The orientation dependence of Br-atom formation in the reaction of the oriented OH radical with the HBr molecule using the hexapole electrostatic field was studied. Experimental results for the orientation dependence in the reaction were analyzed using a Legendre polynomial fit. The results show two reactive sites. It was found that O-end attack is most favored for this reaction, and that H-end attack also shows a pronounced reactivity. The reactivity of the side-ways attack was found to be small. By comparing the results of the orientation dependence in the reaction with studies of inelastic collisions and theoretical calculations, two reaction pathways are proposed. Reaction by O-end attack is followed by a direct abstraction of the H-atom from the HBr molecule. The mechanism for H-end attack may have H-atom migration from HBr to form the water molecule. © 2011 the Owner Societies.

Ab initio potential energy surfaces and the corresponding analytical energy functions of the ground 1A′ and excited 2A′ states for the Li(22P) plus H2 reaction are constructed. Quasiclassical trajectory calculations on the fitted energy functions are performed to characterize the reactions of Li(22P) with H2(v 0, j 1) and H2(v 1, j 1) as well as the reaction when the vibrational energy is replaced by collision energy. For simplicity, the transition probability is assumed to be unity when the trajectories go through the crossing seam region and change to the lower surface. The calculated rotational distributions of LiH(v 0) for both H2(v 0, j 1) and H2(v 1, j 1) reactions are single-peaked with the maximum population at j′ 7, consistent with the previous observation. The vibrational excitation of H2(v 1) may enhance the reaction cross section of LiH(v′ 0) by about 200 times, as compared to a result of 93-107 reported in the experimental measurements. In contrast, the enhancement is 3.1, if the same amount of energy is deposited in the translational states. This endothermic reaction can be considered as an analog of late barrier. According to the trajectory analysis, the vibrational excitation enlarges the H-H distance in the entrance channel to facilitate the reaction, but the excess energy may not open up additional reaction configuration. © 2011 American Institute of Physics.

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In this paper, we study the low-lying energy spectra of a two-dimensional (2D) graphene-based magnetic dot in a perpendicular and radially inhomogeneous magnetic field with the use of the massless Dirac-Weyl equation. Numerical calculations are performed using 2D harmonic basis states for direct diagonalization. Effects of both the dot size and the magnetic field on the low-lying energy spectra are discussed.

Both the size and the magnetic-field dependences of low-lying spectra of two-dimensional (2D) graphene based magnetic dot and ring in perpendicular inhomogeneous magnetic fields, where the magnetic field is zero inside the dot and ring, and constant elsewhere, are studied by the massless Dirac-Weyl equation. Numerical results obtained from direct diagonalization with 2D harmonic basis show that, under nonuniform magnetic fields, the higher Landau levels (N >= 1) for such massless Dirac electron interacting system in general become nondegenerate and split into discrete angular momentum states with level crossings with the lowest one (N=0) being an exception. (C) 2010 American Institute of Physics. [doi:10.1063/1.3435478]

Human telomere contains guanine-rich (G-rich) tandem repeats of single-stranded DNA sequences at its 3' tail. The G-rich sequences can be folded into various secondary structures, termed G-quadruplexes (G4s), by Hoogsteen basepairing in the presence of monovalent cations (such as Na(+) and K(+)). We developed a single-molecule tethered particle motion (TPM) method to investigate the unfolding process of G4s in the human telomeric sequence AGGG(TTAGGG)3 in real time. The TPM method monitors the DNA tether length change caused by formation of the G4, thus allowing the unfolding process and structural conversion to be monitored at the single-molecule level. In the presence of its antisense sequence, the folded G4 structure can be disrupted and converted to the unfolded conformation, with apparent unfolding time constants of 82 s and 3152 s. We also observed that the stability of the G4 is greatly affected by different monovalent cations. The folding equilibrium constant of G4 is strongly dependent on the salt concentration, ranging from 1.75 at 5 mM Na(+) to 3.40 at 15 mM Na(+). Earlier spectral studies of Na(+)- and K(+)-folded states suggested that the spectral conversion between these two different folded structures may go through a structurally unfolded intermediate state. However, our single-molecule TPM experiments did not detect any totally unfolded intermediate within our experimental resolution when sodium-folded G4 DNA molecules were titrated with high-concentration, excess potassium ions. This observation suggests that a totally unfolding pathway is likely not the major pathway for spectral conversion on the timescale of minutes, and that interconversion among folded states can be achieved by the loop rearrangement. This study also demonstrates that TPM experiments can be used to study conformational changes in single-stranded DNA molecules.

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