The hierarchical Yen−Mullins model of Athabasca oil-sands asphaltene is strongly supported by solution-state 1 H NMR relaxation measurements and 2D HSQC-NMR spectroscopy. For the first time, the 1 H T 1 and T 2 relaxation behaviors of specific sites in asphaltene molecules have been studied, and it is shown that the relaxation behavior is in agreement with the hierarchical molecule-nanoaggregate-cluster model proposed by Mullins (Energy Fuels 2012, 26, 3986). The size of the asphaltene nanoaggregate clusters was determined from the biexponential T 2 relaxation behavior of the 1 H nuclei. Assignment of overlapping 1D NMR signals was made possible via HSQC methods, where elusive hallmark long-range aromatic−aliphatic heteronuclear correlations were observed. Alicyclic structures were shown to be more closely associated with the aromatic core than what has been proposed for most archipelago-type structures. The NMR parameters obtained from the deconvolution analysis indicate an average of 6−7 pericondensed aromatic rings present in each molecule, in agreement with the island model. The average molecular weight was determined at ∼720 g/mol, which would be typical for island model structures, hence lending strong support for the Yen−Mullins model.

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The reaction kinetics for the addition of the muonium (Mu = Jl, + e-) atom to C 2 H 4 and C 2 D 4 have been measured over the temperature range 150-500 K at (N 2 ) moderator pressures near 1 atm. A factor of about 8 variation in moderator pressure was carried out for C 2 H 4 , with no significant change seen in the apparent rate constant k app , which is therefore taken to be at the high pressure limit, yielding the bimolecular rate constant k Mu for the addition step. This is also expected from the nature of the Jl,SR technique employed, which, in favorable cases, gives kapp = k Mu at any pressure. Comparisons with the H atom data of Lightfoot and Pilling, and Sugawara et al. and the D atom data of Sugawara et al. reveal large isotope effects. Only at the highest temperatures, near 500 K, is k Mulk H given by its classical value of2.9, from the mean velocity dependence ofthe collision rate but at the lowest temperatures kMulkH ~ 3011 is seen, reflecting the pronounced tunneling ofthe much lighter Mu atom (mIL = 1/9 m p )' The present Mu results should provide accurate tests of reaction theories on currently available ab initio surfaces.

Using the basic muon-spin-rotation technique, the fractions of energetic positive muons thermalizing in diamagnetic environments (f") or as the paramagnetic muonium atom M& ---, M&.Muons are produced with kinetic energies of 4.1 MeV or greater, far larger than the energy regime of atomic interest. During its slowing down process in matter, the p+ undergoes charge exchange with molecules X of the medium, @++X~(p+e ) +X+, in complete analogy with proton charge exchange. The neutral muonium atom (Mu) formed with cross section o. io has as its nucleus a positive muon but otherwise can be regarded simply as a light isotope of the hydrogen atom. ' ' The fraction of muons that thermalize in matter as either "free" p+ or as Mu atoms can easily be measured and interpreted in terms of well-established concepts in proton charge exchange.There are three basic things to be learned from 26 2527 1982 The American Physical Society FLEMING, MIKULA, AND) GARNER the present study.(1) Unlike the proton experiments, the p+ beam stops in the gas so that, in principle, one is able to probe the regime of charge exchange on the approach to thermal energies. In this regard, the information content is similar to that available from protons as thick target yields.