The effect of intramolecular hydrogen bond on the ultraviolet absorption of bi-aryl Schiff bases

Article abstract of DOI:10.1002/poc.4164

Two kinds of Schiff bases, 85 samples of N-(benzylidene)-anilines (ZBAY) and 83 samples of N-(phenyl-ethylene)-anilines (ZAPEY), were used as model compounds, in which the ZBAY contains 13 compounds with 2-OH and the ZAPEY contains 35 compounds with 2-OH

Full text of DOI:10.1002/poc.4164

Received: 26 September 2020
DOI: 10.1002/poc.4164
Revised: 28 October 2020
Accepted: 30 October 2020
R E S E A R C H A R T I C L E
The effect of intramolecular hydrogen bond on the
ultraviolet absorption of bi-aryl Schiff bases
Chao-Tun Cao
| Luyao Li | Chenzhong Cao
| Junlan Liu
Key Laboratory of Theoretical Organic
Chemistry and Function Molecule,
Ministry of Education, School of
Chemistry and Chemical Engineering,
Hunan University of Science and
Technology, Xiangtan, China
Abstract
Two kinds of Schiff bases, 85 samples of N-(benzylidene)-anilines (ZBAY) and
83 samples of N-(phenyl-ethylene)-anilines (ZAPEY), were used as model com-
pounds, in which the ZBAY contains 13 compounds with 2-OH and the
ZAPEY contains 35 compounds with 2-OH (synthesized by this work). The
quantitative correlation analysis ultraviolet absorption spectra of ZBAY and
ZAPEY were performed, and the effect of intramolecular hydrogen bond on
their wave number vmax (cm−1) of the maximum absorption wavelength λmax
(nm) was investigated. The results show that (a) the factors affecting the vmax
of ZBAY and ZAPEY are roughly the same, but their intensities are different.
(b) The vmax move caused by intramolecular hydrogen bond is all red shift for
both ZBAY and ZAPEY, but the red shift value (2,381) of the ZBAY is more
than twice than that (850) of ZAPEY. (c) The effect of intramolecular hydrogen
bond on vmax is only dominated by the parent structure unit of Schiff base,
rather than the substituents in the molecule. For those compounds with the
same parent structure unit, their red shift values of vmax are at fixed value, but
their red shift values of λmax are unequal. Generally, the red shift value caused
by the intramolecular hydrogen bond is larger for the compound with a larger
λmax value. The above observed phenomena are discussed from the view of
molecular coplanarity.
Correspondence
Chenzhong Cao, Key Laboratory of
Theoretical Organic Chemistry and
Function Molecule, Ministry of
Education, School of Chemistry and
Chemical Engineering, Hunan University
of Science and Technology, Xiangtan
411201, Hunan Province, China.
Email: chzcao@163.com
Funding information
National Natural Science Foundation of
China, Grant/Award Numbers: 21672058,
2020JJ5155; Hunan Natural Science
Foundation
K E Y W O R D S
bi-aryl Schiff bases, intramolecular hydrogen bond, proton transfer, substituent effect, ultraviolet
absorption spectra
1 | INTRODUCTION
and the imine moiety linking the other aromatic ring,
which contributes to the photoelectric properties of the
Schiff bases, with their classical π-conjugated system,
showed good ultraviolet (UV) absorption, and this photo-
electric characteristic property has been used in many
applications. The theoretical and experimental study on
Schiff base molecular structure and the change regularity
of its optical property is of great significance in the devel-
opment of photoelectric functional materials.[1–6] The
studied Schiff bases exhibit an intramolecular hydrogen
bond between the OH group of the six-membered ring
bases. This phenomenon has widely attracted the
attention of researchers.[7–13]
There is an enol–keto tautomeric equilibrium in
solution for Schiff bases containing N-salicylideneaniline
(NSA) structural unit,[7–9] as shown in Figure 1. The
transformation of tautomer is realized by intramolecular
hydrogen bond proton transfer, which has been observed
in crystal structure, as shown in Figure 2.[10] This kind of
intramolecular hydrogen bond in Schiff base can
J Phys Org Chem. 2020;e4164.
wileyonlinelibrary.com/journal/poc
© 2020 John Wiley & Sons, Ltd.
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https://doi.org/10.1002/poc.4164

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CAO ET AL.
νmax and contributes a red shift; maybe, the 2-OH, com-
pared with 20-OH, is easier to form a intramolecular
hydrogen bond with the N atom of the bridging bond
C=N; as a result, the intramolecular hydrogen bond con-
tributes a red shift to the νmax of the compounds. It
implies that the UV absorption of aryl Schiff bases with
2-OH has a special performance. Therefore, we initiated
the study of bi-aryl Schiff bases to figure out the influ-
ence of intramolecular hydrogen bond on the νmax of
them and the how the substituents attached to the aryl
groups impact the vmax. This study will contribute
molecular design of optoelectronic materials with NAS
structure.
FIGURE 1 Tautomerization of N-salicylideneaniline between
the enol–imine and keto–enamine tautomers
significantly transform the photoelectric properties of
Schiff base via the proton transfer, such as, the UV
absorptions and photoluminescences,[11–13] the fluores-
cent emissions in water induced the excited state intra-
molecular proton transfer (ESIPT),[14] solid-state
photochromisms and thermochromisms,[15] and pH-
responsive fluorescent sensors[16] and fluorescent probes
In order to explore this topic, we select two
kinds of Schiff bases N-(benzylidene)-anilines (ZBAY)
and N-(phenyl-ethylene) aniline ZArC (Me) = NArY
(ZAPEY) as model compounds to examine the effect of
of reaction[17]
.
intramolecular hydrogen bonds on their νmax.
As we know, both photoluminescence and photoelec-
tric conversion are closely related to the light absorption
properties of the compounds. Meanwhile, the light
absorption property of compounds is intimately con-
nected to their molecular structures. In previous studies
of our research team,[18–25] the effect of substituents on
UV absorption spectra of many compounds were ana-
lyzed in depth, in which these compounds involved
stilbene (XSBY), benzylideneaniline (XBAY), and phe-
nylethylaniline (XPEAY). The obtained results show that
the effect of substituent on the wave number vmax
(cm−1) of the longest wavelength maximum λmax (nm) of
UV has different modes due to different molecular skele-
tons. Recently, Cao et al.[22] explored the effect of
hydroxyl groups, attached to different locations in aryl
Schiff bases XArCH=NArY (XBAY), on the vmax of the
same series of compounds, and observed that hydroxyl
groups at different locations have additional effect on
the vmax. For example, 2-position hydroxyl (2-OH), com-
pared with 20-position hydroxyl (20-OH), declines the
2 | EXPERIMENTAL SECTION
2.1 | Synthesis of model compounds
The model compounds, substituted-N-(phenyl-ethylene)
aniline ZArC (Me)=NArY (ZPEAY) were divided into
two groups. The Group I is 2-H-XArC (Me)=NArY
(Z=2-H, X; abbreviated as 2-H-XPEAY) without ortho-
hydroxyl (2-OH), as shown in Figure 3. And the Group II
is 2-OH-XArC (Me)=NArY (Z=2-OH, X; abbreviated as
2-OH-XPEAY) containing 2-OH. In this paper, the Group
II was synthesized by using the method of the Cao
et al.[26,27] as shown in Figure 4. The crude products were
purified via column chromatography, and all the model
compounds were characterized with nuclear magnetic
1
resonance (NMR) spectra. Their H NMR and 13C NMR
data and spectra can be seen in the supporting
information.
FIGURE 2 Tautomerization of
Naphthalenediimines between the enol–imine
and keto–enamine tautomers (up), and the
crystal structure of keto–enamine tautomer
(down) reported by Kukułka[10]

CAO ET AL.
3 of 9
vmax = 31,142−2,439:43σðXÞ + 2,464:05σðYÞ−308:60Δσ2 ð1Þ
2.2 | Data preparation
X
2
ex
CC
ex
CC
+ 1,090:09
σ
+ 288:99Δσ
The UV absorption data of Group I (2-H-XPEAY) were
taken from Luo et al.[23] (Table 1, Nos. 1–48). In this work,
the model compound solutions with concentration
7.27 × 10−3 mol L−1 were prepared by dissolving the Group
II (2-OH-XPEAY) of 7.27 × 10−5 mol in 10-ml anhydrous
ethanol. And then the model compound solution of 5 μl
was mixed with anhydrous ethanol in a sample cell (kept
total volume 3 ml), and its UV absorption spectrum was
tested with a UV-1800 (Shimadzu, Japan) spectrometer,
scanning range 200–500 nm, and scanning speed
10 nm/s. The Group II UV spectra were recorded, and their
λmax were collected. The spectrum of each target com-
pound was tested for three times (the absorption spectra
can be seen in the supporting information), then the aver-
age value of the three λmax was employed in this work. The
λmax values and the νmax values (νmax = 1 × 107/λmax) of all
target compounds were listed in Table 1 (Nos. 49–83).
R = 0:9880,S = 317:96,F = 344:86,n = 48
In which, σ(X) and σ(Y) are the ground-state elec-
tronic effect constants (Hammett constant) of X and Y
substituent respectively. Δσ2 = [σ(X) − σ(Y)]2, expressing
the substituent specific cross-interaction effect between X
P
ex
ex
and Y.
σeCxC = σ ðXÞ + σ ðYÞ, expressing the sum of
CC
CC
ex
CC
excited-state substituent constants of X and Y. Δσ
= [σ (X) − σ (Y)] , expressing the substituent specific
2
2
ex
CC
ex
CC
cross-interaction effect with excited-state substituent con-
stant between X and Y. Adopting the same parameters of
Equation 1, we carried out a regression analysis for the
vmax values of 83 samples of compounds in Table 1
(Groups
I
=
and II). Here, we took the items
σ(X)
σ(2-OH) and Δσ2(X,OH,
P
σ(X)
+
Y) = [σ(X) + σ(2-OH) − σ(Y)]2 to replace the items
σ(X) and Δσ2 in Equation 1, due to the aromatic alde-
hydes of 2-OH-XPEAY compounds (Group II) containing
2-OH and X groups, and then obtained Equation 2 with
95% confidence levels.
3 | DATA ANALYSIS AND
RESULTS DISCUSSION
vmax = 30,806 – 1,462:3XσðXÞ + 2,142:29σðYÞ
2
ex
ex
CC
+ 455:43Xσ −1,420:19Δσ2ðX,OH,YÞ + 23:14Δσ
ð2Þ
3.1 | Data analysis
CC
R = 0:9437,S = 550:17,F = 125:22,n = 83
3.1.1 | Influence of intramolecular
hydrogen bond on the vmax of ZPEAY
The correlation of Equation 2 is not good. May be, it
is that (a) there is intramolecular hydrogen bond effect
between the 2-OH group of the six-membered ring and
the imine moiety linking the other aromatic ring in the
compounds of Group II; (b) the 2-OH group may influ-
ences the vmax of compounds of Group II via its inter-
acting with the Y group by way of the intramolecular
hydrogen bond. And these two factors are not considered
in Equation 2. To express the effect of above two factors
on the vmax, we dealt with them as following. (a) An indi-
cator variable I2-OH of 2-OH was employed to express the
contribution of the intramolecular hydrogen bond to the
vmax. Here, if the molecule contains 2-OH (Group II),
I2-OH = 1, otherwise (Group I), I2-OH = 0. (b) The item
Δσ2(X,OH,Y) in Equation 2 was further divided into two
terms, one expressing the interaction between X and Y,
namely, Δσ2 = [σ(X) − σ(Y)]2; and the other expressing
the interaction between 2-OH and Y, that is,
Luo et al.[23] used a five-parameters equation to quantify
the vmax of Table 1 (Nos. 1–48) for 2-H-XPEAY (Figure 3)
and obtained Equation 1 with 95% confidence levels.
FIGURE 3 The molecular structure of model compounds
2-H-XPEAY (Group I). Group I: Nos. 1–48; X (m/p)=OMe, Me, H,
Cl, F, CF3, NO2; Y(m/p)=NMe2, OMe, Me, H, Cl, F, CN
FIGURE 4 The synthetic route of model
compounds 2-OH-XPEAY (Group II). Group II:
Nos. 49–83; X(p)=H, 4-OMe; Y(o/m/p)=NMe2,
OMe, Me, H, Br, Cl, F, CF3, NO2, CN, OH

4 of 9
CAO ET AL.
TABLE 1 The UV wavelength of absorption maximum λmax (nm) and its wave number νmax (cm−1) for ZPEAY and substituent
parameters
b
b
c
c
σ(2-OH)a
σ(X)a
σ(Y)a
σ
ðXÞ
σ
ðYÞ
λmax. exp
νmax, exp.
ex
CC
ex
CC
No.
X
Y
Group I (Z=2-H, X)
1
4-OMe
4-OMe
4-OMe
4-OMe
4-OMe
4-Me
4-Me
4-Me
4-Me
H
40-NMe2
40-OMe
40-Me
40-Cl
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
−0.27
−0.27
−0.27
−0.27
−0.27
−0.17
−0.17
−0.17
−0.17
0
−0.83
−0.27
−0.17
0.23
−0.50
−0.50
−0.50
−0.50
−0.50
−0.17
−0.17
−0.17
−0.17
0
−1.81
−0.50
−0.17
−0.22
0.06
356.0
330.0
320.0
316.0
316.0
359.5
337.7
321.1
313.7
330.0
324.7
323.9
373.9
342.2
332.6
328.0
327.0
315.1
361.7
337.7
329.1
322.0
322.8
320.8
313.5
385.9
351.5
340.8
335.0
335.7
336.3
432.8
381.0
363.6
351.5
354.3
353.4
349.0
334.1
329.5
330.9
323.8
28,090
30,303
31,250
31,646
31,646
27,816
29,612
31,148
31,875
30,303
30,802
30,875
26,746
29,223
30,066
30,492
30,581
31,740
27,651
29,616
30,391
31,056
30,984
31,170
31,896
25,911
28,453
29,340
29,850
29,793
29,733
23,108
26,250
27,503
28,454
28,225
28,301
28,653
29,931
30,349
30,221
30,888
(Continues)
2
3
4
5
40-F
0.06
6
40-NMe2
40-OMe
40-F
−0.83
−0.27
0.06
−1.81
−0.50
0.06
7
8
9
40-CN
40-OMe
H
0.66
−0.70
−0.50
0
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
−0.27
0
H
0
0
H
40-F
0
0.06
0
0.06
4-Cl
40-NMe2
40-OMe
40-Me
40-cl
0.23
0.23
0.23
0.23
0.23
0.23
0.06
0.06
0.06
0.06
0.06
0.06
0.06
0.54
0.54
0.54
0.54
0.54
0.54
0.78
0.78
0.78
0.78
0.78
0.78
0.37
0.37
0.37
0.23
0.06
−0.83
−0.27
−0.17
0.23
−0.22
−0.22
−0.22
−0.22
−0.22
−0.22
0.06
−1.81
−0.50
−0.17
−0.22
0.06
4-Cl
4-Cl
4-Cl
4-Cl
40-F
0.06
4-Cl
40-CN
40-NMe2
40-OMe
40-Me
H
0.66
−0.70
−1.81
−0.50
−0.17
0
4-F
−0.83
−0.27
−0.17
0
4-F
0.06
4-F
0.06
4-F
0.06
4-F
40-Cl
0.23
0.06
−0.22
0.06
4-F
40-F
0.06
0.06
4-F
40-CN
40-NMe2
40-OMe
40-Me
H
0.66
0.06
−0.70
−1.81
−0.50
−0.17
0
4-CF3
4-CF3
4-CF3
4-CF3
4-CF3
4-CF3
4-NO2
4-NO2
4-NO2
4-NO2
4-NO2
4-NO2
3-Cl
−0.83
−0.27
−0.17
0
−0.12
−0.12
−0.12
−0.12
−0.12
−0.12
−1.17
−1.17
−1.17
−1.17
−1.17
−1.17
0.02
40-Cl
0.23
−0.22
0.06
40-F
0.06
40-NMe2
40-OMe
40-Me
H
−0.83
−0.27
−0.17
0
−1.81
−0.50
−0.17
0
40-Cl
0.23
−0.22
0.06
40-F
0.06
40-OMe
40-Me
40-F
−0.27
−0.17
0.06
−0.50
−0.17
0.06
3-Cl
0.02
3-Cl
0.02
4-Cl
30-Me
30-Me
−0.07
−0.07
−0.22
0.06
−0.03
−0.03
4-F

CAO ET AL.
5 of 9
TABLE 1 (Continued)
b
b
c
c
ex
ex
No.
43
44
45
46
47
48
X
Y
σ(2-OH)a
σ(X)a
0.78
0.23
0.06
0.78
0
σ(Y)a
−0.07
0.34
σ
ðXÞ
σ
ðYÞ
λmax. exp
355.0
321.0
317.6
344.3
301.5
295.3
νmax, exp.
CC
CC
4-NO2
4-Cl
4-F
30-Me
30-F
0
0
0
0
0
0
−1.17
−0.22
0.06
−1.17
0
−0.03
0.02
0.02
0.02
0.56
0.56
28,173
31,158
31,485
29,049
33,173
33,870
30-F
0.34
4-NO2
H
30-F
0.34
30-CN
30-CN
0.56
4-F
0.06
0.56
0.06
Group II (Z=2-OH,X)
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
H
40-NMe2
40-OMe
40-Me
H
−0.38
−0.38
−0.38
−0.38
−0.38
−0.38
−0.38
−0.38
−0.38
−0.38
−0.38
−0.38
−0.38
−0.38
−0.38
−0.38
−0.38
−0.38
−0.38
−0.38
−0.38
−0.38
−0.38
−0.38
−0.38
−0.38
−0.38
−0.38
−0.38
−0.38
−0.38
−0.38
−0.38
−0.38
−0.38
0
−0.83
−0.27
−0.17
0
−0.10
−0.10
−0.10
−0.10
−0.10
−0.10
−0.10
−0.10
−0.10
−0.10
−0.10
−0.10
−0.10
−0.10
−0.10
−0.10
−0.10
−0.10
−0.60
−0.60
−0.60
−0.60
−0.60
−0.60
−0.60
−0.60
−0.60
−0.60
−0.60
−0.60
−0.60
−0.60
−0.60
−0.60
−0.60
−1.81
−0.50
−0.17
0
364.6
332.2
326.6
324.4
327.4
326.2
332.0
327.2
325.6
333.8
326.0
325.2
325.4
326.4
325.6
325.4
326.8
329.8
323.8
318.6
317.8
321.2
321.6
318.4
329.6
322.0
324.2
331.0
317.0
321.2
321.4
320.0
321.2
323.0
321.8
27,427
30,102
30,618
30,826
30,544
30,656
30,120
30,562
30,713
29,958
30,675
30,750
30,731
30,637
30,713
30,731
30,600
30,321
30,883
31,387
31,466
31,133
31,095
31,407
30,340
31,056
30,845
30,211
31,546
31,133
31,114
31,250
31,133
30,960
31,075
H
0
H
0
H
0
H
40-Br
0
0.23
0.23
0.06
0.54
0.66
−0.37
−0.07
0.39
0.37
0.34
0.43
0.71
0.56
−0.38
−0.27
−0.17
0
−0.33
−0.22
0.06
H
40-Cl
0
H
40-F
0
H
40-CF3
40-CN
40-OH
30-Me
30-Br
0
−0.12
−0.70
−0.19
−0.03
−0.03
0.02
H
0
H
0
H
0
H
0
H
30-Cl
0
H
30-F
0
0.02
H
30-CF3
30-NO2
30-CN
20-OH
40-OMe
40-Me
H
0
0.09
H
0
0.66
H
0
0.56
H
0
−0.10
−0.50
−0.17
0
4-OMe
4-OMe
4-OMe
4-OMe
4-OMe
4-OMe
4-OMe
4-OMe
4-OMe
4-OMe
4-OMe
4-OMe
4-OMe
4-OMe
4-OMe
4-OMe
4-OMe
−0.27
−0.27
−0.27
−0.27
−0.27
−0.27
−0.27
−0.27
−0.27
−0.27
−0.27
−0.27
−0.27
−0.27
−0.27
−0.27
−0.27
40-Br
0.23
0.23
0.06
0.78
0.54
0.66
−0.37
−0.07
0.39
0.37
0.34
0.43
0.71
0.56
−0.33
−0.22
0.06
40-Cl
40-F
40-NO2
40-CF3
40-CN
40-OH
30-Me
30-Br
−1.17
−0.12
−0.70
−0.19
−0.03
−0.03
0.02
30-Cl
30-F
0.02
30-CF3
30-NO2
30-CN
0.09
0.66
0.56
aThe values were taken from other studies.[19–21]
bThe values were taken from other studies.[28–31]
cvmax = 1 × 107/λmax. The values of Nos. 1–48 were from Luo et al.,[23] and those of Nos. 49–83 were determined by this work.

6 of 9
CAO ET AL.
Δσ2(OH,Y) = [σ(2-OH) − σ(Y)]2. The above three items
intramolecular hydrogen bond (I2-OH) has a fixed contri-
bution to the vmax, which reduces the vmax (about
850 cm−1) and makes the λmax a red shift.
I2-OH
,
Δσ2 and Δσ2(OH,Y) were introduced into
Equation 2, removing the item Δσ2(X,OH,Y), and a
regression analysis was carried out once again, then the
Equation 3 with 95% confidence levels was obtained.
3.1.2 | Influence of intramolecular
hydrogen bond on the vmax of ZBAY
X
vmax = 31,058 – 2,613:13
σðXÞ + 2,611:31σðYÞ ð3Þ
X
+ 101:26Δσex 2 −456:02Δσ2
ex
+ 660:77
σ
CC
CC
We notice that the molecular skeleton of compounds
ZArCH=NArY (ZBAY) are very similar to that of
ZPEAY. And the bridging bond is CH=N for the for-
mer and C (CH3)=N for the latter. Chen et al.[24] and
Cao et al.[22] investigated the vmax of 72 samples of
XArCH=NArY compounds (abbreviated as 2-H-XBAY)
and 13 samples of 2-OH-XArCH=NArY compounds
(abbreviated as 2-OH-XBAY) respectively, and obtained
their quantitative equations. However, the topic of
“whether the influence of intramolecular hydrogen
bond on the vmax of ZBAY and ZPEAY has difference”
has not been studied, up to now. Based on the method
of dealing with the compounds ZPEAY in Section 3.1.1,
the vmax values of 85 samples of ZBAY compounds
reported by Cao et al.[21] and Yuan and Cao[19] were
regressed by adopting the same parameters of Equa-
tion 3, and then Equation 9 with 95% confidence
levels was obtained.
– 1,956:30Δσ2ðOH,YÞ−850:17I2−OH
R = 0:9774,S = 355:97,F = 229:22,n = 83
The correlation of Equation 3 is much improved than
that of Equation 2 and the value of F-stat of Equation 3
increases significantly, and the standard deviation of
Equation 3 decreases significantly. It shows that the
above treatment methods are reasonable. We want to
know whether the five substituent parameters in Table 1
are all necessary. Thus, we carried out regression analysis
with less five parameters for the vmax of Group II and got
the results as shown in Table 2. The regression equations
in Table 2 indicate that all models based on less the five
parameters are weaker. Therefore, Equation
recommended to express the variation of vmax
3 is
.
It can also be seen from Equation 3 that the influence
of Δσ2 and Δσ2(OH,Y) on the vmax are unequal. The
TABLE 2 The regression equations with different substituent parameters
Equation no.
Regression equations
Parameters
σ(2-OH), σ(Y), σ ðXÞ, σ ðYÞ, I2-OH
ex
CC
ex
CC
(4)
vmax = 30,513 + 2,511.02σ
P
ex
ex
CC
(Y) + 1,104.42
σ
− 188.92Δσ
[delete σ(X)]
CC
2 + 1,443.62Δσ2(Y) – 2,487.66Δσ2(OH,
Y) + 1,215.21I2-OH R = 0.8629,
S = 845.61, F = 36.94, n = 83
ex
(5)
(6)
(7)
vmax = 31,170 − 2,263.52σ(X) + 1,722.02σ
σ(X), σ(Y), σ ðXÞ
CC
CC
P
ex
ex
ex
(Y) + 574.61
2 – 1,086.32Δσ2 − 497.66I2-OH R = 0.9580,
σ
− 34.79Δσ
σ
ðYÞ, I2-OH [delete σ(2-OH)]
CC
CC
S = 480.12, F = 141.21, n = 83
P
ex
CC
ex
CC
vmax = 31,419 – 2,433.54
σ
σ(X), σ(2-OH), σ ðXÞ, σ ðYÞ, I2-OH
[delete σ(Y)]
P
ex
ex
CC
(X) + 1,155.30
σ
− 238.02Δσ
CC
2 − 560.34Δσ2 – 1,162.69I2-OH R = 0.8959,
S = 738.50, F = 62.64, n = 83
P
ex
CC
vmax = 30,975 – 2,621.96 σ(X) + 2,624.55σ
σ(X), σ(2-OH), σ(Y), σ ðYÞ, I2-OH [delete
ex
ex2
ex
CC
(Y) + 686.03σ ðYÞ + 191.10σ ðYÞ
σ
ðXÞ ]
CC
CC
− 862.86Δσ2 – 1,798.35Δσ2(OH,
Y) − 967.58I2-OH R = 0.9699, S = 410.30,
F = 169.88, n = 83
P
ex
CC
(8)
vmax = 30,885 – 2,192.4 σ(X) + 3,060.98σ
σ(X), σ(2-OH), σ(Y), σ ðXÞ, I2-OH [delete
ex
ex2
ex
(Y) + 82.09σ ðXÞ − 459.52σ ðXÞ
σ
ðYÞ ]
CC
CC
CC
− 838.86Δσ2 – 1,980.16Δσ2(OH,
Y) − 603.51I2-OH R = 0.9681, S = 421.83,
F = 160.13, n = 83

CAO ET AL.
7 of 9
X
HBACN-40 (Cambridge Crystallographic Data Centre
(CCDC)No. 2031443) and 2-OH-HPEACN-40 (CCDC
No. 1868713) in crystals, respectively. Their checkCIF
files were reported in the supporting information. It can
be seen, from Figure 6, that the two benzene rings in
2-OH-HBACN-40 are almost coplanar and the two ben-
zene rings in 2-OH-HPEACN-40 are almost perpendicular
to each other. Therefore, from which, we can infer that
the coplanarity of molecule 2-OH-XBAY may be better
than that of molecule 2-OH-XPEAY in solvent, which is
more conducive to the proton transfer via intramolecular
hydrogen bonds at a lower energy state.
vmax = 32,024−658:94
σðXÞ + 1,174:58σðYÞ
ð9Þ
X
+ 190:92Δσex 2 – 1,101:2Δσ2
ex
CC
+ 1,527:54
σ
CC
−393:58Δσ2ðOH,YÞ−2,381:78I2−OH
R = 0:9807,S = 426:76,F = 276:03,n = 85
Equation 9 has good correlation, and it shows that
the effect of item Δσ2 on the vmax is different from that of
Δσ2(OH,Y) on the vmax, and the intramolecular hydrogen
bond (I2-OH) has a fixed contribution to the vmax, namely,
which decreases the vmax (about 2,381 cm−1), and makes
the λmax a red shift.
It should be pointed out that the intramolecular
hydrogen bond has a fixed contribution to the vmax of
ZBAY and ZPEAY, respectively, that is, reducing the vmax
3.2 | Result discussion
of ZBAY 2,381.78 cm−1 and that of ZPEAY 850.17 cm−1
.
When comparing Equation 3 with Equation 9, it can be
seen that the influence factors of substituents on the vmax
are by largely the same for the two kinds of Schiff bases
ZBAY and ZPEAY. Because the bridging bond of ZBAY
is different from that of ZPEAY, the influence intensities
of variables on the vmax are different for these in
Equations 3 and 9. The change regularities of the vmax of
ZPEAY and ZBAY can be well expressed by Equations 3
and 9 respectively. Figure 5 is the plot of the calculated
vmax values versus the experimental values.
However, it does not mean that the λmax of the com-
pounds can move a fixed value. Because the vmax = 1 × 107/
λmax, the red shift value of the λmax caused by intramolec-
ular hydrogen bond is related to the λmax of the
corresponding molecule. That is,
1 × 107 1 × 107
aI2−OH
=
−
ð10Þ
λmax
λ0max
It should be noted that the coefficient in front of the
variable I2-OH in Equation 3 is −850.17, while the coeffi-
cient in front of I2-OH in Equation 9 is −2,318.79, the
value of the latter is more than twice of the former. It
indicates that the effect of intramolecular hydrogen bond
on the vmax is only dominated by their parent molecular
structure, rather than the substituent in the molecule. As
to why the effect of intramolecular hydrogen bond on the
vmax is much greater in ZBAY than in ZPEAY, it is per-
haps caused by the difference of their molecular copla-
narity. We obtained crystals of some compounds of
2-OH-XBAY and 2-OH-XPEAY and tested the crystals.
Figure 6 shows the molecular configurations of 2-OH-
In Equation 10, the “a” is the coefficient in front of
the variable I2-OH in Equation 3 or Equation 9, the λmax is
the measured value of the compound, and the λ0max is the
theoretical value of the corresponding compound without
intramolecular hydrogen bond. From Equation 10 and
the value of I2-OH = 1, we can deduce Equation 11.
1 × 107λmax
λ0max
=
ð11Þ
1 × 107 −aλmax
By employing Equation 11, we can calculate the λ'max
and further calculate the red shift value Δλ of λmax
,
,
caused by the intramolecular hydrogen bond. That
is, Δλ = λmax-λ'max. Figure 7 shows the relationship
between the red shift values Δλ and the λmax of the
FIGURE 5 Plot of the calculated vmax, cal. with Equations 3
and 9 versus the experimental vmax, exp. (the symbols “o” and “Δ”
represent the ZPEAY and ZBAY, respectively)
FIGURE 6 The molecular configurations of 2-OH-HBACN-40
(left) and 2-OH-HPEACN-40 (right) in crystals

8 of 9
CAO ET AL.
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On the basis of the above research results, we came to
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vmax of ZPEAY and ZBAY are essentially the same, but
their intensities of factors affecting the vmax are different.
(b) In the compounds 2-OH-XPEAY and 2-OH-XBAY,
the effect of the cross-interaction between 2-OH and Y,
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ACKNOWLEDGEMENTS
The project was supported by the National Natural
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ORCID
Chao-Tun Cao https://orcid.org/0000-0002-3390-7587
Chenzhong Cao https://orcid.org/0000-0001-5224-7716
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How to cite this article: Cao C-T, Li L, Cao C,
Liu J. The effect of intramolecular hydrogen bond
on the ultraviolet absorption of bi-aryl Schiff bases.
J Phys Org Chem. 2020;e4164. https://doi.org/10.
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SUPPORTING INFORMATION
Additional supporting information may be found online
in the Supporting Information section at the end of this
article.