Theoretical studies of the vibrational properties of octahedrane (C12H12): A polyhedral caged hydrocarbon molecule.

The vibrational properties of octahedrane (C12H12) are calculated using density-functional theory employing two different computational methods: an all-electron Gaussian orbital approach and a Naval Research Laboratory-tight-binding scheme (NRL-TB) coupled with molecular dynamics (NRL-TBMD). Both approaches yield vibrational densities of states for octahedrane that are in good general agreement with each other. NRL Molecular Orbital Library can also provide accurate infrared and Raman spectra which can be analyzed and compared with experimental results, while NRL-TBMD can be conveniently scaled up for larger finite-temperature simulations. This latter approach is used in our paper to produce a theoretical prediction for a stable room temperature structure of octahedrane.

Access to endodontic care in North Carolina public health and Medicaid settings.

OBJECTIVES: The purpose of this study was to investigate issues related to access to endodontic care in North Carolina for individuals who used dental public health resources such as public health clinics (PHCs) or private practices that accept Medicaid or other government-sponsored reimbursement programs private practices that accept Medicaid (PPM). METHODS: Surveys were sent to 1,195 dentists regarding frequency and type of endodontic conditions encountered, treatments provided, and perceived barriers to care. Results were analyzed using logistic regression with the level of significance set at 0.05. RESULTS: Five hundred forty-six surveys were returned for a 45.7% response rate. Of the respondents, 79% reported frequently encountering an endodontic condition, but only 34% reported performing any type of definitive endodontic procedure. Graduates after the year 2000 were significantly more likely to perform definitive endodontic procedures (P < 0.05). Lack of insurance was the greatest barrier to care with 89% considering it a moderate to major barrier, followed by cost of the endodontic treatment (87%) and cost of the restoration following treatment (86%). PPMs were more likely to consider cost and insurance a major barrier (P < 0.05). CONCLUSIONS: In North Carolina public health and Medicaid settings, the frequency of endodontic treatments provided was much lower than the frequency of endodontic conditions encountered that might have benefited from treatment. Graduation year was the best indicator for the provision of root canal therapy. Additionally, treatment patterns and perceptions of barriers to care are different for PHCs and PPMs.

Optical excitation energies, Stokes shift, and spin-splitting of C24H72Si14.

As an initial step toward the synthesis and characterization of sila-diamondoids, such as sila-adamantane (Si(10)H(16),T(d)), the synthesis of a fourfold silylated sila-adamantane molecule (C(24)H(72)Si(14),T(d)) has been reported in literature [Fischer et al., Science 310, 825 (2005)]. We present the electronic structure, ionization energies, quasiparticle gap, and the excitation energies for the Si(14)(CH(3))(24) and the exact silicon analog of adamantane Si(10)H(16) obtained at the all-electron level using the delta-self-consistent-field and transitional state methods within two different density functional models: (i) Perdew-Burke-Ernzerhof generalized gradient approximation and (ii) fully analytic density functional (ADFT) implementation with atom dependent potential. The ADFT is designed so that molecules separate into atoms having exact atomic energies. The calculations within the two models agree well, to within 0.25 eV for optical excitations. The effect of structural relaxation in the presence of electron-hole-pair excitations is examined to obtain its contribution to the luminescence Stokes shift. The spin-influence on exciton energies is also determined. Our calculations indicate overall decrease in the absorption, emission, quasiparticle, and highest occupied molecular orbital-lowest unoccupied molecular orbital gaps, ionization energies, Stokes shift, and exciton binding energy when passivating hydrogens in the Si(10)H(16) are replaced with electron donating groups such as methyl (Me) and trimehylsilyl (-Si(Me)(3)).

Electronic structure, vibrational stability, and predicted infrared-Raman spectra of the As20, As @ Ni12, and As @ Ni12 @ As20 clusters.

Recently an inorganic fullerine-like [As@Ni(12)@As(20)](3-) onion with near-perfect icosahedral symmetry in the crystalline phase was reported [M. J. Moses, J. C. Fettinger, and B. W. Eichhorn, Science 300, 778 (2003)]. This paper presents a detailed computational study in the framework of density functional theory on various aspects of this molecule. The electronic structure of the As@Ni(12)@As(20) is investigated in its neutral as well as -3 charged state together with its subunits As(20) and As@Ni(12) by the all electron linear combination of Gaussian-type orbitals method. The bonding is studied by examining the integrated charge within atomic sphere, the electron localization function, changes in the electron density distribution, and from vibrational modes. We find that strong covalent As-As bonds seen in isolated As(20) become weaker in the As@Ni(12)@As(20) and strong covalent As-Ni bonds are formed. The structural stability of all four clusters is examined by analyzing the energetics and by calculating the vibrational frequencies. Further, the infrared and Raman spectra is predicted for both the neutral and charged As@Ni(12)@As(20) clusters. Finally, the energy barrier for removal of a single arsenic atom is calculated for the neutral As@Ni(12)@As(20) cluster.