Zenaida Peralta-Inga

Density Functional Tight-Binding Studies of Carbon Nanotube Structures

A density functional tight-binding self-consistent charge approach has been used to study the structures and elastic properties of nine model carbon nanotubes of different helicities and diameters between 5.5 and 10.8 A. The systems contain from 112 to 268 atoms and were optimized under periodic boundary conditions in the axial direction. Both the carbon networks and the overall tube dimensions were optimized. Most of the C—C bond lengths are slightly lengthened relative to graphene (two-dimensional graphite); the others remain essentially the same or are shorter. There is overall a longitudinal compression of the tube. The strain energy per atom, relative to graphene, varies inversely with the square of the tube radius. The Youngs moduli decrease with increasing radius but do not depend upon chirality. The Poisson ratios are nearly constant. The consequences of removing an electron from each system were also investigated. In most instances, the tube dimensions were little affected; in only a few cases is there a change in length or radius (positive or negative) as large as 0.10%. The Youngs moduli remain the same as for the neutral systems, but the Poisson ratios tend to increase for metals and semimetals and to decrease for semiconductors.

Computed molecular surface electrostatic potentials of the nonionic and zwitterionic forms of glycine, histidine, and tetracycline

Three different procedures (HF/STO-3G, HF/3-21G, and B3P86/6-31+G**) were used to compute the electrostatic potentials on the molecular surfaces of glycine, histidine, tetracycline, and their zwitterions. These surface potentials were characterized in terms of a group of statistically defined quantities, which include the average deviation, the positive and negative average potentials, variances and extremes, and a balance parameter. The results were found to be qualitatively reasonably independent of the computational procedure. The zwitterions have much greater internal charge separations than the nonionic forms, although this is progressively diluted as the system increases in size. The statistical properties of the surface potentials permitted the determination of the solvation energetics of both the nonionic forms and the zwitterions.