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

Adam, Waldemar; Platsch, Herbert; Sendelbach, Juergen; Wirz, Jakob

doi:10.1021/jo00058a031

Cited by 10 publications

(5 citation statements)

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“…3. Here, the rate of production of biradical 2 is given as 2 Â 10 7 s À1 , the present time response of our spectrometer, since previous studies [9][10][11] have indicated that 2 is formed within 25 ns of the laser pulse. Its rate of decay is, of course, required to equal the rate of growth of 3.…”

Section: Resultsmentioning

confidence: 99%

“…2(b)] to a single exponential function [k obsd = (3.6 AE 0.4) Â 10 6 s À1 ] that corresponds to a biradical lifetime of 278 AE 30 ns, in very good agreement with previous work. [9][10][11] Kinetics observed at 3050 and 2856 cm À1 were fitted manually [ Fig. 2(a) and (c)] using the traces in Fig.…”

Section: Resultsmentioning

confidence: 99%

“…Since the pioneering experiments of Buchwalter and Closs 6 on direct detection of 2 by low-temperature matrix EPR and the first report of oxygen trapping of 2, 7 the lifetime of this fundamental triplet biradical has been measured by various experimental methods. In solution, these include quantitative oxygen trapping experiments (351 AE 57 ns) 8,9 and photoacoustic calorimetry (258 AE 14 ns 9 and 316 AE 80 ns 10 ), while in the gas phase time-resolved coherent antiStokes Raman spectroscopy has been used (195 ns). 11 In addition, photoacoustic calorimetry experiments have provided a measurement of the bimolecular rate constant for the reaction of 2 with oxygen (k o 2 = 5.3 Â 10 9 l mol À1 s À1 ).…”

Section: Introductionmentioning

confidence: 99%

“…10 Oxygen also catalyzes intersystem crossing of 2 to the singlet state; however, experiments suggest that this process is negligible relative to chemical trapping. 9 The lifetimes of triplet biradicals are usually governed by intersystem crossing rates, which depend on numerous factors including the effects of orbital geometry and singlet-triplet energy gaps on spin-orbit coupling. 12,13 Thus, the relatively long lifetime of 2 is the result of slow intersystem crossing, a direct consequence of limited spin-orbit coupling.…”

Section: Introductionmentioning

confidence: 99%

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

Showalter

Bentz

Ryzhkov

et al. 2000

J. Phys. Org. Chem.

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Section: Resultsmentioning

confidence: 99%

Section: Resultsmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

See 2 more Smart Citations

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

Showalter

Bentz

Ryzhkov

et al. 2000

J. Phys. Org. Chem.

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“…Two factors may be responsible, namely temperature-dependent intersystem crossing S-IIb -* T-IIb at the 1,3 diradical stage22 and/or restricted motion for planarization. 23 Whichever factor dominates is difficult to assess from this data, but clearly the singlet 1,3 diradical S-IIb is puckered. That some intersystem crossing to the planar triplet diradical T-IIb takes place at very low temperatures should be evident from the fact that triplet ESR spectra were observed at 12-30 K for T-IIa;…”

Section: Discussionmentioning

confidence: 94%

Urazole-Bridged Bicyclic Azoalkanes: Generation, Nitroxyl Trapping, and ESR Spectra of the Heterosubstituted 4,5-Diaza-1,3-cyclopentanediyl Diradicals through Direct and Benzophenone-Sensitized Photochemical Deazetation

Adam¹,

Goller²,

Kammel³

et al. 1995

J. Org. Chem.

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The Diels-Alder reaction of isopyrazoles 3 with 4-methyl-1,2,4-triazoline-3,5-dione afforded the azoalkanes 4 in high yields. The stereochemistry of the syn-and anti-diastereomeric derivatives 4 b was established on the basis of spectral data and an X-ray structure determination for syn-4b. Photochemical loss of dinitrogen through direct as well as benzophenone-sensitized photolysis led to the corresponding housanes 5. For the stereolabeled syn-and anti-4b azoalkanes, the respective housanes anti-and syn-Sb were obtained as mixtures, the latter in preference (synlanti > 9O:lO). Above -10 "C, the housanes 5 b isomerized thermally to a 96:4 (at -30 "C) thermodynamic mixture of synlanti, for which a van't Hoff treatment gave AG(243 K) = -4.4 kJ/mol in favor of syn-5b, AH = 4.2 kJ/mol, and A S = 36 J/mol K. AM1 calculations confirm that the syn-Sb diastereomer is the preferred one. The triplet-sensitized photolysis a t -30 "C of the separate syn-and anti-4b azoalkanes gave the same thermodynamically controlled mixture of housanes Sb (syn /anti = 96: 4), which speaks for a planar triplet 1,3 diradical T-IIb as intermediate. The intervention of triplet diradicals T-IIa,d was confirmed by ESR spectroscopy under matrix isolation (2' I 77 K), and the triplet ground state was established by a Curie plot for the dimethyl derivative T-IIa. Since in the direct photolysis of the separate syn-and anti-4b azoalkanes also predominantly the syn-5b housane was obtained, fast intersystem crossing of the singlet 1,3 diradical S-IIb to the triplet species T-IIb is proposed. The small memory effect (retention) in the direct photolysis in solution implies a puckered conformation for the singlet 1,3 diradical S-1%. Indeed, in the direct deazetation under matrix conditions a t -78 "C, the memory efect (retention) is unequivocally established, i.e. synlanti-Sb = 24:76 for azoalkane syn-4b and synlanti > 99:l for anti-4b. In the presence of the nitroxyl radical scavenger 1,1,3,3-tetramethylisoindolin-2-yloxyl, the 1,3 diradicals were efficiently trapped in form of the isomeric bis-alkoxyamines 7 in the triplet-sensitized as well as direct photolyses of azoalkanes 4a,d. These unprecedented results imply that heteroatom substitution does not generally reduce the lifetime of triplet diradicals.

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Structure and Reactivity of Organic Intermediates as Revealed by Time‐Resolved Infrared Spectroscopy

Toscano¹

2001

Advances in Photochemistry

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

Cited by 10 publications

References 1 publication

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

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

Urazole-Bridged Bicyclic Azoalkanes: Generation, Nitroxyl Trapping, and ESR Spectra of the Heterosubstituted 4,5-Diaza-1,3-cyclopentanediyl Diradicals through Direct and Benzophenone-Sensitized Photochemical Deazetation

Structure and Reactivity of Organic Intermediates as Revealed by Time‐Resolved Infrared Spectroscopy

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