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

Abstract

Gas hydrates have long been of interest to the petrochemical industry but there has been growing interest in potential applications for carbon dioxide sequestration and hydrogen storage. This has prompted many fundamental studies of structure and host-guest interactions, but there has been relatively little investigation of chemical reactions of the guest molecules. In previous work we have shown that it is possible to use muon spin spectroscopy to characterize H-atom-like muonium and muoniated free radicals formed in clathrate hydrates. Muonium forms in clathrate hydrates of cyclopentane and tetrahydrofuran, whereas furan and its dihydro-derivatives form radicals. The current work extends studies to clathrates hydrates of other 5-membered heterocycles: thiophene, pyrrole and isoxazole. All form structure II hydrates. In addition to the clathrates, pure liquid samples of the heterocycles were studied to aid in the assignment of radical signals and for comparison with the enclathrated radicals. Similar to furan, two distinct radicals are formed when muonium reacts with thiophene and pyrrole. However, only one muoniated radical was detected from isoxazole. Muon, proton and nitrogen hyperfine constants were determined and compared with values predicted by DFT calculations to aid the structure assignments. The results show that Mu adds preferentially to the carbon adjacent to the heteroatom in thiophene and pyrrole, and to the carbon adjacent to O in isoxazole. The same radicals are formed in clathrates, but the spectra have broader signals, suggesting slower tumbling. Furthermore, additional signals in the avoided levelcrossing spectra indicate anisotropy consistent with restricted motion of the radicals in the clathrate cages.

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).…”

“…The benzene hydrate samples were also structure II clathrates, with benzene occupying the 5 12 6 4 cavities, but with methane or xenon in the 5 12 cavities. These double hydrates were also prepared by the crushed ice method, but in this case the mixture was cooled under gas pressure (methane or xenon), as described for pyrrole-methane and thiophene-xenon (Mozafari , 2017;Mozafari et al, 2018).…”

“…Our previous publications describe studies of muonium in clathrate hydrates of cyclopentane and tetrahydrofuran, and muoniated radicals in clathrates of similar unsaturated molecules: cyclopentene, 2,5dihydrofuran, 2,3,-dihydrofuran, furan, thiophene, pyrrole and isoxazole (Percival et al, 2014;Mozafari et al, 2016Mozafari et al, , 2018. The current work concerns benzene and acetone hydrates.…”

Two-photon chemistry of tetrahydrofuran in clathrate hydrates

“…The atomic radicals quickly decay when our samples are annealed above B120 K, consistent with other reports. 24,26,29,38 Only at these elevated temperatures does the thermally activated diffusion of H /D between cages 59,60,66 enable reaction (R3) with in-tact THF molecules to form the cyclic THF-2yl radicals which we only detect after annealing (Fig. S12 and Table S7, ESI †).…”

“…1b) is granted by the distinct selection rules of 2PA. While this two-photon chemistry (2PC) need not be strictly limited to aqueous matrices, the cavities of clathrate hydrates are highly effective at stabilizing the photochemically produced radicals nearly indefinitely 17–39 making them amenable to observation by spectroscopic methods with inherently low time resolution such as electron spin resonance (ESR).…”