Electronic structure studies on deprotonation of dithiophosphinic acids in water clusters

We report herein a computational study of proton transfer reactions between dithiophosphinic acids (HAs) and water clusters using B3LYP and MP2 methods. The ground-state and transition-state structures of HA-(H(2)O)(n) (n = 1, 2, 3) cluster complexes have been calculated. The influence of water molecules on energy barrier heights of proton transfer reactions has been examined in the gas phase and solution for bis[o-(trifluoromethyl)phenyl]- and bis(2,4,4-trimethylpentyl)dithiophosphinic acids (HA1 and HA2, respectively). Gas-phase calculations indicate that electron-withdrawing substituents and trifluoromethyl groups in the ortho position favor deprotonation of HA1 when three water molecules are included in the cluster. This suggests that at least three water molecules are necessary to solvate the abstracted proton in the presence of the anion. In the case of HA2, the electron-donating groups favor the reverse proton transfer reaction, namely, protonation of dithiophosphinate anion. Bulk solvent effects have been modeled for aqueous and organic media with the CPCM model. The calculated results show that polar solvents can lower the activation energy for less energetically stable transition states that have more localized charges.     

竹红菌甲素光敏活性的量子化学研究

利用密度泛函B3LYP/6-31G^*方法对茈醌类光敏剂竹红菌甲素hypocrellin A(HA)及其模型化合物的结构性质和分子内氢传递(IHT)过程进行了系统的计算研究,比较了各侧链对分子结构和IHT过程的影响.利用CIS/6-31G^*方法对HA的活性中心苝醌(PQ)及与其结构类似的一系列化合物激发态的IHT过程进行了研究.得到的主要结论包括:(1)常温下,处于基态的HA分子能够进行快速的分子内氢传递.(2)HA的几种模型化合物在基态时的IHT势垒、分子内电荷分布与HA差别很小,这说明侧链对IHT过程的影响不大.(3)HA模型化合物的IHT势垒与氧氢键键长变化和氢键键长变化呈良好的线性关系.(4)IHT反应中伴随着一个电荷分离过程,在氢传递过程中占主导地位的是静电相互作用,这说明此IHT过程本质上是质子转移过程.(5)共轭结构对该类分子的激发态IHT过程具有重要影响,总的趋势是对小共轭体系,单重激发态和三重激发态IHT反应的势能曲线分离,随着分子共轭结构的增大,两个激发态势能曲线逐渐接近,发生交叉的可能性以及交叉的程度也随之增大,即从单重态到三重态发生系间窜越的可能性也随之增大.由于分子内氢传递导致单重激发态和三重激发态势能曲线交叉,致使发生系间窜越,使体系三重态量子效率显著提高,从而提高了茈醌类光敏剂的光敏活性.这说明IHT过程对该类分子保持其光敏活性具有重要的意义.     

Theoretical study of intramolecular proton transfer of perylenequinone

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Time-resolved resonance Raman and density functional theory study of the deprotonation reaction of the triplet state of p-hydroxyacetophenone in water solution

[reaction; see text] Picosecond and nanosecond time-resolved resonance Raman (TR(3)) spectroscopy was employed to investigate the deprotonation/ionization reaction of p-hydroxyacetophenone (HA) after ultraviolet photolysis in water solution. The TR(3) spectra in conjunction with density functional theory (DFT) calculations were used to characterize the structure and dynamics of the excited-state HA deprotonation to form HA anions in near neutral water solvent. DFT calculations based on a solute-solvent intermolecular H-bonded complex model containing up to three water molecules were used to evaluate the H-bond interactions and their influence on the deprotonation reaction and the structures of the intermediates. The deprotonation reaction was found to occur on the triplet manifold with a planar H-bonded HA triplet complex as the precursor species. The HA triplet species is generated within several picoseconds and then decays with a approximately 10 ns time constant to produce the HA triplet anion species after 267 nm photolysis of HA in water solution. The triplet anion species was observed to decay with a time constant of about 90 ns into the ground-state anion species that was found to have a lifetime of about 200 ns. The DFT calculations on the H-bonded complexes of the anion triplet and ground-states species suggest that these anion species are H-bonded complexes with planar quinonoidal structures containing two water molecules H-bonded, respectively, with oxygen lone pairs of the carbonyl and deprotonated hydroxyl moieties. A deactivation scheme of the photoexcited HA in regard to the deprotonation reaction in neutral water solutions was proposed. With the above dynamic and structural information available, we briefly discuss the possible implications of the model HA photochemistry in water solutions for the photodeprotection reactions of related p-HP phototrigger compounds in aqueous solutions.     

Change in reaction pathway in the reduction of 3,5-di-tert-butyl-1,2-benzoquinone with increasing concentrations of 2,2,2-trifluoroethanol

The electrochemical reduction of 3,5-di-tert-butyl-1,2-benzoquinone, 1, has been studied in acetonitrile with added 2,2,2-trifluoroethanol, 2. At low concentrations of 2 the reaction proceeds by the following pathway: reduction of the quinone (Q) to its anion radical (Q*-) followed by complexation of the anion radical with 2 (HA) and the further reduction of the hydrogen-bonded complex (Q*- (HA)) to form HQ- and A-. The latter reaction is a concerted proton and electron- transfer reaction (CPET). At higher concentrations of 2, the pathway changes. The first steps remain the same, but now Q*- (HA) is reduced to HQ- via a disproportionation reaction with Q*- along with proton transfer from HA to Q*- to form HQ* which is reduced to HQ-. The only mechanism that could be found which would account for all of the data involves proton transfer to Q*- occurring within a higher complex, Q*-(HA)3.     

The reaction of triplet flavin with indole. A study of the cascade of reactive intermediates using density functional theory and time resolved infrared spectroscopy

As a model for riboflavin, lumiflavin was investigated using density functional theory methods (B3LYP/6-31G* and B3LYP/6-31+G**) with regard to the proposed cascade of intermediates formed after excitation to the triplet state, followed by electron-transfer, proton-transfer, and radical[bond]radical coupling reactions. The excited triplet state of the flavin is predicted to be 42 kcal/mol higher in energy than the singlet ground state, and the pi radical anion lies 45.1 kcal/mol lower in energy than the ground-state flavin and a free electron in the gas phase. The former value compares to a solution-phase triplet energy of 49.8 kcal/mol of riboflavin. For the radical anion, the thermodynamically favored position to accept a proton on the flavin ring system is at N(5). A natural population analysis also provided spin density information for the radicals and insight into the origin of the relative stabilities of the six different calculated hydroflavin radicals. The resulting 5H-LF* radical can then undergo radical[bond]radical coupling reactions, with the most thermodynamically stable adduct being formed at C(4'). Vibrational spectra were also calculated for the transient species. Experimental time-resolved infrared spectroscopic data obtained using riboflavin tetraacetate are in excellent agreement with the calculated spectra for the triplet flavin, the radical anion, and the most stable hydroflavin radical.     

Photochemistry of 1-(N,N-diethylamino)diazen-1-ium-1,2-diolate: an experimental and computational investigation

The aqueous photochemistry of the sodium salt of 1-(N,N-diethylamino)-diazen-1-ium-1,2-diolate (3) has been investigated by both experimental and computational methods. Photolysis results in the formation of the N-nitrosodiethylamine radical anion (5) and nitric oxide (NO) via a triplet excited state. The nitrosamine radical anion either undergoes electron transfer with NO before cage escape to form triplet NO(-) and nitrosamine (minor process) or rapidly dissociates to form an additional molecule of NO and ultimately amine (major process). The production of nitrosamine radical anion 5 upon photolysis of diazeniumdiolate 3 is confirmed by low-temperature EPR spectroscopy. The calculated energetics for the ground and excited states of the parent diazeniumdiolate ion at the CIS and B3LYP levels of theory as well as B3LYP calculations on the fragmentation processes were very effective in rationalizing the observed photodissociation processes.     

Reactions of the hydroperoxide anion with dimethyl methylphosphonate in an ion trap mass spectrometer: evidence for a gas phase alpha-effect

The gas phase degradation reactions of the chemical warfare agent (CWA) simulant, dimethyl methylphosphonate (DMMP), with the hydroperoxide anion (HOO(-)) were investigated using a modified quadrupole ion trap mass spectrometer. The HOO(-) anion reacts readily with neutral DMMP forming two significant product ions at m/z 109 and m/z 123. The major reaction pathways correspond to (i) the nucleophilic substitution at carbon to form [CH(3)P(O)(OCH(3))O](-) (m/z 109) in a highly exothermic process and (ii) exothermic proton transfer. The branching ratios of the two reaction pathways, 89% and 11% respectively, indicate that the former reaction is significantly faster than the latter. This is in contrast to the trend for the methoxide anion with DMMP, where proton transfer dominates. The difference in the observed reactivities of the HOO(-) and CH(3)O(-) anions can be considered as evidence for an alpha-effect in the gas phase and is supported by electronic structure calculations at the B3LYP/aug-cc-pVTZ//B3LYP/6-31+G(d) level of theory that indicate the S(N)2(carbon) process has an activation energy 7.8 kJ mol(-1) lower for HOO(-) as compared to CH(3)O(-). A similar alpha-effect was calculated for nucleophilic addition-elimination at phosphorus, but this process--an important step in the perhydrolysis degradation of CWAs in solution--was not observed to occur with DMMP in the gas phase. A theoretical investigation revealed that all processes are energetically accessible with negative activation energies. However, comparison of the relative Arrhenius pre-exponential factors indicate that substitution at phosphorus is not kinetically competitive with respect to the S(N)2(carbon) and deprotonation processes.     

Experimental and computational studies of hydrogen bonding and proton transfer to [Cp*Fe(dppe)H

The present contribution reports experimental and computational investigations of the interaction between [Cp*Fe(dppe)H] and different proton donors (HA). The focus is on the structure of the proton transfer intermediates and on the potential energy surface of the proton transfer leading to the dihydrogen complex [Cp*Fe(dppe)(H2)]+. With p-nitrophenol (PNP) a UV/Visible study provides evidence of the formation of the ion-pair stabilized by a hydrogen bond between the nonclassical cation [Cp*Fe(dppe)(H2)]+ and the homoconjugated anion ([AHA]-). With trifluoroacetic acid (TFA), the hydrogen-bonded ion pair containing the simple conjugate base (A-) in equilibrium with the free ions is observed by IR spectroscopy when using a deficit of the proton donor. An excess leads to the formation of the homoconjugated anion. The interaction with hexafluoroisopropanol (HFIP) was investigated quantitatively by IR spectroscopy and by 1H and 31P NMR spectroscopy at low temperatures (200-260 K) and by stopped-flow kinetics at about room temperature (288-308 K). The hydrogen bond formation to give [Cp*Fe(dppe)H]HA is characterized by DeltaH degrees =-6.5+/-0.4 kcal mol(-1) and DeltaS degrees = -18.6+/-1.7 cal mol(-1) K(-1). The activation barrier for the proton transfer step, which occurs only upon intervention of a second HFIP molecule, is DeltaH(not equal) = 2.6+/-0.3 kcal mol(-1) and DeltaS(not equal) = -44.5+/-1.1 cal mol(-1) K(-1). The computational investigation (at the DFT/B3 LYP level with inclusion of solvent effects by the polarizable continuum model) reproduces all the qualitative findings, provided the correct number of proton donor molecules are used in the model. The proton transfer process is, however, computed to be less exothermic than observed in the experiment.     

Theoretical investigation of the photosensitization mechanisms of urocanic acid

The photosensitization mechanisms of urocanic acid (UA), the main skin chromophores of ultraviolet (UV) light, are investigated by means of theoretical calculations. The results indicate that the direct photooxidative damage to DNA bases by triplet state UA through electron transfer reaction is not favorable on thermodynamic grounds. However, UA can photogenerate various reactive oxygen species (ROS, e.g., (1)O(2), O(2)(-)) theoretically and the ROS-generating mechanisms are illustrated as follows. Firstly, the (1)O(2)-generating pathway involves direct energy transfer between triplet state UA and (3)O(2). Secondly, UA gives birth to O(2)(-) through two pathways: (i) direct electron transfer between triplet state UA and (3)O(2); (ii) electron transfer between anion radical of UA (generated through autoionization reactions) and (3)O(2).     

The photochemistry of lipoic acid: photoionization and observation of a triplet excited state of a disulfide.

Under short-wavelength UV irradiation, lipoic acid (LipSS) and its reduced form, dihydrolipoic acid (DHLA), undergo photoionization processes through a bi- or monophotonic pathway. After ionization, the LipSS radical cation (LipSS*+) and radical anion (LipSS*-) are generated. LipSS*- can be converted to equimolar amounts of LipSS and DHLA through second-order decay. Triplet acetone can be quenched by LipSS and DHLA with a rate close to the diffusion-controlled limit. The mechanism was further confirmed by continuous irradiation experiments. When LipSS is directly irradiated with UVA light, the first excited triplet state of LipSS is observed, with a lifetime tau=75 ns. Characteristic reactions include triplet energy transfer to oxygen and beta-carotene and addition to isoprene. The lifetime of triplet LipSS is also shortened by addition of water and methanol.     

Photodynamic properties of hypocrellin A, complexes with rare Earth trivalent ions: role of the excited state energies of the metal ions

Fifteen complexes of hypocrellin A (HA) with rare earth trivalent ions (except Pm3+) along with the complex of HA with Sc3+ were prepared, and their photodynamic activities, including absorption in the phototherapeutic window (600-900 nm); water-solubility; triplet lifetime; generation of reactive oxygen species (ROS), such as singlet oxygen (1O2), superoxide anion radical (O2-*), and hydroxyl radical (OH*); generation of semiquinone anion radical; and affinity to DNA, as well as photosensitized damage on calf thymus DNA (CT DNA), were compared in detail using the UV-visible spectrum, fluorescence spectrum, spin-trapping EPR technique, and laser photolysis technique. All complexes exhibit a red-shifted absorption spectrum, an increased absorbance above 600 nm, improved water solubility, and an enhanced affinity to CT DNA over the parent HA. For ions that possess low-energy excited states, including Ce3+, Pr3+, Nd3+, Sm3+, Eu3+, Tb3+, Dy3+, Ho3+, Er3+, Tm3+, and Yb3+, the corresponding complexes show undetectable or nearly undetectable fluorescence, a triplet excited-state lifetime, generation of ROS, and photodamage in CT DNA. In contrast, for ions that do not possess low-energy excited states, including Sc3+, Y3+, La3+, Gd3+, and Lu3+, the corresponding complexes exhibit higher photodamage abilities with CT DNA with respect to HA, benefitting from both their comparable or even higher 1O2 quantum yields and an electrostatic affinity that is higher for DNA than HA.     

Water- and acid-mediated excited-state intramolecular proton transfer and decarboxylation reactions of ketoprofen in water-rich and acidic aqueous solutions

We present an investigation of the decarboxylation reaction of ketoprofen (KP) induced by triplet excited-state intramolecular proton transfer in water-rich and acidic solutions. Nanosecond time-resolved resonance Raman spectroscopy results show that the decarboxylation reaction is facile in aqueous solutions with high water ratios (water/acetonitrile ≥50%) or acidic solutions with moderate and strong acid concentration. These experimental results are consistent with results from density functional theory calculations in which 1) the activation energy barriers for the triplet-state intramolecular proton transfer and associated decarboxylation process become lower when more water molecules (from one up to four molecules) are involved in the reaction system and 2) perchloric acid, sulfuric acid, and hydrochloric acid can shuttle a proton from the carboxyl to carbonyl group through an initial intramolecular proton transfer of the triplet excited state, which facilitates the cleavage of the C-C bond, thus leading to the decarboxylation reaction of triplet state KP. During the decarboxylation process, the water molecules and acid molecules may act as bridges to mediate intramolecular proton transfer for the triplet state KP when KP is irradiated by ultraviolet light in water-rich or acidic aqueous solutions and subsequently it generates a triplet-protonated carbanion biradical species. The faster generation of triplet-protonated carbanion biradical in acidic solutions than in water-rich solutions with a high water ratio is also supported by the lower activation energy barrier calculated for the acid-mediated reactions versus those of water-molecule-assisted reactions.     

Reactions of atomic metal anions in the gas phase: competition between electron transfer, proton abstraction and bond activation

Bare metal anions K(-), Rb(-), Cs(-), Fe(-), Co(-), Ni(-), Cu(-), and Ag(-), generated by electrospray ionization of the corresponding oxalate or tricarballylate solutions, were allowed to react with methyl and ethyl chloride, methyl bromide, nitromethane, and acetonitrile in the collision hexapole of a triple-quadrupole mass spectrometer. Observed reactions include (a) the formation of halide, nitride, and cyanide anions, which was shown to be likely due to the insertion of the metal into the C-X, C-N, and C-C bonds, (b) transfer of H(+) from the organic molecule, which is demonstrated to most likely be due to the simple transfer of a proton to form neutral metal hydride, and (c) in the case of nitromethane, direct electron transfer to form the nitromethane radical anion. Interestingly, Co(-) was the only metal anion to transfer an electron to acetonitrile. Differences in the reactions are related to the differences in electron affinity of the metals and the Δ(acid)H° of the metals and organic substrates. Density functional theory calculations at the B3-LYP/6-311++G(3df,2p)//B3-LYP/6-31+G(d) level of theory shed light on the relative energetics of these processes and the mechanisms by which they take place.     

Ion-molecule reactions of [C,H(3), S](+) ions and evidence for long-lived triplet CH(3)S(+)

Gas-phase [C, H(3), S](+) ions obtained by electron impact from (CH(3))(2)S at 14 eV undergo two distinct low-pressure ion-molecule reactions with the parent neutral: proton transfer and charge exchange. The kinetics of these reactions studied by Fourier transform ion cyclotron resonance (FT-ICR) techniques clearly suggests the [C, H(3), S](+) species to be a mixture of isomeric ions. While proton transfer is consistent with reagent ions displaying the CH(2)SH(+) connectivity, the observed charge exchange strongly argues for the presence of thiomethoxy cations, CH(3)S(+), predicted to be stable only in the triplet state. Charge exchange reactions are also observed in the reaction of these same [C, H(3), S](+) ions with benzene, toluene and phenetole. For these substrates, the CH(2)SH(+) ions can promote proton transfer and electrophilic methylene insertion in the aromatic ring with elimination of H(2)S. The results obtained for the different substrates suggest that the fraction of long-lived fraction of thiomethoxy cations obtained at 14 eV by electron ionization of dimethyl sulfide amounts to ~(22 -/+ 4)% of the [C, H(3), S](+) fragments.     

A DFT study on reaction of eupatilin with hydroxyl radical in solution

Antioxidants scavenge reactive oxygen species and, therefore, are vitally important in the living cells. The antioxidant properties of eupatilin have recently been reported. In this article, the reactions of eupatilin with the hydroxyl radical (OH^?) in solution are studied using density functional theory calculations and the polarizable continuum model. Three mechanisms are considered including: sequential electron proton transfer (SEPT), sequential proton loss electron transfer (SPLET), and hydrogen abstraction (HA). Three solvents with different polarities, that is, benzene, methanol, and water, are used to investigate the effect of the environment on the mechanisms. The relative Gibbs free energies and enthalpies corresponding to different mechanisms are calculated. Our results show that SEPT is thermodynamically favored in aqueous solution. Once the eupatilin anion is produced, the second step in SPLET mechanism is thermodynamically favored in methanol and water. The HA mechanism is thermodynamically favored in gas, benzene, methanol, and water. This mechanism is more energetically favorable to occur in a more polar solvent. The natural bond orbital charges and spin densities as well as the singly occupied molecular orbital are then analyzed. It is concluded that the HA process is governed by proton coupled electron transfer mechanism. The attack of the radical takes place preferentially at position 7 of eupatilin. © 2012 Wiley Periodicals, Inc.     

Revisiting the Origin of Bacterial Bioluminescence: QM/MM Study on Oxygenation Reaction of Reduced Flavin in Protein

Bacterial bioluminescence is initiated by the oxygenation reaction of reduced flavin mononucleotide in luciferase. This enzymatic oxygenation occurs in a wide range of biological processes including cellular redox metabolism, biocatalysis, biosynthesis and homeostasis. However, little is known about the mechanism of the enzymatic reaction between singlet reduced flavin and triplet oxygen. To explore the enigmatic oxygenation, for the first time, the reaction of reduced flavin anion with oxygen was studied in bacterial luciferase by a combined quantum mechanics and molecular mechanics method as well as molecular dynamics simulation. The calculated results demonstrate that the reaction proceeds via a proton-coupled electron transfer (PCET) pathway, and the essential αHis44 acts as a catalytic acid to provide the proton. The currently proposed PCET mechanism clearly describes the initial steps of bacterial bioluminescence, and could be suitable for the other flavin oxygenation reactions in enzymes.     

Kinetics and mechanism of proton transfer between disubstituted benzoic acids and carbinol base of crystal violet in chlorobenzene

An investigation on the detailed kinetics of proton transfer between a set of di- and monofluoro- and chloro- (2,3-, 2,5-, 2,6-, 3,5-, 2-, and 3-) benzoic acids (HA) and Crystal Violet carbinol base in chlorobenzene favors a mechanism in terms of fast equilibrium between HA and D to form a H-bonded complex, D...HA, followed by rate-limiting proton transfer along the H-bond to form the colored ion pair DH+A- under the combined influence of monomer HA catalyst, nonreactive cyclic dimer (HA)2 inhibitor, and hyperacidic homoconjugated complex H(HA2) catalyst through a transition state with nearly 60% charge separation.     

Formation of secondary triplet species after excitation of fluoroquinolones in the presence of relatively strong bases

Laser flash photolysis of 7-(piperazin-1-yl) fluoroquinolones leads to the formation of a triplet excited state (3A*) at the end of the pulse (lambdamax 520, 610, and 620 nm for enoxacin, ciprofloxacin, and norfloxacin, respectively). Phosphate and bicarbonate buffers react with 3A* to form a secondary triplet (3B*, reaction rates (0.8-9.9) x 108 M-1 s-1), whose T-T absorption is red-shifted (lambdamax 670 nm for enoxacin, 700 nm for ciprofloxacin and norfloxacin). The formation of a secondary triplet is not a common process and disagrees with previous work suggesting that electron transfer occurs between phosphate buffer and the primary triplet excited state with the formation of the anion radical of the fluoroquinolone (FQ.-). We have shown that the FQ.- transient absorption spectrum is quite distinct from that of 3B*. The photophysical characteristics of 3B* have been determined by energy transfer to naproxen, and it has been found that its energy is lower than that of 3A*.     

A tr esr study of the reactions of excited organic triplet molecules with inorganic sulfur oxoanions

The reactions of excited triplet ketones including benzophenone, 4,4′-dihydroxybenzophenone, and acetone with inorganic sulfur oxoanions: sulfite (SO₃ ⁼), metabisulfite (S₂O₅ ⁼), and dithionite (S₂O₄ ⁼) in solution were studied by TRESR. Observation of radical anion intermediates polarized by the Triplet Mechanism demonstrated the reactions of electron transfer from the sulfur dianions to the organic triplet states. Thus, electron transfer from SO₃ ⁼ to the triplet organic carbonyl compounds gave rise to the SO₃ ^? radical; reactions with metabisulfite and dithionite were more complex and may be interpreted as occurring via electron transfer to the carbonyl system followed by dissociation of the S-S bond in the resulting sulfur oxoanion radical. The initial radical ion pairs thus formed consist of pairs of anion radicals, one organic and one inorganic.