Gas-phase tautomerism in hydroxy azo dyes-from 4-phenylazo-1-phenol to 4-phenylazo-anthracen-1-ol

Article abstract of DOI:10.1002/rcm.4435

The tautomeric constants of a series of azo dyes were estimated in the gas phase by using electron ionization mass spectrometry. It was shown that the relative amount of the keto tautomer increases from 4-phenylazo-1-phenol to 4-phenylazo-anthracen-1-ol,

Full text of DOI:10.1002/rcm.4435

RAPID COMMUNICATIONS IN MASS SPECTROMETRY
Rapid Commun. Mass Spectrom. 2010; 24: 714–720
Published online in Wiley InterScience (www.interscience.wiley.com) DOI: 10.1002/rcm.4435
Gas-phase tautomerism in hydroxy azo dyes – from
4-phenylazo-1-phenol to 4-phenylazo-anthracen-1-oly
1
Daniela Nedeltcheva , Vanya Kurteva and Ivanka Topalova
1
2
*
1Institute of Organic Chemistry with Centre of Phytochemistry, Acad. G.Bonchev str., bl.9, Sofia 1113, Bulgaria
2Faculty of Chemistry, University of Sofia, 1 James Bourchier avenue, Sofia 1164, Bulgaria
Received 6 November 2009; Revised 23 December 2009; Accepted 24 December 2009
The tautomeric constants of a series of azo dyes were estimated in the gas phase by using electron
ionization mass spectrometry. It was shown that the relative amount of the keto tautomer increases
from 4-phenylazo-1-phenol to 4-phenylazo-anthracen-1-ol, thus confirming the quantum-chemical
predictions. The existence of the enol tautomer of 4-phenylazo-anthracen-1-ol is shown for the first
time by mass spectrometry in the gas phase. This finding is supported by flash photolysis measure-
ments in solution. Copyright # 2010 John Wiley & Sons, Ltd.
The applications of aromatic azo dyes for both conventional1
and high-tech2 purposes are determined by their halo-, photo-
and thermochromism. Therefore, the study of the nature of
these properties is important from both fundamental and
applied points of view. Many of these compounds are
tautomeric and their tautomerism influences or even deter-
mines the properties mentioned above. In this respect, the
study of structural and environmental factors, such as the main
tautomeric backbone, additionally attached substituents,
solvents, temperature, pH and UV-irradiation, which deter-
mine the tautomeric equilibrium of the azo dyes, is important.
The tautomeric compounds investigated in this paper,
representing important basic structures for dyestuff chem-
istry,1 are shown in Scheme 1. The dependence of the
tautomeric ratio (enol E – keto K) in 2 on the solvents,
temperature and substituents has been intensively studied3 by
electron spectroscopy. However, there are only a few studies
that describe the effect of modifying the naphthol unit.4 It is
known that due to aromaticity reasons only the enol form is
present in 1 and the opposite is believed to be true for 3.5
Mass spectrometry (MS) is very suitable for estimation of
the net tautomeric changes under such structural modifi-
cations, because the external factors like solvents, inter-
molecular interactions, etc., can be excluded by transferring
the tautomeric system into the gas phase, where the process
becomes truly unimolecular.6 The use of MS is based on the
assumption that there is no significant interconversion
between the tautomers in the gas phase following electron
ionization (EI).7 Hence, EI-MS provides a snapshot of the
Pihlaja et al.8 in the study of various heterocyclic tautomers,
by Allegretti et al.9 in the investigation of the tautomerism of
some amides and thioamides, by Masur et al.10 in the case of
keto/enol tautomerism in a series of 1- and 3-substituted
acetylacetones, and by Borgerding et al.11 in the application of
fast-atom bombardment tandem MS for the investigation of
the fragmentation pathways of azo dyes. MS has also been
used in the case of 2 and its isomers to reach conclusions
about the relative stability of the individual tautomers.12–15
Recently, we have developed a theoretical model for the
fragmentation of 2 based on ab initio quantum-chemical
calculations.15 The fragmentation pathways of the tautomers
were later revealed by using model compounds (replacing the
tautomeric proton by a methyl group) and stepwise increase of
the ionization energy in the mass spectrometer.16 Finding the
individual fragment ions, corresponding to the tautomeric
forms, allowed us to estimate the tautomeric ratio in the gas
phase for 2 and its isomers, 2-phenylazo-naphthalen-1-ol and
1-phenylazo-naphthalen-2-ol. The purpose of the current
study is to extend this approach towards the tautomerism
of 1 and 3 in order to estimate the effect of the aromatic system
modification. The investigation of 3 represents special interest
because the equilibrium is believed to be fully shifted towards
the keto tautomer at room temperature.5 Consequently, the
decrease in temperature, which could lead to additional
stabilization of 3K as happens in the case of 2,3 could not
provide the conditions for substantial changes in the spectrum.
In this respect, the increase in temperature provided by MS
(the temperature in the ion source is in the range of 200–3008C)
presents the only possibility of shifting the equilibrium in 3 and
provides the opportunity to study the aromaticity effect in
compounds 1–3 under the same conditions.
.
tautomeric gas-phase mixture and normally the Mþ ion does
not undergo tautomerization following its formation in the
ion source. As a result, the different fragmentation pathways
of the tautomeric intermediates can be used for identification
of the individual tautomers. This approach was used by
EXPERIMENTAL
*Correspondence to: D. Nedeltcheva, Institute of Organic Chem-
istry with Centre of Phytochemistry, Acad. G.Bonchev str., bl.9,
Sofia 1113, Bulgaria.
Synthesis
All the reagents were purchased from Aldrich (Taufkirchen,
Germany), Merck (Darmstadt, Germany) and Fluka (Buchs,
Switzerland), and were used without any further purification.
E-mail: dantonova@orgchm.bas.bg
yDedicated to our teacher in mass spectrometry, Prof. DSc
Simeon Popov.
Copyright # 2010 John Wiley & Sons, Ltd.

Gas-phase tautomerism in hydroxy azo dyes 715
Fluka silica gel/TLC-cards 60778 with 254 nm fluorescent
indicator were used for thin-layer chromatography (TLC)
and Rf-value determination. The highperformance flash
chromatography (HPFC) purifications were carried out on
a Biotage HorizonTM system (Charlottesville, VA, USA) on
silica gel. The melting points were determined in capillary
tubes on a MEL-TEMP 1102D-230 VAC apparatus (Dubuque,
IA, USA) without corrections. The nuclear magnetic resonance
(NMR) spectra were recorded on a Bruker AVANCE DRX 250
spectrometer (Rheinstetten, Germany) in deuterochloroform;
the chemical shifts are quoted in ppm in d-value against
tetramethylsilane (TMS) as internal standard and the coupling
constants were calculated in Hz.
Scheme 1. Tautomerism in 1–3.
Compound 2 and its model tautomers, the OMe and NMe
derivatives, were synthesized according to already described
Scheme 2. Fragmentation of 2E.
Copyright # 2010 John Wiley & Sons, Ltd.
Rapid Commun. Mass Spectrom. 2010; 24: 714–720
DOI: 10.1002/rcm

716 D. Nedeltcheva, V. Kurteva and I. Topalova
procedures.16 The same protocol was applied for the
preparation of model tautomers of 1, but only the OMe
derivative was obtained in almost quantitative yield. All
attempts to synthesize the NMe derivative of 1 failed both in
basic and in acidic (Eschweiler-Clarke methylation) con-
ditions. The azo compound 3 was obtained by following the
literature procedure.17 Its OMe5 derivative and the unknown
NMe derivative were prepared as an easy separable mixture
via a one-pot protocol by methylation of 3 in basic media as
follows.
15 min CH3I (5 mmol) were added and the suspension was
stirred at room temperature for 2 h. The solid phase was
filtered off and washed with THF. The solvent was removed
in vacuo and the residue was purified by HPFC on silica gel
(mobile phase with a gradient of polarity from CH2Cl2 to
CH2Cl2/MeOH 90:10). The formation of the OMe derivative
presents evidence for the existence of the enol form of 3 in
basic conditions at room temperature.
OMe derivative of 3
To a solution of hydroxyanthranyl diazene 3 (1 mmol) in
5 mL of tetrahydrofuran (THF) KOH (2 mmol) and after
32% yield; Rf 0.81 (CH2Cl2); orange-red crystals; m.p. 102–
1038C (lit.5 140–1418C); 1H NMR 4.14 (s, 3H, OCH3), 6.82 (d,
Scheme 3. Fragmentation of 2K.
Copyright # 2010 John Wiley & Sons, Ltd.
Rapid Commun. Mass Spectrom. 2010; 24: 714–720
DOI: 10.1002/rcm

Gas-phase tautomerism in hydroxy azo dyes 717
Table 1. Fragmentation of 2 and assignment of the fragment
130.2 (Cquat), 130.4 (CH), 131.4 (Cquat), 132.5 (Cquat), 142.1
(Cquat), 153.5 (Cquat), 158.6 (Cquat).
ions to the individual tautomers
Tautomeric
form
Relative intensity [%]
of the signal
Code
m/z
NMe derivative of 3
43% yield; Rf 0.26 (CH2Cl2); dark-red crystals; m.p. 153–
2-1
2-2
2-3
2-4
248
247
231
219
218
172
171
158
157
156
144
143
142
130
129
128
127
126
115
114
105
93
E/K
E/K
E
100
16.7
1.7
4.9
3.1
1.5
11.3
1.7
2.8
—
13.4
118.1
1.3
1.8
2.7
1.3
0.9
—
33.3
6.6
2.9
3.6
5.9
2.3
5.5
3.0
21.5
5.5
7.0
1
1548C; H NMR 3.85 (s, 3H, NCH3), 6.68 (d, 1H, J 10.5), 7.15
(m, 1H), 7.46 (m, 4H), 7.57 (m, 2H), 7.70 (d, 1H, J 10.5), 8.05 (m,
2H), 8.81 (s, 1H), 8.91 (s, 1H); 13C NMR 46.1 (NCH3), 117.0
(2 ꢀ CH), 122.1 (Cquat), 123.2 (CH), 123.4 (CH), 126.8 (CH),
127.5 (CH), 128.2 (CH), 128.3 (CH), 128.7 (CH), 129.0 (CH),
129.4 (2 ꢀ CH), 130.7 (Cquat), 131.83 (CH), 135.0 (Cquat), 149.5
(Cquat), 151.6 (Cquat), 165.8 (C¼N), 185.2 (C¼O).
K
2-5
2-6
2-7
2-8
K
E
E
K
2-9
K
K
2-10
2-11
2-12
2-13
2-14
2-15
2-16
2-17
2-18
2-19
2-20
2-21
2-22
2-23
2-24
2-25
2-26
2-27
2-28
2-29
E
Instrumentation
E
The absorption spectra were recorded on a JASCO V-570 UV-
Vis-NIR spectrophotometer (JASCO International, Tokyo,
Japan) at room temperature. The raw spectra were processed
by using the Step-by-Step-Filter software (I2MP Group, Sofia,
Bulgaria).18
E
K
K
E
E
E
The mass spectra were recorded on a DFS high resolution
magnetic sector instrument (Thermo Fisher Scientific GmbH,
Bremen, Germany) operating in EI mode at 70 eV with an ion
source temperature of 2008C.
Laser flash photolysis experiments were performed using
a previously described setup.19 Solutions were placed in
quartz cells (4.5 cm long and 1 cm wide) and excited by a
Lambda-Physik EMG 101 (Lambda Physik AG, Goettingen,
E
E/K
E
K
92
91
89
88
77
65
51
K
K
E
E
E/K
K
E/K
Table 3. Fragmentation of 3 and assignment of the fragment
ions to the individual tautomers
1H, J 8.2), 7.55 (m, 5H), 7.88 (d, 1H, J 8.2), 8.08 (m, 4H), 8.90 (s,
1H), 9.50 (s, 1H); 13C NMR 55.9 (OCH3), 101.5 (CH), 112.9
(CH), 121.4 (CH), 122.1 (CH), 122.9 (2 x CH), 124.9 (Cquat),
125.7 (CH), 126.2 (CH), 128.6 (CH), 128.7 (CH), 129.1 (2 x CH),
Relative
Corresponding
Tautomeric
form
intensity [%] of
the signal
Code
code for 1
m/z
3-1
3-2
2-1
2-2
298
297
281
269
268
222
221
208
207
206
194
193
192
180
179
178
177
176
165
164
139
138
105
93
E/K
E/K
E
100
61.2
3.7
12.1
9.4
—
—
3.0
11.8
—
10.7
22.4
—
6.3
3.8
5.2
4.3
—
23.2
9.2
4.2
2.4
4.5
25.9
12.3
3.7
20.4
12.8
2.6
Table 2. Fragmentation of 1 and assignment of the fragment
3-3
3-4
2-3
2-4
ions to the individual tautomers
K
3-5
3-6
2-5
2-6
K
E
Relative
Corresponding
code for 1
Tautomeric
form
intensity [%] of
the signal
3-7
3-8
3-9
2-7
2-8
2-9
E
Code
m/z
K
K
1-1
1-2
2-1
2-2
198
197
181
169
168
122
121
108
107
106
105
94
93
92
80
79
78
77
76
65
E/K
E/K
E
100
1.2
—
1.0
1.0
4.5
71.1
—
0.4
0.4
5.9
8.0
124.1
1.9
0.6
0.6
5.1
72.1
1.5
35.6
16.9
3-10
3-11
3-12
3-13
3-14
3-15
3-16
3-17
3-18
3-19
3-20
3-21
3-22
3-23
3-24
3-25
3-26
3-27
3-28
3-29
2-10
2-11
2-12
2-13
2-14
2-15
2-16
2-17
2-18
2-19
2-20
2-25
2-26
2-21
2-22
2-23
2-24
2-27
2-28
2-29
K
1-3
1-4
2-3
2-4
E
K
E
E
1-5
1-6
2-5
2-6
K
E
K
1-7
1-8
1-9
2-7
2-8
2-9
2-10
E
K
K
K
E
E
1-10
1-11
1-12
1-13
1-14
1-15
1-16
1-17
1-18
1-19
1-20
1-21
K
E
2-21
2-11
2-12, 2-22
2-13, 2-23
2-14
E
E
E/K
E
E
E/K
E/K
K
E
E
K
2-15
2-16
K
E
92
91
77
65
K
2-17, 2-27
2-18
2-19, 2-28
2-20
E/K
E
E/K
E
K
E/K
K
64
51
E/K
Copyright # 2010 John Wiley & Sons, Ltd.
Rapid Commun. Mass Spectrom. 2010; 24: 714–720
DOI: 10.1002/rcm

718 D. Nedeltcheva, V. Kurteva and I. Topalova
Germany) excimer laser operating at 308 nm (XeCl) with
pulse energies of ca. 100 mJ and pulse widths of ca. 30 ns. The
photochemical stabilities of the samples were monitored.
Exposure of the samples to the monitoring light was
minimized by using light shutters and appropriate filters.
The assignment of the mass spectral peaks allows us to
estimate the molar fractions of the individual tautomers and
the tautomeric constants:
P
IE
P
P
P
XE½%ꢂ ¼
XK½%ꢂ ¼
:100
:100
(1)
IE þ IK
P
IK
P
(2)
(3)
IE þ IK
XK
Quantum-chemical calculations
KT ¼
=
All optimized structures were characterized as true minima
by vibrational frequency calculations. All calculations were
carried out with the Gaussian 03 program suite20 at the HF/
6-311þþGꢁꢁ level of theory21 in the gas phase.
XE
where IK and IE are the relative intensities of the peaks
belonging only to the keto and enol forms, respectively, as
taken from Tables 1–3.
The corresponding tautomeric constants at the tempera-
ture of the ion source (2008C), calculated according to the
relative intensities of the individual fragment ions, are 0.03,
0.17 and 1.25, which corresponds to 97%, 85% and 44% enol
tautomer content for 1, 2 and 3, respectively. These results
can be interpreted in the light of the relative stabilities of the
tautomers as given in Scheme 4 and the absorption spectra
shown in Fig. 1.
RESULTS AND DISCUSSION
The fragmentation pathways of 2E and 2K are presented in
Schemes 2 and 3, along with an assignment of the fragment
ions to the individual tautomers, which is given in Table 1.
This assignment is based on the study of the fragmentation of
the individual tautomers by using quantum chemical
calculations and the mass spectra of their model com-
pounds.15,16 This well-developed approach can be extended
to the mass spectra of 1 and 3, by assuming similar
fragmentation pathways. The assignment of the mass peaks
of 1 and 3 to the individual tautomers in analogy to 2 is given
in Tables 2 and 3.
As is known for 2, both tautomeric forms co-exist in
solution (Fig. 1(b)) and the tautomeric equilibrium can be
easily shifted by changing the solvent environment.3,16 The
obtained value of the tautomeric constant corresponds to our
previously published values,16 and to the tautomeric
constant from methylcyclohexane/toluene solvent mixture
recalculated for the currently used temperature.15
Scheme 4. Relative energy (HF/6-311þþGꢁꢁ) of the tautomers of 1–3. The values include the zero
point energy correction.
Copyright # 2010 John Wiley & Sons, Ltd.
Rapid Commun. Mass Spectrom. 2010; 24: 714–720
DOI: 10.1002/rcm

Gas-phase tautomerism in hydroxy azo dyes 719
(a)
value of less than 3% at 2008C for the keto tautomer in the gas
phase results from a few low abundance fragment ions (1%
or less), that can be considered to result from experimental
noise.
According to Scheme 4, in the case of 3 the equilibrium
should be shifted towards the keto tautomer. However, the
relative energy of 2 kcal/mol could not exclude the existence
of the enol tautomer even at room temperature. The
estimated tautomeric constant shows a moderate value at
the temperature used (approximately 40% enol tautomer). If
we assume for 3 the same temperature dependence of the
tautomeric ratio as for 22 a substantial amount of the enol
tautomer should be found at room temperature. The
comparison of the absorption spectrum of 3 with those of
its OMe and NMe derivatives could not exclude the existence
of the enol tautomer. According to the second derivative
spectra the spectrum of 3 contains 3 sub-bands – at 500, 469
and 436 nm. Their shapes are very similar to that of the
corresponding NMe derivative, but the derivative spectrum
of the latter gives bands at 508, 478, 462 and 443 nm. In
addition, the absorption maximum of the OMe derivative at
450 nm (sub-bands at 484, 451, 424 and 400nm) lies in the blue
shoulder of the spectrum of 3; thus if the enol form exists, it
would be hard to detect.
300
400
500
600
WAVELENGTH [nm]
(b)
300
400
500
600
The first evidence for the existence of the enol form is that
its OMe derivative has been synthesized by methylation of 3.
The second piece of evidence comes from the flash photolysis
measurements. According to a previous flash photolysis
study of 2,24,25 upon absorption of a photon the E-form
undergoes trans-cis isomerization (Scheme 5), whereas the
keto tautomer does not show any detectable transient signal.
In addition, the cis enol tautomer does not undergo direct cis-
trans relaxation. It is initially converted into the K-form and
then to the corresponding enol form, restoring the initial
equilibrium tautomeric ratio.
WAVELENGTH [nm]
(c)
The kinetic curve for a solution of compound 3 in benzene,
measured at 500 nm, presented in Fig. 2, shows typical
behavior for a keto-enol mixture. The changes can be
attributed to the initial decrease of the trans enol form,
converted by the laser flash into the cis form, followed by a
rise, corresponding to restoration of the equilibrium state
through the keto form.
300
400
500
600
WAVELENGTH [nm]
Figure 1. Absorption spectra of 1–3 (a–c, bold solid lines)
and their model tautomers (fixed enol form – dashes, fixed
keto form – solid line) in benzene.
Based on the results from flash photolysis experiments, it
can be concluded that the two tautomers of 3 co-exist in
solution, because in the absence of the enol form no transient
signals could be detected. Although this experiment cannot
It is evident that the energy difference (relative energy) of
10.6 kcal/mol between the tautomers in 1 excludes the
existence of the keto tautomer. The absorption spectra of 1
and its OMe derivative, shown in Fig. 1(a), also do not give
evidence for the existence of the keto tautomer. The only
evidence for the existence of 1K can be found at very low
temperatures,22 where it is attributed to stabilization through
dimer formation.23 In the case of 2, the increase of the
temperature in nonpolar solvents shifts the tautomeric
equilibrium towards the enol tautomer.3 If a similar behavior
is assumed for 1, at the temperature of 2008C the keto
tautomer cannot exist, and this is what we actually observe
from the mass spectrum. Generally, an increase in tempera-
ture very rapidly destroys the dimers, which cannot
be observed even at room temperature.22 The calculated
Scheme 5. Kinetic scheme of the enol form relaxation.
Copyright # 2010 John Wiley & Sons, Ltd.
Rapid Commun. Mass Spectrom. 2010; 24: 714–720
DOI: 10.1002/rcm

720 D. Nedeltcheva, V. Kurteva and I. Topalova
0.004
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CONCLUSIONS
The gas-phase tautomerism in 4-phenylazo-1-phenol,
4-phenylazo-naphthalen-1-ol and 4-phenylazo-anthracen-1-ol
has been studied by EI-MS at an ion source temperature of
2008C. The peaks in the mass spectra were assigned to the
individual tautomeric forms by analogy with the fragmenta-
tion of the model (OMe and NMe) tautomers of 4-phenylazo-
naphthalen-1-ol. As expected, the tautomeric equilibrium in
4-phenylazo-1-phenol is fully shifted to the enol tautomer. It
is known that tautomeric benzene ring annelation causes an
increase in the amount of the keto form of the ortho-hydroxy
analogues of the compounds studied, and the same
behaviour might be expected for the compounds in the
current study. However, the results for the tautomeric ratio
in 4-phenylazo-anthracen-1-ol have shown that the amount
of the enol tautomer is substantial. This result was indirectly
confirmed by flash photolysis measurements in benzene.
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Acknowledgements
Financial support from the Bulgarian National Science Fund
(Project TK-X-1716) and the Swiss National Science Founda-
tion (JRP IB7320-110961/1) is gratefully acknowledged. We
thank Prof. J. Wirz and Dr P. Mueller (University of Basel) for
the flash photolysis measurements and the use of computer
resources for the theoretical calculations and Dr F.Theobald
(Thermo Scientific) for help in obtaining the mass spectra.
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Copyright # 2010 John Wiley & Sons, Ltd.
Rapid Commun. Mass Spectrom. 2010; 24: 714–720
DOI: 10.1002/rcm