Browsing by Author "Romero, A.H."

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    Is NMR the tool to characterize the structure of C20 isomers?
    (2002) Romero, A.H.; Sebastiani, D.; Ramírez, R.; Kiwi, M.
    We investigate the feasibility of using nuclear magnetic resonance (NMR) chemical shift calculations as a tool to provide structural information for C-20 fullerene type molecules. NMR chemical shifts are extremely sensitive to the local chemical environment of an atom, reflecting unambiguously its bond lengths and angles as well as its hybridization. Thus, they can distinguish between the different isomers that are candidates for the ground state of this molecule. We calculate the NMR shifts for several C-20 isomers and show that NMR constitutes a potential tool to discriminate and identify experimentally a particular C-20 molecular conformation, and also the level of theory which best describes the experimental structure. (C) 2002 Elsevier Science B.V. All rights reserved.
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    Subsurface bonding of hydrogen in niobium: A molecular-dynamics study
    (1998) Romero, A.H.; Schuller, I.K.; Ramirez, R.
    Binding energy of hydrogen in a (100) niobium slab is investigated through molecular-dynamics simulations. We introduce a hydrogen-niobium interaction potential that contains an attractive as well as a repulsive part, which is valid for surface-diffusion calculations. Simulations for high and low temperatures (compared with theta(Debye)) are presented. For the former case, we found that the hydrogen diffuses in the slab with a diffusion coefficient in agreement with experiment, and in most cases it is trapped inside the slab for T less than or equal to 900 K. Close to room temperature, there is an enhancement of the hydrogen binding energy, which causes self-trapping close to the surface. [S0163-1829(98)07043-X].