the k values are higher than 1 which means that resonance sub-
stituent effect predominates over field effect, and the most pro-
nounced is at C7. The resonance interaction significantly depends
on spatial arrangement of the molecules, i.e. the values of torsional
angle h, and thus, the resonance substituent effect is most effec-
tively transmitted to C7 carbon. The positive values of qF and qR
however, obtained for Cb and C7 atoms, but not for the C atom.
a
This result also reflects that atomic charges and SCS at C atom
a
are sensitive to the substituent effect in a different way.
for the Cb atom (
q
F = 8.15 and
q
R = 17.03) and negative value for
R = ꢂ2.41) show that substituents
p-electron density at carbon atoms
3.3. Comparative LFER analysis of styrene series 1–8
the C atom (
in a different way influence the
q
F = ꢂ1.65 and
q
a
Styrene and its derivatives have been extensively studied in or-
der to investigate field and polarization effects [22]. The transmis-
sion of electronic substituent effects in an aromatic framework
of side-chain group.
The DSP equation does not provide significant improvement in
fits when compared to the single parameter treatment, except for
with side chain, i.e. the concept of
p-polarization is introduced
the C7 atom. The magnitude of the resonance coefficient,
qR, for
by Reynolds [23–25] to rationalize the substituent field effect at
C7 and Cb is larger than that for C for 10 and 7 times, respectively.
the side chain of para- and meta-substituted benzenes. Generally,
a
If one takes the magnitude of the qF values for C as an approxi-
the p-polarization occurs in p-systems of fairly independent units
and close to each other. In such circumstances, the field effect, due
to the presence of a substituent dipole in another part of the mol-
a
mate indicator of the direct (localized)
while qF value for Cb reflects the total
the relative proportion of direct to total
p
p
-polarization component,
-polarization effect, then
-polarization is ca. 1:5
p
ecule, causes subsidiary polarization of
-electronic system without net -electron transfer. In the styrene
systems, the substituent dipole polarizes the vinyl -electronic
system resulting in changes of electron densities by creating oppo-
site charges at each end of the existing -unit. Reynolds has sug-
p-electrons in the second
(i.e. direct to extended is ca. 1:4).
p
p
p
p
gested that, in the case of the styrene type molecules, the polar
effect arises as a result of the substituent dipole induced field ef-
fect, and this effect alters the electron density at Cb by two mech-
anisms: (i) field-induced polarization of the side chain vinyl group
Table 2
Correlations of the SCS values for investigated compounds with SSP equation for C ,
Cb and C7 carbons.
a
Atom
q
h
R
F
sd
(direct
p-polarization), and (ii) field-induced
p-electron transfer
C
rp
–1.73 0.32
–1.02 0.22
–0.15 0.14
–0.39 0.18
0.872
0.842
28
22
0.46
0.50
a
(extended
p-polarization). The second term is considered to be
rpþ
the major effect operative mostly in planar systems. Reynolds con-
clusion that the polar effect is of field, rather than inductive origin,
is supported by the observation that its influence on Cb is approx-
imately the same from the meta- and para- positions [23].
The general styrene framework including polarization
mechanism, localized and extended, together with contribution
Cb
C7
rp
rpþ
10.46 0.84
6.30 0.67
–0.45 0.37
1.06 0.56
0.972
0.952
156
88
1.17
1.53
rp
rpþ
13.43 1.31
8.45 0.54)
–2.86 0.58
–0.75 0.45
0.960
0.982
106
246
1.83
1.23