Tautomerism in solution and solid state, spectroscopic studies and crystal structure of (Z)-1-[(4-amino-2,3,5,6-tetramethylphenylamino)methylene]-1,8a-dihydronaphthalen-2(3H)-one

Article abstract of DOI:10.1080/00387010903284646

A new Schiff base compound-(Z)-1-[(4-amino-2,3,5,6-tetramethylphenylamino)methylene]-1,8a-dihydronaphthalen-2(3H)-one-was synthesized by the reaction of 2-hydroxy-1-naphthaldehyde with 2,3,5,6-tetramethyl-p-phenylenediamine. It was investigated by using elemental analysis, IR, ¹H-NMR, ¹³C-NMR, UV-Visible-spectroscopy, and X-ray-crystallography techniques. Its UV-Visible spectra were examined in polar and nonpolar solvents. The Schiff base studied exists in enol-imine ? keto-amine form in DMSO ethanol, chloroform, and benzene solution. The keto-amine tautomer was found to be dominant in both the phases. The (Z)-1-[(4-amino-2,3,5,6-tetramethylphenylamino)methylene]-1,8a-dihydronaphthalen-2(3H)-one molecule is not planar. It contains a strong intramolecular N-H...O hydrogen bond between the amine and oxo group [N1 and O1 = 2.591(2) A], the H atom being essentially bonded to the N1 and O1 atoms. Taylor & Francis Group, LLC.

Full text of DOI:10.1080/00387010903284646

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Tautomerism in Solution and Solid
State, Spectroscopic Studies and Crystal
Structure of (Z)-1-[(4-amino-2,3,5,6-
tetramethylphenylamino)methylene]-1,8a-
dihydronaphthalen-2(3H)-one
Hüseyin Ünver a & Mustafa Yıldız b
a Department of Physics, Faculty of Science, Ankara University, Beşevler-Ankara, Turkey
b Department of Chemistry, Faculty of Science and Arts, Çanakkale Onsekiz Mart University,
Çanakkale, Turkey
Published online: 17 Mar 2010.
To cite this article: Hseyin nver & Mustafa Yldz (2010) Tautomerism in Solution and Solid State, Spectroscopic Studies
and Crystal Structure of (Z)-1-[(4-amino-2,3,5,6-tetramethylphenylamino)methylene]-1,8a-dihydronaphthalen-2(3H)-one,
Spectroscopy Letters: An International Journal for Rapid Communication, 43:2, 114-121, DOI: 10.1080/00387010903284646
To link to this article: http://dx.doi.org/10.1080/00387010903284646
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Spectroscopy Letters, 43:114–121, 2010
Copyright # Taylor & Francis Group, LLC
ISSN: 0038-7010 print=1532-2289 online
DOI: 10.1080/00387010903284646
Tautomerism in Solution and Solid State,
Spectroscopic Studies and Crystal Structure
of (Z)-1-[(4-amino-2,3,5,6-
tetramethylphenylamino)methylene]-1,
8a-dihydronaphthalen-2(3H)-one
1
¨
Hu¨ seyin Unver
and Mustafa Yıldız2
ABSTRACT A new Schiff base compound—(Z)-1-[(4-amino-2,3,5,6-
tetramethylphenylamino)methylene]-1,8a-dihydronaphthalen-2(3H)-one—
was synthesized by the reaction of 2-hydroxy-1-naphthaldehyde with
2,3,5,6-tetramethyl-p-phenylenediamine. It was investigated by using
1Department of Physics, Faculty
of Science, Ankara University,
Bes¸evler-Ankara, Turkey
2Department of Chemistry,
Faculty of Science and Arts,
C¸ anakkale Onsekiz Mart
1
elemental analysis, IR, H-NMR, 13C-NMR, UV-Visible-spectroscopy, and
X-ray-crystallography techniques. Its UV-Visible spectra were examined in
polar and nonpolar solvents. The Schiff base studied exists in enol-imine
$ keto-amine form in DMSO ethanol, chloroform, and benzene solution.
The keto-amine tautomer was found to be dominant in both the phases.
The (Z)-1-[(4-amino-2,3,5,6-tetramethylphenylamino)methylene]-1,8a-
dihydronaphthalen-2(3H)-one molecule is not planar. It contains a
strong intramolecular N-H . . . O hydrogen bond between the amine
University, C¸ anakkale, Turkey
˚
and oxo group [N1 and O1 ¼ 2.591(2) A], the H atom being essentially
bonded to the N1 and O1 atoms.
KEYWORDS crystal structure, intramolecular hydrogen bond, Schiff base,
spectroscopy, tautomerism
INTRODUCTION
Proton tautomerism plays an important role in many fields of chemistry
and especially biochemistry.[1–3] The ortho hydroxy Schiff bases form the
intramolecular hydrogen bonds, which reveal very interesting properties
like photochromism and thermochromism in the solid state,[4–6] attributed
to the intramolecular proton transfer reaction. It suggests that the proton
transfer state is comparatively easily accessible and that the energy of keto
and enol forms should not be very different.[7] Photochromism is produced
by an intramolecular proton transfer associated with a change in the
p-electron configuration. The presence of ortho hydroxyl group, for
Received 30 July 2008;
accepted 13 July 2009.
Address correspondence to
¨
Hu¨ seyin Unver, Department of
Physics, Faculty of Science, Ankara
University, TR-06100 Bes¸evler-Ankara,
Turkey. E-mail: unver@science.
ankara.edu.tr
114

instance, has been regarded as one of the importance
elements that favor the existence of intramolecular
hydrogen bonding (O-H . . . N and O . . . H-N) and
also the tautomerism that accounts for the formation
of either enol-imino or keto-amino tautomer.[8–12]
The proton transfer process has shown that the posi-
tion of the tautomeric equilibrium depends not only
on the electronic structure of the molecule but also
on its conformation.[13] The interactions between
the nonbonded atoms in molecules, leading to the
strengthening of the hydrogen bond, and the inter-
molecular interactions in the solid state, stabilizing
the planar structure of the molecule, shift the equili-
brium toward the keto-amine form.[14,7]
its structure by using FT-IR, UV-VIS, NMR, HETCOR,
and X-ray crystallographic techniques in order to
study the hydrogen bonding and tautomeric equili-
brium in both the solution and the solid states (Fig. 1).
MATERIALS AND METHODS
Reagents and Techniques
1
The H and 13C-NMR spectra were recorded on a
Bruker DPX FT-NMR spectrometer (Bruker AXS,
Inc.; United States) operating at 400 and 101.6 MHz.
The 1H-NMR and 13C-NMR chemical shifts were
measured using SiMe4 as an internal standard.
Infrared absorption spectra were obtained from a
PerkinElmer BX II spectrometer (PerkinElmer;
United States) in KBr discs and were reported in
cmꢀ1 units. The UV-Visible spectra were measured
In the field of coordination chemistry, this type of
ortho hydroxylated Schiff bases has received over-
whelming attention particularly in the study of com-
plex formation.[15–16] Recently, the liquid crystals
researchers have also made a significant revelation:
The introduction of lateral polar hydroxyl group will
enhance the molecular polarizability as well as stabi-
lize the liquid crystalline compounds.[17–19]
using
a SHIMADZU 1208 series spectrometer
(SHIMADZU, Japan). Carbon, nitrogen, and hydro-
gen analyses were performed on a LECO CHNS-932
analyzer (LECO, USA). Melting points were mea-
sured on an Electro Thermal IA 9100 apparatus
In this work, we synthesized the Schiff base (Z)-1-
[(4-amino-2,3,5,6-tetramethylphenylamino)methylene]-
1,8a-dihydronaphthalen-2(3H)-one and investigated
using
a
capillary tube. 2,3,5,6-Tetramethyl-p-
phenylenediamine, 2-hydroxy-1-naphthaldehyde, THF,
CHCl3, DMSO, ethanol, benzene, and n-heptane
were purchased from Merck (Germany).
Synthesis of (Z)-1-[(4-amino-
2,3,5,6-tetramethylphenylamino)-
methylene]-1,8a-
dihydronaphthalen-2(3H)-one
Tetramethyl-p-phenylenediamine (0.82 g; 5 ꢁ
10ꢀ3 mol) was added to a dry THF (100 mL) solution
of 2-hydroxy-1-naphthaldehyde (0.86 g; 5 ꢁ 10ꢀ3
mol). The mixture was stirred and heated for 2 hr.
Compound was obtained from the evaporation of
THF. It was crystallized from chloroform=n-heptane
as yellow crystals, m.p. 229ꢂC, 1.35 g (85%) yields.
Found: C, 79.01; H, 6.90; N, 8.67. Calcd.: For
C21H22N2O: C, 79.25; H, 6.92; N, 8,81% IR (KBr, cmꢀ1);
(Ar-NH2 and NH; 3423–3357 m,nAr-H; 3058–3030 w,
nAliphatic-H; 2924–2862 w, nArC¼O; 1617 s, nC¼C;
1543 s, nC-N; 1352 m. 1H-NMR(DMSO); d ppm,
3
15.64(d, 1H, Ar-NH-CH¼Ar, JHCNH ¼ 7.00 Hz); 8.20
3
(d, 1H, Ar¼CH-N-Ar, JHNCH ¼ 7.00 Hz); 7.87–6.92
(m, 6H, Ar-H); 4.60 (s, 2H, Ar-NH2), 2.08 (s, 6H,
Ar-CH3); 2.01 (s, 6H, Ar-CH3).
FIGURE 1 Tautomerism of the title compound.
115
Tautomerism in Solution and Solid State, Spectroscopic Studies

atom (N1) was found from difference Fourier map
calculated at the refinement process as a small posi-
tive electron density. The details of the X-ray data
collection, structure solution, and structure refine-
ments are given in Table 1. The final fractional
atomic coordinates and thermal parameters are given
in Table 2. Selected bond distances and angles are
listed in Table 3. The molecular structure with the
atom-numbering scheme is shown in Fig. 2.[23]
Crystallographic data (excluding structure factors)
for the structures reported in this paper have been
deposited with the Cambridge Crystallographic Data
Centre as supplementary publication CCDC
639794.[24]
X-Ray Crystallographic Details
The structure of the title compound was deter-
mined by single-crystal X-ray diffraction. A single
deep plate light yellow crystal with approximate
dimensions of 0.8 ꢁ 0.3 ꢁ 0.15 mm3 was mounted
inside a glass fiber capillary. Data were collected
on a Bruker Smart CCD Diffractometer (Bruker
AXS, Inc.; United States)[20] at 293 K using graphite-
˚
monochromatized MoKa radiation (k ¼ 0.71073 A)
and x-2h-scans technique in the range of
2.69o < h < 38.56ꢂC. Empirical absorption correction
was applied. A total of 25566 (9244 unique, Rint
¼
0.0352) reflections were measured. The structure
was solved by direct methods using the program
SHELXS-97[21] and refined by full-matrix least squares
methods on F2 using the SHELXL-97[22] program
package. Positions of the hydrogen atoms attached
to carbon atoms were fixed at their ideal positions,
and those attached to nitrogen atoms were not
located. The hydrogen atoms attached to nitrogen
RESULTS AND DISCUSSION
Spectroscopic Studies
The FT-IR spectrum of the compound is given in
synthetic procedures. The Ar-NH2 and N-H bond
was observed at 3423–3357 cmꢀ1 (Fig. 3). The
absorption bands assignable to the stretching of
TABLE 1 Crystal and Experimental Data for the Title
Compound
TABLE 2 Fractional Atomic Coordinates and Isotropic
Compound
C21H22N2O
2
Temperature Factors (A ), with Standard Deviations in the Least
˚
Significant Digits in Parentheses for Non-Hydrogen Atoms
Color=shape
Light Yellow=Plate
318.41
Formula weight
Crystal system
Space group
2
˚
Atom
x=a
y=b
z=c
U (A )
Monoclinic
P21=c
C1
C2
0.33079(7) 0.58384(10)
0.42382(8) 0.50873(12)
0.47966(8) 0.40547(12)
0.44920(7) 0.36332(10)
0.50935(8) 0.25548(12)
0.15471(6) 0.01892(16)
0.21711(7) 0.02341(19)
0.18453(7) 0.02354(19)
0.08659(7) 0.01904(16)
0.05518(8) 0.02378(19)
˚
Unit cell dimensions
a ¼ 13.0189(2) A
˚
b ¼ 8.9642(1) A
C3
b ¼ 107.742(1)o
C4
˚
c ¼ 14.8644(2) A
C5
3
˚
Volume
1652.23(4) A
C6
0.48130(8) 0.21344(12) ꢀ0.03838(8) 0.02471(19)
0.39071(8) 0.27939(11) ꢀ0.10302(7) 0.02364(19)
0.33039(8) 0.38555(11) ꢀ0.07387(7) 0.02055(17)
Z
4
C7
Density (calculated)
Absorption coefficient
F(0 0 0)
1.280 g cmꢀ3
0.079 mmꢀ1
680
C8
C9
0.35840(7) 0.43176(10)
0.29953(7) 0.54510(9)
0.02146(6) 0.01637(15)
0.05591(6) 0.01590(15)
C10
C11
N1
Crystal size
0.8 ꢁ 0.3 ꢁ 0.15 mm3
2.69 to 38.56o
ꢀ21 ꢃ h ꢃ 22;
ꢀ15 ꢃ k ꢃ 15; ꢀ26 ꢃ l ꢃ 22
25556
0.21497(7) 0.62388(10) ꢀ0.00800(6) 0.01655(15)
0.15574(6) 0.72611(8) 0.01702(5) 0.01731(14)
h Range for data collection
Index ranges
C12
C13
C14
C15
C16
C17
C18
C19
C20
C21
N2
0.08022(7) 0.81983(9) ꢀ0.05020(6) 0.01531(15)
ꢀ0.02907(7) 0.81492(10) ꢀ0.05423(6) 0.01660(15)
ꢀ0.10277(7) 0.90685(10) ꢀ0.11922(6) 0.01775(16)
ꢀ0.06553(7) 1.00280(10) ꢀ0.17765(6) 0.01816(16)
0.04339(7) 1.00243(10) ꢀ0.17531(6) 0.01833(16)
0.11780(7) 0.91328(10) ꢀ0.10900(6) 0.01611(15)
Reflections collected
Independent reflections
9244
Reflections observed (I > 2r(I)) 5894
Refinement method
Full-matrix least-squares
on F2
ꢀ0.06555(8) 0.70889(11)
0.00861(8) 0.02376(19)
Data=restraints=parameters
Goodness-of-fit on F2
R indices [I > 2r(I)]
5894=0=221
ꢀ0.22220(8) 0.90182(12) ꢀ0.12964(8) 0.0253(2)
0.07838(9) 1.10030(13) ꢀ0.24338(8) 0.0285(2)
0.23660(7) 0.92632(11) ꢀ0.09962(7) 0.02122(17)
ꢀ0.13764(7) 1.09807(10) ꢀ0.24217(6) 0.02395(17)
1.027
R1 ¼ 0.0588; wR2 ¼ 0.1683
R1 ¼ 0.0952; wR2 ¼ 0.1888
R indices (All Data)
ꢀ3
˚
Largest diff. peak and hole
0.751 and ꢀ0.423 e ꢄ A
O1
0.28039(6) 0.68200(9)
0.18764(5) 0.02536(16)
¨
H. Unver and M. Yıldız
116

ꢂ
ꢂ
˚
TABLE 3 Some Selected Bond Lengths (A), Bond Angles ( ) and Torsion Angles ( ) for the Title Compound
C1 O1
1.2806(11)
1.4418(13)
1.4477(13)
1.4564(12)
1.4057(12)
C11 N1
N1 C12
C15 N2
C12 C17
C12 C13
1.3220(11)
1.4395(11)
1.4078(12)
1.4020(12)
1.4064(12)
C1 C10
C1 C2
C9 C10
C10 C11
C1 C10 C9
N1 C11 C10
O1 C1 C10
O1 C1 C2
120.72(8)
C16 C15 C14
C16 C15 N2
N2 C15 C14
C17 C12 C13
C17 C12 N1
C13 C12 N1
121.09(8)
124.19(8)
122.44(8)
120.05(8)
117.50(8)
119.16(8)
118.42(9)
120.46(8)
122.20(8)
119.15(7)
118.65(8)
C10 C1 C2
C11 C10 C1
C2 C1 C10 C9
O1 C1 C2 C3
1.49(13)
ꢀ179.32(10)
174.62(8)
178.23(9)
177.32(8)
0.97(13)
C1 C10 C11 N1
C9 C10 C11 N1
C10 C11 N1 C12
N1 C12 C17 C21
C11 N1 C12 C13
N1 C12 C13 C14
N1 C12 C13 C18
C13 C14 C15 N2
C19 C14 C15 N2
ꢀ4.97(13)
178.48(8)
170.88(8)
ꢀ4.11(13)
120.62(9)
179.57(8)
ꢀ2.20(12)
ꢀ178.82(8)
2.84(13)
C4 C9 C10 C11
C8 C9 C10 C1
N2 C15 C16 C17
C3 C4 C9 C10
O1 C1 C10 C11
N2 C15 C16 C20
N1 C12 C17 C16
4.06(14)
ꢀ1.66(13)
179.00(8)
C¼O bond for compound was observed at frequen-
cies of 1617 cmꢀ1, and this value is less than that
reported in the IR spectrum for substituted aromatic
Schiff base that possesses the formulation of o-OH-
C10H6CH¼NC6H5R.[11] The O-H stretch at 4000–
3500 cmꢀ1 region was not observed in the IR spec-
trum for compound. This observation implies that
the H atom from the O-H group in compound
migrates to azomethine N atom via the N-H . . . O
intramolecular hydrogen bonding in the solid state.
The C¼N bond that is accountable partially for the
existence of keto-amine form can also be inferred
from the IR spectra of compound.
DMSO (Fig. 4). The NH2 proton of the compound
gave a singlet at d ¼ 4.60 ppm. The NH and azo-
methine proton are observed as doublet 15.64 and
9.56 ppm (3 JHNCH ¼ 7.00 Hz) for ligand. The naphthyl
protons resonate at d ¼ 7.87–6.92 ppm multiplet,
respectively, for compound. The ArCH3 proton of
the compound gave a singlet at d ¼ 2.08 ppm and
d ¼ 2.01 ppm, respectively.
According to the 13C NMR spectra, compound has 17
signals (Fig. 5). 13C-NMR data d ppm, 173.97 (C9 ipso,
ArC¼O), 161.23 (C7, Ar ¼ CH-NH), 143.43 (C4ipso,
Ar-NH2) 137.05 (C11), 134.11 (C1 ipso, Ar-N¼CH),
132.60 (C16), 129.38 (C12), 128.53 (C14), 126.72
(C17), 126.43 (C3-C30), 124.03 (C15), 123.25 (C13),
119.68 (C10), 118.09 (C2-C20), 107.56 (C8 ipso,
1
The H NMR data for compound show that the
tautomeric equilibrium favors the keto-amine in
FIGURE 2 The molecular structure of the title molecule. Displacement ellipsoids are plotted at the 50% probability level.[23]
117 Tautomerism in Solution and Solid State, Spectroscopic Studies

FIGURE 3 FT-IR spectrum of the title molecule.
FIGURE 4 1H-NMR spectrum of the title molecule.
FIGURE 5 1H-NMR spectrum of the title molecule.
¨
H. Unver and M. Yıldız
118

FIGURE 6 Solvent effect on UV spectra of the compound, DMSO (——————), ethanol (– ·· –), CHCl3 (–––), benzene (- - - - - - - -).
1
Ar ¼ CH), 15.82 (C5-C50), and 14.27 (C6-C60) (-ArCH3)
has been observed for compound. Assignments of
the protons and carbons were made by two-
dimensional hetero-nuclear-correlated experiments
(HETCOR) using delay values that correspond to
1J(C, H), between the carbons and protons (Fig. 7).
From HETCOR data, Ar ¼ O and ¼ CH-NH carbons
were assigned unambiguously.
In conclusion, UV-VIS, H NMR, and 13C NMR
results show that in DMSO solution, the compound
exists in the keto-amine form.
Crystallographic Study
Two types of intramolecular hydrogen bonds—
N-H . . . O (keto-amine form) and N . . . O-H (enol-
imine form)—can exist in 2-hydroxy Schiff bases.
The Schiff bases derived from salicylaldehyde always
form the N . . . H-O type of hydrogen bonding, regard-
less of the nature of the N substituent.[26] In the com-
pounds derived from 2-hydroxy-1-naphthaldehyde,
both types of hydrogen bonds were found.[27] The
title molecule contains a strong intramolecular hydro-
The UV-VIS spectra of the compound were stu-
died in polar and nonpolar solvents. The Schiff bases
show absorption in the range greater than 400 nm in
polar and nonpolar solvents.[9–12,25] We point out
that the new band belongs to the keto-amine form
of the Schiff bases with OH group in ortho position
to the imino group in polar and nonpolar solvents
in both acidic and basic media.[9–12] The compound
showed absorption above 400 nm in polar (DMSO,
ethanol, and chloroform) and nonpolar (benzene)
solvents (Fig. 6). The keto-amine tautomer is domi-
nant only in DMSO, ethanol, chloroform and
benzene for compound. The calculated keto-amine
form is given in Table 4.
˚
gen bond between the O1 and N1 atoms [2.591(2) A],
the H atom being essentially bonded to the N1 and
O1 atoms as seen in Fig. 1. The sum Van der Waals
˚
radius of the O and N atoms (3.07 A) is significantly
longer than intramolecular hydrogen bond
lengths.[28]
The title compound is not planar. The molecules I
[O1, C1-C11; planar with a maximum deviation of
˚
0.0804(7) A for the C11 atom] and II [N1, C12-C21,
˚
N2; planar with a maximum deviation of 0.0981(9) A
TABLE 4 Effect of Solvent on the UV Spectra of
Compound
for the C20 atom] are inclined at an angle of
68.72(1)o. The phenyl rings show small distortions
from ideal geometry with the C¼C distances for the
Keto-amine
Solvent
tautomer (%)
DMSO
EtOH
91
88
87
67
phenyl rings close to the expected value for aromatic
[28]
˚
rings [1.394(5) A].
X-ray structure determinations
CHCl3
reveal that the keto tautomer is favored over the enol
tautomer. This is evident from the observed bond
Benzene
119
Tautomerism in Solution and Solid State, Spectroscopic Studies

FIGURE 7 HETCOR spectrum of the title molecule.
˚
2. Garnovskii, A. D.; Vasil’chenko, I. S. Tautomerism and various coordi-
nation modes of typical chelating agents with metals. Usp. Khim.
2005, 74(3), 211–234.
3. Raczynska, E. D.; Kosinska, W.; Osmialowski, B.; Gawinecki, R.
Tautomeric equilibria in relation to pi-electron delocalization. Chem.
Rev. 2005, 105(10), 3561–3612.
4. Cohen, M. D.; Schmidt, G. M. J.; Flavian, S. Topochemistry: VI.
Experiments on photochromy and thermochromy of crystalline anils
of salicylaldehydes. J. Chem. Soc. 1964, 2041–2051.
5. Moustakalimavridis, I.; Hadjoudis, E.; Mavridis, A. Structure of thermo-
chromic Schiff-bases: 2. Structures of n-salicylidene-3-aminopyridine
and n-(5-methoxysalicylidene)-3-amiınopyridine. Acta Crystallogr.,
Sect. B: Struct. Sci. 1980, 36, 1126–1130.
6. Hadjoudis, E.; Vitterakis, M.; Moustakalimavridis, I. Photochromism
and thermochromism of Schiff-bases in the solid-state and in rigid
glasses. Tetrahedron 1987, 43(7), 1345–1360.
7. Filarowski, A.; Glowiaka, T.; Koll, A. Strengthening of the intramole-
cular O ꢄ ꢄ ꢄ H ꢄ ꢄ ꢄ N hydrogen bonds in Schiff bases as a result of steric
repulsion. J. Mol. Struct. 1999, 484, 75–89.
distance of 1.281(1) A for O1¼C1 atoms, which is
consistent with the O¼C double bond. Similarly,
˚
the C11-N1 distance of 1.322(1) A is consistent with
the C-N single bond.
Thus, crystal structure and molecular structure have
been investigated by using IR, H-NMR, 13C-NMR,
UV-Visible spectroscopy and X-ray crystallography
1
techniques. The IR, electronic, H, and, 13C NMR
1
spectra indicate that the keto form is favored in both
solution and the solid state. The X-ray structure deter-
mination also confirms that the keto tautomer is
favored over the enol tautomer in the title compound.
8. Elmalı, A.; Kabak, M.; Kavlakoglu, E.; Elerman, Y.; Durlu, T. N. Tauto-
meric properties, conformations and structure of N-(2-hydroxy-5-
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ACKNOWLEDGMENTS
The authors thank C¸anakkale Onsekiz Mart
University Grants Commission for a research grant
(Project 2005=29).
¨
10. Nazır, H.; Yıldız, M.; Yılmaz, H.; Tahir, M. N.; Ulku¨ , D. Intramolecular
hydrogen bonding and tautomerisim in Schiff bases: Structure of
N-(2-pyridil)-2-oxo-1-naphthylidenemethylamine. J. Mol. Struct. 2000,
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Tautomerism in Solution and Solid State, Spectroscopic Studies