High-Resolution ESR Study of the H···CH₃, H···CHD₂, D···CH₂D, and D···CD₃ Radical Pairs in Solid Argon

Abstract

High-resolution electron spin resonance (ESR) spectra of radical pairs of a hydrogen atom that coupled with a methyl radical (H...CH3, H...CHD2, D...CH2D, and D...CD3) were observed for X-ray irradiated solid argon containing selectively deuterium-labeled methanes, CH4, CH2D2, and CD4, at 4.2 K. The double-quartet 1H-hyperfine (hf) splittings of ca. 26 and 1.16 mT at the Deltam(s) = +/-1 and Deltam(s) = +/-2 transitions, which are one-half of the isotropic 1H-hf splittings of an isolated H-atom and a CH3 radical, were attributed to the H...CH3 pair. The 1H-hf splittings at the Deltam(s) = +/-1 transition were further split by the fine structure (fs) due to the electron dipole-dipole coupling. Because of the high-resolution spectra, three different sets of the fs splitting, d, are clearly resolved in the spectra of both the H...CH3 and the D...CD3 pairs. The separation distance (inter-spin distance), R, between the H-atom and the CH3 radical being in pairs was evaluated from the d values based on a point-dipole interaction model. For the case of the H...CH3 pair, the observed d values of 4.2, 4.9, and 5.1 mT yield the respective separations, R = 0.87, 0.83, and 0.82 nm, to probe the trapping site of the pair in an Ar crystalline lattice (fcc). For the pair with R = 0.87 nm, for example, we propose that the CH3 radical occupies a substitutional site and the counter H-atom occupies either the interstitial tetrahedral sites directed away from the CH3 radicals by a distance of 0.87 nm or the interstitial octahedral sites by a distance of 0.88 nm. When a mixture of CH4 and CD4 in a solid Ar matrix was irradiated, only two different radical pairs, H...CH3 and D...CD3, were observed. This result clearly demonstrates that the hydrogen atom and methyl radicals, which undergo a pairwise trapping, can originate from the same methane molecule.

Rotation of Methyl Radicals in Molecular Solids

“…The A and E state contributions are resolved due to the second-order effect that shifts the latter to higher field by 0.024 mT. 19,20 The A/E value is found to drop even below the expected theoretical high-temperature limit of 1/2. At 14 K, both A and E lines were rather broad, ca.…”

The radiation-induced chemistry in solid xenon matrices

“…The formation of radicals from simple guest molecules upon irradiation in solid noble gases has been demonstrated some fifty years ago [7]. This result actually reflects effective charge and/or excitation transfer from noble gas hosts to solutes, which was used for generation of matrix isolated radicals and radical ions characterized mainly by EPR spectroscopy [8][9][10][11][12]. However, the mechanistic aspects were not studied in detail.…”

Hyperfine Coupling in Methyl Radical Isotopomers

“…[16][17][18] The configuration and conformation of a radical can be inferred from measurements of the magnitude, sign, and temperature dependence of the hfcs. There have been many EPR experiments on the methyl radical in different matrices and solvents; [19][20][21][22][23][24][25][26][27][28][29][30][31][32][33][34][35][36] as evident from Table 1, the proton hfc varies with environment and generally decreases with increasing temperature in the liquid phase.…”