Introduction

Proton transfer is the most fundamental and important reactions because it is relevant to many chemical and biological processes [1,2,3,4,5,6,7,8,9,10,11,12,13,14]. Among all kinds of proton-transfer reactions, intermolecular excited-state proton transfer (ESPT) has been a hot topic and drawn much attention for decade years [15,16,17]. ESPT reactions took place in some family of compounds, in which the acidities were strengthened in the excited-state and known as photoacids [18]. In the ESPT process, photoacid transfers a proton from a proton donor (–OH or –NH group) to the proton acceptor (–C = O or –N = N group). A large number of biological phenomena such as DNA damage and some enzymatic reactions are caused by ESPT [19, 20]. Most proton-transfer reactions in proteins often occur along long-distance hydrogen-bonded (H-bonded) chain or network, such as polar amino acids or water molecules [7,8,9,10,11]. However, it is very hard to directly investigate such long-distance proton-transfer reaction along a proton wire experimentally and theoretically at the molecular level owing to the complicated structure and large mass protein [9, 11,12,13]. Hence, building a simplified model to simulate the biological proton-transfer process is very necessary.

Among many photoacid molecules, 7-azaindole (7AI) often serves as a model complex to research ESPT process since it resembles to DNA base pair [21] and has aroused much interest. 7AI has a proton donor (N–H) in the five-membered pyrrole ring and a proton acceptor (= N-) in the six-membered pyridine ring. An intermolecular H-bonded chain can be formed between N–H and = N- upon dimerization and complexation with polar solvents. 7AI occurs ESPT reaction along the H-bonded chain. Excited-state proton-transfer reactions in the 7AI dimer and 7AI-(Solvent)n clusters (Solvent: H2O, CH3OH, NH3, etc.) [22,23,24,25,26,27,28,29,30,31,32,33,34,35,36,37,38,39,40,41] have been widely studied experimentally and theoretically. Many theoretical results showed that ESPT in the 7AI-(Solvent)n complexes in gas happened in the concerted but asynchronous pathway [25, 36, 38, 40, 41]. Yu et al. [42, 43] found that the ESPT processes in the 7AI dimer and (3-methyl-7AI)-7AI complex preferred the concerted but asynchronous mechanism by using the multireference second-order perturbation theory (CASPT2) and time-dependent density functional theory (TDDFT) with long-range correction. Sakota et al. [31, 37] researched the excited-state proton-transfer reactions in 7AI–(H2O)2 and 7AI–(CH3OH)2 complexes experimentally and concluded that triple protons transferred asynchronously but concertedly. They also observed the vibration-mode-specific nature of ESPT. The nature of vibration-mode-specific was reproduced theoretically [40]. Theoretical results indicated that the vibrational-mode selectivity can effectively shorten the ESPT path and accelerate the rate of ESPT.

Detailed investigations on ESPT were focused on understanding the tautomerization dynamics and abstracting useful information related to mechanism in order to regulate proton-transfer process [24, 25, 27, 33, 44]. The dynamics and thermodynamics of ESPT correlate with the kinetics of proton motion and H-bonding strength, namely proton donating (acidity) and accepting (basicity) abilities. Chemical modification by a substituent group R can regulate the H-bonding strength depending on the electronic nature of R. Krygowski et al. [45] studied the substituent effect on proton transfer in para-substituted phenol complexes with seven substituents (-NO, -NO2, -CHO, -H, -CH3, -OCH3, -OH) at the B3LYP/6–31++G(d, p) level. They concluded that the H-bonding strength and the position of proton transfer linearly correlated to the Hammett substituent constants. Chou et al. [46] researched the excited-state proton-transfer process in the 3-cyano-7-azaindole complex, in which the cyano substituted H atom of C3 position in pyrrole ring. They found that the acidity of N–H group in the 3-cyano-7-azaindole complex was increased, and the ESPT rate in water was speeded up due to the electron-withdrawing ability of the cyano group. Tseng et al. [47] investigated excited-state intramolecular proton transfer (ESIPT) in many N-H type H-bonding complexes, in which one of the amino hydrogens was replaced with electron-donating/withdrawing groups. They concluded that the H-bonding strength affected the ESIPT dynamics and thermodynamics. The larger electron-withdrawing ability of the substituent increased the acidity of the NR–H proton, strengthened the H-bond, and quickened ESIPT process. Very recently, Chen et al. [48] studied series of N–H seven-membered-ring H-bonded molecules and confirmed that the strong H-bond was advantageous to ESIPT process thermally.

Substituent effect is also relevant to the position of substituent group, and it is named steric effect. Nazarparvar et al. [49] investigated the substituent effects on the O–H bond dissociation enthalpies (BDEs) of trans-resveratrol derivatives, in which electron-donating and -withdrawing groups were placed in four positions. They found that the BDEs were affected by the mutual positions of substituents and OH groups. Klein et al. [50] studied the reaction enthalpies of the individual steps of two antioxidants action mechanisms in meta- and para-substituted phenols. They found that electron-donating substituents increased the enthalpies of proton dissociation and proton affinity. Electron-withdrawing groups increased the reaction enthalpies of the reactions where electron was abstracted. The reaction enthalpies linearly correlated with Hammett constants of the substituents. And the substituents in meta- position had a great effect on ionization potential and enthalpy of proton dissociation. Solntsev et al. [51] carried out a study on the excited-state dynamics of the meta- and para- isomers of the green fluorescent protein chromophore and its O-methylated derivative. They observed the first excited-state was fast quenched to the ground state by internal conversion in all compounds. Internal conversion may promote by the internal twisting in the para compounds. A similar process emerged slowly in the meta compounds but with obviously slower kinetics. The meta compounds can occur ultrafast intermolecular excited-state proton transfer in aqueous.

It is obvious that substituent group and the position of the substituent can effectively control the kinetics and thermodynamics of ESPT. Thus, we used a cyclic H-bonded complex having been chemically modified with substituent at different positions as a simple model to study the excited-state tautomerization process.

In this work, we presented a detailed study on the excited-state proton-transfer process in the 7AI–H2O derivatives nX7AI–H2O (n = 2~6; X = F, Cl, Br) (see Fig. 1), in which the hydrogen atoms at C2, C3 position in pyrrole ring near N–H···O, and C4, C5, C6 position in pyridine ring near N···H H–Bond were replaced by halogen atom X (X = F, Cl, Br). We aimed to investigate the role of the substituent group and different substituent position to the relative stability of nX7AI–H2O and the ESPT thermodynamics and kinetics.

Fig. 1
figure 1

Molecular structure of the 7AI–H2O complex and its derivatives (denoted as nX7AI–H2O (n = 2~6; X = F, Cl, Br); n value denotes the substituted position

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Computational details

All the structures of the reactant, product, and transition state (TS) in the first excited-state (S1) tautomerization in the 7AI–H2O derivatives were fully optimized in solution with TD-M06-2X [52] method and 6-31 + G(d, p) basis set using Gaussian 09 program [53]. The optimized ground state geometries of 7AI–H2O and its derivatives were obtained at the M06-2X [52]/6-31 + G(d, p) level. 7AI–H2O derivatives are the molecules that the H atoms at C2, C3 position in pyrrole ring near N-H···O H-bond, and C4, C5, C6 position in pyridine ring near N···H H-bond in the 7AI–H2O complex were replaced by halogen atom X (X = F, Cl, Br) and denoted as nX7AI–H2O (n = 2~6, X = F, Cl, Br). The vibrational frequencies were also calculated at the TD-M06-2X/6-31 + G(d, p) level in order to confirm the optimized equilibrium structures were minima (reactant and product) and transition state on the potential energy surface. The optimized reactant/product and transition state had no imaginary frequency and only one imaginary frequency, respectively. M06-2X functional is the suitable method to research main-group themochemistry, thermochemical kinetics, noncovalent interactions, and excited-states [52]. In order to consider the solvent effect, the integral equation formalism polarizable continuum model (IEFPCM) [54,55,56] was used. The atomic radii from the UFF force field were scaled by 1.1 in the IEFPCM model. In the solution calculation, water with a dielectric constant of 78.3 was chosen as solvent. To investigate the chemical bond in 7AI-H2O derivatives, the natural bonding orbital (NBO) analysis [57,58,59] was applied to the optimized structure at the same computational level.

Results and discussion

Relative stability of nX7AI–H2O (n = 2~6, X = F, Cl, Br) complex

The relative energies ΔE and Gibbs free energies ΔG of nX7AI–H2O (n = 2~6; X = F, Cl, Br) complexes are presented in Table 1. The Gibbs free energies were computed at the 298.15 K under 1 atm. Among the isomers of nX7AI–H2O (n = 2~6; X = F, Cl, Br) complexes, 6X7AI–H2O molecule is the most stable one in water. The results in Table 1 indicated that different halogen substitution had different order of the relative stabilities of these isomers. For F-substitutions, the relative stabilized trend is 6F7AI–H2O > 5F7AI-H2O > 3F7AI–H2O > 4F7AI–H2O > 2F7AI–H2O. When the substituted atom X is Cl and Br, the trend of relative stabilities of nX7AI–H2O isomers is 6X7AI–H2O > 4X7AI–H2O > 3X7AI–H2O > 2X7AI–H2O > 5X7AI–H2O. Table 2 listed the Boltzmann population ratios of nX7AI–H2O which were calculated with the IEFPCM model at 298.15 K and 1 atm. As shown in Table 2, among F-, Cl-, or Br-substitutions, the 6X7AI-H2O complex is the main existing form in water.

Table 1 Relative energies (ΔE, kcal/mol) and Gibbs free energies (ΔG, kcal/mol) of 7AI–H2O and nX7AI–H2O (n = 2~6; X = F, Cl, Br) complexes in water
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Table 2 Boltzmann population ratios (%) of nX7AI–H2O (n = 2~6; X = F, Cl, Br) complexes in water at 298.15 K and 1 atm
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Frontier molecular orbitals

The dynamics of the first excited-state tautomerization in the heteroaromatic molecules and their H-bonded clusters are determined by the relative energetics of the Sππ* and Sπσ* states. The ESPT reaction occurs on the ππ* state, and the excited-state hydrogen-atom transfer (ESHT) reaction may occur through πσ* state [60,61,62,63]. In order to understand the nature of the excited-state tautomerization in the nX7AI–H2O (n = 2~6, X = F, Cl, Br) complexes, we analyzed the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) which was obtained at the TD-M06-2X/6-31 + G(d, p) level. The 7AI–H2O complex serves as an example of all complexes, and the HOMO and LUMO of 7AI–H2O are displayed in Fig. 2 (MOs of other complexes nX7AI–H2O (n = 2~6, X = F, Cl, Br) are depicted in Fig. S1).

Fig. 2
figure 2

The frontier molecular orbitals of 7AI–H2O complex in the first excited-state (S1) at the TD-M06-2X/6-31 + g(d, p) level

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By comparing the shapes of HOMO and LUMO, nX7AI–H2O (n = 2~6, X = F, Cl, Br) is an obvious ππ*-type transition. The character of the π → π* transition is the exact proof for proton-transfer reaction. The electron densities of both the HOMO and LUMO completely locate on the 7AI moieties, and no electron density localizes on water, so the H-bonded chain of water still remains in its electronic ground-state during the proton-transfer process. As shown in Fig. 2 and Fig. S1, the substituent halogen atom X at different positions had no effect on the characters of the HOMO and LUMO of nX7AI–H2O complex.

The S0 → S1 adiabatic transition energies of 7AI-H2O and its derivatives nX7AI–H2O (n = 2~6; X = F, Cl, Br) have been calculated on the optimized structures in the ground-state and first excited-state. The electronic excitation energies and the corresponding orbital transition contributions were listed in Table 3. The S1 (ππ*) band origin of 7AI–H2O has been reported experimentally by Hara et al. to 33,340 cm−1 (4.134 eV) [30]. The calculated adiabatic transition energy of 7AI–H2O at the TD-M06-2X/6-31 + G(d,p) level amounts to 4.306 eV, which is comparable with experimental value with a deviation of + 0.157 eV. Our theoretical results of nX7AI-H2O complexes based on TD-M06-2X/6-31 + G(d,p) level are reliable. As shown in Table 3, the electronic transition energies of nX7AI-H2O complexes varied with the substituent X and substituted position.

Table 3 Electronic excitation energy (eV) of S0 → S1, oscillator strengths, and the corresponding orbital transition contributions for nX7AI-H2O (n = 2~6; X = H, F, Cl, Br) in water
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The mechanism of excited-state proton transfer

The structures of stationary points in the cyclic nX7AI-H2O (n = 2~6, X = F, Cl, Br) complexes were fully optimized in water at TD-M06-2X/6-31 + G(d, p) level. Some geometrical parameters of TS are listed in Table 4, and the structures of TS are displayed in Fig. S2. No matter which H atom at the C2, C3, C4, C5, or C6 position of 7AI-H2O complex was substituted for halogen atom X (X = F, Cl, Br), only one concerted but asynchronous TS was obtained without any intermediate for the excited-state double proton-transfer process in the nX7AI-H2O (n = 2~6, X = F, Cl, Br) complexes. As shown in Table 4, N1–H10, H10–O11, O11–H12, and H12–N7 distances in TS of nX7AI-H2O (n = 2~6, X = F, Cl, Br) are in the range of 1.279~1.383, 1.137~1.217, 1.070~1.105, and 1.446~1.523 Å, respectively. It is obvious that N1–H10 distance is averagely 0.128 and 0.223 Å longer than H10–O11 and O11–H12 distances, respectively. These results indicated that H10 proton moves first and transfers more than halfway from N1 to O11, subsequently H12 proton moves a little from O11 to N7, and a H3O+-like portion forms at O11. The ESPT reaction in nX7AI-H2O (n = 2~6, X = F, Cl, Br) complex occurred in a concerted but asynchronous protolysis [65] pattern. The concerted but asynchronous protolysis mechanism of ESPT was verified by NBO charges of the H3O+-like moiety of TS, which were listed in Table S1.

Table 4 Geometric parameters (Å) of transition states for excited-state proton transfer in 7AI-H2O and nX7AI-H2O (n = 2~6; X = F, Cl, Br) complexes in water
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The character of TS during the proton-transfer process can be depicted in a correlation plot between the proton transfer coordinate and the H-bond distance. In A–H…B complex, the rAH and rBH distances conform to the Pauling equations and interrelate with each other, with the hypothesis that the sum of bond orders is conserved during proton-transfer process [66]. Limbach et al. [67,68,69] proposed to combine proton transfer and H-bonding distance in a correlation. The relationship between rAH and rBH in A–H…B complex can be expressed by H-bond coordinates q1 = 1/2(rAH − rBH) and q2 = rAH + rBH. For a linear H-bond, q1 represents the distance from H to the H-bonding center, and q2 represents the distance between two heavy atoms A and B. The property of TS such as bond order, earliness or lateness, and synchronicity during proton-transfer process can be described in this correlation plot. When H moves from A to B in the A–H···B complex, q1 value varies from negative to positive, and q2 situates at q1 = 0 after passing through a minimum. There is a correspondence between late/early TS and positive/negative q1 value. And a tight or loose TS corresponds to a small or big q2 value, respectively. When several protons relay in the synchronous or asynchronous mechanism, the multiple q1 values of TS should be alike or different to each other, respectively.

The correlations between N1–H10 and H10–O11 distances (H10 transfer), and O11–H12 and H12–N7 distances (H12 transfer) in the TS are depicted in Fig. 3. The correlation points of the TS in the nX7AI-H2O (n = 2~6; X = F, Cl, Br) complexes are all at or close to the black line according to Pauling equation, which means that the bond orders are conversed. For the nX7AI-H2O (n = 2~6; X = F, Cl, Br) complexes, the q1 values of H10 transfer at the TS are near zero or slightly positive, which represents that H10 is nearly in the center between N1 and O11. At the same time, the H12 correlation points emerge in the upper-left area; the q1 values of H12 transfer at the TS are very negative, which represent that H12 is close to O11. The correlation plot indicated that the double-proton transfer in the nX7AI-H2O complexes occurred in a concerted but highly asynchronous mechanism (a little late TS for H10 but very early for H12 transfers) and generated a [H3O]+-like portion as part of TS.

Fig. 3
figure 3

Correlation of the H-bond distances, q2 = r1 + r2, with the proton transfer coordinate, q1 = 1/2(r1-r2), for the 7AI-H2O and nX7AI-H2O (n = 2~6; X = F, Cl, Br) complexes in water. Top, H10 transfer; bottom, H12 transfer. All points are for the transition state in S1 optimized at the TD-M06-2X/6-31 + G(d, p) level. The solid lines designate the correlation that satisfies conservation of the bond order. The parameters for Pauling equations were from the literature [67]. The correlation points of 7AI-H2O complex are from the literature [64]. The regions above and below the black line are where the sums of bond orders are smaller and larger than unity, respectively

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The energetics of excited-state proton transfer

The excited-state double proton relay reaction energies (ΔE) and the barrier heights (ΔV) in the nX7AI-H2O (n = 2~6; X = F, Cl, Br) complexes were shown in Table 5. The reaction energies without and with zero-point energy (ZPE) corrections of nX7AI-H2O complexes are in the range of − 5.98~− 19.7 kcal/mol and − 5.96~− 18.8 kcal/mol, respectively. The ESDPT reactions in the nX7AI-H2O complexes are exothermic. The TS structure of nX7AI-H2O would be similar to the reactant. With and without ZPE-corrected barrier heights of ESDPT in the nX7AI-H2O complex are in the range of 1.99~9.30 and 4.43~12.4 kcal/mol, respectively. It is evident that the barrier height of ESDPT is dependent on the substituent halogen atom and substituted position.

Table 5 Reaction energies (ΔE) and barrier heights (ΔV) for excited-state proton transfer in nX7AI-H2O (n = 2~6; X = H, F, Cl, Br) complexes in water
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Substituent effect

In order to investigate the effect of halogen substitution at the different positions of 7AI-H2O complex, we compared the results of ESDPT in the nX7AI-H2O (n = 2~6; X = F, Cl, Br) complexes to those of 7AI-H2O [64] and found some resemblances and differences. The nature of ESPT in the nX7AI-H2O (n = 2~6; X = F, Cl, Br) complex is the π → π* transition no matter which halogen atom X substituted for H atom at C2~C6 position in 7AI-H2O complex. The excited-state proton transfer in the 7AI-H2O complex occurred via a highly asynchronous but concerted protolysis pathway. The nX7AI-H2O (n = 2~6; X = F, Cl, Br) complexes originate from 7AI-H2O complex by replacing H atom at different position (C2, C3, C4, C5 or C6) in the pyrrole and pyridine ring in the 7AI-H2O complex with halogen atom X (X = F, Cl, Br). The substituted halogen atom X did not influence the ESDPT mechanism. The excited-state proton transfer in the nX7AI-H2O complexes still preferred a concerted but asynchronous protolysis path. Besides, there are some differences in ESPT process in the nX7AI-H2O complexes.

Firstly, the S0 → S1 adiabatic transition energies of 7AI-H2O and its derivatives nX7AI-H2O (n = 2~6; X = F, Cl, Br) changed with substituted halogen atom X and position. When the H atom at C2 and C3 positions in the 7AI-H2O complex are substituted for halogen atom X (X = F, Cl, Br), the transition energy averagely increases 0.070 eV and decreases 0.162 eV, respectively, comparing to that value in the 7AI-H2O complex. When the substituted position is C4 and C6, F-, and Cl-, Br-substituted complexes have larger and smaller transition energy than 7AI-H2O complex, respectively. When the substituted position is C5, the transition energy of 5F7AI-H2O increases, whereas the transition energies of 5Cl7AI-H2O and 5Br7AI-H2O decrease. The transition energies in the nX7AI-H2O (X = F, Cl, Br) complexes are linearly dependent on the Pauling electronegativity of halogen atom X (see Fig. S3).

Secondly, halogen atom X as the substituent affected the H-bond distances of reactant and product in the nX7AI-H2O (n = 2~6; X = F, Cl, Br) complexes obviously. The structural parameters of reactant and product in the nX7AI-H2O (n = 2~6; X = F, Cl, Br) complexes were listed in Table S2, and the changes of the H-bond distances were discussed in detail in Supporting Information. The H-bond length is shorter; the H-bond energy is higher, and the proton transfers easier. This is a common rule for a single H-bond. However, estimating the bond energy of a certain H-bond in a relay chain is very difficult. Hence, it is necessary to verify that the common regularity between H-bond distance and energy is applicable to the H-bonded relay chain by using NBO analysis of H-bond [57,58,59]. As an important part of the H-bond, the charge transfer between the lone pair electron of N/O and anti-bonding orbital of N–H/O–H bond can be used to reckon the strength of H-bond. In NBO analysis, the charge-transfer energy infers donor-acceptor (bond-antibond) interactions. The larger the interaction energy of charge transfer, the stronger the H-bond. The H-bond distance and charge-transfer energies of each H-bond in the relay chain were listed in Table S3. We found that between the charge-transfer energy of each H-bond and its length has a good correlation (see Fig. 4), which means that the H-bond strengthens when the H-bond distance shortens. The bonds between the N–H bond and the acceptor O or N atom affected the proton-transfer process obviously [61, 70]. If the distance between two heavy atoms was too far, the barrier height of proton-transfer process was also too large since H-bond compression reduced the barrier height [71]. Namely, the distance between two neighboring end atoms of proton transfer in reactant such as N1–O11 (R1) and O11–N7 (R2) plays a significant part in the tautomerization barrier. The distances of N1–O11 (R1) and O11–N7 (R2) were listed in Table S1. In this work, ∆(R1 + R2) is used to represent the difference between the sum of the N1–O11 and O11–N7 distances of nX7AI-H2O and those values of 7AI-H2O. We found that ∆(R1 + R2) and Pauling electronegativity of halogen atom X have good linear correlation (see Fig. 5). When the substituted position is C2 and C5, ∆(R1 + R2) increases with the decrescent Pauling electronegativity of X. When the substituted position is C3, C4, and C6, ∆(R1 + R2) increases with the incremental Pauling electronegativity of X.

Fig. 4
figure 4

Correlation between the H-bond distance and the charge transfer energy of the H-bonded chain in the 7AI-H2O and nX7AI-H2O (n = 2~6; X = F, Cl, Br) complexes

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Fig. 5
figure 5

Correlation between the change in the sum of the N1–O11 (R1) and O11–N7 (R2) distances (∆(R1 + R2)) in the nX7AI-H2O (n = 2~6; X = F, Cl, Br) complexes and the Pauling electronegativity (ENχ) of X. a Substituted position C2 and C5. b Substituted position C3, C4, and C6

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Thirdly, the structural parameters (see Table 4) in TS of nX7AI-H2O (n = 2~6; X = F, Cl, Br) complexes also changed with the substituted halogen atom X and substituted positions. After the replacement of halogen atom at C2 and C3 positions, the N1–H10 and H10–O11 distances are a little shorter and longer than those distances in the nX7AI-H2O (n = 4~6) complexes. Comparing to the structural parameters of TS in the 7AI-H2O, the halogen substitutions make the N1–H10, H12–N7 and H10–O11, O11–H12 distances of nX7AI-H2O (except 2F7AI-H2O) increase 0.030 Å, 0.054 Å and decrease 0.029 Å, 0.035 Å on average, respectively. In the 2F7AI-H2O complex, the N1-H10, O11-H12, and H10–O11, H12–N7 distances reduced by 0.008 and 0.056 Å and increased by 0.002 and 0.113 Å, respectively.

A few differences appearing in the correlation plot were caused by these small changes on the structures. As shown in Fig. 3a, the H10 and H12 correlation points for TS in the 2X7AI-H2O and 3X7AI-H2O (X = F, Cl, Br) complexes are unchanged and move a little to the upper-left side along the black line, respectively, with the comparison to the corresponding points in the 7AI-H2O. The locations of the TS on the H10 and H12 transfer are invariant and a bit late, respectively. When the halogen substitution is at C4, C5, and C6 positions, the corresponding H10 and H12 points move a bit to the upper-right and upper-left sides along the Pauling line (see Fig. 3b), respectively. As a result, positions of the TS on the H10 and H12 transfer reaction coordinate in the nX7AI-H2O (n = 4~6) complex are a little early and late, respectively. These results indicate that no matter which position in the 7AI-H2O complex is substituted, the asynchronicity of proton transfer is enlarged evidently. The structural changes of the TS are possibly due to the electron-withdrawing ability of halogen atom X, which increases the acidity of N–H group of nX7AI-H2O. The NBO charges of the H3O+-like portion in TS for the nX7AI-H2O complex were in Table S1, and those values for the nX7AI-H2O complex added 0.040 on average with comparison to that value for the 7AI-H2O.

Lastly, the substituent halogen atom and substituent position in the 7AI-H2O complex influenced the barrier height of ESDPT evidently. When the H atom at C2 and C3 position in the pyrrole ring was substituted by halogen atom X, the ZPE-corrected barrier height is in the range of 4.62~6.14 kcal/mol, which is 0.72~2.24 kcal/mol lower than that value of 7AI-H2O complex. If the H atom at C4 and C6 position in the pyridine ring was replaced by halogen atom X, the ESDPT barrier height with ZPE-correction is in the range of 7.20~9.30 kcal/mol. The halogen replacements at C4 and C6 position increase the barrier height of ESDPT by 0.34~2.44 kcal/mol. And the replacement of X at C5 position makes the barrier height rise/reduce due to the substituent halogen atom X. F-substitution increases the barrier height by 0.27 kcal/mol, while Cl- and Br-substitution decreases the barrier height by 4.87 and 4.49 kcal/mol, respectively.

Conclusions

In conclusion, a careful and detailed theoretical study on the halogen substituent effect upon the excited-state tautomerization process in the 7AI-H2O complex in water were carried out at the TD-M06-2X/6-31 + G(d, p) level. The character of the π → π* transition during the tautomerization process could be clearly seen, which could be the exact proof for proton-transfer reaction. For the nX7AI-H2O (n = 2~6; X = F, Cl, Br) complexes, the ESPT reaction preferred a concerted but asynchronous protolysis pathway regardless of halogen atom or substituent position. In this path, H10 proton started the ESPT process and moved more than halfway from N1 to O11; H12 moved a little from O11 to N7, and a H3O+-like portion generated at O11.

The halogen substitution at C2, C3, C4, C5, and C6 positions had little effect on the mechanism and the nature of ESPT process. However, the substitution of halogen atom affected the relative stability, S0 → S1 adiabatic transition energy, structural parameter, and barrier height of nX7AI-H2O during proton-transfer process. Our calculated results showed that the most stable complex of 7AI-H2O derivatives is in the form of 6X7AI-H2O (X = F, Cl, Br) in water. The transition energies in the nX7AI-H2O (X = F, Cl, Br) complexes are linearly dependent on the Pauling electronegativity of halogen atom X. The halogen substitution increases/decreases the H-bond distances in the reactant, product, and structural parameters in the TS, which leads to enlarge the asynchronousity of proton transfer. The ESPT barrier height of nX7AI-H2O is influenced by the halogen atom and substituted position. When the H atom at C2, C3 or C4, and C6 position is replaced by X, the ZPE-corrected barrier height reduces by 0.72~2.24 kcal/mol or increases by 0.34~2.44 kcal/mol. And the replacement of X at C5 position makes the barrier height rise/reduce due to the substituent halogen atom X. The changes in the sum of N1–O11 and O11–N7 distances have good correlations with Pauling electronegativity of halogen atom X.