The effect of solvent on electronic absorption bands of some Benzylideneanilines

Article abstract of DOI:10.1016/j.molliq.2016.09.036

In the present investigation the effects of solvents of varied polarities on the UV–visible spectral bands of some substituted benzylideneanilines and their o/p-hydroxy derivatives were explored. The analyses of the electronic absorption bands indicated t

Full text of DOI:10.1016/j.molliq.2016.09.036

Journal of Molecular Liquids 224 (2016) 53–61
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Journal of Molecular Liquids
journal homepage: www.elsevier.com/locate/molliq
The effect of solvent on electronic absorption bands of
some Benzylideneanilines
Sagarika Panigrahi a, , Pramila K. Misra
⁎
b,⁎⁎
a
Silicon Institute of Technology (Silicon West), Sason Sambalpur, 768200, Odisha, India
Centre of Studies in Surface Science and Technology, School of Chemistry, Sambalpur University, Jyoti Vihar, 768019, Odisha, India
b
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 9 May 2016
Received in revised form 17 August 2016
Accepted 12 September 2016
Available online 18 September 2016
In the present investigation the effects of solvents of varied polarities on the UV–visible spectral bands of some
substituted benzylideneanilines and their o/p-hydroxy derivatives were explored. The analyses of the electronic
absorption bands indicated the long wavelength π-π transition to be due to intramolecular charge transfer orig-
⁎
inating from the 4-methoxyaniline/aniline moiety as source and the\\C_N\\unit as sink. The enol forms of o/p
hydroxy benzylideneanilines were found to be more stable in neat solvents. The keto forms of these compounds
on the other hand, predominated in some of the solvents in both acidic and basic conditions. A maximum of 84%
keto form of p-hydroxybenzylideneaniline was observed in basic DMSO solutions at [NaOH] = 0.002 M. The
tautomerization constants (KT's) of o/p-hydroxybenzylideneanilines at 300 K were determined. A good correla-
tion between the relative permittivities of solvents and KT of p-hydroxybenzylideneaniline in the corresponding
solvent with regression coefficient of 0.998 supported specific interactions between the solvent and the
aldimines. But the occurrence of a scattering distribution of the relative permittivities of solvents with the KT's
of o-hydroxybenzylideneaniline revealed no specific interaction of these molecules with the solvents. The oscil-
lator strength of the charge transfer bands of the o-hydroxybenzylidene-4-methoxyaniline was found to be in the
range 0.16–0.29 L mol−1 cm−2. The energy of the charge transfer band of the o-hydroxy substituted
benzylideneanilines determined from experimental wavelength agreed very well with the theoretical values cal-
culated by using Briegleb relation.
Keywords:
Keto-enol tautomerism
Substituted benzylideneanilines
Solvent cage
Intramolecular H-bond
© 2016 Elsevier B.V. All rights reserved.
1. Introduction
[8–11]. This phenomenon has also similarity with thermochromism [12,
13] and hence the tautomerization in salicylideneaniline and its ana-
Benzylideneanilines belong to a class of compounds called aldimines
having azomethine group (\\CH_N\\) as the characteristic functional
moiety. The interests in the synthesis of aldimines and studies on their
solution behaviour originate mostly due to their wide applications in
various fields ranging from biological to analytical chemistry [1–5].
The exploitation of suitable structural properties of these compounds
is easily accomplished due to their relatively simpler synthetic proce-
dure and synthetic flexibility [6–7]. In addition, the presence of an
ortho hydroxyl group with respect to azomethine linkage of the
aldimines, facilitates the phenomena like intramolecular H-bonding
(O\\H….N and O….H\\N), tautomerism, thereby, promoting the for-
mation of either enol-imino or keto-imino tautomers in those molecules
logues, has been receiving considerable attention. Acquiring character-
istics spectral parameters of the aldimines through UV–visible
technique is one of the expedient techniques that throw deeper insight
into the tautomerism in salicylideneanilines and its p-hydroxy ana-
logues. But pulling of spectral parameters, understanding the possible
electronic transitions and assigning the UV–visible peaks in the absorp-
tion spectra are vital for these molecules. These depend meticulously on
the structure of the aldimines and environments experienced by the
aldimines in their immediate neighbourhoods. Particularly, significant
differences in resolutions of spectra and transitions are exhibited as
the polarity of the surrounding solvent changes. Therefore, the UV–vis-
ible spectra of these compounds have been studied in polar and non
polar solvents in both acidic and basic media [14]. Usually, the manifes-
tation of a new band at or beyond 400 nm for these compounds in the
midst of some polar solvents containing acid/base is an indication of
the formation of keto tautomer of the aldimine [8,10,11,15–16]. But
these types of peaks emerge rarely or do not emerge at all in presence
of some other nonpolar solvents even in acidic or basic conditions
⁎
Correspondence to: S. Panigrahi, Department of Basic Science(Chemistry), Silicon
Institute of Technology, Silicon West, Sason, Sambalpur, 768200, India.
⁎⁎ Correspondence to: P. K. Misra, Centre of Studies in Surface Science and Technology,
School of Chemistry, Sambalpur University, Jyoti Vihar, 768019, India.
E-mail addresses: sagarika_panigrahi2003@yahoo.co.in (S. Panigrahi),
pramila_61@yahoo.co.in (P.K. Misra).
http://dx.doi.org/10.1016/j.molliq.2016.09.036
0167-7322/© 2016 Elsevier B.V. All rights reserved.

54
S. Panigrahi, P.K. Misra / Journal of Molecular Liquids 224 (2016) 53–61
Molecular Structure 1.
[14]. The studies on phenomena like solvatochrosim, keto-enol tautom-
erism of the aldimine in organic solvents of varied polarity are, there-
fore, of considerable importance [17–18].
compounds were recrystallized from ethanol three times to ensure
the purity of the samples. The sharp melting point and distinct singular
spot on TLC plate in each case indicate high purity of the samples. The
generic structures of these aldimines are given in Molecular structure 1.
In the present study, we have synthesized a series of tailor made N-
benzylideneaniline, o/p-hydroxy-benzylideneanilines, N-benzylidene-
4-methoxyaniline and o/p-hydroxy-benzylidene-4-methoxyanilines
and characterized them through spectral techniques comprising of IR,
1H NMR and electronic absorption. The effects of both polar and nonpo-
lar solvents on their electronic absorption bands were explored. DMSO
affected the electronic transition of hydroxyl substituted aldimines to
a greater extent promoting keto-enol tautomerism conspicuously. The
percentages of keto and enol forms and the tautomerization constants
of the o/p-OH substituted aldimines in DMSO were calculated.
2.2. Characterization of the aldimines
The compounds were characterized by different spectral techniques
such as FTIR and NMR spectroscoscopic analyses. FTIR spectra of these
aldimines were recorded in the region 400–4000 cm−1 with the help
of a Schimaduz IR Prestige-21 FTIR spectrophotometer using KBr disk.
1H NMR spectra of the aldimines were recorded using 400 MHZ FT
NMR at SAIF, Madras IIT, Chennai. The FTIR and NMR plots for 1a are
provided in Figs. 1 and 2 and supplementary Figs. S1-S9 (for the rest
aldimines). The spectral data confirmed to their structures [23,24]. The
FTIR spectral data are presented in supplementary Table S1.
2. Experimental
2.1. Synthesis of aldimines
2.3. Effect of solvents on UV–visible spectra of aldimines
The aldimines 1(a-f) were synthesized by refluxing equimolar
amount of aniline/p-anisidine with benzaldehyde/ salicylaldehyde/p-
hydroxybenzaldehyde in minimum amount of ethanol [19–22]. These
The spectral parameters and electronic absorption spectra of the
aldimines 1a to 1f were analyzed in presence of 13 organic solvents
Fig. 1. IR spectrum of 1a (KBr Disk).

S. Panigrahi, P.K. Misra / Journal of Molecular Liquids 224 (2016) 53–61
55
Fig. 4. O\\H…. N intramolecular H-bonding in aldimine 1a.
The aldimines, 1a and 1c (o-hydroxy substituted aldimines) are
quite fascinating set of compounds for studying H-bond properties be-
cause of the possibility of intramolecular hydrogen bonding as a result
of prominent π-electron coupling between their acid and base centers
[Fig. 4, 19–22,26–27]
The electronic absorption spectra of the compounds were obtained
in 13 different organic solvents. Some representative spectra of 1a to
1f are shown in Figs. 5–10. The spectra comprised of several absorption
bands in the range 200 to 500 nm. The analyses of the electronic spectra
of o-hydroxy substituted aldimines 1a and 1c envisaged four distinct
Fig. 2. 1H NMR Spectrum of 1a (Resolution: 399.65 MHz, Solvent: CDCl3).
of varied polarities namely, methanol, ethanol, 1-propanol, 1-butanol
2-propanol, DMSO, DMF, chloroform, THF, 1,4-dioxane, toluene,
hexane, cyclohexane using Shimadzu UV-2450 UV–vis spectropho-
tometer. All reagents used were of spectroscopic grade and distilled
prior to the measurements.
The occurrence of keto-enol tautomerism in hydroxy-substituted
aldimines, 1a to 1d was investigated in presence of both polar and
nonpolar solvents. The aldimines, 1e and 1f did not exhibit keto-
enol taotomerism due to the absence of –OH group in their skeletal
moiety. HCl and NaOH in the range 10−4 M to 2 × 10−3 M were
added to uphold acidic and basic conditions whenever necessary.
The aldimines of 5 × 10−5 M was used throughout the experiments.
The tautomerization constants, KT's were calculated theoretically.
3. Result and discussions
3.1. Electronic absorption spectra in organic solvents
The nitrogen atom of functional moiety i.e. azomethine linkage
(\\C_N\\) of the aldimines is connected exclusively to either an aryl
or an alkyl group [25]. In addition, the aldimines, 1a, 1b and 1e have –
OCH3 group as the donor and\\C_N\\group as the acceptor promot-
ing thereby the vectorial flow of electron from the substituent-OCH3 to
the\\C_N\\center (Fig. 3).
Fig. 5. Electronic absorption spectra of 1a = [5 × 10−5 M] in different solvents.
Fig. 3. Schematic representation of organic D-π- aldimine, where D is electron donor of the
organic aldimine and π is conjugated system that acts as electron acceptor.
Fig. 6. Electronic absorption spectra of 1b = [5 × 10−5 M] in different solvents.

56
S. Panigrahi, P.K. Misra / Journal of Molecular Liquids 224 (2016) 53–61
Fig. 9. Electronic absorption spectra of 1e = [5 × 10−5 M] in different solvents.
solvent polarity [32]. The third band, ‘C′ band located at 266–328 nm
was assigned to π-π* transition involving the π-electrons of the
azomethine (\\CH_N\\) groups [28–30].
Fig. 7. Electronic absorption spectra of 1c = [5 × 10−5 M] in different solvents.
The broad band observed in the range 316–355 nm (D band) is due
to the intramolecular charge transfer band (CT band) originating from
the 4-methoxyaniline/aniline moiety as a source to the\\C_N\\as a
sink [28–29]. This CT band is commonly observed in o-hydroxyl
aldimines and is based on strong intramolecular H-bonding between
the hydroxyl group of the salicylidene unit and the nitrogen atom of
azomethine unit [33]. This peak, although was a well defined one,
but appeared either as shoulders or as distinct splitted peaks. The dis-
tinct broadening of the intramolecular CT band was ascribed to the
existence of enol-keto toutmeric equilibrium originating from the
OH group in o-position to the C_N center [28] as shown in Scheme
1. The forbidden n-π* transition band due to the imino group had in-
significant absorption in case of all aldimines [34].
bands whereas 1b, 1d, 1e, 1f exhibited only three bands in presence of
all solvents (Table 1, Figs. 5–10). The peaks were however, not very dis-
tinct in some cases due to broadening phenomena. The exact bands in
those cases were obtained by fitting Gaussian curve. The representative
spectra for 1a to 1f in cyclohexane and its Gaussian fitting curve were
provided in Figs. 11–12.
The distinct four bands in UV–visible absorption spectra of the o-hy-
droxy substituted aldimines (1a and 1c) obtained in organic solvents
were assigned as A, B, C, D [28–30] whereas the three bands in 1b and
1d were assigned as A, C, D. The ‘B’ band was not distinguished properly
in some cases due to overlapping with ‘A’ band. The band ‘A’ located at
222–271 nm was attributed to the moderate energy π-π* transition of
the aromatic ring [31] while the second band B at 275–295 nm in case
of aldimine 1a and 1c was attributed to low energy π-π* transition of
the aromatic ring [30]. These two bands were sensitive to the substitu-
tion on the aromatic rings but were slightly influenced by changing the
The CT band was confirmed by calculating the energy of charge
transfer band (ECT) theoretically using Briegleb relation [Eq. (1) Ref 35].
ECT ¼ ðIP−EAÞ þ C…
ð1Þ
where IP is the ionization potential of the donor part (aniline and
p-anisidine), EA is the electron affinity of the acceptor part (\\C_N
group) and ‘C’ term is the columbic force of attraction between the
Fig. 8. Electronic absorption spectra of 1d = [5 × 10−5 M] in different solvents.
Fig. 10. Electronic absorption spectra of 1f = [5 × 10−5 M] in different solvents.

S. Panigrahi, P.K. Misra / Journal of Molecular Liquids 224 (2016) 53–61
57
Table 1
Data from UV–Vis spectra of Aldimines (λmax in nm and εmax in L mol−1 cm−1 × 10−4).
Solvent
Compound
Band A
(π-π* Ar)
Band B
(π-π*Ar)
Band C
(π-π*\\C_N group)
Band D
(CT)
λmax
εmax
λmax
εmax
λmax
εmax
λmax
εmax
Cyclohexane
1a
1b
1c
1d
1e
1f
1a
1b
1c
1d
1e
1f
1a
1b
1c
1d
1e
1f
233.00
235.00
266.00
219.00
236.00
247.00
231.00
222.00
271.00
219.00
226.00
215.00
231.00
236.00
270.00
223.00
238.00
215.00
1.75
0.36
1.42
0.99
1.37
1.74
1.69
1.12
1.27
1.04
0.83
1.70
0.76
0.54
1.34
0.75
0.90
1.67
275.00
–
298.00
–
271.00
272.00
285.00
–
300.00
–
280.00
262.00
290.00
–
295.00
–
0.95
–
0.94
–
1.02
0.66
0.77
–
0.97
–
1.18
1.90
0.55
–
1.05
–
319.00
283.80
312.00
266.00
335.00
325.00
319.00
283.80
315.00
277.00
334.00
314.00
321.00
284.00
311.00
279.00
337.00
307.00
1.21
1.22
0.99
0.91
0.89
0.22
1.21
1.22
1.03
0.99
1.07
0.85
0.87
1.01
1.12
0.73
0.85
0.992
355.00
336.00
347.00
317.00
–
1.41
0.45
1.17
0.25
–
–
–
1,4-Dioxane
Methanol
351.00
330.00
343.00
316.00
–
1.58
1.42
1.03
0.79
–
–
–
351.00
335.00
340.00
316.00
–
1.17
1.09
1.21
0.89
–
264.00
260.00
0.93
1.89
–
–
electron transferred and the positive hole left behind. The IP values for
para anisidene and aniline are taken as 7.82 and 7.70, respectively
[36]). The EA is taken as −1.3 eV, [Ref 35] and ‘C’ term is taken as ei-
ther 5.2 or 5.6 eV [Ref 35].
The ECT values was also calculated experimentally from Eq. (2)
[Ref 28].
ECT ðeVÞ ¼ 1241:6=λmaxðnmÞ…
ð2Þ
Fig. 11. Electronic spectra of 1a to 1d ( A) and 1e to 1f ( B) in cyclohexane.
Fig. 12. Gaussian fitting curve for 1a (A) and 1b (B) in cyclohexane.

58
S. Panigrahi, P.K. Misra / Journal of Molecular Liquids 224 (2016) 53–61
where IP was the ionization potential of the molecule, a and b were
constants having the values (i) 4.39 and 0.857 [Ref 38], or (ii) 5.15 and
0.778 [Ref 39] or (iii) 5.11 and 0.701 [Ref. 40], and Emax was the energy
of the lowest electronic transition. The IP values calculated using the
different values for a and b were in good agreement with each other
(Table 2). The mean values for IP were also determined.
Scheme 1. Keto-enol tautomerism in o-hydroxy substituted aldimines.
where, λmax(nm) were experimentally obtained wavelengths of the
3.2. Study of keto-enol tautomerism
CT band. The ECT values thus obtained were compared with ECT values
obtained theoretically from Eq. (1) using the two extreme values of C.
The ECT values obtained in both the ways compared very well with
each other. A representative set of data for aldimine 1a is provided in
Table 2. The oscillator strength, “f” for the CT band was determined by
applying the Eq. (3) [Ref 37].
Usually the keto tautomer (NH-form, Scheme 1) for aldimines,
1a, 1b, 1c, 1d appears at longer wave length (around 400 nm or be-
yond) and the enol tautomer (OH-form, Scheme 1) appeared at
shorter wave length (b400 nm)[8–11,15–16]. The absence of NH ab-
sorption band in the spectra (Figs. 5–10) indicated that the com-
pound 1a existed mainly in the OH form and therefore, the charge
transfer band ‘D’ was assigned to the OH band. Intramolecular proton
transfer occurred, but the equilibrium shifted strongly towards the
enol form (OH\\ form) which was also evident from the IR data.
(Supplementary Table S1).
The absence of splitting peaks or shoulders at longer wave length
(N400 nm) suggested the shifting of keto-enol tutomerism to enol
form and the band was ascribed as OH-band exclusively for 1a and
1c. The probable formation of keto-enol tautomerism was explored
in acidic and basic environments by adding HCl and NaOH to the or-
ganic solvents (Table 3).
−9
f ¼ 4:6 ꢀ 10
ε
Δν1=2:::
ð3Þ
max
where εmax was molar extinction coefficient (L mol−1 cm−1) and
Δν1/2 cm−1 was bandwidth at half value of the absorbance. The cal-
culated values all of these parameters are given in Table 2. The elec-
tronic absorption spectral data of aldimines were also used to
calculate the ionization potentials of the molecules using the general
Eq. (4) (Ref-28)
IP ¼ a þ bEmax
…
ð4Þ
Table 2
UV-Vis data of the aldimine 1a.
ECT(eV) from
E
CT(eV) from
Mean
IP(eV)
f
Eq. (1) when
[C = 5.6 ev]
Solvent
Eq. (2)
IP1(eV)
IP2(eV)
IP3(eV)
L mol−1 cm−2
Cylohexane
Hexane
Tolune
Dioxane
THF
CHCl3
DMF
DMSO
2-Propanol
1-Butanol
1-Propanol
Ethanol
Methanol
3.497
3.497
3.478
3.537
3.532
3.527
3.512
3.507
3.507
3.517
3.522
3.527
3.537
7.660
7.670
7.650
7.720
7.700
7.700
7.690
7.680
7.710
7.700
7.710
7.670
7.700
8.120
8.130
8.110
8.180
8.160
8.160
8.150
8.140
8.170
8.160
8.170
8.130
8.160
7.790
7.790
7.780
7.840
7.820
7.820
7.810
7.800
7.830
7.820
7.830
7.790
7.820
7.860
7.870
7.850
7.910
7.890
7.890
7.880
7.870
7.900
7.890
7.900
7.870
7.890
0.207
0.162
0.225
0.286
0.244
0.261
0.269
0.265
0.232
0.201
0.227
0.248
0.192
3.520
3.520
3.520
3.520
3.520
3.520
3.520
3.520
3.520
3.520
3.520
3.520
3.520
ECT(eV) from Eq. (1) when [C = 5.2 ev] = 3.920.
Table 3
Individual spectral characteristics of both enol and keto tautomers in different solvents (λmax in nm and εmax in L mol−1 cm−1 × 10−4).
Acidic [HCl] = 0.002 M
max/εmax Enol form
Basic [NaOH ] = 0.002 M
max/εmax Enol form
Aldimine
1a
Solvent
λ
λ
max/εmax Keto form
λ
λ
max/εmax Keto form
1,4-Dioxane
CHCl3
DMSO
Methanol
1,4-Dioxane
CHCl3
349.00/1.94
351.00/2.56
352.00/1.55
348.00/1.48
273.00/2.94
332.00/1.08
353.00/0.55
349.00/0.71
322.00/0.61
340.00/1.27
273.00/1.05
324.00/0.274
271.00/2.24
272.00/2.00
287.80/1.75
285.00/1.10
–
–
–
350.00/2.14
351.00/2.69
305.00/0.588
349.00/1.78
330.00/2.26
332.00/1.06
364.00/3.20
331.00/2.32
344.00/0.746
340.00/1.19
272.00/0.96
339.00/0.894
300/.001.02
279.00/0.0.78
312.00/0.788
294.00/0.700
–
–
428.00/1.26
405.00/1.24
–
–
391.00/0.654
385.00/1.34
–
–
1b
1c
1d
–
–
DMSO
389.00/4.48
–
389.00/0.600
–
428.00/1.49
385.00/0.392
352.00/3.35
322.00/0.75
387.00/3.84
349.00/3.09
Methanol
1,4-Dioxane
CHCl3
DMSO
427.00/1.05
Methanol
1,4-Dioxane
CHCl3
DMSO
Methanol
–
–
–
–
–

S. Panigrahi, P.K. Misra / Journal of Molecular Liquids 224 (2016) 53–61
59
Fig. 13. Acid effect on aldimine 1a (A) and 1c (B) in different solvent ([HCl] = 0.002 M).
with alcohols as well as CHCl3 should be energetically more favorable
[41]. Thus, the keto tautomer might be predominated in polar solvent
in 1a and 1c.
But in the present investigation for 1a and 1c the enol tautomer pre-
dominated in all polar neat solvents as well as in acidic conditions of the
solvent ([HCl] = 10−4 M to 2 × 10−3 M). On the other hand in acidic
DMSO having [HCl] = 0.002 M, keto form of 1c outweighed. The elec-
tronic absorption spectra of 1a and 1c in different acidic solvents are
shown in Fig. 13(A and B).
In presence of all polar solvents in acidic conditions, hydrogen bond-
ing between aldimine (1a/1c) and the solvent (tautomer-H…OH) did
not occur indicating the solvation to be due to nonspecific dispersive in-
teraction [41]. Due to the presence of strong electron releasing\\OCH3
group in para position of the aldimine 1a, the strength of intramolecular
hydrogen bond increased [42] making the solute-solvent interaction via
moveable proton and the oxygen of the solvent to be unfavourable. As a
result, the aldimine 1a was preferably present in enol form in the sol-
vent cage as shown in Scheme 2.
DMSO has greater basic character compared to CHCl3, 1,4-dioxane,
methanol etc. and therefore, it has excellent solvation power and capac-
ity to promote hydrogen bonding [43]. The H+ ion released from the
phenolic\\OH of the aldimine 1a and 1c could be easily stabilized by
DMSO. Consequently, keto form of both the aldimine 1a and 1c predom-
inated in basic DMSO (Fig. 14A).
Scheme 2. Localization of o-hydroxybenzylidene-4-methoxyaniline in ethanol.
3.2.1. Tautomerization in case of ortho-hydroxy substituted aldimines, 1a
and 1c
Generally, tautomerization in aldimine depends upon the specific
solute–solvent interactions, e.g. hydrogen bonding, and nonspecific
bulk interactions with the solvent [41]. In polar solvents the equilibrium
shifts towards the keto form [41]. In presence of methanol and to some
extent in CHCl3, the substiuents in donor and acceptor stabilize keto and
enol tautomers. [41]. The dominating tautomer-H…OH-solvent interac-
tion (via movable proton of the aldimines, 1a and 1c and oxygen of the
solvent methanol) would require breaking of the intramolecular hydro-
gen bond. For these molecules a tautomer- O.... H– solvent interaction
In case of aldimine 1c, since the intramolecular hydrogen bonding is
weak in compared to 1a and its keto form exists with lower extinction
coefficient in polar solvents like basic methanol and basic 1,4-dioxane
[Fig. 14B].
Fig. 14. Base effect on aldimine 1a (A) and 1c = (B) in different solvents ([NaOH] = 0.002 M).

60
S. Panigrahi, P.K. Misra / Journal of Molecular Liquids 224 (2016) 53–61
Fig. 15. Acid effect on aldimine 1b in different solvents (A) ([HCl] = 0.002 M) Base effect on aldimine 1b in different solvents (B) ([NaOH] = 0.002 M).
3.2.2. Tautomerization in 1b and 1d
3.3. Conclusion
The keto form of aldimine 1b predominated over its enol form in
acidic DMSO and methanol [Fig. 15(A)]. On the other hand the keto
form existed predominently in basic DMSO [Fig. 15(B)]. The existence
of keto form in acidic methanol and DMSO was attributed to the free
\\OH group of 1b which interacted strongly with the solvent
(Schemes 3 and 4). The keto form of aldimine 1d generally predominat-
ed in basic DMSO (Fig. 16) and enol form was stable in acidic solvent.
The tautomerization in salicylideneaniline and its analogues is of par-
ticular interest due to its close resemblance with thermochromism.
Generally, tautomerization in aldimine depends upon the specific
solute–solvent interaction e.g. hydrogen bonding, and nonspecific bulk
interactions with the solvent. The enol form of aldimines was found to
be more stable in neat solvents in contrast to the keto form which was
stable in acidic or basic solvent system. Rapid enol to keto conversion
has been noted in polar DMSO with the change of pH of the medium. A
maximum 84% conversion of keto form of p-hydroxybenzyledeneaniline
can find support from the tautomerization constant data. But in case of
o-hydroxybenzyledeneaniline only 61% of the keto form was observed
to be converted. The energy of the intramolecular charge transfer band
3.2.3. Description of tautomeric constant, KT
The tautomerization constants KT's (Hydrazo/Azo ratio of the pure
tautomeric forms) are provided in Table 4. Analyses of the data sug-
gested that although there was a trend towards an increase of KT with
increasing relative permittivity of solvent, the parameter in general
could not be correlated with the tautomerization constants of the
ortho substituted aldimines possibly due to the absence of any specific
interactions with the solvent. The plot of relative permittivity of the sol-
vents vs. the tautomerization constants of the aldimines, 1c and 1d (Fig.
17) yielded a good correlation with R2 = 0.998 for the aldimine, 1d . But
in case of ortho substituted aldimine, 1c scattered distribution was ob-
served (Fig. 17).
3.2.4. Percentage calculation of enol and keto form
The % of enol and keto forms was calculated in basic DMSO. The
highest % of keto form was for 1d as given in Table 5.
H3CO
N
Fig. 16. Base effect on aldimine 1d in different solvent ([NaOH] = 0.002 M).
CH
OH.....OH CH3
Scheme 3. Hydrogen bonded structure of 1b with methanol.
Table 4
Measurement of KT of 1c and 1d in different solvents [NaOH] = 0.002 M.
Aldimine
Relative permittivities
Solvents
KT
1c
2.25
32.6
46.7
2.25
32.6
46.7
1,4-dioxane
Methanol
DMSO
1,4-dioxane
Methanol
DMSO
0.804
0.44
1.55
3.29
4.42
4.87
CH3
CH3
H3CO
N
CH
OH O
S
1d
Scheme 4. Hydrogen bonded structure of 1b in DMSO.

S. Panigrahi, P.K. Misra / Journal of Molecular Liquids 224 (2016) 53–61
61
Fig. 17. A plot of the relative permittivities of the solvents vs. the tautomerization constants of 1c (A) and 1d (B).
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The authors express their sincere thanks to CSIR (File No. 09/
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one of the authors (SP). Support of the UGC (DRS) and the FIST (DST)
to the School of Chemistry, Sambalpur University are also gratefully
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