D. NEDELTCHEVA AND L. ANTONOV
with equilibrium constants K (Eqn (1)) and K (a measure for the
T
e
IHB breakage):
0
½
E ꢀ
Ke ¼
(5)
½
Eꢀ
From the structures of 2E and 2E’ we can assume little UV–Vis
spectra difference (otherwise no isosbestic point could be
observed in Fig. 1), which means that the experimentally
determined tautomeric constant in this case can be presented as:
½
Kꢀ
KTobs
¼
(6)
0
½
Eꢀ þ ½E ꢀ
and hence
KT
þ Ke
KTobs ¼ 1
(7)
According to Eqn (7) if a breakage of the IHB occurs and there
are substantial amount of the open E’ form, the experimentally
determined tautomeric constant should be lower than the real
one. Actually this is observed in the case of 2.
Such hypothesis can be easily confirmed or rejected by the
LSER analysis. If we take into account the solvents without proton
acceptor abilities (last four in Table 1) and fit only them, the
e
Figure 5. K values versus b values
predicted value of K in acetone could be the same as if it does
observed, and described in the Introduction Part, experimental
facts.
T
not interact as proton acceptor (i.e., it reflects the equilibrium as
medium through the dielectric constant value). Such fit for 2
yields Eqn (8):
ꢁ
Acknowledgements
log KT ¼ ꢂ0:27ð0:04Þ ꢂ 0:03ð0:13Þp
(
8)
2
The quantum chemical calculations have started under support
of Alexander von Humboldt foundation (postdoctoral fellowship
and return fellowship of LA) and have been finalized along with
the spectral measurements under SCOPES grant from Swiss
National Science Foundation (JRP IB7320-110961/1). We thank
Prof. J. Wirz (University of Basel) for the kind discussion and Dr Y.
Kamdzhilov (University of Basel) for the help concerning the
quantum chemical calculations.
þ 0:84ð0:23Þa; r ¼ 0:968
Using the solvatochromic parameters from Table 1, one can
calculate a value of 0.6 for K in acetone. Such value is reasonable
taking into account the corresponding tautomeric constants in
other solvents with similar polarity. Returning to Eqn (7) it gives
a value of 0.59 for K , which means that in acetone coexist 27%
E ’, 46% 2E, and 27% 2K. Of course these values are only
T
[
5]
e
2
approximation taking into account the unavoidable statistical
deviation in Eqn (8), which give approximate limits for the above
T
predicted K from 0.46 to 0.76. However, even without exact
REFERENCES
values the discussion yields a clear conclusion that in proton
acceptor solvent, like acetone, two different enol form coexist,
namely: 2E stabilized by the action of acetone as medium
[
[
[
1] T. Zincke, H. Bindewald, Berichte 1884, 17, 3026–3033.
2] L. Antonov, D. Nedeltcheva, Chem. Soc. Rev. 2000, 29, 217–227.
3] D. Nedeltcheva, L. Antonov, A. Lycka, B. Damyanova, S. Popov, Curr.
Org. Chem. in press.
(
nonchelated) and 2E ’, where the IHB is broken and the specific
proton acceptor abilities of the solvent play stabilization role
(
chelated).
[4] H. Joshi, F. S. Kamounah, G. van der Zwan, C. Gooijer, L. Antonov,
Using the concept described by Eqns (5–8) an analysis of the
J. Chem. Soc., Perkin Trans. 2 2001, 2303–2308.
[
[
[
5] L. Antonov, S. Stoyanov, T. Stoyanova, Dyes Pigm. 1995, 27, 133–142.
6] L. Antonov, S. Stoyanov, Dyes Pigm. 1995, 28, 31–39.
7] L. Antonov, W. M. F. Fabian, D. Nedeltcheva, F. S. Kamounah, J. Chem.
Soc., Perkin Trans. 2 2000, 1173–1179.
effect of proton acceptor properties of the solvents listed in
Table 1 on the IHB of 2 can be performed. As expected (Fig. 5)
there is a trend between the proton acceptor abilities of the
solvents (b value) and the amount of the open enol form
[8] W. M. F. Fabian, L. Antonov, D. Nedeltcheva, F. S. Kamounah, P. J.
Taylor, J. Phys. Chem. 2004, 108, 7603–7612.
9] L. Antonov, W. M. F. Fabian, P. J. Taylor, J. Phys. Org. Chem. 2005, 18,
(
estimated values of K
If the same LSER hypothesis is applied to 3, the corresponding
equation is:
e
).
[
1169–1175.
[
10] The terms ‘‘chelated’’ and ‘‘unchelated’’ do not have the native
meaning and are used here only to be in conformity with the
explanation of the phenomena in Reference [9]. The idea is that
there are two species (E and E’) interacting in different way with the
solvent through a breakage of IHB.
ꢁ
log KT ¼ ꢂ1:01ð0:04Þ þ 0:65ð0:01Þp
(
9)
2
þ 0:80ð0:02Þa; r ¼ 0:999
and the predicted tautomeric constant is 0.33, which is exactly
the same as determined. The value of K tends to zero.
e
Hence, we can finally conclude that the IHB in 2 is weaker than
that in 3. As result it is partially broken by proton acceptor
solvents, which explains in a reasonable way the previously
[11] D. Nedeltcheva, B. Damyanova, S. Popov, J. Mol. Struct. 2005, 749, 36–
[
7]
44.
[
12] H. Zollinger, Color Chemistry: Syntheses, Properties & Applications of
Organic Dyes & Pigments, 3rd edition, Verlag Helvetica Chimica Acta,
Zurich, 2003. 429–543.
[13] http://www.orgchm.bas.bg/ꢄlantonov/FiNAl.htm
www.interscience.wiley.com/journal/poc
Copyright ß 2008 John Wiley & Sons, Ltd.
J. Phys. Org. Chem. 2009, 22 274–281