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

Abstract

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 behaviour of reactive species in such materials. We have employed muon spin spectroscopy to characterize various organic free radicals which 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.

Characterization of free radicals in clathrate hydrates of pyrrole, thiophene, and isoxazole by muon spin spectroscopy

“…[41][42][43][44] In previous work we showed that two distinct radicals are formed when muonium reacts with furan. 36 Similar behaviour might well be expected for Mu addition to thiophene and pyrrole (Scheme 1) and this is indeed indicated by the TF-µSR spectra ( Fig. 1).…”

“…By analogy with the muoniated radicals formed from furan 31,36 we can confidently assign the stronger signals obtained from thiophene and pyrrole to structures 4 and 6. It is significant that these radicals have smaller muon hfcs than the other two (3 and 5), consistent with the extended unpaired spin distribution in the allylic moieties of 4 and 6.…”

“…Given the relatively large energy differences, we conclude that the relative yields are a result of kinetic control (differing activation energies) rather than thermodynamic. 36 The assignment of the radical signal obtained from isoxazole is not so straightforward. The DFT calculations indicate that 7 is the most stable radical structure, but for unequivocal assignment we turn to the results of the µLCR investigation.…”

“…35,36 It is significant that two distinct radical products were identified for each of furan and 2,3-dihydrofuran, corresponding to Mu addition at inequivalent unsaturated carbon atoms.…”

Investigation of H atom and free radical behaviour in clathrate hydrates of organic molecules

“…In contrast, our own studies (Percival et al, 2014;Mozafari et al, 2016Mozafari et al, , 2017Mozafari et al, , 2018) employ a beam of spinpolarized positive muons (µ + ) which may be stopped in a sample under any desired condition. When the muons thermalize, some of them pick up an electron to form muonium (Mu = µ + e -), which can be considered a light isotope of hydrogen (mµ ≈ mp/9).…”