A theory of localized electronic breakdown in insulating films

A review of measured breakdown properties in insulating films and of electronic theories of breakdown shows both theoretical and experimental inconsistencies. A theory is proposed in which electronic breakdown is caused by local chance events, such as a succession of avalanches at one spot. Successive avalanches sustain the growth of space charges, the local cathode field, and the avalanche rate. When the cathode field becomes large enough to make continuation of avalanching a certainty, instability with current runaway arises, causing breakdown. According to theory, breakdowns occur over a range of fields, their chance increasing very strongly with field; the breakdowns occur randomly in space and in time; the time to instability on a breakdown event decreases as some exponential function of increasing field; the breakdown field can both increase and decrease with temperature; it may be electrode dependent, and it decreases first rapidly and then more slowly with increasing film thickness. Observations made in a number of insulating films support the proposed theory.     

Organotransition-Metal-Mediated Borane-Cage Expansion

Attempts were made to synthesize dinuclear Group 6 metal complexes of the arachno -octahydrotriborate anion. Specifically, the dimer [CpW(CO) 3 ] 2 (Cp = m 5 -C 5 H 5 ) was employed as the metal reagent. However, the formation of dinuclear metallaborane complexes did not ensue. Rather, clean, rapid, systematic borane-cage expansion was observed. The [B 3 H 8 ] m cluster was quantitatively converted to B 5 H 9 ,[B 9 H 14 ] m , or [B 11 H 14 ] m , or both, depending on reaction conditions. Refluxing a 1:1 mixture of [B 3 H 8 ] m and [CpW(CO) 3 ] 2 in toluene or dioxane lead to the rapid, quantitative conversion of [B 3 H 8 ] m to B 5 H 9 . However, refluxing a 2:1 mixture of [B 3 H 8 ] m and [CpW(CO) 3 ] 2 in the same solvents yielded [B 9 H 14 ] m and [B 11 H 14 ] m as the primary products; when glyme was the solvent, only [B 9 H 14 ] m was observed, even after prolonged heating.     

Complexes of N-Phosphorylated Thioureas RNHC(S)NHP(O)(OiPr)2 (R = 2-MeC6H4, 2,6-Me2C6H3, 2,4,6-Me3C6H2) With Nickel(II)

The reaction of O,O′-diisopropylphosphoric acid isothiocyanate (iPrO)₂P(O)NCS with 2-methylaniline 2-MeC₆H₄NH₂, 2,6-dimethylaniline 2,6-Me₂C₆H₃NH₂, or 2,4,6-trimethylaniline 2,4,6-Me₃C₆H₂NH₂ leads to the N-phosphorylated thioureas RNHC(S)NHP(O)(OiPr)₂ (R = 2-MeC₆H₄?, HL^I ; 2,6-Me₂C₆H₃?, HL^II ; 2,4,6-Me₃C₆H₂?, HL^III ). Reaction of the potassium salts of HL^{I –III} with Ni(II) in aqueous EtOH leads to [Ni(L^I–III-N,S)₂] ([NiL^I–III ₂ ]) chelate complexes. The compounds obtained were investigated by ¹H, ³¹P{¹H} NMR spectroscopy and microanalysis. The molecular structure of the thiourea HL^III was elucidated by single crystal X-ray diffraction analysis. Single crystal X-ray diffraction studies showed that HL^III forms both intra- and intermolecular hydrogen bonds, which in turn leads to the formation of polymeric chains. One of the intermolecular hydrogen bonds is of the type N?H…S. Moreover, the formation of intermolecular C?H…η⁶-phenyl interactions was established.

Supplemental materials are available for this article. Go to the publisher's online edition of Phosphorus, Sulfur, and Silicon and the Related Elements to view the free supplemental file.     

Syntheses,Structures and Photoelectron Spectra of Phospha-Alkenes and Phospha-Alkynes and Their Transition Metal Complexes

Abstract

The chemistry of the novel phospha-alkenes RP[dbnd]CR'₂, and phospha-alkynes, RC[tbnd]P, containing 2pπ-3pπ bonds is of current interest.^1,2 Recent molecular orbital calculations,^3,4 suggest that the highest occupied molecular orbital in CH₂[dbnd]PH is of the π-type with the phosphorus lone pair ó-orbital only slightly more stable while the π? lumo orbital is relatively low lying. We now report He (I) photoelectron spectroscopic studies on a variety of RC[tbnd]P molecules^5,6 which indicate that the homo is also of the π-type and the π-σ separation is much greater than that found in the analogous RC[tbnd]N systems.     

Stabilization of Large Adsorbates by Rotational Entropy: A Time‐Resolved Variable‐Temperature STM Study

Investigating the dynamics in an adlayer of the oligopyridine derivative 2‐phenyl‐4,6‐bis(6‐(pyridine‐2‐yl)‐4‐(pyridine‐4‐yl)pyridine‐2‐yl)pyrimidine (2,4′‐BTP) on Ag(111) by fast scanning tunneling microscopy (video‐STM), we found that rotating 2,4′‐BTP adsorbates coexist in a two‐dimensional (2D) liquid phase (β‐phase) in a dynamic equilibrium with static adsorbate molecules. Furthermore, exchange between an ordered phase (α‐phase) and β‐phase leads to fluctuations of the domain boundary on a time scale of seconds. Quantitative evaluation of the temperature‐dependent equilibrium between rotating and static adsorbates, evaluated from a large number of STM images, gains insight into energetic and entropic stabilization and underlines that the rotating adsorbate molecules are stabilized by an entropy contribution, which is compatible with that derived by using statistical mechanics. The general validity of the concept of entropic stabilization of rotating admolecules, favoring rotation already at room temperature, is tested for other typical small, mid‐size and large adsorbates.     

Development and validation studies for determination of phenylpropanoid‐substituted flavan‐3‐ols in semipurified extract of Trichilia catigua by high‐performance liquid chromatography with photodiode array detection

The phenolic compounds are the main phytochemical constituents of the bark of Trichilia catigua and are commonly used for medicinal purposes. An HPLC method for the simultaneous quantification of phenolic compounds (procyanidin B2 (PB2), epicatechin (EPC), chinchonains Ia, Ib, IIa, IIb, catechin, and chrologenic acid) in T. catigua extract was developed and validated. A suitable chromatographic system was selected, which uses a gradient elution with methanol/ACN (75:25), and water both with 0.05% TFA, as mobile phase, column Luna, 280 nm, and flow 0.4 mL/min. Validation of the analytical method was based on the parameters: linearity, precision, LODs and LOQs, accuracy, robustness, and stability. The method showed linearity for PB2 and EPC, in the range 10–120 μg/mL with good correlation coefficients (>0.996). For precision, the repeatability ranged from 1.89 to 3.23%, and the values for accuracy for PB2 and EPC were 95 and 89%, respectively. The LODs and LOQs for PB2 were 1.36 and 4.12 μg/mL, and for EPC were 2.18 and 6.61 μg/mL, respectively. The method was robust under the conditions employed. The proposed method could be employed for quality assessment of T. catigua, as well as pharmaceutical products.     

Enumeration of substitutional isomers with restrictive mutual positions of ligands: I. Overall counts

We address systematics for the enumeration of substitutional isomers when there is constrained positioning of ligands on a molecular skeleton. One constraint involves ‘restrictive ligands’ where two of the same kind are forbidden to occupy adjacent sites in a molecular skeleton. This may arise because of steric hindrance, or because of groups which in neighbor proximity react to eliminate one. For instance, no pair of –OH groups attach to the same C atom in a molecular skeleton. In another case, malonic acid residues decarboxylate leaving no more than one decarboxylation in each residue. The enumeration with such restrictive ligands may be addressed via a Polya-theoretic cycle index hybridized with the graph-theoretic independence polynomial (when there is just a single such neighbor-excluding ligand and another which is not), while more generally a hybridization with the chromatic polynomial is needed. Another substitional-isomer constraint involves bidentate ligands, with each ligand-part occupying adjacent sites, and possibly also with additional separate unidentate ligands. Here, the set of all pure & mixed such ligand placements is analytically represented by a ‘symmetry-reduced’ matching polynomial (which is a hybrid now of the matching polynomial and Polya’s cycle index). This result gives the generating function for isomer enumeration, taking into account every possible so-restricted assortment of the employed ligands. Here we make such novel hybridizations (for these and other graphtheoretic polynomials) to deal with such oft-encountered chemical problems, which nevertheless transcend typical earlier unconstrained formulizations. Further subsymmetry classification & enumerations, along with examples are considered in a further article.     

Mechanistic Insights into the Rhenium‐Catalyzed Alcohol‐To‐Olefin Dehydration Reaction

Rhenium‐based complexes are powerful catalysts for the dehydration of various alcohols to the corresponding olefins. Here, we report on both experimental and theoretical (DFT) studies into the mechanism of the rhenium‐catalyzed dehydration of alcohols to olefins in general, and the methyltrioxorhenium‐catalyzed dehydration of 1‐phenylethanol to styrene in particular. The experimental and theoretical studies are in good agreement, both showing the involvement of several proton transfers, and of a carbenium ion intermediate in the catalytic cycle.