Electronic structure of pyrazine: a study of the geometries in the lowest excited singlet π *← n and π*←π states

Electronic, excitation energies, charge distributions and geometries of pyrazine in the lowest excited singlet π^*←π and π^*←n states have been studied by the VE—PPP, CNDO/2 and CNDO/s-CI molecular orbital methods. Study of the change of geometry in the π^*←n excitation requires localization of the density matrices in the ground and excited states, and with the help of these σ-bond orders are defined. Charge distributions and bond orders in the lowest excited singlet π^*←π and π^*←n states are compared. Whereas in the lowest singlet π*←π excitation the pyrazine ring expands uniformly, in the case of the π^*←n excitation C-C bonds contract whereas C-N bonds elongate. The predictions of theory are in agreement with experimental results, showing that the method used can be employed to obtain reliably the trends of geometry changes following a π^*←π excitation of a molecule before a more complete theoretical or experimental study is performed.     

Bianchi Type-I Cosmological Model with a Varying <Emphasis Type="Italic">Λ</Emphasis> Term in Self Creation Theory of Gravitation

Bianchi type-I cosmological model is investigated in the framework of Barber’s second self creation theory of gravitation, with dust fluid as a source of the gravitational field, in presence of a non-zero time-dependent cosmological term. Various physical and geometrical aspects of the model are also discussed.     

57Fe Mössbauer studies in a new cluster spin glass system RuxFeGa (0.5≦x≦1.5)

⁵⁷Fe Mössbauer and bulk magnetization studies have been carried out in a new magnetic system Ru_xFeGa for 0.5x1.5. The system is seen to freeze into a cluster spin glass phase at 70 K. The freezing temperature and the hyperfine field at Fe at the lowest measured temperature, 190 kOe, is essentially same for all values of x investigated.     

A comparative study of vibrational dynamics of α- and β-forms of poly(β-hydroxybutyrate)

Normal modes and their dispersion are obtained for planar zig-zag form of poly(β-hydroxybutyrate) using Urey-Bradley force field. A comparison is made with the spectra of its helical form. Apart from detailed assignment of modes, various characteristic features of dispersion curves have been explained as arising due to internal symmetry in energy momentum space. The density-of-states have been used to calculate heat capacity in the temperature range 10-450 K using Debye's formalism.     

Ion-exchangers in radioactive waste management Part XIV: Removal behavior of hydrous titanium oxide and sodium titanate for Cs(I)

The removal behavior of hydrous titanium oxide and sodium titanate for Cs(I) from aqueous solutions by radiotracers was studied. Batch experiments revealed that an increase in Cs concentration (10^–8 to 10^–2 mol·dm^–3), temperature (298 to 328 K) and pH (2.50 to 10.20) apparently enhanced the uptake of Cs(I) on hydrous titanium oxide whereas a high degree of uptake of Cs(I) on sodium titanate was almost unaffected by a change in adsorption temperature (298 to 328 K) and pH (2.50 to 10.20). Both systems follow Freundlich adsorption isotherm. Uptake of Cs(I) on hydrous titanium oxide obeys first order rate law. According to thermodynamic data the uptake is endothermic and apparently irreversible in nature.This revised version was published online in November 2005 with corrections to the Cover Date.     

Selective photodissociation of O-H and O-D bonds from ground vibrational state of HOD using simple UV pulses

Selective cleaving of both O-H and O-D bonds in HOD is achieved using reasonably simple UV pulses to excite the HOD molecule in its ground vibrational state to the repulsive first excited A ((1)B(1)) surface. Detailed theoretical analysis of population transfer and flux in the H+O-DH-O+D channels reveals an important preparatory role for the cross-talk between the participating levels and a possible role for the beat structure of the population transfer oscillations in facilitating selective dissociation. Excitation using a 50 fs single color 67,169 cm(-1) laser pulse achieves a branching ratio H+O-DH-O+D=5.64 with 82% flux in the H+O-D channel and 15% in the H-O+D channel. A two color 50 fs laser pulse with frequencies of 54 920 and 52 303 cm(-1) provides a branching ratio of H-O+DH+O-D=2.83 and 52% flux in the H-O+D channel and 18% in the H+O-D channel.