Characterization of Free Radicals in Clathrate Hydrates of Furan, 2,3-Dihydrofuran, and 2,5-Dihydrofuran by Muon Spin Spectroscopy

In addition to their importance as abundant hydrocarbon deposits in nature, clathrate hydrates are being studied as potential media for hydrogen and carbon dioxide storage and as “nano-reactors” for small molecules. However, little is known about the behavior of reactive species in such materials. We have employed muon spin spectroscopy to characterize various organic free radicals that reside as isolated guests in structure II clathrates. The radicals are formed by reaction of atomic muonium (Mu) with the guest molecules furan and two isomeric dihydrofurans. Muonium is essentially a light isotope of hydrogen and adds to unsaturated molecules in the same manner as H. We have determined muon and proton hyperfine coupling constants for the muoniated radicals formed in the clathrates and also in neat liquids at the same temperature. DFT calculations were used to guide the spectral assignments and distinguish between competing radical products for Mu addition to furan and 2,3-dihydrofuran. Relative signal amplitudes provide yields and thus the relative reactivities of the C4 and C5 addition sites in these molecules. Spectral features, hyperfine constants, and reactivities all indicate that the radicals do not tumble freely in the clathrate cages in the same way that they do in liquids

Organic Free Radicals in Clathrate Hydrates Investigated by Muon Spin Spectroscopy

Very little is known about the behavior of free H atoms and small organic radicals inside clathrate hydrate structures despite the relevance of such species to combustion of hydrocarbon hydrates. Muonium is an H atom analog, essentially a light isotope of hydrogen, and can be used to probe the chemistry of H atoms and transient free radicals. We demonstrate the first application of muon spin spectroscopy to characterize radicals in clathrate hydrates. Atomic muonium was detected in hydrates of cyclopentane and tetrahydrofuran, and muoniated free radicals were detected in the hydrates of cyclopentene and 2,5-dihydrofuran, indicating rapid addition of muonium to the organic guest. Muon avoided level-crossing spectra of the radicals in hydrates are markedly different to those of the same radicals in pure organic liquids at the same temperature, and this can be explained by limited mobility of the enclathrated radicals, leading to anisotropy in the hyperfine interactions

Dual Reactivity of a Stable Zwitterionic N-Heterocyclic Silylene and Its Carbene Complex Probed with Muonium

The reactivity of the multifunctional cyclic silylene <b>4</b> and its carbene complex <b>5</b> have been investigated by a combination of muon spin spectroscopy and computation. The free radicals formed by muonium (Mu) addition to <b>4</b> were identified, showing that there are two dominant sites of free radical attack: on the Si atom and on the exocyclic methylene carbon. Reaction of muonium with <b>5</b> also produced two radicals, but with markedly different hyperfine constants. For both compounds avoided level-crossing resonance spectra and calculation of hyperfine constants show that one of the radicals results from Mu addition to the methylene group, yielding radicals <b>4a</b> and <b>5a</b>. Each contains a muoniated methyl group, −CH₂Mu, which undergoes restricted rotation with respect to the plane of the ring. For <b>4</b> the second product is readily assigned as the muoniated silyl radical <b>4b</b>, on the grounds of its high muon hyperfine constant (716 MHz). The second product from <b>5</b> shows instead a very small coupling constant, 19 MHz, assignable to the muoniated complex <b>5b</b>, in which the spin density has been transferred from the silicon to the carbenic carbon