The 1H- and 13C-spectra of some azomethines of 5-amino-2-phenylindole series have been recorded and researched. It shown that, the substituents in benzene ring influenced on the chemical shift of proton on azomethine bond

Duc, Hoang Thanh

Nam, Pham Duy

Tai, Dang Nhu

Thanh, Nguyen Dinh

Vietnam Academy of Science and Technology: Journals Online

642
Journal of Chemistry, Vol. 45 (5), P. 642 - 647, 2007 
1H- AND 13C-NMR SPECTRA OF SOME AZOMETHINES OF  
5-AMINO-2-PHENYLINDOLE SERIES 
Received 27 February 2007 
Dang Nhu Tai, Nguyen Dinh Thanh, Pham Duy Nam, Hoang Thanh Duc 
Faculty of Chemistry, Hanoi University of Science, VNU 
 
summary 
The 1H- and 13C-spectra of some azomethines of 5-amino-2-phenylindole series have been 
recorded and researched. It shown that, the substituents in benzene ring influenced on the 
chemical shift of proton on azomethine bond. 
 
I - Introduction 
Some azomethines of 5-amino-2-
phenylindole have been synthesized from this 
amine and some different substituted benzal-
dehydes in our lab [1]. Their properties, as the 
metallic corrosion inhibition capacity on CT-3 
steel in 2 M HCl medium, were investigated [2]. 
In some previous papers, we indicated that the 
azomethines containing indole ring had some 
interesting spectral properties [3 - 5]. In order to 
continuing these studies, the proton and carbon-
13 NMR spectroscopies of these azomethine 
series have been recorded and discussed.  
II - Experimental 
Azomethines of 5-amino-2-phenylindole 
series what were used in this research have been 
synthesized in according to the method 
described [1]. Their structures have the 
following formulas: 
 
8
9
7
4
6
53
2
N
H
1"
N Ar
H
6"
5"
4"
3"
2" Ar= 5'
4'
6'
3'
1'
2'
5'
O
4'
2'
3'
R or
(I-IX) (I-VIII)
(IX)
1
where, R were m-NO2(I); p-Cl(II); H(III); 3',4'-O2CH2(IV); p-OCH3(V); 
p-OH(VI); p-N(CH3)2(VII); o-OH(VIII). 
 
The 1H and 13C-NMR spectra were recorded 
in AVANCE Spectrometer (BRUKER, German) 
at 500 MHz, using DMSO-d6 as solvent, using 
TMS as the internal reference.  
III - Results and Discussion 
All signals in 1H-NMR spectra of 
benzylidene-5-amino-2-phenylindoles were 
represented in table 1. It shown that total of 
protons in the azomethine molecules was suitable 
to molecular formulas. The sharp resonance 
signal with high intensity that appears in region 
643
of 11.500 ppm to 11.683 ppm belongs to NH-
proton in the indole ring. This signal can be 
easily distinguished from other ones (see figure 
1). The resonance signal of proton in azomethine 
bond (>CH=N–) is the singlet peak and has 
medium intensity. This signal appears in region 
of 8.496 ppm to 9.028 ppm. Both these signals 
NH and >CH=N– are always in the low field.
 
Table 1: Chemical Shifts in 1H-NMR spectra of azomethines of 5-amino-2-phenylindole series  
(in DMSO) 
Chemical Shifts,  (ppm) (J, Hz) 
Substituents
m-NO2 p-Cl H 
3',4'-
O2CH2
p-OCH3 p-OH 
p-
N(CH3)2
o-OH furfuryl
Protons 
(multip.) 
(I) (II) (III) (IV) (V) (VI) (VII) (VIII) (IX) 
N-H (s) 11.652 11.608 11.590 11.558 11.549 11.528 11.500 11.683 11.583
H3 6.955(s) 6.931 (br) 
6.934(d)
J =1 
6.913 
(br) 
6.913 
(br) 
6.901 
(br) 
6.889(d)
J=1 
6.959 
(br) 
6.921(d)
J=1 
H4 7.588(s) 7.510 (br) 
7.509(d)
J=2 
7.507 
(br) 
7.433 
(br) 
7.396(s) 7.372(d)
J = 1.5
7.631 
(br) 
7.906 
(br) 
H6 (dd) 
7.266 
J1=9 
J2=1.5
7.199 
J1=8.5 
J2=1 
7.189 
J1=8.5 
J2=1.5 
7.136 
J1=8.5 
J2=1.5
7.136 
J1=8.5 
J2=2 
7.106 
J1=8.5 
J2=1.5 
7.090 
J1=8.5 
J2=2 
7.281 
J1=8.5 
J2=1.5 
7.141 
J1=8.5 
J2=1.5
H7 (d) 
7.463 
J=9 
7.439 
J=9 
7.439 
J=8.5 
7.405÷
7.435 
7.415 
J=8.5 
7.404 
J=7.5 
7.391 
J=8.5 
7.477 
J=8.5 
7.415 
J=8.5 
Hazomethine (s) 8.755 8.737 8.726 8.600 8.624 8.552 8.496 9.028 8.529 
H2' 
8.917 (s) 7.966(d)
J=8.5 
7.955 
(dd) J1 =
7.5; J2=2
7.405÷
7.435 
7.894 
(d) 
(J=8.5)
7.783(d)
J=8.5 
7.754(d)
J=8.5 
– –
H6' 
8.328 
(dd) J1=
8.5; J2=
1.5 
7.966(d)
J =8.5 
 
7.955 
(dd) 
J1=7.5 
J2=2 
7.405÷
7.435 
7.894 
(d) 
J=8.5 
7.783(d)
J=8.5 
7.754(d)
J=8.5 
7,639 (d)
J=7.5 
–
H3' – 7.575(d)J=8 
7.511(t) 
J=5.5 
– 7.067
(d) J=8.5
6.887(d)
J=9 
6.788(d)
J=9 
6.967(d) 
J=6.5 
7.089(d)
J=3.5 
H5' 
7.806(t)
J=8 
7.575(d)
J=8 
7.511(t) 
J=5.5 
7.048 (d)
J= ??.5
7.067 
(d) J= 
8.5 
6.887(d)
J=9 
6.788(d)
J=9 
6.972(t) 
J=??  
7.458 
J=7 
H4' 
8.370(d)
J= 7.5
– 7.507(t) 
J=6 
– – – – 7.382
J=7.5 
6.694 
(dd) J1 =
3.5; J2
=2 
H2”, H6”  
(d) 
7.869 
J=7.5 
7.871 
J=7.5 
7.874 
J=7.5 
7.864 
J=7.5 
7.866 
J=7.5 
7.862 
J=7.5 
7.860 
J=7.5 
7.881 
J=7.5 
7.865 
J=7.5 
H3”, H5” (t) 7.472 
J=7.5 
7.468 
J=8 
7.471 
J=7.5 
7.465 
J=7.5 
7.466 
J=7.5 
7.463 
J=7.5 
7.461 
J=7.5 
7.476 
J=7.5 
7.465 
J=7.5 
H4” (t) 7.333 
J=7.5 
7.326 
J=7.5 
7.327 
J=7.5 
7.321 
J=7.5 
7.321 
J=7.5 
7.317 
J=7.5 
7.315 
J=7.5 
7.337 
J=7.5 
7.324 
J=7.5 
Other H  CH2:
6.117(s)
CH3:
3.836(s)
OH: 
9.987(s)
CH3:
3.002(s)
OH: 
13.674(s)
(s: singlet; br: broad; d: doublet; dd: double doublet and t: triplet). 
644
The chemical shifts of proton in the 
substituted benzene nucleus in the position 2 of 
indole ring (H2”, H3”, H4”, H5” and H6”) are 
trivially affected by the present of different 
substituents. For example, protons H2” and H6” 
have doublet signals in the narrow region from 
7.860 to 7.881 ppm; protons H3” and H5” have 
triplet signals with chemical shift H = 7,461 ÷
7,476 ppm; and the resonance signal of proton 
H4” is a triplet one in region from 7.315 to 
7.337 ppm. It shown that, the resonance signal 
of proton H3 is easily recognized as a singlet 
one in the high field at 6.889 ÷ 6.959 ppm. 
Protons H6 have double doublet signals and H7 
have doublet signals, with (H6) < (H7), in 
region of 7.090 ÷7.477 ppm; sometimes, these 
signals overlap each other and raise difficulties 
in distinguishing them. 
We recognized that there is an order of 
chemical shifts of substituted benzylidene-5-
amino-2-phenylindoles (I-VIII) as following: p-
N(CH3)2<p-OH<p-OCH3<3',4'-O2CH2<H<p-
Cl<m-NO2. The linear regression between 
chemical shift H and Hammett’s  constants in 
azomethines of 5-amino-2-phenylindole series is 
represented in figure 2. 
Figure 1: 1H-NMR spectrum of benzylidene-5-
amino-2-phenylindole 
This regression can be expressed by 
following equation: X = 0.1896 + 8.6596 (n = 
7, R2 = 0.8377, F = 25.81). 
This equation with the slope  = 0.1896 > 0 
indicated that the electron-donating substituents 
(with  < 0) cause the upfield shift of Hazomethine 
and the electron-withdrawing substituents (with 
 > 0) cause the downfield shift of this chemical 
shift values. However, this influences are 
insignificant which are expressed the small 
value of , so it could indicated the dependence 
of chemical shift of these carbon atoms on their 
density charge. 
 
()= 0.1896+ 8.6596
R2 = 0.8377
8.4
8.5
8.6
8.7
8.8
8.9
-1 -0.75 -0.5 -0.25 0 0.25 0.5 0.75 1
Hammett's 
 (H)
m-NO2p-Cl
H
p-OCH3
p-OH
p-N(CH3)2
3,4-O2CH2
(A) 
(C1') = 7.3041 + 132.0632
n=7; R2 = 0.6726; F=10.271
(Cazomethine) = -2.5233 + 157.3968
n=7; R2 = 0.292; F=2.057
120
130
140
150
160
170
-1 -0.75 -0.5 -0.25 0 0.25 0.5 0.75 1
Cazomethine
C1'
Hammett's 
(C)
 
(B) 
Figure 2: Linear correlation regression between chemical shift H (A) and C (B) with  
the Hammett’s  constants in azomethines of 5-amino-2-phenylindole series 
 
The 13C-NMR spectra of these azomethines 
have been recorded. The chemical shifts on 
carbon atoms in these azomethines are 
represented in table 2.  
From table 2, it has indicated that the 
resonance signal of carbon atom on the 
azomethine bond was always in the low field at 
152.460 ÷ 161.382 ppm. This signal can be 
645
distinguished from another one of other carbon 
atoms. In case of the benzene nucleus in 
position 2 of indole ring, the resonance signals 
of pairs of carbon atoms C2” and C6”, C3” and 
C5” have the more intensive ones and are in 
regions of 124.894 ÷ 125.044 ppm and 
128.686÷128.898 ppm, respectively. The carbon 
atoms C1” and C4”, in turn, have the resonance 
signals in regions of 130.360 ÷ 132.150 ppm 
and 127.408 ÷127.624 ppm, respectively. 
Table 2: Chemical Shifts in 13C-NMR spectra of azomethines of 5-amino-2-phenylindole series  
(in DMSO) 
Chemical Shifts,  (ppm) 
Substituents
m-NO2 p-Cl H
3,4-
O2CH2
p-OCH3 p-OH 
p-
N(CH3)2
o-OH furfuryl
Carbon 
Atom (I) (II) (III) (IV) (V) (VI) (VII) (VIII) (IX) 
C2 138.844 138.700 138.608 138.549 138.496 138.439 138.320 139.042 138.645
C3 99.288 99.194 99.144 99.100 99.073 99.039 98.987 99.250 99.168
C4 112.538 112.010 111.739 111.606 111.585 111.562 111.509 112.308 111.774
C5 142.820 143.422 143.800 143.877 144.213 144.458 144.878 140.223 143.529
C6 116.519 116.478 116.473 116.511 116.493 116.500 116.575 116.074 116.268
C7 111.755 111.677 111.628 111.532 111.376 111.19 110.943 111.961 111.691
C8 136.522 135.455 136.108 135.950 135.864 135.752 135.556 136.480 136.146
C9 129.083 129.097 129.097 129.121 129.128 129.138 129.178 129.153 129.115
Cazomethine 155.024 155.944 157.321 156.617 161.382 160.014 157.171 160.397 152.460
C1' 134.212 135.235 136.569 131.444 129.505 128.065 124.504 118.898 115.284
C2' 122.190 128.829 128.238 106.053 129.946 130.144 129.748 160.238 145.616
C6' 138.224 128.829 128.238 147.924 129.946 130.144 129.748 132.193 –
C3' 148.227 129.822 128.858 149.613 114.163 115.545 111.509 116.436 –
C5' 131.937 129.822 128.858 101.547 114.163 115.545 111.509 119.533 112.302
C4' 124.871 136.252 130.719 108.335 156.781 157.066 151.962 132.413 145.355
C1" 130.360 131.944 132.023 132.063 132.079 132.108 132.150 131.873 132.006
C2", C6" 125.004 124.974 124.950 124.950 124.933 124.952 124.894 125.044 124.952
C3", C5" 128.884 128.876 128.686 128.874 128.865 128.867 128.852 128.898 128.875
C4" 127.557 127.504 127.456 127.45 127.425 127.408 127.353 127.624 127.482
Other 
carbon  
CH2:
39.596
CH3:
55.314
CH3:
39.722 
 
In case of substituted benzene nucleus of 
aldehyde component, the situation can be 
divided into two different groups. For para-
substituted benzene nucleus (II-III and V-VII), 
the pair of carbon atoms C2’ and C6’ give the 
more intensive signals in region 128.238 ÷
130.144 ppm, and carbon atoms C3’ and C5’ 
have similar ones in regions of 111.509 ÷
129,822 ppm. The electron-donating 
substituents in para-position, such as OCH3 (V), 
OH (VI), N(CH3)2 (VII), cause the chemical 
shifts of C3’ and C5’ to move upfield (with  =
111.509 ÷ 115.545 ppm), while the electron-
withdrawing substituents have some influences 
on the contrary.  
The resonance signals in 13C-NMR spectra 
of carbon atoms in indole ring, essentially, were 
influenced by the present of the substituents in 
aldehyde component. The carbon atom C3 has 
lowest chemical shift, with C = 98.987 ÷99.288 
646
ppm and the carbon atom C5 has highest one, 
with C=140.223÷144.878 ppm. The change 
regions of chemical shifts of different carbon 
atoms in azomethines I-VIII can be summarized 
as following from the high field signal to low 
field one (see table 3). 
Similarly in case 1H-NMR spectra, there 
were some linear correlation regression of 
carbon-13 chemical shifts of carbon atoms in 
position C1’ and azomethine-carbon atom with 
Hammett’s  constants of substituents in 
benzene-aldehyde component. These 
relationships can be expressed by equations 
Cazomethine = -2.5233 + 157.3968 (n = 7; R2 =
0.292; F = 2.057) and C1' = 7.3041 + 132.0632 
(n = 7; R2 = 0.6726; F = 10.271), respectively. 
The slopes of those equations are opposite signs 
(Cazomethine < 0 and C1’ > 0), it is indicated that 
the substituents on benzene-aldehyde ring have 
opposite influences on the position of carbon-13 
chemical shift of these carbon type: The 
electron-donating substituents ( < 0) make 
Cazomethine moving to downfield and C1’ moving 
to upfield, and on the contrary, the electron-
withdrawing substituents ( > 0) cause Cazomethine 
to appear upfield and C1’–downfield. 
 
Table 3: Region of chemical shifts in 13C-NMR spectra of azomethines of 5-amino-2-phenylindole 
series (in DMSO) 
Carbon Region (, ppm) Carbon Region (, ppm) Carbon Region (, ppm) 
C2 138.320 ÷139.042 C8 135.455 ÷136.522 C1’ 118.898 ÷136.569
C3 98.987 ÷99.288 C9 129.083 ÷129.138 C2’ 122.190 ÷160.238
C4 111.509 ÷112.538 C1” 130.360 ÷132.150 C3’ 111.509 ÷149.613
C5 140.223 ÷144.878 C2” C6” 124.894 ÷125.044 C4’ 108.225 ÷157.066
C6 116.074 ÷116.575 C3” C5” 128.686 ÷128.898 C5’ 101.547 ÷131.937
C7 110.943 ÷111.76 C4” 127.408 ÷127.624 C6’ 128.238 ÷147.924
Cazomethine 155.024 ÷168.382
N
HNH
H6""H5
"H4
"H3 H2"
H3
H4
H6
H7
H6'
H3'
H4'
H5'
H2'
N
Hazo
 
Figure 3: The C-H interactions in HMBC spectrum of benzylidene-5-amino-2-phenylindole 
 
The long and short C–H interactions can be 
shown in the NMR spectra with two-
dimensional experiment, HSQC and HMBC 
respectively. In the HSQC spectrum of 
azomethine III (R=H), it’s indicated that the 
short-range C–H interactions in each C–H bond, 
as following pairs: C3–H3; Cazomethine–Hazomethine,
C4–H4, C2” and C6” with H2” and H6”; C4”–
H4”; C2’ and C6’ with H2’ and H6’. The 
HMBC spectrum of azomethine III (R=H) that 
is represented in figures 3 and 4 indicated some 
long-range C–H interactions in azomethine 
molecules. 
IV - Conclusions 
The 1H- and 13C-spectra of some 
azomethines of 5-amino-2-phenylindole series 
have been recorded and researched. It shown 
that, the substituents in benzene ring influenced 
647
on the chemical shift of proton on azomethine 
bond. 
 
Figure 4: A part of HMBC spectrum of 
benzylidene-5-amino-2-phenylindole  
This publication is financial supported from 
the National Basic Research Program in 
Natural Sciences. 
References 
1. Dang Nhu Tai, Nguyen Dinh Thanh, Pham 
Duy Nam, Hoang Thanh Duc. Vietnam J. 
Sci., VNU, Vol. XXII, No. 3A PT, P. 159 
(2006). 
2. Dang Nhu Tai, Nguyen Dinh Thanh, Pham 
Duy Nam, Hoang Thanh Duc. Proceedings, 
The second Hanoi Congress of and 
applications, 11/2006.  
3. Dang Nhu Tai, Nguyen Dinh Trieu, Nguyen 
Minh Thao, Nguyen Dinh Thanh. Vietnam 
J. Chem. Vol. 26, No. 4, 1-3 (1988); [C.A., 
Vol. 114, 1991,23308t]. 
4. Dang Nhu Tai, Nguyen Dinh Thanh, Trieu 
Quy Hung. Proceedings, 8th Eurasia 
Conference on Chemical Sciences, Hanoi, 
20/10/2003, P. 1 - 9.  
5. Dang Nhu Tai, Nguyen Dinh Thanh, Tran 
Dinh Phong. Proceedings, 10th Asian 
Chemical Congress, Hanoi, 20/10/2003, P. 
54 - 67. 
 


1H- AND 13C-NMR SPECTRA OF SOME AZOMETHINES OF 5-AMINO-2-PHENYLINDOLE SERIES

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1H- AND 13C-NMR SPECTRA OF SOME AZOMETHINES OF 5-AMINO-2-PHENYLINDOLE SERIES

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