Absorption and scattering characterization of airborne microparticulates by a cavity ringdown technique

Nonresonant cavity ringdown laser absorption spectroscopy (CRLAS) was applied for detection and characterization of airborne particulates. Sensitive detection of a variety of aerosols under ambient conditions was achieved. The method provides, for the first time, time-resolved absolute aerosol concentration, with spatial resolution (along a line). The first report on absorption spectroscopy of monodispersed aerosols (in the size range 100–200 nm) is provided, and comparisons are made with the bulk data. The results indicate the possibility of applying CRLAS for selective analysis of aerosols. A new method for estimating the aerosol refraction index is also obtained from the ringdown data.     

Benchmark policies for utility-carrying queues with impatience

Queueing Systems - Men and jobs alike are characterized by a single trait, which may take on categorical values according to given population frequencies. Men arrive to the system following a...     

Inversion formulas for infinite generalized Toeplitz matrices

Inversion formulas are obtained for a certain class of infinite matrices that possess displacement structure similar to that of finite block Toeplitz matrices. Consequences are symmetric inversion formulas for matrix-valued singular integral operators and infinite Toeplitz plus Hankel matrices.     

Micro-heterometric determination of molybdic acid with α-benzoin oxime

1 mg of molybdenum as molybdic acid in 20 ml solution can be titrated heterornetrically with a-benzoin oxime in the presence of ~ 99% of most bivalent or tri valent cations as well as in excesses of tungstic or vanadic acids. Error: 0–2%.     

Reduction of the allylic substituents in Ni(I)(1,8-dipropenyl-1,4,8,11-tetraazacyclotetradecane)+ by the central Ni(I) in aqueous solutions

The complex Ni(II)(1,8,-di-2-propenyl-1,4,8,11-tetraazacyclotetradecane)(2+), (NiL(1))(2+), was synthesized. X-ray crystallography demonstrates that the complex obtained is the trans-III isomer. The allylic substituents shift the redox couples (NiL(1))(3+/2+) and (NiL(1))(2+/+) anodically relative to the corresponding couples for Ni(II)(1,4,8,11-tetraazacyclotetradecane)(2+), (NiL(2))(2+), as expected. Surprisingly, the lifetime of (NiL(1))(+) in neutral aqueous solutions is shorter than that of (NiL(2))(+). Pulse radiolysis experiments reveal that the allylic substituents are reduced by the central Ni(I) ion. The first step in this reduction is a general acid catalyzed process. The results suggest that this step involves schematically the reaction Ni(I)[bond]NCH(2)CH[double bond]CH(2)(+) + H(+) --> Ni(III)[bond]NCH2CH2CH(2)(2+). The latter transient decomposes slowly with a half-life time of several minutes. Preliminary results support the suggestion that (NiL(2))(+), or other Ni(I)L complexes of this family, might reduce many alkenes present in the solution.     

Biphenalenylidene: the forgotten bistricyclic aromatic ene. A theoretical study

The Lawesson reagent and P(2)S(5) mediated reductive coupling of phenalenone (6) gives LPAH peropyrene (5) in 47% and 54% yields. The mechanism of the reaction involves the formation of phenalenethione (10), Z- and E-1,1'-biphenalenylidene (3), and 9 as intermediates. The electrocyclization reaction of Z-3 to 9, followed by aromatization, gives 5. The results of an ab initio and DFT study of 3 and 2,2'-biphenalenylidene (12) are reported. E-3 and Z-3 have a diradical character with twist angles of 44.8 degrees and 57.8 degrees (at UB3LYP/6-311G**). Delta E(++)(Tot) = 10.2 kJ/mol and Delta G(++)(298) = 10.6 kJ/mol for E-3 <==> Z-3 diastereomerization. These unusually low energy barriers are due to the ground-state diradical destabilization and the aromatic stabilization of the transition state TS-3. Triplet Z-3 is higher in energy than singlets E-3 and Z-3 by 10.4 and 3.1 kJ/mol. In the concealed non-Kekulé 12, singlet 12 is more stable than the triplet by 1.3 kJ/mol. Singlet 12 is more stable than singlet E-3 by 2.0 kJ/mol, and orthogonal singlet TS-12 is lower in energy than singlet TS-3 by 6.0 kJ/mol. The energy barriers for the hexatriene-cyclohexadiene electrocyclization Z-3 --> 9 are Delta E(++)(Tot) = 94.8 and Delta G(++)(298) = 98.3 kJ/mol (at (U)B3LYP/6-31G). The reaction occurs thermally in a conrotatory mode.     

Understanding the reactivity of polycyclic aromatic hydrocarbons and related compounds

This perspective article summarizes recent applications of the combination of the activation strain model of reactivity and the energy decomposition analysis methods to the study of the reactivity of polycyclic aromatic hydrocarbons and related compounds such as cycloparaphenylenes, fullerenes and doped systems. To this end, we have selected representative examples to highlight the usefulness of this relatively novel computational approach to gain quantitative insight into the factors controlling the so far not fully understood reactivity of these species. Issues such as the influence of the size and curvature of the system on the reactivity are covered herein, which is crucial for the rational design of novel compounds with tuneable applications in different fields such as materials science or medicinal chemistry.

This perspective article summarizes recent applications of the combined activation strain model of reactivity and the energy decomposition analysis methods to the study of the reactivity of polycyclic aromatic hydrocarbons and related compounds.     

Binding neutral guests to concave surfaces of molecular hosts. Directional association of water and methylene chloride with hosts containing only cyclic urea binding sites

This paper is concerned with studies of weak intermolecular interactions in molecular inclusion type systems involving uncharged host and guest entities. Three new complexes of synthetic organic ligands with water and methylene chloride have been characterized by single-crystal X-ray diffraction. The hosts are composed of three cyclic urea units whose carbonyl groups are held in convergent positions by bonding their attached nitrogens to one another through two (noncyclic ligand) or three (macrocyclic ligand) rigid spacer units. Conformational organization is further enforced by an aliphatic bridge between two of the phenylene spacers in the macrocyclic hosts and an additional dimerization of the open-chain ligand. The host species were found to be particularly suitable to interact with proton donating H₂O and CH₂Cl₂ guest moieties, as their molecular surface contains appropriately sized polar cavities lined with the carbonyl functions. Association between the interacting components in these complexes is stabilized by O–HO and C–HO hydrogen bonds. In the corresponding crystal structures additional molecules of the solvent are located between units of the complex. The significance of preorganization of the host structure to an efficient guest binding is emphasized by an observation that no stable complexes of a similar but unbridged macrocyclic ligand could be crystallized from the same solvent. The structural features of the inclusion compounds are described in detail, and the host-guest interaction scheme is compared to that observed in complexes of 18-crown-6 with neutral guests. Supplementary Data relating to this article are deposited with the British Library as Supplementary Publication No. SUP 82039 (98 pages)