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Using ab initio spin-density-functional calculations, we investigate the electronic and magnetic structures of a C 60 fullerene during a structural transition to a nanotube segment by a series of Stone-Wales transformations. We find that partly opened intermediate cage structures may acquire a magnetic moment of several Bohr magnetons. Our results offer a possible explanation for the ferromagnetic behavior observed in polymerized C 60 following exposure to high temperatures and pressures, and suggest the use of carbon nanostructures for magnetic measurements.

A few experimental and theoretical studies on the molecular structure of N-acetylproline amide (AP) in D2O solution have been reported recently. However, there is no consensus of the precise structure of AP in D2O because spectroscopically determined structures and a theoretically simulated one have been found to be different from one another. To determine its aqueous solution structure, IR and vibrational circular dichroism spectra of both L- and D-form AP solutions were measured. Molecular dynamics simulations with two different force fields and density functional theory calculations for the trans and cis rotamers of AP were performed to numerically simulate those spectra. Comparisons between experimentally measured and computationally simulated spectra directly suggest that the AP in water adopts a polyproline II-like conformation and that the force field parameter ff03 in the AMBER 8 suite of programs is more realistic and reliable in predicting molecular structure of AP in water than the ff99 in AMBER 7.

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Using ab initio local spin-density-functional formalism, we study the occurrence of spin polarization in quasi-one-dimensional heterostructured C/BN nanotubes. At the zigzag boundary connecting carbon and boron nitride segments of tubes, we find atomiclike states that acquire magnetization when partly filled. Whereas individual C/BN heterojunctions can be used to spin-polarize electrons during transport, periodic arrangements of heterojunctions in doped systems can lead to the formation of a one-dimensional itinerant ferromagnetic state.Following the recent discovery of ferromagnetism at room temperature in an all-carbon system consisting of polymerized C 60 , 1 there has been increased interest in magnetism in metal-free systems. Even though this finding is a significant step in the long-standing search for novel hightemperature magnets, 2 the observed magnetization, corresponding to less than an unpaired spin per thousand atoms, is too small. The origin of this magnetism is still unclear, and possibly linked to defects. This agrees with recent predictions of magnetism at the edge of graphene sheets and in BN/C heterosheets with somewhat artificial interface geometry. 3,4 Here we introduce heterostructured C/BN nanotubes with no undercoordinated atoms as a novel magnetic system with a magnetization a few hundred times larger than that of polymerized C 60 .Nanotubes of carbon, BN, BC 3 , BC 2 N and other stoichiometries have been synthesized successfully. 5-9 Experimental data indicate that these millimeter long, yet only nanometer wide quasi-1D nanostructures 10 cover the entire range from a metallic to an insulating behavior. Their electronic properties can be understood in terms of diameter, chirality, and elemental composition. 11,12 So far, there have been neither experimental indications nor theoretical predictions for the existence of magnetism in nanotubes. Here we study nanotubes consisting of hexagonal BN and C sheets seamlessly connected to form hollow cylinders. When adequately doped, these systems show, as we discuss in the following, magnetic behavior. The p-orbital ferromagnetism that we find follows, as we will show, the atomic Hund's rule in analogy to d-orbital magnetism in transition metals.Our theoretical study of magnetism in C/BN nanotubes is based on the local spin-density-functional formalism ͑LSDA͒. 13 We use ab initio pseudopotentials 14 and the Ceperley-Alder exchange-correlation functional for the local spin density. 15 In our supercell calculations, we alternatively use two kinds of basis sets, namely, plane waves and a realspace grid, depending on the problem size. In both cases, we use a kinetic-energy cutoff of 36 Ry, and find our results to be independent of the basis used. Testing convergence with a higher energy cutoff of 49 Ry, we find the energy difference between ferromagnetic and nonmagnetic solutions to change by less than 1 meV. In plane-wave calculations, we sample the irreducible Brillouin zone by 5-25 k-points along the tube axis. In the real-space multigrid scheme, use...