The concerted mechanism of photo-induced biprotonic transfer in 7-azaindole dimers: Structure, quantum-theoretical analysis, and simultaneity principles

Abstract

Six stable dimer models for 7-azaindole (including the classic C_2h doubly hydrogen-bonded, coplanar, centrosymmetric dimer) are considered to be observable in adiabatic nozzle jet molecular beams. They are analyzed by hybrid density functional theory (DFT), the MP2 ab initio method for the ground electronic state, and the single-excitation configuration interaction (CIS) (over frozen ground state optimized geometries obtained from DFT) excited state calculations, for global potential minima and proton-transfer potential energy curves. Three simultaneity principles are stated: (i) intermolecular coherent excitation molecular exciton simultaneity, (ii) intramolecular acid–base change simultaneity at the pyrrolo-N-H and aza-N proton-donor, proton-acceptor sites, and (iii) intermolecular simultaneity of catalytic proton-donor, proton-acceptor action. It is suggested that the formation of the classic C_2h dimer of 7-azaindole, which is considered exclusively by previous researchers, can be formed from at least one of the several card-pack hydrogen-bonded dimers in a secondary slower step approaching a microsecond scale, instead of the picosecond events at the supersonic nozzle. It is proposed that the complexity of dimerization modes is the basis of the postexcitation, postionization diverse kinetic isotope results.

The photoinduced biprotonic transfer in the classic 7-azaindole (7AI) hydrogen-bonded C_2h dimer (refs. 1 and 2 and references therein) seemingly has revealed contrasting mechanisms according to the experimental techniques applied. Electronic spectroscopic observations and quantum theoretical calculations require a concerted (3) one-step simultaneous intermolecular transfer of the two protons involved in the hydrogen bonding of the 7AI base pair. In contrast, femtosecond laser-pulsed photo-excitation followed by femtosecond laser-pulsed ionization and then time-of-flight (TOF) mass spectrometry (4–6), or with giant laser pulse-induced coulomb explosion (7, 8) followed by TOF mass spectrometry of the resultant ions (and molecular fragments), has suggested to these researchers a two-step neutral H-atom transfer, based on kinetic analysis of the ion appearance and isotopic effects. We shall return to the question of neutral H-atom transfer vs. H⁺ (proton) transfer at the end of the companion paper (9).

In the first part of this paper, we give an outline summary of the spectroscopic demands on simultaneity of the biprotonic transfer for the concerted transfer mechanism, and then in the second part we present an analysis of the structures and proton-transfer potential functions and the energies of formation for the variety of 7AI stable dimers that could form at supersonic velocities in the adiabatic-nozzle supercooled molecular beam. This presentation is suggested as a pathway to resolving the conflict between the spectroscopic and the kinetic results.

Background of 7AI Dimer Biprotonic Transfer

For photo-induced biprotonic transfer we emphasize three strict requirements of simultaneity of action that must exist.

The Three Defining Simultaneity Principles of Photo-Induced Biprotonic Transfer.

(i) Molecular exciton intermolecular simultaneity.

The 7AI classic dimer (Scheme A) is a coplanar centrosymmetric doubly H-bonded dimer (C_2h group). The molecular exciton model of coherent excitation applies (1, 10, 11), with both moieties of the dimer simultaneously excited. This requirement conforms to a core principal of quantum mechanics: commuting operators have simultaneous eigenfunctions. The symmetry operators for the molecule commute with the Hamiltonian energy operator, therefore the electronic wavefunctions must be symmetry adapted. A single moiety excitation of the 7AI dimer would not be possible or acceptable. Accompanying the simultaneous excitation of the two bases is the consequent simultaneous charge redistribution over the molecular dimer skeleton.

(ii) Intramolecular simultaneity of acidity–basicity site stimulation.

The excitation charge redistribution within each moiety of the molecular skeleton of 7AI results in a large transfer of electronic charge to the pyridino-N moiety and a loss of charge at the pyrrolo-N moiety. The consequence of these changes is a simultaneous large increase upon photo-excitation in the basicity of the pyridino-N and a parallel large increase in the acidity of the pyrrolo-N. This chemical synchronicity with the spectroscopic excitation has been explored generally for pyrrolo-aza-aromatics in a spectrothermodynamic correlation study (12). This acidity–basicity change also reinforces the concerted proton-transfer aspect of the biprotonic photo-tautomerization.

(iii) Intermolecular catalytic proton donor–acceptor simultaneity.

It has been long established (1) that a biprotonic transfer in a molecule such as 7AI can have its pyrrolo-H¹ proton transferred while the pyridino-N⁷ site acquires another proton, via an ‘inert’ cyclically H-bonded catalytic agent (13), acting merely as a proton relay center. This biprotonic acid–base reaction (for the excited molecule) is not a typical example of acid–base (two-step) reaction chemistry: a strict simultaneity of the 7AI excited state biprotonic transfer (in time shorter than the excited state lifetime!) must be satisfied. Thus, simple aliphatic alcohols as solvents are effective catalytic agents (1), yielding the proton-transfer tautomer green fluorescence (F₂) instead of the normal tautomer violet fluorescence (F₁). The test of the simultaneity comes with liquid water as solvent: H-bonding independently to H₂O chains at both the pyrrolo and pyridino sites is ensured, but biprotonic transfer does not occur obviously in liquid H₂O solvent at 298 K (1). The needed inductive effect through a cyclically H-bonded catalyst (monomer or dimer) by the solvent to ensure simultaneity is missing in liquid water. However (Fig. 1), in very dilute H₂O in ether solvents the occurrence of water monomers or dimers again provides the simultaneity of proton donor–acceptor to occur (13, 14).

Figure 1.

Requirement of simultaneity in 7AI photo-induced proton transfer. (Left) Cyclical catalytic solvates providing simultaneity. (Right) Uncoupled chain solvates blocking proton transfer in excitation time.

It is clear that in the 7AI doubly H-bonded dimer the 7AI acts as a self-catalyst, the simultaneous intermolecular electronic excitation supplying the intermolecular simultaneity of acid–base site changes required.

The Electronic Spectroscopy of 7AI Dimer.

First, high-resolution electronic spectroscopy yields direct evidence for simultaneous excitation of the two moieties of the 7AI C_2h dimer (15, 16). The essentials of the electronic spectroscopy of 7AI and its C_2h dimer have been critically reviewed in previous papers from this laboratory (1–3, 14, 17). Here a few points are singled out for emphasis. The elegant and definitive high-resolution electronic spectroscopy researches on 7AI by Fuke, Yoshiuchi, and Kaya (15) and Fuke and Kaya (16) confirm perfectly the molecular excitonic character of the 7AI dimer spectroscopy. Their high-resolution gas-phase spectra reveal the origin of the 7AI monomer absorption at 34,639 cm⁻¹. As expected, the 7AI dimer absorption shows a split molecular electronic state; excitation of the lower component is electric-dipole forbidden for the C_2h geometry and is observed as a characteristic biphotonic absorption S₀(¹A_g) → S_1a(²A_g) at 32,254 cm⁻¹. The upper state component for the excitonic pair exhibits an electric-dipole-allowed origin S₀(¹A_g) → S_1b(¹B_u) at 32,290 cm⁻¹ (cf. the energy diagram shown as figure 9 of ref. 17). Both Zewail's group (4–6) and Castleman's group (7, 8) use a femtosecond laser pulse at 305–310 nm, bracketing (32,787–32,258 cm⁻¹) exactly the dipole-allowed molecular exciton band at 32,290 cm⁻¹, yet they do not recognize the simultaneity of the two-molecule excitation implied by that frequency in their mechanism of “single-step” hydrogen transfer.

Second, the π molecular orbital changes (HOMO/LUMO; highest occupied molecular orbital/lowest unoccupied molecular orbital) upon excitation at the pyridino-N site and at the pyrrolo-N site (3) offer dramatic indication of the simultaneity (concertedness) of the biprotonic transfer driving force in the dimer.

Third, the calculated (π,π*) excited state proton-transfer potential functions (3, 9, 17) for 7AI dimer using the highest-level electron-correlation-included computational theory show no intermediate minimum in the proton transfer, contradicting a single-step (stepwise) mechanism.

These clear-cut spectroscopic and quantum theoretical results, together with the experimental evidence for the simultaneity principles enunciated, dictate that we search elsewhere for the resolution of the conflict in the two experimental approaches used in 7AI biprotonic mechanism research.

Theoretical Methods.

The validity of the computational methods used in this paper is provided by comparison of theoretical results with molecular structures and properties of the 7AI monomer that have been measured experimentally. The molecular geometries of 7AI monomer and dimer are both unknown, as the x-ray diffraction studies have revealed that 7AI forms a tetramer in the crystal state (18). Only the 7AI monomer rotational constants and the position of its pyrrole proton in the inertial axis frame are known in the gas phase (19). The dipole moment magnitude in the gas phase and its orientation in the inertial axis frame of 7AI have been determined by using Stark-effect-based methods.

Cane et al. (20) recorded the IR spectrum for 7AI monomer in the gas phase and carefully characterized and assigned 28 of the 39 fundamental frequencies of this molecule. A plot (not shown) of the experimental frequencies vs. the corresponding theoretical ones without using scaling parameters [at hybrid density functional theory (DFT)-B3LYP level with 6-31G** basis set] yields an excellent linear correlation with a regression value r of 0.993 (number of data, n = 28, and SD = 28 cm⁻¹).

From the previous results it follows that B3LYP/6-31G** computations involving optimized geometries are appropriate for studying these molecular systems; thus we shall use them to analyze the dimerization and proton transfer in 7AI.

However, for the sake of calculating the card-pack 7AI dimers, the MP2 method has been used. MP2 appears to give very reliable energies and geometries of card-pack dimers of, e.g., benzene and stacked DNA base dimers (refs. 21–23 and references in ref. 22). The DFT(B3LYP) method does not account sufficiently for van der Waals interactions, which are necessary for stabilizing a card-pack dimer.

In conclusion, the molecular geometries of the non-card-pack dimers for the ground states have been executed with density functional theory (DFT) using the B3LYP (24, 25) hybrid density functional in conjunction with a 6-31G** basis set. Also, for the ground state of the card-pack 7AI dimers, the ab initio MP2 method with 6-31G** basis set has been used. The optimized geometries at the DFT level are implemented in a single-excitation configuration interaction (CIS) ab initio calculation, to calculate the singlet excited state transition energies. The CIS//MP2 (26) and CIS//DFT (27–29) approaches render excellent results, as for example, giving a correct description of the energetic location of the ionic L_a state relative to the covalent L_b state in polyacene (27–29). Vibrational frequency analysis of the non-card-pack dimers at the DFT(B3LYP) level has been applied to determine the nature of the stationary points, and all of these dimers exhibit positive vibrational frequencies. Because of high computational requirements, it was not feasible for us to execute a frequency analysis at the MP2 level (with 6-31G**) of the card-pack dimers. All of the calculations have been executed with the GAUSSIAN 98 program (30).

Dimerization Modes of 7AI in Adiabatic Nozzle Supercooled Molecular Beams.

In the supercooled (1–5 K) molecular beam researches on 7AI it has been assumed tacitly by both main research groups (4–8) that the only dimer formed from the monomer vapor feeding the jet is the C_2h centrosymmetric coplanar, doubly H-bonded dimer. This dimer certainly seems to be the main stable one observed in equilibrium conditions at 298 K in solution studies, and the essential one by which to observe biprotonic transfer dimer fluorescence. Previously, a “nonreactive” (non-proton-transfer) dimer (15, 16) and larger oligomeric forms of 7AI have been suggested (31–33). Fuke and Kaya (16) demonstrated that the relative amount of reactive and nonreactive dimers depends on the stagnation pressure of the carrier gas. Furthermore, a temperature increase of the reservoir to 80°C enhances the abundance of dimer formation relative to other 7AI species in a pulsed supersonic helium expansion (33).

To achieve the delicate-geometry perfect centrosymmetric, coplanar, doubly H-bonded C_2h dimer structure in the adiabatic ultrasonic jet would seem to be one of the least probable of the random collision events. We shall present a model mechanism by which this C_2h coplanar dimer could form as a secondary time-delayed form (companion paper, ref. 9). Under the conditions of the adiabatic nozzle experiment we could expect a considerable variety of transient card-pack and other dimer structures in these ultrarapid transit molecular beams at very low temperatures. If the variety of possible transient dimer structures is considered, we could hope to find a clue to the differential isotope-kinetic results that have been recorded (4–8).

We enumerate the structures that we shall study in the main body of the paper, in order of increasing complexity, and largely in order of increasing stability. A caution must be applied on the assessment of “stability.” We will present data on the depth of the ground state formation potential function minima. Any of these could be regarded as offering “stability” as ultra-short-time transients, as observed in the jet stream emerging from the adiabatic nozzle jet at 1-K to 5-K temperatures. With higher temperatures, and Franck–Condon photo-excitation and observation at microsecond (TOF) or longer time scales, and the possibility of intramolecular rearrangement, equilibration could allow the most stable dimer species (Table 1, reactive C_2h normal tautomer) to dominate finally.

Table 1.

Dimerization energy ΔE calculated at B3LYP or/and MP2 levels with 6-31G^** basis set (Scheme A)

7AI dimer	Level	ΔE, kcal/mol (kJ/mol)
Reactive C2h normal tautomer	B3LYP	−17.23 (−72.09)
	MP2	−20.32 (−85.02)
Nonreactive	B3LYP	−9.15 (−38.28)
C3T	B3LYP	−4.85 (−20.29)
CP3	MP2	−9.24 (−38.66)
CP4	MP2	−8.99 (−37.61)
Dipole–dipole card pack	MP2	−8.35 (−34.94)

ΔE = E_dimer − 2E_monomer. For the 7AI monomer at B3LYP E_monomer = −379.864939 H; at MP2 E_monomer = −378.722963 H [1 hartree (H) = 627.5095 kcal/mol]. 

Thermodynamics of 7AI Dimerization.

Calculations for the 7AI dimerization process, 7AI + 7AI → (7AI)₂, at the DFT(B3LYP)/6-31G** level, indicate that the equilibrium is spontaneous in the gas phase, with ΔG°_298K = −4.42 kcal/mol, which is consistent with reported values in inert solvents. In fact, El-Bayoumi and co-workers (34) obtained a value of −4.5 kcal/mol in 3-methylpentane, and Chou et al. (35) measured −4.54 ± 0.15 kcal/mol in cyclohexane. Notwithstanding the high consistency between the theoretical and experimental data, the ΔH°_298K values obtained from a van't Hoff treatment at temperatures between 260 and 332 K were −9.6 (34) and −9.5 ± 0.9 kcal/mol (35), both of which are much smaller than our calculated value (−15.77 kcal/mol). Thus, this discrepancy must be considered as follows. Taking advantage of the quality of the calculated dimerization free energy value ΔG°_298K, we may evaluate a dimerization energy G°_D over a wide temperature range. Based on the Gibbs expression, there should clearly be a linear relation between G°_D and the corresponding temperatures (T), with an intercept equal to ΔH. Such a relation is of the form G°_D = 0.0368(±0.0002)T − 15.87(±0.05), with (data points) n = 9, r = 0.9999, and SD = 0.06 kcal/mol. The intercept of the best fit of the G°_D vs. T plot, −15.87 ± 0.05 kcal/mol, is highly consistent with the theoretical value obtained in this work for the dimerization ΔH of 7AI, −15.77 kcal/mol, and rather different from the two experimental estimates (34, 35).

Using the equation for G°_D(T), the dimerization equilibrium constant can be estimated for 7AI at 227 K; the resulting value, 1.83 × 10⁷ M⁻¹, suggests that a 10⁻⁴ M 7AI solution at 227 K preserves only 11% of its initial monomers. Similarly, the estimated equilibrium constant at 202 K is 1.45 × 10⁹ M⁻¹, and thus a 10⁻⁴ M 7AI solution at that temperature will contain only 4% of the monomeric species. These results are consistent with our spectroscopic experimental observation (2). It is interesting to note at this point that, in free-jet experiments such as those of Castleman et al. (7, 8), who heated the sample reservoir to 393 K, the dimerization constant is only 6 M⁻¹; this result suggests that almost all of the 7AI emerging from the nozzle must be the monomeric species.

Molecular Structures, Proton-Transfer Potentials, and Energies of Formation of 7AI Dimer Varieties

The calculated electrostatic potential (36, 37) map for 7AI monomer is given in Fig. 2. This map probes the electrostatic field observed by a point positive charge, and may be taken as the electrostatic map for an approaching proton. We then may use this to assess which sites on the molecular skeleton could serve as a good host for an acidic proton (like the pyrrole N-H¹ proton) as a possible H-bonding site in the ground state of the 7AI molecule. The three likely interactive π-electron sites appear to be C³, C⁵, and N¹. This map refines the one published earlier (38).

Figure 2.

Calculated electrostatic potential map of 7AI showing favored C⁵, C³, and N⁷ proton donor interaction sites. Potentials are given in units of hartrees.

The Kaya group (39) included a C5T dimer (see Scheme A) in their search for 7AI dimer structures. Their T-structure has both rings of the perpendicular molecule hovering over the two rings of the horizontal 7AI of their figure. The electrostatic potential maps for each 7AI would seem to indicate repulsion at the aza-N of the perpendicular (donor) 7AI(a) with respect to the pyrrole ring of the horizontal-lying 7AI(b) in their model.

We have calculated the energy and structure for a C5T dimer of 7AI. Here the pyrrolo-H of molecule 7AI(a) is H-bonded to C⁵ of 7AI(b). This perpendicular structure can be described as the C5T dimer, with the donor pyrrolo-H of 7AI(a) in a plane perpendicular to the horizontal plane 7AI(b) containing the acceptor C⁵ atom (see Scheme A). DFT(B3LYP) with 6-31G** basis set calculation indicates a final optimized geometry with a potential energy for the ground state S₀ with a small binding energy of −2.93 kcal/mol. However it is determined, this seeming energy minimum represents a saddle point, because the vibrational frequency analysis reveals one negative vibrational frequency.

The C3T dimer of 7AI (Scheme A).

Here the pyrrolo-H of molecule 7AI(a) is H-bonded to C³ of 7AI(b). The electrostatic potential map (Fig. 2) indicates that carbon C³ is the skeletal carbon most favored in 7AI to interact with a pyrrole proton of an approaching donor 7AI. The structure of this C3T dimer complex is similar to the perpendicular geometry structure of the C5T dimer; however, the C3T dimer exhibits a tilted T-structure and the H-bonding energy is expected to be greater, thus the DFT calculation shows a potential energy minimum depth of −4.85 kcal/mol (Table 1). This model gives all positive vibration frequencies and hence does represent a global minimum for the C3T dimer.

The proton-transfer potential functions for the C3T dimer of 7AI are given in Fig. 3. The excited state S₁(π,π*) barrier for proton transfer is almost as high as for the C5T dimer, reaching 30 kcal/mol, inhibiting the photo-induced proton transfer.

Figure 3.

The 7AI C3T dimer proton-transfer potentials for the S₀–S′₀ ground state, S₁(π,π*)–S′₁(π,π*), and π-electron-ionized doublet state D as a function of N–H distance.

Dipole–dipole card-pack C_i 7AI dimer (Scheme A).

As the 7AI monomer (with carrier gas) emerges supersonically from the adiabatic nozzle, the most powerful intermolecular forces acting on the monomer pairs would be expected to be the van der Waals dispersion force and the electrostatic dipole–dipole force. In the ground state of 7AI these would be expected to dominate over the H-bonding force.

The long-range effect of the permanent dipole moments of a pair of 7AI, especially in the case that the dipoles are pointing in opposite directions, would permit a centrosymmetric card-pack pair with inversion symmetry to reach an energy minimum. Such pair should have a relatively high stability, even when subject to π-electron ionization, and could survive microsecond TOF-MS study. However, such a dimer would, of course, show no intermolecular proton-transfer tendency.

Our MP2 calculations results indicate a potential minimum for the S₀ state of this dimer with an energy stability of −8.35 kcal/mol. However, the final optimized geometry (Scheme A and Fig. 4) does not correspond to that of the above-described one, since the symmetry belongs to the C_i group. The computer time for this large calculation is extensive. The Kaya group (39) considered a card-pack “stack structure,” but they chose pyrrole/pyrrole and pyridine/pyridine stacking, with a C₂ long axis, which would tend to introduce dipole–dipole repulsion.

Figure 4.

Dipole–dipole card-pack 7AI dimer structure.

The singly H-bonded N-H/N card-pack 7AI (CP4) dimer (Scheme A).

This card-pack dimer shows a structure (at MP2 level with 6-31G** basis set) that exhibits a slightly tilted 7AI unit over another 7AI unit. Thus, there exists a hydrogen bond between the N-H group of the upper 7AI(a) unit and pyridinic nitrogen of the lower 7AI(b) unit. The stabilization energy results in −8.99 kcal/mol.

The doubly H-bonded-coplanar “nonreactive” 7AI dimer (Scheme A).

It was noted that in molecular beam laser-induced-fluorescence studies that a non-proton-transfer fluorescent dimer of 7AI was generated in part in the adiabatic nozzle expansion. The name “nonreactive” dimer (Fuke and Kaya) (16) was used for 7AI dimer with no observable proton-transfer-tautomer fluorescence. The Kaya group (39) suggested four possible structures for such a dimer. One of these structures labeled “(b3) single-bonded structure” resembles in part one that we have derived (40) to fit the “non-proton-transfer dimer” of 7AI. The doubly H-bonded non-proton-transfer 7AI dimer we describe as having a 7AI(a) (upper) pyrrole-H bond to N⁷ of 7AI(b) (lower), and a C⁶-H bond of molecule 7AI(b) to the N⁷ of 7AI(a). Of course, the acceptor site N⁷ of 7AI(b) has a favorable electrostatic potential negative charge (Fig. 2); the C⁶-H may also permit a proton donor role. The relatively large ΔE for the formation of a dimer of such a hetero-doubly H-bonded structure (−9.15 kcal/mol) indicates relatively stable formation energetics, with all of the vibrational frequencies as positive. The one proton-transfer (N-H, N) potential for this coplanar “nonreactive” dimer (Fig. 5) shows an S₁(π,π*)–S′₁(π,π*) double-well with a barrier of 13 kcal/mol. If only the N-H proton were transferred, with an unfavorable barrier magnitude for the C-H transfer, an anion–cation pair would result (3) precluding the photo-ionization step, leading to the confusion of ions previously remarked (3, 17).

Figure 5.

Doubly H-bonded coplanar “nonreactive” dimer proton-transfer potentials S₀–S′₀ ground state, S₁(π,π*)–S′₁(π,π*), and π-electron-ionized doublet state D.

The singly H-bonded card-pack 7AI Dimer (CP3, Scheme A).

This dimer possesses a clear-cut hydrogen bond between each C³ position of the lower 7AI(b) monomer unit and the N-H position of the upper 7AI(a) monomer unit. In addition, the geometry is such that it allows two more stabilizing interactions: between C³-H of upper 7AI monomer and the pyridinic -N⁷ of lower 7AI monomer; and between the -N⁷ pyridinic of the upper 7AI monomer and C⁴-H of the lower 7AI monomer. This dimer framework provides a stabilization energy, −9.24 kcal/mol, and thus proves its very feasible formation at low temperature in the free-jet adiabatic expansion.

The doubly H-bonded coplanar centrosymmetric 7AI dimer (C_2h symmetry) (Scheme A).

This classic C_2h dimer has been exhaustively researched (ref. 2 and references therein), and the mechanism of its photo-induced biprotonic transfer has, after some 30 years of research, become the cause celebré. We have stated the case for a concerted, one-step simultaneous biprotonic mechanism. The spectroscopic and quantum theoretical background would seem to confirm this mechanism of biprotonic phototransfer to be necessary and in the long run incontrovertible.

We have shown that the necessarily coherent molecular excitonic character of dimer spectroscopy leads to simultaneity of electronic orbital redistribution, with consequent acid–base changes at (aza)-N: and (pyrrole)-N-(H) sites. The HOMO-LUMO orbital coefficient changes (3) give convincing evidence of what happens at the N-reaction sites upon excitation.

Other dimers could form in the supersonic jet expansion, but they would be obviously unstable. Examples include the mirror card-pack stacking, which has enormous dipole–dipole repulsion for the 7AI pairs. A related stacking can be pictured in which one of the two monomers of the mirror card-pack pair is rotated 180° about its long axis, yet still has a rather large dipole–dipole repulsion. Various single attachment random geometries H-bonded with little stability could have a transient. Such “transient-existence” structures could further contribute confusion in kinetic observations.

In the following companion paper (9), we shall present a model for the mechanism of secondary formation of the classic C_2h dimer from a card-pack H-bonded 7AI dimer with two plane-to-plane H bonds, initially formed on the picosecond time-scale at the supersonic nozzle, which can act as a hinge axis going progressively from a 0° to a 90° to the 180° form of the doubly H-bonded classic C_2h dimer.

As a final remark, in case of the stable card-pack dimers, for instance, the C_i dipole–dipole dimer, and the CP3, CP4, and C3T dimers, the possibility of hydration of these dimers would lead to further energy stabilization, because water molecules will be subjected to bonding, by means of intermolecular hydrogen bonds, to the lone-pair electrons of the pyridinic nitrogens, for example, see refs. 13 and 23.

Conclusion

We present evidence that fosters a concerted mechanism in the excited state of the doubly H-bonded C_2h 7AI dimer. We have unraveled the existence of six stable 7AI dimer species at adiabatic nozzle conditions in free-jet experiments, which can explain the two kinetic rates obtained with femtosecond pump-probe experiments. Various 7AI card-pack dimers are generated, and some of them exhibit hydrogen bonds, which may result in the transfer of a proton, thereby influencing the kinetic transient observations.

It could be emphasized here that most researchers consider the classic C_2h 7AI dimers as the starting point in formulating mechanisms for the photo-excited biprotonic transfer. The large variety of more readily accessible structures of more limited symmetry requirements, which could form in the supersonic nozzle expansion with the extreme cooling (to 7 K or less) observed in the carrier gas, suggests that these simpler dimers are the primary species. The multiple pathways that then become available could directly influence the transient kinetics and species observed in the TOF spectra.

Scheme A.

Stable 7AI dimer configurations (cf. Table 1).

Abbreviations

7AI

7-azaindole

TOF

time-of-flight

DFT

density functional theory