A barrier-free molecular radical-molecule reaction: {^{3}C_{2} (a^{3}\Pi) {+} O_{2} (X^{3} \Sigma)}

The reaction of ³C₂ (a³Π) radical with O₂ (X³Σ) molecule has been studied theoretically using ab initio Quantum Chemistry method. Both singlet and triplet potential energy surfaces (PES) are calculated at the CCSD(T)/aug-cc-pVDZ//B3LYP/6-311+G(d) + ZPE and G3B3 levels of theory. On the singlet PES of the title reaction, it is shown that the most feasible pathway should be the O-atom of O₂ attacking the C-atom of the  ³C₂ molecule first to form the adduct 1 CCOO, followed by the O-shift to give intermediate 2 CC(OO), and then to the major products P1 (2CO). Alternatively, 1 can be directly dissociated to P1 via transition state TS1-P1. The other reaction pathways are less competitive due to thermodynamical or kinetic factors. On the other hand, the pathways on the triplet PES are less competitive than those on the singlet PES in low temperature range, whereas it is not the case in high temperature ranges. On the basis of the analysis of the kinetics of all pathways through which the reactions proceed, we expect that the competitive power of reaction pathways may vary with experimental conditions for the title reaction. The reaction heats of formation calculated are in good agreement with that obtained experimentally.     

Singlet Biradical Versus Triplet Biradical/Zwitterion Characteristics in Isomers of C6−C5−C6−C7−C6-Fused Pentacyclic Aromatic Hydrocarbons Revealed through Reactivity Patterns

Among various polycyclic aromatic hydrocarbons, C₆−C₅−C₆−C₇−C₆ fused pentacyclic aromatic hydrocarbons have the unique potential to adopt quinonoid, zwitterion, singlet, or triplet biradical electronic configurations. Two such hybrid structures between pentacene and azulene were synthesized and their ground state electronic configurations were deduced from the reactivity patterns they exhibit respectively. Compound 6 , where the radicaloid carbons are linked through a para-phenylene, forms a head-to-head dimer like a singlet biradical. In contrast, isomer 7 , where the para-linkage was switched to meta, reacts readily with oxygen which resembles the reactivity of a triplet state. The oxidized intermediate(s) then undergoes rearrangement to furnish the C₆−C₅−C₆−C₆−C₆ ring contraction product 13 . Cation 14 , the protonated form of 7 , was synthesized, which implies 7 also reacts like a zwitterion. It was revealed the oxidative rearrangement takes place even with mesityl dibenzotropylium cation despite its perceived aromaticity. DFT calculations confirm the most stable forms of 6 and 7 are singlet and triplet diradical, which is consistent with the observed reactivity of respective molecules.     

Photochemical formation of chrysanthemic acid and cyclopropylacrylic acid derivatives: A case of multiplicity-dependent regiospecificity in the di-π-methane rearrangement

Photochemical transformations of methyl E- and Z - 4,4,6 - trimethyl - 2,5 - dienoate (5a) and of the corresponding acids (5b) have been investigated. In contrast to similar pure hydrocarbon di-π-methanes, these aliphatic α,β-unsaturated car?ylic acid derivatives react from both the singlet and the triplet excited states and form different types of vinylcyclopropanes depending on multiplicity. When irradiated in acetone, both 5aE and 5aZ give methyl E - 3(2,2,3,3 - tetramethylcyclopropyl)prop - 2 - enoate (8aE). The same is observed for the corresponding acids. Direct irradiation of 5aE, but not of 5aZ, readily gives methyl E-chrysanthemate (4aE). Only singlet excited 5Z reacts by internal hydrogen abstraction to cyclopropylacetic add derivatives (7). Procedures for independent synthesis of 7 and 8 are described.     

A theoretical study of the reaction of N(4 S) with nitrogen dioxide on the N2O2 potential energy surface

The reaction of N(⁴ S) radical with NO₂ molecule has been studied theoretically using density functional theory and ab initio quantum chemistry method. Both singlet and triplet electronic state [N₂O₂] potential energy surfaces (PESs) are calculated at the G3B3 level of theory. Also, the highly cost-expensive coupled-cluster theory including single and double excitations and perturbative inclusion of triple excitations CCSD(T)/cc-pVTZ single-point energy calculation is performed on the basis of the geometries obtained at the Becke??s three parameter Lee-Yang-Parr B3LYP/6-311++G(d, p) level. On the singlet PES of the title reaction, it is shown that the most feasible pathway should be as follows. The atomic radical N attacking the NO bond of the NO₂ molecule first to form the adduct 1 N(NO)O, followed by one of the NO bond broken to give intermediate 2 ONNO, and then to the major products P1 (2NO). On the triplet PES of the title reaction, it is shown that the most favorable pathway should be the atomic radical N attacking the N-atom of NO₂ firstly to form the adduct 7 NN(OO), followed by one of the NO bonds breaking to give intermediate 8 NNOO, and then leading to the major products P2 (O₂ + N₂). As efficient routes to the reduction of NO₂ to form N₂ and O₂ are sought, both kinetic and thermodynamic considerations support the viability of this channel. All the involved transition states for generation of (2NO), (³O + N₂O), and (O₂ + N₂) lie much lower than the reactants. Thus, the novel reaction N + NO₂ can proceed effectively even at low temperatures and it is expected to play a role in both combustion and interstellar processes. The other reaction pathways are less competitive due to thermodynamical or kinetic factors. On the basis of the analysis of the kinetics of all path-ways through which the reactions proceed, we expect that the competitive power of reaction pathways may vary with experimental conditions for the title reaction. The calculated reaction heats of formation are in good agreement with that obtained experimentally.     

Photoinduced cross-coupling reaction of 5-bromo-1,3-dimethyluracil to electron-rich aromatics

On irradiation with Pyrex-filtered light, 5-bromo-1,3-dimethyluracil 1 coupled with methyl- and methoxynaphthalenes 2–7 to give 5-naphthyl-1,3-dimethyluracils 8–13. No coupling product was formed by triplet sensitization except in the cases of 2,3-dimethioxynaphthalene 3 and 2-methoxynaphthalene 5. indicating that the singlet excited states of the naphthalenes are involved in the unsensitized coupling reaction. The k_qτ values of the fluorescence quenching of 1,4-dimethoxynaphthalene 2 and 1-methoxynaphthalene 4 by 1 in acetonitrile were comparable with those obtained from the kinetics of the coupling reactions. On the basis of this fact and the fluorescence-quenching rate constants k_q in acetonitrile ranging from 10⁸ to 10⁹ M^?1 sec^?1, involvement of an electron-transfer process possibly via a singlet exciplex is proposed for this cross-coupling reaction.     

Excited state intramolecular proton transfer (ESIPT) from phenol OH (OD) to adjacent “aromatic” carbons in simple biphenyls

This work reports results of further studies on a new class of excited state intramolecular proton transfer (ESIPT), from phenol OH to adjacent aromatic carbon atoms of suitably designed biphenyl systems. For this purpose, a number of 2-phenylphenols 3–6 with methyl and methoxy substituents on the adjacent proton accepting phenyl ring were synthesized. In particular, we were also interested in studying the effect of an acetyl (ketone) substituent on the proton accepting ring (biphenyl 7) and the effect on the photochemistry when the ketone is reduced to alcohol (biphenyl 8). All compounds except for 7 were found to undergo deuterium exchange (Ф_ex = 0.019–0.079) primarily at the 2′-position on photolysis in 1:3 D₂O–CH₃CN. This is consistent with a reaction mechanism involving initial ESIPT from the phenol OH to the 2′-position of the adjacent phenyl ring, to generate a biphenyl quinone methide intermediate which rapidly tautomerizes back to starting material. Biphenyl 8 also undergoes a competing photosolvolysis reaction (overall loss of water). Both photosolvolysis and ESIPT reactions react via isomeric quinone methide intermediates and are best interpreted as arising from an excited singlet state that possesses a large degree of charge transfer character, from the phenol ring to the attached phenyl ring. The failure of 7 to react may be due to two possible reasons: (i) high intersystem crossing rate to a non-polarized triplet excited state and/or (ii) a polarized singlet state that is now much more basic at the carbonyl oxygen. The results are consistent with qualitative examination of calculated HOMOs and LUMOs (AM1).     

A fullerene core to probe dendritic shielding effects

Dendrimers with a C₆₀ core have been obtained by cyclization of dendritic bis-malonate derivatives at the carbon sphere. The resulting bis-methanofullerene derivatives have been characterised by electrospray (ES) and/or MALDI-TOF mass spectrometries. UV-VIS absorption spectra, fluorescence spectra, and fullerene singlet excited state lifetimes have been determined in solvents of different polarity (toluene, dichloromethane, acetonitrile). These data suggest a tighter core/periphery contact upon increase of solvent polarity and dendrimer size. In all the investigated solvents, the fullerene triplet lifetimes are steadily increased with the dendrimer volume, reflecting lower diffusion rates of O₂ inside the dendrimers along the series. Measurements of quantum yields of singlet oxygen sensitization indicate that longer lived triplet states generate lower amounts of singlet oxygen (¹O₂^∗) in dichloromethane but not in apolar toluene suggesting a tighter contact between the dendritic branches and the fullerene core in CH₂Cl₂. In acetonitrile, the trend in singlet oxygen production is peculiar. Effectively, enhanced singlet oxygen production is monitored for the largest dendrimer. This reflects specific interactions of excited ¹O₂^∗ molecules with the dendritic wedges, as probed by singlet oxygen lifetime measurements, possibly as a consequence of trapping effects.     

ab initio molecular orbital and density functional analysis of acetylene + O2 reactions with CHEMKIN evaluation

A number of researchers have indicated that a direct reaction of acetylene with oxygen needs to be included in detailed reaction mechanisms in order to model observed flame speeds and induction times. Four pathways for the initiation of acetylene oxidation to chain propagation are considered and the rate constants are compared with values used in the mechanisms:

• 1 ³O₂ + HCCH to triplet adduct and reaction on the triplet surface
• 2 ³O₂ + HCCH to triplet adduct, conversion of triplet adduct to singlet adduct via collision in the reaction environment, with further reaction of the singlet adduct
• 3 ¹O₂ + HCCH to singlet adduct
• 4 Isomerization of HCCH to vinylidene and then vinylidene insertion reaction with ³O₂

Elementary reaction pathways for oxidation of acetylene by addition reaction of O₂(³Σ) on the triplet surface are analyzed. ab initio molecular orbital and density functional calculations are employed to estimate the thermodynamic properties of the reactants, transition states, and products in this system. Acetylene oxidation reaction over the triplet surface is initiated by addition of molecular oxygen, O₂(³Σ), to a carbon atom, forming a triplet peroxy‐ethylene biradical. The reaction path to major products, either two formyl radicals or glyoxal radical plus hydrogen atom, involves reaction through three transition states: O₂(³Σ) addition to acetylene (TS1), peroxy radical addition at the ipso‐carbon to form a dioxirane (TS2), and cleavage of O O bond in a three‐member ring (TS3). Single‐point QCISD(T) and B3LYP calculations with large basis sets were performed to try to verify barrier heights on important transition states. A second pathway to product formation is through spin conversion of the triplet peroxy‐ethylene biradical to the singlet by collision with bath gas. Rapid ring closure of the singlet peroxy‐ethylene biradical to form a four‐member ring is followed by breaking of the peroxy bond to form glyoxal, which further dissociates to either two formyl radicals or a glyoxal radical plus hydrogen atom. The overall forward rate constant through this pathway is estimated to be k_f = 2.21 × 10⁷ T^1.46e^{−33.1(kcal/mol)/RT}. Two additional pathways from the literature, HCCH + O₂(¹Δ) and pressure‐dependent isomerization of acetylene to vinylidene and then vinylidene reaction with O₂(³Σ), are also evaluated for completeness. CHEMKIN modeling on each of the four proposed pathways is performed and concentration profiles from these reactions are evaluated at 0.013 atm and 1 atm over 35 milliseconds. Through reaction on the triplet surface is evaluated to be not important. Formation of the triplet adduct with conversion (via collision) to a singlet and the vinylidene paths show similar and lower rates than those used in mechanisms, respectively. Our implementation of the HCCH + O₂(¹Δ) pathway of Benson suggests the need to include: (i) reverse reaction, (ii) barriers to further reaction of the initial adduct plus (iii) further evaluation of the O₂(¹Δ) addition barrier. The pathways from triplet adduct with conversion to singlet and from vinylidene are both recommended for initiation of acetylene oxidation. © 2000 John Wiley & Sons, Inc. Int J Chem Kinet 32: 623–641, 2000     

Photochemical and thermal rearrangements of a benzoylnaphthobarrelene-like system

8-Benzoyl-9-deuterionaphtho[de-2.3.4]bicyclo[3.2.2]nona-2,6,8-triene (12a)rearranges in a photochemical di-π-methane-type process to the l-benzoylatho[de-[2.3.4]tricyclo[4.3.0.0^2,9]nona-3,7-dienes 14a-c.The dihydro derivate 13a and the hydroxypheoylmethyl analogs 21a and 22a undergo similarly regioselective rearrangements to 15a+c, 23a-c, and 24a, respectively. At 298 K the primary photoreaction directly occus from the S₁(n,π^*) and T₂(n,π^*) states, and it proceeds from T₁(π,π^*) and from S₂(π,π^*) either directly or via T₂. At lower temperature on direct irradiation. S₁→T intersystem crossing and triplet reaction compete with reaction from the singlet. The rearrangement 12a→14a-c proceeds along three reaction paths evolving from the two primary photochemical processes of naphthyl-vinyl and vinyl-vinyl bonding in β-position to the CO (12→25+29). Two ground-state triplet diradical intermediates such as 25 and 27 have been shown to intervene consecutively—for the first time in di-π-methane photochemistry. Each has been characterized by ESR and IR, and the second one additionally by fluorescence and fluorescence excitation, and by laser flash photolysis.The failure of products 14a-c to interconvert photochemically is ascribed to efficient energy dissipation through thermally reversible pbotocleavage of the 3-membered ring.Compounds 12 and 14 thermally interconvert in the dark which constitutes the first example of a ground-state counterpart of a di-π-methane photorearrangement. The thermal reaction includes a path with highly regioselective (and possibly concerted) product formation competing with a stepwise process causing positional scrambling. The sequence 12→14 (photochemically; Φ = 1.0 at 366 nm and 298 K) and an electrophile-catalyzed reversal 14→12 in the dark is a model of a chemical light energy storage cycle which can be conducted without loss of reactants.     

Nonempirical Investigation of Equilibrium Geometry and Electronic Structure of Kynurenine and 3-Hydroxykynurenine Molecule

By Hartree-Fock-Roothaan method with complete geometry optimization in the basis 6-31G* ab initio calculations of equilibrium geometry and electronic structure were performed for kynurenine C₁₀H₁₂N₂O₃ and 3-hydroxykynurenine C₁₀H₁₂N₂O₃ molecules in the singlet ground state and the first triplet excited state. The molecules in the triplet state can react at the oxygen of the carbonyl group adjacent to the aromatic ring by quite different pathway compared to the molecules in the ground singlet state.     

Electron correlation in weakly coupled binuclear transition metal compounds: Cyclopentadienyl-allyl-cyclobutadiene-dinickel

The Hartree—Fock (HF) instabilities of the binuclear 3d complex cyclobutadiene-allyl-cyclobutadiene-dinickel (1) have been investigated by means of a semi-empirical INDO approach. It is demonstrated that the HF solution is unstable with respect to singlet, non-singlet and complex variations. The MO fluctuation leading to the singlet instability is predominantly localized in the region of Ni₂ and the four-membered ring. The lowest eigenvalue of the triplet instability corresponds to a spin decoupling between both Ni centers. Consequences arising from HF instabilities in binuclear transition metal compounds for the comparison between theoretical model calculations and experimental data (e.g. vertical ionization potentials, geometries, geometrical preferences, transport properties) are shortly discussed.     

Theoretical study on the atmospheric reaction of CH3O2 with OH

A quantum chemical investigation on the reaction mechanism of CH₃O₂ with OH has been performed. Based on B3LYP and QCISD(T) calculations, seven possible singlet pathways and seven possible triplet pathways have been found. On the singlet potential energy surface (PES), the most favorable channel starts with a barrierless addition of O atom to CH₃O₂ leading to CH₃OOOH and then the O? O bond dissociates to give out CH₃O + HO₂. On the triplet PES, the calculations indicate that the dominant products should be ³CH₂O₂ + H₂O with an energy barrier of 29.95 kJ/mol. The results obtained in this work enrich the theoretical information of the title reaction and provide guidance for analogous atmospheric chemistry reactions. © 2015 Wiley Periodicals, Inc.     

CIDNP 1H study of the photolysis of 7-sila- and 7-germa-norbornadienes

The photochemical decomposition of 7-sila- and 7-germa-norbornadienes (Ia,b) was studied by the CIDNP ¹H technique. The reactions proceeds by a two-step mechanism via the reversible formation of singlet biradicals, II. The triplet biradical (II), formed as a result of S-T conversion of (II)(S), irreversibly decomposes giving Me₂E (E = Si, Ge). The insertion of Me₂E into the CBr bond of PhCH₂Br and the SnCl bond of Me₃SnCl occurs via a radical mechanism, as deduced from the CIDNP effects observed in these reactions.     

Ab initio quantum chemical studies on the reactions of CF3O2 with OH

The potential energy surface for the CF₃O₂ + OH reaction has been theoretically investigated using the DFT (B3LYP/6-311G(d,p)) level of theory. Both singlet and triplet potential energy surfaces are investigated. The reaction mechanism on the triplet surface is simple. However, the reaction mechanism on the singlet surface is more complicated. It is revealed that the formation of CF₃O + HO₂ is the dominant channel on the triplet surface. The potential energy surface (PES) for this reaction has been given according to the relative energies calculated at the DFT/B3LYP/6-311G(d,p) level. Because this reaction involves both triplet and singlet states, triplet–singlet intersystem crossing (ISC) crossing also have been investigated in this paper.     

Oxidation of 7-ethyl-2,3,5,6,8-pentahydroxy-1,4-naphthoquinone (echinochrome A) by atmospheric oxygen 1. Structure of dehydroechinochrome

The nondestructive oxidation of 7-ethyl-2,3,5,6,8-pentahydroxy-1,4-naphthoquinone by atmospheric oxygen in the ground (triplet ³Σ_g^–) and excited (singlet ¹Δ_g) states in different solvents (acetone, dioxane, ethanol, aqueous ethanol, water) at room temperature involves the initial formation of 7-ethyl-5,6,8-trihydroxy-2,3-dioxo-2,3-dihydro-1,4-naphthoquinone (dehydroechinochrome) accompanied by the release of an H₂O₂ molecule into the reaction medium. Dehydroechinochrome, being highly susceptible to hydration, successively reacts with two H₂O molecules to form 7-ethyl-2,2,3,3,5,6,8-heptahydroxy-2,3-dihydro- 1,4-naphthoquinone as the relatively stable final product. In this form, it can be isolated from the mixture of reaction products.     

Singlet and Triplet Excited States and Intersystem Crossing in Free‐Base Porphyrin: TDDFT and DFT/MRCI Study

Extensive time-dependent DFT (TDDFT) and DFT/multireference configuration interaction (MRCI) calculations are performed on the singlet and triplet excited states of free-base porphyrin, with emphasis on intersystem crossing processes. The equilibrium geometries, as well as the vertical and adiabatic excitation energies of the lowest singlet and triplet excited states are determined. Single and double proton-transfer reactions in the first excited singlet state are explored. Harmonic vibrational frequencies are calculated at the equilibrium geometries of the ground state and of the lowest singlet and triplet excited states. Furthermore, spin–orbit coupling matrix elements of the lowest singlet and triplet states and their numerical derivatives with respect to nuclear displacements are computed. It is shown that opening of an unprotonated pyrrole ring as well as excited-state single and double proton transfer inside the porphyrin cavity lead to crossings of the potential energy curves of the lowest singlet and triplet excited states. It is also found that displacements along out-of-plane normal modes of the first excited singlet state cause a significant increase of the 2|H_so|S₁>, 1|H_so|S₁>, and 1|H_so|S₀> spin–orbit coupling matrix elements. These phenomena lead to efficient radiationless deactivation of the lowest excited states of free-base porphyrin via intercombination conversion. In particular, the S₁→T₁ population transfer is found to proceed at a rate of ≈10⁷ s⁻¹ in the isolated molecule.     

Spin non-conservation in low-energy molecular charge-transfer reactions

The reaction H₂O⁺(²B)+NO₂(²A) → H₂O(¹A) + NO₂⁺(¹Σ) occurs at near the collision rate constant 1.2 × 10^?9 cm³ s^?1, in spite of the fact that the reactants produce both a singlet and a triplet state and the products correlate only with the singlet state. This would be expected to yield a statistical weight factor of $\text{1}\text{4}$ to be multiplied by the collision rate constant to obtain the maximum charge-tranfer rate constant. The triplet products of the charge transfer are clearly endothermic. The singlet—triplet intersection has not been identified but the available information about the singlet and triplet states of the intermediate protonated nitric acid molecule is discussed. Four other examples of apparent “spin violation” charge-transfer reactions have been noted H₂O⁺ + NO, N₂O⁺ + NO.CO⁺ + NO and CH₄⁺ + O₂.     

Effect of the excitation wavelength and the structure of nitrated 1,2-dyhydroquinolines on dynamics of primary photophysical and photochemical processes

Dynamics of transformations of excited states and active transient species generated in the photolysis of nitrated 1,2-dihydroquinolines (N-DHQ) has been studied by femto- and nanosecond laser pulse photolysis. Spectral and kinetic parameters of primary photophysical and photochemical processes have been determined, and their dependence on the substituent position at the aromatic ring of 1,2-dihydroquinoline (DHQ) and on the wavelength of excitation light has been established. The lifetime of the excited singlet state S₁ in N-DHQ is ca. 100 and 500 fs for 8- and 6-nitro-substituted DHQ, respectively, which is shorter in comparison with DHQ without the nitro group by a factor of 10⁴ and more. The major decay channel of the S₁ state is the successive formation of three transient species with lifetimes of 0.5 to 16 ps. A triplet state is generated only upon excitation of the short-wavelength band by UV light. The quantum yield of the triplet state depends on the structure of N-DHQ.     

Synthesis,photophysical and photochemical properties of tetra- and octa-substituted gallium and indium phthalocyanines

The synthesis, photophysical and photochemical properties of the tetra- and octa-[4-(benzyloxyphenoxy)] substituted gallium(III) and indium(III) phthalocyanines are reported for the first time. The new compounds have been characterized by elemental analysis, IR, ¹H NMR spectroscopy and electronic spectroscopy. General trends are described for quantum yields of photodegredation, fluorescence quantum yields and lifetimes, triplet lifetimes and triplet quantum yields as well as singlet oxygen quantum yields of these compounds in dimethylsulfoxide (DMSO). Substituted indium phthalocyanine complexes (7b–9b) showed much higher quantum yields of triplet state and shorter triplet lifetimes, compared to the substituted GaPc derivatives due to enhanced intersystem crossing (ISC) in the former. The gallium and indium phthalocyanine complexes showed phototransformation during laser irradiation due to ring reduction. The singlet oxygen quantum yields (Φ_Δ), which give an indication of the potential of the complexes as photosensitizers in applications where singlet oxygen is required (Type II mechanism) ranged from 0.51 to 0.94. Thus, these complexes show potential as photodynamic therapy of cancer.     

Fullerene C60 Bound to Insoluble Hydrophilic Polymer: Synthesis,Photophysical Behavior,and Generation of Singlet Oxygen in Water Suspensions

Fullerene C₆₀ has been covalently bound to an insoluble hydrophilic polymeric matrix: Sephadex ^® G‐200. The new polymeric equivalent of C₆₀ swells in H₂O to form gel‐like suspensions. The transient photochemical behavior of this polymeric fullerene has been studied in dry and H₂O‐suspended samples. Both samples show a transient absorption similar to the absorption of the parent C₆₀ solution. There is a lack of triplet‐triplet annihilation and of a O₂‐quenching process in the dry sample. On the contrary, the O₂‐quenching process is very efficient in the H₂O‐suspended samples (k_q(O₂)=(1.9±0.5)×10⁸ dm³ mol⁻¹ s⁻¹) and results in the formation of singlet oxygen, which is detected by its emission at 1270 nm. These results point to this hydrophilic polymeric equivalent of C₆₀ as a good candidate for use as a singlet‐oxygen solid sensitizer in H₂O suspensions.