DFT based computational study on the excited state intramolecular proton transfer processes in o-hydroxybenzaldehyde

Potential energy (PE) curves for the intramolecular proton transfer in the ground (GSIPT) and excited (ESIPT) states of o-hydroxybenzaldehyde (OHBA) were studied using DFT-B3LYP/6-31G(d) and TD-DFT-B3LYP/6-31G(d) level of theory, respectively. Our calculations suggest the non-viability of ground state intramolecular proton transfer in this compound. Excited states PE calculations support the ESIPT process in OHBA. The contour PE diagram and the variation of oscillator strength along the proton transfer co-ordinate support the dual emission in OHBA. Our calculations also support the experimental observations of Nagaoka et al. [S. Nagaoka, U. Nagashima, N. Ohta, M. Fujita, T. Takemura, J. Phys. Chem. 92 (1988) 166], i.e. normal emission of the title compound comes from S(2) state and the red-shifted proton transfer band appears from the S(1) state. ESIPT process has also been explained in terms of HOMO and LUMO electron density of the enol and keto tautomer of OHBA and from the potential energy surfaces.     

DFT and MP2 study of isomery scheme in Formazan and intermolecular and intramolecular proton transfer between its tautomers

DFT and Moller Plesset (MP2) calculations were applied to study of isomery scheme in formazan. Formazan that can be presented by three tautomers and eight isomers has various applications in dyes, complexes, and biochemistry. The structures of its isomers and related transition states were optimized, and important molecular parameters, IR frequencies, and energetic results were extracted. The relative stabilities of formazan isomers in the gas phase were found to be as 1EZ >1ZZ >1EE >1ZE >2EE > 3 >2EZ >2ZZ >TS1 >TS3 >TS2. Thermodynamic data confirms that tautomer 1 is major tautomer, and all possible tautomerism interconversions have small rates at room temperature. Then, relative stabilities were calculated in different solvents (chloroform, tetrahydrofuran, acetone, and water). The relative stabilities and thermodynamic data in solvents are nearly similar to those in the gas phase, but the rate constants are slightly more than that in the gas phase. Moreover, relative stabilities of formazan isomers and intermolecular proton transfer in presence of one to three molecules of water have been studied. The results showed that activation barriers in water‐assisted tautomerisms are in general lower than those in the gas phase, but the relative energies of isomers do not change importantly by water clusters. © 2011 Wiley Periodicals, Inc. Int J Quantum Chem, 2012     

Theoretical investigation (DFT and MP2) of the intermolecular proton transfer in the supersystems uracil-(H2O) n and uracil-(CH3OH) n (n = 1, 2)

Twelve binary and eight ternary H-bonded systems between uracil and water/methanol were investigated at the B3LYP and MP2 theoretical levels using 6 − 31 + G(d) basis functions. The binary and ternary systems that contain the hydroxo-uracil tautomer H-bonded with water and methanol were found to be the most stable complexes. The calculated energy barriers of the intermolecular proton exchange showed that the methanol molecule provokes larger reduction of the energy barrier of the intermolecular proton exchange reactions than the water molecule. Correspondence: Vassil B. Delchev, Department of Physical Chemistry, University of Plovdiv, Plovdiv, Bulgaria     

Assisted intramolecular proton transfer in (uracil)2Ca2+ complexes

The structure and relative stability of the complexes between uracil dimers and Ca²⁺, as well as the proton transfer (PT) processes within these dimers, have been investigated by the density functional theory methods. Although in uracil dimers PT occurs as an almost synchronous double PT processes that connect the diketo dimer with a keto-enol dimer, the process within the most stable (uracil)₂Ca²⁺ complexes is much more complicated, and the product of the reaction looks like the result of an intramolecular PT from one of the NH groups of one monomer to one of the carbonyl groups of the same monomer. An analysis of the force profile along the reaction coordinate shows that the intimate mechanism implies three elementary steps, two intermolecular PTs, and an in-plane displacement of one monomer with respect to the other. The result of this so-called assisted intramolecular proton transfer is the formation of a dimer in which only one monomer is a keto-enol derivative, the other monomer being apparently unchanged, although it suffers significant structural rearrangements along the reaction coordinate. Quite importantly, this dimer is significantly stabilized upon Ca²⁺ association; therefore, while the most stable uracil dimers correspond systematically to associations involving only the diketo forms, in (uracil)₂Ca²⁺ complexes the most stable structures correspond to those in which one of the monomers is a keto-enol uracil isomer.     

Probing excited-state dynamics and intramolecular proton transfer in 1-acylaminoanthraquinones via the intermolecular solvent response

The dynamics of a series of 1-acylaminoanthraquinones with varying degrees of excited-state intramolecular proton transfer are studied in acetonitrile and dichloromethane. Events are followed via changes in the third-order intermolecular Raman response as a function of time after resonant excitation of the chromophore. Compared to electronically resonant probes of the solute, measuring the ultrafast dynamics using the nonresonant solvent response offers a new and complementary perspective on the events that accompany excitation and proton transfer. Experimentally observed changes in the nuclear polarizability of the solvent follow dynamic changes in the solvent-solute interactions. Reorganization of the solvent in response to the significant changes in the intermolecular interactions upon proton transfer is found to play an important role in the reaction dynamics. With transfer of the proton taking place rapidly, the solvent controls the dynamics via the time-dependent evolution of the free energy surface, even on subpicosecond time scales. In addition, the solvent response probes the effects of intermolecular energy transfer as energy released during the reactive event is rapidly transferred to the local solvent environment and then dissipates to the bulk solvent on about a 10 ps time scale. A brief initial account of a portion of this work has appeared previously, J. Am. Chem. Soc. 2004, 126, 8620-8621.     

Role of the intermolecular and intramolecular hydrogen bonding on the excited-state proton transfer behavior of 3-aminophthalimide (3AP) dimer

In this work, both the intermolecular and intramolecular hydrogen bonding of 3-aminophthalimide (3AP) dimer complex in the electronically excited state have been investigated theoretically using the time-dependent density functional theory (TDDFT) method. The calculated infrared spectrum of the hydrogen-bonded 3AP dimer complex for the S₁ state shows that the CO and H–N bonds involved in the intramolecular hydrogen bond C₃O₅?H₈–N₆ and intermolecular hydrogen bond C₁O₄?H_7′–N_2′ which are markedly red-shifted compared with those predicted for the ground state. The calculated length of the two hydrogen bonds C₃O₅?H₈–N₆ and C₁O₄?H_7′–N_2′ are significantly shorter in S₁ state than in the ground state. However, the bond lengths of the intramolecular hydrogen bond C_3′O₅?H_8′–N_6′ and intermolecular hydrogen bond C_1′O_4′?H₇–N₂ nearly unchanged upon electronic excitation to the S₁ state. Thus, the intramolecular hydrogen bond C₃O₅?H₈–N₆ and intermolecular hydrogen bond C₁O₄?H_7′–N_2′ of the hydrogen-bonded 3AP dimer complex are stronger in the electronically excited state than in the ground state. Moreover, it has been demonstrated that the excited-state proton transfer reaction is facilitated by the electronic excited-state hydrogen bond strengthening.     

Barrier for excited-state intramolecular proton transfer and proton tunneling in bis-2,5-(2-benzoxazolyl)-hydroquinone

The excitation spectra of dual fluorescence for isolated bis-2,5-(2-benzoxazolyl)-hydroquinone at low temperatures in a supersonic jet is reported. The vibronic structure near the electronic origin for the 410 nm band is attributed to proton transfer. Proton transfer was observed for the vibrationally cold excited state. From the relative fluorescence quantum yields in organic glasses below 100 K, a barrier for the excited-state proton transfer or 121 ± 17 cm^?1 is obtained. It is concluded that proton tunneling occurs. The relative yield of the usual Stokes fluorescence in an organic glass, as a function of temperature. is compared with the relative yield in the supersonic jet as a function of excitation energy. This leads to estimates of the temperature of the isolated molecule in the excited state.     

Thermochemical aspects of proton transfer in the gas phase

The beginning of the twentieth century saw the development of new theories of acidity and basicity, which are currently well accepted. The thermochemistry of proton transfer in the absence of solvent attracted much interest during this period, because of the fundamental importance of the process. Nevertheless, before the 1950s, few data were available, either from lattice energy evaluations or from calculations using the emerging molecular orbital theory. Advances in mass spectrometry during the last 40 years allowed studies of numerous systems with better accuracy. Thousands of accurate gas-phase acidities or basicities are now available, for simple atomic and molecular systems and for large biomolecules. The intrinsic effect of structure on the Br?nsted basic or acidic properties of molecules and the influence of solvents have been unravelled. In this tutorial, the basics of the thermodynamic principles involved are given, and the mass spectrometric techniques are briefly reviewed. Advances in the design and measurements of gas-phase superacids and superbases are described. Recent studies concerning biomolecules are also evoked.     

Excited-state intramolecular proton transfer (ESIPT) in 2-(2′-hydroxyphenyl)-oxazole and -thiazole

The azoles 2-(2′-hydroxyphenyl)oxazole (HPO) and 2-(2′-hydroxyphenyl)-4-methylthiazole (HPT) have been synthesised and studied in order to compare their photophysical characteristics. Their absorption and emission properties are reported in non-polar, alcoholic and aqueous media. Ground and excited state pK data were determined by spectroscopy and a model is proposed to explain the behaviour of HPT and HPO as a function of the pH. Excitation spectra and quantum chemical calculations suggest an equilibrium of ground state conformers. The calculations also predict a small energy barrier for rotation in the first excited singlet state for the proton transferred tautomers. The resulting twisted structure of the tautomer form possesses a biradicaloid nature, and is near-degenerate in energy with the first excited triplet state.     

Excited-state intermolecular proton transfer reactions of 7-azaindole(MeOH)(n) (n = 1-3) clusters in the gas phase: on-the-fly dynamics simulation

Ultrafast excited-state intermolecular proton transfer (PT) reactions in 7-azaindole(methanol)(n) (n = 1-3) [7AI(MeOH)(n=1-3)] complexes were performed using dynamics simulations. These complexes were first optimized at the RI-ADC(2)/SVP-SV(P) level in the gas phase. The ground-state structures with the lowest energy were also investigated and presented. On-the-fly dynamics simulations for the first-excited state were employed to investigate reaction mechanisms and time evolution of PT processes. The PT characteristics of the reactions were confirmed by the nonexistence of crossings between S(ππ*) and S(πσ*) states. Excited-state dynamics results for all complexes exhibit excited-state multiple-proton transfer (ESmultiPT) reactions via methanol molecules along an intermolecular hydrogen-bonded network. In particular, the two methanol molecules of a 7AI(MeOH)(2) cluster assist the excited-state triple-proton transfer (ESTPT) reaction effectively with highest probability of PT.     

Intra- and intermolecular proton transfer reactions in 2-phenyl substituted benzazoles

The present review describes the salient features of inter- and intramolecular proton transfer reactions of 2-(2′-aminophenyl)-, 2-(3′-aminophenyl)-, 2-(4′-aminophenyl)-, 2-(2′-hydroxyphenyl)-, 2-(3′-hydroxyphenyl)- and 2-(4′-hydroxyphenyl)-benzimidazoles, benzoxazoles and benzothiazoles. Fluorescence quantum yield of the phototautomer produced by the intramolecular hydrogen bonding decreases on going from benzimidazole to benzoxazole to benzothiazole. This indicates that the rate of internal conversion increases in the order of compounds as mentioned above. The biprotonic phototautomerism or the presence of intermolecular proton transfer has led to the formation of (i) nonfluorescent zwitterions in case of hydroxyphenyl derivatives and the ground state precursor of this species in neutral molecules, (ii) nonfluorescent monoanions from fluorescent monoanions and (iii) nonfluorescent monocations from monocations in case of aminophenyl derivatives. In the case of 2-(4′-aminophenyl)-substituted compounds, the first protonation has always led to the formation of two types of monocations; one by protonating the amino group and the other by protonating the tertiary nitrogen atom. The former is more stable in aqueous media and the latter in non-polar media.     

Intra- and intermolecular proton transfer in 1H(2H)-1,2,3-triazole based systems

Hartree-Fock ab initio and density functional theory calculations suggest that intra- and intermolecular proton transfer between neutral and protonated triazole tautomers play a vital role in the observed dramatic proton conduction in triazole-based polymer electrolyte membranes.     

Rotamerism, tautomerism, and excited-state intramolecular proton transfer in 2-(4'-N,N-diethylamino-2'-hydroxyphenyl)benzimidazoles: novel benzimidazoles undergoing excited-state intramolecular coupled proton and charge transfer

The solvent and temperature dependence of the phototautomerization of 1-methyl-2-(2'-hydroxyphenyl)benzimidazole (4) and the novel compounds 2-(4'-amino-2'-hydroxyphenyl)benzimidazole (1), 2-(4'-N,N-diethylamino-2'-hydroxyphenyl)benzimidazole (2), and 1-methyl-2-(4'-N,N-diethylamino-2'-hydroxyphenyl)benzimidazole (3), together with the ground-state rotamerism and tautomerism of these new compounds, have been studied by UV-vis absorption spectroscopy and steady-state and time-resolved fluorescence spectroscopy. A solvent-modulated rotameric and tautomeric equilibrium is observed in the ground state for 1, 2, and 3. In cyclohexane, these compounds mainly exist as a planar syn normal form, with the hydroxyl group hydrogen-bonded to the benzimidazole N3. In ethanol, the syn form is in equilibrium with its planar anti rotamer (for 1 and 2), with the phenyl ring rotated 180 degrees about the C2-C1' bond and with a nonplanar rotamer for compound 3. In aqueous solution, a tautomeric equilibrium is established between the anti normal form (or the nonplanar rotamer for 3) and the tautomer (with the hydroxyl proton transferred to the benzimidazole N3). The syn normal form of these compounds undergoes in all the solvents an excited-state intramolecular proton-transfer process from the hydroxyl group to the benzimidazole N3 to yield the excited tautomer. The tautomer fluorescence quantum yield of 2, 3, and 4 shows a temperature-, polarity-, and viscosity-dependent radiationless deactivation, connected with a large-amplitude conformational motion. We conclude that this excited-state conformational change experienced by the tautomer is associated with an intramolecular charge transfer from the deprotonated dialkylaminophenol or phenol (donor) to the protonated benzimidazole (acceptor), affording a nonfluorescent charge-transfer tautomer. Therefore, these compounds undergo an excited-state intramolecular coupled proton- and charge-transfer process.     

Matching-pursuit split-operator Fourier-transform simulations of excited-state intramolecular proton transfer in 2-(2'-hydroxyphenyl)-oxazole

The excited-state intramolecular proton-transfer dynamics associated with the keto-enolic tautomerization reaction in 2-(2(')-hydroxyphenyl)-oxazole is simulated according to a numerically exact quantum-dynamics propagation method and a full-dimensional excited-state potential energy surface, based on an ab initio reaction surface Hamiltonian. The reported simulations involve the propagation of 35-dimensional wave packets according to the recently developed matching-pursuit/split-operator-Fourier-transform (MP/SOFT) method by Wu and Batista. The underlying propagation scheme recursively applies the time-evolution operator as defined by the Trotter expansion to second order accuracy in dynamically adaptive coherent-state expansions. Computations of time-dependent survival amplitudes, photoabsorption cross sections, and time-dependent reactant(product) populations are compared to the corresponding calculations based on semiclassical approaches, including the Herman-Kluk semiclassical initial value representation method. The reported results demonstrate the capabilities of the MP/SOFT method as a valuble computational tool to study ultrafast reaction dynamics in polyatomic systems as well as to validate semiclassical simulations of complex (nonintegrable) quantum dynamics in multidimensional model systems.     

Theoretical investigation of the intermolecular H-bonding and proton transfer in cytosine assisted by water and methanol

Abstract  

This investigation is devoted to study of the electronic structure of several H-bonded complexes of cytosine tautomers with water and methanol. The stability of these “supersystems” (aggregates) was estimated by calculating the bonding energies ΔE and ΔE _b. The energy barriers of water/methanol-assisted tautomeric conversions were calculated (intermolecular proton transfer), and the electronic structures of the transition states were studied. Each transition state of the proton-transfer reaction was determined as a first-order saddle point on the potential energy surface (full coordinate hyperspace). The crystal structure of the tetramer of cytosine monohydrate was also investigated. It was found that this structure is close to a conical intersection.     

Excited-state proton transfer in 2-(2'-hydroxyphenyl)benzothiazole: formation of the anion in polar solvents

Excited-state proton transfer in 2-(2'-hydroxyphenyl)benzothiazole (HBT) dissolved in pyridine is investigated. New absorptions and fluorescence bands are detected after addition of water or NaOH to the solution. The spectra are identical to the HBT anion absorption and fluorescence. Picosecond spectroscopy is used to determine the kinetics of electronically excited states. The excited-state lifetime of the anion is 3.5 ns. The tautomeric fluorescence of HBT after proton transfer builds up within 4 ps after excitation and decays with a time constant of 20 ps.     

Excitation energy dependence on the intramolecular excited-state proton transfer of 3-hydroxyflavone in the vapor phase

3-hydroxyflavone in the vapor phase exhibits long-wavelength flourescence (λ_max = 530 nm) due to a tautomer generated by intramolecular excited-state proton transfer. Excitation energy dependence on the flourescence quantum yield and lifetimes of the tautomer was observed suggesting proton transfer in the upper vibrational S₁ state of 3-hydroxyflavone.     

Excited-state intramolecular proton transfer in 2-(3'-hydroxy-2'-pyridyl)benzoxazole. Evidence of coupled proton and charge transfer in the excited state of some o-hydroxyarylbenzazoles

The influence of solvent, temperature, and viscosity on the phototautomerization processes of a series of o-hydroxyarylbenzazoles was studied by means of ultraviolet-visible (UV-vis) absorption spectroscopy and steady-state and time-resolved fluorescence spectroscopy. The compounds studied were 2-(2'-hydroxyphenyl)benzimidazole (HBI), 2-(2'-hydroxyphenyl)benzoxazole (HBO), 2-(2'-hydroxyphenyl)benzothiazole (HBT), 2-(3'-hydroxy-2'-pyridyl)benzimidazole (HPyBI), and the new derivative 2-(3'-hydroxy-2'-pyridyl)benzoxazole (HPyBO), this one studied in neutral and acid media. All of these compounds undergo an excited-state intramolecular proton transfer (ESIPT) from the hydroxyl group to the benzazole N3 to yield an excited tautomer in syn conformation. A temperature- and viscosity-dependent radiationless deactivation of the tautomer has been detected for all compounds except HBI and HPyBI. We show that this radiationless decay also takes place for 2-(3-methyl-1,3-benzothiazol-3-ium-2-yl)benzenolate (NMeOBT), the N-methylated analog of the tautomer, whose ground-state structure has anti conformation. In ethanol, the radiationless decay shows intrinsic activation energy for HPyBO and HBO; however, it is barrierless for HBT and NMeOBT and controlled instead by the solvent dynamics. The relative efficiency of the radiationless decay in the series of molecules studied supports the hypothesis that this transition is connected with a charge-transfer process taking place in the tautomer, its efficiency being related to the strength of the electron donor (dissociated phenol or pyridinol moiety) and electron acceptor (protonated benzazole). We propose that the charge transfer is associated with a large-amplitude conformational change of the tautomer, the process leading to a nonfluorescent charge-transfer intermediate. The previous ESIPT step generates the structure with the suitable redox pair to undergo the charge-transfer process; therefore, an excited-state intramolecular coupled proton and charge transfer takes place for these compounds.