Triplet Diradicals Research Articles

ChemInform Abstract: Benzophenone-Sensitized Two-Photon Chemistry of Azoalkanes in the “ Laser/Liquid Jet”: Evidence for Photoinduced Hydrogen 1,2-Shift in 1,3- Cyclopentadiyl Triplet Diradicals.

Abstract While the triplet-sensitized photolysis of the bicyclicazoalkanes 1 led on denitrogenation to the corresponding bicycloalkanes 2, the laser/liquid jet photochemistry gave additionally also the cyclopentenes 3 by hydrogen 1,2-shift; the amount of the latter increased with increasing lifetime of the intermediary cyclopentadiyl triplet diradicals 4, which suggests that two-photon chemistry may operate.

ChemInform Abstract: The EPR Spectroscopic D Parameter of Localized Triplet Diradicals as Probe for Electronic Effects in Benzyl-type Monoradicals

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ChemInform Abstract: The EPR D Parameter as a Measure of Spin Delocalization in Cyclopentane-1,3-diyl Triplet Diradicals with Extended Conjugation π-Type Substituents.

π-Substituted cyclopentane-1,3-diyl triplet diradicals 2 were readily prepared from the corresponding azoalkanes 1 by photodenitrogenation in a 2-methyltetrahydrofuran matrix at 77 K. The D values of these triplet diradicals, obtained from the EPR spectra under matrix isolation, depend on the π substituent in the order butadienyl > 2-anthryl > vinyl > 2-naphthyl > phenyl. Good linear correlations of the D values have been obtained with the reported hyperfine coupling constants (α-hfc), with the ab initio (B3LYP) spin densities for the corresponding monoradicals 3, and with the radical stabilization energies (RSE). The present results for these extended π systems are compared with those for previously reported heteroaryl-substituted triplet diradicals.

Very Strongly Ferromagnetically Coupled Diradicals from Mixed Radical Centers. II. Nitronyl Nitroxide Coupled to Tetrathiafulvalene via Spacers

We predict large and positive intramolecular magnetic exchange coupling constants (J) for coupled diradicals constructed from nitronyl nitroxide and tetrathiafulvalene monoradical moieties. These diradicals have the general formula TTF-coupler-NN, where the couplers are mostly aromatic systems. Unrestricted density functional methodology (UB3LYP) has been used to optimize the molecular geometries of the triplet diradicals using the 6-311 g(d,p) basis set. This has been followed by single-point UB3LYP calculations for triplet and broken symmetry (BS) states using 6-311++g(3df,3pd) basis and the optimized triplet geometries. We find that the species comprising of ethylene (geminal coupling) and pyridine as couplers have singlet ground states whereas the other species have triplet ground states. These findings are in support of the spin alternation rule. The largest J value we predict is 648.6 cm(-1) for the molecule with the spacer pyrrole. We also determine the percent weightings of triplet and singlet components in the BS state, estimate the diradical nature, and calculate the relative weights of different singlet and triplet component functions in the BS solution in each case.

Electronic structure of the members of the electron transfer series [NiL]z (z = 3+, 2+, 1+, 0) and [NiL(X)]n (X = Cl, CO, P(OCH3)3) species containing a tetradentate, redox-noninnocent, Schiff base macrocyclic ligand L: an experimental and density functional theoretical study

The electronic structure of the four members of the electron transfer series [NiL](z) (z = 3+, 2+, 1+, 0) have been established experimentally (EPR spectroscopy and X-ray crystallography) and by density functional theoretical (DFT) calculations using the B3LYP functional in conjunction with a conductor-like screening model (COSMO) for acetonitrile solvent effects. L represents a generic designation of the tetradentate macrocycle 2,12-dimethyl-3,7,11,17-tetraazabicyclo[11.3.1]-heptadeca-1(17),2,11,13,15-pentane where the true oxidation level is not specified; (L(Ox))(0) represents its neutral form, (L )(1-) is the one-electron reduced pi radical anion, and (L(Red))(2-) is the singlet (or triplet) diradical dianion of this ligand. It is shown that the above series consists of square planar [Ni(III)(L(Ox))](3+) (S = 1/2), [Ni(II)(L(Ox))](2+) (S = 0), [Ni(II)(L )](1+) (S = 1/2), [Ni(II)(L(Red))](0) (S = 0). The structure of [Ni(II)(L(Red))](0) has been determined by X-ray crystallography. The electrochemistry of [Ni(II)(L(Ox))](PF(6))(2) in the presence of hard chloride anions shows the presence of trans-[Ni(III)(L(Ox))Cl(2)](+), the EPR spectrum of which has been recorded and calculated, and of trans-[Ni(II)(L(Ox))Cl(2)](0) (S = 1). Upon further reduction the coordinated Cl(-) ligands dissociate and [Ni(II)(L )](1+) and [Ni(II)(L(Red))](0) are successively generated. Similarly, in the presence of good pi-acceptor ligands such as CO or P(OCH(3))(3) the following five-coordinate, square base pyramidal species are found to be stable: [Ni(I)(L(Ox))(X)](1+) (S = 1/2), [Ni(I)(L )(X)](0) (S = 0, 1) (X = CO, P(OCH(3))(3)). As shown by EPR spectroscopy in the work of J. Lewis and M. Schröder, J. Chem. Soc., Dalton Trans., 1982, 1085, the monocations consist of a central nickel(i) ion (d(9), S(Ni) = 1/2). These spectra have been faithfully reproduced by the calculations. The neutral complexes [Ni(I)(L )(X)](0) are singlet or triplet diradicals comprising a central nickel(i) ion and a pi radical anion (L )(1-). Interestingly, six-coordinate species trans-[Ni(L)(X)(2)](n) (n = 2+, 1+, 0) are computationally not stable in the gas phase or in solution. No experimental evidence has been found for their existence.

Theory of chemical bonds in metalloenzymes XIII: Singlet and triplet diradical mechanisms of hydroxylations with iron‐oxo species and P450 are revisited

AbstractElectronic structures of the Compound I (CpdI) in P450 are investigated on the basis of spin coupling forms of iron‐oxo (Fe(IV)O) cores and radical ligand (•L) groups to generalize previous singlet and triplet diradical (TD) mechanisms for oxygenations of alkanes with Fe( IV)O. Orbital interaction schemes for four lower‐lying spin configurations of CpdI with HC bond of substrate are examined to elucidate how magnetic coupling modes correlate with radical reaction pathways for hydroxylation reactions on the basis of the broken symmetry (BS) molecular orbital (MO) model. The configuration correlation diagrams for the four configurations model are depicted on the basis of the isoelectronic analogy among O, O2, and Fe( IV)O, in addition to Coulomb exchange energy on the iron site, which determines its local spin configuration. Important role of ligand spin (•L) of CpdI for regulation of hydroxylation mechanisms is clarified with the aid of the spin coupling forms. Transition states for one quartet and three doublet configurations under the BS MO approximation are examined on the basis of potential curve crossings along reaction pathways. The four transition structures and corresponding radical intermediates for methane and trimethyl methane with CpI are located by the BS hybrid Kohn–Sham density functional theory (DFT) (B3LYP) method to confirm the orbital interaction schemes. Spin density populations obtained by the BS B3LYP calculations are found to be consistent with the theoretical predictions based on the four configurations model. The configuration and state correlation diagrams by BS B3LYP before and after spin projection are also consistent with the BS MO interaction schemes, which provide local SD and TD mechanisms of hydroxylation with CpdI. The present BS MO‐theoretical framework is useful for systematic understanding of a lot of recent BS hybrid DFT computational results for hydroxylation reactions with CpdI and configuration correlation diagrams reported by several groups. Implications of the present theoretical and computational results are discussed in relation to several experimental characteristics of hydroxylation reactions with iron‐oxo species and P450. © 2009 Wiley Periodicals, Inc. Int J Quantum Chem, 2009

Non-Kekulé N-Substituted m-Phenylenes: N-Centered Diradicals versus Zwitterions

The relationship between the structures and electronic ground state properties of non-Kekulé-type N-substituted m-phenylenes was studied utilizing density functional theory (DFT), with the aim of determining the factors that lead to ground state triplet diradicals. At B3LYP/6-311G(d,p)//B3LYP/6-311G(d,p) we identified octahydropyridoquinoline with an exceptionally large singlet-triplet energy separation of +27.9 kcal mol(-1), in favor of the triplet. The high-spin structures can readily be obtained by interruption of the full cyclic pi-delocalization that avoids cross-conjugation of the nitrogen radical centers. Introduction of additional heteroatoms, on the other hand, preferentially stabilizes the singlet zwitterionic resonance contributors in these systems. The identified diradicals show strong ferromagnetic exchange interactions between two radical centers.

Coupling of Fischer carbene complexes with conjugated enediynes featuring radical traps: Novel structure and reactivity features of chromium complexed arene diradical species

The reaction of Fischer carbene complexes with conjugated enediynes that feature a pendant alkene group has been examined. The reaction proceeds through carbene–alkyne coupling to generate an enyne–ketene intermediate. This intermediate then undergoes Moore cyclization to generate a chromium complexed arene diradical, which then undergoes cyclization with the pendant alkene group. The radical cyclization prefers the 6-endo mode unless radical-stabilizing groups are present to favor the 5-exo mode. The intermediate diradical species were evaluated computationally in both the singlet and triplet configurations. Arene triplet diradicals feature minimal spin density at oxygen and delocalization to chromium. The 6-endo cyclization product was kinetically and thermodynamically favored.

Exo-selective formation of bicyclic oxetanes in the photocycloaddition reaction of carbonyl compounds with vinylene carbonate: the important role of intermediary triplet diradicals in the stereoselectivity

The photochemical (2+2) cycloaddition reaction, the Paterno-Buchi (PB) reaction, of carbonyl compounds 1a-d with vinylene carbonate (2) was used to synthesize oxetanodioxolanones 3. The reaction involves the exo-selective (exo/endo > 83/17) formation of the bicyclic oxetanes 3b-d. Computational studies were performed to reveal the equilibrium structures of the intermediary triplet diradicals and their conformational distribution, which play an important role in determining the stereoselectivity of such PB reactions.

Notable substituent and temperature effects on the regioselectivity and the efficiency in Paternò-Büchi reaction of 4,4′-disubstituted benzophenones with 1,3-dimethyluracil and 1,3-dimethylthymine

The photochemical [2 + 2] cycloadditions (the so-called Paternò-Büchi (PB) reaction) of 1,3-dimethyluracil (DMU) and 1,3-dimethylthymine (DMT) with benzophenone ( 1d) and its derivatives (BPs) with 4,4′-disubstituted groups including CH 3O ( 1a), CH 3 ( 1b), t-Bu ( 1c), F ( 1e), Cl ( 1f) and CN ( 1g) generate two series of regioisomeric oxetanes, A and B, and reveal notable substituent effects on the regioselectivity ( A: B) and the photochemical efficiency, that is a higher ratio of A: B and lower efficiency for the BPs with electron-donating groups (EDGs), and a contrary effect for the BPs with electron-withdrawing groups (EWGs), such as A: B ≥ 95:5 for 1a to 32:68 for 1g in DMU photochemical system. The PB reactions of DMU with three BPs ( 1d, 1e and 1g) were performed at various temperatures, and temperature effects on the regioselectivity and the efficiency were observed. The ratio of photoproducts ( A: B) in the DMU systems decreases with increasing the reaction temperature, such as DMU– 1d system, ranging from 75:25 at −30 °C to 55:45 at 60 °C. On the basis of computational studies, the substituent effects were relationalized in terms of stability and charge distribution of intermediary triplet diradicals. The EWGs can stablize triplet diradicals especially the diradicals causing oxetanes B, and a destablization for EDGs. It was demonstrated that temperature effects originate from the conformational properties of intermediatary triplet diradicals.

Captodative Stabilization in Geminally Substituted Allylic Radicals Assessed by Means of the EPR‐Spectral D Parameter of 1,3‐Cyclopentanediyl Triplet Diradicals

AbstractThe EPR‐spectral zero‐field parameter D of low‐temperature persistent triplet diradicals T was used to assess the electronic substituent effects in EA/ED and ED/ED geminally substituted allylic radicals. From these EPR data, the interaction parameter (ΔS) was calculated for a set of doubly functionalized radicals, which expresses the ability of the unpaired electron to interact either individually with the functional groups or simultaneously in either a synergistic or antagonistic way. We show that the ED/ED combinations interact antagonistically with the odd electron, whereas the R2N/EA and the MeO/EA pairs display a pronounced captodative delocalization. Compared to MeO/EA, the MeS/EA functionalities manifest just an additive effect of the individual substituents. These results are rationalized in terms of appropriate electron‐transfer‐type mesomeric structures. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2004)

Mechanism of stereo- and regioselectivity in the Paternò-Büchi reaction of furan derivatives with aromatic carbonyl compounds: importance of the conformational distribution in the intermediary triplet 1,4-diradicals.

Temperature and substituent effects on the stereo- and regioselectivity have been investigated in the photochemical [2 + 2] cycloaddition reaction, the so-called Paternò-Büchi (PB) reaction, of unsymmetrically substituted furans 2a,b (2-methyl- and 3-methylfuran) with aromatic carbonyl compounds 1a,b (benzaldehyde and benzophenone). The regio-random but stereoselective (exo/endo > 97/3) formation of lower substituted oxetane 3a and higher substituted oxetane 4a is found in the reaction with benzaldehyde (1a). The exclusive stereoselectivity is not dependent on the position of methyl substituent on the furan ring and the reaction temperature. The double-bond selection (3a versus 4a) is slightly dependent on the reaction temperature (3a/4a = 55/45 to 40/60). The Eyring plots of the regioselectivity are linear. Contrastively, in the reaction with benzophenone (1b), the double-bond selection (3b versus 4b) largely depends on the reaction temperature. The Eyring plots are not linear, but the inflection points are observed. The transient absorption spectroscopic analyses (picosecond time scale) clarify the intervention of triplet 2-oxabutane-1,4-diyls in the photochemical processes. Computational studies reveal the equilibrium structures of the triplet diradicals, energy barriers between the conformers, and the equilibrium constants. A rational mechanism is herein proposed by the support of both experimental and computational investigations to account for not only the exclusive formation of the exo-configured oxetanes 3a and 4a but also the nonlinear Eyring plots observed in the reaction with 1b.

Singlet Oxygen Quenching Effect of Quercetin in Erythrosine-Sensitized Photooxidation of Oil-in-Water Emulsion

Oxidation reaction can be initiated by either diradical triplet oxygen or non radical singlet oxygen. The singlet oxygen can be formed infoods from triplet oxygen by photosensitized reaction. This research was intended to study the quenching effect of quercetin on lipid oxidation rate in the erythrosine-sensitized photooxidation of oil-in-water emulsion. Palm oil-in-water emulsion, containing erythrosine 100 ppm and quercetin 0, 25, 50, 75 and 100 ppm, were prepared with polyoxyelhylene 100 stearyl ether (Brij 700) or polyoxyethylene sorbitan monolaurate (Tween 20). Structurally Brij 700 has 5 times longer polyoxyethylene groups than Tween 20. The mixture were stored under 4000 luxfluorescent light for 10 h and peroxide values were measured at 2 h interval. Erythrosine effectively sensitized the photooxidation of palm oil-in-water emulsion, as expected. Lipid oxidation rates, as determined by the formation of lipid hydroperoxides and headspace oxygen, in palm oil-in-water emulsions containing erythrosine decreased with increasing quercetin concentration. At pH 3, the peroxide value was higher than at pH 7. Brij 700 decreased production of lipid hydroperoxides from palm oil-in-water-emulsions compared to emulsions stabilized by Tween 20. The results indicate that quercetin is an efective singlet oxygen quencher in palm oil-in-water emulsion and the surfactant headgroup size could be an important determinant in the oxidative stability of food emulsions.

Enhanced formation of inverted housane through steric effects by rotationally unsymmetric bridgehead substituents in the ring closure of triplet cyclopentane-1,3-diyl diradicals, generated photolytically from 2,3-diazabicyclo[2.2.1]heptene(DBH)-type azoalkanes

The photolysis of the cyclopentene-annelated DBH-type azoalkanes 1c,d with the n-propyl and acetoxymethyl substituents at the bridgehead positions affords under singlet conditions (high-temperature direct photolysis) predominantly the retained housanes anti-2c,d, while under triplet conditions (low-temperature direct or benzophenone-sensitized photolysis) the inverted diastereomer syn-2c,d is favored. The latter novel stereochemical feature of the triplet pathway is rationalized in terms of the unsymmetric nature of the n-propyl and acetoxymethyl substituents in regard to rotation about the bridgehead position of the planar cyclopentane-1,3-diyl triplet diradicals 33DR(c,d). In their lowest-energy conformation (AM1 calculations), the rotationally unsymmetric substituents prefer to be located opposite to the annelated cyclopentene ring. The ratio of the diastereomeric syn and anti housanes is determined by the equilibrium distribution of the bridgehead-substituent conformers for the triplet diradicals on ring closure after ISC.

Spin delocalization by triple-bonded functionalities in propargyl and heteropropargyl radicals, assessed from the EPR-spectral D parameter of 1,3-cyclopentanediyl triplet diradicals.

The cyclopentane-1,3-diyl triplet diradicals T and T' with the triplet-bonded acetylene, cyano, and isocyano functionalities at one of the radical sites are readily prepared from the corresponding azoalkanes by photodenitrogenation in a 2-methyltetrahydrofuran (2-MTHF) matrix at 77 K. The EPR-spectral D values of these triplet diradicals show that the spin delocalizing ability of the triple-bonded pi substituent follows the order -Ctbd1;CH > -NC approximately -CN. Good correlations of the D values have been obtained with the hyperfine coupling constants (a(H)) and with the calculated spin densities (PM3/AUHF-CI method) of the corresponding monoradicals M. The propargyl-type mesomeric structure is favored over the allenyl-type contributor for all three triple-bonded functionalities; spin delocalization is less pronounced in the heteropropargyl derivatives due to the electronegativity effect of the nitrogen atom.

Chemistry and Reaction of Singlet Oxygen in Foods.

Singlet oxygen is a highly reactive, electrophilic, and nonradical molecule. It is different from diradical triplet oxygen in its electron arrangement. Photosensitizers can form singlet oxygen from triplet oxygen in the presence of light. The reaction rate of singlet oxygen with foods is much greater than that of triplet oxygen due to the low activation energy. Singlet oxygen oxidation produces undesirable compounds in foods during processing and storage. However, carotenoids and tocopherols in foods can minimize singlet oxygen oxidation. The in-depth scientific knowledge on the formation, reactions, quenching mechanisms, and kinetics of singlet oxygen can greatly improve the quality of foods by minimizing the oxidation during processing and storage. The single oxygen oxidation of foods has contributed to the explanation of several important chemical reactions in the reversion flavor in soybean oil, sunlight flavor in milk products, and the rapid losses of vitamin D, riboflavin, and ascorbic acid in milk under light storage.

Spin delocalization in 1-heteroallyl monoradicals as a measure of radical stabilization by heterovinyl substituents assessed through the EPR-spectral zero-field D parameter of 1,3-cyclopentanediyl triplet diradicals.

The EPR-spectral zero-field splitting parameter D of the localized heterovinyl-substituted 1,3-cyclopentanediyl triplet diradicals T, generated in a 2-methyltetrahydrofuran (MTHF) glass matrix at 77 K through the photochemical deazetation of the corresponding azoalkanes 1-13, is a quantitative measure of the spin-density (rho) variation by the substituents at the radical site in the 1-heteroallylic radicals. From these data, the radical-stabilizing ability of a variety of nitrogen-containing groups has been assessed, which includes imino and hydrazonyl functionalities. The radical stabilization in the heteroallylic radical fragment follows the order X = O < NMe < CH2 < CHMe << NOH approximately NOMe << NNHCHO approximately NNHC(O)NH2 < NPh approximately NNMe2 < NNH2 < CHPh < NNHPh. The lowest D values have been found for the hydrazonyl-substituted derivatives, which implies the lowest spin density at the carbon center and, thus, the most efficacious radical stabilization through spin delocalization. This superdelocalization may be rationalized in terms of nitrogen-centered radical-cationic structures. Localization of the spin at the terminal atom is resisted through the electronegativity effect (O < N < C).

Relative Stabilites of the Crossed vs. Linear σ-Conjugated Group 14 Triplet Diradicals E4H8 and E5H10 (E = C, Si, Ge, Sn): Orbital Phase Predictions

Abstract Orbital phase theory was applied to predict that the cross σ-conjugated isomers of acyclic Group 14 triplet diradicals E4H8 1 and E5H10 3 (E = C, Si, Ge, Sn) are more stable than the linear ones, 2 and 4, respectively. This prediction was confirmed by ab initio molecular orbital (MO) and density functional theory (DFT) calculations. It was also found that the radicals are preferentially in conjugation with the C-H bond for E = C (1a and 2a), but with the E-E bond for E = Si, Ge, Sn.

Theoretical investigations on bond enthalpy additivity deviations in the formation of triplet diradicals from hetero-substituted 1,3-dimethylenecyclobutanes

In our continuing effort to understand better the cause of the large positive deviation from bond enthalpy additivity (BEA) observed in the formation of triplet 1,3-dimethylenecyclobutane-2,4-diyl (1c) from 1,3-dimethylenecyclobutane (1a), we performed B3LYP/6–31G* calculations on a series of 1,3-disubstituted cyclobutanes, where the methylene groups in 1a are replaced by hetero groups, specifically O, NH, S, PH and SiH2. The size of the deviation from BEA (ΔBEA) is found to increase consistently with decreasing bond strength of the exocyclic π bonds. DFT calculations were also carried out on related hydrocarbons and the ΔBEA value in each case was found to be in close agreement with that predicted by simple Hückel theory. All of the computational results provide convincing evidence that antiaromaticity is absent in these diradicals, and the positive deviations from BEA, as large as 30.3 kcal mol−1, are due to the more substantial increase in the π electron delocalization in the monoradicals than that in the diradicals. Copyright © 2001 John Wiley & Sons, Ltd.

Synthesis and characterisation of some novel hetero-diradicals containing linked hydrazyl and aminoxyl (nitroxide) moieties

We report the synthesis and characterisation of some novel hydrazine-derived monoradicals incorporating nitroxide 14, nitronyl-nitroxide15 and imino-nitroxide 16 functions, as well as the corresponding diradicals assigned the hydrazyl-nitroxide 17, hydrazyl-nitronyl-nitroxide 18 and hydrazyl-imino-nitroxide 19 structures. EPR, UV-Vis and electrochemical (CV) characteristics are reported, along with MS and NMR data where appropriate; pKa and BDE values are calculated as well as the average interelectronic separation in the triplet diradicals 18 and 19 obtained via solid-state EPR measurement and calculations. The reactions of these compounds with short-lived radicals generated by photolysis, as well with NO and NO2, are also reported.