Experimental and modeling study of the oxidation of xylenes

This paper describes an experimental and modeling study of the oxidation of the three isomers of xylene (ortho‐, meta‐, and para‐xylenes). For each compound, ignition delay times of hydrocarbon–oxygen–argon mixtures with fuel equivalence ratios from 0.5 to 2 were measured behind reflected shock waves for temperatures from 1330 to 1800 K and pressures from 6.7 to 9 bar. The results show a similar reactivity for the three isomers. A detailed kinetic mechanism has been proposed, which reproduces our experimental results, as well as some literature data obtained in a plug flow reactor at 1155 K showing a clear difference of reactivity between the three isomers of xylene. The main reaction paths have been determined by sensitivity and flux analyses and have allowed the differences of reactivity to be explained. © 2006 Wiley Periodicals, Inc. Int J Chem Kinet 38: 284–302, 2006     

Experimental investigation of the concept of molecular migration within sheared polystyrene

Several important aspects of the flow in polymer melts through capillaries remain unexplored. This paper examines experimentally one such effect associated with the radial shear-stress gradient in capillaries. During capillary melt flow of a polymer with a wide molecular weight distribution, migration of the large molecules away from the region of highest shear stress, i.e., at the capillary wall, has been predicted but only modestly investigated. This effect has the potential to produce a molecular weight spectrum over the cross section of extruded polymer. Studies of distribution in shear were conducted on a well-characterized wide-distribution polystyrene (M?_w = 234,000). An Instron Rheometer equipped with a long capillary (length/diameter ratio of 66.7) was used to perform the extrusion at temperatures of 160–250°C. A solvent coring procedure was used to dissolve away concentric layers of polymer from the extrudate for molecular weight analyses. The method has been shown to cut clean sections without selective extraction. Values of M?_w, M?_n and M?_w/M?_n were calculated from complete molecular weight distribution data obtained by calibrated gel permeation chromatography. For a wide range of shear rates and temperatures, no evidence for molecular fractionation was observed. Shear degradation of this polymer was found to be small. However, at high shear rates at 250°C, evidence indicating extensive shear-induced thermal degradation was found. No evidence for oxidative degradation at the extrudate surface was found at either low or high shear rates at this temperature.     

Experimental and theoretical investigation of the Li is spectra of molecular lithium halides

The Li 1s absorption spectra of molecular LiF and LiCl have been studied for the first time by experimental as well as by theoretical methods. The strong absorption bands at the onset of the Li 1s excitations can qualitatively be interpreted in terms of a simple ionic model. The spectra are analyzed and assigned in more detail on the basis of SCF and Cl calculations, and also within the framework of the Z + 1 core analogy model. The limitations of this model are discussed. Li 1s ionization potentials of LiF, Li₂F₂, LiCl, and Li₂Cl₂ are derived.     

Stability of molecular ions

Physical Institute, Leningrad State University. Translated from Zhurnal Strukturnoi Khimii, Vol. 32, No. 5, pp. 133–134, September–October, 1991.     

Experimental and theoretical investigation of the molecular and electronic structure of anticancer drug camptothecin

The Fourier Transform Infrared spectrum of (S)-4 ethyl-4-hydroxy-1H-pyrano [3',4':6,7]-indolizino-[1,2-b-quinoline-3,14-(4H,12H)-dione] [camptothecin] was recorded in the region 4000-400 cm(-1). The Fourier Transform Raman spectrum of camptothecin (CPT) was also recorded in the region 3500-50 cm(-1). Quantum chemical calculations of geometrical structural parameters and vibrational frequencies of CPT were carried out by MP2/6-31G(d,p) and density functional theory DFT/B3LYP/6-311++G(d,p) methods. The assignment of each normal mode has been made using the observed and calculated frequencies, their IR and Raman intensities. The harmonic vibrational frequencies were calculated and the scaled values have been compared with experimental FT-IR and FT-Raman spectra. Most of the computed frequencies were found to be in good agreement with the experimental observations. The isotropic chemical shifts computed by (13)C and (1)H NMR analysis also show good agreement with experimental observations. Comparison of calculated spectra with the experimental spectra provides important information about the ability of computational method to describe the vibrational modes of large sized organic molecule.     

Structure of the alkali-metal-atom + strontium molecular ions: towards photoassociation and formation of cold molecular ions

The potential energy curves, permanent and transition dipole moments, and the static dipolar polarizability, of molecular ions composed of one alkali-metal atom and a strontium ion are determined with a quantum chemistry approach. The molecular ions are treated as effective two-electron systems and are treated using effective core potentials including core polarization, large gaussian basis sets, and full configuration interaction. In the perspective of upcoming experiments aiming at merging cold atom and cold ion traps, possible paths for radiative charge exchange, photoassociation of a cold lithium or rubidium atom and a strontium ion are discussed, as well as the formation of stable molecular ions.     

Microwave spectroscopy of molecular ions

In this paper a brief overview of the research in microwave spectroscopy of molecular ions done at the University of Wisconsin will be given, with major emphasis on work done in the past year. Five molecular ions (CO⁺, HCO⁺, HNN⁺, HCS⁺, and HOC⁺) have been studied in this work, and all of them have also been detected by radioastronomy. Molecular structures (r_s for HCO⁺, HOC⁺, and HNN⁺ and r_e for HCO⁺) have been determined and important dynamical information has been obtained from pressure broadened linewidths (Δν/P), Doppler shifts, and relative intensity data. In particular the Δν/P values have been shown to correspond to the Langevin cross-section, indicating the monopole-induced dipole interaction is the pertinent intermolecular force.     

Polarizability of negative molecular ions

Theoretical and Experimental Chemistry -     

Estimation of xylenes with chloramine-T

The reaction between xylenes and chloramine-T has been found to proceed quantitatively over a wide range of experimental conditions. A back-titration procedure in which xylenes are reacted with excess of chloramine-T in 1M perchloric acid-0.03M sodium chloride medium has been developed. Inorganic ions other than chloride do not interfere. The method can be used for estimating xylenes in the presence of some other petrochemicals.     

Charge migration in the molecular ions of isoxazole derivatives

The effect of charge migration in the molecular ion on the dissociative ionization of isoxazole derivatives was examined. It is shown that charge redistribution between the substituent in the 3 position and the oxygen atom of the heteroring precedes isomerization of the molecular ion and its disintegration. The intensity of the peaks of the RCO⁺ ions in the mass spectra of 5-R-3-arylisoxazoles increases as the acceptor effect of the aryl substituent becomes stronger. It is shown that the average internal energy with which the nitrobenzoyl ions are formed decreases as the probability of their formation increases, i.e., as the intensity of charge migration from the aryl substituent to the oxygen atom increases.     

Redox-inactive ions control the redox-activity of molecular vanadium oxides

Polyoxometalates are key materials for energy conversion and storage due to their unique chemical tunability and electrochemical reactivity. Herein, we report that functionalization of molecular vanadium oxides, polyoxovanadates, with redox-inert Ca²⁺ cations leads to a significant increase in their electron storage capabilities. The electrochemical performance of the Ca²⁺-functionalized dodecavanadate [Ca₂V₁₂O₃₂Cl(DMF)₃]²⁻ (={Ca₂V₁₂}) was thus compared with that of the precursor compound (H₂NMe₂)₂[V₁₂O₃₂Cl]³⁻ (={V₁₂}). {Ca₂V₁₂} can store up to five electrons per cluster, while {V₁₂} only shows one reversible redox transition. In initial studies, we demonstrated that {Ca₂V₁₂} can be used as an active material in lithium-ion cathodes. Our results show how redox-inert cations can be used as structural and electrostatic stabilizers, leading to major changes in the redox-chemistry of polyoxovanadates.

The enhanced redox-activity of a molecular vanadium oxide cluster upon functionalization with redox-inert Ca²⁺ ions is reported together with initial insights into its performance as a lithium ion battery cathode.     

Reciprocity in the degeneracies of some tetra-atomic molecular ions

Various ab initio computations, as, e.g., in G. J. Hala?sz and A?. Vibo?k, Int. J. Quantum Chem. 111, 342 (2011), have shown that in molecules of the type (HCCH)(+), when the extremal H atoms are distorted from a linear form but maintain a planar geometry, a pair of conical intersections (ci) occur at such positions that the ratios of the distortional coordinates of the two atoms are in the two ci's reciprocals of each other. These computations have here been extended to locate the ci's also for HCNH. The two groups of results are explained by simple analytic perturbational expressions for the energy differences of the lowest adjacent electronic states, with inclusion of excited state effects.     

On the existence of free doubly negative molecular ions

We present a theoretical analysis on the existence of doubly negative molecular ions in the gas phase. A thorough investigation involves the discussion of two main problems: the stability of the system with respect to dissociation and the stability to the loss of an electron. An adequate treatment of both problems requires the choice of proper basis sets as well as the application of higher levels of theory. As an example the BeF ₄ ^2? dianion is studied in detail. Here we use configuration interaction and many-body Green's function approaches. Strong basis set dependencies in the calculations are observed which have no analogy in neutral systems.     

Reversal of the regioselectivity of the birch reduction of xylenes

The “Photo-Birch” reduction of o-, m-, and p-xylene, using NaBH₄, 1,3-dicyanobenzene, and photolysis, gives 1,4-dienes as products. The regioselectivity of these reactions is greatly different from the normal Birch reduction.