J. Phys. Chem. 1996, 100, 3933-3941
3933
Photophysics of 2-Hydroxypyridine: An ab Initio Study
Andrzej L. Sobolewski
Institute of Physics, Polish Academy of Sciences, 02 668 Warsaw, Poland
Ludwik Adamowicz*,†
Department of Theoretical Chemistry, Chemical Center, UniVersity of Lund, S-22100 Lund, Sweden
ReceiVed: March 24, 1995; In Final Form: NoVember 14, 1995X
The potential energy (PE) surfaces of the electronic ground and the lowest excited states relevant to photophysics
of interconversion of the 2-hydroxypyridine/2(1H)-pyridone system are characterized by ab initio calculations.
The energy calculations at the optimized geometries are performed with the aid of the second-order perturbation
theory, employing the complete-active-space self-consistent-field (CASSCF) wave function as the reference
(CASPT2). Results confirm the earlier hypothesis based on the CASSCF calculations (Chem. Phys. Lett.
1993, 211, 293) that the photoinduced dissociation-association (PIDA) mechanism is probably responsible
for excited-state tautomerization observed in this system. Absorption of the second photon in the excited
electronic state is suggested to provide the “driving” force for the PIDA mechanism.
1. Introduction
The photomeric tautomerism in the 2-hydroxypyridine/2(1H)pyridone (2HP/2PY) molecular system has been the subject of
many studies because it is representative of a large number of
heterocyclic molecules that are relevant to biological functions.1
The 2HP/2PY tautomeric system is one of the few systems
where the energy difference between the two tautomers is
sufficiently small for both species to be observed in the gas
phase and in inert matrices,2,3 whereas in polar solvents and in
the crystal phase the lactam 2PY form predominates.3,4 The
experimental measurements lead to the conclusion that under
isolated molecule conditions (e.g. supersonic jet, gas phase, inert
matrices) the free energy difference between the lactam, 2PY,
and lactim, 2HP, forms is about 0.02-0.04 eV in favor of the
former one.2,3,5-7 Theoretical studies of the reaction, although
generally supporting the experimental findings, have resulted
in a wide range of relative energies of the two forms.8-19 All
the studies, however, were consistent in predicting a large energy
barrier for the proton transfer (PT) reaction in the ground state
(=2 eV).10,15,18,19 This is a direct consequence of lack of an
intramolecular hydrogen bond in the system which could
facilitate a PT reaction.
The optical excitation within the lowest singlet manifold
generally changes the acidity and basicity of molecular moieties
and thus can significantly change the tautomeric equilibrium.
The so-called excited-state intramolecular proton transfer
(ESIPT) reaction occurs when the subtrate (stable in the ground
state) becomes unstable (or metastable) with respect to the
product on the excited-state potential energy (PE) surface (the
reverse asymmetry). The majority of the ESIPT reactions
involve transfer of a proton (or hydrogen) from the oxygen
donor to the oxygen or nitrogen acceptor along the intramolecular hydrogen bond (for a recent review see ref 20). The
existence of a hydrogen bond is a necessary condition for the
ESIPT process to occur. This can be either an intramolecular
hydrogen bond20 or an intermolecular bond in the hydrogenbonded complex.21,22
The considered 2HP/2PY molecular system belongs to an
important class of compounds in which the excited-state PT
† Permanent address: Department of Chemistry, University of Arizona,
Tucson, AZ 85721.
X Abstract published in AdVance ACS Abstracts, February 1, 1996.
0022-3654/96/20100-3933$12.00/0
reaction has been observed,3,23 but which under isolatedmolecule condition does not fulfill any of the two abovementioned conditions for ESIPT; that is, it does not have an
intramolecular hydrogen bond, nor do its PE functions have
desired asymmetry with respect to the PT reaction coordinate.
However, the transfer of the hydrogen atom in this system does
occur upon optical excitation in a low-temperature inert gas
matrix, as it was confirmed via analysis and assignment of the
IR spectra obtained before and after irradiation.3,23 Because it
manifests itself differently, as compared to a typical ESIPT
system, where the occurrence of the PT reaction results in redshifted fluorescence, we propose to consider the process as an
example of the photoinduced proton transfer (PIPT). In a typical
PIPT reaction, contrary to the ESIPT process, the hydrogen atom
is transferred exclusively from the nitrogen in the ring to the
oxygen (or sulfur) acceptor, i.e., against the existing endothermicity on the excited-state PE surface.23 This kind of transfer
happens in the 2HP/2PY system where an excitation of the
lactam 2PY form at λ = 300 nm increases the amount of the
lactim 2HP form in the matrix,3 despite a large endothermicity
(=0.75 eV) of the PT reaction on the lowest ππ*-state PE
surface.24 The significantly different photophysics of typical
ESPIT and typical PIPT systems, as mentioned above, lead to
the conclusion that the physical mechanism of the PT reactions
in the ESPIT and PIPT systems should be fundamentally
different.
In a series of papers published over the last 2 years, the results
of extensive ab initio calculations of the PE surfaces relevant
for the ESIPT reaction have been presented.25-29 The general
conclusion which emerges from the studies is the relevance of
near-degeneracies and multidimensional surface crossings of the
excited-state PE surfaces. It seems that in many ESIPT systems
the reaction dynamics is largely determined by the intersection
of the barrierless 1ππ* PE surface with the 1nπ* surface, which
usually exhibits a significant barrier to PT. Therefore, the
established paradigm of the reaction dynamics on a single
isolated adiabatic surface is not appropriate for the ESIPT
systems.
In the parallel theoretical studies of the PT reaction in systems
without intramolecular hydrogen bonds (the PIPT systems), a
qualitatively new reaction mechanism was postulated.18,30 It
has been proposed that the PT reaction in these systems is
© 1996 American Chemical Society
3934 J. Phys. Chem., Vol. 100, No. 10, 1996
initiated by the motion of the “mobile” hydrogen atom on the
predissociative PE surface toward dissociation. Strong nonadiabatic interaction present near the conical intersection between
the PE surface of the “dissociative” 1A′′(πσ*) state and the
ground state causes a nonradiative relaxation of the wave packet
to the ground state, with a finite probability to set down in the
minimum corresponding to the rare tautomeric form. The
photoinduced dissociation-association (PIDA) mechanism of
PT was initially proposed for the 2HP/2PY system,18 but
recently it was also found to occur in formamide.30 In the
meantime, Barone and Adamo19 theoretically reinvestigated the
2HP/2PY system and concluded, contrary to the conclusions
of ref 18, that the PIDA mechanism for proton transfer is not
possible in this system.
In the present paper we report new results of calculations of
the 2HP/2PY system performed with the ab initio completeactive-space self-consistent-field (CASSCF) method and with
the second-order perturbation theory method employing the
CASSCF wave function as the reference (CASPT2). This more
sophisticated theoretical treatment, which allows us to account
for both the dynamic and static electron correlation effects,
confirms the previous conclusion obtained at the CASSCF
level18 with respect to the relevance of the PIDA mechanism
for the PT reaction in the 2HP/2PY system. It is shown that
the conclusion of ref 19 results from artifacts of the approximate
theoretical approach (configuration interaction scheme with only
single-electron excitation from the Hartree-Fock reference
(CIS)) used for description of the excited-state PE surfaces.
2. Theoretical Methodology
2.1. Reaction Path Concept. When considering reaction
paths on the PE surfaces of excited states, as required for the
characterization of photochemistry,31 two major complications
arise. First, reliable ab initio energy calculations for excited
states are typically much more involved than the ground-state
calculations. Secondly, multidimensional surface crossings are
the rule rather than the exception for the excited electronic states.
The concept of an isolated Born-Oppenheimer (BO) surface,
which is usually assumed from the outset in the reaction-path
theory, is thus not appropriate for the excited-state dynamics.
At surface crossings (so-called conical intersections31-34), the
adiabatic PE surfaces exhibit nondifferential cusps, which
preclude the application of the established methods of the
reaction-path theory.35-37 As an alternative to nondifferentiable
adiabatic PE surfaces, so-called diabatic surfaces38 may be
introduced, which are smooth functions of the nuclear coordinates. However, the definitions of these diabatic surfaces and
of the associated wave functions are not unique and involve
some subtleties.39-41
Despite the lack of a comprehensive reaction-path theory for
excited-state surfaces, the reaction-path concept can fruitfully
be applied at a pragmatic level to advance the understanding
of photochemical dynamics. There are two general issues of
the reaction-path concept widely used in theoretical explorations
of the PE surfaces. These are the concerted reaction-path (CRP)
approach based on the straight-line, least-motion intramolecular
coordinate which interpolates linearly between the equilibrium
geometries of the reactant and the product,42 and the minimum
energy reaction path (MEP),43 which is defined as the steepest
descent path from the transition state down to the local minima
of the equilibrium geometries of the reactant and the product.
With respect to a quantitative characterization of the PE function
along the reaction coordinate, in terms of local minima and
barriers between them, the MEP concept is more relevant.
However, it is well-known that for many reactions (PT process
Sobolewski and Adamowicz
among them) the MEP is very sharply curved, so that the
relevant dynamical motion can deviate far away from it. The
CRP approach is free from such a curvature problem by the
virtue of its definition, but can provide only a qualitative
characterization of the PE surface. In the following presentation
we use one of the two methods depending on the particular
(qualitative or quantitative) features of the PE surface considered.
2.2. Computational Details. The ground-state molecular
geometries of all molecular systems considered in this work
were optimized at the Hartree-Fock (HF) level, whereas in the
optimizations of the excited-state geometries, the configuration
interaction scheme with single excitations (CIS) and the
complete-active-space self-consistent-field (CASSCF) method
were used. Most of the geometry optimizations were performed
with the Cs symmetry constraints (the systems were kept planar
unless explicitly specified otherwise). The planarity of the
system helps to perform the excited-state calculations. The
lowest excited singlet states of all the systems considered in
this work result from the ππ* and nπ* excitations and as such
fall into two distinct symmetry representations, A′ and A′′, in
the Cs point group. Any out-of-plane deformation destroys the
Cs symmetry and mixes the A′ and A′′ states together. It then
often becomes practically impossible to get a converged solution
for the higher of the two lowest close-lying excited electronic
states of the same multiplicity and the same symmetry, which
in the Cs symmetry are distinct (i.e, the A′(ππ*) and A′′(nπ*)
states) and much easier to converge. Restricting the system to
the Cs symmetry in the calculations of the PE functions along
the reaction coordinate also has important implications for
treatment of the reaction dynamics. It is convenient in such a
treatment to consider any distortion from the Cs symmetry as a
pseudo-nonadiabatic nuclear-electron interaction. This, of
course, makes sense only if the distortions from the Cs symmetry
have a negligible influence on the energy. This point was
carefully checked for the systems considered in this work.
The split-valence Gaussian 3-21G basis set44 was employed
in geometry optimizations, and optimizations at the HF and CIS
levels of theory were performed with the use of the GAUSSIAN92 program package,45 whereas for those performed at the
CASSCF approximations the GAMESS issue of programs was
used.46 The single-point energy calculations were performed
for the ground state with the use of the Moller-Plesset (MP2)
perturbation theory and the double-ζ Gaussian basis set with
polarization functions (the D95G** basis set of GAUSSIAN).
The excited-state energies were calculated by means of the
CASSCF method,47 and the double-ζ-valence Gaussian basis
set of Dunning and Hay48 with polarization functions (DZVP
basis set) was used. The remaining dynamic correlation effects
were added in the subsequent step with the use of the secondorder perturbation theory, with the CASSCF wave function as
the reference (CASPT2).49 CASPT2 calculations were performed with the CASSCF function optimized for each state
separately (unless otherwise specified) with the use of the
nondiagonal zero-order Hamiltonian as it is implemented in the
MOLCAS-3 quantum chemistry software.50 All the calculations
were performed on the IBM RS/6000-590 workstation.
3. Results of Calculations and Discussion
3.1. Energies of Electronic Transitions. The molecular
systems considered in this study are presented schematically in
Figure 1. The geometrical parameters computed at the HF/
3-21G and CIS/3-21G approximation for the ground and for
the lowest excited singlet states of the 2HP/2PY system are wellknown (see for instance ref 19), so we do not present them in
Photophysics of 2-Hydroxypyridine
J. Phys. Chem., Vol. 100, No. 10, 1996 3935
TABLE 1: CASSCF and CASPT2 Energies, Energies
Relative to the Ground State (∆E), and the Weight of the
CASSCF Reference Function in the First-Order Wave
Function (ω) Calculated with the DZV P Basis Set for 2PY
and 2HP at Molecular Geometries Optimized in Different
Electronic States for Both Tautomeric Forms
Figure 1. Oxo (lactam) and hydroxy (lactim) forms of 2-pyridone.
this paper. The point we would like to discuss in more detail
concerns the (presumed) planarity of these systems in the lowest
excited singlet states (the structures are essentially planar in
the ground state). At the CIS/3-21G level, the 1A′′(nπ*) state
is the lowest singlet excited state of both 2HP and 2PY forms.
The normal mode analysis performed at the Cs-optimized
geometry of the 1A′′ state gives two imaginary frequencies for
both forms, and they mostly involve the out-of-plane deformation of the OH or NH groups. The out-of-plane energy-lowering
distortions of these molecular moieties were confirmed by the
CIS/3-21G optimizations without any symmetry constraints (C1).
The stabilization energy with respect to the Cs-optimized
geometry was about 300 cm-1 for 2HP and 800 cm-1 for 2PY
at this level of theory. The “bright” (for optical excitation from
the ground state) 1A′′(ππ*) state is the second singlet state in
both forms at the CIS/3-21G level. The normal mode analysis
performed at the Cs-optimized geometries gives all real frequencies for the 2PY form and all but one (the imaginary frequency
corresponds to the OH out-of-plane bending) real frequency for
2HP. An attempt to optimize 2HP in the 1A′ state without
symmetry constraints causes a collapse of the system to the
lowest close-lying 1A′′ state. The energetical ordering of the
two lowest excited singlet states, (nπ*) and (ππ*), is reversed
at the CIS/6-31G** level. Now the 2HP form remains planar
in the 1A′(ππ*) state, while 2PY shows one imaginary frequency
corresponding to the out-of-plane deformation of the ring. The
C1-optimized geometry of 2PY shows, however, only minor
energetical stabilization of the S1 state (e100 cm-1). The “soft”
out-of-plane deformation of 2PY in the 1A′(ππ*) state, suggested
by our calculations, is in accord with experimental observation.51
In conclusion we can say that, although the out-of-plane
deformations in the lowest singlet states may be present, they
do not seem to cause significant effects on the energy scale
relevant to this study. Thus, the system were kept planar in
this work, if not explicitly specified otherwise.
The single-point energy calculations of the ground and lowest
excited singlet states were performed at the Cs-optimized
geometries according to the methods specified in the preceding
section. In the CASSCF calculations, 10 electrons (of a total
number of 50) were correlated in nine active MOs. The active
space in this case is denoted as (20,0/2,7), where the first two
numbers indicate the number of core (doubly occupied in each
configuration) molecular orbitals in the A′ and A′′ symmetry
representations of the Cs point group, respectively, and the
second two numbers indicate the similar symmetry distribution
for the active orbitals. In other words, the active space includes
all the (first shell) π orbitals (from 1a′′ to 7a′′) and two σ orbitals
(21a′ of the n type and 22a′ of the σ* type). The double-ζvalence Gaussian basis set of Dunning and Hay48 with polarization functions (DZVP) was employed in the energy calculations.
The exponents of the polarization functions were 0.75, 0.80,
0.85, and 1.0 for carbon, nitrogen, oxygen and hydrogen atoms,
respectively. Calculated energies are listed in Table 1.
Inspecting the CASPT2 results presented in Table 1, one can
notice that the lactim 2HP form is by 0.07 eV more stable than
state[geometry]
CASSCF
(au)
S0[S0]
1
A′[S0]
1A′′[S ]
0
S0[1A′]
1
A′[1A′]
1
A′′[1A′′]
-321.7094
-321.5156
-321.5274
-321.6946
-321.5449
-321.5732
S0[S0]
1A′[S ]
0
1
A′′[S0]
S0[1A′]
1A′[1A′]
1A′′[1A′′]
-321.7080
-321.5157
-321.4962
-321.7051
-321.5301
-321.5263
∆E
(eV)
CASPT2
(au)
ω
∆E
(eV)
5.27
4.95
0.40
4.47
4.31
-322.6010
-322.4404
-322.4177
-322.5955
-322.4659
-322.4542
0.779
0.758
0.772
0.774
0.759
0.774
0.07
4.44
5.06
0.22
3.75
4.07
2HP
0.04
5.27
5.80
0.12
4.88
4.98
-322.6037
-322.4291
-322.3988
-322.6002
-322.4462
-322.4191
0.778
0.762
0.769
0.775
0.760
0.770
4.75
5.57
0.0
4.28
5.02
2PY
the lactam 2PY form. This is in reasonable agreement with
the experimental observations,2-3,5-7 as well as with the other
theoretical results.8-19 At this level of theory, the lowest excited
singlet state in both systems is the A′(ππ*) state. This is the
state allowed for absorption from the ground state. The abovelying 1A′′(nπ*) state is essentially “dark” in this respect. The
quantities which can be directly compared to the experiment
are the energies of the “vertical” electronic excitations, i.e., the
differences between the energies of the ground and the excited
singlet states, calculated at the ground-state equilibrium geometries. The CASPT2 energies of the S0 f S1(ππ*) transition
calculated for the 2HP and 2PY forms (∆E ) 4.75 eV and ∆E
) 4.37 eV) can be compared to the maxima of the first
absorption bands observed in solution for these systems at 4.59
and 4.20 eV, respectively.2 Having optimized the structures in
the S1 state for both tautomers, we can estimate the energies of
the 0-0 lines in absorption. The obtained values (Table 2) are
∆E ) 4.28 eV and ∆E ) 3.68 eV for 2HP and 2PY,
respectively. These can be compared with ∆E ) 4.47 eV and
∆E ) 3.71 eV observed by Nimlos et al.24 in a molecular jet.
One sees that theoretically predicted values are in good
agreement with the experiment. Thus, it is reasonable to
conclude that the theoretical methods selected for the geometry
optimizations and for the energy calculations of the low-lying
electronic states of the 2HP/2PY system provide results which
are consistent with experimental observations within a fraction
of an electronvolt. Therefore, we can expect similar precision
in the description of the PE surfaces relevant to the photophysics
of these systems.
3.2. PT Potential Energy Functions. In characterization
of the reaction path for PT in terms of local minima and the
transient points between them, we follow the simplified version
of the MEP approach. In this approach one of the 3N-6 (where
N is the number of atoms) intramolecular degress of freedom
of the system is defined as the reaction coordinate, and the
remaining (3N-7) coordinates are optimized at each step of
the reaction. There are no strict rules for choosing the reaction
coordinate, and in principle, this can be any of the 3N-6
intramolecular coordinates, since in this static approach all the
kinetic energy terms are neglected. In practice, this should be
the coordinate which changes the most when the reaction
proceeds. In a typical ESIPT system, the PT reaction, where
the light hydrogen nucleus is moving between two heavier (X
and Y) heteroatoms, being chemically bonded to one of them
and forming a hydrogen bond to the other and Vice Versa, can
3936 J. Phys. Chem., Vol. 100, No. 10, 1996
Sobolewski and Adamowicz
TABLE 2: CASSCF/3-21G and UHF/3-21G Optimized Parameters of Geometry (Bond Lengths in Angstroms; Bond Angles in
Degrees) and Energies (au) of 2PY and 2HP and Their Prefulvenic Forms
1A′′(nπ*)
prefulvenic form
parameter
2PY
2HP
2PY
2HP
2PY
2HP
N1C1
C1C2
C2C3
C3C4
C4C5
C5N1
C3C5
C1O1
C2H
C3H
C4H
C5H
N1H(a)
C1N1C5
C1C2C3
C5N1Ha
C2C1O1
C3C2H
C5C4H
C4C5H
C2C3H
C1C2C4C4
C3C4C5N1
C5N1C1C2
C5N1C1O1
C5C3C2H
C3C5N1Ha
N1C2C3H
C2N1C5H
C2C3C4H
1.373
1.329
1.457
1.400
1.377
1.415
2.418
1.388
1.068
1.068
1.071
1.066
0.995
119.4
118.6
120.6
124.2
121.2
118.5
116.2
120.0
0.000
0.000
0.000
180.0
180.0
180.0
180.0
180.0
180.0
1.365
1.339
1.464
1.407
1.381
1.410
2.429
1.360
1.067
1.069
1.072
1.064
0.964
128.1
118.9
113.5
123.9
122.0
118.4
118.4
119.3
0.000
0.000
0.000
180.0
180.0
0.000
180.0
180.0
180.0
1.440
1.406
1.428
1.378
1.411
1.350
2.424
1.230
1.068
1.068
1.070
1.069
1.000
122.1
121.7
122.5
128.2
121.4
117.8
117.9
120.1
0.000
0.000
0.000
180.0
180.0
180.0
180.0
180.0
180.0
1.353
1.406
1.420
1.404
1.416
1.359
2.454
1.334
1.067
1.067
1.071
1.069
0.970
115.0
118.2
110.2
118.2
123.0
118.7
116.6
121.6
0.000
0.000
0.000
180.0
180.0
0.000
180.0
180.0
180.0
1.384
1.456
1.482
1.507
1.488
1.440
1.545
1.226
1.066
1.071
1.068
1.070
0.995
113.4
110.0
123.3
127.5
126.5
129.7
117.1
120.1
60.1
92.7
-3.14
176.1
176.1
168.7
-141.3
140.3
148.3
1.325
1.403
1.497
1.499
1.492
1.470
1.543
1.361
1.066
1.070
1.068
1.067
0.966
107.2
107.5
110.9
122.8
126.7
128.7
115.8
120.8
61.8
95.8
0.26
178.2
-177.0
-0.09
-141.7
144.7
150.6
-319.5528
-319.5473
-319.5745
-319.5343
-319.6323
-319.6095
energy
a
1A′(ππ*)
These parameters should for the 2HP form be replaced by O1H, C1O1H, and N1C1O1H, respectively.
Figure 2. Cartesian trajectories of the hydrogen atom along the CIS/
3-21G optimized MEP in the A′(ππ*) (a) and in the A′′(nπ*) (b) excited
singlet states for stretching of the NH (circles) and OH (squares) bond
lengths. Triangles in (a) denote trajectory obtained for fixing the NCH
bond angle.
Figure 3. PE functions of the ground (circles) and the A′(ππ*) excited
singlet (squares) states along the MEP for proton transfer in the 2HP/
2PY molecular system calculated in the MP2/D95** and in the
CASPT2/DZVP approximations for both states, respectively. The
reaction coordinate is the NCH bond angle.
be described with the use of the reaction coordinate of the type
X-H‚‚‚Y. It is then quite natural to choose the X-H distance
as the reaction coordinate.27,28 In the 2HP/2PY system there is
no intramolecular hydrogen bond which the proton follows when
the reaction proceeds. Thus, it is not obvious that the choice
of the OH (or NH) stretching provides the optimal reaction
coordinate for the process. An alternative to this would be, for
instance, to chose as the reaction coordinate the angle which
specifies the position of the hydrogen nucleus with respect to
the ring. To compare MEPs for different choices of the reaction
coordinate for MEP, in Figure 2a we present the CIS/3-21G
optimized Cartesian trajectories of the “mobile” hydrogen atom
in the S1(ππ*) state for the three reaction coordinates: the NH
and OH bond lengths and the NCH bond angle. Inspection of
the results leads to the conclusion that the MEP Cartesian
trajectory depends generally on the reaction coordinate. There
are, however, three points common to all the reaction coordinates. These are the equilibrium geometries of the two
tautomeric forms and the transition point between them. This
means that although the shape of the PE function generally
depends on the reaction coordinate, the energetics at the extrema
do not, as expected from the definition of MEP. This provides
a justification that the MEP approach as used in this work is
indeed capable of tracing the reaction path through its saddle
point. In Figure 3 the PE functions for the PT reaction in the
ground state and in the S1(ππ*) state calculated along MEP vs
the NCH bond angle are presented. The PE function of the
excited state was calculated in the CASPT2/DZVP approximation (at the CIS/3-21G geometries) with the active space as
defined in the preceeding section, while the ground-state PE
function was obtained at the MP2/D95G** level (at the HF/
3-21G geometries). One sees, upon insection of the results
Photophysics of 2-Hydroxypyridine
presented in Figure 3, that both tautomeric forms are separated
by a moderate barrier (=1.5 eV) on the ground-state PE surface.
This is in agreement with other theoretical predictions.10,15,18,19
On the S1(ππ*) PE surface, the 2PY f 2HP tautomerization
reaction is endothermic by about 0.54 eV (see Table 1) and the
transient point is 1.25 eV higher above the local minimum of
the 2PY form. Qualitative features of both PE functions are
similar to those reported in ref 18 and obtained at a lower level
of theory. However, the appearance of a shallow minimum in
the middle of the PE function for the S1 state requires a
comment. This minimum, or rather a plateau, appears only at
the CASPT2 level of theory, whereas in the CIS and CASSCF
calculations a well-defined maximum in this region of the
reaction path was found. It is likely that the minimum is an
artifact of the perturbational treatment, because at this point we
also observe a noticeable decrease of the weight of the CASSCF
reference in the first-order wave function. The general conclusion which emerges from the results presented in Figure 3 and
in Table 1 is that the thermally or optically induced 2PY f
2HP reaction is rather unlikely in either the S0 or S1 state.
Let us now tern our attention back to the 1A′′(nπ*) state.
This state is essentially “dark” for absorption from the ground
state and lies just above the “bright” 1A′(ππ*) state (Table 1).
MEP calculated for this state along the NH and OH stretching
coordinates gives proton trajectories presented in Figure 2b. One
sees that for small displacements they are similar to the
trajectories obtained in the 1A′(ππ*) state, but for larger
displacements they are qualitatively different. There is a sharp
(discontinuous) change in each of the trajectories at R(NH) and
at R(OH) of about 1.4 Å related to passing over the barrier.
Further stretch of either one of the reaction coordinates leads
to a local minimum (near the intersection of both trajectories
of Figure 2b), after which the energy increases. There is no
“real” PT reaction observed along either one of the reaction
coordinates. This is, to some extent, in accord with the result
of Barone and Adamo,19 who claim that they were unable to
detect a saddle point for the PT reaction on the 1A′′(nπ*) PE
surface. There is a simple reason for that; the proton trajectory
obtained at the 1A′′(nπ*) PE surface results from artifacts of
the CIS approximation which cannot properly handle the
dissociation. In Figure 4a we present proton trajectories vs the
NH and OH stretches obtained at the CASSCF/3-21G level of
theory with the (20,2/2,4) active space. The first few points of
both trajectories are very similar to those obtained at the CIS
level (Figure 2b), but after passing the barrier the proton freely
dissociates on the CASSCF 1A′′ PE surfaces. This is qualitatively the same picture as presented in ref 18 and obtained at a
more approximate level of theory.
Finally, we conclude this section with a discussion of the
results which we obtained by performing symmetry-unrestricted
search for transition states in the lowest singlet excited states
of the 2PY and 2HP systems with the use of the CIS method
with the 6-31G** basis. The purpose of this search was to
verify whether or not restricting the system to a planar
configuration, which is done in the majority of the calculations
presented in this work, introduces any error in the results. The
transition-state search performed for the 2HP system in the
lowest singlet state resulted in a structure with the hydrogen
atom of the hydroxy group significantly displaced from the
molecular plane (with ROH ) 1.277 Å). The dihedral angle
between the hydrogen, oxygen, and C1 and C2 atoms was equal
to 31°. The analysis of the symmetry of the wave function along
the optimization path from the 2HP equilibrium to the transition
state indicated that for most of the path the structure was planar,
and only near the transition state did a sharply increasing
J. Phys. Chem., Vol. 100, No. 10, 1996 3937
Figure 4. Cartesian trajectories of the hydrogen atom along the
CASSCF/3-21G optimized MEP on the A′′ PE surface (a) calculated
for stretching of the NH (black circles) and the OH (black squares)
bond lengths. Circles and squares denote positions of hydrogen at the
successive steps of energy optimization without any geometry constraints. In (b) positions of hydrogen atom are marked by geometry
optimizations on the A′′ PE surface with the NH and OH distances
fixed.
contribution from a πσ* configuration to the essentially pure
ππ* wave function cause an out-of-plane bend of the hydrogen
atom. After passing the transition point the structure returned
back to the planar conformation with the wavefunction dominated by the πσ* configuration. It was apparent from the results
of the calculation that the transition state was located on the
dissociation path of hydrogen and not the path leading to the
2PY tautomer. A very similar result was obtained in the search
for a transition state on the lowest singlet excited state surface
of the 2PY system. Here also the transition state corresponded
to an out-of-plane displacement of the hydrogen atom bonded
to N2 (by 26.5° with the length of the NH bond to 1.557 Å).
The molecule, initially planar in the ππ* state, was distorted
from planarity near the transition state due to a sharply
increasing contribution from a πσ* configuration, which after
passing the transition point starts to demoniate the wave function
and the molecule becomes planar again. The above investigation indicates that from a narrow domain around the transition
point the PT process in the 2PY/2HP system can be studied
with Cs-constrained geometries. Also, it indicates that in the
lowest excited single state of both tautomers, both transition
states correspond to dissociation of the hydrogen from the
system, and not to an intramolecular proton transfer.
3.3. Photophysics of the “Mobile” Proton. Behavior of
the “mobile” hydrogen atom on the 1A′′ PE surface of the 2HP/
2PY system presented in Figure 4a follows qualitatively the
same pattern as revealed in ref 18. It was suggested in that
paper that there is an intersection between the “bonding” 1nπ*
and “dissociative” 1πσ* PE surfaces along the NH and/or OH
stretching coordinates. When one tries to visualize the PE
functions along the trajectories of Figure 4a, a technical problem
arises. Namely, there is a range of the NH (or OH) bond length
where there are two solutions for the MEP at a given value of
the reaction coordinate. It is the region of the NH (OH) distance
near the barrier, where the MEP depends on the direction the
barrier is approached from (stretching or compression of the
bond length). This indicates that the reaction path for dissociation of the proton is probably sharply curved near the barrier,
and the MEP approach does not describe properly the phenomenon. This problem can be avoided to some extent in the CRP
treatment. Two points (molecular geometries) are needed in
order to define the CRP. For the case on hand, the lower limit
of the CRP reaction coordinate (Q ) 0) is defined to be the
equilibrium geometries of 2HP and 2PY systems on the 1A′′
3938 J. Phys. Chem., Vol. 100, No. 10, 1996
Figure 5. CASPT2/DZVP PE functions of 2PY (a) and 2HP (b) for
the ground (circles), 1A′ (squares), 1A′′ (triangles connected via solid
line), and 3A′′ (triangles connected via dashed line) states calculated
along the CRP for hydrogen dissociation. The CRP is spanned by
equilibrium geometry of a given tautomeric form in the 1A′′ state (Q
) 0) and by geometry with the hydrogen atom in positions 1′ and 3′
of Figure 4b for (a) and (b), respectively. The CASSCF/3-21G
geometries were optimized on the 1A′′ PE surface.
PE surface. The upper limit of the CRP reaction coordinate
for the molecular system with the “mobile” hydrogen atom
should correspond to its dissociation. This is, of course, the
situation of minor importance from the spectroscopic point of
view, because the most interesting region of the reaction is near
the barrier. Comparing trajectories of the proton presented in
Figures 2b and 4a, one sees that the calculated trajectory is rather
sensitive to the theoretical method used in determination of the
MEP. The CASSCF approximation, which handles properly
the near-degeneracy effects, but takes into account only a small
part of the dynamic electron correlation, predicts almost “radial”
dissociation of the proton with respect to the molecular frame.
On the other hand, the CIS approximation, which does not
handle properly the near-degeneracy effects resulting from
breaking the chemical bond, predicts essentially “angular”
motion of the proton instead of dissociation. The proper
description of both effects, i.e., the near-degeneracy and dynamic
correlation effects will probably result in a proton trajectory
which is closer to the CASSCF result. Thus, in the following
we decided to optimize the molecular geometry for the upper
limit of CRP by fixing the distance of the proton from both
heteroatoms. The points at which the proton was located with
respect to the molecular frame in the optimization procedure
are indicated in Figure 4b. The geometry optimizations at these
points were performed at the CASSCF/3-21G level with the
(20,2/2,4) active space. Among them, points 1′ and 3′ were
used for the definition of the upper limits of the CRP coordinates
(Q ) 1) for dissociation of the proton from the 2PY and 2HP
molecules, respectively. As is usually the case, the CRP was
defined as the vector of the internal displacement (bond lengths
and bond angles) that connects the initial and final geometries
of the two tautomeric forms. Next, a number of intermediate
nuclear configurations were generated by incremental increase
of the reaction coordinate from Q ) 0 to Q ) 1, and the energies
of the ground and lowest excited states were calculated at each
configuration. The energies were calculated at the CASPT2/
DZVP level with the (20,0/2,7) active space.
The resulting PE functions are presented in Figure 5, and
they look similar for 2HP and 2PY and are not qualitatively
different from the appropriate functions of ref 18 obtained at
the CASSCF/DZV level of theory (Figure 4b,c of ref 18). One
can notice upon inspection of the results presented in Figure 5
that only PE surfaces of the A′′ state and the ground state are
relevant to the proton (pre)dissociation. The energy of the A′
Sobolewski and Adamowicz
Figure 6. CASPT2/DZVP PE functions of the ground (circles), the
1
A′′ (squares with solid line), and the 3A′′ (squares with dashed line)
states of the 2HP/2PY system calculated with the hydrogen atom fixed
in positions 1-3 and 1′-3′ of Figure 4b for (a) and (b), respectively.
state rises sharply along the reaction coordinate and does not
lead to dissociation. The first conclusion resulting from the
results presented in Figure 5 is that both tautomeric forms are
well protected in the A′′ state against dissociation of the proton.
Barriers of about 1.5 eV can be estimated for both 2HP and
2PY systems with respect to their equilibrium in the 1A′′(nπ*)
state. Let us notice that geometries of the transition structures
were not optimized, so the real saddle point may have a
somewhat lower energy. We do not expect, however, that this
can significantly lower the barrier. In any case, it seems that
the barriers are too high for any noticeable tunneling or for any
thermal transfer of the proton to the “dissociative” part of the
PE surface. If, however, the wave packet can be optically
“prepared” on the excited PE surface to effectively tunnel
through the barrier and to reach the repulsive A′′(πσ*) PE
surface, it will exhibit a significant tendency toward dissociation
of the hydrogen atom. On the other hand, just behind the barrier
there is an intersection between the excited A′′ and groundstate PE surfaces. This intersection occurs only when the Cs
symmetry (the planarity) of the system is conserved. Any outof-plane deformation of the system will cause an avoided
crossing of the two surfaces, resulting in a conical intersection.
Strong nonadiabatic interactions present near such an intersection will cause an irreversible nonradiative relaxation of the
wave packet to the ground state. In the condensed phase the
system should be effectively cooled down in the ground state
to a local minimum corresponding to one of the two tautomeric
forms. One may wonder, Is there a nonzero probability for the
wave packet to reach the other tautomeric minimum due to such
a process?
In Figure 6 we present energies calculated for the 2HP/2PY
system with the hydrogen atom placed in one of the positions
indicated in Figure 4b. In the CASSCF/3-21G optimization
procedure, only the relative position of the hydrogen atom with
respect to the heteroatoms (N and O) was fixed. The rest of
intramolecular degrees of freedom were fully optimized by
minimizing the energy of the 1A′′(πσ*) state. The first cross
section through the PE surfaces (shown in Figure 6a) is just
behind the barrier on the A′′ PE surface, and the second (shown
in Figure 6b) corresponds to the hydrogen atom being shifted
by 1 Å toward its dissociation. It is interesting to note that
there is an energy gradient on the A′′(πσ*) PE surface (singlet
or triplet) attracting the hydrogen atom toward the middle point
(of roughly equal distances from nitrogen and oxygen atoms).
There is a region where the ground-state PE surface approaches
the A′′ PE surface and where the conical intersection is located.
It is thus likely that the wave packet propagated on the repulsive
A′′(πσ*) PE surface can reach the ground-state PE surface near
Photophysics of 2-Hydroxypyridine
J. Phys. Chem., Vol. 100, No. 10, 1996 3939
Figure 8. Prefulvenic forms of 2PY (a) and 2HP (b) optimized in
their ground state at the UHF/3-31G approximation.
Figure 7. CASSCF/DZVP PE functions of the four lowest excited
singlet states of A′′ symmetry calculated along the MEP optimized in
the 1A′′ state at the CASSCF/3-21G approximation for 2PY (a) and
for 2HP (b) vs stretching of the NH and OH bond lengths, respectively.
its transition point for PT, and thus the system can further relax
into one of the local minima. Thus, in principle, the PT reaction
can take place.
The scenario sketched above is similar to the photon-induced
dissociation-association (PIDA) mechanism of PT proposed
in ref 18. The question remains, How can the wave packet
propagated near the equilibria on the excited A′′ or A′ PE
surfaces reach the repulsive A′′(πσ*) PE surface? The barrier
on the 1A′′ PE surface are rather prohibitive with respect to
effective tunneling or a thermal excitation. The experiments
where the photoinduced PT reaction was detected3,23 were
performed under stationary conditions. One thus cannot exclude
that a second photon is absorbed by the system during its
lifetime in the excited state. In relation to that, it would be
interesting to explore higher excited states, in particular the πσ*
states, which can be adiabatically correlated to the repulsive
part of the A′′ PE surface. In Figure 7 we present the PE
functions of the four lowest singlet states of the A′′ symmetry
calculated for different stretchings of the NH (Figure 7a) and
the OH (Figure 7b) bonds. Because all the states belong to the
same symmetry representation in the Cs point group, it is almost
impossible to obtain converged CASSCF wave functions for
all the (but the lowest) roots. The PE functions presented in
Figure 7 were obtained in the state-averaged CASSCF/DZVP
calculations with the (20,2/2,7) active space. We should
mention that even the state-averaged approach has a convergence
problem in the region of strong mixing between the states.
Fortunately, the first three points presented in Figure 7 allow
us to qualitatively characterize the PE functions of the higher
excited states of the A′′ symmetry. The two lowest states have
near the equilibrium geometry a mostly nπ* character, and the
next two are dominated by the πσ* electronic configurations.
Among them, the state number three (when counting from the
bottom) correlates adiabatically to the “dissociative” part of the
A′′ PE surface for a larger stretching of the reaction coordinate.
The wave packet excited (presumably by the second photon)
to this state is accelerated in the direction of the proton
dissociation. Thus, there is a chance to pass through the region
of strong nonadiabatic interactions between the PE surfaces and
to reach the “dissociative” 1A′′(πσ*) PE surface. This may
provide the “driving” force for the PIDA mechanism to operate
in the proton-transferring system without an intramolecular
hydrogen bond.
3.4. Nonradiative Deactivation Channel. The photophysics of the 2HP/2PY system discussed in the preceding section
results in a conclusion that the PIPT reaction via the PIDA
mechanism should be essentially symmetric with respect to the
“doorway” tautomeric form. Thus, longer wavelengths of
exciting light should induce the lactam (2PY) to lactim (2HP)
transformation, whereas a shorter wavelength should induce the
reverse process. While there are some experimental works
reporting the lactam-to-lactim PIPT transformation in 2HP/2PY
and in similar systems,3,23 there exists no evidence for the reverse
reaction. A possible explanation for this, as was already
suggested in ref 18, is that an efficient radiation-less transition
to the ground state takes place after excitation of the lactim
(aromatic) form within the lowest ππ* singlet manifold. It has
been postulated that strong nonadiabatic interactions between
the excited and ground states of the aromatic 2HP molecule
along the reaction path to its biradical prefulvenic form may
be responsible for its efficient internal conversion to the ground
state. The above-mentioned postulate of ref 18 has been drawn
by analogy to benzene and pyrazine, where such interactions
determine the excited-state dynamics.52,53 More recently, this
type of internal conversion has also been elucidated for other
systems.25,28 In the following we present results of explorations
of the prefulvenic reaction path for the 2HP/2PY system
interconversion.
First, it appears that the full aromaticity of the (aza)aromatic
ring is not the necessary condition for the formation of a stable
(or metastable) prefulvenic form, as has been previously
suggested. In Figure 8 we schematically present the geometry
of one, among the six possible, prefulvenic form of 2PY and
2HP molecules. Both tautomeric forms represent local minima
on the PE surface of the ground state at the UHF/3-21G
approximation. The UHF/3-31G optimized parameters of their
geometries are listed in Table 2 and are very close to those
reported previously for other similar systems.25,52,53 A characteristic feature of such structures is a strong out-of-plane
distortion of one of the ring atoms (C4 for the case on hand)
and formation of a new chemical bond between a pair of
neighboring atoms (C3 and C5 in Figure 8). The biradical
character of such structures is due to localization of one electron
on the out-of-plane tilted atom (C4) and localization of another
electron on the aryl part of the ring (N1-C1-C2), which is
almost planar.
Both structures pictured in Figure 8 belong to the C1 point
group, and there is no symmetry distinction between their
excited electronic states. This means that it would be difficult
to apply the MEP approach to study the reaction path on the
excited PE surface, since there is a high probability that the
optimized wave function of the higher electronic state collapses
to the ground state. Thus, CRP is the method of choice for
characterization of this reaction path. The initial and final CRP
points are the geometry of the CASSCF/3-21G optimized
1A′(ππ*) state (Q ) 0) and the geometry of the UHF/3-21G
optimized ground state of the prefulvenic form (Q ) 1). The
geometrical parameters of the initial and final 2PY and 2HP
structures are listed in Table 2. The CASPT2/DZVP PE
functions of the ground and first excited singlet states calculated
3940 J. Phys. Chem., Vol. 100, No. 10, 1996
Figure 9. CASPT2/DZVP PE functions of the ground and the first
excited singlet state of 2PY (a) and 2HP (b) calculated along the CRP
from the minimum of the S1 state at planar geometry (Q ) 0) to the
minimum of the prefulvenic form (Q ) 1).
Figure 10. CASSCF/DZVP adiabatic (solid) and diabatic (dashed) PE
functions calculated near the avoided crossing region of Figure 9a (a)
and the NAC element (b).
with respect to the state-averaged CASSCF wave functions are
presented in Figure 9.
Upon inspection of the PE functions presented in Figure 9,
one can notice that they look qualitatively similar for both
systems; that is, their shape results from avoided crossing of
two electronic states. A different degree of configuration mixing
has a dramatically different effect on the photophysics of the
two molecules. First of all, 2PY is rather well protected in its
(almost) planar form at the equilibrium of the S1(ππ*) state.
This is not the case for the 2HP form. The nuclear frame of
this molecule is much “softer” with respect to the out-of-plane
deformation along the reaction coordinate. Also, the barrier
which separates the equilibrium in the S1(ππ*) state from the
region of the avoided crossing with the ground state is much
lower in this case. Let us notice that the transient point on the
S1 PE surface was not optimized in the present treatment. Thus,
the barrier may be lower than indicated in Figure 9b. In other
words, the wave packet propagated on the S1 PE surface of 2HP
can rather easily reach the region of strong coupling to the
ground state and relax due to nonadiabatic interactions.
To visualize the effect of the interstate coupling, the stateaveraged CASSCF/DZVP PE functions of the ground and
excited singlet states near the interaction region are presented
in Figures 10a and 11a for 2PY and 2HP, respectively. The
CASSCF adiabatic PE functions can approximately be “diabatized” by a block-diagonlization of the configuration interaction
matrix (see for details refs 25, 54, and 55). The quasi-diabatic
PE functions are also presented in Figure 10a,b, and they
intersect each other as expected. The derivative of the adiabaticto-diabatic mixing angle ν (R) with respect to the reaction
coordinate R determines the nonadiabatic-coupling (NAC)
element. This derivative is presented in Figure 10b and 11b
Sobolewski and Adamowicz
Figure 11. CASSCF/DZVP adiabatic (solid) and diabatic (dashed) PE
functions calculated near the avoided crossing region of Figure 9a (a)
and the NAC element (b).
for 2PY and 2HP, respectively. Because there is no real
intersection between adiabatic PE surfaces, the NAC element
does not show any singularity at the intersection as it does for
the conical intersection.54 Nevertheless, the NAC element
reaches a much higher value for 2HP than for 2PY. Moreover,
any additional out-of-plane deformation of the system will
amplify the coupling. Thus, one can except that photophysics
of 2HP in the first excited singlet state will follow a similar
pattern as predicted for benzene and pyrazine; that is it will be
dominated by radiation less decay to the ground state.
In closing, we can confirm the hypothesis of ref 18 that
nonadiabatic interactions between the S1 and S0 states in 2HP
provide a channel for an efficient nonradiative decay of
electronic excitation to the ground state. This process is
ineffective in depopulating the lowest excited states of 2PY
because this molecule is well protected by barriers near its
equilibrium geometry.
4. Conclusions
Extensive ab initio explorations of multidimensional PE
surfaces, reported in this work, were performed with the
intention of developing a better understanding of the photophysical behavior of the 2HP/2PY molecular system. Among
many possible photoreaction channels, we have focused our
attention on those which are expected to be relevant to the
tautomerization (PT) reaction due to optical excitation. The
picture of photophysics of the 2HP/2PY system which emerges
from our study is rather complex. Present results, obtained with
the aid of the state-of-art ab initio technology for excited states
of larger polyatomic systems, confirm generally the earlier
suggestion that the excited-state dissociation followed by the
ground-state association of the hydrogen atom is the intrinsic
feature of the photoinduced proton transfer (PIPT) reaction in
systems without intramolecular hydrogen bonds. The direction
of the PT reaction, which according to the experimental
observations, occurs exclusively from the lactam (oxo) form to
the lactim (hydroxy) form, is explained by the existence of an
efficient channel for radiation less deactivation of the lowest
excited singlet state of 2HP. This deactivation process is due
to strong nonadiabatic interactions with the ground state along
the reaction path leading to the prefulvenic form. This channel
is supposed to be fast enough to effectively compete with any
other reaction which can occur after optical excitation within
the lowest singlet manifold of the lactim form. The lactam 2PY
form is protected by a significant barrier on the PE surface of
the lowest excited singlet state from the region of strong
nonadiabatic interactions with the ground state, and thus internal
conversion to the ground state seems to be unimportant for the
dynamics on the excited-state PE surface.
Photophysics of 2-Hydroxypyridine
Both tautomeric forms are protected by rather large barriers
against dissociation of the “mobile” hydrogen atom on the
excited-state PE surface. Our results lead to the conclusion that
excitations (via a second photon) to higher excited electronic
states may provide a driving force for dissociation of the
hydrogen atom and promote the PT reaction. On the other hand,
excitation to higher electronic states can open many additional
reaction channels, and this practically prohibits any serious
theoretical consideration of the relative efficiency of those
channels. This hypothesis can, in principle, be verified in a
two-photon pump-probe experiment or in a study on the
dependency of the yield of the tautomerization reaction on the
intensity of the light.
Acknowledgment. This study was supported by a grant from
the Office of Health and Environmental Research of the
Department of Energy (No. DEFG 0393ER61605) and by a
grant from the Committee for Scientific Research of Poland
(No. 2 2395 92 03). L.A. would like to thank the Natural
Science Research Council of Sweden for supporting his stay at
the Theoretical Department, University of Lund. L.A. also
wishes to thank Prof. Björn Roos for his hospitality.
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