Excited-State Dynamics of the 2-Hydroxypyridine−Ammonia Complex

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Excited-State Dynamics of the 2-Hydroxypyridine-Ammonia Complex† M. Esboui,‡ C. Jouvet,*,§ C. Dedonder,§ and T. Ebata| Laboratoire de Spectroscopie Atomique, Mole´culaire et Applications, De´partement de Physique, Faculte´ des Sciences de Tunis, 1060 Tunis, Tunisia, Laboratoire de Photophysique Mole´culaire du CNRS et Centre laser de l’UniVersite´ Paris-Sud (CLUPS), UniVersite´ Paris-Sud 11, 91405 Orsay, France, and Department of Chemistry, Graduate School of Science, Hiroshima UniVersity, Higashi-Hiroshima 739-8526, Japan ReceiVed: July 14, 2009; ReVised Manuscript ReceiVed: NoVember 4, 2009

The excited-state lifetimes of different vibrational levels of the 2-hydroxypyridine-ammonia complex have been recorded with the picosecond pump probe technique. These lifetimes decrease with increasing energy. The decrease of the lifetime is discussed based on the possible enol keto isomerization computed using RICC2 methods. 1. Introduction The enol f keto tautomerization between 2-hydroxypyridine (2HP) and 2-pyridone (2PY) constitutes one of the simplest models for DNA base units and for molecules involved in the biological life cycle, where tautomerization through hydrogen or proton transfer causes genetic damage.1 2PY and 2HP have complementary adjacent H-bonding sites, the NH donor and CdO acceptor sites in 2PY (keto form) and the nitrogen acceptor and OH donor sites in 2HP (enol form) that can form stable hydrogen bonded complexes with solvent molecules such as water, ammonia, or methanol.2-12 However, most of the previous studies on clusters of 2HP/2PY such as one-color resonanceenhanced multiphoton ionization, fluorescence excitation, and dispersed fluorescence spectra, in particular with ammonia and water, are not directly connected to the dynamics of the enol f keto tautomerization. In the 2PY and 2HP complexes with one ammonia molecule, Nimlos et al. have observed a mixing of the nπ* and ππ* electronic states.11 In these clusters, the ammonia molecule acts as both a proton donor and an acceptor,2-5,7 which is unusual.13 Held et al. have analyzed the rotationally resolved S1 r S0 fluorescence excitation spectrum of the hydrogen-bonded complex of 2PY with ammonia.7 They have found that in the monosolvated complex (2PY-NH3) correlation between the hydrogen bond distances and the barriers to internal rotation of NH3 in both electronic states suggests the existence of two hydrogen-bonding interactions. The 2PY-(NH3)2 cluster exhibits three hydrogen bonds and there is no evidence for any internal rotation of either ammonia molecule in this cluster in both electronic states. The two tautomers can also form three chemically distinct and strongly bound dimers; the 2PY2 and 2HP2 homodimers and 2HP-2PY mixed dimer, a model for double proton transfer in base pairs.14-17 Theoretical calculations of structures and energetics of enol and keto forms and their interactions with ammonia molecules * Corresponding author. E-mail: [email protected]. † Part of the “Benoît Soep Festschrift”. ‡ Laboratoire de Spectroscopie Atomique, Mole´culaire et Applications, De´partement de Physique, Faculte´ des Sciences de Tunis. § Laboratoire de Photophysique Mole´culaire du CNRS et Centre laser de l’Universite´ Paris-Sud (CLUPS), Universite´ Paris-Sud 11. | Department of Chemistry, Graduate School of Science, Hiroshima University.

without symmetry constraints have been investigated in S0 and S1 states in previous studies.2-5 It was found that proton transfer occurs in these clusters in both S0 and S1. CIS/6-31G(d,p) calculations show that at least three ammonia molecules are needed to produce a stable excited ion pair cluster.4 Recently, chromophore to ammonia excited state proton transfer has been found to compete with a reaction classified as a hydrogen or coupled electron-proton transfer, where the formation of a radical pair is the primary step in these types of reactions.18-24 These processes could both be present in 2HP/2PY clustered with ammonia. In the present study, we concentrate on the excited state dynamics in the 2HP-NH3 complex, performing lifetime measurements and ab initio calculations on the hydrogen/proton transfer via the ammonia molecule resulting in a tautomerization reaction in the complex. Recently Chmura et al. have studied the 2PY/2HP photoinduced isomerization with CC2 and CASSCF methods in the free molecule.25 They have shown that this reaction proceeds on a surface of A″ symmetry, through the πσ* state, repulsive with respect to the hydrogen atom detachment, where the system evolves to a conical intersection with the ground electronic state. At the intersection the N-H bond is elongated so that the system can be treated as a biradical consisting of the departing hydrogen atom and the remaining molecular radical. The system then relaxes on the ground electronic state PE surface toward one of the two (enol or keto) ground-state minima. The process is thus a hydrogen rather than a proton transfer in the bare molecule, but the situation may be different in the complex if the intersection with the ground state is removed as in the case of phenol or indole complexed with ammonia.18,21,22 This system is then one of the smallest systems in which we can study the competition between the solvent-induced hydrogen (H) or proton (P) intramolecular reaction and H/P transfer from the molecule to the solvent. The question of proton or hydrogen transfer reaction remains an open question. For example in the 7-hydroxyquinoline-(NH3)3, the enol - keto tautomerization mediated by the NH3 wire has first been assigned to an excited state proton transfer reaction,26 then to hydrogen transfer from CIS calculations,20 and lately to proton transfer from CASSCF/ CASPT2 calculations.27 In a similar system, 7-azaindole with solvent molecules, CC2 calculations seem to show that the solvent-induced transfer should be a sequential proton transfer

10.1021/jp906652y  2010 American Chemical Society Published on Web 12/04/2009

Excited-State Dynamics of 2HP-NH3 in the case of 7-azaindole(NH3)228 and a concerted double proton transfer in the case of 7-azaindole(methanol)229,30 and 7-azaindole(water) clusters.31 2. Experimental Setup The experimental setup for picosecond pump-probe spectroscopy has already been described in previous papers.32 Briefly, the fundamental output of a mode-locked picosecond Nd3+:YAG laser (Ekspla PL2143B) is frequency tripled to 355 nm and split into two parts to pump two OPG/OPA and SHG systems (Ekspla PG401SH) so as to obtain two independently tunable UV laser beams. Typical bandwidth and output power of the UV light is 12 cm-1 and 50-100 µJ, respectively. Jet-cooled molecules and their clusters are generated by a supersonic expansion of the sample vapor seeded in a mixture of He/NH3 carrier gas (typically 1 to 3% ammonia in helium) at a total pressure of 3 bar into vacuum through a pulsed nozzle (General Valve) with a 0.8 mm aperture. The free jet is skimmed by a 0.8 mm diameter skimmer (Beam Dynamics) located 30 mm downstream of the nozzle. The UV lasers are introduced into the vacuum chamber in a counterpropagated manner and cross the supersonic beam 50 mm downstream of the skimmer. The molecules and clusters in the supersonic beam are ionized by 1 + 1 resonance enhanced multiphoton ionization (REMPI) via the S1 state, and the ions were repelled perpendicularly to the direction of the molecular beam and the laser beams. The ions are mass analyzed by a 50 cm time-of-flight tube and detected by an electron multiplier (Burle 4700). The ion signals are integrated by a digital boxcar integrator (PAR 4402) connected to a computer. A computer-controlled optical delay line sets the delay time between the tunable S1-S0 UV excitation laser and the UV ionization laser pulses. 3. Computational Methods All calculations were carried out with the TURBOMOLE program package, version 5.8.33 The ground state structures of 2HP-NH3 and 2PY-NH3 in Cs symmetry were optimized at the second-order Moller-Plesset MP2 level within resolution of the identity (RI) approximation for the electron repulsion integrals34 (RI-MP2). For all atoms, the correlation-consistent polarized valence of double-ζ basis set cc-pVDZ quality augmented with diffuse functions aug-cc-pVDZ was used. The use of diffuse basis functions is required to describe correctly low-lying Rydberg πσ* excited states. Some complementary calculations for the ground-state structure were done using the TZVP basis set on all atoms. The equilibrium geometries and the reaction path in the lowest excited singlet states of the enol f keto tautomerization reaction have been calculated at the second-order approximate coupled cluster (CC2) method employing the resolution of the identity (RI) approximation,35,36 using the aug-cc-pVDZ basis set. The goal of the theoretical part of this work is to provide support for the experimental observation. To do this, one has to explore the excited-state potential energy (PE) surface in the vicinity of the Franck-Condon region. For this purpose CC2 is the method of choice since it allows excited-state optimization at the electron-correlated level and is free of drawbacks of the TDDFT method. Potential energy (PE) profiles have been calculated along the minimum energy paths (MEP) for an elongation of the R(O-H) or R(N-H)) stretching coordinate in 2HP-NH3 and 2PY-NH3 to identify the reaction path for hydrogen or proton transfer: for a given value of the stretching coordimate R(O-H) or R(N-H)), all remaining coordinates have been optimized for

J. Phys. Chem. A, Vol. 114, No. 9, 2010 3061 the S1 excited state, under Cs symmetry. It should however be emphasized that the CC2 method may fail in the description of a conical intersection between excited states or between ground and the electronically excited state, but this is not the main point of this study. 4. Results 4.1. Experimental Results. As in the case of phenol or indole derivatives clustered with ammonia, we have searched for the formation of NH4(NH3)n products through a delayed ionization of the reaction products. Although we employed the same methodology and laser power as for phenol or indole, the NH4(NH3)n photoproduct signals were very small. It is difficult to make really quantitative comparisons, and it seems that the H transfer process from 2-hydroxypyridine to the ammonia cluster is less efficient than in the case of phenol. For the 1:1 complex, the two-color excitation spectrum presented in Figure 1a is essentially the same as that reported previously.11 The band origin of the complex is red-shifted by 942 cm-1, and the spectrum presents a number of vibrational bands, most of which can be assigned to intramolecular modes: the 6a10, 6b10, 110, and 1210 vibronic bands corresponding to optically active in-plane modes are located at 511, 541, 810, and 954 cm-1 above the 2-hydroxypyridine origin, and the intense bands in the complex excitation spectrum located at +507, +564, +810, and +969 cm-1 are the analogues. The band around 420 cm-1 could correspond to the in plane OH bending vibration (calculated at 415 cm-1 in the ground state) while the 210 cm-1 band may be either an intramolecular mode that is weak in the bare molecule or an intermolecular vibration (stretching). Above 1200 cm-1, the vibrational structure becomes diffuse. The lifetime of the 2HP monomer has been measured on the S1 band origin to be 935 ps, in good agreement with the 170 MHz line width measured by Borst et al.37 The 2HP-NH3 complex lifetime has been measured for different vibrational bands, indicated by an asterisk on Figure 1a, to test the existence of a reaction (P/H transfer, enol keto isomerization) and of an eventual barrier on the reaction path and are presented in Figure 1b-d. The lifetime measured on the band origin is 720 ps, shorter than that for the bare molecule, which may be indicative of a supplementary nonradiative process, and when higher vibrational states are excited, the lifetime shortens to 250 ps at 1200 cm-1 higher in energy and to 150 ps at 2000 cm-1 above the band origin. The decrease is however less abrupt than in the case of the phenol-NH3 complex and seems rather monotonous, without mode specificity (Figure 1d). 4.2. Theoretical Results. The ground-state structures and vibrational modes of 2HP-NH3 and its tautomer under Cs symmetry have been calculated at the RI-MP2 level. When a basis set aug-cc-pVDZ on all atoms is used, 2HP-NH3 is more stable than 2PY-NH3 by 0.04 eV at the RI-MP2 level (0.05 eV when the ZPE is taken into account) but is less stable than 2PY-NH3 by 0.05 eV at the RI-CC2 level used for excitedstate calculations. With the TZVP basis set, 2HP-NH3 is more stable than 2PY-NH3 at both levels. These results are in agreement with previous calculations by Dkhissi using MP2/ 6-31++G** methods that give 2HP-NH3 lower than 2PY-NH3 by 0.068 eV (0.076 with ZPE).3 The vertical excitation energies have been calculated with the RI-CC2 method, starting from the RI-MP2 optimized geometry of the ground state. In the enol form (2HP-NH3), the first excited state corresponds to a state with A′ symmetry (ππ*) calculated at an energy of 4.79 eV and the second to the A″ (nπ*) state at 5.41 eV. In the keto tautomer (2PY-NH3),

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Figure 1. Excitation spectrum and excited-state lifetimes for the 2HP-NH3 complex: (a) excitation spectrum of 2HP (red) and 2HP-NH3 (black), lifetimes have been measured on the bands marked with an asterisk; (b) lifetime recorded on the 2HP-NH3 band origin; (c) lifetime recorded 1200 cm-1 above the band origin; (d) variation of the lifetime with excess energy in 2HP-NH3 compared to the phenol-NH3 complex.

TABLE 1: Relative Stabilities (in eV) of the 2HP-NH3 and 2PY-NH3 Complexes in Their Ground-State and Excited-State Vertical Transition Energies under Cs Symmetry 2HP-NH3 S0 MP2 S0 CC2 S1 A′(ππ*) CC2 vertical transition S2 A″(nπ*) CC2 vertical transition

2PY-NH3

aug-cc-pVDZ

TZVP

aug-cc-pVDZ

TZVP

0 0 4.79

0 0 4.90

0.04 -0.05 4.23

0.093 0.006 4.44

5.41

5.61

4.48

4.66

the vertical transition energy to the A′ (ππ*) state is at 4.23 eV, and to the A″ (nπ*) state at 4.48 eV. The geometries of the first excited states of A′ (ππ*) and A″ (nπ*) symmetry have been optimized in keeping the Cs symmetry. The 2HP-NH3 A′ (ππ*) and A″ (nπ*) geometries are compared to the ground state RI-MP2 optimized structure in Figure 2. The O-H · · · N hydrogen bond of the enol complex is shortened by 0.20 Å in the first A′(ππ*) excited state, and there is a similar shortening (0.14 Å) of the distance between the pyridine nitrogen and the nearest hydrogen of the ammonia molecule; overall the ammonia molecule comes closer to the 2-hydroxypyridine moiety. For the analogous phenol compound, the RHF and CIS calculations of Yi and Scheiner38 and the CASSCF calculations of Sobolewski and Domcke21 under Cs symmetry also show that the hydrogen bond is shortened upon excitation to the S1 A′(ππ*) state. In the S2 A″(nπ*) excited

Figure 2. Optimized structure of the 2HP-NH3 enol complex in its ground and first excited states of A′ and A″ symmetry.

state, on the other hand, the optimization leads to an O-H · · · N hydrogen bond only slightly shorter (0.05 Å) than in the ground state, while the distance to the pyridine nitrogen increases. In the keto form, while the A′(ππ*) state is lower than the A″(nπ*) for vertical excitation, optimization changes the ordering and the first excited state in Cs symmetry is the A″(nπ*) state in which the N-H · · · N hydrogen bond is slightly longer (0.02 Å) than in the ground state and the distance between the carbonyl oxygen and the ammonia molecule greatly increases by 0.59 Å (Figure 3). In the S2 A′ (ππ*) state the N-H · · · N hydrogen bond is shortened by 0.06 Å, and the distance between the carbonyl oxygen and the ammonia molecule increases by 0.2 Å. The hydrogen bond change in the keto tautomer agrees with B3LYP and CIS calculations that 2PY-NH3 hydrogen bonds

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Figure 3. Optimized structure of the 2PY-NH3 complex in its ground and first excited states of A′ and A″ symmetry.

TABLE 2: Relative Energies (eV) and Oscillator Strengths of the Lowest Singlet States Computed at the RI-CC2 Level with the aug-cc-pVDZ Basis Set for the Relevant Structure in Cs Symmetrya 2HP-NH3

S0 1A′ 1A″ 2A′ 2A″ 3A′ 3A″

2PY-NH3

energy (eV)b

oscillator strength

energy (eV)b

oscillator strength

0.0 4.40 (ππ*) 4.83 (nπ*) 5.91 (ππ*) 5.13 (nπ*) 6.76 (ππ*) 6.34 (πσ*)

0.0714 0.001 0.176 0.0015 0.0399 0.003

-0.05 3.57 (ππ*) 3.22 (nπ*) 5.21 (nσ*) 5.03 (nπ*) 5.48 (ππ*) 5.42 (πσ*)

0.057 6.3 × 10-5 0.005 2.8 × 10-5 0.226 0.003

a S0, 1A′ and 1A″ states have been optimized, 2A′ and 3A′ are calculated at the 1A′ geometry while 2A″ and 3A″ are calculated at the 1A″ geometry. b All the energies are scaled with respect to the ground-state energy of the 2HP-NH3 complex.

are elongated upon excitation to the S1 state4 indicating a decreasing bond strength consistent with the blue shift of the transition. For this complex, the RI-CC2 computed intermolecular distances N-N and N · · · H are 2.868 and 1.844 Å, only slightly shorter than the experimental values (3.02 and 2.06 Å, respectively), while for the NNH3-H · · · O bond in which NH3 is a hydrogen donor, the RI-CC2 computed bond length (2.367 Å) is in disagreement with the experimental value (3.05 Å).7 The energy and the corresponding oscillator strengths of the lowest excited states of both A′ and A″ symmetries in the enol and keto tautomers, obtained at the RI-CC2 level of theory, are given in Table 2. In the enol form, the calculated adiabatic excitation energy under Cs symmetry for the 1A′ (ππ*) is 4.40 eV, which corresponds to a stabilization of 0.39 eV from the vertical excitation energy, and the first 1A″(nπ*) state lowers to 4.83 eV. In the keto form, the 1A″(nπ*) state is the lowest excited state at 3.22 eV, 1.2 eV below the excited state of the enol form. This state has a negligible oscillator strength, and the next state is an optically accessible 1A′(ππ*) at 3.57 eV (0.35 eV above the nπ* state and 0.83 eV below the corresponding 1A′(ππ*) of the enol isomer) that bears the oscillator strength. Figures 4 and 5 show potential energy profiles computed for the 1A′ and 1A″ excited states (Cs symmetry) in 2HP-NH3 and its 2PY-NH3 tautomer at the RI-CC2 level, obtained in elongating the OH or NH distances and optimizing the other coordinates. These potential energy profiles were calculated to find out if the enol f keto tautomerization could take place through consecutive proton transfer (PT) or hydrogen atom transfer (HT) reaction.

In the potential energy function of A′ symmetry, starting from the optimized ππ* state of the enol complex and increasing the OH distance, the ππ* stays the lowest state, which will correspond to a proton transfer from the 2HP to the ammonia part of the complex. The energy of the ππ* state slightly increases up to the OH distance of 1.2 Å and decreases afterward, indicating a barrier for the proton transfer reaction from 2HP to ammonia. When the OH distance exceeds 1.4 Å, the proton is attached to the ammonia molecule, and a second proton transfer from the ammonium to the nitrogen of the hydroxypyridine occurs without barrier, leading to the keto form. When starting from the geometry with an OH bond fixed at 1.6 Å (energy 3.88 eV) and releasing the constraint, the system returns to the optimized structure of the ππ* state of the 2PY-NH3 complex at 3.57 eV. Thus in A′ symmetry, the enol f keto isomerization can take place if the energy is sufficient to overcome the barrier of 0.033 eV for the proton transfer from the 2-hydroxypiridine part to the ammonia molecule. The geometry at the transition state corresponds to a proton at middistance between the oxygen atom and the nitrogen of the ammonia molecule and a short distance from the ammonia molecule to the pyridine nitrogen (Figure 4). In Figure 4, the right part of the figure represents the back reaction when the NH distance in the 2PY-NH3 complex is stretched from the optimized geometry. Following this path, the longer distance calculated does not reach yet the transition state which occurs at NH ) 1.99 Å but it shows a smooth increase of the energy toward the transition state following this path. The route from the enol to the keto form is different under A″ symmetry. In A″ symmetry (Figure 5), the first excited state is the nπ* state, but the increase of the O-H bond in the enol complex induces a stabilization of the πσ* state that crosses the nπ* state at an OH distance of 1.08 Å. The σ* orbital being localized on the ammonia part, this crossing corresponds to a coupled electron-proton transfer to form a hydrogen transferred (HT) intermediate of the form C5H4NO• · · · •NH4 (intermediate 1 on Figure 5). The energy of the πσ* state at the minimum is 4.00 eV. Alternatively, starting from the keto form and increasing the N-H bond results in a stabilization of the πσ* state that crosses the lowest nπ* state (d(N-H) ) 1.65 Å). The optimized structure of the πσ* state, intermediate 2 in Figure 5, has the same energy (within 0.01 eV) as intermediate 1 and is also a C5H4NO• radical hydrogen bonded to a NH4 radical, but the geometry is different. In the two intermediates, the ammonium radical is rather far from the hydroxypyridine radical, much farther than in the ππ* transition state. The path from intermediate 1 to intermediate 2 would go through an in-plane rotation of the ammonium radical. Thus in A″ symmetry, if the energy is sufficient to reach the initial crossing between the nπ* and πσ* state, the enol to keto isomerization could proceed through a hydrogen transfer (HT) complex. This reaction path stays however energetically above the A′ proton transfer reaction, as in the case of 7-azaindole and 7-hydroxyquinoline clustered with ammonia.28 In the reverse direction starting from the keto form with enough energy to overcome the nπ*/πσ* crossing may lead to a hydrogen transfer intermediate. In bare molecules, CASSCF and CASPT2 calculations suggest that photoinduced tautomerization occurs from the keto form to the enol form with an energy barrier about 1.25 eV and via the photoinduced dissociation-association (PIDA) mechanism.25 There is no evidence for the reverse reaction because there are strong nonadiabatic interactions between the excited and ground states of 2HP molecule along the first half of tautomerization

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Figure 4. Potential energy profiles for the enol f keto tautomerization in the first exited state of A′ symmetry. Left panel stretching of the OH coordinate in the 2HP-NH3 (enol) complex. Right panel stretching of the NH coordinate in the 2PY-NH3 (keto) complex. The energy of the enol ground state is taken as the zero energy reference. The geometry of the transition state between enol and keto forms (hexagon) is represented in the center of the figure.

Figure 5. Potential energy profiles for the excited states of A″ symmetry. Left panel stretching of the OH coordinate in the 2HP-NH3 (enol) complex. Right panel stretching of the NH coordinate in the 2PY-NH3 (keto) complex. The energy of the S0 ground state enol is taken as the zero energy reference. The geometries of the intermediate states of πσ* character are represented on the sides of the figure.

reaction path, which is responsible for its efficient internal conversion to the ground state. In the calculations performed here, the ground state energy stays far below the excited state energies, i.e., there is no crossing between the ground state and the excited state that would lead to internal conversion. However, one crossing has been found between the A″ (πσ*) state and the ground state, when the HP-NH4 distance is stretched at a very long distance

of 7 Å. The effect of solvation of 2HP with NH3 is to thus remove the S1-S0 conical intersection and allow the enol f keto photoinduced tautomerization reaction. 5. Discussion The calculations are in good agreement with experiment for the excited-state transition energies: in 2HP-NH3, the electronic

Excited-State Dynamics of 2HP-NH3 origin is experimentally observed at 4.36 eV and the optimized ππ* transition is calculated at 4.40 eV. For the 2PY-NH3 complex, the observed transition at 3.72 eV is a ππ* state as deduced from the rotational analysis7 and the calculated adiabatic transition for the ππ* state is in agreement (within 0.1 eV) with the experimental transition origin. However, in that 2PY-NH3 complex, the nπ* state calculated to be the lowest excited state, does not appear in the excitation spectra, but the oscillator strength for this transition is very low (1000 times lower than the ππ* state oscillator strength), which may explain why it is not observed. The 2HP-NH3 excitation spectrum is similar to that of the monomer when the excitation energy is less than 0.15 eV above the band origin and the lifetimes are decreasing smoothly with increasing energy. We expected to observe clear changes in the excited-state lifetime and/or mode specificity as in the cases of phenol-NH339 or 7-azaindole (CH3OH)2,29 where the excitedstate hydrogen or proton transfer occurs. These results thus seem to indicate that the vibrational bands observed are not coupled to a reaction coordinate, in particular the +200 cm-1 band does not seem to be an intermolecular stretching vibration. Vibrational frequencies have been calculated for the 2HP molecule, and the bands observed in the 2HP and 2HP-NH3 spectra can be assigned on the basis of these calculations. The 420 cm-1 band corresponds to an in-plane bending of the OH group, while the other in-plane modes are calculated at 511, 536, 802, and 925 cm-1 and correspond to the vibrational bands observed at 511, 541, 810, and 954 cm-1 in 2HP and 507, +564, +810, and +969 cm-1 in the 2HP-NH3 complex. The band at 210 cm-1, which is strong in the complex and weak in the molecule, corresponds to an out-of-plane wagging motion calculated at 210 cm-1 in the ground and 192 cm-1 in the excited state that may be more active in the complex if the geometry is slightly nonplanar. The reaction expected in the 2HP-NH3 complex can be hydrogen or proton transfer to the ammonia molecule and isomerization to the keto form via a second hydrogen or proton transfer. Calculations indicate that hydrogen as well as proton transfer reactions are possible. Hydrogen atom transfer to ammonia can occur in A″ symmetry with a barrier of 0.7 eV, but since the A″ states are higher in energy than the A′ states and have weak oscillator strengths, this reaction seems unlikely. Proton transfer can occur on the A′ ππ* excited surface with a low barrier (0.03 eV) on the entrance channel of a reaction that induces the enol to keto isomerization and should be the preferred reaction path. One can tentatively assign the disappearance of the vibrational bands in the excitation spectrum (around 1200 cm-1, 0.15 eV) to the barrier height of the proton transfer reaction in the excited ππ* state. Under this hypothesis, the experimental barrier is higher than the calculated one, but the inconsistency stays in the expected accuracy of the calculations. A different hypothesis can be proposed for the observed decrease of the lifetime: (a) It can be due to the opening of the proton transfer reaction, but then one would have expected a strong variation of the lifetime as the energy in the system is approaching the barrier, which is not the case. (b) The most probable process is the evaporation of the ammonia molecule. The evaporation cannot occur directly from the enol form if the excitation energy stays below the dissociation limit in 2HP + NH3. However, after the transition state on the A′ surface, the system will be above the dissociation limit in 2-pyridone + NH3 (which is around 4.0 eV) so that

J. Phys. Chem. A, Vol. 114, No. 9, 2010 3065 evaporation of the ammonia molecule may occur. The energy excess available for dissociation being between 0.36 and 0.50 eV, this evaporation process should not be very fast. (c) Another possibility is that, since the A′ and A″ surfaces get very close when the OH distance is lengthened, the A″ state being lower in energy than the A′ state for R(OH) ) 1.4 Å, the complex may also stay trapped in the biradical intermediate C5H4NO• · · · •NH4 for some time before it can eventually undergo an internal conversion to the ground state if the C5H4NO• · · · •NH4 distance reaches more that 7 Å. It is worthwhile to note that from the theoretical point of view the presence of one ammonia molecule changes the nature of the enol f keto isomerization. The work of Chmura et al. on the isomerization process in the free molecule25 indicates that isomerization occurs on the A″ surface; i.e., it is a concerted proton/electron transfer (hydrogen transfer). It seems from our calculations that the proton transfer mechanism occurring on the A′ surface is more favored in the complex, although the hydrogen transfer is also possible. This shows once more that the competition between hydrogen and proton transfer is a quite subtle mechanism, which cannot be so easily predicted. It should be also pointed out again that the CC2 method is very efficient to optimize excited states in the Franck-Condon region but fails in the description of conical intersection between the ground and the electronically excited state and between different excited states. Multireference calculations (CASPT2 for example) with diffuse orbitals to take into account the Rydberg character of the σ* orbital and with excited state optimization would be very useful to assess more firmly our calculations and to compare with experiment. Simulations of dynamics on the multidimensional PE surface would shed some light on the reaction mechanism involved in this system. Conclusion The enol f keto isomerization has been investigated in the 2-hydroxypyridine-NH3 complex both experimentally and theoretically. The calculations seem to show that an excitedstate proton transfer can proceed through a small barrier on the A′ surface and induce the isomerization reaction via a second proton transfer. Experimentally the dynamics observed does not present a clear evidence of the P/H transfer reaction and the decrease of the lifetime is probably due to the evaporation of the ammonia molecule following the initial proton transfer. Acknowledgment. This work has been supported by a France/Japan SAKURA cooperation program. References and Notes (1) Friedberg, E. C. Nature 2003, 421, 436. (2) Del Bene, J. E. J. Am. Chem. Soc. 1995, 117, 1607. (3) Dkhissi, A.; Adamowicz, L.; Maes, G. J. Phys. Chem. A 2000, 104, 5625. (4) Esboui, M.; Jaidane, N.; Lakhdar, Z. B. Chem. Phys. Lett. 2006, 430, 195. (5) Esboui, M.; Nsangou, M.; Jaidane, N.; Lakhdar, Z. B. Chem. Phys. 2005, 311, 277. (6) Florio, G. M.; Gruenloh, C. J.; Quimpo, R. C.; Zwier, T. S. J. Chem. Phys. 2000, 113, 11143. (7) Held, A.; Pratt, D. W. J. Am. Chem. Soc. 1993, 115, 9718. (8) Matsuda, Y.; Ebata, T.; Mikami, N. J. Chem. Phys. 1999, 110, 8397. (9) Matsuda, Y.; Ebata, T.; Mikamia, N. J. Chem. Phys. 2000, 113, 573. (10) Matsuda, Y.; Ebata, T.; Mikami, N. J. Phys. Chem. A. 2001, 105, 3475. (11) Nimlos, M. R.; Kelley, D. F.; Bernstein, E. R. J. Phys. Chem. A 1989, 93, 643. (12) Sakota, K.; Tokuhara, S.; Sekiya, H. Chem. Phys. Lett. 2007, 448, 159.

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