A density matrix variational calculation for atomic Be

The ground-state energy of the beryllium atom is calculated using a variational procedure in which the elements of the two-body reduced density matrix (particle–particle matrix) are the variational parameters. It is shown that, for this problem and with the limited number of spin-orbitals used, the trace condition and the simultaneous nonnegativity conditions on the particle–particle, the particle–hole, and the hole–hole matrices form a complete solution to the N-representability problem. The energy obtained is – 14.61425 a.u., practically identical to the value given by a configuration interaction calculation which uses the same states. The effects of weakening the nonnegativity conditions on each of the matrices in turn were also explored.     

Electroinitiated polymerization of vinylic monomers in polar systems. III. Polymerization of acrylates and methacrylates in alcohol solutions

Methyl, ethyl, and n-butyl acrylates and methacrylates are polymerized by electroinitiation in methanol–, ethanol–, and n-propanol–electrolyte mixtures in which the monomers are soluble whereas the polymers obtained are insoluble. The technique of changing the polarity of the electrodes described earlier was used. The relationships between molecular weights and polymer yields as function of current density, initial monomer concentration and dielectric constant of the solvent are described. A kinetic scheme for the initiation, propagation, and termination is given.     

On a new partitioning of the Hamiltonian in many-body calculation of pair-correlation energies in closed-shell systems

We introduce here a new partitioning of the Hamiltonian in calculating pair-correlation energies using many-body perturbation theory, by which we are able to eliminate the off-diagonal particle–hole (p–h) ladders exactly to all orders in the perturbation expansion. In this formulation, the particle states turn out to be different for each distinct pair of hole states in the correlation energy calculation. We have also included the contributions of the diagonal particle–particle (p–p) and hole–hole ladders exactly to all orders. The effect of the off-diagonal p–p ladders has been estimated for each pair by computing the third-, foruth- and fifth-order energies. For highly symmetric systems the present partitioning yields in general symmetry-broken orbitals. Here one may use an average kind of partitioning for all the partners of the degenerate sets, which restores the symmetry and at the same time ensures cancellation of the p–h ladders exactly at the lowest order and approximately at the higher orders. Results are presented for a selection of 6π-electron conjugated systems. The correlation energy for each pair is in excellent agreement with that obtained from a partial CI calculation involving all double excitations from this pair. The advantages of implementing the present scheme in larger systems has been discussed.     

Electroinitiated polymerization of vinylic monomers in polar systems. II. Polymerization of ethyl methacrylate in methanol solutions

Ethyl methacrylate is polymerized by electroinitiation in methanol–electrolyte mixtures in which the monomer is soluble whereas the polymer obtained is insoluble. A technique of changing the polarity of the electrodes is used. With this technique a polymer of high molecular weight can be obtained. The relationships between molecular weight and monomer concentration, current densities, and time of the reaction as well as the yield as a function of current density, time of the reaction and initial monomer concentration are given. A free-radical mechanism is proposed for the reaction.     

Molecular quantum similarity measures and N-dimensional representation of quantum objects. II. Practical applications

Newly developed classification and graphical representation techniques are discussed in order to visualize quantum similarity computational results. Some examples of molecular quantum similarity applications involving the fields of (a) structure–property relationships, (b) structure–activity relationships, (c) MO taxonomy, and (d) conformer taxonomy are presented.     

Equation of state of poly(dimethylsiloxane) melts

The pressure–volume–temperature (PVT) properties of three commercial samples of poly(dimethylsiloxane) are studied experimentally and theoretically in the temperature range 25–150°C and for pressure to ∼ 3 kbar. The Tait equation is employed to represent the data at elevated pressure. Isothermal compressibilities are computed for the three samples. The melt data are analyzed in terms of the Simha–Somcynsky hole theory, and scaling parameters of pressure, volume, and temperature are obtained. Satisfactory agreement between theory and experiment is found over the entire range of experimental pressures. © 1998 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 36: 841–850, 1998     

Hole polarons in pure BaTiO3 studied by computer modeling

Self‐trapped hole polarons in technologically important perovskite‐type ceramic of BaTiO₃ have been modeled by means of the quantum chemical method modified for crystal calculations. The computations are carried out in the self‐consistent field (SCF) manner using the embedded molecular cluster model. The spatial configuration of a hole polaron, displacement of defect‐surrounding atoms, and wave functions of the polaron ground and excited states are obtained and analyzed. The probability of spontaneous hole self‐trapping is estimated in the perfect lattice of the BaTiO₃ crystal by calculating the value of the hole self‐trapping energy as a difference of the atomic relaxation energy and the hole localization energy. This value is found to be negative, −1.49 eV, which demonstrates the preference of the self‐trapped polaron state. The calculated polaron absorption energy, 0.5 eV, is discussed in light of the available experimental data. © 2000 John Wiley & Sons, Inc. Int J Quant Chem 79: 358–366, 2000     

Analysis of reduced density matrices in the coordinate representation. II. The structure of closed-shell atoms in the restricted Hartree–Fock approximation

The electron density and the Fermi hole of the ground states of the atoms He, Be, Ne, Mg, Ar, Ca, Zn, and Kr are studied in the restricted Hartree–Fock approximation. The use of single Slater-type orbitals for the free atoms is also discussed.     

Organic light‐emitting diodes with multiple photocrosslinkable hole‐transport layers

We report on photocrosslinkable hole‐transport polymers and their use as photodefinable hole‐transport layers in organic light‐emitting diodes. The polymers were obtained by copolymerization of bis(diarylamino)biphenyl‐based acrylate monomers with cinnamate‐functionalized acrylate moieties. Polymers with a range of redox potentials were obtained by varying the substitution patterns of the bis(diarylamino)biphenyl units. The 2 + 2 cycloaddition of the cinnamate moieties following UV irradiation renders the material insoluble. This allows for patterning of the polymer and simultaneously enables the fabrication of multilayer structures from solution. Hole mobilities were measured in these copolymers with the time‐of‐flight technique. Their performance as hole‐transport layers in light‐emitting diodes, with tris(8‐hydroxyquinolinato)aluminum as the emitter and electron‐transport layer, is evaluated. Electroluminescent devices with multiple hole‐transport layers having different ionization potentials were fabricated from solution, and the quantum efficiency of these devices was greater than that for devices based on a single hole‐transport layer. © 2003 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 41: 2726–2732, 2003     

Elementary steps for charge transport in DNA: thermal activation vs. tunneling

Using stacks of Watson–Crick base pairs as an important example of multichromophoric molecular assemblies, we studied charge migration in DNA with special emphasis on the mechanism of hole hopping between neighboring guanines (G) connected by the adenine–thymine (AT) bridge. The tight-binding model proposed for this elementary step shows that for short AT bridges, hole transfer between two G bases proceeds via quantum mechanical tunneling. By contrast, hopping over long bridges requires thermal activation. The condition for crossover between tunneling and thermal activation near room temperature is specified and applies to the analysis of experimental data. We show that thermal activation dominates, if the bridge between two G bases contains more than three AT pairs. Our theoretical findings predict that the replacement of AT base pairs by GC pairs increases the efficiency of hole transport only in the case of short base pair sequences. For long sequences, however, the opposite effect is expected.     

Prediction and estimation of solvent diffusivities in polyacrylate and polymethacrylates

The relationship between polymer side‐chain length and the hole free volume that is effective for solvent diffusion was investigated for polyacrylates and polymethacrylates on the basis of free‐volume theory. Measurements of a polymer's viscoelasticity and solvent diffusivity provided experimental evidence for polymer segment mobility, and the results indicated that hole free volume in a linear polymer increases with hydrocarbon side‐chain length. Because the molecular mechanisms of polymer viscoelasticity and diffusivity are identical, the free‐volume parameters obtained for polyacrylates and polymethacrylates by measuring the polymer viscoelastic‐temperature dependence can reliably be used in predicting the solvent diffusion coefficient. © 2003 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 41: 1393–1400, 2003     

Evaluation of gamma camera collimator for regional cerebral blood flow measurements using a 133Xe inhalation method

In the measurements of regional cerebral blood flow (rCBF) using inhalation of 133Xe gas, the activity present is generally limited in lower levels than those of usual brain scintigraphy. Measurements with low count-rate are usually resulted in diminishing the accuracies of results obtained. Therefore, it is necessary to make measurements using a high sensitive collimator for getting as much count-rate as possible when a gamma camera is used. The relationships among sensitivity and structures of multi-parallel collimator were mathematically analyzed. The results of analysis suggested that sensitivity usually increased by using a collimator with holes of reduced height and diameter. A prototype multi-parallel collimator with holes of low height and small diameter was made in our laboratory for testing sensitivity and resolution. The collimator possessing 1141 holes of 6 mm phi in hole diameter, 1.5 cm is hole height and septal thickness of 1 mm lead showed 24 times more sensitive than those of a general all purpose collimator supplied by the manufacturer. However, resolution measured in FWHM was of 9 to 14 mm at the collimator face and of 29 to 38 mm at 5 cm from the face. The results indicated that this collimator was useful enough in rCBF measurements with 133Xe inhalation using a gamma camera. The mathematical analysis however, suggested that optimum collimator for rCBF measurements was approximate 4.5 mm phi in hole diameter and 1.0 cm in hole height.     

Hollow spheres of aromatic polyamide prepared by reaction‐induced phase separation

Hollow spheres of aromatic polyamide are obtained by the reaction‐induced phase separation during polymerization of 5‐hydroxyisophthalic acid and 1,4‐phenylene diamine in an aromatic solvent at a concentration of 1–2% at 320 °C without stirring. The hollow sphere has a dimple hole and the diameters of the hollow spheres are 3–4 μm. The droplets are initially generated via liquid–liquid phase separation and then rigid cross‐linked network structure formed the rigid skin layer on the surface of the droplets. The solidification of the droplets occurred owing to the further polymerization in them with maintaining the morphology to form the hollow spheres. The hollow spheres exhibit outstanding thermal stability. © 2012 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2013     

Dynamics and energetics of single-step hole transport in DNA hairpins

The dynamics of single-step hole transport processes have been investigated in a number of DNA conjugates possessing a stilbenedicarboxamide electron acceptor, a guanine primary donor, and several secondary donors. Rate constants for both forward and return hole transport between the primary and secondary donor are obtained from kinetic modeling of the nanosecond transient absorption decay profiles of the stilbene anion radical. The kinetic model requires that the hole be localized on either the primary or the secondary donor and not delocalized over both the primary and the secondary donor. Rate constants for hole transport are found to be dependent upon the identity of the secondary donor, the intervening bases, and the location of the secondary donor in the same strand as the primary donor or in the complementary strand. Rate constants for hole transport are much slower than those for the superexchange process used to inject the hole on the primary donor. This difference is attributed to the larger solvent reorganization energy for charge transport versus charge separation. The hole transport rate constants obtained in these experiments are consistent with experimental data for single-step hole transport from other transient absorption studies. Their relevance to long-distance hole migration over tens of base pairs remains to be determined. The forward and return hole transport rate constants provide equilibrium constants and free energies for hole transport equilibria. Secondary GG and GGG donors are found to form very shallow hole traps, whereas the nucleobase deazaguanine forms a relatively deep hole trap. This conclusion is in accord with selected strand cleavage data and thus appears to be representative of the behavior of holes in duplex DNA. Our results are discussed in the context of current theoretical models of hole transport in DNA.     

Photochemical hole-burning in photosynthetic reaction centers

The change in absorbance (hole spectrum) of the primary electron donor (P870) in bacterial photosynthetic reaction centers has been studied at 1.5–2.1 K following narrow-band excitation at several wavelengths within the P870 absorption band. The hole width is very large, suggesting a homogeneous linewidth on the order of 200–300 cm⁻¹. Possible interpretations of this highly unusual result, including ultra-fast excited-state decay, are discussed.     

Binding energies of ionized donor–exciton complexes in polar crystals

Binding energies of ionized donor–exciton complexes in crystals of CdSe, CdTe, and ZnSe are calculated using the most realistic effective potentials available for the electron–hole interaction.     

How Does Bridging Core Modification Alter the Photovoltaic Characteristics of Triphenylamine-Based Hole Transport Materials? Theoretical Understanding and Prediction

Perovskite solar cells have gained immense interest from researchers owing to their good photophysical properties, low-cost production, and high power conversion efficiencies. Hole transport materials (HTMs) play a dominant role in enhancing the power conversion efficiencies (PCEs) and long diffusion length of holes and electrons in perovskite solar cells. In hole transport materials, modification of π-linkers has proved to be an efficient approach for enhancing the overall PCE of perovskite solar cells. In this work, π-linker modification of a recently synthesized H−Bi molecule ( R ) is achieved with novel π-linkers. After structural modifications, ten novel HTMs ( HB1–HB10 ) with a D−π−D backbone are obtained. The structure–property relationship, and optoelectronic and photovoltaic characteristics of these newly designed hole transport materials are examined comprehensively and compared with reference molecules. In addition, different geometric parameters are also examined with the assistance of density functional theory (DFT) and time-dependent DFT. All the designed molecules exhibit narrow HOMO–LUMO energy gaps (E_g=2.82–2.99 eV) compared with the R molecule (E_g=3.05 eV). The designed molecules express redshifting in their absorption spectra with low values of excitation energy, which in return offer high power conversion efficiencies. Further, density of states and molecular electrostatic potential analysis is performed to locate the different charge sites in the molecules. The reorganizational energies of holes and electrons are found to have good values, suggesting that these novel designed molecules are efficient hole transport materials for perovskite solar cells. In addition, the low binding energy values of the designed molecules (compared with R ) offer high current charge density. Finally, complex study of HB9:PC₆₁BM is also undertaken to understand the charge transfer between the molecules of the complex. The results of all analyses advocate that these novel designed HTMs are promising candidates for the construction of future high-performance perovskite solar cells.     

Fracture surface morphology and phase relationships of polystyrene/poly(methyl methacrylate) systems. I. Low-molecular-weight polystyrene in poly(methyl methacrylate)

Samples of low-molecular-weight polystyrene (PS) in poly(methyl methacrylate) (PMMA) were prepared by first dissolving PS in methyl methacrylate monomer and then polymerizing the monomer. Forty-three specimens of varying number-average molecular weight (2100–49,000) and composition (5–40 wt %) of PS were prepared, and the surface morphology and phase relationships studied by scanning electron microscopy. Four distinct types of phase relationships were observed: (i) a single phase consisting of PS dissolved in PMMA; (ii) PS dispersed in PMMA; (iii) PMMA dispersed in PS; and (iv) regions of PS dispersed in PMMA coexisting with regions of PMMA dispersed in PS. Values of the size and population density of the dispersed particles are reported. Finally, the size and distribution of the dispersed particles and the various types of phase relationships are discussed in terms of the ternary polystyrene/poly(methyl methacrylate)/methyl methacrylate phase diagram.     

A general theory for the formation of a new phase. Static cavitation in a nonvolatile liquid

The rate of formation of cavities in a nonvolatile liquid subjected to negative pressure is calculated. The pressure is assumed to be sufficiently moderate, so that the critical bubble (hole) can be treated macroscopically. If this condition is satisfied one may apply the Zel'dovich saddle-point method. However, in distinction from that method, the present paper makes use of a construction of Gibbs grand canonical ensemble, which allows one to avoid the ambiguities that appear in the calculation of the microscopic initial stage of formation of the hole and yields a more accurate value of the kinetic (preexponential) factor. A more accurate value of the work required to form a critical hole, the value which occurs in the exponent, is obtained by taking into account the dependence of the surface tension of the hole on the curvature of the surface. For this purpose we use an expansion of the surface tension in powers of the curvature of the surface of the hole. Since the surface tension occurs in the exponent of the expression for the probability or rate of cavitation, it is necessary to retain the expansion terms which are linear or quadratic in the curvature, whereas the correction factors to the cavitation rate, due to higher-order terms tend to unity as the expansion of the liquid decreases, in contradistinction from the first terms. It is important that for large expansions, and hence for small curvatures of the surface of the critical hole, the series for the surface tension diverges.     

Speciation of organotin compounds by tandem mass spectrometry

The collison-induced dissociation of a range of organotin compounds has been examined by tandem (MS–MS) mass spectrometry in order to investigate the potential of the technique for speciation without prior chromatographic separation. Tables of parent–daughte ion relationships are prepared for a group of six closely related compounds. The results obtained are sufficiently characteristic to distinguish between the isolated compounds and to identify the individual components of a simple mixture. This procedure has potential applications for less volatile compounds and for quantitative analysis.