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Muonium–the Second Radioisotope of Hydrogen: A Remarkable and Unique Radiotracer in the Chemical, Materials, Biological and Environmental Sciences

“…than in cyclohexadienyl type radicals. In a single crystal study of related Ph 2 C • -OMu radicals 165 , formed by Mu addition to benzophenone, dipolar couplings of approximately -15, -6, +21 MHz were determined; in comparison with which, values of ca 2 MHz for δA indicate that the type of reorientational motion for sorbed PhCH • -OMu radicals can only involve fairly small-angle jumps, whose amplitude is limited by such Mu-bonding as proposed earlier. In contrast, Me 2 C • -OMu radicals apparently undergo extensive averaging of the muon hyperfine anisotropy, as sorbed in the zeolite NaX at 298 K 166 .…”

“…Hydroxylic solvents will tend to stabilise (II), as will electron withdrawing substituents in the ring; electron releasing substituents will increase the weighting of structure (I), so the coupling rises over that for the unsubstituted PhCH • -OMu radical (12.6 MHz). A detailed study has been made of the related radical, Ph 2 C • -OMu, formed in a single crystal of benzophenone, in which all components of the muon hyperfine tensor have been determined: this is found to be dominated by the spin-density on the oxygen atom 72 .…”

“…The C=O compound most often studied by TF-μSR is acetone, which yields the "muoxy-iso-propyl" radical, Me 2 C • OMu, by addition to the oxygen atom of the C=O group [58][59][60][61][62][63][64][65][66][67][68][69] . Assuming that the relative order of bond energies for species X-Mu is approximately the same as those for X-H, it might reasonably be concluded that the driving force for this is the formation of an O-Mu bond, which should be 80 kJ mol -1 stronger than a C-Mu bond.…”

Muonium—the second radioisotope of hydrogen—and its contribution to free radical chemistry

“…The C᎐ ᎐ O compound most often studied by MuSR is acetone, which yields the "muoxy-isopropyl" radical, Me 2 C ؒ -OMu, by addition to the oxygen atom of the C᎐ ᎐ O group. [55][56][57][58][59][60][61][62][63][64][65][66] Assuming that the relative order of bond energies for species X-Mu is approximately the same as that for X-H, it might reasonably be concluded that the driving force for this is the formation of an O-Mu bond, which might be 80 kJ mol Ϫ1 stronger than a C-Mu bond. Compared with most of the radicals so far considered, the muon coupling is very small, in the region of 2.8 G, its precise value depending both on solvation effects and temperature.…”

The first observation of a muonium–carbonyl adduct with a negative muon coupling constant

“…There have been a number of studies [1][2][3][4][5][6][7][8][9][10][11][12] made of free radicals formed by addition of the light hydrogen isotope, muonium (a hydrogen atom with a positive muon as its nucleus) to compounds containing a single carbonyl (CNO) group, which include aldehydes, ketones, esters and amides. All these radicals possess the common structural feature of a muonium atom which is bound close to the radical plane (C • -O-Mu), but which can make vibronic (out-of-plane) excursions from it: the observed coupling is now a compromise between two contributions; one providing negative spin density in the muonium 1s orbital, arising from spin polarisation of the O-Mu bond by spin density at the oxygen atom, and the second, which is a positive transfer of spin density to the muon, as the out-of-plane vibration permits a degree of overlap with the spin bearing formal carbon 2p z orbital.…”