Free Radicals of N-Donor-Stabilized Silicon(II) Compounds Probed by Muon Spin Spectroscopy

Abstract

Transverse-field muon spin rotation (TF-μSR) spectra have been obtained for muoniated free radicals formed by positive muon irradiation of a series of four-membered cyclic silylenes. Muon-electron hyperfine coupling constants (A μ ) were determined from the spectra; values range from 28.5 to 59.3 MHz. The radicals are formed by muonium (Mu) addition to the silylenes, in a fashion similar to the expected reaction of the H atom. However, it is unclear a priori which is the site of addition. Accordingly, density functional theory (DFT) calculations were carried out for two classes of radicals: silyl radicals formed by H addition to the backbone C atom of the four-membered ring and alkyl radicals formed by H addition to the unsaturated Si(II) center. By comparison of the predicted proton hyperfine constants with the experimentally determined A μ values it is possible to assign structures to each detected radical. Examples of both addition sites were found. ■ INTRODUCTIONIt is now over three decades since transverse-field muon spin rotation (TF-μSR) was first used to detect muoniated free radicals. 1 This rather exotic spectroscopy relies on the use of spin-polarized muons and particle physics techniques, in addition to the principles of magnetic resonance more familiar to chemists. With a lifetime of 2.2 μs, muons are relatively short lived, and it takes specialized facilities to generate the necessary intense beams of muons, direct them to the sample of interest, and monitor the subsequent evolution of the muon spin polarization. Thus, muon spin spectroscopy is available at only a few places in the world. Its application to chemistry is primarily due to the existence of muonium (Mu = [μ + e − ]), a single-electron atom with the positive muon (μ + ) as its nucleus. Because Mu is chemically equivalent to an H atom (identical electron configuration and similar reduced mass), it can be considered a light isotope of hydrogen. 2 Thus, Mu and H undergo similar chemical reactions, such as addition to unsaturated molecules to form free radicals. With its monatomic, nonpolar nature, muonium has been successfully established as an unbiased probe of radical reactivity, and muon spin spectroscopy has been applied to study a broad range of issues in free radical chemistry. 3 The TF-μSR technique relies on the precession of spinpolarized muons in a transverse magnetic field. In common with other types of magnetic resonance, the local field experienced by the spin probe depends on molecular effects as well as the applied field. Thus, different precession frequencies arise from the specific chemical environment of the stopped muon, either as muonium, muoniated radicals, or muons incorporated in diamagnetic compounds. In the studies described here, about 65% of the muons end up in diamagnetic molecules and consequently the associated spin population precesses at the muon Larmor frequency. The remaining muons capture an electron to form muonium atoms, which quickly react to form muoniated radicals. Provided that the rate of reaction...

Free Radicals Formed by H Atom Addition to Allenes as Determined by Muon Spin Spectroscopy

“…As part of an on-going examination of the reactivity of lowvalent main-group compounds towards free radical addition, [1][2][3][4] we planned to carry out comparative reactivity studies of some cumulated dienes, C=C=M (M = C, Si, Ge). As a first step we examined two substituted allenes: 1,1-dimethylallene (1) and 1-methoxyallene (2).…”