Conformational preferences of α-functionalised keten-S,N-acetals: Potential role of SO and SS interactions in solution

PMR spectra of carbonyl compounds 2a-k reveal significant variations in the population of E and Z isomers on changing the solvent from CDCl3 to DMSO-d6. In non-polar media, the intramolecular N-H…. O hydrogen bonded form is exclusively observed. In DMSO-d6, the alternative Z form is also populated. A similar conformational switch is also noted in the corresponding thiones. Different interpretations are critically analysed. The most consistent explanation is suggested to involvean interplay of N-H….X hydrogen bonding and S…X attractive interaction (X = O,S) in these systems. Ab initio calculations support this interpretation.

Stabilisation of unusual simultaneous binding of four cytosine nucleobases to copper(II) by a novel network of bifurcated hydrogen bonding

The crystal structure of tetrakis(cytosine)copper(II) perchlorate dihydrate has been determined. All the hydrogen atoms were obtained from Fourier-difference synthesis. The geometry around. copper is a bicapped octahedron (4 + 2 + 2*). The adjacent cytosine rings are oriented head-to-tail with respect to each other and are roughly at right angles to the co-ordination plane. The exocyclic oxo groups form an interligand, intracomplex hydrogen-bonding network above and below the co-ordination plane with the exocyclic amino groups of alternate cytosine bases. The EPR and electronic spectra are consistent with the retention of the solid-state structure in solution. The steric effect of the C(2)=O group of cytosine is offset by the presence of the intracomplex hydrogen-bonding network. The trend in Ei values of Cu-II-Cu-I couples for 1.4 complexes of cytosine, cytodine, pyridine, 2-methylpyridine and N-methylimidazole suggests that both steric effects and pi-delocalization in imidazole and pyridine ligands and the steric effect of C(2)=O in pyrimidine ligands are important in stabilising Cu-I relative to Cu-II.

Systematic crystallographic investigation of hydrogen-bonded networks involving monohydrogen tartrate-amine complexes: Potential materials for nonlinear optics

Crystal structures of six binary salts involving aromatic amines as cations and hydrogen tartrates as anions are presented. The materials are 2,6-xylidinium-L-monohydrogen tartrate monohydrate, C12H18O6.5N, P22(1)2(1), a = 7.283(2) Angstrom, b = 17.030(2) Angstrom, c = 22.196(2) Angstrom, Z = 8; 2,6-xylidinium-D-dibenzoyl monohydrogen tartrate, C26H25O8N, P2(1), a = 7.906(1) Angstrom, b = 24.757(1) Angstrom, c = 13.166(1) Angstrom, beta = 105.01(1)degrees, Z = 4; 2,3-xylidinium-D-dibenzoyl monohydrogen tartrate monohydrate, C26H26O8.5N, P2(1), a = 7.837(1) Angstrom, b = 24.488(1) Angstrom, c = 13.763(1) Angstrom, beta = 105.69(1)degrees, Z = 4; 2-toluidinium-D-dibenzoyl monohydrogen tartrate, C25H23O8N, P2(1)2(1)2(1), a = 13.553(2) Angstrom, b = 15.869(3) Angstrom, c = 22.123(2) Angstrom, Z = 8; 3-toluidinium-D-dibenzoyl monohydrogen tartrate (1:1), C25H23O8N, P1, a = 7.916(3) Angstrom, b = 11.467(6) Angstrom, c = 14.203(8) Angstrom, alpha = 96.44(4)degrees, beta = 98.20(5)degrees, = 110.55(5)degrees, Z = 2; 3-toluidinium-D-dibenzoyl tartrate dihydrate (1:2), C32H36O10N, P1, a = 7.828(3) Angstrom, b = 8.233(1) Angstrom, c = 24.888(8) Angstrom, alpha = 93.98 degrees, beta = 94.58(3)degrees, = 89.99(2)degrees, Z = 2. An analysis of the hydrogen-bonding schemes in terms of crystal packing, stoichiometric variations, and substitutional variations in these materials provides insights to design hydrogen-bonded networks directed toward the engineering of crystalline nonlinear optical materials.

Structure of diaquadichlorobis(hydrazinium)iron(II) dichloride

[Fe(N2H5)2(H2O)2Cl2].Cl2, M(r) = 299.65, monoclinic, P2(1)/c, a = 8.027 (1), b = 5.725 (2), c = 11.430 (2) angstrom, beta = 97.08 (1)-degrees, V = 521.3 (2) angstrom 3, Z = 2, D(m) = 1.92, D(x) = 1.910 g cm-3, lambda(Mo K-alpha) = 0.71069 angstrom, mu = 24.5 cm-1, F(000) = 304, T = 295 K, final R = 0.0242 and wR = 0.0292 for 1411 significant [F(o) > 5.0-sigma(F(o))] reflections. The crystal contains discrete Cl- ions and complex [Fe(N2H5)2(H2O)2Cl2]2+ cations. In the complex cation, the Fe atom is bonded to two hydrazinium cations, two Cl atoms and two water molecules. The coordinated atoms are trans to each other. The ions are connected by both N-H...Cl and O-H...Cl type hydrogen bonds.

Thermal Stability of Spherical Nanoporous Aggregates and Formation of Hollow Structures by Sintering-A Phase-Field Study

Nanoporous structures are widely used for many applications and hence it Is important to investigate their thermal stability. We study the stability of spherical nanoporous aggregates using phase-field simulations that explore systematically the effect of grain boundary diffusion, surface diffusion, and grain boundary mobility on the pathways for microstructural evolution. Our simulations for different combinations of surface and GB diffusivity and GB mobility show four distinct microstructural pathways en route to 100% density: multiple dosed pores, hollow shells, hollow shells with a core, and multiple interconnected pores. The microstructures from our simulations are consistent with experimental observations in several different systems. Our results have important implications for rational synthesis of hollow nanostructures or aggregates with open pores, and for controlling the stability of nanoporous aggregates that are widely used for many applications.

Electrical characterization of surface defects in GaSb created by hydrogen plasma

Using steady state and transient capacitance measurements, the electrical characteristics of a defect layer on the surface of bulk GaSb created during the hydrogen plasma treatment is presented. The trap density, activation energies, and the thickness of the defect layer have been calculated. The trap densities are comparable in magnitude to the carrier concentration. The defects introduce multiple energy levels in the band gap. Typical defect layer thicknesses range from a few angstroms to a fraction of a micron. © 1995 American Institute of Physics.

Evaluation of relative growth limitation due to depletion of glucose and oxygen during fungal growth on a spherical solid particle

A mathematical model for glucose and oxygen consumption, and cell growth during fungal growth on a single solid particle is developed. A moving biofilm is assumed to be present on the surface of the solid particle. Initially only glucose is assumed to be growth limiting and later oxygen transferred from the gas phase on to the biofilm is also assumed to be growth limiting. Glucose is found to be severely growth limiting when assumed to be the only growth limiting factor and its limiting levels far less severe when oxygen limitation is also included. The objective of the model is to gain a better understanding of the mass transfer and relative growth limiting characteristics of glucose and oxygen in fungal growth systems. The results obtained from the model proposed here will be the subject of future work.

Further studies on Norrish type II reactions including a reaction in the first excited singlet state and cyclization of 1:4 biradicals

The Norrish type II processes of methyl-2,2-dimethyl- cyclopropyl ketone, alpha-alkoxy acetones and alkyl pyruvates have been examined using the AM1 semi-empirical molecular orbital method with complete geometry optimization at the partial configuration interaction level in the restricted Hartree-Fock (RHF) frame. The results reveal that the methyl-substituted cyclopropyl ketone has a constrained geometry favourable for hydrogen abstraction from the gamma-position relative to the carbonyl group in the excited singlet state. The presence of the ether oxygen atom in the beta-position relative to the carbonyl group in alkoxy acetones and alkyl pyruvates leads to increased reactivity relative to alkyl monoketones and diketones respectively. The cyclization of 1:4 biradicals has been studied in the unrestricted Hartree-Fock (UHF) frame, and the results reveal that the 1:4 biradical derived from alkoxy acetones readily cyclizes to form oxetanols. On the other hand, in the 1:4 biradicals derived from methyl-substituted cyclopropyl ketone, the three-membered ring breaks readily to form an enol intermediate. Delocalization of an odd electron in 1:4 biradicals derived from alkyl pyruvates is thought to make cyclization difficult.

Optical and electrical properties of hydrogen-passivated gallium antimonide

The effect of hydrogen-plasma passivation on the optical and electrical properties of gallium antimonide bulk single crystals is presented. Fundamental changes of the radiative recombination after hydrogenation in undoped, zinc-doped, tellurium-doped, and codoped (with Zn and Te) GaSb are reported. The results of optical measurements indicate that passivation of acceptors is more efficient than that of the donors and, in general, the passivation efficiency depends on the doping level. Passivation of deep nonradiative centers is reflected by the gain of photoluminescence intensity and decrease in deep-level transient spectroscopy peak height. Extended defects like grain boundaries and dislocations have also been found to be passivated. The thermal stability of the passivated deep level and extended defects is higher than that of the shallow level. The kinetics of thermally released hydrogen in the bulk has been studied by reverse-bias annealing experiments.

Ultraviolet photoelectron spectroscopy of the hydrogen bonded heterodimer H2Scdots, three dots, centeredHCl

The HeI photoelectron spectrum of the hydrogen bonded hetero-dimer H2Scdots, three dots, centeredHCl shows two vertical ionization energies at 10.91 and 12.16 eV. Ab initio MO calculations reveal that these features are due to the sulphur and chlorine lone pair ionizations respectively. Results show that while the ground ionic state is repulsive the first excited ionic state is strongly bound. The photoelectron spectrum of the diethyl sulphidecdots, three dots, centeredHCl complex is similar to that of H2Scdots, three dots, centeredHCl.

Hydrogen-Bond-Directed Nonlinear-Optical Organic Materials - Evidence For The Control Of 2nd-Harmonic Generation Activities From The X-Ray Crystal-Structures Of The Complexes Of L-Tartaric Acid With M-Anisidine and P-Toluidine

Several substituted anilines were converted to binary salts with L-tartaric acid. Second harmonic generation (SHG) activities of these salts were determined. The crystal packing in two structures, (i) m-anisidinium-L-tartrate monohydrate (i) and (ii) p-toluidinium-L-tartrate (2), studied using X-ray diffraction demonstrates that extensive hydrogen bonding steers the components into a framework which has a direct bearing on the SHG activity

Vesicle formation from dimeric surfactants through ion-pairing. Adjustment of polar headgroup separation leads to control over vesicular thermotropic properties

Eight new vesicle-forming dimeric surfactants are synthesized: the polar headgroup separation in such dimeric amphiphiles strongly influences their vesicular thermotropic phase-transition behaviour.