Triplet Diradicals Research Articles

ChemInform Abstract: UV Laser Photochemistry of Azoalkanes: Surprising Effects of Phenyl Substitution on the Lifetimes of 1,3‐Cyclopentanediyl and 1,4‐Cyclohexanediyl Triplet Diradicals.

AbstractThe direct and semitized (Ph2CO) photolysis of azoalkanes (I) leads to biradicals 3(II).

UV‐Laser Photochemistry: Trapping of the Norrish Type II Triplet Diradical 1‐Hydroxy‐4‐methyl‐1‐phenyl‐1,4‐pentanediyl with Molecular Oxygen

AbstractThe triplet lifetime (3ι = 131 ± 5 ns) of diradical 2, produced in the laser photolysis (334 nm) of 4‐methyl‐1‐phenyl‐1‐pentanone (1) in methanol, was determined by dioxygen trapping. This lifetime agrees well with the value determined by laser flash photolysis (3ι = 97 ns), thereby establishing the reliability of the trapping technique. The trapping product was prepared by independent synthesis and characterized as 3‐hydroxy‐1,2‐dioxolane 9, the ring tautomer of hydroperoxide 6.

UV-laser photochemistry of azoalkanes: surprising effects of phenyl substitution on the lifetimes of 1,3-cyclopentanediyl and 1,4-cyclohexanediyl triplet diradicals

UV-laser photochemistry of azoalkanes: surprising effects of phenyl substitution on the lifetimes of 1,3-cyclopentanediyl and 1,4-cyclohexanediyl triplet diradicals

Preparative UV‐VIS laser photochemistry: Photocycloadditions of methylenelactones with benzophenone and p‐benzoquinone. Oxygen trapping of paterno‐Büchi triplet 1,4‐diradicals as model reactions for quinghaosu‐type 1,2,4‐trioxanes

AbstractThe UV‐VIS laser irradiation of 4‐penten‐4‐olide (1) with benzophenone (BP) or with p‐benzoquinone (BQ) under an argon atmosphere afforded the acetal‐type oxetanes 2‐BP and 2‐BQ, respectively. Only for BP was the oxetane regioisomer 3‐BP obtained, actually as major product, while 3‐BQ appears to be photolabile under VIS‐laser photolysis conditions and thus does not accumulate in sufficient amounts for detection. Under an oxygen atmosphere, besides these oxetane products also the 1,2,4‐trioxanes 5‐PB and 5‐BQ, the oxygen trapping products of the intermediary preoxetane triplet diradicals 12, were isolated and fully characterized. These results demonstrate that spirolactone‐type 1,2,4‐trioxanes as quinghaosu analogues are accessible via trapping of Paterno‐Büchi triplet 1,4‐diradicals by molecular oxygen. In the case of diketene as enol‐lactone partner, the Paterno‐Büchi reaction with BQ gave a complex product mixture even under an argon atmosphere, leading to the oxetane 6 as major product, and the spirocyclobutanone 7 and spirobioxetane 8 as minor products. Under an oxygen atmosphere also the spiroacetal 9 was obtained, but no trioxanes (oxygen trapping) could be detected in this VIS‐laser photolysis.

ChemInform Abstract: UV‐Laser Photochemistry of the Azoalkane 1‐Phenyl‐2,3‐diazabicyclo(2.2.2)oct‐2‐ene: Detection and Lifetime Determination of the Triplet Diradical 1‐Phenyl‐1,4‐cyclohexadiyl by Time‐Resolved UV Spectroscopy (Flash Photolysis) and by Trapping with Molecular Oxygen.

AbstractDirect laser photolysis of azoalkane (I) at 364 nm gives the diene (III), presumably via the singlet diradical S‐(II).

UV laser photochemistry: Evidence for “through-bond coupling” effects in the lifetimes of 2,2-dimethyl-1,3-cyclopentadiyl triplet diradicals.

The 2,2-dimethyl substituted 1,3-diradicals 5a-c possess significantly shorter triplet lifetimes (τ t) than the parent 1,3-cyclopentadiyl 3 , which is being rationalized on the basis of changes in the “through-bond” coupling and thus singlet-triplet energy splitting between the radical sites.

ChemInform Abstract: Preparative UV‐VIS Laser Photochemistry: Molecular Oxygen Trapping of the Paterno‐Büchi Triplet Diradicals Derived from 1,4‐Dioxene.

Abstract1,4‐Dioxen (I) addiert die Ketone (II) unter Laser‐Bestrahlung zu den Bicyclen (III).

Preparative UV-VIS laser photochemistry; molecular oxygen trapping of the Paterno-Büchi triplet diradicals derived from 1,4-dioxene.

Argon ion laser irradiation of CCl 4 solutions of 1,4-dioxene and benzophenone or p-benzoquinone under oxygen pressure affords the 1,2,4-trioxanes 4a,b ( cis ring junction by NOE 1H-NMR differential spectroscopy), resulting from trapping of the corresponding triplet preoxetane diradicals.

Sensitized UV/Laser Photolysis of Azoalkanes: Conformational Influences on Intersystem‐Crossing and Lifetimes of Triplet Diradicals

Sensitized UV/Laser Photolysis of Azoalkanes: Conformational Influences on Intersystem‐Crossing and Lifetimes of Triplet Diradicals

Methodological traps encountered in theroretical treatments of the photoisomerization of triplet butadiene

The relative triplet energies of various conformations of butadiene were computed with reference to the ground state using a variety of conventional unrestricted Hartree—Fock (UHF) and restricted Hartree—Fock (RHF) methodologies correlationally corrected at the Møller-Plesset second- and third-order levels. It is shown that the geometries and relative energies of the various triplet species cannot be adequately estimated at either the UHF or the RHF level alone. In particular, at the RHF open-shell level, the geometry of the singly-twisted allyl—methylene diradical triplet shows the same properties as those encountered in the classic RHF instability of the allyl radical. However, the relative energies computed at the UHF level alone are poor, possibly reflecting the differing levels of spin contamination obtained for different geometries. Therefore, in contrast to the case of the ethylene triplet, where the relative energies of the triplet conformations are not correlation sensitive, the relative energies of the various conformations of the butadiene triplet require a correlation-energy correction.

ESR analysis of the initial reaction products in the solid state photopolymerization of diacetylenes

In order to investigate the mechanism of the solid state photopolymerization of single crystals of the bis(p-toluene sulphonate) ester of 2,4-hexadiyne-1,6 diol (TSHD) the low temperature UV-photoproducts are analyzed by ESR-spectroscopy. In this paper we report the fine structure, hyperfine structure and temperature dependence of a series of thermally activated triplet diradicals. The structure of these diradical dimer and trimer initiators of the UV-photopolymerization process is deduced from the analysis of the ESR data.

Unsaturated bridgehead silicon intermediates. Dipolar character of unsaturated siliconnitrogen bonds in such intermediates

Photolysis of 1-azido-1-silabicyclo[2.2.1]heptane and 1-azido-1-silabicyclo[2.2.2.]octane give loss of nitrogen and rearrangement with ring enlargement yielding silaimine reaction intermediates which were trapped by reaction with methyltrimethoxysilane and triethoxysilane. Evidence is presented which indicates that the unsaturated siliconnitrogen bonds in the intermediates have considerable polar character and do not behave like triplet diradicals. The balance of the evidence favors, but does not completely prove, that the intermediates have significant π-bonding.

On the mechanism of aromatic hydroxylation by O( 3P) atoms

It is shown that O( 3P) oxidation of aromatic compounds in the liquid phase leading to phenols may involve the intermediacy of triplet diradicals.

Unrestricted Hartree–Fock (UHF) calculations of singlet and triplet diradicals: Nitrene peroxide (HNOO)

It is shown that the eight different diradical states of nitrene peroxide (HNOO) are well represented by the spin density wave (SWD) solutions which are equivalent to the unrestricted Hartree–Fock (UHF) solutions for this radical. (AIP)

EXCITED STATES OF ACETYLENE AND THEIR ROLE IN PYROLYSIS

Previous theories of acetylene pyrolysis are reviewed in the light of recent work by Minkoff, Newitt, and Rutledge. It is shown that the relatively large rates observed at the beginning of the induction period do not agree with mechanisms involving the intervention of comparatively stable dimers. The required kinetic form is obtained, however, if a triplet diradical is produced on the surface in a bimolecular process, followed by gas phase polymerization, with some chain ending on the surface. The detailed mechanism closely resembles Flory's scheme for liquid phase vinyl polymerization. The shape of the radical is discussed, and it is suggested that the trans-configuration leads to polymerization, while the cis facilitates dehydrogenation.