Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 202 (2018) 58–64

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Spectrochimica Acta Part A: Molecular and BiomolecularSpectroscopy

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Effect of different alkyl groups on excited-state tautomerization of7AI-azaindole-H2O: A theoretical study

Jiacheng Yi, Hua Fang ⁎Department of Chemistry and Material Science, College of Science, Nanjing Forestry University, Nanjing 210037, People's Republic of China

⁎ Corresponding author.E-mail address: susanfang20@gmail.com (H. Fang).

https://doi.org/10.1016/j.saa.2018.05.0371386-1425/© 2018 Elsevier B.V. All rights reserved.

a b s t r a c t

a r t i c l e i n f o

Article history:Received 3 December 2017Received in revised form 21 April 2018Accepted 9 May 2018Available online 16 May 2018

The effect of substituted alkyl groups at different substituted position on the first excited-state proton transfer ofnR7AI-H2O (n = 2–6; R = CH3, C2H5, CF3) complexes were theoretically investigated at the TD-M06-2X/6–31+ G(d, p) level. Here n value denoted the substituted position Cn of R group. The replacement of alkyl R grouphad no effect on the features of HOMO and LUMO, but influenced the S0 → S1 adiabatic transition energies ofthe nR7AI-H2O complex. Through computation, we found that the double proton transfer took place in a con-certed but asynchronous protolysis pattern regardless of substituted group R and substituted position in thenR7AI-H2O complex. The vibrational-mode specific nature of ESPT was verified. The alkyl group R changed thegeometrical parameters of TS, and resulted in enlarging/narrowing the asynchronousity of ESPT. The ESPT barrierheight was also affected by the substituted group and position.

© 2018 Elsevier B.V. All rights reserved.

Keywords:Excited-stateProton transferSubstituent effectConcertedAsynchronous

1. Introduction

Non-covalent interactions are important in determining the molec-ular structure, the functionalities and chemical activities of proteins. Hy-drogen bonding (H-bonding) is one of themost important types of non-covalent interactions, and plays a significant part in photochemistry,photophysics, and photobiology [1–5]. Hydrogen bond (H-bond) is asite-specific interaction between hydrogen-donating and hydrogen-accepting groups. The photophysical and photochemical properties ofmany photofunctional molecules can be regulated by the dynamics ofH-bonding.

A great deal of photochemical reactions such as excited-state protontransfer (ESPT) [6], fluorescence quenching [7], tuning effects [8] can becontrolled by the excited-state H-bonding. Excited-state proton transfer(ESPT) as a key step in many chemical and biological reactions usuallyoccurs along the long-distance H-bonded chain, and has attractedmuch attention [9–11]. H-bonded chain over long distance acting as aprotonwire could offer an effective pathway for rapid protonmigration.Much valuable informationwould be provided by simulating ESPT reac-tions which took place widely in biological systems.

ESPT dynamics along H-bonded chain has been extensively studiedfor 7-azaindole (7AI), in which has both proton donor (N\\H) and ac-ceptor (_N\\) and is as a model molecule of DNA bases [12–33].Many studies on the ESPT dynamics of the 7AI dimer have been per-formed, and different ESPT mechanisms of the 7AI dimer have been re-ported. Zewail et al. [34–37] studied the 7AI dimer in gas and in solution

with time-resolved spectra and concluded the stepwise mechanism ofESPT. Castleman et al. [38–40] also supported the stepwise mechanism.However, Takeuchi and Tahara [41,42] proposed a concerted mecha-nism by using time-resolved fluorescence decay. Catalán et al. [43–45]and Sekiya et al. [46,47] presented evidence and conformed the con-certed mechanism. Theoretical investigations revealed that ESPT pro-cess in the 7AI complex with small protic solvent molecules (e.g. H2O,CH3OH, NH3, etc.) in gas occurred in an asynchronous but concertedpath [16,27,29,32]. The proton transfer dynamics of the 7AI dimer and(3-methyl-7AI)-7AI complex has been reported to take place concert-edly but asynchronously [48,49]. The vibrational-mode selectivity forexcited-state triple proton transfer in the 7AI(H2O)2, 7AI(CH3OH)2 and7AI(C2H5OH)2 complex has been found experimentally [22,26,28], andproved theoretically [31,50]. The excitation of a specific vibrational-mode is in accordance with the collaborative motion of H-bond net-work, and promotes ESPT process effectively.

Proton relay in the excited-state was caused by photoexcitation,along with geometrical changes and electron redistributions, andshowed specific fluorescence emission and large Stokes shift. There isa correlation between the dynamics of ESPT process and the strengthof H-bond (proton-donating (acidity) and –accepting (basicity) ability)and the kinetics of proton motion. Krygowski et al. [51] reported thesubstituent effect on proton transfer in phenol complexes with\\NO,\\NO2,\\CHO,\\H,\\CH3,\\OCH3,\\OH groups theoretically. Boththe position of proton transfer and H-bonding strength were linearlydependent on the Hammett constants of substituent group. Chou et al.[52] investigated ESPT reaction in the 3-cyano-7-azaindole complexby measuring electronic spectra, and found that the acidity of N\\Hgroup in the pyrrole ring increased due to the electron-withdrawing

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ability of the cyano group, which led to the acceleration of ESPT rate.Excited-state intramolecular proton transfer (ESIPT) in a series ofN\\H type H-bonding complexes, which H atom belonging to one ofthe amino groups was replaced by electron-donating/withdrawinggroup, has been researched [53]. The results showed that the ESIPT dy-namics and thermodynamics were influenced by the H-bondingstrength. The larger the electron-withdrawing ability of the substituentgroup, the bigger the acidity of the NR-H proton, the stronger the H-bond, and the faster the ESIPT rate. Recent studies on many N\\Hseven-membered-ring H-bonding molecules confirmed that the stron-ger H-bond contributed to ESIPT reaction thermally [54]. Solntsevet al. [55] performed a study on the excited-state dynamics of themeta- and para- isomers of the green fluorescent protein and its O-methylated derivatives. They concluded that the meta- compoundscould occurred ultrafast intermolecular ESPT in aqueous solution.

ESPT dynamics is highly sensitive to the chemical substitution. It hasbecome a good strategy to modify chemical structures by introducingdifferent functional groups or using alkyl substituents. This strategyhas been proved to be very significant for the design of ESPT moleculesfor specific purposes. Chou et al. [56] carried out a systematic study onpKa, pKa⁎, the fluorescence spectroscopy and dynamics of a series of7AI analogues and their methylated derivatives. They concluded thatthe derivatives with an electron-donating substituent (CH3) at C3 oc-curred water-catalyzed ESPT to form an excited proton transfer tauto-mer (T⁎). T⁎ generated an excited cationic species (TC⁎) after rapidprotonation, TC⁎ then deactivated quickly to the normal species in theground state. Teimouri et al. [57] found that protonmigrationwas easierin pyrazoles with electron-donating groups (such as NH2, CH3). Chouand co-workers [58] researched a series of 2-pyridyl pyrazole deriva-tives, and found that an electron-donating group on pyridine availedthe ESIPT process by strengthening the intramolecular H-bond.

Therefore, to further study the correlation between substituent andthe ESPT process, we presented an in-depth theoretical study of ESPTprocess in the 7AI-H2O derivatives, which based on the substitution ofdifferent alkyl groups R (R=CH3, C2H5, CF3) at different substituted po-sitions (C2–C6) of the pyrrole and pyridine ring (see Fig. 1). The 7AI-H2Oderivatives was denoted as nR7AI-H2O (n = 2–6; R = CH3, C2H5, CF3).Here n value denotes the substituted position of R group, for instance,substituted position C2means that n equals 2.We analyzed the electroneffect of the substituents and substituted position on the ESPT process,and investigated the relationship between the dynamic features ofESPT and the substituents. Our resultswill provide detailed understand-ing into the role of the alkyl groups to the ESPT.

Fig. 1. Molecular structure normal of the nR7AI-H2O (n = 2–6; R = H, CH3, C2H5, CF3).

2. Computational Details

The equilibrium geometries of the first excited-state (S1)tautomerization in the nR7AI-H2O (n= 2–6; R = CH3, C2H5, CF3) com-plexes in solution were fully optimized with TD-M06-2X [59] methodand 6–31 + G(d, p) basis set using Gaussian 09 program [60]. Manystudies have confirmed that M06-2X method can give reasonable re-sults of main-group thermochemistry, kinetics, noncovalent interac-tions, and electronic excitation energies to valence and Rydberg states[59]. All minima (reactant and product) and transition state (TS) inthe S1 state were verified by normal-mode analysis at the same compu-tation level. There are no imaginary frequency and only one imaginaryfrequency for the minima and TS along the potential surface of ESPT.Zero-point energy (ZPE) in solution was also calculated at the TD-M06-2X/6–31 + G(d, p) level. In order to consider solvation effect, theintegral equation formalism polarizable continuum model (IEFPCM)[61–63]was used in this study. All H atomshad their individual spheres,and the atomic radii from the UFF force field were scaled by 1.1 inIEFPCM model. Water with a dielectric constant of 78.3 was chosen assolvent. To analyze the chemical bonding in the nR7AI-H2O complexes,the natural bonding orbital (NBO) analysis [64–66] was applied.

3. Results and Discussion

3.1. Frontier Molecular Orbital

The TD-M06-2X method with 6–31 + G(d, p) basis set has beenadopted to optimize the structures of minima (reactant and product)and TS in theH-bonded nR7AI-H2O (n=2–6; R=CH3, C2H5, CF3) com-plexes without any constraints. The optimizations of the structureswere confirmed by analyzing the frequencies. Water has been chosenas the solvent in the IEFPCM model to consider the solvent effect. Inthe heteroaromatic molecules and their H-bonded clusters, the relativeenergy of the Sππ⁎ and Sπσ⁎ states determined the dynamics of thetautomerization in the first (S1) excited-state. The ESPT reaction occursin the ππ* state, and the excited-state hydrogen-atom transfer (ESHT)reaction may occur in the πσ* state [67–70]. Therefore, it is necessaryto qualitatively analyze the nature of electron distribution of thenR7AI-H2O (n = 2–6; R = CH3, C2H5, CF3) complex before discussingthe proton transfermechanism in S1 state. As examples of all complexes,the frontier molecular orbitals (HOMO and LUMO) of 7AI-H2O and2R7AI-H2O (R = CH3, C2H5, CF3) complexes are displayed in Fig. 2.The HOMOs and LUMOs of other complexes nR7AI-H2O (n = 3–6; R= CH3, C2H5, CF3) are depicted in Fig. S1. It is obvious that HOMO andLUMO show the π and π* character, respectively, which means thatthe S1 state has the evident ππ* characteristics. The transition featureis the proof for proton transfer reaction. The electron densities of theHOMO and LUMO completely locate on the 7AI part and no electrondensity is distributed on water, so water in the H-bonded chain is stillin its electronic ground-state (S0) during the proton transfer process.As shown in Fig. 2 and Fig. S1, the substituent R and substituted positionhad no effect on the π and π* characters of the HOMO and LUMO in thenR7AI-H2O complex, respectively.

We calculated the S0 → S1 adiabatic transition energies of 7AI-H2Oand its derivatives nR7AI-H2O (n = 2–6; R = CH3, C2H5, CF3) basedon the fully optimized structures in the S0 and S1 state. The electronicexcitation energies and the corresponding orbital transition contribu-tionswere listed in Table 1. Hara et al. reported the S1 (ππ*) band originof 7AI-H2O is 33340 cm−1 (4.134 eV) in experiment [71]. The calculatedadiabatic transition energy of 7AI-H2O at the TD-M062X/6–31 + G(d,p) level is 4.306 eV, which is comparable to the experimental valuewith a deviation of +0.157 eV. Our theoretical results of nR7AI-H2Ocomplexes based on TD-M062X/6–31 + G(d, p) level are crediable.The adiabatic transition energies of nR7AI-H2O (n = 2–6; R = CH3,C2H5, CF3) complexes are in the range of 3.979–4.538 eV. As shown in

7AI-H2O 2(CH3)7AI-H2O 2(C2H5)7AI-H2O 2(CF3)7AI-H2O

HOMO

LUMO

Fig. 2. The frontier molecular orbitals of 2R7AI-H2O (R = H, CH3, C2H5, CF3) complex in the first excited-state (S1) at the TD-M06-2X/6–31 + g(d, p) level.

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Table 1, the substituent R and substituted position affected the elec-tronic transition energies of nR7AI-H2O complexes.

3.2. The Mechanism of Excited-state Proton Transfer

For the nR7AI-H2O (n = 2–6; R = CH3, C2H5, CF3) complexes, nomatter which H atom at the C2, C3, C4, C5 or C6 position of 7AI-H2O com-plex was substituted for alkyl group R (R= CH3, C2H5, CF3), only one TSwas obtained without any intermediate for ESPT reaction. TS structureswere confirmed by frequency calculations and intrinsic reaction coordi-nate (IRC) calculations. Some structural parameters of TS and minima(reactant and product) were listed in Table 2 and Table S1, respectively.

As shown in Table 2, the N1-H10, H10-O11, O11-H12 and H12-N7 dis-tances in TS of nR7AI-H2O (n = 2–6; R = CH3, C2H5, CF3) complexesare in the range of 1.242–1.349 Å, 1.161–1.267 Å, 1.061–1.183 Å and1.328–1.551 Å, respectively. N1–H10 distance is averagely 0.162Å longerthan O11-H12 distance. These results indicate that H10 proton triggersthe ESPT process and moves more than halfway from N1 to O11, subse-quently H12 proton migrates and shifts less than halfway from O11 toN7. The H10 proton is closer O11 atom than N1 atom, the H12 proton isalso nearer to O11 atom than N7 atom. In the TS, a H3O+-like portionwith partial positive charge is present at O11. This would be a typicalTS in the concerted but asynchronous protolysis proton transfer

Table 1Electronic excitation energy (eV) of S0 → S1, oscillator strengths, and the corresponding orbita

Complex Electronic excitation energy

7AI-H2Oa 4.1347AI-H2O 4.3062(CH3)7AI-H2O 4.2352(C2H5)7AI-H2O 4.2232(CF3)7AI-H2O 4.4733(CH3)7AI-H2O 4.0973(C2H5)7AI-H2O 4.0963(CF3)7AI-H2O 4.5384(CH3)7AI-H2O 4.3654(C2H5)7AI-H2O 4.3484(CF3)7AI-H2O 3.9795(CH3)7AI-H2O 4.2735(C2H5)7AI-H2O 4.2905(CF3)7AI-H2O 4.1626(CH3)7AI-H2O 4.2566(C2H5)7AI-H2O 4.2596(CF3)7AI-H2O 4.090

a The experimental value for S1 0-0 band of 7AI-H2O has been adopted from Ref. [71].

mechanism [73]. The NBO charges [64–66] of the H3O+-like part of TS(see Table 3) confirmed the asynchronous protolysis pathway.

We applied correlation plot between the hydrogen bond length andthe proton transfer coordinate to display the TS characteristics duringproton relay process. In A–H…B complexes, rAH and rBH lengths corre-late with each other and satisfy Pauling equations, on the assumptionthat two bond orders (nAH, nBH) in total is conserved.

nAH ¼ exp − rAH−rAH0� �=bAH

� � ð1Þ

nBH ¼ exp − rBH−rBH0� �=bBH

� � ð2Þ

where rAH0 /rBH0 is the equilibrium bond length in the free AH/BH, and

bAH/bBH is the decay parameter of bond valence. The hydrogen bond co-ordinates q1 = (1 / 2)(rAH− rBH) and q2 = rAH + rBH can be used to de-scribe the correlation between rAH and rBH inA\\H…B complex [74–76].Proton transfer and H-bond length are united in a same correlation. Fora linear H-bond, q1 and q2 denote the distance from H to H-bond centerand from A to B, respectively. Bond length relates to bond energy andbond order. A long rAH and short rBH distances represent a strong H-bond. The “bond energy bond order” correlation [77,78] is usually ap-plied to study the bond order and synchronicity of TS during protontransfer process. When H shifts from A to B in the A\\H…B complex,q1 varies fromnegative to positive, q2 positions at q1=0via aminimum.

l transition contributions for nR7AI-H2O (n = 2–6; R = H, CH3, C2H5, CF3) in water.

Oscillator strengths Orbital contributions

0.2670 HOMO → LUMO 98.4%0.2982 HOMO → LUMO 98.5%0.3168 HOMO → LUMO 98.2%0.3378 HOMO → LUMO 98.3%0.1904 HOMO → LUMO 98.3%0.1900 HOMO → LUMO 98.4%0.3163 HOMO → LUMO 98.3%0.3406 HOMO → LUMO 98.3%0.3367 HOMO → LUMO 98.1%0.2376 HOMO → LUMO 98.5%0.2631 HOMO → LUMO 98.4%0.2668 HOMO → LUMO 98.3%0.1754 HOMO → LUMO 98.2%0.2912 HOMO → LUMO 98.6%0.2961 HOMO → LUMO 98.4%0.2390 HOMO → LUMO 97.7%

Table 2Geometric parameters (Å) of transition states for excited-state proton transfer in nR7AI-H2O (n = 2–6; R = H, CH3, CF3, C2H5) complexes in water.

System r(N1-H10) r(H10-O11) r(O11-H12) r(H12-N7)

7AI-H2Oa 1.287 1.215 1.126 1.4102(CH3)7AI-H2O 1.242 1.267 1.183 1.3282(C2H5)7AI-H2O 1.252 1.255 1.173 1.3402(CF3)7AI-H2O 1.298 1.201 1.061 1.5513(CH3)7AI-H2O 1.259 1.246 1.153 1.3673(C2H5)7AI-H2O 1.260 1.245 1.152 1.3683(CF3)7AI-H2O 1.315 1.187 1.082 1.4974(CH3)7AI-H2O 1.276 1.228 1.144 1.3804(C2H5)7AI-H2O 1.274 1.230 1.144 1.3814(CF3)7AI-H2O 1.326 1.179 1.086 1.4865(CH3)7AI-H2O 1.282 1.220 1.129 1.4045(C2H5)7AI-H2O 1.283 1.220 1.129 1.4045(CF3)7AI-H2O 1.328 1.177 1.087 1.4826(CH3)7AI-H2O 1.282 1.221 1.137 1.3936(C2H5)7AI-H2O 1.281 1.223 1.140 1.3886(CF3)7AI-H2O 1.349 1.161 1.082 1.491

a Data from Ref. [72].

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Anegative/positive q1 value of TSmeans anearly/late TS, and a small/bigq2 value of TS means a tight/loose TS. When multiple protons transportin the synchronous/asynchronous pattern, q1 values of TS are similar/different. The correlations between N1-H10 and H10-O11 distances (H10

transfer), and O11-H12 and H12-N7 distances (H12 transfer) in the TSwere shown in Fig. 3. The TS correlation points are at or approachedto the Pauling line, namely the bond orders at those points are approx-imately conserved. During the double proton transfer process, H10 andH12 correlation points of TS appear in the middle/upper-right side andin the upper-left side, respectively. The q1 values of H10 and H12 at theTS are near zero/slightly positive and very negative, respectively,which indicates that both H10 and H12 protons of TS are close to O11,and a H3O+-like segment emerges as part of TS. The ESDPT in thenR7AI-H2O (n = 2–6; R = CH3, C2H5, CF3) complex takes place in ahighly asynchronous but concerted path.

3.3. The Energetics of Excited-state Proton Transfer

Excited-state tautomerization energy (ΔE) and barrier heights (ΔV)of the nR7AI-H2O (n= 2–6; R= CH3, C2H5, CF3) complexes were listedin Table 4. The tautomerization energies of the nR7AI-H2O complex arein the range of−9.32 to −18.4 kcal/mol and−9.05 to−17.8 kcal/molwithout and with zero-point energy (ZPE) corrections, respectively.The ESDPT processes in the nR7AI-H2O (n = 2–6; R = CH3, C2H5, CF3)

Table 3The distance (R1, R2) between two neighboring heavy atoms in the reactant of nR7AI-H2O(n=2–6; R=H, CH3, C2H5, CF3) complexes, and theNBOcharges of theH3O+-likemoietyof the TS in water.

System r(N1-O11) r(O11-N7) Sum H3O+ charge

R1/Å R2/Å (R1 + R2)/Å

7AI-H2Oa 2.814 2.758 5.572 0.5772(CH3)7AI-H2O 2.825 2.747 5.572 0.5152(C2H5)7AI-H2O 2.827 2.745 5.572 0.5262(CF3)7AI-H2O 2.753 2.800 5.553 0.6403(CH3)7AI-H2O 2.811 2.739 5.550 0.5433(C2H5)7AI-H2O 2.812 2.743 5.555 0.5453(CF3)7AI-H2O 2.783 2.792 5.575 0.6284(CH3)7AI-H2O 2.822 2.759 5.581 0.5574(C2H5)7AI-H2O 2.821 2.757 5.578 0.5584(CF3)7AI-H2O 2.783 2.790 5.574 0.6305(CH3)7AI-H2O 2.808 2.760 5.568 0.5715(C2H5)7AI-H2O 2.810 2.758 5.568 0.5715(CF3)7AI-H2O 2.785 2.784 5.569 0.6296(CH3)7AI-H2O 2.825 2.758 5.583 0.5656(C2H5)7AI-H2O 2.825 2.752 5.577 0.5626(CF3)7AI-H2O 2.790 2.796 5.586 0.645

a Data from Ref. [72].

complexes are exothermic. The structure of TS would resemble thestructure of reactant. The barrier heights without ZPE correction are inthe range of 7.47–11.4 kcal/mol. After ZPE-correction, the barrierheights are in the range of 4.71–7.76 kcal/mol by decreasing3.60 kcal/mol on average. The barrier height of ESDPT in the nR7AI-H2O (n = 2–6; R = CH3, C2H5, CF3) complex could be modified by thesubstituent R and substituted position.

3.4. Vibrational-mode Selectivity of Tautomerization

Sakota et al. [22,26,30] found that the 7AI-(ROH)2 (R = H, CH3,C2H5) complex had a selective vibrational-mode during proton transferprocess. The ESPT rate is noticeably enhanced by the excitation ofvibrational-mode, which is relevant to the in-plane intermolecularstretching displacement of the H-bonded network. For the nR7AI-H2O(n = 2–6; R = CH3, C2H5, CF3) complex, the specific vibrational-modeduring the ESDPT reaction are in the range of 175–192 cm−1. Thevibrational-mode of frequency in 2R7AI-H2O (R = CH3, C2H5, CF3) andnR7AI-H2O (n = 3–6; R = CH3, C2H5, CF3) were displayed in Fig. 4and Fig S2, respectively. As shown in Fig. 4 and Fig. S2, the nature ofthe vibrational-mode selectivity is a heavy-atom breathing motionwithout hydrogenic displacement, which makes N1, N7 and O11 of 7AIapproach, brings the correlation points of H10 and H12 protons close tothose points of TS, shortens the reaction pathway to reach TS, and pro-motes the ESPT reaction.

3.5. Substituent Effect

7AI is awell-knownmodelmolecule for studying the ESPT process ofbiological system, and has received much attention experimentally andtheoretically [12,22,23,26,28]. The properties of 7AI, such as acidity of N-H group and the strength of intermolecular H-bond, were affected bythe substituent and substituted position. The effect of different alkylgroups at different substituted position on the ESDPT process could beobtained by comparing the results in the nR7AI-H2O (n = 2–6; R =CH3, C2H5, CF3) complexes to those in 7AI-H2O complex [72]. Thereare some similarities and differences in the proton transfer process inthe nR7AI-H2O complex. The nature of π → π* transition of nR7AI-H2Ocomplex could be found regardless of substituent and substituted posi-tion. The ESPT in 7AI-H2O complex preferred a highly asynchronous butconcerted protolysis pathway. No matter which H atom at C2–C6 posi-tion in 7AI-H2O complex was replaced by CH3, C2H5 and CF3 groups,the mechanisms of ESPT in these clusters were still a concerted butasynchronous protolysis pattern. The vibrational-mode selectivity ofthe ESDPT was obtained in the low internal energy region in both 7AI-H2O and nR7AI-H2O (n = 2–6; R = CH3, C2H5, CF3) complexes. TheESDPT process was speeded up with this selective vibrational-mode.Nevertheless, some differences during proton transfer process arediscussed as below.

At first, the S0 → S1 adiabatic transition energies of 7AI-H2O and itsderivatives nR7AI-H2O (n = 2–6; R = CH3, C2H5, CF3) varied with sub-stituent R and substituted position. When the H atom at C2 and C3 posi-tions in the 7AI-H2O complex are replaced by CH3, C2H5 and CF3, thetransition energy averagely decreases 0.143 eV and increases0.200 eV, respectively, comparing to that value in the 7AI-H2O complex.When the substituted position is C5 and C6, CH3- and C2H5-, CF3-substituted complexes all have smaller transition energies than 7AI-H2O complex. When the substituted position is C4, the transition ener-gies of 4(CH3)7AI-H2O, 4(C2H5)7AI-H2O and 4(CF3) 7AI-H2O complexesincrease and decrease, respectively.

Secondly, the geometrical parameters of minima in the nR7AI-H2O(n = 2–6; R = CH3, C2H5, CF3) were influenced by the substitutedgroup. The structural parameters of reactant and product in thenR7AI-H2O (n = 2–6; R = CH3, C2H5, CF3) complexes were listed inTable S1 and discussed in detail in Supplementary Information. Ashorter H-bond length means a higher H-bond energy. The stronger

Fig. 3.Correlation of theH-bond distances, q2= r1+ r2, with the proton transfer coordinate, q1=1/2(r1− r2), for the 7AI-H2O and nR7AI-H2O (n=2–6; R=CH3, C2H5, CF3) complexes inwater. a: CH3 and C2H5 groups at C2 and C3 positions; b: CH3 and C2H5 groups at C4–C6 position; c: CF3 at C2–C6 position. Top, H10 transfer; bottom, H12 transfer. All points are for thetransition 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 forPauling equations were from the literature [75]. The correlation points of 7AI-H2O complex are from the literature [72]. The regions above and below the black line are where thesums of bond orders are smaller and larger than unity, respectively.

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H-bond is conducive to proton transfer. This is a common rule for a sin-gle bond. But evaluating the H-bond energy of a given H-bond in a pro-ton relay chain is not easy. Therefore, we use natural bond order (NBO)[64–66] analysis of H-bond to verify the common principle between H-bond length and energy in the H-bonded chain. As a significant part ofthe H-bond, the charge transfer between the lone pair electron of N/Oand anti-bonding orbital of N\\H/O\\H bond could be used to estimatethe strength of H-bond. In NBO analysis, the charge transfer energy rep-resents donor-acceptor (bond-antibond) interactions. The larger thecharge transfer energy, the stronger the H-bond. The H-bond lengthand charge transfer energies of each H-bond in the relay chain areshown in Table S2. The charge transfer energies of each H-bond are lin-early dependent on its length (see Fig. 5), which means that H-bondstrengthens as the H-bond length shortens in the H-bonded chain. Thedistances between the N\\H bond and the acceptor O or N atom hadan evident effect on the proton transfer process [68,79]. The longerthe distance between two heavy atoms, the higher the barrier heightof tautomerization reaction. H-bond compression reduced the barrierheight [80]. Namely, the distance between two neighboring end atomssuch as N1-O11 (R1) and O11-N7 (R2) distances played a significant partin the tautomerization barrier. As shown in Table 3, the N1-O11 andO11-N7 distances in the nR7AI-H2O (n = 2–6; R = CH3, C2H5, CF3)

Table 4Reaction energies (ΔE) and barrier heights (ΔV) for excited-state proton transfer in nR7AI-H2O

Species ΔV

Hb CH3 C2H5 CF3

2R7AI-H2Ob 10.6(6.86)

11.4(7.14)

11.4(7.30)

7.47(4.71)

3R7AI-H2O 10.6(6.86)

10.4(6.33)

10.5(6.37)

9.61(6.59)

4R7AI-H2O 10.6(6.86)

11.3(7.61)

11.2(7.22)

10.0(6.90)

5R7AI-H2O 10.6(6.86)

10.6(6.96)

10.6(6.91)

10.1(6.92)

6R7AI-H2O 10.6(6.86)

11.3(7.50)

11.3(7.66)

10.8(7.76)

a The numbers in parentheses include zero-point energies. Energies are in kcal/mol.b Ref. [72].

complex varied with the substituted group. The longer the sum of N1-O11 and O11-N7 distances, the higher the ZPE-corrected barrier heightof tautomerization. When the substituted position is C2, C3 and C4, thesum of N1-O11 and O11-N7 distances in the nR7AI-H2O (n = 2–6; R =CH3, C2H5, CF3) complex correlates with the ESPT barrier height linearly(see Fig. 6). When H atom at C5 or C6 is replaced, there is no linear cor-relation between (R1 + R2) and ZPE-corrected barrier height.

Thirdly, the structural distances (see Table 2) of TS in nR7AI-H2Ocomplex also variedwith the substituted group R. After the replacementof electron-donating group (CH3, C2H5) at C2–C6 position, the N1-H10,H12-N7 and H10-O11, O11-H12 distances averagely decrease 0.018 Å,0.035 Å and increase 0.020 Å, 0.022 Å, respectively, comparing tothose values in the 7AI-H2O complex. When the substituent is CF3, theN1-H10, H12-N7 and H10-O11, O11-H12 distances averagely increase0.036 Å, 0.091 Å and decrease 0.034 Å, 0.046 Å, respectively. Some dif-ferences emerging in the correlation plot were caused by these changeson the structures. As shown in Fig. 3, the H10 and H12 correlation pointsfor TS in the n(CH3)7AI-H2O and n(C2H5)7AI-H2O (n=2–6) complexesmoves a little to the upper-left side and upper-right side along the Pau-ling line, respectively, with the comparison to the corresponding corre-lation points in the 7AI-H2O complex. The positions of the TS on the H10

and H12 transfer are a little late and early, respectively. These results

(n = 2–6; R = H, CH3, CF3, C2H5) complexes in watera.

ΔE

Hb CH3 C2H5 CF3

−13.6(−13.2)

−12.4(−11.9)

−12.2(−11.9)

−18.4(−17.8)

−13.6(−13.2)

−14.0(−13.6)

−13.9(−13.6)

−14.6(−14.2)

−13.6(−13.2)

−12.9(−12.1)

−12.9(−12.5)

−12.6(−12.2)

−13.6(−13.2)

−13.7(−13.3)

−13.8(−13.5)

−12.0(−11.7)

−13.6(−13.2)

−12.5(−12.1)

−12.3(−11.5)

−9.32(−9.05)

2(CH3)7AI-H2O 2(C2H5)7AI-H2O 2(CF3)7AI-H2O

179 cm-1 175 cm-1 190 cm-1

Fig. 4. Normal mode of vibration of frequency in the 2R7AI-H2O (R = CH3, C2H5, CF3) complex.

63J. Yi, H. Fang / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 202 (2018) 58–64

mean that the substituted electron-donating group (CH3, C2H5) for Hatom at C2–C6 position narrows the asynchronicity of proton transfer.When the substituent is electron-withdrawing group (CF3), the corre-sponding H10 and H12 points move a little to the upper-right side andupper-left side along the Pauling line, respectively. The locations of theTS on the H10 and H12 transfer are a bit early and late, respectively,which enlarges the asynchronicity of proton transfer in the n(CF3)7AI-H2O (n = 2–6) complex.

Lastly, the substituent R had an effect on the barrier height of ESDPTin the nR7AI-H2O complex evidently. When the H atom at C2, C4 posi-tion in the 7AI-H2O complex was replaced by the electron-donatinggroup (CH3, C2H5), the ZPE-corrected barrier height of ESDPT is 7.14,7.61 and 7.30, 7.22 kcal/mol for n(CH3)7AI-H2O and n(C2H5)7AI-H2Ocomplexes, and 0.28, 0.75 and 0.44, 0.36 kcal/mol lower than thatvalue of 7AI-H2O complex, respectively. And the replacement of theelectron-withdrawing CF3 group at C2, C4 position in the 7AI-H2Ocomplex makes the ZPE-corrected barrier height of ESDPT reduceby 2.15 kcal/mol and remain unchanged. When the substituent posi-tion is C3 and C6, the barrier height of CH3-, C2H5- and CF3-substituted complexes all decreases 0.43 kcal/mol and increases0.78 averagely, respectively. The replacing of H atom at C5 positionhad no effect on the barrier height of ESDPT.

Fig. 5. Correlation between the H-bond distance and the charge transfer energy in the H-bonded chain of the nR7AI-H2O (n = 2–6; R = H, CH3, C2H5, CF3) complexes.

4. Conclusions

In conclusion, a detailed theoretical study on the effect of differentsubstituted groups and positions on excited-state tautomerization pro-cess in the nR7AI-H2O (n= 2–6; R= CH3, C2H5, CF3) complex in waterwere carried out at the TD-M06-2X/6–31+G(d, p) level. Our investiga-tions showed that ESPT in the nR7AI-H2O complexes occurred in anasynchronous but concerted protolysis pathway. In this path, H10 pro-ton triggered the ESPT process and migrated more than halfway fromN1 to O11, subsequently H12 proton moved less than halfway from O11

to N7, and a H3O+-like portion generated at O11. The vibrational-modeselectivity of ESDPT was verified in the model molecules. This specificvibrational-mode contributed to shorten the reaction path and acceler-ate the reaction rate of proton transfer. The ESPT mechanisms were notaffected by the different substituted group and position. However, thesubstituting H atom for electron-donating group (CH3, C2H5) andelectron-withdrawing group (CF3) changed the structural parametersof TS evidently. These variations made the asynchronicity of protontransfer narrow and enlarge by CH3, C2H5 and CF3 groups, respectively.The sum of N1-O11 andO11-N7 distances in the nR7AI-H2O (n=2–4; R=CH3, C2H5, CF3) complex correlated with the ESPT barrier height linearly.The longer the sum of N1-O11 and O11-N7 distances, the higher the ZPE-

Fig. 6. Correlation between the sum of the N1-O11 (R1) and O11-N7 (R2) distances and theZPE-corrected barrier height of ESDPT in the nR7AI-H2O (n = 2–4; R = CH3, C2H5, CF3)complexes.

64 J. Yi, H. Fang / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 202 (2018) 58–64

corrected barrier height of tautomerization. The ESDPT barrier heightwith ZPE-correction varied with substituted group and positionobviously.

Acknowledgments

This work was supported by grants from the National Natural Sci-ence Foundation of China (21403114), the Natural Science Foundationof Jiangsu Province (BK20140970).

Appendix A. Supplementary data

Supplementary data to this article can be found online at https://doi.org/10.1016/j.saa.2018.05.037.

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