Biradical Lifetimes Research Articles

Ring strain release as a strategy to enable the singlet state photodecarbonylation of crystalline 1,4‐cyclobutanediones

Challenging most chemists' intuition, highly reactive dialkyl biradicals can be reliably generated in the solid state by taking advantage of the photodecarbonylation of cyclic ketones. However, it has been shown that radical stabilizing groups with resonance‐delocalizing abilities at the α‐carbons of the precursor are required to facilitate the α‐cleavage reaction, and that triplet state reactivity is essential to slow down the combination of the intermediate acyl–alkyl biradical back to the starting ketone. Relatively long triplet acyl–alkyl biradical lifetimes give a chance for the loss of CO to occur. Looking for additional strategies to generate transient biradicals in solids, we studied the solid state photochemistry of four aliphatic, dispiro‐substituted 1,4‐cyclobutandiones (1a–d) that were expected to react from the singlet state. We hypothesized that the release of ring strain from the small ring carbonyl would make the reverse acyl–alkyl combination disfavored, allowing for the loss of CO to occur efficiently and irreversibly. We report here the results of studies carried out in solution, bulk (powder) crystals, and nanocrystalline photochemistry. We have recently shown that excitation of dispirocyclohexyl‐1,3‐cyclobutanedione 1c led to the trapping of the intermediate oxyallyl with a half life of about 42 min. Our studies with the three other crystalline derivatives revealed that, while all react efficiently, the remarkably long lifetime of oxyallyl is unique to crystals of 1c. Copyright © 2011 John Wiley & Sons, Ltd.

Introduction to the Special Issue in Honor of J. C. (Tito) Scaiano

In August 2005, Tito Scaiano’s coworkers, friends and colleagues gathered in Ottawa for Reactive Intermediates in Photochemistry, a truly international symposium held to celebrate Tito’s achievements on the occasion of his 60 birthday. The symposium provided a wonderful opportunity to pay tribute to a scientist who is a world leader in studying the kinetics and mechanisms of organic reactions involving short-lived intermediates, who has been a guiding force in physical organic chemistry over the last quarter century and who has made exceptional contributions to training young chemists. Tito’s remarkable record of achievements includes the publication of almost 600 scientific papers, book chapters and patents, and recognition with numerous awards and honors. His research extends from fundamental mechanistic studies to applications in diverse areas such as pulp and paper chemistry, microelectronics and health sciences. Juan Cesar (Tito) Scaiano was born in 1945, in Buenos Aires, Argentina. He credits his high school science teachers with introducing him to the excitement and power of chemistry with many practical, hands-on demonstrations. Tito earned an undergraduate degree from the University of Buenos Aires in 1967. After a brief stint in the Argentinean army, much of which was fortunately spent on clerical assignment, he moved to the University of Chile to start graduate studies with Eduardo Lissi. His PhD research on reactions of methyl radicals and bromine atoms with triethylborane marked the start of a lifelong interest in reactive intermediates. Shortly after moving to Chile he married Elda, and their first son, Gus, was born before Tito finished his PhD. In 1970 Tito accepted a postdoctoral position to study in the laboratory of Alwyn Davies at University College London, where he continued his studies on organoborane chemistry. After 2 years he and his family, now including his daughter, Veronica, returned briefly to Chile and then moved to the University of Rio Quarto in Argentina. The political climate in Argentina eventually convinced the Scaianos that it was an opportune time for a visit to North America, and they arrived in Ottawa on a very cold New Year’s Eve in 1974. Tito spent a year and a half with Keith Ingold at the National Research Council (working on kinetic studies of radical reactions using electron spin resonance) before moving to his first faculty position in the Radiation Laboratory at the University of Notre Dame. There he had the good fortune to have the use of a nitrogen laser, which he coupled with a HeNe laser and a computer, and developed his first laser flash photolysis system. His recognition of the potential of laser techniques for studies of reaction mechanisms is a landmark in his career. He was among the first to apply laser flash photolysis to a wide range of organic reactions, and the automated, user-friendly equipment that he developed made the technique accessible to a wide range of physical organic and mechanistic chemists. Tito’s early research at Notre Dame focused primarily on studies of biradicals and alkoxyl radicals. He was the first to determine the lifetime of short-lived biradicals involved in the Norrish Type II reaction, using the novel paraquat probe technique that he invented for the task. This was the first of many textbook intermediates that would be detected—directly or indirectly—and characterized over the next 2 decades by Tito and the many coworkers and collaborators who were introduced to laser flash photolysis in his laboratory. In 1979 Tito returned to the National Research Council in Ottawa. After having lived in five countries with children born in the United Kingdom (Gus), Argentina (Veronica), Chile (Adriana) and the United States (Martin), he and Elda were ready to settle in Canada. Tito’s research group grew rapidly during the next few years; the synergy between his group and the researchers associated with Ingold and Griller and Tito’s boundless energy and enthusiasm for science provided a stimulating environment for research on reactive intermediates and for training young scientists. Many studies focused on free radical kinetics, most notably with the determination of the first absolute rate constants for reactions of Tito Scaiano after receiving the NSERC Herzberg Gold Medal.

High Magnetic Field Effects on Biradical Lifetimes: Evaluation of Magnetic Field Dependence and Chain Length Dependence Using Calculated g and Hyperfine Tensors

Abstract The g and hyperfine tensors of radicals were theoretically estimated by quantum chemical calculations, and compared with experimentally obtained ones. From the comparison, it was found that the theoretically calculated tensors can reproduce the experimental ones with accuracy. By using those tensors, therefore, high magnetic field dependences of the lifetimes of biradicals, which were generated by a photo-induced electron transfer in α-cyclodextrin inclusion complexes of chain-linked molecules of phenothiazine–(CH2)n–viologen (PhnV2+, n = 12, 10, and 8) and carbazole–(CH2)12–viologen (Cz12V2+), were quantitatively analyzed on the basis of the relaxation mechanism of the generated bi-cationic biradicals (Ph•+nV•+ and Cz•+12V•+). The analysis conclusively showed that two extremely small correlation times of each radical motion were still necessary for reversals characteristic of the high magnetic field dependences, and also that the order of the obtained correlation time was Cz•+ > V•+ > Ph•+. The order in the correlation time was discussed while referring to the rigidity of the radical frameworks.

Stereodifferentiation in the decay of triplets and biradicals involved in intramolecular hydrogen transfer from phenols or indoles to pi,pi aromatic ketones.

Laser flash photolysis studies on (R,S) and (S,S) diastereoisomers of the bichromophoric compounds 1-6 have been used to investigate the possible chiral discrimination in the quenching of triplet excited ketones, resulting in formal hydrogen abstraction. Deuterium isotopic effects show that triplet deactivation in these bichromophores is dominated by hydrogen atom transfer. A remarkable stereodifferentiation is found in the intramolecular quenching of the ketone triplets of 1-3 and 5 by the phenolic or indolic moieties, either in methanol or acetonitrile as solvents. This indicates the existence of specific structural requirements for hydrogen transfer. On the other hand, the lifetimes of the generated biradicals show large solvent dependence; solvation appears to slow their reversion to the starting ketone. The considerable stereodifferentiation observed for the biradical lifetimes suggests that the kinetics of biradical decay is faster when the approach of the two radical termini becomes easier.

Laser flash photolysis of [3,n]paracyclophan-2-ones. Direct observation and chemical behavior of 4,4'-(1,n-alkanediyl)bisbenzyl biradicals.

The 4,4'-(1,n-alkanediyl)bisbenzyl biradicals (2b-d) have been generated from the Norrish type-I reaction of [3,n]paracyclophan-2-ones (1b-d) giving the paracyclophanes 3b-d as the only reaction products. The behavior of biradicals 2b-d has been studied in detail and compared with the previously reported biradical 2a. The lifetimes increase as the chain length decreases and are affected by the solvent viscosity, thus showing the effect of the length of the chain on the conformations of the biradicals. Quenching with persistent radicals such as TEMPO resulted in length-dependent rate constants. Finally, the study of the magnetic field effects on the biradical lifetimes suggest that ISC control determines biradical lifetimes for long-chain systems.

Spin and Molecular Dynamics in Acyl-Containing Biradicals: Time-Resolved Electron Paramagnetic Resonance and Laser Flash Photolysis Study

A combination of time-resolved electron paramagnetic resonance (TREPR) and laser flash photolysis (LFP) studies of flexible acyl-containing biradicals over a wide temperature range is reported. In contrast to previous reports, it is shown that the main channel of intersystem crossing in these biradicals is the electron spin relaxation of the acyl moiety rather than spin−orbit interaction in the biradical. This relaxation determines the decay rate of the electron spin polarization at low temperatures and the biradical lifetime at high temperatures. The relaxation mechanism is attributed to the spin-rotation interaction, associated with the rotation of the carbonyl group about the neighboring C−C bond axis. From a model simulation of the time profile of the spin-polarized TREPR signal based on the numerical solution of the stochastic Liouville equation of the spin density matrix in frame of realistic model of biradical, the Arrhenius parameters for correlation times of spin rotation interaction and activation energies for molecular and spin dynamics were determined in two solvents, 2-propanol and hexane.

Dynamics of the biradical mediating vinylcyclopropane–cyclopentene rearrangement

A previous direct dynamics trajectory study of the thermal vinylcyclopropane–cyclopentene rearrangement [C. Doubleday, J. Phys. Chem. A, 2001, 105, 6333], based on an AM1-SRP potential energy surface (PES), is extended to study details of the intramolecular and unimolecular dynamics in the biradical region of the PES. The branching ratio of the 4 diastereomeric products and the time dependence of their formation depend on the transition state at which the trajectories are initialized and its vibrational temperature or mode specific excitation. For the small amount of reaction occurring after ∼600 fs, there is considerable stereorandomness in the product yield, which contrasts with the short-time nonstatistical and mode specific reaction. Varying the initial amount of energy in reaction coordinate translation has only a minor effect on the biradical lifetime, but may have an appreciable effect on the relative product yield. Changing the rotational temperature has a negligible effect on the reaction dynamics. This study illustrates the utility of the quasiclassical trajectory method for studying short-time unimolecular dynamics and the importance of including zero-point energy motion in the trajectory initial conditions.

Effect of High Magnetic Field on the Lifetime of Biradicals Generated from Benzophenone and Diphenylamine Bifunctional Chain Molecules

Abstract The effect of high magnetic fields (≤ 13 T) on the lifetime of triplet biradicals (BPH·-O-(CH2)n-O-DPA·, n = 2, 4, 8, 16) composed of a protonated diphenylketyl radical (BPH·) and a diphenylaminyl radical (DPA·) is studied in N,N-dimethylformamide at room temperature by laser flash photolysis. On increasing the magnetic field from zero to ca. 2 T, the lifetime of BPH·-O-(CH2)n-O-DPA· (n = 4, 8, 16) increases steeply, and then decreases gradually in higher magnetic fields. The lifetime of BPH·-O-(CH2)2-O-DPA· decreases gradually with increasing the magnetic field with a shallow dip at ca. 2 T. The reversal of the effect observed in the lifetime of BPH·-O-(CH2)n-O-DPA·, n = 4, 8, 16 is interpreted in terms of spin-lattice relaxation which is induced by the anisotropic Zeeman interaction. The dip observed in the magnetic field dependence of the lifetime of BPH·-O-(CH2)2-O-DPA· is attributable to the S-T_ level crossing.

Time-resolved infrared studies of triplet 1,3-cyclopentanediyl

Triplet-sensitized photolysis of 2,3-diazabicyclo[2.2.1]hept-2-ene (1) in argon- or oxygen- saturated acetonitrile-d3 solutions results in the formation of bicyclo[2.1.0]pentane (3), a ring closure product arising from an intermediate 1,3-cyclopentanediyl triplet biradical (2). Time-resolved infrared (TRIR) spectroscopy was used to monitor the kinetics of bicyclopentane 3 production. This analysis provides a measurement of the triplet biradical lifetime and an estimate of the bimolecular reaction rate between biradical 2 and oxygen, both in good agreement with previous investigations. Our studies also indicate that certain IR bands due to 3 in the C–H stretching region overlap with corresponding bands in biradical 2. This interpretation is supported by computational investigations.Copyright © 2000 John Wiley & Sons, Ltd.

Effects of High Magnetic Field on the Lifetime of Chain-Linked Triplet Biradicals Composed of Xanthone Ketyl and Xanthenyl Radicals

The magnetic field dependence (0−13.1T) of the lifetime of the chain-linked triplet biradicals, composed of xanthone ketyl and xanthenyl radicals, has been studied as a function of chain length. Upon increasing a magnetic field, its lifetime increases steeply to the respective maximum value at ca. 2 T. Upon further increasing the field, the lifetime of a long chain biradical decreases slightly, whereas those of all other biradicals are almost constant. Chain length dependence of the biradical lifetimes in a low magnetic field (0.1 to ca. 2 T) are discussed in terms of the spin−lattice relaxation (SLR) due to the dipole−dipole interaction between two radicals and the spin−orbit-induced intersystem crossing from triplet states of biradicals. A slight decrease in the lifetime, observed for a long chain-linked biradical in high magnetic field, is explained by the SLR due to the anisotropic Zeeman interaction.

Influence of Molecular Structure on the Rate of Intersystem Crossing in Flexible Biradicals

Laser flash photolysis (LFP), low-field chemically induced dynamic nuclear polarization (CIDNP), and time-resolved electron paramagnetic resonance (TREPR) techniques have been used for the comparative study of magnetic field and spin effects in acyl−ketyl and bis(ketyl)biradicals formed during the photolysis of 2-hydroxy-2,12-dimethylcyclododecanone (1-OH) and 2,12-dihydroxy-2,12-dimethylcyclododecanone (2-OH), respectively. The short biradical lifetime, the small magnetic field effect (MFE) on a biradical lifetime, and the low intensity of spin-correlated radical pair (SCRP) polarization observed during the photolysis of 1-OH indicate that the main channel of intersystem crossing in acyl−ketyl biradical is spin−orbit coupling (SOC). For bis(ketyl) biradicals observed during the photolysis of 2-OH, SOC is of minor importance, and both MFE and SCRP polarization are much larger. It is shown that the presence of significant SOC in biradicals can result in an increase of the CIDNP intensity at low magnetic fi...

High Magnetic Field and Magnetic Isotope Effects on Lifetimes of Triplet Biradicals Consisting of Two Equivalent Benzophenone Ketyls Linked by Methylene Chains

External magnetic field and magnetic isotope effects on the lifetimes of several isotope-labeled triplet biradicals (BPH•−O−(CH2)n−O−BPH•, n = 8, 10, and 12) composed of two equivalent benzophenone ketyl radicals (BPH•s) were studied by laser flash photolysis in magnetic fields up to 14 T. The biradical lifetimes increased with increasing a magnetic field below ca. 3 T and then slightly decreased at 4−14 T. Below 4 T, a large magnetic isotope effect on the biradical lifetimes was observed in 13C isotope substitution at a benzylic carbon of BPH•, whereas no magnetic isotope effect was observed above 4 T. The magnetic isotope effect in magnetic fields up to 4 T can be explained only by the spin−lattice relaxation mechanism due to the anisotropic hyperfine interaction of the biradicals.

Magnetic Field and Isotope Effects on Photochemistry of Chain-Linked Compounds Containing Benzophenone and Hydrogen-Donor Moieties

The magnetic field effects on the decay kinetics for chain-linked triplet biradicals have been observed by the use of a laser photolysis apparatus with a conventional electromagnet and a pulsed magnet. Biradical lifetimes increase steeply to their maxima from the zero field to ca. 2 T. The biradical decay times tend to decrease gradually with further increase of the magnetic field intensity to 14 T. The decreased lifetimes for triplet biradicals can be explained in terms of spin−lattice relaxation due to hyperfine anisotropy and to g tensor anisotropy.

External magnetic field and magnetic isotope effects upon the lifetimes of biradicals derived from benzophenone derivatives

The decay profile of several isotope-labelled biradicals containing a benzophenone ketyl and a hydrogen-donor radical has been examined by means of nanosecond laser flash photolysis in the presence of an external magnetic field. Noticeable magnetic isotope effects are observed on the decay kinetics: deuteration of the particular CH bond in the hydrogen donor results in retardation of the biradical recombination, whereas C-13 substitution of the benzylic carbon in the hydrogen donor shortens the biradical decay time. The observed results can be interpreted in terms of the relaxation mechanism and molecular motion.

A laser flash photolysis study of effects of high magnetic fields on the chain-linked biradical lifetime

ADVERTISEMENT RETURN TO ISSUEPREVArticleNEXTA laser flash photolysis study of effects of high magnetic fields on the chain-linked biradical lifetimeMasahiro Mukai, Yoshihisa Fujiwara, Yoshifumi Tanimoto, and Masaharu OkazakiCite this: J. Phys. Chem. 1993, 97, 49, 12660–12662Publication Date (Print):December 1, 1993Publication History Published online1 May 2002Published inissue 1 December 1993https://doi.org/10.1021/j100151a003RIGHTS & PERMISSIONSArticle Views60Altmetric-Citations33LEARN 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 (328 KB) Get e-Alerts

Norrish Type I Reaction of Diphenyltetrahydrobenzocycloheptenones. Conformational Effects on Biradical Lifetime

Abstract Isomeric 6,6-diphenyl-6,7,8,9-tetrahydro-5H-benzocyclohepten-7-one and 7,7-diphenyl-6,7,8,9-tetrahydro-5H-benzocyclohepten-6-one (2) were photolyzed in methanol to give rise to unsaturated aldehydes as well as diphenyltetrahydronaphthalene, a decarbonylation product (for 2). Lifetimes of the intermediate triplet biradicals were dependent on their structures, indicating that cyclic conformations play an important role in the intersystem crossing of biradicals.

Determination of the triplet lifetimes of 1,3-cyclopentanediyl biradicals derived from the photodenitrogenation of azoalkanes with time-resolved photoacoustic calorimetry

The triplet lifetime of the parent 1,3-cyclopentanediyl (2a) and the 4-methylene and the 1-phenyl derivatives 2b and 2c were determined by time-resolved photoacoustic calorimetry (PAC). For the biradical 2b, PAC was the only suitable method to date for determining its hitherto inaccessible triplet lifetime. A comparison with results for biradicals 2a,c obtained in earlier studies shows that PAC is a reliable method for measuring triplet lifetimes of biradicals. The effect on the triplet lifetimes of allyl and benzyl conjugation at one radical center in 2b,c proved to be insignificant

Temperature dependence of lifetime of chain-linked biradicals

With increasing the temperature from about 250 K, the lifetime of the chain-liked triplet biradicals composed of the xanthone ketyl and xanthenyl radicals decreases to a minimum (140–170 ns) at 270–300 K, and then increases slightly (200–210 ns at 350 K). This anomaly may result from the difference in the temperature dependence of the rate constants of chain motion and end-to-end recombination reaction.

Lifetimes of biradicals produced in the Norrish type I reaction of methyl-substituted 2-tetralones

The Norrish Type I biradicals derived from 2-tetralones decay by a competition of recyclization to the starting material, intramolecular disproportionation to yield aldehydes and decarbonylation. The last path is only favored in the case of 3,3-dimethylated derivatives while, in the other extreme, the parent 2-tetralone is essentially photostable. The biradical lifetimes are around 140 ns in benzene and 40 ns or less in non-aromatic solvents; they show little sensitivity to the solvent polarity and hydrogen bonding ability. The remarkable stabilization of the biradical in benzene is probably due to specific interactions between the acyl center in the biradical and benzene, since similar effects have not been observed in other biradicals.

Investigation of the kinetic window for generation of carbon-13 T0-S CIDNP derived from long-chain biradicals by tuning the rates of bimolecular scavenging and intersystem crossing

In the absence of radical scavengers, the products of biradicals generated from photolysis of 2-phenylcycloalkanones exhibit no CIDNP. Addition of a radical scavenger such as CCI4 or CBrCI, gives rise to strong CIDNP derived from To-S intersystem crossing. The CIDNP intensity depends on the scavenger concentration, the molecular chain length of the biradial, and the reaction temperature. The results are interpreted in terms of a simple kinetic scheme which implies a kinetic window within which To-S CIDNP is efficiently generated in biradicals. Bimolecular scavenging is sufficient to provide the strong competition with intersystem crossing needed to produce To-S CIDNP. From a knowledge of the dependence of CIDNP intensity on scavenger concentration and the previously measured biradical lifetimes, the rate constant of halogen atom abstraction by the acyl radical moiety is estimated to be 1 X 106 M-' s-' for CI abstraction from CCI4 and 1 X IO9 M-' s-' for Br abstraction from CBrCI,.