Single-phase primary electron spin polarization transfer in spin-trapping reactions

Turro, Nicholas J.; Khudyakov, I. V.

doi:10.1016/0009-2614(92)85846-3

Cited by 23 publications

(42 citation statements)

References 23 publications

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“…Photolysis of acylphosphine oxide 1a (25 mM) in ethyl acetate produce, after laser excitation, a pair of acyl (1b) and phosphinoyl radicals (1c) as shown in eq 1 41,46 The TR-ESR spectrum produced by photolysis of 1a is presented in Figure 1a. Similar spectra were observed 43,[46][47][48][49] and analyzed earlier. 41,48 The spectrum is in polarized net absorption due to the triplet mechanism and shows a broad absorptive singlet near the centerfield assigned to benzoyl radicals 1b and a doublet of multiplets (see inset in Figure 1) due to the phosphinoyl radical 1c (a(P R ) ) 36.26 mT, a(H o ) ) 0.086 mT, a(H m ) ) 0.072 mT, a(H p ) ) 0.079 mT).…”

Section: Tr-esr Experimentssupporting

confidence: 87%

Electron Spin Resonance and Laser Flash Photolysis Study of Radical Addition to Vinyl Acrylate and Related Alkenes

2001

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Addition of two radicals (diphenyl phosphinoyl and 2-hydroxy-2-propyl) to the bifunctional alkene, vinyl acrylate, was studied by both time-resolved (TR) and steady-state (SS) ESR and laser flash photolysis (LFP). The adduct radicals are predominately a result of tail addition (addition to the unsubstituted carbon atom) of the acrylate double bond. Chemical structures of the adducts were established by comparison of the observed ESR spectra with those of adducts of the same reactive radicals to structurally related alkenes, tert-butyl acrylate and vinyl pivalate, which have only one type of double bond. Adducts of bulky phosphinoyl radicals to the acrylates demonstrate hindered rotation and a cis-trans isomerization at room temperature. The structure of the adduct radicals and the reactivity of the two radicals are discussed. Absolute rate constants for the addition of the phosphinoyl radical to the alkenes were measured by LFP in ethyl acetate at 296 K. A rate constant of k add ) (33 ( 1) × 10 6 M -1 s -1 was found for vinyl acrylate. The latter value is ∼1.5 times higher than that for tert-butyl acrylate (k add ) (22 ( 1) × 10 6 M -1 s -1 ) and ∼17 times higher than that for vinyl pivalate (k add ) (2.0 ( 0.1) × 10 6 M -1 s -1 ). These rate constants are consistent with conclusions derived from the ESR data. The results provide some insights into free radical polymerization of vinyl acrylate and vinyl ethers.

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Section: Tr-esr Experimentssupporting

confidence: 87%

Electron Spin Resonance and Laser Flash Photolysis Study of Radical Addition to Vinyl Acrylate and Related Alkenes

2001

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“…Similar spectra were observed 48,74,79,87,[91][92][93] and analyzed earlier. 92,93 The spectrum is polarized in net absorption due to the triplet mechanism 45,[94][95][96][97][98] and shows a doublet of singlets due to the diphenylphosphinoyl radical 1a.…”

Section: S(τ)supporting

confidence: 86%

“…30, 35 The photochemistry and photophysics of bis(acyl)phosphine oxides (2 and 3) are similar to those of the mono(acyl)phosphine oxides (1) and are fairly well understood. 30,79,[86][87][88][89][90] After excitation with light, photoinitiators 2 and 3 undergo an efficient R-cleavage from the excited triplet state to produce strongly spin-polarized acylphosphinoyl (2a, 3a) and benzoyl (2b, 3b) radicals:…”

Section: S(τ)mentioning

confidence: 99%

A Novel Approach for Measuring Absolute Rate Constants by Pulsed Electron Spin Resonance: Addition of Phosphinoyl and 2-Hydroxy-2-propyl Radicals to Several Alkenes^,

Weber

Turro

2003

J. Phys. Chem. A

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A time-resolved Fourier transform electron spin resonance (TR FT ESR) method for determining absolute rate constants of reactive, free radical reactions is described. The phase memory times T M of a family of reacting radicals were measured precisely by electron spin-echo (ESE) detection. In all cases the ESE signal decays show a simple pseudo-first-order behavior. Rate constants of the reactions were extracted from the linear relationship between the reciprocal of the T M values and the alkene concentrations. The validity of the method is demonstrated for the measurement of the rate constant for addition of different phosphinoyl and 2-hydroxy-2-propyl radicals to several acrylates, methyl methacrylate, and vinyl pivalate in organic solvents at room temperature. The results are in excellent agreement with those obtained by other kinetic investigations employing time-resolved steady-state electron spin resonance (TR CW ESR) and optical absorption spectroscopy. The technique is shown to be applicable to systems with rate constants between 10 4 and 10 8 M -1 s -1 , a range covered by the alkenes examined.

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“…In the experiments involving photosensitizer, ITX was added to TMDPO solutions in IBOA; the range of [ITX] was 8.0 × 10 -3 -4.5 × 10 -2 M. Figure 1 presents TR EPR spectra of TMDPO in ethyl acetate. Quite similar spectra were observed [9][10][11][12][13] and analyzed 12 earlier. Acyl and bis(acyl)phosphine oxides such as TMDPO and BAPO undergo efficient photolysis (R-cleavage) via the triplet state to produce substituted benzoyl (referred to as benzoyl for the remainder of this report) and phosphorus centered radicals as shown in eq 1: [9][10][11][12][13] Two mechanisms of CIDEP are usually considered to be responsible for the most common polarization patterns observed for radicals produced by photolysis in homogeneous solutions: the triplet mechanism (TM) and the S-T 0 radical pair mechanism (RPM).…”

Section: Methodssupporting

confidence: 78%

“…Quite similar spectra were observed [9][10][11][12][13] and analyzed 12 earlier. Acyl and bis(acyl)phosphine oxides such as TMDPO and BAPO undergo efficient photolysis (R-cleavage) via the triplet state to produce substituted benzoyl (referred to as benzoyl for the remainder of this report) and phosphorus centered radicals as shown in eq 1: [9][10][11][12][13] Two mechanisms of CIDEP are usually considered to be responsible for the most common polarization patterns observed for radicals produced by photolysis in homogeneous solutions: the triplet mechanism (TM) and the S-T 0 radical pair mechanism (RPM). [9][10][11][12][13] In systems involving radical pairs possessing very large hyperfine coupling (HFC) constants, a third mechanism, S-T_RPM is sometimes invoked.…”

Section: Methodssupporting

confidence: 78%

Direct and Sensitized Photolysis of Phosphine Oxide Polymerization Photoinitiators in the Presence and Absence of a Model Acrylate Monomer: A Time Resolved EPR, Cure Monitor, and PhotoDSC Study

et al. 2000

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The effect of triplet sensitizers on the photoinitiated polymerization (cure) of a model acrylate monomer, isobornyl acrylate (IBOA), has been investigated. Time-resolved electron paramagnetic resonance (TR EPR) spectroscopy was employed to investigate the initiation of polymerization. Cure monitoring and photodifferential scanning calorimetry (photoDSC) were employed to follow the course of the polymerization. Thioxanthen-9-one (TX) and 2-isopropylthioxanthen-9-one (ITX) were found to be effective sensitizers of the photopolymerization, which was initiated by radicals produced from (2,4,6-trimethylbenzoyl)diphenylphosphine oxide (TMDPO) and bis(2,4,6-trimethylbenzoyl)phenylphosphine oxide (BAPO). TR EPR experiments demonstrated that the mechanism of sensitization involves T-T energy transfer from TX (or ITX) to TMDPO or BAPO followed by formation of radicals by R-cleavage of the photoinitiators. Direct photolysis of TMDPO and BAPO results in an absorptive chemically induced dynamic electron polarization (CIDEP) pattern due to the triplet mechanism (TM) of polarization of the substituted benzoyl and P-centered radicals produced by R-cleavage of the photoinitiators. TR EPR demonstrates that the same radicals were produced during direct and sensitized photolysis. However, a different CIDEP pattern is produced by photosensitization, namely an emissive/absorptive (E*/A) pattern. A TR EPR study of solutions containing phosphine oxide initiators and IBOA under direct and photosensitized conditions demonstrated that the polarized primary P-centered radicals add to the double bond of IBOA with the formation of polarized secondary radical adducts. Both primary and secondary radicals exhibit the same polarization pattern as the primary radical precursors, i.e., A in direct photolysis and E*/A in the presence of a sensitizer. The rate of polymerization of neat IBOA was followed by cure monitoring. In the presence of ITX the rate of cure increases significantly compared to direct photolysis of same. The heat evolved in the polymerization of IBOA photoinitiated (direct and sensitized) with TMDPO was monitored by photoDSC, and at early times was found to be higher in the sensitized photopolymerization. Time-intermittent UV irradiation allowed an estimation of the ratio of termination to propagation rate constants (k t /k p ) during dark periods of polymerization. The observed decrease of k t /k p with the progress of polymerization is discussed. The results suggest that photosensitization may provide a means of manipulating and controlling the parameters of photocuring of acrylates.

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Single-phase primary electron spin polarization transfer in spin-trapping reactions

Cited by 23 publications

References 23 publications

Electron Spin Resonance and Laser Flash Photolysis Study of Radical Addition to Vinyl Acrylate and Related Alkenes

Electron Spin Resonance and Laser Flash Photolysis Study of Radical Addition to Vinyl Acrylate and Related Alkenes

A Novel Approach for Measuring Absolute Rate Constants by Pulsed Electron Spin Resonance: Addition of Phosphinoyl and 2-Hydroxy-2-propyl Radicals to Several Alkenes^,

Direct and Sensitized Photolysis of Phosphine Oxide Polymerization Photoinitiators in the Presence and Absence of a Model Acrylate Monomer: A Time Resolved EPR, Cure Monitor, and PhotoDSC Study

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Single-phase primary electron spin polarization transfer in spin-trapping reactions

Cited by 23 publications

References 23 publications

Electron Spin Resonance and Laser Flash Photolysis Study of Radical Addition to Vinyl Acrylate and Related Alkenes

Electron Spin Resonance and Laser Flash Photolysis Study of Radical Addition to Vinyl Acrylate and Related Alkenes

A Novel Approach for Measuring Absolute Rate Constants by Pulsed Electron Spin Resonance: Addition of Phosphinoyl and 2-Hydroxy-2-propyl Radicals to Several Alkenes,

Direct and Sensitized Photolysis of Phosphine Oxide Polymerization Photoinitiators in the Presence and Absence of a Model Acrylate Monomer: A Time Resolved EPR, Cure Monitor, and PhotoDSC Study

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A Novel Approach for Measuring Absolute Rate Constants by Pulsed Electron Spin Resonance: Addition of Phosphinoyl and 2-Hydroxy-2-propyl Radicals to Several Alkenes^,