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Protic vs Aprotic Solvent Effect on Proton Transfer in 3-Hydroxyisoquinoline: A Theoretical Study Bo Xiao, Yan-Chun Li, Xue-fang Yu, and Jian-Bo Cheng J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/acs.jpca.5b10180 • Publication Date (Web): 23 Nov 2015 Downloaded from http://pubs.acs.org on November 27, 2015

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The Journal of Physical Chemistry

Protic vs Aprotic Solvent Effect on Proton Transfer in 3Hydroxyisoquinoline: A Theoretical Study

Bo Xiao,a Yan-chun Li,b Xue-fang Yu*,a and Jian-bo Cheng,a a

The Laboratory of Theoretical and Computational Chemistry, School of Chemistry and Chemical Engineering, Yantai University, Yantai 264005, China b

Institute of Theoretical Chemistry, Jilin University, Changchun 130021, China

Abstract In this work, the structures, energetics and the tautomerizations in 3-hydroxyisoquinoline (3HIQ) in both the ground state and the excited state have been theoretically investigated by the MP2, TDDFT and CASPT2 methods, respectively. The solvent effect including the implicit solvent and explicit solvent on the structures, energetics and tautomeizations are revealed. We found that the explicit solvent plays more important roles in the structures, energetics and tautomerizations in 3HIQ than implicit solvent in both the ground state and excited state. The proton transfer is more facilitated in explicit solvent (water or methanol) compared to that in the gas phase and in the implicit solvent in the excited state, and the reactive role of the molecular solvent is found to be related with the two linear hydrogen bonds.

* To whom correspondence should be addressed. E-mail: [email protected].

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1. Introduction Proton transfer (PT) reactions play important roles in chemistry and biology1-3, and occur within molecules containing proton donor and proton acceptor. Intra-molecular PT occurs within one molecule with both proton donor and proton acceptor sites in one moiety, such as proton jumping from O atom to N atom in 7-azaindole monomer, while inter-molecular PT occurs within dimers or complexes, such as 7-zaindole dimers and 7-azaindole(H2O)2-3 where proton transfers from O atom in one monomer to N atom belonging to the other monomer or solvent4-6. The explanation and the understanding of the intra-molecular PT or inter-molecular PT, especially the excited state PT (ESPT) have been extensively investigated in recently7-20. There are two different commonly accepted excited state intra-molecular PT mechanisms for systems with hydrogen bond (7-azaindole) and the ones without hydrogen bond (hydroxyflavone). The former process occurs on a "standard" adiabatic potential energy surface of the first excited state, while the latter is non-adiabatically mediated by a 'dissociative' πσ* state. The intra-molecular PT mechanism of this reaction, the photo-induced dissociation-association, is explored in J. Phys. Chem. A, 112 (2008) 13655, for instance. It has been well known that the excited state proton transfer (ESPT) process usually can be triggered by the enhanced O-H...N or N...O-H hydrogen bonds, and the process is accompanied by large stoke shift during the tautomerization. This phenomenon can be utilized in the design of fluorescent sensors.21,22 3-hydroxyisoquinoline (3HIQ) is a prototype for studying PT reactions, because it contains a proton donor (OH) and a proton acceptor (N), see scheme 1 for the proton transfer from normal form (Enol) to tautomer form (Keto). It has received particular attention recently, due to the complex tautomerization/thermodynamics of the conformers in different solvents in both the ground state and excited state. Evans et al. reported that the Keto form and Enol form exist in water and diethyl ether, respectively, and both conformers exist in some specific nonpolar solvents and polar solvents23. Wei et al. indicated that the existence of specific conformer strongly depends on the concentration of 3HIQ in non-polar solvent, with Enol form and Keto form formation in low and high concentration, respectively. Moreover, the conjugated dual hydrogen-bonding (CDHB) effect plays key role to govern the ground state equilibria toward the Keto form. In addition the CDHB complex (3HIQ with acid) is found to undergo a fast excited state proton transfer reaction24. 2


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Recently, Joshi et al. reported the spectra and photophysical properties of 3HIQ in various protic and aprotic solvents through steady state and resolved fluorescence. They reported that both the ground state PT and excited state PT could occur, with the processes governed by polarity of the solvent25. In particular they found that intra-molecular PT can take place in the excited state in the monomer form of 3HIQ. This is in great contrast with previous studies who suggest that the proton transfer occurs in dimer or complex with solvent molecule24. Theoretically, Gerega et al. studied the stability of Enol and Keto forms, and found that Enol form is more stable than Keto form at the QCISD/cc-pVDZ and QCISD(T)/cc-pVDZ levels based on the geometries optimized at the B3LYP/cc-pVDZ level26. Recently, Pan et al. investigated the thermodynamics of the neutral and cationic forms of Enol and Keto, and confirmed that neutral Enol is more stable than neutral Keto at higher level. The relative energy of the neutral Keto with respect to neutral Enol, and the energy barrier for this tautomerization is calculated to be 0.3 eV and 1.7 eV at CCSD(T)-F12 level based on the equilibrium structures optimized at PBE0/6-311+G(d,p) level27, respectively. Ramos et al. studied the process of tautomerization of 3HIQ from Enol form to Keto form in the first excited state at the CIS/631G(d) level. They revealed that the cooperation of a single water molecule or acetic acid (AA) considerably decreases the energy barrier for the tautomerization of 3HIQ from 51.1 kcal/mol to 23.8 kcal/mol in 3HIQ-H2O, and to 10.1 kcal/mol in 3HIQ-AA, respectively28. The solvent effect plays important roles in both the ground state and excited state25,28. To reveal the reactive roles of the solvent in 3HIQ, we theoretically study the protic and aprotic solvent effect on the energetics and structures as well as the proton transfer processes (tautomerizations) in 3HIQ in both the ground state (S0) and the first excited state (S1). The aprotic solvent effect is simulated based on Polarizable Continuum Model (PCM) using the integral equation formalism variant (IEFPCM)29-32, where dichloromethane and diethyl ether are used as aprotic solvents. The protic solvent effect is simulated with cluster model. The complexes of 1:1 3HIQ with common solvent water (3HIQ-W) and 1:1 3HIQ with methanol (3HIQ-M) are constructed to include the specific interaction of solvent and solute. The present work provides a direct picture of the solvent effect on the tautomerizations of 3HIQ in both S0 state and S1 state.

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2. Computational Details Equilibrium geometries of 3HIQ, 3HIQ-M and 3HIQ-W in S0 state were determined by the Møller-Plesset second-order perturbation (MP2) method, while geometries of minima and transition states in S1 state were determined by the time-dependent density functional theory (DFT) with B3LYP33-34 functional (TD-)B3LYP. The coulomb attenuate B3LYP (CAMB3LYP35) which includes the long-range correction was also used for geometry optimization and energy calculations to check the results dependence on the functional. For comparison, the complete-active-space self-consistent filed (CASSCF) method was used for geometry optimization without symmetry constraints. The wave function was constructed with the active space of 10 electrons distributed in 5 π and 3 π* orbitals, with averaging the lowest three electronic states with equal weights. We choose this active space in order to get the consistence of the orbitals not only in the tautomerizations of the monomer, but also for the clusters. The single-point energy calculations were performed at complete active space second order perturbation theory (CASPT236) (single-state CASPT2) level to include the dynamic electron correlation, where a level shift of 0.337 au was applied to avoid the intruder state problems. In calculations, all minima and transition states in S0 and S1 states were confirmed by normal mode analysis at the respective computational levels, except CASSCF calculations. The intrinsic reaction coordinate (IRC) calculations were also performed starting from the respective transition states in the S0 and S1 state to determine the reaction pathways at the respective levels. In this study, several different basis sets were used to test the results dependence on basis set. 6-31G(d), 6-311+G(d), cc-pVDZ and aug-cc-pVDZ basis sets were used in MP2 calculations in the S0 state, 6-31G(d), 6-311+G(d) and cc-pVDZ basis sets were used in the S1 state calculations. The calculated results show that energies are not so dependent on basis set. MP2 calculations and CASPT2 calculations were performed with Gaussian 09,38 and MOLPRO 2010.139, respectively, while (TD-)B3LYP and (TD-)CAM-B3LYP calculations were performed with GAMESS40, except for the calculations for exited state aprotic solvent effect using IEFPCM performed with Gaussian 09.

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3. Results and Discussion 3.1. Protic and aprotic solvent effect on ground state proton transfer in 3HIQ The thermodynamics of 3HIQ in the gas phase have been studied by Hochlaf et al. very well at CCSD(T)-F12 level of theory with core-valence and scalar-relativistic corrections using the equilibrium structures optimized at PBE0/6-31+G(d, p) level27. They revealed that intramolecular isomerization does not occur due to the high energy barrier (1.71 eV) and the relatively higher energy in Keto form (0.30 eV) with respect to Enol form. In this part, we focus on the solvent effect on the structures and energetics in the intra-molecular isomerization in 3HIQ. For this purpose, the equilibrium structures of the normal form (Enol) and the tautomer form (Keto), and the transition state (TS) between them in the gas phase (denoted as EMS0 and KMS0, TSMS0, for later use) and those in the implicit solvents (dichloromethane and diethyl ether) are located at the MP2/cc-pVDZ level. All the structures show Cs symmetry of the molecular plane, see EMS0, KMS0 and TSMS0 in Figure 1 as an example. In the Enol form and Keto form, the hydrogen atom H18 is bonded with O17 and N16, respectively. The equilibrium structures in the gas phase and in the implicit solvents are very similar, in particular in Enol form. The bond distances of N16...H18 and O17...H18 in EMS0 and KMS0 are about 2.21 Å and 2.43 Å, respectively. The values do not change even in the implicit solvents. In TSMS0, the hydrogen atom H18 is just located between O17 and N16, with the bond distances of O17...H18 and N16...H18 about 1.41 Å and 1.25 Å, respectively. These values become to 1.40 Å and 1.26 Å in the implicit solvents, respectively. The implicit solvent effect on the energetics for the intra-molecular isomerization in 3HIQ is also not evident. The energy of the Enol form is set to be zero as reference. The energies of KMS0 and TSMS0 in the gas phase are calculated to be 8.98 kcal/mol and 38.31 kcal/mol at MP2/cc-PVDZ level, respectively. Including the zero-point energy correction, these values become 9.00 kcal/mol and 35.32 kcal/mol, respectively. Obviously the high activation energies for the tautomerizations from EMS0 to KMS0 (38.31 kcal/mol) as well as from KMS0 to EMS0 (29.33 kcal/mol) prevent proton transfer from either side. Our results in the gas phase are very consistent with previous work by Hochlaf et al.(the energy barrier is about 1.71 eV and the relative energy of Keto with respect to Enol is about 0.30 eV) at CCSD(T)-F12 level of theory with core-valence and scalar-relativistic corrections using the equilibrium structures optimized at 5



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PBE0/6-31+G(d, p) level 27, and the small energy difference indicates our calculation method is also reliable. In dichloromethane (and diethyl ether) solvent, the relative energies of KMS0 and TSMS0 are 5.71 kcal/mol (and 6.38 kcal/mol) and 37.36 kcal/mol (and 37.55 kcal/mol), respectively. The energy differences between EMS0 and KMS0 in both solvents become smaller compared to the one in the gas phase, indicating that KMS0 is a little stabilized in the solvents, although not evidently. In the meanwhile, the relatively smaller energies in TSMS0 in the solvents compared to that in the gas phase indicate that the proton transfer is somewhat facilitated in these solvents. Although these aprotic solvents do influence the energy difference between the conformers, the energy stability order remains with EMS0 still being the most stable form in these solvents, and the activation energy for the intra-molecular isomerization is still too high for proton transfer from any side. This result is consistent with previous theoretical study on the tautomerization of the monomer41. However, in recent experiment, joshi et al. observed the absorption spectra featured at ~405 nm in dichloromethane, and attributed it to the Keto form25. The existence of Keto was explained by the proton transfer from Enol to Keto in the S0 state. As shown in this study, the energy barrier for the proton transfer in dichloromethane is calculated to be as high as 37.36 kcal/mol, the proton transfer can not take place by overcoming such high energy barrier forming the Keto form, while one possible explanation for the existence of the Keto form might result from the quantum tunneling. To investigate the roles of the specific interaction of solvent with 3HIQ, we studied the micro-solvation which counts the interaction like hydrogen bond. The cluster models of 1:1 3HIQ with water denoted as 3HIQ-W, and 3HIQ with methanol denoted as 3HIQ-M were constructed to estimate the specific effect of the molecular solvent on the structures, energetics, and tautomerizations in 3HIQ. The optimized structures of the normal form cluster (Enol cluster), the tautomer form cluster (Keto cluster) as well as the transition state between them in 3HIQ-M, denoted as EC-MS0, KC-MS0 and TSC-MS0 are shown in Figure 2a-c. The optimized structures of the Enol cluster, the Keto cluster as well as the transition state between them in 3HIQ-W, denoted as EC-WS0, KC-WS0 and TSC-WS0 are shown in Figure 2d-f. In both Enol clusters (ECMS0 and EC-WS0), and Keto clusters (KC-MS0 and KC-WS0), two hydrogen bonds O...H-O and O-H...N are formed connecting the molecular solvents and 3HIQ. In addition the structures of 3HIQ in the 3HIQ-M and 3HIQ-W are very similar to the structure of 3HIQ monomer, indicating that 3HIQ is little disturbed by the molecular solvents. In TSC-MS0 and TSC-WS0, two hydrogen

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atoms (one from the O-H bond in 3HIQ and the other from methanol or water) are located on the midway of the two hydrogen bonds. The corresponding energies of the above structures in 3HIQ-W and 3HIQ-M are shown in Table. 1 calculated at MP2/cc-pVDZ level. The energies of KC-MS0 in 3HIQ-M, and KC-WS0 in 3HIQ-W are still higher than the corresponding Enol clusters, indicating that the energy stability order dose not change in these clusters, while the energy differences between EC-MS0 and KC-MS0 in 3HIQ-M as well as between EC-WS0 and KC-WS0 in 3HIQ-W are about 2 kcal/mol decreased from that in the gas phase. The decrease of the energy difference once again manifests that the solvent can help to stabilize the tautomer form in the S0 state. In addition, the energy barriers for the tautomerizations are largely decreased from 38.31 kcal/mol in the gas phase to 14.55 kcal/mol in 3HIQ-M and 16.06 kcal/mol in 3HIQ-W at MP2/cc-pVDZ level. The large decrease of the energy barriers in the clusters are mainly owing to the specific interaction of the solvents with 3HIQ. In the cluster models, the specific solvents (water and methanol) act both as proton donor and proton acceptor, which promote the proton transfer in 3HIQ. However, in the gas phase and in implicit solvent, the proton transfer is a simple and direct jumping process from O atom to N atom within 3HIQ only. The detail reason for the reactive role of the solvent is discussed later.

3.2. Protic and aprotic solvent effect on excited state proton transfer in 3HIQ For the excited state calculation, TDDFT method is a compromise of accuracy and computational cost. However the results of the TDDFT calculations are questionable especially when treating with the charge transfer state. Thus choosing a proper functional is very important for getting the meaning results. In this part, we use not only the most popular functional B3LYP, but also the long-range corrected functional CAM-B3LYP for geometry optimization and the excited state energy calculations. In the meanwhile, CASPT is also performed for single-point energy calculations based on the CASSCF geometry optimization. To test the energy and structure dependences on basis set, 6-31G(d), 6-311+G(d) and cc-pVDZ basis sets were used under the B3LYP functional for geometry optimization and energy calculations. The results show that the calculated energies with different basis sets are very similar, and consistent with each other, indicating the results are not so dependent on the basis set. Figure 3 shows the gas 7



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phase equilibrium structures of the Enol form, Keto form, as well as the transition state between them in the S1 state at the (TD-)B3LYP/6-31G(d) level, denoted as EMS1, KMS1, and TSMS1, respectively. The electronic character of the above structures is local excitation (LE), where electron is excited from HOMO to LUMO, see the frontier orbitals of EMS1 as an example in Figure 4. Compared to the ground state structures, the bond lengths of C11-C14 and C5-C6 and C1-C2 are increased due to the reduction of bonding character by excitation from HOMO, while the bond lengths of C1-C6, C3-C10 and C14-N16 are decreased due to the enhancement of bonding character by excitation into the LUMO in EMS1. The similar character can also be found in KMS1. The lengths of the hydrogen bonds N16...H18 and O17...H18 are about 2.20 Å and 2.40 Å in EMS1 and KMS1, respectively. In TSMS1, the H18 atom is located on the way for proton transfer from O atom toward N atom with bond distances of O...H and N...H 1.29 Å and 1.34 Å, respectively. The optimized structures of the Enol form, Keto form, and the transition state in dichloromethane and diethyl ether in the S1 state are very similar to that in the gas phase shown in Figure 1, indicating the implicit solvent effect is small in the S1 state. Table 2. shows the energies of EMS1 and KMS1 as well as TSMS1 calculated at various computational levels. The energy of EMS1 is set to zero as reference. In contrast to the S0 state, all the methods show that KMS1 is lower in energy than EMS1, at least 14.74 kcal/mol lower in energy at CASPT2 level. The calculated energies at (TD-)B3LYP/6-31G(d) and (TD-)CAMB3LYP/6-31G(d), and CASPT2 levels are very similar, with the latter being slightly larger. The energy barrier for the proton transfer from EMS1 to KMS1 is calculated to be 23.9 kcal/mol at (TD-)B3LYP/6-31G(d) level, this value becomes higher at (TD-)CAM-B3LYP (25.6 kcal/mol ) and CASPT2 (31.3 kcal/mol) levels. Such high energy barrier is unfavorable for proton transfer in 3HIQ monomer despite the fact that the tautomerization is an exothermic process. This result is consistent with the experimental observation of the emission spectra from the Enol monomer only after excitation when the concentration of 3HIQ is low, indicating that ESIPT in 3HIQ monomer dose not take place in the S1 state24. KMS1 is 19.76 kcal/mol (or 19.50 kcal/mol) lower in energy than EMS1 in dichloromethane (or diethyl ether) solvent, and the energy barrier for the tautomerization is 25.87 kcal/mol (or 25.52 kcal/mol) calculated with the relaxed geometries in explicit solvent in the S1 state. The result shows that both dichloromethane and diethyl ether solvents are somewhat favorable for the stabilization of the Keto form in the S1 state. Although the energy barrier for the 8


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tautomerization is influenced (slightly increased compared to that in the gas phase), the solvent effect is still too small to change the behavior of proton transfer from that in the gas phase in the S1 state. Finally, we consider the explicit solvent effect on the structures, energetics and tautomerizations in 3HIQ using cluster models 3HIQ-W and 3HIQ-M in the S1 state. The optimized structures of the Enol cluster, the Keto cluster as well as the transition state between them in 3HIQ-M, denoted as EC-MS1, KC-MS1 and TSC-MS1 are shown in Figure 5a-c calculated at (TD-)B3LYP/6-31G(d). The corresponding structures in 3HIQ-W, denoted as ECWS1, KC-WS1 and TSC-WS1 are shown in Figure 5d-f. The structures of 3HIQ in EC-MS1 and EC-WS1 as well as the ones in KC-MS1 and KC-WS1 are very similar to the structures of EMS1 and KMS1 in the gas phase, respectively, indicating that the photo excitation is localized on 3HIQ. The electronic character of 3HIQ-M and 3HIQ-W in the S1 state is local excitation where electron is excited from HOMO to LUMO, inducing the electron redistribution from O atom to N atom, see frontier orbitals in EC-MS1 and EC-WS1 shown in Figure 6-7. In EC-MS1 the lengths of the two hydrogen bonds are considerably decreased from 1.71 Å and 1.92 Å in the S0 state to 1.65 Å and 1.87 Å in the S1 state, which means the enhancement of the hydrogen bonds, expected to facilitate the proton transfer from EC-MS1 in the S1 state. While in KC-MS1, the lengths of the two hydrogen bonds are considerably increased from 1.77 Å and 1.80 Å in the S0 state to 1.91 Å and 1.88 Å in S1 state, which means the weakness of the hydrogen bonds. Similar behavior can also be found in EC-MS1 and KC-MS1. In both TSC-MS1 and TSC-WS1, the H atom from 3HIQ and the H atom from water or methanol are located on the midway for proton transfer. It is noted that no intermediate can be found on the pathways for the tautomerizations in the cluster models, indicating that the double proton transfer following the concerted mechanism in both 3HIQ-M and 3HIQ-W. The calculated energies at various levels are shown in Table 1. Keto clusters (KC) are very stable in both 3HIQ-M and 3HIQ-W, about 14.87 kcal/mol and 14.84 kcal/mol lower in energy than the respective Enol clusters (EC) at (TD-)B3LYP/6-31G(d) level. The values become 13.30 kcal/mol (and 12.30 kcal/mol) and 13.29 kcal/mol (and 12.44 kcal/mol) at (TD)CAM-B3LYP/6-31G(d) level (and CASPT2 level). We note that these values are larger than the ones in the implicit solvents, which again emphasis the important roles of the specific interaction of solvent and solute in the S1 state. The energy barriers for the tautomerization processes are

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estimated to be 5.76 kcal/mol and 6.14 kcal/mol in 3HIQ-M and 3HIQ-W at (TD-)B3LYP/631G(d) level, respectively. Including zero-point energy correction, the values becomes 1.89 kcal/mol and 2.41 kcal/mol, respectively. Such small energy barriers are very favorable for proton transfer occurring. Compared with the energy barriers calculated in 3HIQ-M and 3HIQW in the gas phase and in implicit solvent, both water and methanol molecules are able to facilitate the proton transfer reactions in both the S0 state and S1 state. We have to point out that the calculated energies at different levels ((TD-)B3LYP and (TD-)CAM-B3LYP) are very similar, and this means that the chosen calculation methods are consistent with each other. Including the long-range correction scheme, the results using CAM-B3LYP functional do not change very much with that using B3LYP functional, which manifests that the charge transfer plays minor roles in the tautomerization process in 3HIQ. In addition, the TDDFT methods can basically qualitatively reproduce the results at CASPT2 level in this system. To explore the reactive role of the solvent molecules (methanol and water), the ground state and the excited state steepest intrinsic reaction coordinates (IRCs) for the proton transfer processes in the monomer, clusters were constructed in Figure 8 and Figure 9. The energy of the Enol form is set to zero in each case. We take Figure 8 as an example. Including the protic solvents (methanol and water), both the transition states and the tautomer forms are stabilized, with the energies of the former decreased even more. In all the cases, IRC reaction path before the transition state can be divided into two parts (the flat increase part and the sharp increase part). The flat part before the reaction coordinate (-0.8) is a process for the approaching of the solvent molecule with 3HIQ in cluster model or the approaching of N atom and O atom in 3HIQ monomer for the proper position for the H transfer. Apparently, much higher energy is needed in the approaching process of N atom and O atom in the monomer due to the strain opposed to the neighbor ring. After getting prepared for the proton transfer, the potential energy increases very sharply leading to high energy barrier in each case. In cluster model, with the help of a solvent, the H1 atom and H2 atom transfer almost through the linear hydrogen bonds O-H-O and N-H-O, and the skeleton of the 3HIQ, especially the O-C-N angle, hardly changes during the proton transfer. While in the monomer, the proton transfer is not only related with the bond distance of O-H and N-H or O-N, but also with the angle of O-C-N. In the process of proton transfer, fourmember ring (H, N, C, O) is necessary, making a non linear proton transfer path. As a consequence, additional energy is required due to the nonlinearity of the hydrogen bond as well 10


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as the distorted N-C-O angle. This behavior is also observed by Ramos et al. in previous study 28. Other effect, such as electronic character of the transition state, does not play important roles in stabilizing the energy. Similar behavior can also be found in the reactive role of the solvent in the excited state. In the ground state or excited state, the calculated number of imaginary frequency in TSS0 or TSS1 is about 1788.98 i cm-1 or 1878.4 i cm-1 in the monomer in the gas phase, with vibrational mode of H atom transferring from O atom to N atom. The value is decreased to 1473.40 i cm-1 (or 1421.0 i cm-1) and 1383.16 i cm-1 or (1421.0 i cm-1) in TSC-WS0 (or TSC-WS1) and TSC-MS0 (or TSC-MS1), respectively. The slightly smaller value in 3HIQ-M compared to the one in 3HIQ-W indicates that the energy barrier in 3HIQ-M is smaller compared to that in 3HIQW. Other possible reason for the smaller energy barrier in 3HIQ-M than 3HIQ-W might because that methanol is more acid than water, and methyl group has more inductive effect, thus H transfer can be facilitated more effectively than water. To further explore the reason for the decreased energy barrier in the S1 state compared to the S0 state, we investigated the atomic dipole corrected hirshfeld population (ADCH) of 3HIQM and 3HIQ-W in both the S0 state and S1 state42. In 3HIQ-W, we found that the electron distribution on O atom belonging to 3HIQ obviously increases from -0.43 in the S0 state to -0.31 in the S1 state, while the electron distribution on the other atoms only slightly changes. The loss of electron on O atom could enhance the proton donating ability in 3HIQ. The same behavior occurs in 3HIQ-M. This result is very consistent with the electron redistribution from O atom to N atom. The increased proton donating ability in 3HIQ is expected to promote the proton transfer from 3HIQ to water or methanol.

4. Conclusions The present work has provided a comprehensive study on the structures, energetics and the tautomerizations in 3HIQ in both the ground state and the excited state by MP2, TDDFT, and CASPT2 methods, respectively. We confirmed that the normal form (Enol) is stable against the tautomer form (Keto) in the gas phase and the proton transfer process in the monomer is forbidden due to the high energy barrier of about 38.31 kcal/mol at the MP2/cc-pVDZ level. 11



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Including the implicit solvent or explicit solvent effect, the energy difference between the Enol and Keto decreases, but the energy order keeps. The proton transfer is facilitated in the explicit solvent due to the largely decreased energy barrier compared to the one in the gas phase and in the implicit solvent. In the excited state, Keto becomes more stable than Enol energetically in the gas phase as well as in the solvents, and the tautomerizations are endothermic processes. However the activation energies for the proton transfer processes in the gas phase as well as in the implicit solvent are still high, about 23.95 kcal/mol in the gas phase and 25.52 kcal/mol in diethyl ether, forbidding the proton transfer in them. The situation is changed in the cluster models considering the explicit solvent-solute interaction. The activation energies in 3HIQ-W and 3HIQ-M are just calculated to be 2.41 kcal/mol and 2.09 kcal/mol, respectively. Such small energy barriers, which probably originate from the enhanced hydrogen bonds as well as the electron redistribution on O atom in Enol clusters, are very favorable for proton transfer occurring in clusters. The reactive role of the molecular solvent is found to be related with the two linear hydrogen bonds in 3HIQ-W and 3HIQ-M.

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Acknowledgement The authors sincerely thank Prof. Z-G Lan and the referees for valuable comments on the manuscript. This work was supported by Grant No.ZR2015BQ013 from Shandong province in China.

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Figure Captions Scheme. Scheme of proton transfer from normal form to tautomer form in 3HIQ. Figure 1. Minimum-energy geometries of 3HIQ in the S0 state optimized at the MP2/cc-pVDZ level: (a) EMS0, (b) KMS0, and (c) TSMS0. Bond lengths are in angstroms. Figure 2. Minimum-energy geometries of 3HIQ-M and 3HIQ-W in the S0 state optimized at the MP2/cc-pVDZ leve: (a) EC-MS0, (b) KC-M S0, (c) TSC-MS0, (d) EC-WS0, (e) KC-WS0, and (f) TSC-WS0. Bond lengths are in angstroms. Figure 3. Minimum-energy geometries of 3HIQ in the S1 state optimized at the (TD-)B3LYP/631G(d) level: (a) EMS1, (b) KMS1, and (c) TSMS1. Bond lengths are in angstroms. Figure 4. Frontier orbitals of EMS1 at B3LYP/6-31(d) level. Figure 5. Minimum-energy geometries of 3HIQ-M and 3HIQ-W in the S1 state optimized at the TD-B3LYP/6-31G(d) level: (a) EC-MS1, (b) KC-M S1, (c) TS-MS1, (d) EC-WS1, (e) KC-WS1, and (f) TS-WS1. Bond lengths are in angstroms. Figure 6. Frontier orbitals of EC-MS1 at B3LYP/6-31(d) level. Figure 7. Frontier orbitals of EC-WS1 at B3LYP/6-31(d) level. Figure 8. The IRC paths from Enol to Keto in the S0 state calculated at the MP2/cc-pVDZ level. The black line, green line, and the red line denote the IRC paths in the 3HIQ monomer, in 3HIQW, and 3HIQ-M, respectively. Figure 9. The IRC paths from Enol to Keto in the S1 state calculated at the (TD-)B3LYP/631G(d) level. The black line, green line, and the red line denote the IRC paths in the 3HIQ monomer, in 3HIQ-W, and 3HIQ-M, respectively.

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Table 1. Relative energies (in kcal/mol) of stationary points in the ground state and excited state at various computational levels, the energy of EC-MS0/S1 and EC-WS0/S1 are set to be zeroa,b. MP2/cc-PVDZ

TD-B3LYP/6-

TD-CAM-

CASPT2/6-

31G(d)

B3LYP/6-31G(d)

31G(d)

EC-MS0/S1

0.00

0.00(0.00)

0.00

0.00

KC-MS0/S1

6.99

-14.87(-14.77)

-13.30

-12.30

TSC-MS0/S1

14.55

5.76(1.89)

6.31

9.74

EC-WS0/S1

0.00

0.00

0.00

0.00

KC-WS0/S1

6.84

-14.84(-14.79)

-13.29

-12.44

TSC-WS0/S1

16.06

6.14(2.41)

6.71

10.75

a

Numbers in parentheses denote the values corrected for zero-point energy.

b

Numbers in bracket denote the values calculated at B3LYP/6-311+G(d) level.

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Table 2. Relative energies (in kcal/mol) of stationary points in the first excited state at various computational levels, the energy of EMS1 is set to be zero.

B3LYP/6-

B3LYP/6-

CAM-B3LYP/

CASPT2

31G(d)

311+G(d)

6-31G(d)

/6-31G(d)

EMS1

0(LE)

0(LE)

0(LE)

0(LE)

KMS1

-18.61(LE)

-19.19(LE)

-16.6(LE)

-14.74(LE)

TSMS1

23.95(LE)

26.7(LE)

25.6(LE)

31.34(LE)

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Scheme. Protic vs a protic solvent effect on proton transfer in 3HIQ by Bo Xiao et al

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Figure 1. Protic vs a protic solvent effect on proton transfer in 3HIQ by Bo Xiao et al

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The structures, energetics and the tautomerizations in 3-hydroxyisoquinoline (3HIQ) in both the ground state and the excited state have been theoretically investigated by the MP2, TDDFT and CASPT2 methods, respectively. The solvent effect including the implicit solvent and explicit solvent on the structures, energetics and tautomeizations are revealed. We found that the explicit solvent plays more important roles in the structures, energetics and tautomerizations in 3HIQ than implicit solvent in both the ground state and excited state. The proton transfer is more facilitated in explicit solvent compared to that in the gas phase and in the implicit solvent in the excited state. In particularly, the reactive role of the molecular solvent is revealed, and found to be related with the two linear hydrogen bonds. On the contrary, the proton transfer in the monomer is very unfavorable, owing to the nonlinearity of the hydrogen bond as well as the distorted N-C-O angle. 78x64mm (150 x 150 DPI)


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Protic vs Aprotic Solvent Effect on Proton Transfer ... - ACS Publications

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