Ground and excited state hydrogen atom transfer reactions and cyclization of 2-acetylbenzoic acid

Article abstract of DOI:10.1021/jp048330j

We present experimental and theoretical studies of the ring-chain tautomerism (H-atom transfer and cyclization) for 2-acetylbenzoic acid at both ground and electronically first excited states. ¹H and ¹³C NMR studies in solution confirm the existence of equilibrium between the open and ring structures at the ground state, with the ring one being dominant (～90%). Temperature-dependent ¹H NMR experiments allowed obtaining the thermodynamic and kinetic parameters at the coalescence temperature (380 K). Fluorescence measurements disclose the involvement of highly efficient nonradiative processes in agreement with the theoretical data. Electronic calculations for the ground state give additional information on the different conformers of the open tautomer. In agreement with the experiment the most stable structure is of the closed ring tautomer, and it is obtained after additional internal rotations of the -COOH and -CO(CH₃) fragments. Intrinsic reaction coordinate calculations indicate that the ring formation/breaking and the H-atom transfer are taking place in a concerted but not synchronous manner. At S₁ the most stable form is the open one, for which different conformers are also found. The influence of the solvent is also accounted for through a model that considers the solvent as a continuum at both the ground and excited electronic states. No major differences were observed when comparing both gas and condensed phase results, so calculations of the isolated molecule should give a picture of the reaction which is experimentally observed in solution.

Full text of DOI:10.1021/jp048330j

J. Phys. Chem. A 2004, 108, 9331-9341
9331
ARTICLES
Ground and Excited State Hydrogen Atom Transfer Reactions and Cyclization of
-Acetylbenzoic Acid
2
†
‡
,§
|
§
,‡
L. Santos, A. Vargas, M. Moreno,* B. R. Manzano, J. M. Lluch, and A. Douhal*
Departamento de Qu ´ı mica-F ´ı sica, Facultad de Ciencias Qu ´ı micas, UniVersidad de Castilla-La Mancha,
3071 Ciudad Real, Spain, Departamento de Qu ´ı mica-F ´ı sica, Secci o´ n de Qu ´ı micas, Facultad de
1
Ciencias del Medio Ambiente, UniVersidad de Castilla-La Mancha, AVenida Carlos III, S.N.
4
5071, Toledo, Spain, Departament de Qu ´ı mica, Facultat de Ci e` ncies, UniVersitat Aut o´ noma de Barcelona,
0
8193 Bellaterra, Barcelona, Spain, Departamento de Qu ´ı mica Inorg a´ nica, Org a´ nica y Bioqu ´ı mica,
Facultad de Ciencias Qu ´ı micas, UniVersidad de Castilla-La Mancha, 13071 Ciudad Real, Spain
ReceiVed: April 16, 2004; In Final Form: July 9, 2004
We present experimental and theoretical studies of the ring-chain tautomerism (H-atom transfer and cyclization)
1
13
for 2-acetylbenzoic acid at both ground and electronically first excited states. H and C NMR studies in
solution confirm the existence of equilibrium between the open and ring structures at the ground state, with
1
the ring one being dominant (∼90%). Temperature-dependent H NMR experiments allowed obtaining the
thermodynamic and kinetic parameters at the coalescence temperature (380 K). Fluorescence measurements
disclose the involvement of highly efficient nonradiative processes in agreement with the theoretical data.
Electronic calculations for the ground state give additional information on the different conformers of the
open tautomer. In agreement with the experiment the most stable structure is of the closed ring tautomer, and
it is obtained after additional internal rotations of the -COOH and -CO(CH
coordinate calculations indicate that the ring formation/breaking and the H-atom transfer are taking place in
a concerted but not synchronous manner. At S the most stable form is the open one, for which different
3
) fragments. Intrinsic reaction
1
conformers are also found. The influence of the solvent is also accounted for through a model that considers
the solvent as a continuum at both the ground and excited electronic states. No major differences were observed
when comparing both gas and condensed phase results, so calculations of the isolated molecule should give
a picture of the reaction which is experimentally observed in solution.
1. Introduction
SCHEME 1
In molecules bearing both proton (or hydrogen atom) donor
and acceptor groups, an intramolecular hydrogen bond is
generally formed and observed when the involved groups are
1
in suitable positions. Thermal or photonic activation of these
systems may lead to a proton (or hydrogen atom) transfer
reaction and thus produces new products. Therefore, great effort
has been made for a better understanding and control of this
2
reaction. The phenomenon of ring-chain tautomerism is also
possible and plays an important role in many aspects of
chemistry and biology.3 Such tautomerism has been shown to
occur in a number of oxocarboxylic acids giving rise to both
closed (ring) and open (chain) forms. Molecules showing this
kind of reaction can be used as starting materials for potential
use in nanotechnology such as molecular memories. In this
work, we will focus on 2-acetylbenzoic acid (AB), a molecule
which exhibits such a reaction (Scheme 1).
,4
The whole reaction implies a rearrangement in which the acid
hydrogen atom (H12) migrates from the carboxylic group to the
acetyl one. The process yields the formation of a new C-O
bond so that an additional ring is formed. The dynamics and
issues of this kind of reaction are determined by the shape of
*
Corresponding
authors.
Fax:
+34-925-268840.
E-mail:
1,5
the multidimensional potential energy surface (PES). Surpris-
abderrazzak.douhal@uclm.es (A.D.); Miquel.Moreno@uab.es (M.M.).
†
ingly, little work on the ring-chain tautomerism of oxocar-
Departamento de Qu ´ı mica-F ´ı sica, Facultad de Ciencias Qu ´ı micas,
6-9
Universidad de Castilla-La Mancha.
boxylic acids has been reported. Experimental determination
‡
Departamento de Qu ´ı mica-F ´ı sica, Secci o´ n de Qu ´ı micas, Facultad de
of the equilibrium constant for the ring-chain tautomeric
Ciencias del Medio Ambiente, Universidad de Castilla-La Mancha.
6
equilibrium was done by Finkelstein et al. They obtained the
§
Universitat Aut o´ noma de Barcelona.
1
|
UV-IR absorption spectra and also the H NMR spectra of
Departamento de Qu ´ı mica Inorg a´ nica, Org a´ nica y Bioqu ´ı mica, Facultad
de Ciencias Qu ´ı micas, Universidad de Castilla-La Mancha.
2-acetylbenzoic acid (and of some 6-derivatives), concluding
1
0.1021/jp048330j CCC: $27.50 © 2004 American Chemical Society
Published on Web 10/02/2004

9332 J. Phys. Chem. A, Vol. 108, No. 43, 2004
Santos et al.
1
that the ring tautomer is the dominant form. The IR and H
NMR spectra of the same molecule were also reported by
7a
Bowden and Taylor. Their results agree with the previous one
and show that the closed structure is the dominant one. The
relative populations of both tautomers and their equilibrium
constants depend on the experimental conditions (nature of the
solvent and the initial concentration of the 2-acetylbenzoic acid).
The structure of AB was studied by X-ray diffraction, and it
8
corresponds to the phthalide (ring) form. The intermolecular
bond in the crystal is made up of only one type of hydrogen
bond formed between the hydrogen-12 of one molecule and
oxygen-8 of the carbonyl group of the neighboring molecule
(Scheme 1).
More recently, Fabian and co-workers have carried out a
9
theoretical work on the subject. They restricted the study to
the relative stability of the two tautomers and analyzed the
possible conformers that result from rotations around the
C′1-C′7 and C′2-C′10 single bonds in the open form (Scheme
Figure 1. Room-temperature UV-visible absorption (Abs.) and
emission (Em., λex.) 260 nm) (s) spectra of 2-acetylbenzoic acid (AB)
1
). The calculations were performed at several ab initio and
DFT levels of theory, and the solvent effect was also included
by means of a continuum method by using the dielectric constant
of water. Their results, in terms of Gibbs free energy differences,
fail to reproduce the experimental observations as one of the
open conformations was always the most stable structure in the
gas phase. However, upon introduction of the solvent effect,
the stability pattern was reversed and the ring tautomer became
the lowest energy structure in water solution.
The purpose of this work is to analyze the mechanism of the
ring-chain tautomerism of 2-acetylbenzoic acid. Therefore,
results of experiments devoted to evaluating not only the
equilibrium constant but also the kinetic parameters of the
reaction are presented. From the theoretical point of view,
calculations were done to analyze the potential energy change
in order to understand the mechanism of such a reaction.
Molecules related to 2-acetylbenzoic acid such as the derivatives
of salicylic acid have recently been the focus of attention
because upon irradiation the difference in stability of the
different tautomers makes these molecules potential material
for optical memories.10 In this sense, we have not restricted the
study of 2-acetylbenzoic acid to the ground electronic state, but
we have also analyzed the first singlet excited electronic state
behavior. Given that experiments are carried out in liquid phase,
the solvent effect is also accounted for through the use of a
continuum model.
in methanol. Excitation spectrum (---) of AB in methanol for λobs
)
3
50 nm.
approach.12,13 For the excited S1 state, a configuration interaction
all-single-excitations with a spin-restricted Hartree-Fock refer-
ence ground state (CIS) has been used to optimize geometries.14
Single point energies of the excited state have also been
calculated by the time-dependent formalism within the B3LYP
15
functional (TD-DFT (density functional theory) calculations).
All calculations have been done with the split-valence 6-31+G(d)
basis set, which includes a set of d polarization functions and
16
a set of sp diffuse functions on atoms other than hydrogens.
For comparative purposes, we have also used the complete
17
active space self-consistent field method (CASSCF) to opti-
mize the geometries in both the ground and the excited electronic
states.
Full geometric optimization and direct location of stationary
points (minima and transition states) have been carried out by
18
means of the Schlegel gradient optimization algorithm by using
redundant internal coordinates as implemented in the Gaussian
11
9
8 package. Diagonalization of the energy second-derivative
matrix has been performed to disclose the nature of each
stationary point: no negative eigenvalues indicate a minimum
whereas one negative eigenvalue identifies a transition state.
To establish the connection between the transition states and
the corresponding equilibrium structures, the reaction pathways
were followed using the intrinsic reaction coordinate procedure
2
. Experimental and Theoretical Details
.1. Experimental Section. 2-Acetylbenzoic acid (Sigma-
1
9
(
IRC). Diagonalization of the second-derivative matrix also
2
provides the vibrational harmonic frequencies that were used
for evaluating the thermodynamic corrections at 1 atm and
Aldrich, >99%) was sublimed and recrystallized in cyclohexane,
and its purity was checked and confirmed by fluorescence
spectroscopy. Sigma-Aldrich (>99%) solvents were spectro-
grade. Absorption and emission spectra were recorded on Varian
2
98.15 K to the initially obtained electronic energy by using
the standard statistical formulas, assuming that the system
behaves as an ideal gas and that the rotational and vibrational
degrees of freedom are well described through the rigid rotor
(
Cary E1) and Perkin-Elmer (LS-50B) spectrophotometers,
1
13
1
respectively. H and C{ H} NMR spectra (CDCl3 or DMSO-
d6) were recorded on a Varian Unity 300 spectrometer. Chemical
shifts (ppm) are given relative to TMS. Coupling constants are
given in hertz. H- C COSY spectrum was produced with a
standard pulse sequence with an acquisition time of 0.076 s, a
pulse width of 90°, a relaxation delay of 1 s, number of scans
of 20, and number of increments of 160. For variable temper-
ature spectra the probe temperature ((1 K) was controlled by
a standard unit calibrated with a methanol reference.
2
0
and harmonic approximations, respectively. The bulk effect
of the solvent has been introduced through the isodensity surface
2
1
1
13
polarized continuum model (IPCM). We have used an
electronic density of 0.0005 au to define the cavity in this model.
The IPCM calculations have been carried out both in S0 and S1
without reoptimization of the geometries.
3
. Results and Discussion
2
.2. Theoretical Calculations. Ab initio calculations have
3.1. Experimental Results. AB is very poorly soluble in
apolar solvents, so the experiment has been done in a polar
medium. Therefore, in methanol (Figure 1) the molecule shows
11
been performed using the Gaussian 98 series of programs. The
S0 state has been studied using the B3LYP density functional

H-Atom Transfer Reactions and Cyclization of AB
J. Phys. Chem. A, Vol. 108, No. 43, 2004 9333
TABLE 1: Values of Free Energy of Activation for the Ring T Chain Transformation of AB in 1,1′,2,2′-Tetrachloroethene-d
and of Other Parameters Deduced from NMR Experiment (See Text)
2
a
b
c
δνd
195
e
f
∆G°#(RfO)g
19
∆G°#(OfR)h
18
T
3
c
P
O
P
R
k
93
RfO
k
OfR
408
80
0.18
0.82
a
Coalescence temperature in K. b Relative population of the open form. Relative population of the ring form. Difference in chemical shift
c
d
e
-1
f
-1
(
in Hz) between the open and ring forms. Rate constant (in s ) for the ring f open reaction. Rate constant (in s ) for the open f ring reaction.
Gibbs free energy of activation (in kcal/mol) for the ring f open reaction. Gibbs free energy of activation (in kcal/mol) for the open f ring
g
h
reaction.
a structured UV absorption band with peaks at 278 and 282
6
nm, a result that agrees with the findings of Finkelstein et al.
However, we found that this molecule has a very small emission
-
5
quantum yield (<10 ), so it was not possible to separate the
weak emission of the two forms. The maximum of emission in
methanol is observed at 320 nm (Figure 1). The emission band
extends its blue tail up to 500 nm, suggesting a very weak
emission around 450 nm and probably originating from the open
structure. However, we failed to isolate this blue emission band.
Further experiments are in progress to detect this emission by
use of time-resolved emission spectroscopy. The very low
emission quantum yield is due to the involvement of bond
breaking and twisting processes and mixing of n,π* and π,π*
excitations at the S1 state, as supported by the theoretical data
of this work. The excitation spectrum of the main emission band
shows a structured shape similar to that of the absorption one.
However, the small difference in the relative peaks intensities
suggests the involvement of an efficient nonradiative process
at higher energy of excitation. It may also suggest the existence
of several conformers at the ground state, besides a strong
solvation due to the polar and H-bonding effects of solvent
methanol solution. Both suggestions agree with the theoretical
data (vide infra). Note also that the emission band is not a mirror
image of the absorption (excitation) spectrum, in agreement with
the above suggestions. Finally, relaxation to the ground state
might occur via intersystem crossing, with a possible involve-
ment of triplet state(s), and internal conversion to the ground
states of the formed isomers through bond breaking, twisting,
and H-atom transfer (see theoretical sections).
1
13
H and C NMR studies confirm the existence of equilibrium
1
Figure 2. (a) Aromatic part of H NMR (300 MHz, 273 K) spectrum
of AB in CDCl
. (b) Aromatic part of 13C NMR (300 MHz, 298 K)
spectrum of AB in CDCl
1
between the two forms (Figure 2). In the H NMR spectrum of
AB in CDCl3 at 298 K the broadening of some signals prevents
an unambiguous identification of both isomers. They are clearly
separated at 273 K in an 89:11 ratio (Figure 2a). The C{ H}
NMR spectrum (298 K) also confirms the presence of both
forms (see Figure 2b). The assignment of the resonances was
3
3
. The carbon atom numeration is that used in
Scheme 1.
1
3
1
room temperature, broadened as the temperature increased. The
coalescence was achieved at Tc ) 380 K. Because the signals
come from different populations, the calculation of the free
energy of activation was made using the Shanan-Atidi and Bar-
1
13
made using information obtained from H- C COSY and NOE
2
2
spectra. The major isomer corresponds to the ring structure,
and the ratio of the two structures changes slightly with the
temperature and the solvent, although the ring form is always
the most stable structure. The existence of equilibrium between
both forms follows from the following points: (i) a saturation
transfer between the methyl resonance of both isomers was
found when the NOE spectra were reordered; (ii) an increase
in the temperature of the CDCl3 solution from 273 K gives rise
to a broadening of the resonances.
2
3
Eli method. Using the coalescence temperature (Tc), the
relative population of both signals (PO ) population of the open
form, PR ) population of the ring form), and their difference
in chemical shift expressed (δν), the rate constants (kRfO) for
the ring f open and (kOfR) open f ring transformations could
be obtained (Table 1). Finally, the Gibbs free energies of
activation for both transformations were also calculated using
the thermodynamic formulation of the transition state theory.23
3.2. Theoretical Results. We will divide this section in two
parts. The first one is devoted to the electronic ground state
(S0), where we are mainly interested in the mechanism. The
second one deals with the first singlet excited electronic state
(S1), where we are also interested in the mechanism of the ring-
chain tautomerism.
Previous NMR studies were limited to the observation of both
species in DMSO-d6 at room temperature, and calculations of
equilibrium constants were based on the averaged values of
chemical shifts estimated from the expected ones of the
separated species. To our knowledge, our study is the first
temperature-dependent NMR experiment that gives relevant
information about energetic and kinetic parameters. Therefore,
,1′,2,2′-tetrachloroethane-d2 was chosen as solvent because its
high boiling point allows a heating of the sample to 413 K.
The two methyl signals of both isomers, observed separately at
6
7
Electronic Ground State. The results for the ground state are
depicted in Figure 3 where the geometries of the stationary
points are shown along with a scheme of their relative energies.
The related data are presented in Tables 2 and 3. The most stable
1

9334 J. Phys. Chem. A, Vol. 108, No. 43, 2004
Santos et al.
TABLE 2: Values of Potential (V) and Free (G°) Energies Relative to Those of R′′ Structure and of Dipole Moments of the
Stationary Points Located at the Ground Electronic State S
0
parameter
R′′
TS
1
′′
O
1
′′
TS
2
′′
O
2
′′
TS
3
′′
3
O ′′
V (gas phase)a
G° (gas phase)
0.0
0.0
0.0
7.9
6.92
40.6
38.2
31.8
8.71
11.91
3.7
2.1
1.4
7.42
9.73
17.2
13.5
10.2
6.43
8.87
3.8
1.0
0.6
4.85
6.43
9.3
6.9
6.2
3.0
3.4
0.5
0.8
2.17
3.09
b
b
G° (condensed phase)
c
µ (gas phase)
c
µ (condensed phase)
4.30
a
Relative potential energy in kcal/mol. b Relative Gibbs free energy in kcal/mol. c Total dipole moment in debyes.
TABLE 3: Selected Geometric Parameters of the Stationary Points Located at the Ground Electronic Potential Energy Surfacea
parameter TS ′′ ′′ TS ′′ ′′ TS ′′ ′′
R′′ d
r(7,9)b
1.37
1
O
1
2
O
2
3
O
3
1.29
2.40
1.30
1.00
2.44
1.80
61.1
-2.4
31.8
1.33
3.18
1.23
1.62
2.58
0.99
25.8
-31.7
9.9
1.39
3.11
1.22
3.69
2.78
0.97
18.2
55.8
82.9
1.35
3.08
1.22
3.31
2.85
0.98
23.7
54.9
181.7
1.35
3.08
1.22
3.12
2.87
0.98
52.8
1.36
(
1.36)
1.46
1.42)
1.40
1.38)
0.97
0.95)
2.34
2.29)
2.53
2.47)
119.3
114.5)
-0.5
-1.4)
36.4
31.2)
r(9,10)b
2.73
(
r(10,11)b
1.22
(
r(11,12)b
4.25
(
r(9,11)b
3.41
(
r(9,12)b
0.98
(
R(1-2-10-11)c
R(2-1-7-9)c
R(1-7-9-12)c
124.9
-15.6
180.0
(
-54.9
(
178.4
(
a
The numeration of the atoms is that of Scheme 1. b Interatomic distance in angstroms. c Dihedral angle in degrees. d Numbers in parentheses
refer to the CASSCF optimization.
form and are called O1′′, O2′′, and O3′′. The last is the more
stable one of the open structures in the gas phase, although the
energy difference between the three isomers is quite small (Table
2
). Three transition states are also depicted in Figure 3, TS1′′,
TS2′′, and TS3′′. As explained in the methodological section,
IRCs and full optimizations have been carried out in order to
ascertain the connection between the minima through each
transition state. These connections are indicated by dotted lines
in Figure 3. The first transition state TS1′′ connects the closed
R′′ structure with the first open tautomer O1′′. The other two
transition states TS2′′ and TS3′′ correspond to the isomerization
of O1′′ toward O2′′ and O2′′ toward O3′′, respectively. We have
also found an isomer of the R′′ structure corresponding to an
internal rotation of the C10-O11H bond. As this structure is
higher in energy and it is not directly connected with the
cyclization and proton transfer process, we have omitted it in
Figure 3 for clarity. Therefore, the whole process shown in
Figure 3 corresponds to the formation of the more stable open
isomer (O3′′) starting from the more stable closed (R′′) form of
2
-acetylbenzoic acid.
The values of the energies of the whole process are presented
in Table 2, and are relative to that of the more stable R′′ form.
The potential energy values of the structures shown in Figure
3
are given in the first row of Table 2, whereas the second row
includes all the thermodynamic corrections that give the Gibbs
free energy (G°) according to the formalism outlined in the
methodological section. Inclusion of thermodynamic parameters
does not greatly modify the energies. The only relevant point
is that in terms of G° all the open forms are stabilized by 1-3
kcal/mol. This is mainly due to the entropic term that clearly
favors the more disordered open structures with respect to the
closed isomer. The third row of Table 2 gives the free energy
of the structures when adding the bulk effect of the solvent
through the IPCM cavity method. The dielectric constant of
Figure 3. Schematic free energy (G°) profiles for the ring-chain
0
tautomerism of AB at the S state. The values of G° (kcal/mol) are
relative to that of R′′ taken as a reference.
structure in the ground state (for clarity, the ground state
structures are labeled by X′′, while those of the S1 state are
labeled by X′) corresponds to the closed form called R′′ (we
will use this nomenclature from now on). The other three
minima shown in Figure 3 correspond to isomers of the open

H-Atom Transfer Reactions and Cyclization of AB
J. Phys. Chem. A, Vol. 108, No. 43, 2004 9335
1 1 0
Figure 4. (a) Snapshots of selected geometries along the IRC R′′ f TS ′′ f O ′′ at S . (b) Relative positions of the potential energy (V) of selected
points along the whole IRC energy profile at S . The numbers in parentheses are those of V (kcal/mol) relative to that of R′′.
0
water has been used for these calculations. Note that the solvent
greatly diminishes the energy barrier for the ring-chain
tautomerism. Also, the open structures suffer an additional
lowering with the exception of O3′′, the more stable one in the
gas phase, which is slightly destabilized. As a consequence, in
water the more stable open form is no longer O3′′ but O2′′,
although the differences are not significant (0.2 kcal/mol), and
if geometry optimizations were carried out within the cavity
method it is quite possible that the ordering would be reversed
again.
The last two rows in Table 2 give the dipole moment value
of the structures at each stationary point in both the gas and
condensed phases. As expected, in the condensed phase the
dipole moments are larger as solvent polarizes the system (R′′
is an exception). The relative stabilization of the structures upon
solvation is correlated with the dipole moment of each structure.
This is in fact the case when comparing the different open
structures so that the smaller stabilization of O3′′ correlates with
the smallest dipole moment values of this isomer. The large
lowering of the TS1′′ energy is also accounted for by the large
dipole moment of this structure. However, R′′, which has a quite
large dipole moment value, is almost the less stabilized structure.
An explanation of this abnormal behavior can be found in the
fact that the closed form is less able to be solvated than the
open ones. The molecular frame of the latter structure extends
over a larger portion of the space so that more interactions with
external (solvating) molecules are possible.
The structures of the stationary points are depicted in Figure
3. The quantitative values of some selected geometric parameters
of these points are given in Table 3. In particular, the table gives
the values of the distances between the atoms that are directly
involved in the cyclization and the proton transfer reaction along
with some dihedral angle values that help to understand the
internal rotations taking place in the interconversion of the
different open structures. The most interesting process is the
one that goes from R′′ to O1′′ (Figure 3). In this step both the
breaking of the bond between the oxygen-9 and the carbon-10
and the proton transfer (hydrogen-12 is transferred from oxygen-
11 to oxygen-9) occur in a concerted way. However, the two
processes are not simultaneous. This process was further
analyzed with the help of the IRC calculations. The IRC starts
from the transition structure TS1′′ and goes in the reactant side
toward R′′ and in the product side toward O1′′ (Figure 4a, Table
3). Even if the reaction in the IRC scheme begins at the
transition state, it is easy to understand the process considering
the reactant structure as the starting point and with the IRC
followed in the reverse sense up to the transition state. At this
point the other half of the IRC toward products can be followed
in a direct manner. At the early steps of the reaction the relevant
motion implies the cleavage of the O9-C10 bond so that at the

9
336 J. Phys. Chem. A, Vol. 108, No. 43, 2004
Santos et al.
transition state (TS) this bond is totally broken (the O9-C10
distance is 2.40 Å at the TS). This breaking is accompanied by
a rotation of the C2-C10 bond so that the transferring hydrogen-
favor of the closed structure. The predominance of the closed
one is, in fact, a general trend of all the reported works,6 and
it is also consistent with our theoretical results. The closed form
R′′ lies below O3′′, the more stable open isomer in the gas phase.
In terms of ∆G°, the difference is quite small: 0.5 kcal/mol
(0.8 kcal/mol in the condensed phase, where the O2 structure is
the more stable open isomer). Larger differences of free energy
would imply a very small relative population of the less stable
species, unable to be detected.
,7
1
2 gets closer to the accepting oxygen-9 (Table 3). At this point
the system has reached the transition state where all the energy
climb was done, and all the way is energetically downhill to
O1′′ (Figure 4b). This second part of the reaction almost
exclusively involves the motion of the hydrogen atom that is
transferred (snapshots 6 and 7 in Figure 4a). Additionally, the
O9-C10 angle rotates a little further, and the C1-C7 bond
slightly turns to reach the minimum energy conformation of
the product, O1′′. Figure 4b shows in a pictorial way the relative
potential energy (V) position of the selected points of the IRC.
On the other hand, dynamic NMR experiments have allowed
the measurement of the kinetics and energetics of this
ring-chain tautomerism (Table 1). The reported values of
#
∆
G° (RfO) ) 19 kcal/mol for the direct (closed to open form)
#
Prior to the analysis of the rest of the reactions, it is interesting
to consider the charges of the transferring hydrogen atom and
the acceptor and donor oxygen atoms along the R′′ to O1′′
process. The Mulliken analysis shows that the transferring
hydrogen atom has a positive charge of 0.480 au in R′′ which
increases slightly along the whole path, passing through the
transition state TS1′′ (0.523 au) and ending up in the product
O1′′ (0.553 au). As for the charges on the oxygen atoms, they
are clearly negative and range between -0.3 and -0.6 au. The
oxygen atom directly bounded to the hydrogen is always the
more negative one. Therefore, the relative order reverses along
the path (it has already reversed in the transition state). At this
point it is of interest to discuss the nature of the transfer (i.e.,
whether it is a proton or hydrogen atom transfer). Given the
asynchrony of the transfer, in the transition state the O9-C10
bond is almost broken whereas the H12 atom is still bonded to
the O11 atom. Keeping in mind that our theoretical procedure
excludes open-shell structures, this means that the transition state
should be a zwitterionic species with a negative charge in the
carboxylate fragment and a positive charge on the C10 atom. In
fact, the analysis of charges reveals an important increment of
negative charge in the carboxylate (C7O8O9) fragment by
reaction and ∆G° (OfR) ) 18 kcal/mol for the reverse one imply
that both forms are separated by 1 kcal/mol in terms of ∆G°.
The calculated difference in free energy in the condensed phase
of 0.8 kcal/mol between both tautomers (R′′ and O3′′) (Table
2) fits very well with the experimental one (1 kcal/mol). Results
in Table 2 also allow a direct comparison with the NMR kinetic
data. The higher transition state (TS1′′) found along the whole
reaction path implies energy barriers (in terms of G° and in the
condensed phase) of 31.8 and 31.0 kcal/mol for the direct and
reverse processes, respectively. In this case the agreement with
experiment is only qualitative. Inclusion of the solvent effect
greatly diminishes the Gibbs free energy of the transition state.
If optimization of the geometry in condensed phase were
allowed, a further decrease of the energy barrier would probably
show up. Note that theoretical calculations of the Gibbs free
energy barrier are subject to quite drastic model approximations
such as the use of the harmonic oscillator and the rigid rotor
for the vibrational and rotational degrees of freedom of the
molecule, respectively. Also, the condensed phase is introduced
through an (approximate) continuum model that only takes into
account the effect of the solvent molecules as a bulk. Finally,
the values of the energetics and kinetics parameters deduced
from the experiments are also subject to the approximations of
the thermodynamic formulation of the transition state theory,
-0.559 au, while the positive charge on the C10 atom increases
only slightly from +0.067 to +0.099 au. However, it should
be noted that the positive charge can be delocalized through
the π system of the benzene ring as well as to the O11 atom.
This fact is partially taken into account by considering the global
charge of all the atoms directly bonded to C10. The calculation
reveals an increment of positive charge of 0.418 au between
the R′′ and TS1′′ structures. Once the transition state has been
reached, the reaction proceeds by the transfer of a proton so
that globally a hydrogen atom transfer takes place along the
whole reaction coordinate.
22
the Shanan-Atidi and Bar-Eli Method, and supposition of only
two equilibrated structures: open and ring forms.
First Singlet Excited Electronic State. Before analyzing the
potential energy curves obtained for the first singlet excited
electronic state (S1), it is important to know the exact nature of
the electronic transition. As there are no symmetry elements
along the whole reaction path, we have always followed the
first excited state irrespective of the nature of the excitation.
For the S1 closed structure, the electronic excitation is almost
exclusively constituted by the HOMO-LUMO π,π* transition.
Figure 5a depicts the shape of these orbitals. While the HOMO
is mainly a π orbital of the benzene ring, the LUMO has some
nonnegligible contribution from the remaining heavy atoms. In
particular, the LUMO presents significant contributions from
C7, O9, and O11 atoms. The description of the excited state
arising from the open form (O3′′) is, however, more complicated
as there are three orbitals involved in the excitation which could
be described as a combination of the HOMO-LUMO and the
The subsequent reactions that transform the open structure
O1′′ to the progressively more stable O2′′ and O3′′ rotamers can
be easily traced by changes in the dihedral angle values shown
in Table 3. In this way the process that starts from O1′′ to O2′′
mostly involves a rotation of the C7-O9 bond (that is, the
conformation of the hydroxyl group bonded to C7 is changed;
see Figure 4). In addition to that, there is also a significant
change of the dihedral angle that involves rotation around the
C1-C7 bond. Thus, this reaction involves a global reorganization
of the COOH group. In the following step that goes from O2′′
to O3′′ the C1-C7 bond rotates again, but now in the reverse
sense ending at a value quite similar to the one of the initial
conformer (O1′′). However, the main component of this step
involves motion of the CO(CH3) group that rotates almost 90°
in order to adopt the conformation yielding the final O3′′ product.
(
5
HOMO-2)-LUMO excitations. These are depicted in Figure
b. The HOMO and LUMO orbitals are qualitatively similar
to the ones described for the closed tautomer, although clear
differences in the contributions of the atomic orbitals are
observed when both figures are compared. The shape of the
other involved orbital, the HOMO-2, is different. It seems to
be mainly due to a lone electron pair located at the oxygen-11.
In this case the usual belief that n and π orbitals do not mix
does not apply. The reason for that lies in the particular geometry
Let us compare now these theoretical results with the
experimental data reported above. Our NMR spectra at 273 K
confirm the existence of the two forms with a ratio of 89:11 in

H-Atom Transfer Reactions and Cyclization of AB
J. Phys. Chem. A, Vol. 108, No. 43, 2004 9337
Figure 6. Schematic potential energy (V, in kcal/mol) profile for ring-
1
chain tautomerism of AB at the S state. The indicated values are those
of V relative to that of O ′.
3
atomic distances and dihedral angles) of the stationary points
are given in Table 5.
One well-known drawback of CIS and TD-DFT methods is
their inability to treat double and multiple excitations. This
problem can be avoided using the more flexible CASSCF
method. To see whether this problem is affecting our result,
we have performed CASSCF calculations on the reactant (ring
form). The active space considered included 12 electrons and
11 orbitals (six occupied and five virtual), so all the π system
and the highest σ and lowest σ* orbitals are considered. The σ
orbital roughly corresponds to a lone pair of the oxygen-9,
whereas the σ* is predominantly C10-O9 antibonding. With this
quite large system we have optimized the geometry of the
reactant in both S0 and S1 electronic states. The obtained
geometries are posted in Tables 3 and 5 so we can compare
them with the DFT and CIS results, respectively. At the S0 level,
there are small differences. However, at S1 the CIS and CASSCF
geometries are almost equal. We have also analyzed the CAS
wave function to ascertain that double excitations have a very
small contribution to the whole wave function which is mainly
constituted, as in the CIS case, by the HOMO-LUMO single
excitation. We have also tried to localize the transition state at
the CASSCF level for the S1 state. However, the optimization
was unsuccessful because the orbitals made sudden changes
along the optimization so that following a coherent active space
proved fruitless. In any case, analysis of the CASSCF wave
function at fixed (CIS) geometry also reveals a main contribution
of single excitations. There is also an important contribution
from the HOMO-1 f LUMO excitation, a fact that is also
present in the TD-DFT calculation. This probably indicates a
crossing of π orbitals along the reaction coordinate. In any case,
the contributions of double (and higher) excitations to the
CASSCF S1 wave function are very small, so a diradical
character of the process seems to be excluded.
Figure 5. (A) Shape of orbitals involved in S
excitation for the closed form (R′′). (B) Shape of orbitals involved in
f S electronic excitation for the open form (O ′′).
0 1
f S electronic
S
0
1
3
of the open isomer. As we are considering here the most stable
conformer O3′′ in gas phase, this conformer presents the
CO(CH3) fragment lying in a plane almost perpendicular to the
benzene ring (see Figure 3 and Table 3). Thus, the lone pair n
orbitals of the oxygen atom in the carbonyl group are not both
orthogonal to the π system of the benzene ring: one of them is
located in almost the same plane. Therefore, the excited state
now is not π, π* or n, π* but a mixing of both. The mixing of
these states and the involvement of cyclization and proton
transfer explain the lower emission quantum yield of AB in
solution, as said above. The shape of these orbitals can also
explain the main differences, both geometric and energetic,
observed between the ground and the excited electronic states.
For the excited state S1, we have carried out the same kind
of calculations performed for the ground state. As explained in
the methodological section, we have used the CIS method to
explore the potential energy surface at S1. As the CIS energies
may be subject to errors,24 we have recalculated the energy of
the stationary points by using the TD-DFT method within the
B3LYP functional.15 This method is quite recent, but it has
already been proved to give much more accurate results for
intramolecular proton transfer reactions, comparable in some
cases to the ones obtained with the very computing demanding
CASPT2 method.25 Results are shown schematically in Figure
6
, where the different stationary points and the reaction paths
linking them are shown. The actual values of the relative
potential energies and dipole moments of each structures are
given in Table 4. Some selected geometric parameters (inter-
Figure 6 reveals a general shape of the energy profile quite
similar to the one of the ground state depicted in Figure 3. The

9338 J. Phys. Chem. A, Vol. 108, No. 43, 2004
Santos et al.
TABLE 4: Potential Energies and Dipole Moment Values of the Stationary Points Located in the First Singlet Excited
Electronic State S
1
parameter
R′
TS
1
′
O
1
′
TS
7.2
98.75
12.16
77.20
5.9
2
′
O
2
′
TS
3
′
3
O ′
V(CIS)a
E(CIS)
20.0
131.87
26.2
107.17
14.2
1.09
47.6
107.26
33.6
75.73
38.0
2.38
4.5
96.28
7.8
73.11
2.4
4.35
5.60
3.6
11.6
0.0
104.14
0.0
75.28
0.0
5.4
b
∆
93.69
7.63
71.65
2.4
6.93
9.02
103.99
10.52
73.71
1.0
7.35
10.39
a
V(TD-DFT)
b
∆
E(TD-DFT)
a
V (condensed phase)
c
µ (gas phase)
6.50
8.34
c
µ (condensed phase)
1.50
3.20
7.55
a
Relative potential energy in kcal/mol. b Difference in energy between S . c Total dipole moment in debyes.
and S
0 1
TABLE 5: Selected Geometric Parameters of the Stationary Points Located in the First Singlet Excited Electronic State
Potential Energy Surfacea
parameter
r(7,9)b
R′ d
1.36
1.36)
1.42
1.42)
1.38
1.38)
0.95
0.95)
2.29
2.29)
2.45
2.46)
115.1
112.7)
1.7
TS
1
′
O
1
′
TS
2
′
O
2
′
TS
3
′
3
O ′
1.29
1.94
1.34
0.95
2.44
2.39
64.1
3.1
1.33
3.01
1.26
2.49
2.84
0.95
39.5
36.8
-6.1
1.32
2.98
1.26
1.96
2.87
0.95
1.33
2.94
1.26
3.11
3.92
0.95
1.36
1.33
2.90
1.26
4.75
3.90
0.95
(
r(9,10)b
2.92
1.26
4.29
3.93
0.95
(
r(10,11)b
(
r(11,12)b
(
r(9,11)b
(
r(9,12)b
(
R(1-2-10-11)c
R(2-1-7-9)c
R(1-7-9-12)c
-68.8
17.5
1.2
-152.3
32.7
-158.3
28.6
-156.3
28.1
(
(1.5)
23.0
44.8
-1.0
86.5
176.8
(29.1)
a
The numeration of the atoms is that adopted in Scheme 1. b Interatomic distance in angstroms. c Dihedral angle in degrees. d Numbers in parentheses
refer to the CASSCF optimization.
closed form R′ undergoes a hydrogen atom transfer reaction
and a bond-breaking process so that open tautomers are formed.
Analysis of Mulliken charges along the whole process reveals
the same pattern observed for the ground state. The charge of
the hydrogen is almost constant as it goes from +0.50 to +0.49
au from R′ to O1′ with an irrelevant increase to +0.53 au at the
transition state TS1′. A charge analysis similar to the one
performed for the ground electronic state reveals also patterns
similar to the ones discussed before, although now an important
fraction of the charge separation produced by the heterolytic
cleavage of the O9-C10 bond in the transition state tends to
concentrate in the benzene ring rather than in the carboxylate
and C10 fragments. In this way the increment of negative charge
of the C7O8O9 carboxylate fragment is -0.288 au, whereas the
increase of positive charge of the fragment including C10 and
neighboring atoms is only +0.148 au, but for instance, the
charge on C1 (in the phenyl ring) changes from -0.403 to
excited states.28 This comes from the very weak overlap between
the orbitals involved in the charge transfer transition so that a
local functional can hardly have an effect on the excitation
energy. The use of hybrid functionals which include nonlocal
Hartree-Fock exchange, such as the B3LYP method used here,
is expected to lead to more reliable results. In fact, we are not
dealing with a charge transfer excited state, so this problem is
not greatly affecting our results. It may be that the first transition
state TS1′ is too stabilized at the TD-DFT level (as it has
noticeable charge separations although it is not actually a charge
transfer state as previously discussed). This would only mean
that the energy barrier for the first process will be slightly higher
than the value reported in Table 4 and Figure 6 (7.4 kcal/mol).
Depending on the internal rotations of the -COOH and
-CO(CH3) groups, AB progressively passes from the initial
O1′ tautomer to O2′ and O3′ forms, each one being more stable
than the previous structure. However, a more careful analysis
reveals some differences from the S0 results. The minima (stable
structures) and the transition states connecting them are different.
This is clearly seen by looking at the actual values posted in
Table 4 (and comparing them with the ones corresponding to
S0 given in Table 2). The first row in Table 4 gives the potential
energy values obtained with the CIS calculations whereas the
third row contains the values recalculated with the TD-DFT
method at fixed geometry. There are no qualitative differences
between both levels of calculation, although some quantitative
differences are worth noting. At both levels of calculations the
closed R′ structure is no longer the more stable one as the O3′
structure has a clearly lower energy. This inversion of the
stability of tautomers upon electronic excitation has been
+0.201 au.
At this point some comments about the validity of the TD-
DFT method to deal with proton transfer reactions are in order.
In a very recent contribution,26 the TD-DFT method has been
successfully used for the study of excited state reactivity
problems, at least when the geometries were optimized at the
CASSCF level. Another recent work has shown that, when the
ground state is well described by DFT methods (such as in our
case), the TD-DFT excited state description is comparable to
that obtained in ground state DFT calculations.2 Given that in
the case presented here CIS and CASSCF geometries are very
similar, we believe that our theoretical procedure is giving quite
reliable results for the system considered here. An important
drawback of the TD-DFT method, recently described, is that it
systematically underestimates the energy of charge transfer
7
2
9
observed in many related systems. The first transition state
that connects the closed R′ structure with the first open

H-Atom Transfer Reactions and Cyclization of AB
J. Phys. Chem. A, Vol. 108, No. 43, 2004 9339
1 1 1
Figure 7. Snapshots of selected geometries along calculated IRC R′ f TS ′ f O ′ for the S excited state.
conformer O1′ is the higher point in energy along the path. TD-
DFT clearly lowers this barrier, which is only 7.4 kcal/mol. In
any case, TD-DFT does not modify the fact that this is the
highest energy point along the whole reaction path. As for the
rest of the path, it consists of internal rotations of the -COOH
and -CO(CH3) groups, as noted above. Energetically, the gaps
are now larger than the corresponding ones in the ground state
solvation even if it has the lowest dipole moment among the
values reported in Table 4. Note that in the ground state the
closed R′′ structure has one of the largest dipole moments but
it was almost the less stabilized structure. In any case, the fact
that solvent does not significantly affect the energy profile for
the H-atom transfer and ring-breaking process is an additional
proof of the process not being zwitterionic (this also applies to
the ground state results previously analyzed). This fact also
validates the TD-DFT/CIS strategy as erroneous descriptions
of the excited state (not taking into account double excitations)
would bias the calculation toward zwitterionic transition states.
Other interesting data to discuss here are the excitation
energies along the PES. The differences in energies between
S and S at the stationary points of S are given in Table 4 in
(compare again results in Tables 2 and 4). As discussed above,
the TD-DFT energy barrier for R′ f O1′ is probably too small,
but the CIS value is too high because it is well-known that the
CIS method tends to overestimate the barriers for the proton
2
4
transfer reactions. As explained, we have been unable to
localize the transition state at the CASSCF level but a single
CASSCF calculation upon CIS optimized structures gives an
energy barrier of 28.0 kcal/mol, quite close to the CIS value
0
1
1
kilocalories per mole at both the CIS and TD-DFT levels
(second and fourth rows). Note that excitation energies are
always quite high and that they do not change abruptly along
the different regions of the PES where the stationary points are
found. CIS excitation energies are higher (always above 90 kcal/
mol) and those from TD-DFT slightly lower, but even there
the excitation energy is never less than 70 kcal/mol. Besides,
these high values are also maintained along the different sections
of the calculated IRC. These results then clearly demonstrate
that there are no intersections between the ground and excited
states, at least not near the lower energy zones of the PES, the
only ones of chemical interest. To compare the theoretical
excitation energies with the experimental absorption and
fluorescence spectra, data in Table 4 are not useful as they give
the differences in energy at the stationary points of S1, whereas
the absorption process takes place from the minimum energy
structures of the ground state S0. The vertical transition
absorption band from the most stable closed tautomer is
calculated to appear at 247 nm at the TD-DFT level. On the
other hand, the adiabatic 0-0 transition is predicted at 260 nm
if the zero point energy is assumed to be equal in both electronic
states. These results are to be compared with the CASSCF
calculations which predict the bands of the vertical and adiabatic
transitions at 259 and 273 nm, respectively. All these values
are in quite good agreement with the experimentally observed
absorption maximum found at 278-282 nm. As for the
absorption of the most stable open form O3′′, which could be
(
(
27.6 kcal/mol). Of course, introduction of dynamic correlation
a CASPT2 calculation, for instance) would certainly lower this
25c
value so that the actual energy barrier is expected to lie
between 8 and 25 kcal/ mol.
We have also analyzed the reaction path that goes from R′
to O1′ at the excited state in order to see whether the breaking
of the cycle and the H-atom transfer processes are taking place
simultaneously. The IRC calculations reveal that the whole
process in S1 is qualitatively very similar to the one in S0. This
fact is pictorially demonstrated in Figure 7, where the geometries
of some selected points along the IRC are presented so that a
direct visual comparison with the same reaction in S0 can be
done (Figure 4a).
The bulk effect of a polar solvent (dielectric constant of water)
has also been considered at S1. Corrections of the CIS energies
with the solvent effect are given in the third row of Table 4.
Again, there are no significant differences, so the relative
ordering of energies is maintained. All the relative energies are
lowered. This means that upon solvation the reference O3′
structure is the less stabilized one. Total dipole moments values
in the gas phase and in the condensed phase are given in the
last two rows of Table 4. The loose relationship between solvent
stabilization and the dipole moment values observed in the
ground state no longer applies at S1. This suggests that higher
electric moments (quadrupole, octupole, ...) may play a major
role at S1. The closed R′ structure is now largely stabilized upon

9340 J. Phys. Chem. A, Vol. 108, No. 43, 2004
Santos et al.
in equilibrium with the closed tautomer in the ground state, the
Franck-Condon vertical transition is calculated at 308 nm
within the TD-DFT method.
additional conformer O2′′ and two transition states that cor-
respond to internal rotations of the -COOH and -CO(CH3)
fragments. We have also analyzed the IRC for the tautomer-
ization process. Both H-atom transfer and ring breaking/
formation are occurring in a concerted but not synchronous
manner.
Less evident is the comparison of the fluorescence spectrum
with our excitation energies because it is not known where along
the whole path (proton transfer + ring opening + internal
rotations) the system comes back to the ground state. Given
that the open forms have the lower energy, it is reasonable to
assume that electronic deactivation proceeds from the open
isomers; the energy range for light emission should correspond
to the electronic excitations of the open forms that lie around
In the first singlet excited electronic state S1, the whole energy
profile is qualitatively similar to the S0 one. However, the more
stable tautomer at S1 is not the ring form but one of the open
conformers O3′. The energy barrier for the ring-chain tautom-
erism is 7.4 kcal/mol at our best level of calculation (TD-DFT).
The vertical excitation structure is found 5.8 kcal/mol above
the minimum at the TD-DFT level. Therefore, the ring-chain
tautomerism can be considerably enhanced upon irradiation at
the appropriate wavelength. This opens the door to potential
applications of proton transfer and ring-chain tautomerism in
different fields of science and technology such as photo-
chromism and for building materials able to store information
at the molecular level and for controlling structural changes in
supramolecular environments by using ultrafast (i.e., fem-
tochemistry) techniques.
7
0-80 kcal/mol. This corresponds to wavelengths ranging
between 360 and 410 nm, again in a reasonable agreement with
the experimental fluorescence spectra shown in Figure 1. Note
that our theoretical results are for isolated molecules (i.e., gas
phase), whereas the emission spectra are obtained in solution.
Therefore, part of the discrepancy between computed and
observed bands might be attributed to the solvent effect on both
ground and excited state potential energy surfaces and on
solvent-dependent emission quantum yield.
To end the description and discussion of results at S1, let us
now consider a few geometric parameters of the stationary points
posted in Table 5. As at S0, the first step at S1 is the one
involving the proton transfer and the opening of the pyrrolic
cycle. Both processes take place in a concerted way (that is, in
a single kinetic step), but they are not synchronous. An
enlargement of the O9-C10 distance takes place in the first stages
of the reaction so that in the transition state the bond is almost
broken. Interestingly, the O9-C10 distance in TS1′ is clearly
shorter than the one in TS1′′. However, this difference in both
states can be solely attributed to the fact that in S1 the transition
state occurs earlier than in S0 because the reactant has higher
energy than the product, and so the Hammond postulate
Finally, we have found that solvent does not greatly affect
the gas phase results. The energy profiles obtained from gas
phase electronic calculations are qualitatively similar to the
condensed phase ones even if the dipole moments may change
considerably along the involved reactions.
Acknowledgment. We are grateful for financial support from
the Spanish Ministerio de Ciencia y Tecnolog ´ı a through Project
Nos. MAT-2002-01829 and BQU2002-00301, the JCCM (PAI-
0
2-004), the Fondo Europeo de Desarrollo Regional, the use of
the computational facilities of the CESCA, and the computa-
tional service of the Universidad de Castilla-La Mancha.
3
0
applies. Once the transition state TS1′ has been reached, the
reaction path proceeds to the transfer of hydrogen-12. The
energy goes down monotonically, and the first open tautomer
O1′ is reached. The rest of the reaction path can be followed by
checking the dihedral parameters given in Table 5. From O1′
to O2′ structure, the C2-C10 bond turns upside down (the
dihedral angle changes almost 180°), whereas the final step that
transforms O2′ in the final O3′ product involves also a whole
References and Notes
(1) (a) Caldin, E. F.; Gold, V. Proton-Transfer Reactions; Chapman
and Hall: London, 1975. (b) Douhal, A.; Lahmani, F.; Zewail, A. H. Chem.
Phys. 1996, 207, 477 and references therein. (c) Hydrogen Transfer:
Experiment and Theory; Limbach, H., Manz, J., Eds.; Ber. Bunsen-Ges.
Phys. Chem. 1998, 102.
(2) (a) Jones, P. R. Chem. ReV. 1963, 53, 461. (b) Valters, R. E.; Flitsch,
W. Ring Chain Tautomerism; Plenum Press: New York, 1985.
(
3) Bowden, K. Chem. Soc. ReV. 1995, 431.
1
80° turn of the O-H group (indicated by the 1-7-9-12
(4) Bowden, K. Organic ReactiVity: Physical and Biological Aspects;
dihedral angle in Table 5). In parallel with the S0 results, we
have found additional transition state structures that correspond
to different internal rotations of the open tautomer. Because they
are not directly involved in the process that goes from the closed
R′ structure to the more stable open tautomer O3′, they are not
posted in Figure 6.
Golding, B. T., Griffin, R. J., Maskill, H., Eds.; Royal Society of
Chemistry: Cambridge, 1995; p 123.
(
5) Felker, P. M.; Lambert, W. R.; Zewail, A. H. J. Chem. Phys. 1982,
7, 1603.
6) Finkelstein, J.; Williams, T.; Toome, V.; Traiman, S. J. Org. Chem.
1967, 32, 3229.
7
(
(7) (a) Bowden, K.; Taylor, R. G. J. Chem. Soc B 1971, 1390. (b)
Bowden, K.; Taylor, R. G. J. Chem. Soc B 1971, 1395. (c) Bowden, K.;
Henry, M. P. J. Chem. Soc., Perkin Trans. 2 1972, 201. (d) Bowden, K.;
Henry, M. P. J. Chem. Soc., Perkin Trans. 2 1972, 206. (e) Bowden, K.;
Malik, F. P. J. Chem. Soc., Perkin Trans. 2 1993, 635. (f) Bowden, K.;
Byrne, J. M. J. Chem. Soc., Perkin Trans. 2 1996, 1921. (g) Bowden, K.;
Byrne, J. M. J. Chem. Soc., Perkin Trans. 2 1997, 123. (h) Bowden, K.;
Hiscocks, S. P.; Perjessy, A. J. Chem. Soc., Perkin Trans. 2 1998, 291. (i)
Bowden, K.; Raja, J. J. Chem. Soc., Perkin Trans. 2 1999, 39. (j) Bowden,
K.; Misic-Vukovic, M. M.; Ranson, R. J. Collect. Czech. Chem. Commun.
4
. Conclusions
In this paper we have presented experimental and theoretical
results that help to understand the complex mechanism involved
in the ring-chain tautomerism of 2-acetylbenzoic acid at both
S0 and S1 states.
1
999, 64, 1601.
From the experimental side, our results clearly show the
existence of an equilibrium between the ring and open forms,
the ring one being clearly predominant (∼90%). The emission
quantum yield is very weak, in agreement with the theoretical
data showing a mixing between π,π* and n,π* states, and the
involvement of proton transfer and cyclization processes.
Electronic calculations have also disclosed the presence of
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