Hamiltonian Structure of Poincaré Gauge Theory and Separation of Non-Dynamical Variables in Exact Torsion Solutions

The canonical Hamiltonian of the Poincaré gauge theory of gravity is reanalyzed for generic Lagrangians. It is shown that the time components e₀^α and Γ₀^αβ of the tetrad and the linear connection fields of a Riemann-Cartan space-time U₄ constitute gauge degrees of freedom which remain non-dynamical during the time evolution of the system. Whereas the e₀^α are to be identified with the lapse and shift functions N^α known from the ADM formalism in Einstein's theory, the additional Lorentz degrces of freedom Γ₀^αβ are pertinent to Poincaré gauge models. These non-dynamical variables are instrumental in the derivation of exact torsion solutions obeying modified double duality conditions for the U₄-curvature. Thereby, in the case of spherical symmetry and for the charged Taub-NUT metric, we obtain the most general torsion configuration for a large class of quadratic Lagrangians. Previously found solutions are contained therein and can be recovered after fixing special “gauge”.     

Study of alternating current conduction mechanism in polypyrrole-magnesium ferrite hybrid nanocomposite through correlated barrier hopping model

In the present work, both polypyrrole (PPy) and optimized polypyrrole–magnesium ferrite (PPy-MgFe₂O₄) hybrid nanocomposite were synthesized separately by simple oxidative chemical polymerization method and then structurally characterized using Fourier transform infrared spectroscopy (FTIR), X-ray diffraction (XRD) and scanning electron microscopy (SEM) techniques. The FTIR spectrum of the composite showed the presence of characteristic absorption bands of both PPy and MgFe₂O₄ in the composite confirming interfacial interaction of PPy with MgFe₂O₄. That this interaction is not affected by crystalline behaviour of predominant MgFe₂O₄ particles but that MgFe₂O₄ has embedded in PPy matrix was confirmed by XRD studies. Agglomerated granular spherical morphology of the composite was confirmed by SEM studies. Decrease in AC conductivity of the composite as compared to PPy due to the formation of interfacial heterojunction barrier between p-type PPy and n-type MgFe₂O₄ was confirmed experimentally and well supported theoretically by calculating binding energy, hopping distance and density of states at Fermi level of PPy and the composite as per CBH model.     

Accurate vibrational-rotational partition functions and standard-state free energy values for H2O2 from Monte Carlo path-integral calculations

Accurate quantum mechanical partition functions and absolute free energies of H(2)O(2) are determined using a realistic potential energy surface [J. Koput, S. Carter, and N. C. Handy, J. Phys. Chem. A 102, 6325 (1998)] for temperatures ranging from 300 to 2,400 K by using Monte Carlo path integral calculations with new, efficient polyatomic importance sampling methods. The path centroids are sampled in Jacobi coordinates via a set of independent ziggurat schemes. The calculations employed enhanced-same-path extrapolation of trapezoidal Trotter Fourier path integrals, and the paths were constructed using fast Fourier sine transforms. Importance sampling was also used in Fourier coefficient space, and adaptively optimized stratified sampling was used in configuration space. The free energy values obtained from the path-integral calculations are compared to separable-mode approximations, to the Pitzer-Gwinn approximation, and to values in thermodynamic tables. Our calculations support the recently proposed revisions to the JANAF tables.     

Surface species in the direct amination of methanol over Brønsted acidic mordenite catalysts

Quantitative in situ infrared spectroscopy in combination with kinetic analysis is utilized to derive mechanistic aspects for the reaction of methanol with ammonia on Brønsted acidic mordenite. Under non-reactive conditions, a coadsorption complex between methanol and ammonia is found, in which only ammonia is in direct interaction with the Brønsted acid sites of the zeolite. This complex is proposed to be the precursor for the formation of protonated methylamines in the zeolite pores which are formed in sequential order up to tetramethylammonium ions. These methylamines are unable to desorb under reaction conditions in the absence of ammonia. They leave the surface either by ammonia adsorption assisted desorption or by scavenging of methyl groups from protonated methylamines by ammonia. Both steps are concluded to be potentially rate determining.     

Chemically Initiated Electron Exchange Luminescence of Silyloxyaryl-Substituted Spiroadamantyl Dioxetanes: Kinetics and Excited State Yields

Abstract— The kinetics of the fluoride-induced decomposition of the thermally stable silyloxyaryl-substituted spiroadamantyl dioxetanes 1a,b and the excited state formation of this chemically initiated electron exchange luminescence (CIEEL) have been investigated. Two limiting kinetic regimes flash and glow have been identified, which depend on the fluoride concentration, the first at high, the second at low [F^-] triggering, whose detailed kinetic analysis affords the rate constants for the deprotected dioxetanes 2a,b cleavage in acetonitrile and dimethyl sulfoxide and chemiluminescence measurements the CIEEL and phen-olate 4 (CIEEL emitter) excitation yields. Chloro-substi-tution in the spiroadamantyl dioxetane does not affect the deprotection step k ₂ but leads to a ca five-fold faster cleavage of the deprotected dioxetane 2, while the chemiexcitation yield is the same for both dioxetanes. The energies of the first excited singlet and triplet states of the emitting phenolate 4 were estimated by AM1 configuration interaction calculations with explicit consideration of acetonitrile as solvent (self-consistent reaction field approach). The first excited singlet and triplet state of the CIEEL emitter phenolate 4 possess π,π* character, as suggested by the π-type molecular orbitals and the large singlet-triplet energy gap. The chemiexcitation of both singlet and triplet states of the excited phenolate 4 is feasible during the dioxetanes 1a,b cleavage, but the experimentally determined high singlet excitation yields suggest that preferentially the phenolate 4 singlet state is populated in the fluoride ion-triggered CIEEL process.     

Photochemical and Photobiotogical Studies with Acridine and Phenanthridine Hydroperoxides in Cell-free DNA

Abstract— The acridine and phenanthridine hydroperoxides 3 and 7 were synthesized as photochemical hydroxyl radical sources for oxidative DNA damage studies. The generation of hydroxyl radicals upon UVA irradiation (Λ. = 350 nm) was verified by trapping experiments with 5,5-di-methyl-1-pyrroline N -oxide and benzene. The enzymatic assays of the damage in cell-free DNA from bacteriophage PM2 caused by the acridine and phenanthridine hydroperoxides 3 and 7 under near-UVA irradiation revealed a wide range of DNA modifications. Particularly, extensive single-strand break formation and DNA base modifications sensitive to formamidopyrimidine DNA glycosylase (Fpg protein) were observed. In the photooxida-tion of calf thymus DNA, up to 0.69±0.03% 8-oxo-7,8-dihydroguanine was formed by the hydroperoxides 3 and 7 on irradiation, whose yield was reduced up to 40% in the presence of the hydroxyl radical scavengers mannitol and fert-butanol. The acridine and phenanthridine hydroperoxides 3 and 7 also induce DNA damage through the type I photooxidation process, for which photoinduced electron transfer from 2'-deoxyguanosine to the singlet states of 3 and 7 was estimated by the Rehm-Weller equation to possess a negative Gibb's free energy of cα -5 kcal/ mol. Control experiments with the sensitizers acridine 1 and the acridine alcohol 4 in calf thymus and PM2 DNA confirmed the photosensitizing propensity of the UVA-ab-sorbing chromophores. The present study emphasizes that for the development of selective and efficient photochemical hydroxyl radical sources, chromophores with low photosensitizing ability must be chosen to avoid type I and type II photooxidation processes.