Ab initio study on mechanisms of singlet oxygen reaction with ethylene

The reaction of singlet oxygen with ethylene has been studied at the ab initio level with both HF/3-21G and HF/6-31G basis sets, fully optimizing the geometries of the critical points. The transition state leading to the intermediate peroxirane from the initial reactants is found,Iocated at 81.71 kJ/mol above the dissociation limit. The vibrational analysis is done with two basis sets. From the results it can be seen that the frequencies have not been made an improvement obviously with augmented 6-31G compared to 3-21G basis; it follows that main reason for too high HF/3-21G frequency could mainly be the vibrational anharmonicity. The eigenvector corresponding to the single imaginary vibrational frequency is dominated by the larger O-O distance. The finding of the transition state confirms that the peroxirane minimum can be reachable passing through a peroxirane-like saddle point. In addition , the mechanisms of the reaction forming oxirane are discussed as well.     

Induced ring currents and aromaticity of planar singlet π-electronic monocyclic carbenes

The induced -electronic ring currents and the relative diamagnetic susceptibilities of planar singlet -electronic monocycic carbenes with the general formula C_nH_2n–1(n = 3–10) were calculated within the framework of a linked variant of perturbation theory using the PPP method; this permitted their inclusion in the general aromaticity-antiaromaticity scale of organic compounds. It was shown that these carbenes can act as intermediates in a number of thermal rearrangements.Translated from Teoreticheskaya i Éksperimental'yana Khimiya, Vol. 29, No. 1, pp. 4–48, January–February, 1993.     

Theoretical study of some bis-verdazyl diradicals: singlet–triplet energy gap

The bis-verdazyl diradical (BVD) system is closely examined by using the multiconfiguration wavefunctions as well as the density functional theory (DFT). The totally symmetric singlet ground state turns out to have strong multiconfiguration character at all levels of theory. The singlet ground state takes on the planar structure while the most stable triplet state corresponds to the twisted form. The MCSCF+MCQDPT2 calculations are shown to be sufficient to predict the singlet–triplet energy gap which is insensitive to the electronic characters of the ring substituents.     

Modulating the singlet oxygen generation property of meso-β directly linked BODIPY dimers

We report the synthesis, X-ray analysis and singlet oxygen generation properties of a set of meso-β directly linked BODIPYs with the meso-aryl group (φ(1)) and meso-BODIPY component (φ(2)) free to rotate or constrained.     

Diastereoselective carbenes: Stereoelectronic control of bent singlet trans-2′-substituted cyclobutylcarbenes

The UM06-2X/aug-cc-pVTZ//UM06-2X/6-311G(d) theoretical models of representative singlet trans-2′-substituted cyclobutylcarbenes were computed. The :CH-group of carbenes with an ERG is bent toward the c-C₄H₇-ring’s proximal C1′–C2′ single bond. The :CH-group of carbenes with an EWG is bent toward the c-C₄H₇-ring’s distal C1′–C4′ single bond. The contrasting stereoelectronic effect was computationally tested using trimethylsilyl vs. bromo substituents to determine if exo-1- and exo-2-substituted bicyclo[2.1.0]pentanes would be produced in diastereomeric excess.     

Electronic structure and aromaticity of planar singlet π-electron monocyclic π2 carbenes

Induced -electron ring currents and relative diamagnetic susceptibilities of planar singlet -electron monocyclic ² carbenes are calculated in the coupled variant of perturbation theory in the CNO method. It is shown that the parity theorem holds for charged planar ² and ² carbenes formally generated by the detachment of a proton or a hydride ion from alternant hydrocarbons. This leads, in such carbenes, to the same energies of singlet-singlet and singlet-triplet transitions, electric polarizabilities, conjugation energies, -electron ring currents, diamagnetic susceptibilities and current contributions to chemical shifts, whereas the charge contributions to the latter are of opposite signs.Donbas State Academy of Building and Architecture, 1 Derzhavina St., 339023 Makeevka, Donetsk Region, Ukraine. Translated from Teoreticheskaya i Éksperimental'naya Khimiya, Vol. 32, No. 3, pp. 157–162, May–June, 1996. Original article submitted December 5, 1994.     

Quantum-chemical study of the singlet potential energy surface of the nitrene—dioxygen system

The potential energy surfaces of the HN—O₂ and PhN—O₂ systems were calculated by the MP2 and B3LYP methods. The mechanism of photooxidation of azides was refined. Photooxidation produces the nitrene—O₂ adducts with dioxaziridine and non-cyclic structures. The parameters of IR spectra of the adducts were calculated. The rearrangement of dioxaziridine to a nitro compound is likely a reason for chemiluminescence accompanying the photooxidation of azides.     

Chiroptical spectra of bicyclic selenolactams. Optical activity of the singlet–triplet transition

Two selenolactams with rigid 2-azabicyclo[2.2.1]heptane skeletons were prepared by reaction of the corresponding lactams with P₄Se₁₀. A comparison of their UV–vis and CD spectra with those of the carbonyl and thiocarbonyl analogues showed a similar character of the lowest-energy electronic transitions and the same signs of the corresponding Cotton effects. The MCD spectra of selenolactams revealed that their n–π* absorption band is dominated by the singlet–triplet component. A very weak CD corresponding to this component has an opposite sign to its much stronger singlet–singlet counterpart observed at the blue edge of the n–π* band.     

O?+?C2H4 potential energy surface: lowest-lying singlet at the multireference level

Extensive density functional theory (DFT) calculations have been performed to develop a force field for the classical molecular dynamics (MD) simulations of various azobenzene derivatives. Besides azobenzene, we focused on a thiolated azobenzene’s molecular rod (4′-{[(1,1′-biphenyl)-4-yl]diazenyl}-(1,1′-biphenyl)-4-thiol) that has been previously demonstrated to photoisomerize from trans to cis with high yields on surfaces. The developed force field is an extension of OPLS All Atoms, and key bonding parameters are parameterized to reproduce the potential energy profiles calculated by DFT. For each of the parameterized molecule, we propose three sets of parameters: one best suited for the trans configuration, one for the cis configuration, and finally, a set able to describe both at a satisfactory degree. The quality of the derived parameters is evaluated by comparing with structural and vibrational experimental data. The developed force field opens the way to the classical MD simulations of self-assembled monolayers (SAMs) of azobenzene’s molecular rods, as well as to the quantum mechanics/molecular mechanics study of photoisomerization in SAMs.     

The acid—base equilibria of dimethylphenols in the ground and the first excited singlet states

Measurements of the acidity constants in the ground (pK_a(S₀)) and the lowest excited singlet (pK_a(S₁)) states for 2,3-, 2,4-, 2,5-, 2,6-, 3,4- and 3,5-dimethylphenols in aqueous solution have been carried out spectrophotometrically at 25°C. The pK_a values in S₀ have been derived from the absorption spectra and the pK_a values in S₁ were estimated by means of the Forster cycle. It is found that the hydroxy group is more acidic in the first excited singlet than in the ground state.     

Synthesis of 2-aryl-3H-indol-3-ones via trapping reaction in singlet oxygenation of 2-arylindoles

2-Aryl-3H-indol-3-ones (3a-o) have been synthesized via methanol trapping reaction in singlet oxygenation of 2-arylindoles (1a-o), followed by thermal decomposition of the resultant 2-aryl-2-methoxy-3-oxo-2,3-dihydroindoles (2a-o) in good yields.     

Effect of secondary MO interaction on the regiospecificity in the reactions of α-ionones with singlet oxygen

The eme reactions of α-ionones with singlet oxygen were examined to ascertain theeffect of secondary Me interaction between the reactants on the reaction regiospecificity.Exclusive formation of 3-hydroxy-γ-ionones found in the reactions reveals favorable interaction ofsinglet oxygen with the acyclic α-hydrogen atom,On the other hand,no formation of 3-hy-droxy-β-ionones implies that the steric requirement was not met for the bond formation betweenzwitterionic perepoxide with C_1-hydrogen in the process.The MMP21 and MNDO calculationsindicate minus value of the secondary interaction energy for the aeyelie α-hydrogen abstrac-tion and a repulsion between the oxygen with C_1-hydrogen atom.Twisting tilting of the doublebond may account for favorable attack of singlet oxygen on C_3.An explanation of the excellentregiospecificity was addressed and placed in proper mechanic prospective.     

Styrylbenzimidazolium dye–borate complex as an effective,singlet state photoinitiator in an argon laser-induced TMPTA photopolymerization

Dye photoinitiators consisting of styrylbenzimidazolium cations, acting both as light absorbers and as electron acceptors and n-butyltriphenylborate anions being electron donors were tested in order to reveal the effect of a dye structure on the efficiency of multiacrylate photoinitiated polymerization. The efficiency of tested systems depends on the ΔG_el of electron transfer between borate anion and hemicyanine cation. The latter value was experimentally determined for 9 photoredox pairs. The relationship between the rate of polymerization and the free energy of activation of electron-transfer reaction shows the dependence predicted by the classical theory of electron-transfer phenomena. The kinetic studies clearly demonstrated that the styrylbenzimidazolium borate photoredox pairs are very promising photoinitiators for multiacrylates free radical polymerization. They initiate the polymerization reaction with the efficiency comparable to RBAX, Rose Bengal derivative, common triplet state initiator.     

Access to a stable Si2N2 four-membered ring with non-Kekulé singlet biradical character from a disilyne

The reactions of 1,1,4,4-tetrakis[bis(trimethylsilyl)methyl]-1,4-diisopropyltetrasila-2-yne 1 with an equivalent amount of trans- and cis-3,3',5,5'-tetramethylazobenzenes produced a Si(2)N(2) four-membered ring biradicaloid [RSi(μ-NAr)(2)SiR] 2 (R = Si(i)Pr[CH(SiMe(3))(2)](2), Ar = 3,5-Me(2)C(6)H(3)), which was isolated as air- and moisture-sensitive dark purple crystals. Compound 2 displays no EPR signal, and the molecular structure of 2 was characterized by NMR spectroscopy and X-ray crystallography, revealing that 2 has a planar centrosymmetric Si(2)N(2) four-membered ring. The Si1-Si1' distance is 2.63380(9) ?, and there is no bond interaction between the Si1 and Si1' atoms of 2. The reactions of 2 with methanol and carbon tetrachloride show that 2 has both closed-shell and radical-type reactivity.     

Cycloaddition of alkynes RCCR′ with free singlet dimethyl germylene Me2Ge

Alkynes RCCR′ exhibit a high and specific reactivity towards free dimethyl germylene Me₂Ge as a typical singlet heavy carbene analogue. New and novel unsaturated² 4-,5-, and 6-membered germa heterocycles with 1–3 Ge atoms are formed, dependent on R and R′ in the 7 alkynes used. Corresponding reaction mechanisms are proposed.     

Solvent- and temperature-controlled inversion of π-facial selectivity in the 1,2-cycloaddition of singlet oxygen to hydroxyphenyl-substituted cyclohexadihydrofurans

Singlet-oxygenation of 3-hydroxyphenyl-substituted dihydrofurans fused with a cyclohexane 1a?c exclusively gave the corresponding syn/anti-stereoisomeric mixtures of dioxetanes 2a?c. The syn/anti-π-facial selectivity in the 1,2-cycloaddition of singlet oxygen (¹O₂) was found to be remarkably sensitive to the solvent as well as the reaction temperature. In fact, the solvent effect was so conspicuous that inversion of the syn/anti-π-facial selectivity was observed in different solvents, such as chloroform and toluene. An LSER (linear solvation energy relationships) analysis suggested that the Lewis-acidity/basicity and HBD (hydrogen-bond donor)/HBA (hydrogen-bond acceptor) ability as well as dipolarity/polarizability of the solvent played an important role in this change in syn/anti-π-facial selectivity. An investigation of the temperature-dependency of the singlet-oxygenation suggested that the syn/anti-π-facial-selective 1,2-cycloaddition of ¹O₂ to 1 was a conformationally-(entropy-) controlled process.     

Theoretical study on the singlet and triplet potential energy surfaces of NH (X3Σ−) + HCNO reaction

Both the singlet and triplet potential energy surfaces (PESs) of the NH (X³Σ^?) + HCNO reaction have been investigated at the BMC-CCSD level based on the UB3LYP/6-311++G(d, p) structures. The results show that the title reaction is more favorable through the singlet potential energy surface than the triplet one. For the singlet potential energy surface of the NH (X³Σ^?) + HCNO reaction, the most feasible association of NH (X³Σ^?) with HCNO is found to be a non-barrier nitrogen-to-carbon attack forming the adduct a (trans-HNCHNO), which can isomerize to the adduct b (cis-HNCHNO). The most feasible channel is that the 1, 3-H shift with N2–H2 and C–N1 bonds cleavage associated with the N1–H2 bond formation of adduct a leads to the product P ₁ (HCN + HNO). Moreover, P ₂ (HNC + HNO) should be the competitive product. The other products, including P ₃ (NH₂ + NCO) and P ₄ (N₂H₂ + CO), are minor products. The product P ₁ can be obtained through two competitive channels Path 1: R → a → P ₁ and Path 3: R → b → d → P ₁ , whereas the product P ₂ can be formed through Path 2: R → b → d → P ₂ . At high temperatures, the nitrogen-to-nitrogen approach may become feasible. For the triplet potential energy surface of the NH (X³Σ^?) + HCNO reaction, the Path 10: R → ³ a → ³ a ₁ → P ₁ should be the most feasible pathway due to the less reaction steps and lower barriers. These conclusions will have impacts on further experimental investigations.