ESR, ENDOR, and ESEEM investigations on UV-irradiated 2,4,6-tri-tert-butyl phenol crystals

Abstract

Electron spin resonance (ESR), electron nuclear double resonance (ENDOR), and electron spin echo envelope modulation (ESEEM) measurements were carried out for UV-irradiated 2,4,6-tri-tert-butyl phenol in the polycrystalline state. The radical produced in the crystal was detected by ESR and identified to be the corresponding phenoxyl radical, which is well characterized in the chemical oxidations in solutions. ENDOR and ESEEM spectra were unambiguously analyzed in terms of the hyperfine coupling constants determined from well-resolved ESR in solutions. Radical pairs in the crystals were also ascertained, and together with the single-crystal study the analysis disclosed zero-field splitting parameters in the triplet states. ESEEM time decays gave relaxation times T, = 5.94 and T2 = 1.12 µs at room temperature. These appropriate values permit an easy detection of the spin echoes, and therefore this radical matrix can be used as a useful standard for pulsed ESR investigations.

Electron Spin-Lattice Relaxation Processes of Radicals in Irradiated Crystalline Organic Compounds

“…The EPR spectrum (Figure 1A) of the radical in irradiated 2,4,6-tri-tert-butyl-phenol (2,4,6-t-Bu-phenol) is a single line with unresolved hyperfine coupling to the ring protons and to the tert-butyl methyl protons. 40 The EPR spectrum of the radical in irradiated glycylglycine (Figure 1B) exhibits resolved coupling to a single proton with A H ∼ 18 G. 41,42 The room-temperature EPR spectrum of the dominant stable radical in irradiated L-alanine (Figure 1C) exhibits approximately equal coupling (∼25 G) to a unique proton and to the protons of a methyl group. For the radical in irradiated 4-methyl-2,6-di-tert-butyl-phenol (4-Me-2,6-t-Bu-phenol), the coupling to the three equivalent protons of the 4-methyl group in the fast methyl-rotation regime is about 11 G. 17,18,43,44 Comparison of the relaxation processes for the radical in L-alanine and for the 4-Me-2,6-t-Bu-phenoxy radical with those for the 2,4,6-t-Bu-phenoxy radical shows the differences between the effects on T 1e of methyl protons that give resolved hyperfine splittings and more weakly coupled methyl protons.…”

“…To evaluate the effectiveness of methods for measuring T 1e in samples with various spectral diffusion processes, radicals with different proton hyperfine coupling patterns were selected (Figure ). The EPR spectrum (Figure A) of the radical in irradiated 2,4,6-tri- tert -butyl-phenol (2,4,6- t -Bu-phenol) is a single line with unresolved hyperfine coupling to the ring protons and to the tert -butyl methyl protons . The EPR spectrum of the radical in irradiated glycylglycine (Figure B) exhibits resolved coupling to a single proton with A H ∼ 18 G. , The room-temperature EPR spectrum of the dominant stable radical in irradiated l -alanine (Figure C) exhibits approximately equal coupling (∼25 G) to a unique proton and to the protons of a methyl group.…”

Spin-lattice and spin-spin relaxation times of 2,4,6-tri-tert-butyl phenoxyl and 2,6-di-tert-butyl-4-methyl phenoxyl in diamagnetic crystals: Relaxation mechanism and influence of molecular motions

“…ESR and ENDOR studies have revealed that the radical produced by UV irradiation is TTBP and is dispersed homogeneously in a diamagnetic matrix of the corresponding phenol [5]. The EPR spectrum of DTBMP is also anisotropic due to the g-values but is essentially composed of four lines with an intensity ratio of 1:3:3:1, which can be assigned to "three methyl pr 0-tons at the para-position.…”

“…Because of the difficulty of radical production, the irradiation time for DTBMP was doubled. Details of the sample preparation were also desc¡ in a previous paper [5]. The radical concentrations determined from CW-EPR spin counting were 5.0" l017 and 6.4.…”

“…In this paper, we report motional effects on 7"1 and T m for two phenoxyl radicals dispersed in a diamagnetic matrix with a special emphasis on different substitutions on the para-position, that is, tert-butyl or methyl substitutions. EPR, electron nuclear double resonance (ENDOR), and electron spin echo envelope modulation investigations on UV-irradiated 2,4,6-tri-tert-butyl phenol crystals were carried out, implying an oxidation of the phenol to a corresponding phenoxyl radical [5]. It was found that 2,4,6-tri-tert-butyl phenoxyl is dispersed in the diamagnetic phenol crystal and that this radical matrix works asa useful standard for CW as well as pulsed EPR investigations.…”