Double Proton Transfer Mechanism Research Articles

Theoretical Insights Into the Excited State Double Proton Transfer Mechanism of Deep Red Pigment Alkannin.

As the most important component of deep red pigments, alkannin is investigated theoretically in detail based on time-dependent density functional theory (TDDFT) method. Exploring the dual intramolecular hydrogen bonds (O1-H2···O3 and O4-H5···O6) of alkannin, we confirm the O1-H2···O3 may play a more important role in the first excited state than the O4-H5···O6 one. Infrared (IR) vibrational analyses and subsequent charge redistribution also support this viewpoint. Via constructing the S1-state potential energy surface (PES) and searching transition state (TS) structures, we illuminate the excited state double proton transfer (ESDPT) mechanism of alkannin is the stepwise process that can be first launched by the O1-H2···O3 hydrogen bond wire in gas state, acetonitrile (CH3CN) and cyclohexane (CYH) solvents. We present a novel mechanism that polar aprotic solvents can contribute to the first-step proton transfer (PT) process in the S1 state, and nonpolar solvents play important roles in lowering the potential energy barrier of the second-step PT reaction.

The role played by ethanol in achieving the successive versus simultaneous mechanism of excited-state double proton transfer in dipyrido[2,3-a:3′,2′-i]carbazole

The excited-state double proton transfer (ESDPT) process of dipyrido[2,3-a:3',2'-i]carbazole (DPC) in ethanol (EtOH) solvent is investigated using density functional theory (DFT) and time-dependent density functional theory (TDDFT). The computational results provide convincing evidence that proton transfer did not occur spontaneously for the DPC monomer due to the lack of hydrogen bonds. Interestingly, after adding EtOH to DPC, two intermolecular hydrogen bonds were formed in the ground-state, and the intermolecular hydrogen bonds were strengthened in the excited-state, as confirmed by comparing the changes in the primary bond parameters. In addition, the charge transfer was observed in the DPC-EtOH complex compared with the DPC monomer. In particular, a reliable ESDPT process occurs within the system upon photoexcitation, which was monitored by the formation and disappearance of characteristic peaks in the IR spectrum. All results adequately proved that the participation of EtOH exerts a remarkable effect on the ESDPT process. Overall, our work not only comprehensively elaborated the simultaneous mechanism of ESDPT but can also pave the way towards the design and synthesis of novel molecules.

Solvent effect on the intermolecular proton transfer of the Watson and Crick guanine-cytosine and adenine-thymine base pairs: a polarizable continuum model study.

Herein we present our results on the study of the double proton transfer (DPT) mechanism in the adenine-thymine (AT) and guanine-cytosine (GC) base pairs, both in gas phase and in solution. The latter was modeled using the polarizable continuum method (PCM) in different solvents. According to our DFT calculations, the DPT may occur for both complexes in a stepwise mechanism in condensate phase. In gas phase only the GC base pair exhibits a concerted DPT mechanism. Using the Wigner's tunneling corrections to the transition state theory we demonstrate that such corrections are important for the prediction of the rate constants of both systems in gas and in condensate phase. We also show that (i) as the polarity of the medium decreases the equilibrium constant of the DPT reaction increases in both complexes, and (ii) that the equilibrium constant in the GC complex is four orders of magnitude larger than in AT. This observation suggests that the spontaneous mutations in DNA base pairs are more probable in GC than in AT.

Effects of C5-substituent group on the hydrogen peroxide-mediated tautomerisation of protonated cytosine: a theoretical perspective

ABSTRACTThe direct tautomerism (path A) and H2O2 as a catalyst (path B) have been studied in conversion of Cyt2t+ into CytN3+ isomer. The protonated 5-carboxycytosine (5-caCyt) is represented and has been further explored in the presence of H2O2 (path C). In going from a four-membered-ring transition state in the case of the direct tautomerism to the six-membered ring for H2O2, the H2O2 significantly contributes to decreasing the free energy barrier of tautomerisation. Although the carboxylic substituent of 5-carboxycytosine has certain affected on the electron distribution of the pyrimidine ring, the six-membered-ring transition state has not changed. This result illustrates that the C5-carboxylation has no significant effect on the H2O2-mediated isomerisation of Cyt2t+ to CytN3+ isomer. Meanwhile, these paths A–C have been further explored in the presence of two water molecules. Use of implicit solvent models (PCM) does not significantly alter the energetics of water-mediated paths A–C compared to those in gas phase. Furthermore, the rate constant with Wigner tunnelling correction of path A is obviously smaller than those of paths B and C. Finally, the lifetime τ99.9% of paths B and C is 10−5 s, which is implemented by the mechanism of the concerted synchronous double proton transfer.

Insights into the Mechanism of Ground and Excited State Double Proton Transfer Reaction in Formic Acid Dimer.

The mechanism of ground and excited state double proton transfer reaction in formic acid dimer has been analyzed with the help of reaction force and the reaction electronic flux. The separation of reaction electronic flux in terms of electronic activity and reactivity, NBO, and dual descriptor lends additional support for the mechanism. Interestingly we found that the ground state double proton transfer mechanism is concerted synchronic, whereas the excited state double proton transfer is concerted asynchronic in nature.

A TD-DFT investigation of photoinduced excited-state intermolecular multi-proton transfer dynamics of a 3-hydroxy-2-(thiophen-2-yl) chromen-4-one in methanol

Density functional theory (DFT) and time-dependent density functional theory (TDDFT) methods have been performed to investigate the ground and exited states intermolecular multiple protons transfer process of 3-hydroxy-2-(thiophen-2-yl)chromen-4-one (3-HTC) in methanol solvent. We demonstrated that the intermolecular hydrogen bonds formed between 3-HTC and CH3OH are strengthened at the first single excited state by comparing the bond parameters and infrared vibrational spectra with those in S0 state, which provides the possibility for exited state intermolecular multiple protons transfer reaction. Based on the potential energy surface/curve at S0 state and S1 state, a CH3OH-assist exited state simultaneous double proton transfer mechanism has been proposed, whose reaction barrier is slightly lower than the case of intramolecular single proton transfer reaction reported previously (RSC advances, 2016, 6, 96147) and the case of two CH3OH assisted exited state multi-proton transfer reaction; in comparison, the high potential barrier heights among the local minima on the S0 surface imply that the ground state double proton transfer reaction is significantly inhibited. Moreover, the absorption and fluorescence spectra of 3-HTC in CH3OH solvent were simulated for the first time, which haven't been reported in the previous experimental and theoretical studies. Our calculations predicted the dual fluorescence behavior, which corresponds to Stokes shifted emissions of 3-HTC-CH3OH complex and its double protons transfer phototautomer, respectively.

Controlling Excited State Single versus Double Proton Transfer for 2,2'-Bipyridyl-3,3'-diol: Solvent Effect.

In this work, we theoretically investigate the sequential excited state double proton transfer (ESDPT) mechanism of a representative intramolecular hydroxyl (OH)-type hydrogen molecule 2,2'-bipyridyl-3,3'-diol (BP(OH)2). We mainly adopt three kinds of different polar solvents (nonpolar cyclohexane (CYH), polar acetonitrile (ACN), and moderate chloroform (CHCl3)) to explore solvent effects on this system. Two intramolecular hydrogen bonds of BP(OH)2 are testified to be strengthened in the S1 state, which provides possibility for ESDPT process. Explorations of charge redistribution and potential energy surfaces (PESs) reveal ESDPT process. Searching transition state (TS) structures in different polar aprotic solvents, we successfully regulate and control the stepwise ESDPT behaviors of BP(OH)2 through solvent polarity.

Insight into the excited-state intramolecular double-proton transfer of the 2,5-bis(benzoxazol-2-yl)thiophene-3,4-diol: one-step or stepwise mechanism?

The excited-state double-proton transfer mechanism of 2,5-bis(benzoxazol-2-yl)thiophene-3,4-diol has been theoretically investigated based on the methods of density functional theory and time-dependent density functional theory. Geometric structure comparison and infrared vibrational spectra analysis confirm that the strengthening of the intramolecular hydrogen bond in the first excited state (S1) has facilitated the proton transfer process. The frontier molecular orbitals analysis illustrates that the nature of the hydrogen bond enhancement lies in the charge redistribution upon photo-excitation, which is further confirmed by NBO charge analysis quantitatively. The reduced dimensionality (two-dimensional) potential energy surfaces have been scanned for both the ground state (S0) and the first electronic excited state to reveal the mechanism and pathway of the two protons being transferred. Compared with the concerted path of proton transfer, the stepwise path with one proton being transferred first and then a second one followed exhibits a relatively mild energy barrier and seems to be more rational and favorable.

Elaboration and controlling excited state double proton transfer mechanism of 2,5-bis(benzoxazol-2-yl)thiophene-3,4-diol

In the present work, we theoretically illuminate the excited state double proton transfer (ESDPT) process about a novel synthesized system 2,5-bis(benzoxazol-2-yl)thiophene-3,4-diol (BBTD). Minor changes of both structure and charge redistribution deriving from photoexcitation result in obviously different excited state dynamical process. Exploration about our constructed S1-state potential energy surface (PES) indicates a stepwise ESDPT mechanism for BBTD. In addition, we present a new mechanism about regulating and controlling stepwise ESDPT process via external electric field.

Theoretical investigation of excited-state single and double proton transfer mechanisms for 2,5-bis(benzoxazol-2-yl)thiophene-3,4-diol

In this work, excited-state intramolecular single and double proton transfer mechanisms for 2,5-bis(benzoxazol-2-yl)thiophene-3,4-diol were investigated using time-dependent density functional theory (TDDFT) method for the first time. Our calculations showed that the intramolecular hydrogen bond N⋯HO is strengthened in the excited state, which provides a driving force to effectively facilitate the proton transfer process. The constructed potential energy surface of the excited state demonstrated that the double proton transfer reaction occurs more readily in both dynamics and thermodynamics aspects, and it is implemented by simultaneous double proton transfer, or successive single transfers. Furthermore, the assignments of multiple fluorescence bands in experiment were confirmed by fluorescence spectral simulation. The calculated emission spectra indicated that the experimental fluorescence maxima at 475 nm should be attributed to normal Stokes shifted emission; the large Stokes shifted fluorescence with peaks at 550 nm originates from the double proton transfer phototautomer emission, and the experimentally observed shoulder peak at ∼493 nm results from the excited state single proton transfer tautomer.

Insight into the excited-state double proton transfer mechanisms of doxorubicin in acetonitrile solvent

Doxorubicin (DXR) is theoretically investigated with an aim to explore the excited-state intramolecular double proton transfer (ESIDPT) mechanism regarding stepwise versus synchronous double proton transfer.

Solvent effects on the excited-state double proton transfer mechanism in the 7-azaindole dimer: a TDDFT study with the polarizable continuum model.

Solvent effects on the excited-state double proton transfer (ESDPT) mechanism in the 7-azaindole (7AI) dimer were investigated using the time-dependent density functional theory (TDDFT) method. Excited-state potential energy profiles along the reaction paths in a locally excited (LE) state and a charge transfer (CT) state were calculated using the polarizable continuum model (PCM) to include the solvent effect. A series of non-polar and polar solvents with different dielectric constants were used to examine the polarity effect on the ESDPT mechanism. The present results suggest that in a non-polar solvent and a polar solvent with a small dielectric constant, ESDPT follows a concerted mechanism, similar to the case in the gas phase. In a polar solvent with a relatively large dielectric constant, however, ESDPT is likely to follow a stepwise mechanism via a stable zwitterionic intermediate in the LE state on the adiabatic potential energy surface, although inclusion of zero-point vibrational energy (ZPE) corrections again suggests the concerted mechanism. In the meantime, the stepwise reaction path involving the CT state with neutral intermediates is also examined, and is found to be less competitive than the concerted or stepwise path in the LE state in both non-polar and polar solvents. The present study provides a new insight into the experimental controversy of the ESDPT mechanism of the 7AI dimer in a solution.

A theoretical study of the potential energy surfaces for the double proton transfer reaction of model DNA base pairs.

The excited-state double proton transfer (ESDPT) mechanism in a model DNA base pair, 7-azaindole (7AI) dimer, has been debated over the years. Recently, Otero and coworkers concluded that the stepwise mechanism is not possible and the concerted mechanism dominates the dynamics (Chem. Sci., 2015, 6, 5762). In this work, the potential energy surfaces of the 7AI dimer in the ground state (S0) and the lowest energy excited singlet state (S1) were constructed. After vertical excitation to the S1 state, the single proton transfer can occur. The second proton transfer process in the stepwise mechanism is blocked by a high potential barrier (36.4 kcal mol-1), which is consistent with the result proposed by Otero and coworkers. However, the single proton transfer process is compatible with the concerted mechanism and we show that the single proton transfer process rather than the concerted mechanism dominates the dynamics. The concerted process is unfavorable in the S1 state compared with the barrierless single proton transfer process. In addition, the proton transfer process in the S0 state is revealed. The single proton transfer tautomer in the S1 state returns to the S0 state and transfers the second proton via a barrierless process. Finally, the double proton transfer tautomer in the S0 state can recover to the normal dimer through the reverse proton transfer reaction.

TD-DFT Study of the Double Excited-State Intramolecular Proton Transfer Mechanism of 1,3-Bis(2-pyridylimino)-4,7-dihydroxyisoindole.

The 1,3-bis(2-pyridylimino)-4,7-dihydroxyisoindole (BPD) is chosen to investigate the excited-state double proton transfer process (ESDPT). The IR spectra, bond distance, and angle analyses show that the two intramolecular hydrogen bonds in the BPD molecule, formed between hydroxyl group and pyridine-type nitrogen atom, are significantly strengthened in the S1 state. The potential energy surfaces in both S0 and S1 states are scanned with varying O-H bond lengths to visually investigate the double proton transfer mechanism. Compared with previous investigations, the proton transfer process can be interpreted in more detail. The hydrogen bond strengthening promotes the proton transfer in the S1 state effectively. The large Stocks shift observed in the experiment can be explained more comprehensively according to the ESDPT mechanism.

Double-proton transfer mechanism in 1,8-dihydroxydibenzo[a,c]phenazine: a TDDFT and ab initio study

The double-proton transfer mechanism in the newly synthesized molecule 1,8-dihydroxydibenzo[a,c]phenazine containing two hydrogen bonds is investigated by means of the time-dependent density functional theory and resolution-of-identity second-order approximate coupled-cluster methods. Both reaction paths of the concerted and the stepwise mechanisms are examined, and the results show that the double-proton transfer is likely to occur in the excited state following the stepwise mechanism in the gas phase at the lowest excitation energy. Based on the calculated potential energy scheme, the single-proton transfer and the double-proton transfer are likely to coexist, with the former is expected to occupy in a significant number. The concerted reaction path of C 2 symmetry is found to be less preferred compared with the stepwise reaction path, and also could be dynamically open due to the small activation energy.

New Excited-State Proton Transfer Mechanisms for 1,8-Dihydroxydibenzo[a,h]phenazine

The excited state intramolecular proton transfer (ESIPT) mechanisms of 1,8-dihydroxydibenzo[a,h]phenazine (DHBP) in toluene solvent have been investigated based on time-dependent density functional theory (TD-DFT). The results suggest that both a single and double proton transfer mechanisms are relevant, in constrast to the prediction of a single one proposed previously (Piechowska et al. J. Phys. Chem. A 2014, 118, 144-151). The calculated results show that the intramolecular hydrogen bonds were formed in the S0 state, and upon excitation, the intramolecular hydrogen bonds between -OH group and pyridine-type nitrogen atom would be strengthened in the S1 state, which can facilitate the proton transfer process effectively. The calculated vertical excitation energies in the S0 and S1 states reproduce the experimental UV-vis absorption and fluorescence spectra well. The constructed potential energy surfaces of the S0 and S1 states have been used to explain the proton transfer process. Four minima have been found on the S1 state surface, with potential barriers between these excited-state minima of less than 10 kcal/mol, which supports concomitant single and double proton transfer mechanisms. In addition, the fluorescence quenching can be explained reasonably based on the proton transfer process.

Promoting the "water-wire" mechanism of double proton transfer in [2,2'-bipyridyl]-3,3'-diol by porous gold nanoparticles.

The effect of nanopores in porous gold nanoparticles (Au NPs) on excited-state double proton transfer (DPT) in [2,2'-bipyridyl]-3,3'-diol (BP(OH)2) in an aqueous environment is the main focus of the present work. DPT in BP(OH)2 is known to take place through two mechanisms. In a bulk environment, an open solvated molecule facilitates the process and emits at 460 nm whereas, in a confined situation, formation of a "water wire" between the prototropic centers leads to the transfer of protons. It has been shown spectroscopically in the present study that in the nanovessels provided by nanoporous Au NPs, the unconventional mechanism of DPT through the formation of a "water wire" is promoted due to the presence of a limited number of water molecules around the probe. Experiments in the presence of solid pure Au, Ag and Au/Ag NPs were performed to support our proposition. Time-resolved fluorescence spectral changes confirm our findings.

Competitive excited-state single or double proton transfer mechanisms for bis-2,5-(2-benzoxazolyl)-hydroquinone and its derivatives.

The excited state intramolecular proton transfer (ESIPT) mechanisms of 2-(2-hydroxyphenyl)benzoxazole (HBO), bis-2,5-(2-benzoxazolyl)-hydroquinone (BBHQ) and 2,5-bis(5'-tert-butyl-benzoxazol-2'-yl)hydroquinone (DHBO) have been investigated using time-dependent density functional theory (TDDFT). The calculated vertical excitation energies based on the TDDFT method reproduced the experimental absorption and emission spectra well. Three kinds of stable structures were found on the S1 state potential energy surface (PES). A new ESIPT mechanism that differs from the one proposed previously (Mordzinski et al., Chem. Phys. Lett., 1983, 101, 291. and Lim et al., J. Am. Chem. Soc., 2006, 128, 14542.) is proposed. The new mechanism includes the possibility of simultaneous double proton transfer, or successive single transfers, in addition to the accepted single proton transfer mechanism. Hydrogen bond strengthening in the excited state was based on primary bond lengths, angles, IR vibrational spectra and hydrogen bond energy. Intramolecular charge transfer based on the frontier molecular orbitals (MOs) also supports the proposed mechanism of the ESIPT reaction. To further elucidate the proposed mechanism, reduced dimensionality PESs of the S0 and S1 states were constructed by keeping the O-H distance fixed at a series of values. The potential barrier heights among the local minima on the S1 surface imply competitive single and double proton transfer branches in the mechanism. Based on the new ESIPT mechanism, the observed fluorescence quenching can be satisfactorily explained.

A questionable excited-state double-proton transfer mechanism for 3-hydroxyisoquinoline.

Two excited state proton transfer mechanisms of 3-hydroxyisoquinoline (3HIQ) in cyclohexane and acetic acid (ACID) were investigated based on the time-dependent density functional theory (TDDFT), suggesting a different double-proton transfer mechanism from the one proposed previously (J. Phys. Chem. B, 1998, 102, 1053). Instead of the formation of keto-enol complexes for 3HIQ self-association in cyclohexane, our theoretical results predicted that 3HIQ self-association exists in two forms: the normal form (enol/enol) and the tautomer form (keto/keto) in cyclohexane. A high barrier (37.023 kcal mol(-1)) between the 3HIQ enol monomer and 3HIQ keto monomer form indicated that the 3HIQ keto monomer in the ground state should not exist. In addition, the constructed potential energy surfaces of the ground state and excited state have been used to explain the proton transfer process. Upon optical excitation, the enol/enol form is excited to the first excited state, then transfers one proton, in turn, transition to the ground state to transfer another proton. A relatively low barrier (8.98 kcal mol(-1)) demonstrates two stable structures in the ground state. In view of the acetic acid solvent effect, two protons of 3HIQ/ACID transfer along the dihydrogen bonds in the first excited state, which is a different transfer mechanism to 3HIQ self-association. In addition, the proton transfer process provides a possible explanation for the fluorescence quenching observed.

Using the reaction force and the reaction electronic flux on the proton transfer of formamide derived systems

In this work, we present a theoretical study of the mechanism of double proton transfer in formamide, formamide-thioformamide and thioformamide dimers. The reaction mechanisms were analyzed in terms of the energy profile and novel concepts such as the reaction force profile and reaction electronic flux, along with local electronic properties such as NBO analysis. All systems were characterized computationally using DFT/B3LYP 6-311G** on Gaussian09. These results show that the electronic processes take place mostly in the transition state for all the systems; in the formamide and thioformamide dimers, electron transfer has a synchronic nature, while the electron transfer is asynchronic in the formamide-thioformamide dimer.