Influences of polarizability effect of alkyl group and homoring competition effect of substituents on the NMR spectra of salen-type Schiff base

Article abstract of DOI:10.1002/mrc.5131

Salen-type Schiff bases are a kind of important compounds and are widely used. In order to explore the effect of alkyl groups and substituents attached to aromatic ring on the chemical shifts, 63 title compounds were synthesized. Their ¹H NMR and ¹³C NMR spectra were obtained; and the effects of the alkyl chain length and substituents on the chemical shifts (δ_H(CH=N), δ_C(CH=N), δ_H(OH), and δ_C(C-OH)) were studied. The results show that (1) the alkyl polarizability effect index (PEI) has an important influence on the chemical shifts of the above four atoms, with the increase of PEI, the values of δ_H(CH=N) and δc(CH=N) decrease, and the values of δ_H(OH) and δ_C(C-OH) increase. (2) The influence of substituent X attached to aromatic ring on the chemical shift is related to its position by taking OH or CH=N as reference. As for the effect of substituent on the chemical shifts, the effect of Hammett constant σ(X)_-OH and excited-state substituent parameter (Formula presented.) with OH as reference are different from that ofσ(X)_-CH=N and (Formula presented.) with CH=N as reference, and there is a “homoring competition effect” of the substituent. (3) The effect of the cross-interaction between X and OH on the chemical shift is also significantly different due to the different position of X. Quantitative correlation equations against chemical shifts were built for the four atoms, and the stability and prediction ability of the obtained equations were confirmed by leave-one-out cross validation.

Full text of DOI:10.1002/mrc.5131

Received: 4 October 2020
DOI: 10.1002/mrc.5131
Revised: 31 December 2020
Accepted: 3 January 2021
R E S E A R C H A R T I C L E
Influences of polarizability effect of alkyl group and
homoring competition effect of substituents on the NMR
spectra of salen-type Schiff base
Bai-ying Wei
| Chen-zhong Cao | Chao-tun Cao
School of Resource Environment and
Safety Engineering, School of Chemistry
and Chemical Engineering, Key
Laboratory of Theoretical Organic
Chemistry and Function Molecule,
Ministry of Education, Hunan University
of Science and Technology, Xiangtan,
China
Abstract
Salen-type Schiff bases are a kind of important compounds and are widely
used. In order to explore the effect of alkyl groups and substituents attached to
aromatic ring on the chemical shifts, 63 title compounds were synthesized.
Their 1H NMR and 13C NMR spectra were obtained; and the effects of the alkyl
chain length and substituents on the chemical shifts (δH(CH N), δC(CH N),
δH(OH), and δC(C OH)) were studied. The results show that (1) the alkyl
polarizability effect index (PEI) has an important influence on the chemical
shifts of the above four atoms, with the increase of PEI, the values of
δH(CH N) and δc(CH N) decrease, and the values of δH(OH) and δC(C OH)
increase. (2) The influence of substituent X attached to aromatic ring on the
chemical shift is related to its position by taking OH or CH N as reference. As
for the effect of substituent on the chemical shifts, the effect of Hammett con-
Correspondence
Chen-zhong Cao, School of Resource
Environment and Safety Engineering,
School of Chemistry and Chemical
Engineering, Key Laboratory of
Theoretical Organic Chemistry and
Function Molecule, Ministry of
Education, Hunan University of Science
and Technology, Xiangtan 411201, China.
Email: czcao@hnust.edu.cn
stant σ(X)
and excited-state substituent parameter σeCxCðXÞ
with OH as
with CH N
OH
OH
reference are different from that ofσ(X)
and σeCxCðXÞ
CH
N
Funding information
CH
N
Hunan Natural Science Foundation,
Grant/Award Number: 2020JJ5155;
National Natural Science Foundation of
China, Grant/Award Number: 21672058
as reference, and there is a “homoring competition effect” of the substituent.
(3) The effect of the cross-interaction between X and OH on the chemical shift
is also significantly different due to the different position of X. Quantitative
correlation equations against chemical shifts were built for the four atoms, and
the stability and prediction ability of the obtained equations were confirmed
by leave-one-out cross validation.
K E Y W O R D S
alkyl polarizability effect, homoring competition effect, NMR chemical shift, salen-type Schiff
base, substituent effect
1 | INTRODUCTION
applied to materials,[1,2] catalysis,[3–7] biomedicine,[8–10]
and other fields.[11,12] For example, Zhang et al.[1] pro-
Salen-type Schiff bases refers to a class of compounds
containing carbon–nitrogen double bond (C N), con-
densated by salicylaldehyde derivative with diamine.
Because the molecular central position contains four
coordination atoms of O, N, N, and O, which can be used
as a good ligand for certain metals, they were widely
vided a comprehensive introduction for many salen-type
Schiff bases metal complexes, to be used as soluble conju-
gated materials, as converters for light/electricity signals
in organic light-emitting diodes and dye-sensitized solar
cells, energy storage, and potential conductive thermo-
electric materials. He pointed out that metal Schiff base
Magn Reson Chem. 2021;1–12.
wileyonlinelibrary.com/journal/mrc
© 2021 John Wiley & Sons, Ltd.
1

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WEI ET AL.
chemistry can and will make important contribution to
the global energy problem. Cozzi[5] pointed out that
Schiff base ligands are able to coordinate many different
metals, enabling the use of Schiff base metal complexes
for a large variety of useful catalytic transformations.
Mahapatra et al.[6] synthesized a kind of salen-type Schiff
base and its complex containing mononuclear copper,
prepared two kinds of new trinuclear complexes, and
investigated their catalytic activities, then proposed firstly
the mechanisms of these biomimetic oxidase reactions
involving any heterometallic catalyst. Damercheli et al.[8]
indicated that it is possible to use bioactive salen-type
Schiff bases transition metal complexes as anticancer
agents. The work of Onami et al.[9] showed that the
salen-type Schiff bases metal complexes can increase
the degradation of protein.
In addition to metal ions and coordination modes, the
structure and properties of ligands also have an impor-
tant influence on the properties and applications of the
complexes. Abu-Surrah et al.[13] synthesized a series of
salen-type Schiff bases with different substituents and
obtained some complexes with metals Fe(III) and Co(III).
He further studied the catalytic activities of the obtained
complexes. The obtained results showed that the iron(III)
complexes catalysts bearing electron-withdrawing substit-
uents on the salen ligands showed the highest catalytic
activity under similar reaction conditions. Rao et al.[14]
synthesized four Schiff bases which differ in the chain
lengths of the substituents and their Pd(II) complexes,
investigated the catalytic activity in Suzuki–Miyaura
reaction, and found that the catalytic activity is related to
the alkyl chain length of substituents. Gupta et al.[15]
studied the effect of diamines with different chain lengths
(carbon atom number n = 2, 3, 4) on the formation of
complexes with salen-type Schiff bases and Co(III) and
observed that the nature of the final product is sensitive
to small changes in the length of the spacer in the initial
diamine. Nathan et al.[16] designed and synthesized a
series of salen-type Schiff bases with different diamine
chain lengths and studied their coordination with copper.
It was found that the coordination modes of salen-type
Schiff bases with different diamine chain lengths were
different. It can be seen that the substituent and the
chain length of the diamine play an important role for
the properties of salen-type Schiff bases.
length, but there is a lack of systematic study on the
effect of substituent and carbon chain length on the
charge density distribution of these Schiff bases, espe-
cially on the quantitative correlation analysis of NMR
spectra. Studies of Cao et al.[17–19] showed that the elec-
tronic effect of substituents had an important effect on
the NMR spectra of single Schiff bases and showed a
good quantitative relationship. However, the effect of
substituent on the NMR chemical shift of salen-type
Schiff bases and the influence of diamine chain length on
the NMR chemical shift of salen-type Schiff bases are still
unclear. Therefore, a series of salen-type Schiff bases with
different diamine chain length (carbon atom number
n = 2–8) were designed and synthesized in this study.
The change regularity of NMR chemical shifts was dis-
cussed in terms of both the electronic effect of substitu-
ents and the change of alkyl chain length, which may
provide a reference for understanding the charge distri-
bution of salen-type Schiff bases.
2 | EXPERIMENTAL METHODS
2.1 | Materials synthesis
Sixty-three samples of diamine-salicylaldehyde conden-
sates (abbreviated as Cn-X) were synthesized according to
You et al.[20] The ethanol solution (50 ml) of diamine
compound H2N(CH2)nNH2 (n = 2–8) (1.0 mmol) was
slowly added into the ethanol solution (50 ml) of
substituted salicylaldehyde (2.0 mmol). The mixture was
stirred for 30 min at room temperature, and yellow pre-
cipitate appeared. Some of the solvent was evaporated
and cooled until there was a great deal of the yellow pre-
cipitate. After filtering and recrystallizing with ethanol,
the target products were obtained with yield of 90% to
95%. After vacuum drying, their molecular structures
were confirmed. The synthetic route is as shown in
Figure 1.
2.2 | Spectroscopic measurement
Using CDCl3 as solvent, NMR spectra of the target com-
pound were recorded at 298 K by using a Bruker AV
500-MHz spectrometer and measured relative to the sig-
nals for residual chloroform (7.26 ppm for 1H and
77.00 ppm for 13C) in the deuterated solvent. And the
chemical shifts were referenced to TMS (0.00 ppm). The
acquisition parameters for the 1H NMR were pulse
sequence, zg30; spectral width, 13,020 Hz; number of
scans, eight; and acquisition time, 1.50 s. The acquisition
parameters for the 13C NMR were pulse sequence,
Theoretically, the coordination ability of salen-type
Schiff bases with metals depends on the charge density
on the N and O atoms in the molecule. For a series of
compounds with similar parent structures, the chemical
shifts are an effective method to express the relative
density of charges on the atoms of interest. A lot of salen-
type Schiff bases and their complexes have been investi-
gated by researchers by changing substituent and chain

WEI ET AL.
3
FIGURE 1 Synthesis route of target compound Cn-X
zgpg30; spectral width, 29,762 Hz; number of scans,
20–200; and acquisition time, 1.10 s. Data were processed
by using Bruker's TOPSPIN-NMR software version 2.1
(Bruker, Germany). A total of 63 samples were character-
the benzene ring with substituent X, C N bond and OH
group can form a six-membered quasi-ring containing
atoms HOCCCN, which has some aromaticity.[30–32] In
this quasi-ring, there are lone pairs of electrons on atom
of O and N. The electron transfer direction is shown in
Figure 2. If the hydroxyl group OH is used as reference
and X is at C-5 (Figure 2), it is in para-position to OH;
and when it is at C-4, it is in meta-position to
OH. However, if the reference is CH N, X is at C-5
(Figure 2), it is in the meta-position to CH N; and when
it is at C-4, it is in the para-position to CH N. Therefore,
we use both OH and CH N as reference for locating X
group. The Hammett constants values of the substituents
X with about two locating methods are expressed as
1
ized with H and 13C NMR. The chemical shift value of
the hydrogen atom on the CH N is expressed as
δH(CH N), and that of the carbon atom on the CH N is
expressed as δC(CH N); the chemical shift value of the
hydrogen atom of hydroxyl group (OH) connected to the
benzene ring is expressed as δH(OH), and the chemical
shift value of the carbon atom connected to the OH is
expressed as δC(C OH). The obtained chemical shift
values of hydrogen atoms and carbon atoms are listed
in Table 1. Additionally, mass spectra of some new
compounds were performed with a Waters Xevo Q-
TOF-electronic spray ion mass spectrometer. The NMR
and mass spectra were presented in the Supporting
Information.
σ(X) OH and σ(X)
N, listed in Table 1.
CH
2.3.3 | Excited-state substituent
ex
parameters (σ )
CC
ex
2.3 | Parameters introduction
2.3.1 | Alkyl PEI
Excited-state substituent parameter (σ ) was proposed
CC
by Cao et al.[23] and is a molecular descriptor characteriz-
ing the stability of the excited state. It was extensively
applied to the QSPR studies[29,33–35] and plays an impor-
tant role in the study of the chemical shifts.[33,36,37] We
The alkyl polarizability effect index (PEI) was proposed
by Cao and Li,[21] to quantify the relative magnitude of
the polarizability effect of alkyl groups, and was exten-
sively applied.[25–27] The PEI used in this paper was
directly calculated by the method reported in Cao and
Li,[21] listed in Table 1.
ex
use both OH and CH N as reference and express the σ
CC
ex
of the substituent X as σ ðXÞ
CC
ex
and σ ðXÞ
CC
by
N
OH
CH
adopting the locating method of substituent X in
Section 2.3.2, listed in Table 1.
2.3.4 | The cross-interaction parameter
between substituents (Δσ2)
2.3.2 | Hammett parameter (σ)
Hammett parameter (σ)[22] is a well-known substituent
parameter characterizing the ground-state electronic
effect, which has been extensively applied to the quanti-
tative structure–property relationship (QSPR) stud-
ies.[18,22,28,29] The value of the Hammett constant of a
substituent is related to its position on the aromatic ring.
If a group is in ortho (o)-, meta (m)-, or para (p)-position,
its Hammett constant is, expressed by σo, σm, and σp,
respectively. The molecular structure of the target com-
pound, affected by the intramolecular hydrogen bonding,
The cross-interaction parameter between substituents
(Δσ2) is a parameter used to express the cross-interaction
effect between two or more substituents in an
organic molecule. In this study, the cross-interaction
parameter between substituents is directly calculated
by the method reported in Cao et al.[38] The cross-
interaction parameter between hydroxyl group
(OH) and substituent (X) has two expressions Δσp2
ꢀ
ꢁ
2
and Δσ2m , that is, Δσp2 = σpðXÞ OH −σpðOHÞ
and
ꢀ
ꢁ
2
Δσ2m = σmðXÞ OH −σmðOHÞ .

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3.2 | The effect of substituent X on
chemical shift
As shown in Table 1, compounds with alkyl chains with
equal number of carbon atoms but different substituents
have different chemical shift values. The type of substitu-
ent on the aromatic ring and their relative position on
the aromatic ring have an important influence on the
NMR chemical shift value.
FIGURE 2 Electron transfer diagram of the target compounds
3 | RESULTS AND DISCUSSION
3.1 | Influence of alkyl chain length on
the chemical shift
3.3 | Quantitative correlation of the
chemical shift
It was found that the NMR chemical shifts of salen-
type Schiff bases with the same substituent X and
different alkyl chain length are different. In order to
investigate the relationship between the chemical shift
and the alkyl chain length, we took δH(OH) and
δC(CH N) and plotted the chemical shift values of
δH(OH) and δC(CH N) against the chain length
n (Figure 3).
It can be seen from Figure 3 that as the number n of
carbon atoms in the alkyl chain changes, the chemical
shift value also changes, but there is no linear relation-
ship. The alkyl group is nonpolar, in which each C atom
of CH2 is in sp3 hybrid state and cannot transfer the
conjugation effect. Under the action of the hydrogen
bond O HꢀꢀꢀN and the partial charge of the nitrogen
atom, the alkyl group is polaeized, which affect the
chemical shift. Therefore, we plotted the δH(OH) and
δC(CH N) in Figure 3 against the PEI of the alkyl group
and obtained Figure 4.
We used the alkyl PEI; the substituent electronic
effect parameters σ(X) OH, σ(X)
N, σeCxCðXÞ OH , and
CH
ex
σ
ðXÞ
; and the cross-interaction parameter
N
CC
CH
between substituents Δσ2p and Δσm2 as variables and per-
formed regression analysis for δH(CH N), δC(CH N),
δH(OH), and δC(C OH) values (Table 1). The obtained
QSPR models are shown in Table 2. Leave-one-out cross
validation is also carried out to test the stability and pre-
dictive ability for each model and its performances, mar-
ked by “cv,” were also listed in Table 2. That is, RCV and
SCV represent the correlation coefficient and standard
deviation of the leave-one-out.
For δH(CH N), δC(CH N), δH(OH), and δC(C OH)
discussed in this work, we established QSPR models
(Table 2) by carrying out regression analysis and
employing different parameters. In order to investigate
the contribution of each parameter to the chemical shift,
we calculated the parameter contributions to each model
according to Equations 5 and 6.[27,39] Ψγ and Ψf are the
relative and fractional contributions of the parameter to
the model. In Equations 5 and 6, mi and Xi are the regres-
sion coefficient and the average value of each parameter
It can be seen from Figure 4 that the chemical shifts
show a good linear relationship with PEI. Therefore, it is
preliminarily judged that the alkyl PEI has a direct influ-
ence on the NMR chemical shift value.
FIGURE 3 Plot of the NMR chemical sift value versus the number of carbon atoms n

8
WEI ET AL.
FIGURE 4 Plot of the NMR chemical shift value versus the alkyl polarizability effect index (PEI)
in the QSPR model, respectively, and R is the correlation
coefficient. The physical meanings of each model will be
discussed in the following sections.
substituent X on δH(CH N) can play a role by different
ways via OH and CH N groups, which shows the com-
plexity of substituent effect. In order to discuss the contri-
bution of each parameter in Equation (1) to δH(CH N),
we calculated the parameter contributions and fractional
contributions to Equation (1) according to Equations 5
and 6 and listed in Table 3.
It can be seen from Table 3 that the alkyl PEI is the
most important parameter in Equation (1) with fractional
contribution of 70.10%. The second is the excited-state
ΨγðiÞ = miXi
R2jΨγðiÞj
P
Ψf ðiÞ =
× 100%:
jΨγðiÞj
i
ex
substituent parameters σ ðXÞ
which is located by
N
CC
CH
the reference group of CH N. By comparing the
ex
CC
3.3.1 | Influence of alkyl PEI and various
substituent parameters on δH(CH N)
contributions of parameters σ ðXÞ
with that of
OH
ex
σ
ðXÞ
N, as well as the contributions of parameters
CH
CC
Δσ2p with that of Δσ2m, it can be preliminarily considered
that, for the influence of X group on the δH(CH N),
there is a competitive effect when using OH and CH N
as reference groups.
From Equation (1), we can see that δH(CH N) of the
target compound have good correlations with alkyl
ex
PEI, σ(X) OH, σ(X)
N, σeCxCðXÞ OH, σ ðXÞ
,
N
CH
CC
CH
Δσ2p, and Δσ2m , the correlation coefficient R is 0.9244,
and the standard error S is 0.03 ppm. The leave-one-out
cross validation gives similar result. It indicates that
Equation (1) can quantitatively describe the change of
δH(CH N). Equation (1) shows that δH(CH N)
decreases as PEI increases (i.e., the alkyl chain increases).
The δH(CH N) is mainly affected by the Hammett con-
3.3.2 | Influence of alkyl PEI and various
substituent parameters on δH(OH)
From Equation (2), we can see that δH(OH) is mainly
ex
affected by PEI, σ(X)
,
σ(X)
,
CH N
σ
ðXÞ
,
OH
OH
CC
ex
stant σ(X)
with hydroxyl (OH) as reference position-
σ
ðXÞ
N , Δσ2p , and Δσ2m . The correlation is good,
OH
CC
CH
ing group, and the δH(CH N) decreases as σ(X)
the correlation coefficient R is 0.9931, and the standard
error S is 0.04 ppm. The leave-one-out cross validation
gives similar result. It indicates that Equation (2) can
quantitatively describe the δH(CH N) change. The
δH(CH N) increases as PEI increases, the δH(OH) is
OH
increases. Different from the influence of Hammett con-
stant, the δH(CH N) was simultaneously affected by the
excited-state substituent parameters with OH and CH N
ex
as reference positioning group
σ
ðXÞ
and
CC
OH
ex
σ
ðXÞ
N , and δH(CH N) increases as σeCxCðXÞ
simultaneously affected by σ(X)
and σ(X)
N, but
CH
OH
CC
CH
OH
ex
and σ ðXÞ
increase. The cross-interaction param-
N
the effect of the two parameters on δH(OH) are just oppo-
CC
CH
eter between substituent X and OH, the δH(CH N) was
affected by Δσ2p and Δσm2 , and δH(CH N) increases as
Δσ2p and Δσ2m increase. In summary, the effect of
site. The δH(OH) is affected by both σeCxCðXÞ
and
OH
ex
CC
σ
ðXÞ
N , and the effects of the two parameters on
CH
δH(OH) are also opposite. The δH(CH N) is affected by

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9

10
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TABLE 3 The relative and
ex
ex
Parameters
Ψγ
PEI
σ(X)
σ
ðXÞ
σ
ðXÞ
Δσ2p
Δσ2m
0.0307
4.65
OH
CC
OH
CC
CH
N
fractional contributions of parameters
in Equation (1)
−0.4633
70.10
−0.0121
0.0073
0.0386
0.0128
1.94
Ψf (%)
1.82
1.10
5.84
TABLE 4 The relative and fractional contributions of parameters in Equation (2)
ex
ex
Parameters
Ψγ
PEI
5.9800
93.36
σ(X)
σ(X)
σ
ðXÞ
σ
ðXÞ
Δσ2p
Δσ2m
OH
CH
N
CC
OH
CC
CH
N
0.1765
2.76
−0.0757
0.0161
−0.0176
−0.0235
0.37
−0.0277
0.43
Ψf (%)
1.18
0.25
0.27
Δσp2 and Δσm2 and decreases as Δσp2 and Δσ2m increase.
The parameter contributions and fractional contributions
of Equation (2) were listed in Table 4.
Table 4 shows that the alkyl PEI contributes 93.36%
to δH(OH), which is the main contribution. The second
contribution is σ(X) OH. By comparing the contributions
It can be seen from Table 5 that the alkyl PEI is the
most important parameter in Equation (3) with the frac-
tional contribution of 86.99%. For the influence of the
substituent on the δC(CH N), there is also a competitive
effect between σ(X) OH and σ(X)
N (Table 5).
CH
of parameters σ(X)
with that of σ(X)
N, we can
CH
OH
see that, for the influence of substituent X on the
δH(CH N), there is also a competitive effect between the
3.3.4 | Influence of alkyl PEI and various
substituent parameters on δC(C OH)
σ(X) OH and σ(X)
.
N
CH
It can be seen from Equation (4) that δC(C OH) of the
compound is mainly affected by PEI, σ(X)
,
OH
ex
ex
3.3.3 | Influence of alkyl PEI and various
substituent parameters on δC(CH N)
σ(X)
,
N
σ
ðXÞ
,
σ
ðXÞ
,
N
and Δσ2m
CH
CC
OH
CC
CH
together. The correlation is good, the correlation coeffi-
cient R is 0.9926, and the standard error S is 0.36 ppm.
The leave-one-out cross validation gives similar result.
This indicates that Equation (4) can quantitatively
describe the change of δC(C OH). The δC(C OH)
increases as PEI increases. The δC(C OH) is affected not
The result of Equation (3) shows that δC(CH N) of the
compound is mainly affected by PEI, σ(X)
,
OH
σ(X)
N, σeCxCðXÞ
N , and Δσ2m . The correlation is
CH
CH
good, the correlation coefficient R is 0.9447, and the stan-
dard error S is 0.28 ppm. The leave-one-out cross valida-
tion gives similar result. It indicates that Equation (3) can
quantitatively describe the change of δC(CH N). From
Equation (3), it can be seen that δC(CH N) decreases as
PEI increases. The δC(CH N) is affected not only by
only by σ(X) OH but also by σ(X)
N, and the effects of
CH
the two parameters on δC(C OH) are just opposite. The
δC(C OH) is simultaneously affected by σeCxCðXÞ
and
OH
ex
CC
σ
ðXÞ
, the effects of the two parameters are
N
CH
also opposite. For the cross-interaction between substitu-
ents, the δC(C OH) is mainly affected by Δσ2m . The
parameter contributions and fractional contributions of
Equation (4) were listed in Table 6.
σ(X)
but also by σ(X)
N, which is negatively cor-
CH
OH
related with σ(X)
only affected by σ ðXÞ
and σ(X)
N. The δC(CH N) is
CH
OH
ex
CC
N , whereas the influence of
CH
ex
σ
σ
ðXÞ
ðXÞ
on the δC(CH N) can be ignored. When
increases, δC(CH N) also increases. The
N
From Table 6, it can be seen that the alkyl PEI is the
most important parameter in Equation (4) with fractional
contribution of 75.30%, and the second is σ(X) OH, with
CC
ex
OH
CC
CH
δC(CH N) is mainly affected by Δσ2m and increases as
Δσ2m increases. The parameter contributions and frac-
tional contributions of Equation (3) were listed in
Table 5.
fractional contribution of 11.81%. Comparing the contri-
ex
butions of σ(X)
ex
versus σ ðXÞ
CC
versus σ(X)
and σ ðXÞ
OH
CH N
CC
OH
to δC(C OH), it can also be
CH
N
TABLE 5 The relative and
fractional contributions of parameters
in Equation (3)
ex
Parameters
Ψγ
PEI
σ(X)
σ(X)
σ
ðXÞ
Δσ2m
OH
CH
N
CC
CH
N
−23.6295
86.99
−0.0943
−0.0576
−0.2818
0.1807
0.67
Ψf (%)
0.35
0.21
1.04

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11
TABLE 6 The relative and fractional contributions of parameters in Equation (4)
ex
ex
Parameters
Ψγ
PEI
9.6693
75.30
σ(X)
σ(X)
σ
ðXÞ
σ
ðXÞ
Δσ2m
OH
CH
N
CC
OH
CC
CH
N
1.5160
11.81
−0.8105
0.0713
−0.0987
−0.4851
3.78
Ψf (%)
6.31
0.56
0.77
FIGURE 5 Plot of the calculated chemical shifts versus the experimental ones of target compound. (a) The δH(CH N) (4) and
ꢁ
δH(OH) (□) of the hydrogen atoms and (b) the δC(CH N) ( ) and δC(C OH) (☆) of the carbon atom
concluded that for the influence of substituent on
δC(C OH), there are competitive effects among the
4 | CONCLUSION
ex
σ(X)
versus σ(X)
and σ ðXÞ
versus
In summary:
OH
CH N
CC
OH
ex
σ
ðXÞ
.
CC
CH
N
Through the analysis of the above four regression
1. The alkyl PEI has an important influence on the
chemical shift of the mentioned four atoms.
The δH(CH N) and δC(CH N) decrease with the
increase of PEI (the alkyl chain increases); and
δH(OH) and δC(C OH) increase with the increase
of PEI.
2. The chemical shift values of different atoms are
affected by the different electron effect parameters
of substituent X. Regression analysis were per-
formed on the chemical shift values δH(CH N),
δC(CH N), δH(OH), and δC(C OH) by using five to
seven parameters, and good quantitative correlation
equations were obtained. The leave-one-out cross
validation shows that these quantitative equations
have good stability and predictability and can be
used to predict chemical shift values of the title
compounds.
3. When the substituent X attached to the aromatic ring
can be located by two reference groups, there are two
ways of expressing its electronic effect. The two
expressions may increase or decrease the chemical
shift value of the specified atom and show different
influence intensity on the chemical shift. We name
this phenomenon “homoring competition effect” of
the substituent.
equations, it can be found that the influence of the sub-
stituents on the chemical shift values of the four atoms of
the target compound in this work is different, but they
have one thing in common. That is, if X group is located
with OH or CH N as reference, it shows different elec-
tronic effect and different influence intensity on the
chemical shifts, and there appears a competitive effect.
The above results show that in conjugated organic mole-
cules with multiple substituents in the same aromatic
ring, if the position of a substituent on the aromatic ring
can be designated with two ways, such as to designate as
para- or meta-position, the electronic effect of the substit-
uent can show the electronic effect of para-substituent
and meta-substituent. The intensities of the two elec-
tronic effects are different, and there is competition with
each other. We name this phenomenon as “homoring
competition effect.”
In order to further exhibit the relationship between
the calculated chemical shifts and the experimental ones,
we plotted the calculated values against the experimental
ones for the δH(CH N), δC(CH N), δH(OH), and
δC(C OH), as shown in Figure 5. It can be seen that the
calculated chemical shifts are in good agreement with the
experimental ones.

12
WEI ET AL.
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The project was supported by the National Natural
Science Foundation of China (21672058) and Hunan
Natural Science Foundation (2020JJ5155).
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SUPPORTING INFORMATION
Additional supporting information may be found online
in the Supporting Information section at the end of this
article.
How to cite this article: Wei B, Cao C, Cao C.
Influences of polarizability effect of alkyl group
and homoring competition effect of substituents on
the NMR spectra of salen-type Schiff base. Magn
Reson Chem. 2021;1–12. https://doi.org/10.1002/
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