A laser flash photolysis study of pivalothiophenone triplets: steric and electronic effects in thione photoreaction kinetics

Upon laser pulse excitation (Aex = 532 nm) into the lowest-lying '(n,a*) band system, pivalothiophenones in benzene solutions give rise to short-lived triplets (Ama: = 325-335 nm, em: = (1 1-15) X lo3 M-' cm-I) with quantitative intersystem crossing efficiencies. The triplet yields decrease slightly (by 10-30%) upon changing A, to 308 nm (Le., upon excitation into S2). Kinetic data are presented for intrinsic triplet lifetimes, self-quenching, and quenching by oxygen, di-tert-butylnitroxy radical, and various reagents capable of interacting with the triplets via energy, electron, or hydrogen-atom transfer and by biradical formation (possibly leading to cycloaddition). The mechanisms of the quenching processes are discussed. Relative to rigid aromatic thiones, namely, xanthione and thiocoumarin, the interaction of pivalothiophenone triplets with most of the quenchers are kinetically inefficient. This is interpreted primarily as a manifestation of the steric crowding at positions a to the thiocarbonyl group.

Photochemical Oxidation of Thioketones: Steric and Electronic Aspects

Oxidation of diaryl, aryl alkyl, and dialkyl thioketones by singlet oxygen generated via self-sensitization and other independent methods yielded the corresponding ketone and sulfine in varying amounts. A zwitterionic/ diradical intermediate arising out of the primary interaction of singlet oxygen with the thiocarbonyl chromophore is believed to be the common intermediate for the ketone and sulfine. While closure of the zwitterion/diradical to give 1,2,3-dioxathietane would lead to the ketone, competing oxygen elimination is believed to lead to the sulfine. This partitioning is governed by steric and electronic factors operating on the zwitterionic/diradical intermediate.

Photochemical oxidation of thio ketones: steric and electronic aspects

Oxidation of diaryl, aryl alkyl, and dialkyl thioketones by singlet oxygen generated via self-sensitization and other independent methods yielded the corresponding ketone and sulfine in varying amounts. A zwitterionic/ diradical intermediate arising out of the primary interaction of singlet oxygen with the thiocarbonyl chromophore is believed to be the common intermediate for the ketone and sulfine. While closure of the zwitterion/diradical to give 1,2,3-dioxathietane would lead to the ketone, competing oxygen elimination is believed to lead to the sulfine. This partitioning is governed by steric and electronic factors operating on the zwitterionic/diradical intermediate.

Chlorinated Hypoelectronic Dimetallaborane Clusters: Synthesis, Characterization, and Electronic Structures of (eta(5)-C5Me5W)(2)B5HnClm (n=7, m=2 and n=8, m=1)

Pyrolysis of (eta(5)-C5Me5WH3)B4H8, 1, in the presence of excess BHCl2 center dot SMe2 in toluene at 100 degrees C led to the isolation of (eta(5)-C5Me5W)(2)B5H9, 2, and B-Cl inserted (eta(5)-C5Me5W)(2)B5H8Cl, 3, and (eta(5)-C5Me5W)(2)B5H7Cl2, (four isomers). All the Chlorinated tungstaboranes were isolated as red and air and moisture sensitive solids. These new compounds have been characterized in solution by H-1, B-11, C-13 NMR, and the structural types were unequivocally established by crystallographic analysis of compounds 3, 4, and 7. Density functional theory (DFT) calculations were carded out on the model molecules of 3-7 to elucidate the actual electronic structures of these chlorinated species. On grounds of DFT calculations we demonstrated the role of transition metals, bridging hydrogens, and the effect of electrophilic substitution of hydrogens at B-H vertices of metallaborane structures.

Self Assembly and Electronic Structure of ZnO Nanocrystals

In this paper, we report the synthesis and self assembly of various sizes of ZnO nanocrystals. While the crystal structure and the quantum confinement of nanocrystals were mainly characterized using XRD and UV absorption spectra, the self assembly and long range ordering were studied using scanning tunneling microscopy after spin casting the nanocrystal film on the highly oriented pyrolytic graphite surface. We observe self assembly of these nanocrystals over large areas making them ideal candidates for various potential applications. Further, the electronic structure of the individual dots is obtained from the current-voltage characteristics of the dots using scanning tunneling spectroscopy and compared with the density of states obtained from the tight binding calculations. We observe an excellent agreement with the experimentally obtained local density of states and the theoretically calculated density of states.

Conformational Analysis and Electronic Structure of Monothiodiacetamide: Normal Coordinate Analysis and Molecular Orbital Study

The infrared spectra of monothiodiacetamide (MTDA, CHaCONHCSCH3) and its N-deuterated compound in solution, solid state and at low temperature are measured. Normal coordinate analysis for the planar vibrations of MTDAd o and -dl have been performed for the two most probable cis-trans-CONHCSor -CSNHCO-conformers using a simple Urey-Bradley force function. The conformation of MTDA derived from the vibrational spectra is supported by the all valence CNDO/2 molecular orbital method. The vibrational assignments and the electronic structure of MTDA are also given.

Crystal Structure, Ultraviolet Spectrum, and Electronic Structure of a Strongly Twisted Push-Pull Ethylene, approaching an Amidinium-Vinylogous Dithioate Zwitterion

The crystal structure of 1,3-di benzyl -2 - (4,4-dimet hyl- 2,5- bist hioxocyclo hexylidene) imidazolidine (2) shows a twist of 80.8(5)' about the inter-ring bond, which has a length of 1.482(6) A. The near orthogonality of the donor and acceptor parts of this formal push-pull ethylene makes the structure approach that of a zwitterion, as evidenced by bond lengths indicating strong electron delocalization. The acceptor part approaches a vinylogous dithioate structure, the donor part an amidinium system. The U.V. spectrum shows an n + R and a R + R transition, at 51 1 and 41 7.5 nm, respectively; according to CNDO/S calculations these are located entirely in the [S-C-C-C-SI- part. Two bands at shorter wavelength are ascribed to transitions from combinations of the lone-pair orbitals on the sulphur atoms to a n* orbital in the [N-C-N] + part; this is facilitated by the near perpendicularity of the two parts of the molecule.

Study of Structural Stability and Electronic Structure of Nonstoichiometric CdS Nano Clusters from First Principles

Structural stability of small sized nonstoichiometric CdS nano clusters between zincblende and wurtzite structures has been investigated using first-principles density functional calculations. Our study shows that the relative stability of these two structures depends sensitively on whether the surface is S-terminated or Cd-terminated. The associated band gap also exhibits non-monotonic behavior as a function of cluster size. Our findings may shed light on contradictory reports of experimentally observed structures of CdS nano clusters found in the literature.

Metal complexes of thiophene 2-thiocarboxamide. Proton and 13C NMR, IR and electronic spectral studies

The complexes of thiophene 2-thiocarboxamide (TTCA) with some metal chlorides and bromides [M = Ni(II), Zn(II), Cd(II), Hg(II) and Cu(I)] are described. Elemental analyses, magnetic susceptibilities and conductance studies, electronic, IR, proton and 13C magnetic resonance spectra are reported. The results suggest exclusive coordination of TTCA through the thiocarbonyl sulfur. The influence of the thiophene ring on the donor properties of the thioamide are discussed.

Vertex-Fused Metallaborane Clusters: Synthesis, Characterization and Electronic Structure of [(eta(5)-C5Me5Mo)(3)MoB9H18]

The reaction of the [(eta(5)-C5Me5)MoCl4] complex with [LiBH4 - TH F] in toluene at - 70 degrees C, followed by pyrolysis at 110 degrees C, afforded dark brown [(eta(5)-C5Me5Mo)(3)MoB9H18], 2, in parallel with the known [(eta(5)-C5Me5Mo)(2)B5H9], 1. Compound 2 has been characterized in solution by H-1, B-11, and C-13 NMR spectroscopy and elemental analysis, and the structural types were unequivocally established by crystallographic studies. The title compound represents a novel class of vertex-fused clusters in which a Mo atom has been fused in a perpendicular fashion between two molybdaborane clusters. Electronic structure calculations employing density functional theory yield geometries in agreement with the structure determinations, and on grounds of density functional theory calculations, we have analyzed the bonding patterns in the structure,

Roofed polyquinanes: synthesis and electronic structure

Starting from readily available norbornenobenzoquinone 7 and employing a photothermal metathesis reaction as the main strategy, novel "roofed" polyquinane bisenones 3 and 13 have been synthesized. Among these, the former is potentially serviceable for further elaboration to dodecahedrane 1. Catalytic hydrogenation of 3 provided the dione 12, which fully inscribes the circumference of dodecahedrane sphere. The "roofed" C-16-bisenone 3 has been successfully annulated to C19-bisenone 24 and C19-trisenone 26 by employing the Greene methodology and Pauson-Khand reaction, respectively. The molecular structures of 3 and 13 were computed using molecular mechanics and semiempirical MO methods. The nonbonded distances between the double bonds vary strongly with the method employed. The interactions between the pi-MO's were, therefore, probed by means of photoelectron (PE) spectroscopy. Comparison with the PE spectra of a series of model systems with increasing complexity enabled an unambiguous assignment of the observed peaks. The symmetric and antisymmetric combinations of the pi-MO's of the enone moieties of 3 and 13 show large splittings, characteristic of propano-bridged systems in which through-space and through-bond effects act in concert.

meso-substituted octabromoporphyrins: Synthesis, spectroscopy, electrochemistry and electronic structure

The free-base, copper(II) and zinc(II) derivatives of 5,10,15,20-tetraarylporphyrin (aryl = phenyl, 4-methylphenyl or 4-chlorophenyl) and the corresponding brominated 2,3,7,8,12,13,17,18-octabromo-5,10,15,20-tetraarylporphyrin derivatives have been synthesized and their spectral and redox properties compared by UV/VIS, H-1 NMR, ESR and cyclic voltammetric methods. Substitution with the electron-withdrawing bromine groups at the pyrrole carbons has a profound influence on the UV/VIS and H-1 NMR spectral features and also on the redox potentials of these systems. On the other hand, electron-withdrawing chloro or electron-donating methyl groups at the para positions of the four phenyl rings have only a marginal effect on the spectra and redox potentials of both the brominated and the non-brominated derivatives. The ESR data for the copper(II) derivatives of ail these systems reveal that substitution at either the beta-pyrrole carbons and/or the para positions of the meso-phenyl groups does not significantly affect the spin-Hamiltonian parameters that describe the metal centre in each case. Collectively, these observations suggest that the highest-occupied (HOMO) and lowest-unoccupied molecular orbitals (LUMO) of the octabromoporphyrins involve the porphyrin pi-ring system as is the case with the non-brominated derivatives.-Investigations have been carried out to probe the electronic structures of these systems by three different approaches involving spectral and redox potential data as well as AMI calculations. The results obtained suggest that the electron-withdrawing beta-bromine substituents stabilize the LUMOs and, to a lesser degree, the HOMOs and that the extent of these changes can be fine-tuned, in a subtle way, by substituting at the meso-aryl rings of a given porphyrin.

Structural evolution and electronic properties of La1+xSr2-xMn2O7

A detailed study of the layered manganite La1+xSr2-xMn2O7 has been performed, establishing that within the composition range 0.1 less than or equal to x less than or equal to 0.45 the phases crystallize in the I4/mmm space group. The evolution of structural parameters with x: in this composition range has been followed using a novel application of an existing program for the Rietveld analysis of powder diffraction data. The structure, a familiar intergrowth of rock-salt (La,Sr)O slabs and double perovskite (La,Sr)(2)Mn2O6 units, is characterized by a reluctance to deform the latter. This manifests as a ''pumping'' of the larger Sr-II ion into the 12-coordinate site of the structure as x is increased. We report these features of the structure as well as electrical transport and magnetic properties, in light of recent observations of giant, negative magnetoresistance in these systems.

Optical and Electronic Properties of Conjugated Polymer -Nanocluster Semiconductor Hybrid Systems

Conjugated polymers are intensively pursued as candidate materials for emission and detection devices with the optical range of interest determined by the chemical structure. On the other hand the optical range for emission and detection can also be tuned by size selection in semiconductor nanoclusters. The mechanisms for charge generation and separation upon optical excitation, and light emission are different for these systems. Hybrid systems based on these different class of materials reveal interesting electronic and optical properties and add further insight into the individual characteristics of the different components. Multilayer structures and blends of these materials on different substrates were prepared for absorption, photocurrent (Iph), photoluminescence (PL) and electroluminscence (EL) studies. Polymers chosen were derivatives of polythiophene (PT) and polyparaphenylenevinylene (PPV) along with nanoclusters of cadmium sulphide of average size 4.4 nm (CdS-44). The photocurrent spectral response in these systems followed the absorption response around the band edges for each of the components and revealed additional features, which depended on bias voltage, thickness of the layers and interfacial effects. The current-voltage curves showed multi-component features with emission varying for different regimes of voltage. The emission spectral response revealed additive features and is discussed in terms of excitonic mechanisms.

Synthesis, Characterization, and Electronic Structure of New Type of Heterometallic Boride Clusters

The reaction of [Cp*TaCl(4)], 1 (Cp* = eta(5)-C(5)Me(5)), with [LiBH(4)center dot THF] at -78 degrees C, followed by thermolysis in the presence of excess [BH(3)center dot THF], results in the formation of the oxatantalaborane cluster [(Cp*Ta)(2)B(4)H(10)O], 2 in moderate yield. Compound 2 is a notable example of an oxatantalaborane cluster where oxygen is contiguously bound to both the metal and boron. Upon availability of 2, a room temperature reaction was performed with [Fe(2)(CO)(9)], which led to the isolation of [(Cp*Ta)(2)B(2)H(4)O{H(2)Fe(2)(CO)(6)BH} ] 3. Compound 3 is an unusual heterometallic boride cluster in which the [Ta(2)Fe(2)] atoms define a butterfly framework with one boron atom lying in a semi-interstitial position. Likewise, the diselenamolybdaborane, [(Cp*Mo)(2)B(4)H(4)Se(2)], 4 was treated with an excess of [Fe(2)(CO)(9)] to afford the heterometallic boride cluster [(Cp*MoSe)(2)Fe(6)(CO)(13)B(2)(BH)(2)], 5. The cluster core of 5 consists of a cubane [Mo(2)Se(2)Fe(2)B(2)] and a tricapped trigonal prism [Fe(6)B(3)] fused together with four atoms held in common between the two subclusters. In the tricapped trigonal prism subunit, one of the boron atoms is completely encapsulated and bonded to six iron and two boron atoms. Compounds 2, 3, and 5 have been characterized by mass spectrometry, IR, (1)H, (11)B, (13)C NMR spectroscopy, and the geometric structures were unequivocally established by crystallographic analysis. The density functional theory calculations yielded geometries that are in close agreement with the observed structures. Furthermore, the calculated (11)B NMR chemical shifts also support the structural characterization of the compounds. Natural bond order analysis and Wiberg bond indices are used to gain insight into the bonding patterns of the observed geometries of 2, 3, and 5.