Triplet Intermediates Research Articles

Unimolecular Decomposition of Chemically Activated Pentatetraene (H2CCCCCH2) Intermediates: A Crossed Beams Study of Dicarbon Molecule Reactions with Allene

The reactions dynamics of the dicarbon molecule C2 in the 1Sigma (g)+ singlet ground state and 3Pi(u) first excited triplet state with allene, H2CCCH2(X1A1), was investigated under single collision conditions using the crossed molecular beam approach at four collision energies between 13.6 and 49.4 kJ mol(-1). The experiments were combined with ab initio electronic structure calculations of the relevant stationary points on the singlet and triplet potential energy surfaces. Our investigations imply that the reactions are barrier-less and indirect on both the singlet and the triplet surfaces and proceed through bound C5H4 intermediates via addition of the dicarbon molecule to the carbon-carbon double bond (singlet surface) and to the terminal as well as central carbon atoms of the allene molecule (triplet surface). The initial collision complexes isomerize to form triplet and singlet pentatetraene intermediates (H2CCCCCH2) that decompose via atomic hydrogen loss to yield the 2,4-pentadiynyl-1 radical, HCCCCCH2(X2B1). These channels result in symmetric center-of-mass angular distributions. On the triplet surface, a second channel involves the existence of a nonsymmetric reaction intermediate (HCCCH2CCH) that fragments through atomic hydrogen emission to the 1,4-pentadiynyl-3 radical [C5H3(X2B1)HCCCHCCH]; this pathway was found to account for the backward scattered center-of-mass angular distributions at higher collision energies. The identification of two resonance-stabilized free C5H3 radicals (i.e., 2,4-pentadiynyl-1 and 1,4-pentadiynyl-3) suggests that these molecules can be important transient species in combustion flames and in the chemical evolution of the interstellar medium.

The Photochemistry of 4‐Chlorophenol in Water Revisited: The Effect of Cyclodextrins on Cation and Carbene Reactions

The photochemistry of 4-chlorophenol (1) in water and in the presence of cyclodextrins has been studied by means of steady-state and time-resolved experiments. These have shown that 1 undergoes photoheterolysis of the C--Cl bond in the triplet state to yield the 4-hydroxyphenyl cation (3)2 in equilibrium with 4-oxocyclohexa-2,5-dienylidene, (3)3. These triplet intermediates scarcely react with a n nucleophile, such as water, nor abstract hydrogen from this solvent, thus they are long-lived (approximately 1 micros). Specific trapping of both intermediates has been achieved. The cation adds to 2-propenol, k(add) approximately 1.3 x 10(8) m(-1) s(-1), to form the long-lived phenonium ion 11 (with lambda(max) = 290 nm), which then converts to 3-(4-hydroxyphenyl)propane-1,2-diol (10). Carbene (3)3 is trapped by oxygen to give benzoquinone and is reduced by D-glucose (k(q) = 8.5 x 10(6) m(-1) s(-1)) to give the phenoxyl radical (8) and phenol (9). Cyclodextrins have been found to trap the intermediates much more efficiently (k(q) = 9.4 x 10(8) m(-1) s(-1) with beta-CD), which indicates that inclusion is involved. Ground state 1 forms inclusion complexes with 1:1 stoichiometry and association constants of 140 and 300 M(-1) with alpha- and beta-CD, respectively. Complexation does not change the efficiency or the mode of photofragmentation of 1; however, it does influence the course of the reaction because the major portion of the intermediates are reduced to phenol within the cavity (k'(red)> or = 5 x 10(7) s(-1)) either via a radical 8 or via a radical cation 9(+)(.). Under these conditions, neither 2-propenol nor oxygen trap the intermediates to a significant extent.

Mechanistic and dynamic investigations for multi-channel reaction of CH 3O 2+NO

All species involving the reaction system of CH 3O 2+NO→CH 3O+NO 2 have been optimized extensively using gaussian94 package at the B3LYP/6-311++G** level, and four singlet and two triplet intermediates, three singlet and a triplet transition states were found in gas phase. The overall profile of potential energy surface was drawn out. By vibrational mode analysis and population analysis we found that a singlet channel CH 3O 2+NO→TS3→IM4→CH 3O+NO 2 and a triplet channel CH 3O 2+NO→ 3IM1→ 3TS1→ 3IM1→CH 3O+NO 2 are energetically most favorable.

Inter- and Intramolecular [2 + 2]-Photocycloaddition of Tetronates ? Stereoselectivity, Mechanism, Scope and Synthetic Applications.

For Abstract see ChemInform Abstract in Full Text.

Inter‐ and Intramolecular [2+2]‐Photocycloaddition of Tetronates − Stereoselectivity, Mechanism, Scope and Synthetic Applications

Abstract[2+2]‐photocycloaddition reactions of various tetronates (3−8, 12, 16, 20, 37) have been examined. The tetronates were prepared from the corresponding tetronic acid by O‐alkylation with an alkyl bromide or with an alcohol (Mitsunobu protocol). If the photocycloaddition reaction (λ = 254 nm, solvent: diethyl ether or tert‐butyl alcohol) was carried out in an intramolecular fashion the tri‐ and tetracyclic products 21−23, 27−36 were obtained in good yields (52−75%) and with excellent simple diastereoselectivity. The reaction tolerates a high degree of substitution at both the tetronate and at the alkene double bond. High facial diastereoselectivities were observed if stereogenic centers were present in the alkene tether (e.g. 7a → 33) or if the alkenyl substituent was attached to the stereogenic C5 carbon atom of the tetronate irradiation precursor (e.g. 20 → 32). The reaction was also conducted in an intermolecular fashion. The methoxyethoxymethoxy (MEM) protecting group was suitable to protect the free tetronate hydroxy group. The photocycloaddition of tetronate 8 to cyclopentene was highly selective delivering the tricyclic products 39 in an 8:1 exo/endo ratio (59% yield). The products obtained from the intermolecular reaction underwent a de Mayo‐like ring expansion delivering oxepane‐diones 45−47 in good yields (74−84%). In mechanistic studies it was found that the formation of the photoproduct was supressed upon addition of piperylene. Furthermore, in the presence of acetone as a sensitizer the reaction could be conducted at λ = 300 nm. At this wavelength no conversion occurred under direct irradiation. These observations suggest the involvement of triplet intermediates in the reaction. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2004)

Triplet excited states and radical intermediates formed in electron pulse radiolysis of amino-substituted fluorenones

Electron pulse radiolysis of four differently substituted amino derivatives of fluorenone, namely, 1-amino-, 2-amino- 3-amino-, and 4-aminofluorenone, has been carried out to study the effect of structure on the spectroscopic and kinetic characteristics of the triplet excited states as well as the transient free radical intermediates formed under reducing and oxidizing conditions. The triplet states of these compounds have been generated in benzene by pulse radiolysis and in other solvents by flash photolysis technique and their spectral and kinetic properties have been investigated. Hydrated electron (e aq −) has been found to react with these fluorenone derivatives to form the anion radical species with a diffusion-controlled rate constant. The spectral and kinetic properties of the transient ketyl and anion radicals have been studied by generating them in aqueous solutions of suitable pH. The pK a values of ketyl⇌anion radical equilibria are in the range of 6.8–7.7 for these derivatives. The oxidized species have been generated by reaction with the azide radical. Hydrogen atom adducts as well as the cation radicals of these derivatives have also been generated by pulse radiolysis and characterized.

Solution and crystal lattice effects on the photochemistry of 6-substituted cyclohexenones(1)(,)(2).

The photochemistry of 13 4,4-diphenylcyclohexenones, substituted at carbon-6, was investigated in solution and in the crystalline state. The stereoselectivity was of particular interest. In the solution photochemistry of C-6 monosubstituted enones in benzene, there was a unique preference for migration of the cis-phenyl group with formation of bicyclo[3.1.0]hexanone photoproducts, with the original 6-substituent having an endo configuration at carbon-3 of the product. In methanol the reaction was diverted to afford 3,4-diphenylcyclohex-2-enes understood as arising from a hydrogen-bonded zwitterionic intermediate. The solid-state photochemistry was also investigated. There was a dramatic absence of the 3,4-diphenylcyclohex-2-ene products in accord with the absence of the hydrogen bonding encountered in methanol. Further, the solid-state reactivity correlated with a vector analysis using X-ray atomic coordinates. This established that the migrating phenyl group required an orientation facing the enone beta-carbon. While the interesting preference for the cis-endo migration was not intuitively predicted, ab initio computations on the alternative phenyl-bridged triplet intermediates did lead to an understanding of the selectivity.

The Triplet Potential Energy Surface of s-trans-2,4-Hexadiene. A Comparison of Theory and Experiment

The mechanism of s-trans-2,4-hexadiene triplet interconversion has been explored using the B3LYP method with 6-31+G(d,p) and cc-pVTZ basis sets. Nine stationary points are located on the T1 surface and five on the S0 surface of s-trans-2,4-hexadiene. The results are consistent with experimental observations that indicate that the triplet-state photoisomerization of the 2,4-hexadiene involves the interconversion of allylmethylene triplet intermediates. Furthermore, this equilibration is predicted to proceed via readily accessible planar triplet intermediates which are nearly isoenergetic. The accessibility of planar triplet intermediates is consistent with triplet excitation transfer steps between the diene isomers that are responsible for the observed quantum chain process in the photoisomerization of the 2,4-hexadienes.

Triplet Organometallic Reactivity under Ambient Conditions: An Ultrafast UV Pump/IR Probe Study

The reactivity of triplet 16-electron organometallic species has been studied in room-temperature solution using femtosecond UV pump IR probe spectroscopy. Specifically, the Si-H bond-activation reaction of photogenerated triplet Fe(CO)(4) and triplet CpCo(CO) with triethylsilane has been characterized and compared to the known singlet species CpRh(CO). The intermediates observed were studied using density functional theory (DFT) as well as ab initio quantum chemical calculations. The triplet organometallics have a greater overall reactivity than singlet species due to a change in the Si-H activation mechanism, which is due to the fact that triplet intermediates coordinate weakly at best with the ethyl groups of triethylsilane. Consequently, the triplet species do not become trapped in alkyl-solvated intermediate states. The experimental results are compared to the theoretical calculations, which qualitatively reproduce the trends in the data.

A Laser Flash Photolysis Study on Fenofibric Acid

AbstractFenofibric acid (FA) is a photosensitizing drug used in the treatment of hyperlipidemia. This compound follows two different photodegradation pathways: the free acid exhibits the typical benzophenone photoreactivity, while its sodium salt undergoes photodecarboxylation via a triplet biradical, that undergoes intramolecular electron transfer to form a carbanion, or cyclization to give an intramolecular light‐absorbing transient (LAT). The obtained photoproducts are explained as the result of pro‐tonation of the carbanion, ring opening of the LAT with rearrangement or oxygen trapping of any of the triplet intermediates. The above mechanism is supported by direct detection of the triplet state of FA and two long‐lived intermediates in laser flash photolysis experiments. The triplet lifetime of the carboxylate form in methanol is 0.06 μ.s; by contrast, in the case of the free acid, it is 10 times longer. The benzophenone moiety is clearly the key chromophore involved in the photobehavior of FA.

Wavelength- and Isomer-Dependent Photochemistry of Styryl- and (Phenylethynyl)fluorenones

Fluorenones substituted with cis- and trans-styryl and phenylethynyl groups at C-1, C-2, and C-4 were synthesized, as were the trans-2- and trans-4-(3,4-dimethoxystyryl) and β-methyl-2-styryl derivatives. The static absorption and fluorescence spectra of these compounds were measured, as were their redox potentials in acetonitrile. Transient spectra in a range of solvents were obtained on a microsecond time scale. The compounds showed marked solvent-dependent behavior. In solvents of low polarity, the principal photochemical process for the styryl derivatives is cis−trans isomerization via triplet intermediates. In polar solvents, singlet processes become more important, including photoionization from upper excited states. There are also significant regiochemical differences. The triplets from the 2-substituted fluorenones have microsecond lifetimes and λmax values in the 400−500-nm range, indicating that they have a trans configuration. By contrast, 1- and 4-substituted fluorenones give rise to triplets ...

Photochemistry of Quinone-Bridged Amino Acids. Intramolecular Trapping of an Excited Charge-Transfer State

Linkages to amino acids that feature a chlorinated p-benzoquinone as a bridging group have been prepared by reaction of 2,5-dichloro-p-benzoquinone or chloranil with the free amine groups of alanine and proline. Electron-transfer capabilities of these peptide conjugates have been assessed through cyclic voltammetry and laser flash photolysis measurements. Phototransients are observed (λmax = 410 nm) that cannot be assigned to either triplet or radical anion intermediates normally associated with quinones. These intermediates are associated with novel biradical zwitterions that result from intramolecular trapping of an aminoquinone excited charge-transfer (CT) state.

Data analysis techniques for monitoring excited state kinetics

Progress in understanding photoinitiated reactions requires that kinetics of small dynamic populations of triplet state initiators and intermediates be distinguished from one another and from larger populations of precursor and solvent molecules. In this work, several approaches for analysis of multiwavelength data of photochemical reactions are described. The first addresses the analysis of time-resolved delayed-fluorescence and phosphorescence spectra of dye molecules in an inhomogeneous glassy matrix; a global analysis of the time- and wavelength-resolved emission data is used to resolve the spectra and kinetics of two discrete populations of molecules in the sample. The resolved kinetic parameters are then related to the wavelength dependence of the resolved spectra. A second application describes multidimensional least squares and factor analysis techniques for resolving transient Raman spectra of triplet state photoinitiators from Raman bands of the ground state and solvent. Raman spectra of solutions containing the ground-state photoinitiator and solvent are acquired using a series of laser pulses which simultaneously initiate and probe triplet state formation. The kinetics of optical excitation produce a higher order dependence on laser intensity for the triplet state permitting resolution of the component spectra.

Quantum chemical study of the intramolecular transfer of hydrogen in o- methylacetophenone and 1- alkylanthraquinones

The geometries and formation enthalpies of reagents, products, and triplet intermediates in the reaction of photoenolization of o- methylacetophenone and 1- methylanthraquinone are calculated by semiempirical quantum chemical methods. It is shown that the triplet intermediate is a triplet Σπ- biradical. The geometries and formation enthalpies of the transition state of the thermal transfer of the hydrogen atom in 9- hydroxy- 1,10- anthraquinone- 1- methide are calculated. The MNDO semiempirical methods give significantly exaggerated activation enthalpies in the thermal transfer of the hydrogen atom.

Photochromism of Quinonfs Experimental and Quantum Chemical Study

There are three types of photochromic processes for substituted quinones. They are characterized by photoinduced isomerization of the colorless or yellow para-quinone form to the ana-quinone colored form. Photochemical hydrogen and acyl migration in photochromic 1-alkyl- and 1-acylanthraquinones is the adiabatic process occurring on the triplet potential energy surface. Quantum chemical calculations of the structure of triplet intermediates in the photochromic reactions of phenoxy quinones were performed. It was concluded, that the most preferable mechanism of photochemical phenyl migration is the non-adiabatic process with the formation of the triplet biradical spiro-form. Variation of the lifetime of photoinduced ana-quinones by the orders of magnitude was achieved by the modification of quinone structures.

Photosubstitution of Two Iron Pentacarbonyl CO's in Solution via a Single-Photon Process: Dependence on Dispersed Ligands and Role of Triplet Intermediates

ADVERTISEMENT RETURN TO ISSUEPREVArticleNEXTPhotosubstitution of Two Iron Pentacarbonyl CO's in Solution via a Single-Photon Process: Dependence on Dispersed Ligands and Role of Triplet IntermediatesSuresh K. Nayak, Gerard J. Farrell, and Theodore J. BurkeyCite this: Inorg. Chem. 1994, 33, 10, 2236–2242Publication Date (Print):May 1, 1994Publication History Published online1 May 2002Published inissue 1 May 1994https://doi.org/10.1021/ic00088a028RIGHTS & PERMISSIONSArticle Views272Altmetric-Citations28LEARN ABOUT THESE METRICSArticle Views are the COUNTER-compliant sum of full text article downloads since November 2008 (both PDF and HTML) across all institutions and individuals. These metrics are regularly updated to reflect usage leading up to the last few days.Citations are the number of other articles citing this article, calculated by Crossref and updated daily. Find more information about Crossref citation counts.The Altmetric Attention Score is a quantitative measure of the attention that a research article has received online. Clicking on the donut icon will load a page at altmetric.com with additional details about the score and the social media presence for the given article. Find more information on the Altmetric Attention Score and how the score is calculated. Share Add toView InAdd Full Text with ReferenceAdd Description ExportRISCitationCitation and abstractCitation and referencesMore Options Share onFacebookTwitterWechatLinked InReddit PDF (907 KB) Get e-AlertsSupporting Info (1)»Supporting Information Supporting Information Get e-Alerts

Effects of complexation by cyclodextrins on the photoreactivity of Rose Bengal and Erythrosin B. A laser flash photolysis investigation

The effect of complexation by cyclodextrins on the photoreactivity of the halogenated xanthenes Rose Bengal and Erythrosin B has been studied by detecting the change induced in the yield and reactivity of triplet and radical states. Solutions of Rose Bengal and Erythrosin B in γ-cyclodextrins, containing 90% of complex in the ground state, and solutions of erythrosin B in α-cyclodextrins, containing 60% of complex in the ground state, were studied by laser flash photolysis. Other complexes could not be prepared in useful fractions, either because of the low solubility of the complexing cyclodextrin or because of the low values of the association constants. The spectrum and the yield of triplet and radical intermediates appear to be identical in water and in systems containing the complexed form. The rates of reaction of the triplets and radicals in the complexed system are slower than in water. The exit rate constant of triplet is less than 2 × 105 s–1 and the inclusion complex of triplet and radicals has an equilibrium constant similar to that of the ground state. A stoichiometry ratio of 1 : 1 was confirmed for all complexes, but there is an indication of some contribution from 2 : 1 (host : guest) in the γ cyclodextrin–erythrosin B system.

Experimental and theoretical studies of the reaction of atomic oxygen with silane

The flash-photolysis resonance-fluorescence technique has been employed to measure the rate constant for O+SiH4→products from 295–565 K, and yielded k1=1.23×10−10 exp(−14.6 kJ mol−1/RT) cm3 s−1 with an accuracy of about ±15%. The transition state for direct H-atom abstraction has been characterized at up to the Gaussian-2 ab initio level of theory. With small adjustments it is possible to model kinetic data for O+SiH4 in terms of an abstraction channel leading to OH+SiH3. This agreement does not rule out minor participation by addition or insertion channels, but there is no theoretical evidence for bound triplet intermediates in the potential energy surface. A transition state theory analysis suggests that k1 at 1000 K is 16 times larger than previously thought.

Photodimerization of dibenz[ b,f] azepine derivatives and their reaction intermediates

It is clearly demonstrated that the photodimerization of 5-acyl dibenz[ b,f]azepine (DBA) derivatives occurs via excited triplet intermediates. The excited triplet state of 5-acyl DBA derivatives is barely formed on direct excitation, but is formed efficiently by triplet—triplet (T—T) energy transfer from a triplet sensitizer. Benzophenone-sensitized photoexcitation gave a unique reaction product, the anti-cyclobutane dimer of DBA. Under the sensitized conditions, the quantum yield for the photodimerization was estimated to be 0.15 at 25°C ([DBA]=2.0×10 −3 mol l −1). The triplet intermediate of this reaction was detected by nanosecond laser photolysis and the kinetic parameters of the triplet species were determined.

Reactive scattering of a supersonic oxygen-atom beam: O + C2H4, C2H2

Reactive scattering of O atoms with C2H4 and C2H2 molecules has been studied at an initial translational energy E≈ 27 kJ mol–1 using a supersonic beam of O atoms seeded in He. Reactive scattering of C2H3O and C2HO radicals is found to be nearly isotropic but slightly favours forward and sideways scattering, respectively. High product translational energies E′av≈ 30 kJ mol–1 are observed in both cases. The total reaction cross-sections were measured as a function of initial translational energy, using a He/Ne buffer-gas mixture to vary the O-atom velocity. The threshold energy E0= 5 ± 3 kJ mol–1 for O + C2H4 is in good agreement with the activation energy for the overall reaction of O atoms with C2H4 but the threshold energy E0= 8 ± 3 kJ mol–1 for O + C2H2 is slightly lower than the activation energy Ea= 12.5 kJ mol–1. Hence the H-atom displacement reactions represent important reaction pathways for the O + C2H4, C2H2 reactions which are initiated by bonding of the electrophilic O atom to the unsaturated bonds of the hydrocarbon molecules. The structures of the triplet intermediates for these reactions are compared with that proposed for the O + H2S reaction.