Hydrogen-Bonded Channel Dependent Mechanism of Long Range Proton Transfer in the Excited-State

Hydrogen-Bonded Channel Dependent Mechanism of Long Range Proton Transfer in the Excited-State Tautomerization of 7-Hydroquinoline: A Theoretical Study

Hua Fang a,* and Yongho Kim b,*

a Department of Chemistry and Material Science, College of Science, Nanjing Forestry University, Nanjing 210037, People’s Republic of China

b Department of Applied Chemistry and Institute of Natural Sciences, Kyung Hee University, Yongin si, Gyeonggi do, 17104, Korea

*Corresponding author e-mail: (Hua Fang)

(Yongho Kim)

Supplementary

Table SI Some geometric parameters of reactant, product, and TS for the first excited-state proton transfer in the 1:2 7-hydroxyquinoline: ROH (ROH: H2O, CH3OH) complex at the TD-M06-2X /6-311+g(d, p) level with SMD solvent model in solutiona. Bond distances are in Å

Table SII Reaction energies and barrier heights for the ground-state proton transfer in the 7HQ-(ROH)2,3 (ROH: H2O, CH3OH) complexes at the M06-2X/6-311+g(d, p) level with SMD solvent model in heptane. The numbers in parentheses are the barrier heights of proton transfer including zero-point energies

Table SIII Some geometric parameters of reactant, product, and TS for the first excited-state proton transfer in the 1:3 7-hydroxyquinoline: ROH (ROH: H2O, CH3OH) complex at the TD-M06-2X/SMD/6-311+g(d, p) level in heptane. Bond distances are in Å

Table SI Some geometric parameters of reactant, product, and TS for the first excited-state proton transfer in the 1:2 7-hydroxyquinoline: ROH (ROH: H2O, CH3OH) complex at the TD-M06-2X /6-311+g(d, p) level with SMD solvent model in solutiona. Bond distances are in Å

a 7HQ-(H2O)2 complex is in DPE (di-n-propyl ether). 7HQ-(CH3OH)2 complex is in heptane.

For the 7HQ-(H2O)2 complex in DPE as shown in Table SI, the H-bond distances were 1.714, 1.821, 1.865 Å, for 2H-3O, 4H-5O, 6H-7N in the reactant, respectively, and 1.857, 1.878, 2.013 Å, for 1O-2H, 3O-4H, 5O-6H in the product, respectively. When the H-bonded chain molecule was changed from water to methanol, the H-bond distances 2H-3O, 4H-5O in the reactant and 3O-4H, 5O-6H in the product in heptane averagely decreased 0.05 Å, while 6H-7N in the reactant and 1O-2H in the product increased 0.05 Å on average. The shorter the H-bond length, the higher the H-bond energy, hence the easier the proton transfer process. In the TS of 7HQ-(H2O)2 complex, the 1O-2H and 5O-6H distance in DPE were 1.075 Å and 1.377 Å, which were 0.085 Å and 0.390 Å longer than the corresponding distances in the reactant, respectively. The longer 5O-6H distance indicated that the 6H of the second water molecule connecting to the N atom of 7HQ moved first and more than halfway from 5O to 6N atoms. However, the 2H of the O-H group of 7HQ moved little, which resulted in an OH− like moiety at 5O. For the 7HQ-(CH3OH)2 complex, the 1O-2H and 5O-6H distance were 1.422 Å and 1.051 Å, respectively. The 2H moved first and fast from 1O to 3O, while 4H and 6H moved little, which generated a CH3OH2+-like moiety at 3O in the TS.

Table SII Reaction energies and barrier heights for the ground-state proton transfer in the 7HQ-(ROH)2,3 (ROH: H2O, CH3OH) complexes at the M06-2X/6-311+g(d, p) level with SMD solvent model in heptane. The numbers in parentheses are the barrier heights of proton transfer including zero-point energies

The reactant, transition state (TS) and product for the ground-state tautomerization in the 7HQ-(ROH)2,3 (ROH: H2O, CH3OH) were fully optimized at the M06-2X/6-311+G(d,p)/SMD level in Gaussian 09 program [S1-S3] in heptane. All optimized minima and transition states were confirmed to have no imaginary frequency for reactants and products, and only one imaginary frequency for the transition state by normal-mode analysis at the respective computation level. The barrier heights for the ground-state tautomerizations in the 7HQ-(ROH)2,3 (ROH: H2O, CH3OH) complexes were high, the ground-state tautomerization energies of 7HQ-(ROH)2,3 (ROH: H2O, CH3OH) were all positive, it meant that the reactant was more stable than the product, so that ground-state proton transfer in the 7HQ-(ROH)2,3 (ROH: H2O, CH3OH) complex would not take place. Our results are consistent with the experimental observations [S4-S7].

Table SIII Some geometric parameters of reactant, product, and TS for the first excited-state proton transfer in the 1:3 7-hydroxyquinoline: ROH (ROH: H2O, CH3OH) complex at the TD-M06-2X/SMD/6-311+g(d, p) level in heptane. Bond distances are in Å

The hydrogen bonded distances 2H-3O, 4H-5O, 6H-7O, 8H-9N in the reactant and 1O-2H, 3O-4H, 5O-6H, 7O-8H in the product of 7HQ-(H2O)3 obtained were 1.552 Å, 1.635 Å, 1.641 Å, 1.858 Å and 1.713 Å, 1.713 Å, 1.721 Å, 1.779 Å, respectively. In the 7HQ-(CH3OH)3 complex, the H-bonded distances 2H-3O, 4H-5O, 6H-7O, 8H-9N in the reactant and 1O-2H, 3O-4H, 5O-6H, 7O-8H in the product were 1.484 Å, 1.640 Å, 1.677 Å, 1.689 Å and 1.726 Å, 1.724 Å, 1.729 Å, 1.744 Å, respectively. The hydrogen bonded distances in the reactant and product of 7HQ-(CH3OH)3 were quite similar to those of 7HQ-(H2O)3. The intramolecular O-H and N-H covalent bond lengths in the reactant and product of 7HQ-(ROH)3 (ROH: H2O, CH3OH) were about 1.000 ± 0.030 Å, which were almost same to those of 7HQ-(ROH)2 (ROH: H2O, CH3OH). However, all the H-bond distances in the reactant and product of 1:3 7HQ: ROH (ROH: H2O, CH3OH) complexes were about 0.10-0.40 Å shorter than those of 1:2 7HQ: ROH (ROH: H2O, CH3OH) complexes. As a result, the distance between two end atoms of H-transfer in reactant, such as 1O-3O or 3O-5O, was reduced by about 0.15 Å.

Analyzing the structural parameters of TS in Table 7, the 1O-2H, 3O-4H, 5O-6H and 7O-8H distances of 7HQ-(H2O)3 were 1.102 Å, 1.078 Å, 1.147 Å, 1.343 Å, respectively, which were increased by 0.093 Å, 0.088 Å, 0.158 Å, 0.353 Å, respectively, compared with the corresponding distances in the reactant. It was obvious that, in TS, 7O-8H distance was 0.241 Å longer than 1O-2H, which means that 8H of water moved first to the N of 7HQ forming an OH−-like moiety at the 7O, and the other protons, 6H, 2H and 4H moved in turn.

For the 7HQ-(CH3OH)3 complex, the 1O-2H, 3O-4H, 5O-6H and 7O-8H distances in TS were 1.396, 1.193, 1.065, 1.059 Å, respectively,. The 1O-2H distance was 0.337 Å longer than 7O-8H, namely the 2H of OH in 7HQ moved first to methanol, a CH3OH2+-like moiety formed at 3O atom, and other protons transferred subsequently. A concerted and asynchronous protolytic pathway was preferred for the ESQPT of 7HQ-(CH3OH)3.

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