Photophysics of fluorinated benzene. III. Hexafluorobenzene

A theoretical study of the photoabsorption spectroscopy of hexafluorobenzene (HFBz) is presented in this paper. The chemical effect due to fluorine atom substitution on the electronic structure of benzene (Bz) saturates in HFBz. State- of-the-art quantum chemistry calculations are carried out to establish potential energy surfaces and coupling surfaces of five energetically low-lying electronic (two of them are orbitally degenerate) states of HFBz. Coupling of these electronic states caused by the Jahn-Teller (JT) and pseudo-Jahn-Teller (PJT) type of interactions are examined. The impact of these couplings on the nuclear dynamics of the participating electronic states is thoroughly investigated by quantum mechanical methods and the results are compared with those observed in the experiments. The complex structure of the S(1) ← S(0) absorption band is found to originate from a very strong nonadiabatic coupling of the S(2) (of πσ* origin) and S(1) (of ππ* origin) state. While S(2) state is orbitally degenerate and JT active, the S(1) state is nondegenerate. These states form energetically low-lying conical intersections (CIs) in HFBz. These CIs are found to be the mechanistic bottleneck of the observed low quantum yield of fluorescence emission, non overlapping absorption, and emission bands of HFBz and contribute to the spectral width. Justification is also provided for the observed two peaks in the second absorption (the unassigned "c band") band of HFBz. The peaks observed in the third, fourth, and fifth absorption bands are also identified and assigned.     

Magnetic and electrical transport properties in the self-doped manganite La0.9Mn0.9M0.1O3 (M=Mn,Zn and Ti)

The magnetic and electrical transport properties of La_0.9Mn_0.9M_0.1O₃ (M=Mn, Zn and Ti) were investigated. The temperature and magnetic field dependence of electrical resistivity (ρ) and dc magnetization were studied. All the compounds are found in rhombohedral structure. The excess oxygen in all three compounds was detected through iodometric titration. A modification in resistivity is observed when M=Mn is replaced by M=Zn and Ti. The high temperature resistivity above T_C follow variable range hopping model for both Zn and Ti compounds. For Zn doping, the observation of large field-cool effect and decrease in resistivity at room temperature and is assumed to be due to the implant of Mn⁴⁺ in Mn³⁺ matrix, which favor Mn³⁺/Mn⁴⁺ double exchange. The ferromagnetic behavior below T_C for the compound with M=Ti is correlated to the excess oxygen in it, which implants Mn⁴⁺ and thus incorporates ferromagnetic interactions. The substitutions lead to a reduction of T_c and magnetization.     

Low-lying Resonance State of 15 C: Application of Supersymmetric Quantum Mechanics

Calculation of the low-lying resonance state of ¹⁵C is performed using a two-body model (¹⁴C+n) with the application of Supersymmetric Quantum Mechanics (SSQM). The effective two-body potential presents a shallow well followed by a low and wide barrier. Large spatial extension of the halo state causes loss of accuracy due to numerical difficulties in calculating low-lying resonances. Use of SSQM permits one to construct an isospectral potential with a deep well followed by a high barrier which can effectively trap the system and hence facilitates the numerical calculation of resonance states more accurately than the original shallow potential. This method gives quite accurate result for the resonance energy of the ${\frac{3}{2}^{+}}$ state of ¹⁵C. Calculated width of this state also agrees fairly well within the experimental error bar.     

Oblique stagnation-point flow and heat transfer towards a shrinking sheet with thermal radiation

An analysis is made of steady two-dimensional oblique stagnation-point flow and radiative heat transfer of an incompressible viscous fluid towards a shrinking sheet which is shrunk in its own plane with a velocity proportional to the distance from a fixed point. Here the axis of the stagnation flow and that of the shrinking sheet are not aligned. A similarity transformation reduces the Navier-Stokes equations to a set of non-linear ordinary differential equations and are solved numerically using a shooting technique. The analysis of the results obtained shows that multiple solutions exist for a certain range of the ratio of the shrinking velocity to the free stream velocity. The effect of non-alignment for the wall shear stress and the horizontal velocity components are discussed. Streamline patterns are also shown for shrinking at the sheet with aligned and non-aligned cases. It is found that the temperature at a point in the fluid decreases with increase in effective Prandtl number (Pr _eff ). The results pertaining to the present study indicate that as Pr _eff increases, the rate of heat transfer also increases. The reported results are in good agreement with the available published work in the literature.     

Network structure and thermal property of a novel high temperature seal glass

In this study, novel glasses based on SrO–La₂O₃–Al₂O₃–B₂O₃–SiO₂ system are investigated for solid oxide fuel and electrolyzer cells. The network structure evolution of the glasses with increasing B₂O₃:SiO₂ ratio was studied using Raman spectroscopy. The thermal properties of the glasses, including glass transition temperature T _g and glass softening temperature T _d , were studied using dilatometry. The thermal stability of the glasses was investigated using X-ray diffraction. The study shows that as the B₂O₃:SiO₂ ratio increases, the SrO–La₂O₃–Al₂O₃–B₂O₃–SiO₂ glass micro-heterogeneity and the amount of non-bridging oxygen atoms increase. Correspondingly, the T _g of the SrO–La₂O₃–Al₂O₃–B₂O₃–SiO₂ glasses changes from 635 to 775°C, and the T _d changes from 670 to 815°C. Glass thermal stability decreases with B₂O₃:SiO₂ ratio increase. The glass without B₂O₃ is thermally stable after being kept at 850°C for 200 hrs.     

Fluorescence sensing of theobromine by simple 2,6-diamino-pyridine and the novel cyclic chair-like hydrogen-bonded tetramer of its diacetyl derivative

2,6-Diaminopyridine is found to be a simple fluorescent sensor for theobromine and its diacetyl derivative 2 also effectively binds theobromine. The receptor-binding sites are based on the co-operative hydrogen-bonding abilities of secondary amides. An unprecedented hydrogen-bonded self-organised cyclic tetrameric supramolecular network is shown for one such small molecule 2 containing one heterocyclic ring in contrast to the binuclear substrates like guanine or pterin which usually form cyclic tetrameric structures.     

Observation of a universal aggregation mechanism and a possible phase transition in Au sputtered by swift heavy ions

Two exponents delta for the size distribution of n-atom clusters, Y(n) approximately n{-delta}, have been found in Au clusters sputtered from embedded Au nanoparticles under swift heavy ion irradiation. For small clusters, below 12.5 nm in size, delta has been found to be 3/2, which can be rationalized as occurring from a steady state aggregation process with size independent aggregation. For larger clusters, a delta value of 7/2 is suggested, which might come from a dynamical transition to another steady state where aggregation and evaporation rates are size dependent. In the present case, the observed decay exponents do not support any possibility of a thermodynamic liquid-gas-type phase transition taking place, resulting in cluster formation.