Role of Rydberg States in the Photostability of ... - ACS Publications

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Role of Rydberg States in the Photostability of Heterocyclic Dimers: The Case of Pyrazole Dimer Shmuel Zilberg* and Yehuda Haas Institute of Chemistry and the Farkas Center for Light Induced Processes, The Edmond Safra Campus, The Hebrew University of Jerusalem, Jerusalem, Israel ABSTRACT: A new route for the nonradiative decay of photoexcited, H-bonded, nitrogen-containing, heterocyclic dimers is offered and exemplified by a study of the pyrazole dimer. In some of these systems the N(3s) Rydberg state is the lowest excited singlet state. This state is formed by direct light absorption or by nonradiative transition from the allowed ππ* state. An isomer of this Rydberg state is formed by H atom transfer to the other component of the dimer. The newly formed H-bonded radical pair is composed of two radicals (a H-adduct of pyrazole, a heterocyclic analogue of the NH4 radical) and the pyrazolium π-radical. It is calculated to have a shallow local minimum and is the lowest point on the PES of the H-pyrazole/pyrazolium radical pair. This species can cross back to the ground state of the original dimer through a relatively small energy gap and compete with the H-atom loss channel, known for the monomer. In both Rydberg dimers, an electron occupies a Rydberg orbital centered mostly on one of the two components of the dimer. This Rydberg Center Shift (RCS) mechanism, proposed earlier (Zilberg, S.; Kahan, A.; Haas, Y. Phys. Chem. Chem. Phys. 2012, 14, 8836), leads to deactivation of the electronically excited dimer while keeping it intact. It, thus, may explain the high photostability of the pyrazole dimer as well as other heterocyclic dimers.

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INTRODUCTION The excellent photostability of DNA1 is believed to be one of the reasons for evolution’s choice for storing the genetic information in this particular polymer. It was further found that the photostability is observed also in the isolated four bases2,3 as well as in H-bonded pairs,4 spurring considerable theoretical work.5,6 The first electronic excited states in many nitrogen heterocycles have a very short lifetime,7 indicating ultrafast radiationless processes. In the case of collision-free nitrogen heterocyclic molecules, the NH bond cleavage reaction is the dominant nonradiative route (provided the photon carries the energy required).8−11 Models based on small molecules such as pyrrole and pyrimidine were developed to account for these rapid processes. Domcke and Sobolewski12 proposed a unified mechanism suitable for many heterocyclic molecules in which the key step is rapid conversion of the initially excited ππ* state to a πσ* that becomes the lowest excited state for large NH distances. In condensed matter the NH bond cleavage rarely happens. The interaction with the nearest neighbors is likely to affect the potential energy surface as well as the none-radiative processes. In particular conical intersection subspaces, expected to be one of the major loci in which such interactions are important, may be modified, and new ones may be formed. One of the simplest and most useful systems for studying the nature of such interaction are dimers (either homo- or heterogeneous). A dimer clearly is not a truthful representation of a liquid or a solid, but comparison with the monomer usually reveals some © XXXX American Chemical Society

interesting trends. This approach is especially useful in the study of bimolecular interactions, as found in H-bonded systems. When H-bonding is possible, it is often a major factor determining the structure of the most stable ground state isomer. In the absence of H-bonding, ππ interactions are important. Thus, most data indicate that the stable dimer of benzene has a perpendicular structure13,14 due to hydrophobic aromatic−aromatic interaction (the π−π interaction). Another structure, the parallel displaced one, is calculated to be stable but slightly higher in energy. In the case of nitrogen heterocycles, H-bonding is always possible, although in some molecules in-line H bonds are not realizable due to steric congestion. Thus, the most stable structure of the pyrrole dimer15 is similar to the T-shaped one of benzene, with the nitrogen side of one ring directed to the π electron system of the other ring establishing a weak hydrogen bond, except that the angle between the two monomers is 55° rather than 90°. Pyrazole has two neighboring N-atoms in the five-membered ring. The dimer was observed in a supersonic slit nozzle,16,17 and its IR spectrum recorded. It was found that a planar C2h structure best fits the experimental data. For steric reasons, the NH···N bond is bent. Special Issue: Jörn Manz Festschrift Received: May 4, 2012 Revised: July 2, 2012

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dx.doi.org/10.1021/jp304325x | J. Phys. Chem. A XXXX, XXX, XXX−XXX

The Journal of Physical Chemistry A

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In this paper the energy level diagram of the low-lying electronic states of the pyrazole dimer is investigated, in an attempt to account for its photostability. High level ab initio calculations help to propose possible routes for the fast nonradiative decay of the first excited state. This work follows the analysis of the photochemistry of the pyrrole−ammonia complex.18−20 The lowest singlet state S1, a Rydberg type one is shown to provide a possible effective funnel to the ground state. The electronic excited states of pyrazole have been extensively studied using VUV absorption and electron energy loss (EEL) spectroscopies.21 Summaries of the current accepted energy level diagram in the FC region have been given by Ashfold et al.10 and by Palmer et al.22 The lowest dipoleallowed states are ππ* (at about 6 eV), and optical transitions from the ground state are the most intense ones. At about the same energy, forbidden Rydberg states are calculated and possibly observed. The lowest excited states are triplet states21,22 at 3.9 and 5.1 eV. The photophysics and photochemistry of the pyrazole monomer were studied in a supersonic jet by Ashfold et al. using H atom photofragment spectroscopy.10 The data are similar to those observed in pyrrole and imidazole.10 The translational energy distribution of the ejected H-atom is bimodal. The fast component is thought to be formed by the πσ* repulsion model, whereas the origin of the “slow” H-atoms is not yet agreed upon. To our knowledge, the photochemistry of the pyrazole dimer has not been studied yet. In this paper the dimer is studied as model H-bonded system.

Scheme 1. Schematic Presentation of the Two Rydberg Isomer Dimers along the H-Shift Coordinate (Initially Excited One on the Left, the Radical Pair [•DN···H-AN•] on the Right)a

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In the ground state the same coordinate leads to the PES region described mainly by the zwitterion (DN‑···H-AN+) VB structure (highlighted in bold) and further on to ion separation (a CT excited state DN− + H-AN+). The H-adduct dissociation coordinate separates between the two components of the radical pair. As the distance between these components increases, the electrostatic attraction within the zwitterion weakens and the energy of the separated ion pair is larger than that of the separated radical pair, (which is nearly constant along this coordinate) and a crossing takes place (dotted lines on the right). This crossing and its vicinity form a new funnel to the ground state (highlighted in bold).

The charge transfer (CT) form of radical pair (DN+···H-AN‑) is at infinite separation an electronically excited state. As the two ions approach each other, the coulomb interaction lowers the energy of the CT pair, and at the geometry of H-bonding Hshifted complex (DN···H-AN), it becomes the ground state. Thus, a curve crossing takes place, somewhere along the (DN•···H-AN•) radical−radical separation coordinate. This locus provides the necessary funnel for S1/S0 nonradiative transition. Once on the ground state, charge recombination, back H atom transfer and fast relaxation recovers the original (DN-H···AN). Thus, the zwitterion (DN−···H-AN+) is a nonstationary species on the ground state surface. Although we are not aware of experimental evidence for Rydberg radicals except NH4•, that is, H-adducts of the parent azole, it is proposed that these species do exist and have a minimum, which may be shallow. These Rydberg radicals are the key species of the model. They may open up new nonradiative pathways in the dimer in addition to the H-dissociation route.28 Rydberg excited state of nitrogen-containing heterocycle molecules have been discussed by several authors,11,28 but the H-transfer from one Rydberg center to another one considered here in the dimer was not recognized as a possible important factor in a mechanism leading to radiationless transitions. It is expected that the structure of the two Rydberg isomers will be similar to that of the corresponding cations. We therefore proposed the term Rydberg Center Shift (RCS) for the H-transfer reaction connecting the two dimer isomers. The RCS mechanism was first used for the analysis of H-shift in the pyrrole−ammonia complex.18 Scheme 1 depicts the essential features of the model.

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MODEL The model we propose applies to an H-bonded complex DNH···AN in which both the donor DN-H and the acceptor AN contain a nitrogen atom. (In the case of the pyrazole dimer (and some other complexes), the two H-bonded dimer is somewhat more stable than the single-bonded one, yet the latter is the dominant species in the excited states and, therefore, is used as the starting point of the model.) The unique role of nitrogen atoms is due to the low ionization potential of the monomers containing them. Consequently, the Rydberg states (especially the N(3s) one) have low energy compared for instance with other heterocyclic molecules; indeed, in many of these molecules, the 3s Rydberg state is the lowest singlet excited state.23 Excitation of one of the components, DN-H, for instance, to the 3s level results in formation of a Rydberg state, which may be considered as a cation plus a relatively diffuse orbital (though still primarily around the DN-H component), with one electron occupying the N(3s) orbital. This species is essentially a proton donor Rydberg monomer ([DN-H]Ryd) bound to a proton acceptor AN. An isomer of this Rydberg dimer may be formed by transferring a proton to the other component, creating a [HAN+] protonated cation, with the 3s electron following to form a [H-AN•] Rydberg radical, while the proton donor fragment becomes the π-radical •DN. The H-AN• radical is a ground state species, similar to NH4•. A neutral ammonium NH4 is the ground state Rydberg radical that was observed experimentally24,25 and studied theoretically.26,27 The properties of the radical−radical pair (•DN···H-AN•) depend on the geometry, especially their distance. At infinite separation, this pair is the ground state of the system. As they approach each other, they form a shallow minimum stabilized by the H-bonding energy relatively to the separated radicals (•DN + H-AN•; Scheme 1).

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COMPUTATIONAL RESULTS: APPLICATION TO THE PYRAZOLE DIMER We carried out extensive CASSCF calculations on the ground, lowest singlet states, the lowest triplet states, the cation-radical of pyrazole ((Prz-H, I) and the H-bonded dimer (dim-Prz-H, B



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II) . The CASSCF29 methodology as implemented in the PCGAMES30 program suite has been used to study these systems. Several selected points were also calculated using CASMP2 as implemented in the GAMESS software. Ten orbitals, the six occupied orbitals (three π and three σ MOs) and the four unoccupied ones (two π*, one σ* and 3s Rydberg MO), were included in the active space of the CASSCF calculations of the pyrazole I. Eleven orbitals, the six occupied orbitals (four π and two σ MOs) and the five unoccupied ones (two π*, two σ* and one 3s Rydberg MO), were included in the active space of the CASSCF calculations of the dimer II. The pyrazolyl radical (Prz•, III) and pyrazole with H-atom connected to the second N-atom (H-Prz-H, IV) were calculated on the same CAS level as an initial monomer I: 10 orbitals occupied by 11 or 13 electrons, respectively. All calculations were performed under the imposed constrain of Cs symmetry. This restriction makes possible the distinction between the Rydberg states (π → 3s, A″ symmetry) and the valence π → π* states of A′ symmetry. The aug-cc-pVDZ basis set was used for calculations. The CASSCF results, which include the adiabatic energies of different species, including, minima, transition states, H-transfer, and dissociation products on the ground and excited states and conical intersections between them, are summarized in Table 1.

The high C2h symmetry of the ground state is broken for all excited states species of dimer II: π → π* valence state, n → π* state, and Rydberg type π → 3s state. The lowest excited state of the dimer is a Rydberg species (V, 11A″, π → 3s, Figure 2a), but has a considerably lower energy, −0.8 eV, than the monomer’s Rydberg state. It is found that on electronic excitation, one H bond of the original dimer is broken, while the other becomes very strong (∼1.8 Å). This bond is very similar to that found in the corresponding cation (Figure 2). The second kind of Rydberg state species (VI, Figure 2b) is an H-bonded complex between pyrazolyl radical (Prz•,III) and H-pyrazole adduct radical (H-Prz-H, IV) (11A″, π → 3s, Figure 4b,c). The two Rydberg type isomers V and VI are separated by a 0.26 eV barrier from the side of initial Rydberg species V (X, TS structure of H-shift, Figure 3a). Both Rydberg type isomers are the lowest excited singlet (S1) states of the respective species, according to CAS and also CAS-MP2 calculations. The barrier to N−H dissociation (to produce Prz-H···Prz + H VII, Table 1) is of the same order as the H-shift barrier (XI, TS structure of N−H dissociation, Figure 3b). Adduct IV is a Rydberg type radical in the ground state. When the unpaired electron of IV occupies the valence π* orbital, it leads to a π-excited radical −0.34 eV above the ground state Rydberg type adduct IV. Our attempts locate an H-adduct isomer with two H-atoms on the same nitrogen were unsuccessful. The calculated N−H dissociation energy of pyrazole (4.83 eV) is in excellent agreement with recent experimental estimations −4.65 eV.10 The analogues computational value for H dissociation of the dimer is 4.50 eV. In the case of pyrazole and its dimer, like in the well-studied case of pyrrole, as the NH bond lengthens, the n → π * state becomes lower than the lowest lying state 3s Rydberg state; this leads to the cleavage of the NH bond subsequent to the S0/S1 crossing analogous to the various N- and O-containing heterocycles.12 The separation (RIII−IV > 5 Å) between pyrazolyl radical (Prz•, III) and H-pyrazole adduct (H-Prz-H, IV) leads to the S0/S1 touching at about 5.5 eV. This funnel is a 0.4 eV higher than H-shifted complex VI. The lowest optimized ππ* valence state is an excited singlet state S2, which is ∼1 eV above the lowest Rydberg S1 state (ΔΔE ππ* = 6.13 eV vs ΔΔERydberg = 5.19 eV, according to the CAS results (Table 1)). The nπ* excited state of the dimer is of A″ symmetry, as the Rydberg state, but lies at considerably higher energy. CAS-MP2 corrections maintain the order of the excited states, although the relative stabilization of the valence ππ* state is larger than that of the Rydberg state.

Table 1. Absolute (au) and Relative (Adiabatic) Energies (eV) of the Pyrazole Dimer (II) and Some of Its Excited state’s Species. CAS(12e/11o)/aug-cc-pVDZ Results for Dimer II. Roman Numerals Relate to Species Whose Structures are Shown in Figures 1−4 state GS Rydberg π → 3s S1 11A″ π → π* n → π* N−H dissociation

pyrazole dimer ΔE(eV) II S0 C2h dimer V Rydberg VI Rydberg-H 21A′S2 21A″S3 VII Prz-H···Prz + H X TS H-shift XI TS NHdissociation XII CI H-adduct XIII CI NHdissociation

−449.79054 5.19 5.08 6.13 6.69 4.50 5.45 5.50 5.49 4.50

CAS calculations of the pyrazole monomer show that the energies of the lowest Rydberg state and the valence ππ* state are approximately the same, which is in a good agreement with experiment and previous theoretical studies.10,22,31,32 The ground state dimer (II) has a C2h structure with two typical NH···N hydrogen bonds (∼2.2 Å; Figure 1b). The pyrazole unit is a little bit distorted from original monomer structure in the dimer (Figure 1a vs b).

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DISCUSSION Whereas the two pyrazole moieties are equivalent in the ground state dimer II (C2h structure), all excited states of the dimer are composed of one excited and one ground state fragment (Figure 1 vs Figure 2). This phenomenon can be rationalized in a natural way by VB crossing model.33 In the spirit of this two state approach, in the absence of an external factor preserving the symmetric structure, excitation of the symmetric dimer II leads to distortion of one monomer, and thus to the annihilation of the symmetric dimer structure. In principle, the exciton model could be also applied in this case, but it is effective for systems keeping the symmetry, which allows to distinguish between combination and anticombination of the two states and to estimate their corresponding coupling. H-bonding affects all electronic excited states, but the influence is relatively small: 0.1−0.2 eV, for the valence excited

Figure 1. Calculated structures of (a) monomer I and (b) the Hbonded dimer II. C



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Figure 2. Calculated structures of the (a) Rydberg state of the initially formed dimer (V), (b) H-shifted isomer of dimer (VI), (c) cation radical of pyrazole dimer (VIII), and (d) H-shifted isomer of cation radical of pyrazole dimer (IX).

Rydberg state is the lowest lying excited state of pyrrole (for a detailed discussion of computed and experimental excitation energies of pyrrole, see refs 34−38). The 3s Rydberg state10,21,22 of pyrazole is also low lying, whose energy is similar to that of the ππ* state. Our calculations strongly support the Rydberg state character of the lowest excited state of the pyrazole dimer. The structural and electronic features of the cation-radicals, in general, are similar to those of the analogous Rydberg states (Figure 2). Cations VIII and IX have nearly the same energy (ΔΔE = 0.09 eV). Addition of an electron to the Rydberg 3s nitrogen orbital of the cations leads to the isoenergetic corresponding Rydberg type species V and VI (ΔΔE = 0.09 eV). Scheme 2 presents a cartoon of the proposed model. The H-bonding energy of complex VI is 0.4 eV (estimated from the difference between the energies of VI and the two separated radical fragments (H-Prz-H• + Prz•; IV + III). The H-pyrazole adduct is a Rydberg type radical [H-Prz-H]• IV. It is a minimum on the PES. The barrier to N−H dissociation of the H-shifted complex VI of the dimer is 0.44 eV (Figure 3a, structure XI). The initial Rydberg complex V dissociates to the same products IX (Prz-H···Prz• + H), as VI. Our attempts to locate the corresponding TS were unsuccessful because it leads to simultaneous H-shift through the transient structure X. So, N−H dissociation is going through the bifurcated transient structure X (for a discussion of the bifurcating reaction pathways, see refs 39−42) connecting products VII (PrzH···Prz• + H) with the two “reactants” V and VI (Figure 5a). The H-dissociation reaction is a strongly exothermic process; the products (Prz-H•···Prz• + H) are calculated to be 0.76 eV below complex VI. Different mechanisms involving hydrogen-bonded Rydberg states were recently offered to account for the interpretation of experimental results for the pyrrole-ammonia cluster.18−20 The

Figure 4. Calculated structures of (a) the S1 Rydberg state of pyrazole, (b) the H-pyrazole adduct (H-Prz-H, IV), and (c) the pyrazolyl radical (Prz•, III).

states ππ* and nπ*. In contrast, the H-bond effect on Rydberg state is considerably larger ∼0.9 eV analogous to the strong stabilization of Rydberg state of pyrrole−ammonia complex.18 This is a direct result of the cationic character of the Rydberg states in which the N···H bond is very strong and very short −1.85 Å in both Rydberg species V and VI (Figure 2a,b). Comparison of the two Rydberg species, V and VI, to the corresponding cation radicals, VIII and IX (Figure 2c,d), reveals very similar structures. In both cases, Rydberg excitation or cation center are localized on one of the pyrazole rings and the H-shifted isomers are 0.1 eV more stable than parent Rydberg type complex V or the corresponding cation. The ionization potential of the Rydberg species V and VI is 2.1 eV and 1.7 eV, respectively. Our definition of V and VI as Rydberg type species is based on the analysis of orbital population. In both cases (V and VI) the leading configuration is ···(π)1(3s)1. Moreover, it is easy to distinguish between the Rydberg state and the nπ* state. Formally, both have the same state symmetry (A″), but the MO occupation is different. The molecule has N π electrons in the ground state, but (N − 1) π electrons in Rydberg state versus (N + 1) π electrons for the nπ* one. Consider the electronic structure of the two Rydberg state species. In several heterocycles with saturated N-atom the lowest singlet state is a Rydberg type, for example, the (3s)

Figure 3. Calculated structures of (a) the transition state of the H shift reaction (X, TS H-shiftRyd) between the Rydberg species V and VI and (b) the transition state for N−H dissociation (XI, TSN−H). D



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Scheme 2. Schematic Presentation of the Correlations between the Isomeric Cation-Radicals VIII and IX and Their Corresponding Rydberg Species V and VI

Figure 5. Schematic energy level diagram of the pyrazole dimer H-bonded complex along two coordinates in the pyrazole dimer system. The H-shift coordinate connecting the two Rydberg isomers (V and VI) appears in both (a) and (b). The second coordinate in (a) is the H-dissociation coordinate, leading to the pyrazole−pyrazolyl radical and an H atom (Prz-H···Prz• + H, VII); in (b) it is the H-adduct separation coordinate, leading to pyrazolyl radical and H-pyrazole adduct radical (H-Prz-H• + Prz•; CAS (12/11)/aug-cc-pVDZ results, please see Table 1).

RCS mechanism18 applied to the pyrazole dimer assumes that a proton could be located on one of the two units, either the pyrazole or the H-pyrazole adduct. The proton shift is escorted by the shift of a Rydberg orbital from the initial Rydberg state excited monomer to the second one. Thus, the key aspect of the RCS mechanism is the creation of an H-shifted isomer of

at the geometry of the H-bonded complex. The zwitterion Prz−···H-Prz-H+ becomes energetically preferable over the biradical Prz•···H-Prz-H due to three major energy factors determining the S0−S1 gap ΔE(S0 − S1) = −Ecoul + IPIV − EAIII: First, the low ionization potential (IP) of H-adduct Rydberg type radical IV, IPIV = 2.1 eV; second, the relatively large (1.5 eV) electron affinity (EAIII) of III, and last, the coulomb attraction energy (Ecoul) between the charged components, which is ∼2.5 eV.43 Note that the zwitterion Prz−···H-Prz-H+ spontaneously collapses to the initial dimer II from the Hshifted species VI. Such back H-transfer could be a key factor, increasing the photostability of H-bonded N-heterocyclic complexes, by adding an effective S0−S1 funnel. This situation would be changed as a result of extra stabilization of zwitterionic form, for example, in the case O-containing acceptor groups. Evidence for the existence of H-bonded radical pair has been recently advanced for a different system.44 Experimental studies revealed that phenol-ammonia clusters with more than five ammonia molecules are proton transferred species in the ground state. A long-lived excited electronic state (50−100 ns)

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the dimer complex Prz ···H-Prz-H• (VI). The pyrazolyl radical (Prz•,III) is a π-radical in the ground state, and the H-adduct (H-Prz-H•, IV) is a Rydberg radical in its ground state. The radical pair (III + IV) is a lowest ground singlet state at a large distance between two fragments (Figure 5, on the right). H-bonding stabilizes the complex VI between two radicals (the energy of the two separated radicals III + IV is ∼0.4 eV higher), but this H-bonded structure VI is an excited state species (see Figure 5 and Scheme 1). The ground state at this configuration VI is the zwitterion Prz−···H-Prz-H+, with a calculated dipole moment μ = 18D. The energy order inversion of the biradical Prz•···H-Prz-H• versus the zwitterion Prz−···HPrz-H+ states along the coordinate connecting the two fragments may be explained by the change of Coulomb interaction between the charged species (Prz− and H-Prz-H+) E



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component of the dimer. Thus, the RCS mechanism is an additional process accounting for the unique photostability of H-bonded Nitrogen heterocyclic systems.

was observed and assigned as a H-bonded radical pair based on ab initio calculations. Generalization of the Model to the Study of Photostability of H-Bonded N-Heterocyclic Complexes. Photoinduced proton transfer plays an essential role in the photostability mechanism of H-bonded dimers.45,46 The standard model currently used is based on a mechanism of nonradiative decay of the monomers, accompanied by H-atom elimination via a repulsive πσ* state.1,9,11,12 The H-cleavage process is undesirable as it counteracts photostability. The cleavage may be frustrated in condensed systems, where cage geminate recombination leads back to the original molecule. However, in dimers, there is second N−H bond, not shielded by H-bonding, which may be cleaved. Barbatti et al. proposed a ring deformation mechanism, which is complementary to the πσ* channel, thus, offering a more complete interpretation of the different heterocycles.36,47 A photoprotection mechanism has been proposed for single proton transfer in H-bonded complexes, in the case of the guanine−cytosine pair.48 Groenhof et al.49 proposed for the same system an ultrafast nonradiative channel based on proton transfer from the guanine to the cytosine, followed by charge transfer. All these models considered only valence orbitals, the lowest excited state being ππ*. The authors of reference45 pointed that in the case of heterocyclic dimers the valence ππ* states is generally below the Rydberg-type biradical state. Our calculations of the Hshifted isomer of the dimer complex Prz•···H-Prz-H• (VI) on the CAS(12/10)/aug-cc-pVDZ level and also CAS-MP2 estimations show that Rydberg state is lower than ππ* state. Thus, in this case, the lowest excited state is due to the transition from one Rydberg isomer to the other one, accompanied by proton transfer. The RCS mechanism therefore provides an alternative S1/S0 funnel leading to the initial dimer. This channel is a result of the crossing between zwitterion and nonpolar Rydberg state along the H-shift coordinate, and is applicable to all N-heterocyclic dimers in which the Rydberg state is lower than the ππ* one. As this route is operative without undesirable NH-cleavage, it can explain the enhanced photostability of dimers compared to monomers even in the gas phase. Dynamic simulations are necessary to support this conjecture.

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AUTHOR INFORMATION

Notes

The authors declare no competing financial interest.

ACKNOWLEDGMENTS Financial support from the DFG within the trilateral project Germany-Israel-Palestine Ma-515/22-3 is gratefully acknowledged. The Minerva Farkas Center for Light Induced Processes is supported by the MinervaGesellschaft GmbH, Munich, Germany.

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CONCLUSIONS The RCS model introduced in ref 18 and presented in this paper is shown to account for the photostability of H-bonded dimers of pyrazole; it may apply also to some other Nheterocyclic molecules. The dimer remains bound throughout the process, no adverse photochemical reactions, such as NH bond cleavage is involved. The key step is isomerization of the initially excited Rydberg state [DN-H]Ryd···AN to a H-bonded radical pair [•DN···H-AN•]. This biradical isomer is a local minimum, but at the geometry of the minimum, the ground state of the original dimer lies at a lower energy, in a zwitterion form. As the distance between the two radicals increases, the order is inverted and curve crossing takes place creating a funnel to the ground state. In the case of the pyrazole dimer it is shown computationally that this funnel may compete effectively with the standard N−H stretching channel one. According to the proposed mechanism, the driving force of the isomerization is proton transfer, escorted by the shift of the singly occupied Rydberg orbital. Thus, rather than being spread over the entire dimer, the Rydberg orbital is located on either F



Article

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