Tautomerism in hydroxynaphthaldehyde anils and azo analogues: A combined experimental and computational study

Article abstract of DOI:10.1021/jp048035z

The enol imine ? enaminone tautomerization constants, K _T, and thermodynamic parameters, ΔH_T and ΔS_T, of 1-hydroxy-2-naphthaldehyde Schiff bases are determined by UV/vis spectroscopy. Polar solvents shift the equilibrium toward the quinone form (for the unsubstituted derivative 1c, K_T = 0.20 (cyclohexane) and K_T = 1.49 (ethanol)). Both donor (MeO, NMe₂) and acceptor (CN, NO₂) substituents lead to a decreased K_T independent of solvent polarity. In apolar solvents, for all derivatives 1a-1e, the enol imine ? enaminone equilibria are endergonic but exothermic. Linear solvation energy relationships allow extrapolation of ΔG _T to the gas phase. Density functional theory calculations (B3LYP/6-311+G**) yield good agreement with these extrapolated ΔG_T values. Solvent effects on le are also successfully reproduced by the calculations. Geometric (0...N distance) and energetic criteria (conformer energy differences, homodesmotic reactions) establish the importance of intramolecular hydrogen bonding for the tautomerism of these compounds. The results obtained for 1a-1e are compared with tautomeric properties of the isomeric naphthaldehyde anils 2-4, the monocyclic analogues 5 and 6, the corresponding azo derivatives 7-9, and the N-alkyl derivative 10.

Full text of DOI:10.1021/jp048035z

J. Phys. Chem. A 2004, 108, 7603-7612
7603
Tautomerism in Hydroxynaphthaldehyde Anils and Azo Analogues: a Combined
Experimental and Computational Study
Walter M. F. Fabian,*,† Liudmil Antonov,‡ Daniela Nedeltcheva,§ Fadhil S. Kamounah,| and
Peter J. Taylor
Institut fu¨r Chemie, Karl-Franzens UniVersita¨t Graz, Heinrichstr. 28, A-8010 Graz, Austria, National Forestry
UniVersity, Faculty of Ecology, 10 Kliment Ohridski AVenue, BG-1756 Sofia, Bulgaria, Institute of Organic
Chemistry, Bulgarian Academy of Sciences, Akad. G.BoncheV Street, bl.9, Sofia 1113, Bulgaria, CISMI,
Department of Chemistry, UniVersity of Copenhagen, UniVersitetsparken 5, DK-2100 Copenhagen,
Denmark, and AstraZeneca, Mereside, Alderley Park, Macclesfield, Cheshire SK10 4TG, England
ReceiVed: May 7, 2004; In Final Form: June 15, 2004
The enol imine a enaminone tautomerization constants, KT, and thermodynamic parameters, ∆HT and ∆ST,
of 1-hydroxy-2-naphthaldehyde Schiff bases are determined by UV/vis spectroscopy. Polar solvents shift the
equilibrium toward the quinone form (for the unsubstituted derivative 1c, KT ) 0.20 (cyclohexane) and KT
) 1.49 (ethanol)). Both donor (MeO, NMe2) and acceptor (CN, NO2) substituents lead to a decreased KT
independent of solvent polarity. In apolar solvents, for all derivatives 1a-1e, the enol imine a enaminone
equilibria are endergonic but exothermic. Linear solvation energy relationships allow extrapolation of ∆GT
to the gas phase. Density functional theory calculations (B3LYP/6-311+G**) yield good agreement with
these extrapolated ∆GT values. Solvent effects on 1c are also successfully reproduced by the calculations.
Geometric (O‚‚‚N distance) and energetic criteria (conformer energy differences, homodesmotic reactions)
establish the importance of intramolecular hydrogen bonding for the tautomerism of these compounds. The
results obtained for 1a-1e are compared with tautomeric properties of the isomeric naphthaldehyde anils
2-4, the monocyclic analogues 5 and 6, the corresponding azo derivatives 7-9, and the N-alkyl derivative
10.
Introduction
so far, it has been difficult to obtain reliable quantitative
experimental data on these equilibria.18a,b In the following, we
Intramolecular proton transfer in the ground and/or excited
state of Schiff bases has important consequences on their
electronic structure which can be exploited for their thermo-
chromic and/or photochromic behavior,1,2 the design of molec-
ular electronic devices,3 and their suitability as agents in
photodynamic cancer therapy.4 Furthermore, intramolecular
hydrogen bonding in 1,3-diketones, â-enaminones or keto-
hydrazones,5-7 and related heterodienes8 is of fundamental
importance in the theory of hydrogen bonding.9-11
In view of this general interest in the prototropic properties
(enol imine (phenol, A) a enaminone (keto amine, quinone,
H) tautomerism) of hydroxy naphthaldehydes, Schiff bases, or
salicylidene anils, as well as the long-known azo a hydrazo
tautomerism of hydroxy azo compounds,12 numerous experi-
mental investigations on such compounds (X-ray crystal-
lography, UV/vis, fluorescence, and NMR spectroscopy, as well
as combinations thereof) have been performed; as a complement
to these experiments, a variety of computational chemistry
methods also has been applied.1-8,13-29 Unfortunately, estimating
tautomeric equilibria by computational methods can be fiend-
ishly difficult,30 necessitating calibration of calculated tautomer-
ization energies with the aid of experimental results. However,
present a combined experimental (chemometric analysis of UV-
Vis spectra18a,b) and computational investigation of the tauto-
meric equilibria of substituted 1-hydroxy-2-naphthaldehyde anils
1 (Scheme 1). Special emphasis will be put not only on the
effect of substituents (donor vs acceptor) on the tautomerization
constant, KT ) [quinone]/[phenol], but also on the influence of
solvation, the importance of intramolecular hydrogen bonding,
positional isomerism, and the effect of the additional benzene
ring in naphthyl derivatives as opposed to salicylidene anils or
hydroxy azobenzenes. Thus, to put the results obtained for 1
into a broader context, comparison will be made with the
isomeric anils of 2-hydroxy-1-naphthaldehyde, 3-hydroxy-2-
naphthaldehyde, and 4-hydroxy-1-naphthaldehyde 2-4,
salicylidene anil 5, and 4-phenyliminomethylphenol 6. In
addition, phenylazonaphthols 7-9 are also included (Scheme
1). The effect of the N-aryl group on KT is estimated by
comparison with the alkyl derivative 10.
Results
Experimental Results. In this section, first, tautomerization
constants KT ) [quinone]/[phenol] for compounds 1a-e will
be presented and compared to the related structures 2-10
(Scheme 1). Second, a more detailed description of solvent
effects on KT of 1c, as well as their treatment by linear solvation
energy relationships, will be given. Finally, thermodynamic
parameters of the imino phenol a enaminone (azo a hydrazo)
tautomeric equilibria will be presented.
* Corresponding author: E-mail: walter.fabian@uni-graz.at. URL:
http://www-ang.uni-graz.at/∼fabian/.
† Karl-Franzens Universita¨t Graz.
‡ National Forestry University.
§ Bulgarian Academy of Sciences.
| University of Copenhagen.
AstraZeneca.
10.1021/jp048035z CCC: $27.50 © 2004 American Chemical Society
Published on Web 08/24/2004

7604 J. Phys. Chem. A, Vol. 108, No. 37, 2004
Fabian et al.
SCHEME 1: Structures of the Investigated Compounds
1. Tautomerization Constants. Observed (or, estimated, see
Experimental section) UV/vis absorption bands of the individual
tautomeric forms (imino phenol A and enaminone H) for Schiff
isomeric 2-hydroxy-1-naphthaldehyde anils 2a-e.27a Time de-
pendent density functional calculations (TD-B3LYP/6-31+G**)
are in line with these experimental findings (Table 1). Whereas
band positions of the UV/vis spectra are essentially independent
of solvent polarity, tautomerization constants of 1a-e show a
clear increase in the order cyclohexane < acetonitrile < ethanol,
in line with the expected higher polarity of the quinone tautomer
and, thus, preferential stabilization by polar solvents. The
dependence of the overall shape of the UV/vis spectra of
compound 1c on solvent polarity is shown in Figure 1.
bases 1a-e, as well as tautomerization constants, KT
)
[quinone]/[phenol], derived therefrom via chemometric analysis
of the spectral data, are summarized in Table 1. KT values for
all five derivatives were determined in apolar (cyclohexane),
polar aprotic (acetonitrile), and polar protic (ethanol) solvents.
Both donor (1a, 1b) and acceptor substituents (1d, 1e) cause
bathochromic shifts of the longest wavelength absorption bands
of either tautomeric form. A similar dependence of the UV/vis
spectra on substituents has previously also been found for the
Thereby, the different tautomer content 1cA vs 1cH clearly
can be seen. Also, the negligible solvent shift of the various

Hydroxynaphthaldehyde Schiff Base Tautomerism
J. Phys. Chem. A, Vol. 108, No. 37, 2004 7605
TABLE 1: UV/Vis Data, Enol Imine a Keto-Amine Equilibrium Constants KT, Experimental (in parentheses) and Calculated
∆GT Values of Compounds 1a-1e in Various Solvents
A form
max/nma max/nmb
H form
max/nma max/nmb
cyclohexane
acetonitrile
ethanol
KT(∆Gexp
7.85d
c
c
c
compd
R
λ
λ
λ
λ
KT(∆Gexp
)
∆Gcalc
-0.97
KT(∆Gexp
)
∆Gcalc
-2.07
)
∆Gcalc
1a
N(CH3)2 406 (390,
410, 432)
387
359
354
373
407
477 (452,
481, 507)
400
392
397
417
445
0.11 ( 0.02
0.88 ( 0.09
-3.70
(1.31 ( 0.10) (-1.78)
0.10 ( 0.01 -0.75
(0.08 ( 0.06) (-3.71) (-1.22)
0.63 ( 0.03 -1.64 1.21 ( 0.05
(-3.61)
-2.64
1b
1c
1d
1e
OCH3
387
383
397
403
457 (438,
460, 485)
(1.36 ( 0.06) (-1.64)
0.20 ( 0.01 -1.14
(0.27 ( 0.03) (-3.58) (-0.11 ( 0.02) (-3.56)
1.24 ( 0.06 -2.07 1.49 ( 0.08 -2.42
H
454 (433,
458, 482)
(0.95 ( 0.03) (-1.95) (-0.13 ( 0.03) (-3.85) (-0.24 ( 0.03) (-3.78)
0.07 ( 0.01 -1.02 0.57 ( 0.05 -1.76 0.64 ( 0.07 -2.09
CN
463 (448,
467, 492)
(1.57 ( 0.08) (-1.84) (0.33 ( 0.05) (-3.81) (0.26 ( 0.06) (-3.72)
0.06 ( 0.01 -1.79 0.56 ( 0.08 -2.44 0.60 ( 0.09 -2.73
NO2
475 (461,
478, 504)
(1.66 ( 0.09) (-2.65) (0.34 ( 0.08) (-4.69) (0.30 ( 0.08) (-4.54)
a Spectral characteristics of the individual tautomeric forms (sub-bands are given in parentheses) estimated according to ref 18a,b. The values in
all solvents are practically the same (see Figure 1). b Calculated values (TD-B3LYP/6-31+G**). c PCM-B3LYP/6-311+G**//HF-6-31G* + p(H)
results (PB-SCRF-B3LYP/6-311+G** ∆G values are given in parentheses). d Due to strongly overlapping bands, this value is only approximate.
Figure 1. Absorption spectra of 1c in 1, CH; 2, 80/20 CH/EtOH; 3,
60/40 CH/EtOH; 4, 40/60 CH/EtOH; 5, 20/80 CH/EtOH; 6, EtOH; 7,
80/20 EtOH/Water; 8, 60/40 EtOH/Water; and 9, 40/60 EtOH/Water.
Figure 2. Individual spectra of A (filled circles) and H (filled
diamonds) forms of 1c along with the decomposed individual bands
(empty characters) obtained through chemometric data processing of
the data from Figure 1.
band maxima of the individual tautomers is evident. As an
example, the decomposition of the UV/vis spectrum of the 1cA
a 1cH tautomeric mixture into individual bands according to
the procedure described in detail previously18a,b is given in
Figure 2.
Proton transfer from the imino enol tautomer to the keto
amine form is accompanied by a benzenoid-quinonoid struc-
tural change with a concomitant loss of aromaticity. In 1 and
2, this change is restricted to only one ring of the naphthalene
moiety, whereas in 3, both are involved (see structures in
Scheme 1), making 3H considerably less stable than 3A. Unlike
hydroxyazobenzenes or salicylidene anils, which mainly or
almost exclusively exist as azo (imino enol) tautomers,24a there
is a significant or even predominant quinone content in the
naphthol derivatives. Only recently has it been possible by the
more sensitive fluorescence excitation spectroscopy to determine
tautomerization constants in 5,26a and they were found to be at
least an order of magnitude lower than those in 1 or 2. The
same argument as abovescomplete loss of aromaticity rather
than only for one ringshas been put forward to explain this
difference.15a,b Arylazonaphthols are slightly more prone to form
the hydrazo tautomers (KT ) 1.5-2.5 for 2-phenylazo-1-
naphthol 727b and 0.5-1.6 for 1-phenylazo-2-naphthol 827c).
Considerably larger KT values (KT ) 2-9) have been reported
for the N-alkyl derivative 10.31
Except for KT of 1a in ethanol, which because of strongly
overlapping bands is only approximate, substituentssespecially
acceptors (CN, NO2)sdecrease the amount of enaminone
content in the equilibrium mixture. Thus, for all derivatives,
the imino enol form predominates in cyclohexane; in acetonitrile,
the enaminone form of the unsubstituted derivative 1c and, in
ethanol also, of the donor (MeO, NMe2) substituted derivatives
is prevalent. The effect of the substituent on KT is, however,
quite small as also found in the case of 2-hydroxy-1-naphthal-
dehyde Schiff bases 227a and salicylidene anils 5.26a In these
latter two derivatives, contrary to 1, donor substituents appar-
ently stabilize the quinone tautomer, at least in polar protic
media (EtOH, MeOH). Positional isomerism, i.e., 1 vs 2, has
only little influence on KT: for R ) H, KT values obtained in
different solvents are in the range 0.20-1.25 in 1 (Table S1,
Supporting Information), those in 2 are in the range 0.1-1.1.27a
In complete contrast, anils of 3-hydroxy-2-naphthaldehyde 3
have been reported to exist exclusively as the imino enol
tautomer.14c
2. Linear Solvation Energy Relationships. Although cor-
relation of KT with solvent polarity as described by its relative
permittivity, D (Table S1), is modest (r2 ) 0.73), polar solvents

7606 J. Phys. Chem. A, Vol. 108, No. 37, 2004
Fabian et al.
TABLE 2: Thermodynamic Parameters for the Enol Imine a Keto-amine Equilibrium of Compounds 1, 2, 7-9 in
Methylcyclohexane/Toluene (Results for Ethanol Are Given in Parentheses)
∆HT
∆ST
∆HT
∆ST
compd
[kcal mol-1
]
[cal mol-1K-1
]
compd
[kcal mol-1
]
[cal mol-1K-1
]
1a
-0.535 ( 0.053
-0.442 ( 0.055
-6.44 ( 0.26
-6.36 ( 0.28
2a
-0.198 ( 0.013
(-0.598 ( 0.038)
-0.584 ( 0.021
(-0.384 ( 0.034)
-0.463 ( 0.004
(-0.089 ( 0.012)
-0.115 ( 0.014
(0.526 ( 0.033)
-5.43 ( 0.08
(-2.18 ( 0.25)
-1.19 ( 0.09
(-0.31 ( 0.17)
-2.95 ( 0.02
(-0.98 ( 0.05)
-4.60 ( 0.01
(-1.65 ( 0.11)
1b
1ca
1d
7a
8a
9a
-0.481 ( 0.067
(-0.664 ( 0.043)
-0.553 ( 0.026
-4.76 ( 0.32
(-1.43 ( 0.16)
-7.36 ( 0.12
1e
-0.836 ( 0.043
-8.62 ( 0.21
a Data from ref 18d.
clearly increase the amount of the enaminone tautomer. A
similar result is also found for the imino enol a keto amine
tautomerism of isomeric 2-hydroxy-1-naphthaldehyde anils 227a
(r2 ) 0.75) and the azo a hydrazo tautomerism of 1-phenylazo-
2-naphthols 827c (r2 ) 0.81). In striking contrast, both 2-phen-
ylazo-1-naphthol27b and 4-phenylazo-1-naphthol27d lack any
clear correlation of KT with solvent polarity (relative permittivity
D, r2 ) 0.05 and r2 ) 0.21, respectively).
A more satisfactory description of solvent effects on KT can
be achieved by using the solvatochromic parameters R (proton
donor ability of the solvent), â (proton acceptor ability of the
solvent), and π* (polarity-polarizability) in linear solvation free
energy relationships (eq 1, Table S2).32
Figure 3. ∆GT of compounds 1a-1e as a function of the temperature
(1a -]-; 1b -0-; 1c -*-; 1d -b-; 1e -2-).
from the monocyclic analogues 5: both donor and acceptor
substituents decrease ∆HT in the case of 1 and 2, i.e., favor the
quinonoid structure; in the case of compound 5, only donor
substituents decrease ∆HT, whereas acceptors lead to an increase
in ∆HT. In both naphthyl and phenyl derivatives, the entropic
contribution is larger for compounds substituted by more polar
groups. The influence of solvent polarity (methylcyclohexane/
toluene vs ethanol) on ∆HT is different for naphthaldehyde
Schiff bases compared to the corresponding arylazonaphthols
7-9: ∆HT values are more negative in ethanol than in
methylcyclohexane/toluene for 1 and 2 and less negative, or
even positive (9), in the case of the azo compounds.
Finally, the temperature dependence of ∆GT (in methylcy-
clohexane/toluene) for substituted derivatives of 1 is funda-
mentally different from that of the unsubstituted derivatives 1c,
2, 7, 8, and 9: the latter all show two distinct regions (300-
200 and 200-120 K, respectively), yielding two sets of
thermodynamic parameters ∆HT and ∆GT; in the case of the
substituted Schiff bases 1a, b, d, and e, no such behavior is
found. The temperature dependence of ∆GT in the range 100-
300 K of compounds 1a-e (methylcyclohexane/toluene) is
shown in Figure 3. For 2 and 7-9, previously ∆GT vs T curves
similar in shape to that of 1c (and different to the substituted
derivatives 1a, b, d, and e) have been obtained.18d
Computational Results. In the following, the intrinsic (gas
phase) stability of the individual tautomeric forms will be treated
by quantum chemical (ab initio HF and MP2, as well as density
functional theory (B3LYP)) methods. First, the reliability of the
computational procedures used will be assessed. In the second
section, various solvation models will be applied to estimate
the effect of solvent polarity on the imino phenol a enaminone
(azo a hydrazo) tautomerism. Finally, several calculated
structural and energetic parameters will be used to assess the
influence of intramolecular hydrogen bonding on KT.
log KT ) c + sπ* + aR + bâ
(1)
Anils 1 and 2 show significant terms in π* and R but not in
â. The absence of a â term can be attributed32b to strong
intramolecular hydrogen bonding (resonance-assisted H-bonds5).
As evidenced by the value of the coefficient b, this intra-
molecular hydrogen bonding is weaker in azo compounds 7 and
8 and can be broken by proton-accepting solvents. The negative
sign indicates stronger hydrogen bonding of the solvent to the
azo than the hydrazo tautomer. 4-Phenylazonaphthol 9, where
no intramolecular hydrogen bonding is possible, shows the most
negative coefficient of the â term. No such comparison can be
made in the case of Schiff bases 1, 2, and 4 since, because of
the frequently observed17d extensive dimerization/oligomeriza-
tion, KT for 4 could not be determined. All six compounds
(Table S2) show positive R and π* terms, meaning greater
stabilization of the quinone tautomer. The effect of R can be
attributed to stronger hydrogen bonding between the solvent
and the carbonyl oxygen than with the phenolic oxygen and/or
aza-nitrogen of the bridge, while the sign of π* indicates a
greater polarity of the enaminone (hydrazo) tautomers.
3. Thermodynamic Parameters. Thermodynamic parameters
of the enol imine a enaminone tautomeric equilibria for 1a-
1e obtained from the temperature dependence of KT in meth-
ylcyclohexane/toluene are summarized in Table 2. For com-
parison purposes, the corresponding data for 2 and 7-9
determined previously18d are also listed there. In apolar solvents,
e.g., methylcyclohexane/toluene, for all compounds, formation
of the quinonoid tautomer is exothermic (∆HT < 0). It is entropy
which makes the imino enol a enaminone (azo a hydrazo)
tautomeric equilibria endergonic (∆GT > 0) in apolar solvents
for all systems but 2-phenylazo-1-naphthol 7. An important
entropy contribution has also been found for salicylidene anils
5.26a However, the above-mentioned complete loss of aromaticity
makes imino enol a enaminone tautomerization of 5 not only
endergonic but also endothermic (∆HT > 0). Hydroxynaph-
thaldehyde Schiff bases 1 and 2 differ also in another respect
1. Intrinsic Tautomer Stability. First, the reliability of
different computational procedures (HF, B3LYP, MP2) and
basis sets with respect to the calculation of tautomerization
energies, will be addressed. Since the tautomerization enthalpy,

Hydroxynaphthaldehyde Schiff Base Tautomerism
J. Phys. Chem. A, Vol. 108, No. 37, 2004 7607
TABLE 3: Calculated Relative Enthalpies, ∆Ha, for
Different Conformations of the Enol Imine and Enaminone
Tautomer and Transition State for Proton Transfer of
Compound 1c
TABLE 4: Comparison of Calculated Structural Data of
Compound 1c Pertinent to Intramolecular Hydrogen
Bonding with Averaged X-Ray Dataa
Ib
IIc
IIId
1c
Ib
IIc
IIId
IVe
1c-A1
R(C1-O9)
1.325
1.380
1.461
1.264
0.958
1.853
2.684
1.338
1.409
1.446
1.297
1.006
1.682
2.593
1.348
1.405
1.447
1.286
0.937
1.764
2.592
A1
A2
A3
A4
H1
H2
TS
0.0
15.0
10.5
10.7
-0.3
7.2
0.0
14.4
9.9
9.5
4.6
12.6
1.6
0.0
0.0
14.9
10.7
10.5
5.5
14.6
3.2
R(C1-C2)
15.5
11.4
11.1
-0.1
8.3
R(C2-C11)
R(C11-N12)
R(O9-H10)
R(N12-H10)
R(O9-N12)
<(O9-H10-N12)
-0.4
1.2
143.6
148.5
146.3
e
∆
0.041
1.220
1.461
1.368
1.331
1.912
1.002
2.676
0.025
1.260
1.463
1.393
1.339
1.738
1.036
2.606
a In kcal mol-1; for structures of conformers, see Scheme 2.
b B3LYP/6-311+G**//HF/6-31G* + p(H). c MP2/6-311+G**//HF/6-
31G* + p(H). d B3LYP/6-311+G**//B3LYP/6-31+G**. e MP2/6-
311+G**//B3LYP/6-31+G**.
1c-H1
R(C1-O9)
1.295
1.428
1.421
1.304
1.740
0.939
2.558
R(C1-C2)
R(C2-C11)
R(C11-N12)
R(O9-H10)
R(N12-H10)
R(O9-N12)
<(O9-H10-N12)
∆HT, of 1c is remarkably insensitive to the solvent used (Table
2), it should be a good approximation to the corresponding
calculated gas-phase value. Consequently, we use this datum
to estimate the reliability of our computational procedures.
Extensive testing (for total energies, see Tables S3-S5 and for
tautomerization energies, see Table S6 of the Supporting
Information) indicates that (i) except at the lowest levels, the
basis set and procedure (HF, B3LYP) used for geometry
optimization are of little relevance; (ii) with B3LYP, a slightly
greater stability of the quinone (enaminone) tautomer is
obtained, whereas HF, and even more so, MP2 predict a greater
stability of the phenol (enol imine) form of 1c (Table 3).
Interestingly, for the bis-azo dye Sudan III not only B3LYP
but also MP2 calculations predict the hydrazo (quinone)
tautomer to be more stable,25a indicating that care has to be
taken in generalizations, even for closely related systems. As a
reasonable tradeoff between computational costs and reliability
of tautomerization energies, we suggest using B3LYP/
6-311+G** single point calculations at HF/6-31G* + p(H)
geometries (here, p(H) indicates a basis set with p-type
polarization functions (R ) 1.1) added only to the moveable
hydrogen). However, energies are only one aspect; intra-
molecular hydrogen bonding (resonance assisted hydrogen
bonding5) is expected to play a crucial role for the tautomer-
ization equilibria as demonstrated for simple enaminones7 or
nitroethylenes.8 Apparently, for a correct description of the
geometry of such systems, electron correlation has to be taken
into account by MP2 or DFT methods.33 A comparison of
calculated pertinent structural parameters for the intramolecular
hydrogen bond with averaged X-ray data from a CSD search34
is shown in Table 4. Clearly, B3LYP geometries are generally
superior to those obtained by Hartree-Fock calculations,
whereas B3LYP energies are surprisingly insensitive to the
geometry used (Table 3).35 Most important, B3LYP/6-311+G**
results (∆HT ) -0.1 kcal mol-1 (B3LYP/6-31+G** geometry)
and ∆HT ) -0.3 kcal mol-1 (HF/6-31G* + p(H) geometry),
Table 3) are in good agreement with the experimental tautomer-
ization enthalpy for 1c determined in methylcyclohexane/toluene
(∆HT ) -0.5 kcal mol-1, Table 2).
130.8
0.077
138.5
0.040
144.2
e
∆
a Distances in Å, angles in degrees. b HF/6-31G* + p(H). c B3LYP/
6-31+G**. d X-ray. e Mean unsigned error.
TABLE 5: Calculated Gas Phase Tautomerization
Enthalpies for 1-9a
Ib
IIc
Ib
IIc
1a
1b
1c
1d
1e
2
0.06
0.15
-0.33
-0.02
-0.15
0.60
0.19
0.25
-0.12
0.16
5a
5b
5c
5d
5e
6
7
8
9
5.38
5.60
5.27
5.85
5.78
4.43
4.67
4.50
4.98
4.94
0.00
0.81
8.64
7.98
d
3
4
9.55
1.25
-
-2.28
-1.57
0.62
-1.90
-0.99
-0.52
1.93
a In kcal mol-1 b B3LYP/6-311+G**//HF/6-31G* + p(H). c B3LYP/
.
6-311+G**//B3LYP/6-31+G**. d With B3LYP/6-31+G**, no stable
minimum for the quinone tautomer could be found.
TABLE 6: Estimateda (∆GTgas) and Calculated Gas Phase
Tautomerization Gibbs Free Energiesb
gas
∆GT
Ic
IId
1c
2c
7
8
9
1.961
-0.76
0.09
-2.33
-1.26
0.64
-0.87
0.12
-1.94
-1.06
-0.60
2.859
-0.027
0.585
3.063
a From linear solvation energy relationships. b In kcal mol-1 c B3LYP/
.
6-311+G**//HF/6-31G* + p(H). d B3LYP/6-311+G**//B3LYP/6-
31+G**.
and r2 ) 0.997 (B3LYP/6-31+G** geometry)) provided, for
1c, the low-temperature value of ∆HT is used.
Gas-phase tautomerization free energies, ∆GT, can be ex-
trapolated with the aid of the linear solvation energy relation-
ships described above (eq 1, π* ) -1.23 in vacuo32c) (Table
6). As expected, in the gas phase, the equilibrium is shifted
toward the less polar enol imine (azo) tautomer (∆GT ) 1-3
kcal mol-1); only for 2-phenylazo-1-naphthol 7, even in the gas
phase, the hydrazo form should slightly be predominant.
Calculated ∆GT values generally are too low, but the trend is
in good to excellent (HF/6-31G* + p(H) geometry, r2 ) 0.971;
Having established an appropriate computational procedure,
we now turn to the substituent effect on the tautomeric equilibria
of 1-hydroxy-2-naphthaldehyde Schiff bases 1a-1e. Calculated
tautomerization enthalpies for 1-9 are collected in Table 5 and
Table S7 of the Supporting Information. Generally, the stability
of the enaminone tautomer is underestimated (calculated ∆HT
values less negative than experimental ones). The calculated
influence of substituents on KT of 1-hydroxy-2-naphthaldehyde
Schiff bases, however, is in excellent agreement with the
experimental results (r2 ) 0.966 (HF/6-31G* + p(H) geometry)
gas
calc
∆GT ) 1.14∆GT + 2.52) agreement with experiment.
For anil 4, which until now has eluded experimental deter-
mination of KT, the above regression equation yields KT ≈ 0.004.
Completely in line with experimental data,24a,26a the calculations
result in considerably larger ∆GT values, i.e., greater stability

7608 J. Phys. Chem. A, Vol. 108, No. 37, 2004
Fabian et al.
TABLE 7: Solvent Effect on Tautomerization Gibbs Free
Energies of Compound 1c
intramolecular hydrogen bonds, several criteria, e.g.,
geometric, energetic and topological, have been
suggested,5a,b,7c,8,10,25e,29,39 with each one having some advantages
or shortcomings.10a Several of these criteria (N‚‚‚O distance,
conformational energy differences, barrier to proton transfer,
homodesmotic reactions and NBO analysis) will be described
in the following.
exp
solvent
∆GT
Ia
IIb
IIIc
IVd
Ve
H2O
DMSO
acetonitrile
methanol
ethanol
acetone
CH2Cl2
toluene
cyclohexane
Et2O
-0.67 -4.78 -2.50 -4.54 -4.74 -4.29
0.11 -3.85 -2.03 -4.06 -2.81 -3.50
-0.13 -3.85 -2.07 -4.07 -2.76 -3.52
-0.09 -3.94 -2.45 -3.98 -4.55 -3.70
-0.24 -3.78 -2.42 -3.97 -4.47 -3.48
0.47 -3.72 -2.01 -3.89 -2.68 -3.38
0.25 -3.35 -1.81 -3.52 -2.45 -3.13
0.71 -2.17 -1.34 -2.18 -1.92 -1.94
0.95 -1.95 -1.14 -2.01 -1.66 -1.70
0.84 -2.92 -1.83 -3.00 -2.75 -2.51
1.96 -0.76 -0.76 -0.87 -0.87 -0.87
3.1. Geometric Criteria for Hydrogen Bonding. Among the
geometric criteria, the N‚‚‚O distance apparently gives a good
indication of H-bond strengths, whereas the N-H lengthening
due to strong (resonance-assisted) H-bond formation is small
or even irrelevant.5b We find that the differences between the
calculated (B3LYP/6-31+G** geometry) N‚‚‚O distances (Table
S10) of the enol imine and enaminone tautomers, respectively,
of compounds 1a-e, 2c, 5a-e, 7, 8, and 10 correlate quite
nicely with the corresponding KT values (r2 ) 0.979). It is also
evident from these data that the stronger hydrogen bond (shorter
N‚‚‚O distance) is associated with the less stable tautomer.5a,b,7,8
Interestingly, despite less reliable absolute structural parameters,
a similarly good correlation is also obtained with HF/6-31G*
+ p(H) geometries (r2 ) 0.981). For the aliphatic analogue 10
of the 2-hydroxy-1-naphthaldehyde Schiff base 2, the calculated
(HF/6-31G* + p(H) geometries) N‚‚‚O distances indicates
stronger intramolecular hydrogen bonding than in case of
compound 2. The difference, however, is comparable to the anils
and, thus, this derivative fits quite well to the r(N‚‚‚O) vs log
KT correlation (log KT(CHCl3) ) 0.40,31 r2 ) 0.980). The
importance of intramolecular hydrogen bonding on KT is further
evidenced by r(N‚‚‚O) calculated for 1c in different solvents
(B3LYP/6-31+G**, IEF-PCM, r2 ) 0.859). For the solvent
effect on vibrational frequencies,6c we find for 1c a decrease
for ν (O-H) and an increase for ν (N-H), respectively, with
increasing solvent polarity (Table S11). Since both ν (O-H)
and ν (N-H) are linearly correlated with the respective N‚‚‚O
distance and, thus, inversely with the strength of the respective
intramolecular H-bond, this behavior indicates strengthening of
the O-H‚‚‚N and weakening of the N-H‚‚‚O H-bond by polar
solvents with a concomitant shift of the tautomeric equilibrium
toward the enaminone form. This exactly is observed experi-
mentally.
3.2. Conformer Energies. Despite some shortcomings,7,8 the
difference between energies of conformers with and without
intramolecular hydrogen bonds (Scheme 2) or rotational barriers
has widely been used as an energetic criterion for H-bond
strengths.5a,b,7,8 In the case of the parent substituted ethylenes,7,8
by judiciously choosing the respective conformations (E/Z and
syn/anti, Scheme 2), at least some of the inherent drawbacks
can be accounted for; in the case of the naphthaldehyde Schiff
bases or azo derivatives, not all E/Z isomers exist. Furthermore,
severe steric hindrance can also cause strong repulsive non-
bonded interactions in syn/anti isomers, making H-bond energies
evaluated in this way largely meaningless.5b Especially in the
quinone tautomers of 2 and 8, peri interactions lead to
considerably distorted structures for 2c-H2 and 8-H2. However,
despite these deficiencies, conformational energy differences,
summarized in Table S12, can provide at least some crude
estimate of intramolecular H-bond strengths for these com-
pounds. It is well-known that the hydrogen-bond strength
decreases with increasing solvent dipole moment; for instance,
in â-aminoacrolein, a reduction from ca. 5 kcal mol-1 in the
gas phase to ca. 2-3 kcal mol-1 in aqueous solution (PCM,
MP2/6-31G**) was found.7a Therefore, we have evaluated
conformational energy differences also for aqueous, and in some
selected cases, acetonitrile solution (PCM, B3LYP/6-311+G**
gas phase
a PB-SCRF/B3LYP/6-311+G**//HF/6-31G* + p(H). b PCM/B3LYP/
6-311+G**//HF/6-31G* + p(H). c PB-SCRF/B3LYP/6-311+G**//
B3LYP/6-31+G**. d PCM/B3LYP/6-311+G**//B3LYP/6-31+G**.
e IEF-PCM/B3LYP/6-31+G**//IEF-PCM/B3LYP/6-31+G**.
of the aromatic phenol tautomer, for salicylidene anil 5 and the
corresponding para isomer 6.
2. Solvent Effects on Calculated Tautomerization Ener-
gies. Solvent polarity profoundly affects the imino enol a
enaminone (azo a hydrazo tautomerism) in Schiff bases (azo
phenols) by stabilizing the more polar quinone forms, although
frequently no clear-cut relation between KT and solvent polarity
(relative permittivity, D) could be established.27 To estimate
solvent effects on calculated tautomerization energies, both bulk
solvation models,7a,27a as well as explicit solvent molecules, have
been used.26a,27a,e,f In the following, several variants36 of the
polarizable continuum (PCM,36b,c IEF-PCM,36d CPCM,36e
SCIPCM36f) and the Poisson-Boltzmann SCRF (PB-SCRF)37
solvation model will first be applied to the tautomeric equilib-
rium of 1c. In addition to single point calculations on gas-phase
geometries, the effect of optimization in solvent (IEF-PCM/
B3LYP/6-31+G**) on calculated KT values will also be
addressed (Tables 7, S8, and S9).
We find that even for apolar solvents, e.g., cyclohexane, bulk
solvation models greatly overestimate the stabilization by solvent
of the enaminone tautomer yielding considerably too negative
∆GT values. In contrast, the general trend exerted by solvents
of quite different polarity and protic/aprotic nature can be
described very successfully by these models (Table 7).
Optimization in solvent (IEF-PCM, B3LYP/6-31+G**) does
not further improve the results. Previously, a detailed investiga-
tion on the solvent effect (aqueous solution) on tautomer and
conformer stabilities, geometries, and strength of intramolecular
hydrogen bonds in the prototype â-aminoacrolein7a also had
indicated that full optimization in solvent is not really necessary.
Instead, inclusion of explicit solvent molecules, even at a
moderate level of theory, appears more promising.26a,27a In
contrast to the substituted salicylidene anils 5a-e (Table S8),
for the analogous Schiff bases of the isomeric hydroxy naph-
thaldehydes, 1a-e and 2a-e, neither the polarizable continuum
nor the PB-SCRF model correctly describes the solvent effects,
even in cyclohexane (!), on KT values (Table S9 of the
Supporting Information).
3. Effect of Intramolecular Hydrogen Bonding. Intramo-
lecular O-H‚‚‚N and O‚‚‚H-N hydrogen bonding is a promi-
nent feature of hydroxy azobenzenes and the analogous Schiff
bases, comparable to, if not even more important than, the
homonuclear O-H‚‚‚O bond in enols of â-dicarbonyl com-
pounds.5,6,9,14d,20,21,38 Computationally, the importance of in-
tramolecular hydrogen bonding on the position of the tautomeric
equilibria of the parent â-aminoacrolein and in nitroethylenes
has been studied in detail.7,8 To address the strength of

Hydroxynaphthaldehyde Schiff Base Tautomerism
J. Phys. Chem. A, Vol. 108, No. 37, 2004 7609
SCHEME 2: Conformations Used for Estimating the
Strength of Intramolecular Hydrogen Bonds
sensitive to the problems associated with conformer energy
differences. From the data given in Table S12, one can deduce
gas-phase PT barriers of 1.43, 1.35, 3.76, 0.62, and 0.68 kcal
mol-1 for the phenol tautomers of compounds 1c, 2c, 5c, 7,
and 8, respectively, and of 2.30, 1.23, -0.15, 2.56, and 1.74
kcal mol-1 for the corresponding quinonoid forms. In the case
of the naphthalene derivatives, the phenol (imino enol (1) or
azo (7, 8)) tautomer has a lower or equal (2) barrier (stronger
hydrogen bond) than the enaminone (hydrazo) forms in line
with the slightly greater stability of the latter tautomeric forms.
In contrast, for the salicylidene anil 5 the PT barrier for the
quinonoid form essentially vanishes. Not surprisingly, for
salicylidene anils, detectable amounts of the enaminone tautomer
are present only in highly polar media (see PCM data in Table
S12).
3.4. Homodesmotic Reaction. As an alternative to conforma-
tional energy differences, homodesmotic reactions have been
proposed to assess the strength of intramolecular hydrogen
bonding.39a Relevant reaction energies (see Scheme 3 for the
reactions used) are summarized in Table S13. For enol imine
(azo) tautomers of para derivatives (4, 6, and 9) the analogous
homodesmotic reaction energies are quite small (<0.1 kcal
mol-1), indicating that, essentially, pure hydrogen-bonding
energies are described thereby. For the corresponding enaminone
(hydrazone) tautomers, larger reaction energies are obtained
(<2.5 kcal mol-1 for 4 and 9; 5.2 kcal mol-1 for 6), making
these data less reliable as measures for H-bond strengths.
Although the data in Table S13 are corrected for this para effect,
hydrogen bonding appears to be overestimated for keto amine
(hydrazo) tautomers. Comparison within one tautomeric series
(enol imine (azo) or enaminone (hydrazo)), however, should
be possible. For both tautomeric species, the azo analogues are
predicted to have weaker hydrogen bonds than the corresponding
Schiff bases (8A vs 2c-A, 8H vs 2c-H; 7A vs 1c-A; and 7H
vs 1c-H, Table S13) as also deduced from the experimentally
determined solvent effects on KT. Replacing phenyl by alkyl
(2 vs 10) results in stronger hydrogen bonding.
3.5. NBO Analysis. Based on a detailed experimental (X-ray
crystallography, IR, and NMR spectroscopy), as well as a DFT
computational study,5a-c an essentially covalent nature for the
H-bond in the proton-transfer TS and a more covalent character
for the O-H‚‚‚N than the N-H‚‚‚O one has been inferred.5a
Recently, the second-order stabilization energy resulting from
charge transfer of the hydrogen acceptor lone pair orbital (lp-
(X)) toward the antibonding σ* (Y-H) bond of a Y-H‚‚‚X
hydrogen bond, E(2) (lp(X) f σ* (Y-H)), as derived from an
NBO analysis,40 has been used for H-bond characterization.41
In line with the above conclusions, the largest values for E(2)
are obtained for TSs (E(2) ) 150-250 kcal mol-1), to be
single point calculations, Table S12). Generally, a drop of ca.
4 kcal mol-1 (in some cases up to 15 kcal mol-1) for
conformational energy differences is obtained by the PCM
model. Only for transition states is almost no effect found.
Apparently, in TSs the intramolecular hydrogen bond is strong
enough to resist being broken by solvent.
3.3. Barriers to Proton Transfer. The importance of the proton
transfer (PT) transition state on the strength, electrostatic/
covalent nature, and single- vs double-well character of
resonance-assisted intramolecular hydrogen bonds in enols of
â-dicarbonyl compounds and/or enaminones and related het-
erodienes, as well as for classification of hydrogen bonds in
general, has been stressed recently.5a,c Chemical modification
results in a splitting of the two competing H-bonds to a higher-
energy tautomer with a stronger hydrogen bond, being closer
to the PT transition structure, and a more stable tautomer, which
is farther from the barrier and, thus, has weaker hydrogen
bonding.5a Such an inverse relationship between the proton-
transfer barrier and the strength of the hydrogen bond has
already been established earlier by Scheiner.29a Proton-transfer
barriers as an index of hydrogen bond strength should be less
SCHEME 3: Homodesmotic Reactions Used for Estimating the Strength of Intramolecular Hydrogen Bonds

7610 J. Phys. Chem. A, Vol. 108, No. 37, 2004
Fabian et al.
compared to those for O-H‚‚‚N (E(2) ) 30-36 kcal mol-1
)
Computational Details. Geometries of all structures were
completely optimized for the gas phase using ab initio (HF,
MP242) and density functional (B3LYP43) methods. Thermo-
dynamic quantities (∆H and ∆G corrections) were calculated
by the standard rigid rotor-harmonic oscillator approximation
and are unscaled. Effects of a variety of solvents of different
polarity were estimated by single point calculations on the gas-
phase geometry using several variants of the polarizable
continuum model36a (PCM,36b,c IEF-PCM,36d CPCM,36e and
SCIPCM36f) and the Poisson-Boltzmann (PB-SCRF)37 model.
For some selected compounds, optimization with inclusion of
solvent effects (IEF-PCM) was also done. The programs used
were Jaguar44 and Gaussian 98.45
and N-H‚‚‚O (E(2) ) 16-21 kcal mol-1) bridges. However,
no clear three-center bonds were detected by the NBO analysis.
Conclusion and Outlook
Imino enol a keto amine tautomerization constants KT and
thermodynamic parameters ∆HT and ∆ST for 1-hydroxy-2-
naphthaldehyde Schiff bases 1a-e have been determined by a
chemometric analysis of the respective UV/vis spectra. Both
donor and acceptor substituents in the para position of the N-aryl
moiety decrease KT. Polar solvents shift the equilibria toward
the quinone tautomer. Ab initio (HF, MP2) and density
functional theory (B3LYP) calculations are used to rationalize
the experimental findings. Among the computational procedures,
B3LYP calculations yield the best agreement with experimental
∆HT values. Hartree-Fock and, especially, MP2 overestimate
the stability of the phenol tautomers. Comparison is made with
Schiff bases of the positional isomers 2-hydroxy-1-naphthal-
dehyde 2a-e, 3-hydroxy-2-naphthaldehyde 3, and 4-hydroxy-
1-naphthaldehyde 4, as well as the corresponding azo analogues
7-9. Replacing the N-aryl group by N-alkyl (2 vs 10) strongly
decreases KT. Intramolecular hydrogen bonding is characterized
by geometric (r(N‚‚‚O)) and energetic (conformer energies,
homodesmotic reactions, proton-transfer barriers) criteria. In line
with conclusions drawn from linear solvation energy relation-
ships, the intramolecular hydrogen bond in azo analogues is
found to be weaker than in the corresponding Schiff bases. The
importance of intramolecular hydrogen bonds on the tautomeric
equilibria is indicated by a clear correlation between KT and
the differences between r(N‚‚‚O) of the respective enaminone
(hydrazo) and imino enol (azo) tautomers. Solvent effects on
substituted derivatives of salicylidene anils 5a-e, as well as
on 1c, can be quite successfully described by the polarizable
continuum model (PCM) or the Poisson-Boltzmann SCRF (PB-
SCRF) procedure. In contrast, solvent effects on the substituted
derivatives of 1 and 2 are rather poorly described by either
solvation model. This drawback needs further scrutiny, possibly
by inclusion of explicit solvent molecules in the calculations.
Investigations along these lines are in progress.
Acknowledgment. L.A. thanks The European Community-
Access to Research Infrastructures action of The Improving
Human Potential (contract/grant no. HPRI-CT-1999-00064) for
financial support.
Supporting Information Available: Tables for solvent
dependence of KT of 1c (Table S1), LSER correlations (S2),
total (S3-S5) and relative energies (S6-S7), PCM results for
5 (S8) and 1 (S9), N‚‚‚O distances (S10), solvent effect on
r(N‚‚‚O) and vibrational frequencies (S11), conformational
energies (S12), and homodesmotic reaction energies (Table S13).
This material is available free of charge via the Internet at
http://pubs.acs.org.
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Experimental Section
Synthesis and Determination of KT. Compounds 1a-e have
been synthesized by a conventional condensation method and
then purified by multiple recrystallizations to constant melting
point and, if needed, by column chromatography. The purity
has been checked by GC/MS.
The solvents used (listed in Table 7) were of spectral or extra
pure grade. The absorption spectra at room temperatures were
recorded on a Varian Carry 50Bio UV/vis spectrophotometer.
The low-temperature spectra (100-300 K) were recorded on a
home-built spectrophotometer using a specially constructed 6
mm thick cell cooled by a Cryodyne (CTI Cryogenics) model
21 closed-cycle helium cryostat. An ordinary deuterium lamp
was used as light source, the signal was detected by a PC2000
Miniature Fiber Optic Spectrometer (Ocean Optics, Inc.) and
initially processed by OOIBase 32 (Ocean Optics, Inc.) software.
The temperature was measured with 2-3K precision. At each
temperature, reference spectra of the solvent were recorded.
EtOH and methylcyclohexane/toluene 1:1 were used as solvents
because they form a transparent glass upon cooling. All spectra
were corrected for solvent contraction and processed as de-
scribed in the literature.18d
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Hydroxynaphthaldehyde Schiff Base Tautomerism
J. Phys. Chem. A, Vol. 108, No. 37, 2004 7611
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