Thermochemical properties and group values for nitrogen-containing molecules

Gas-phase thermochemical group additivity values were derived from CBS-QB3 computational chemistry calculations for 105 noncyclic C/H/O/N molecules. The molecules contain nitrile, nitro, nitroso, nitrite, nitrate, amine, imino, and azo functional groups. The enthalpy of formation, entropy, and heat capacity values for 49 atom-centered groups were derived. The effect of hindered internal rotations was included via rotor potential energy scans and solution of the one-dimensional Schrodinger equation. The average 95% confidence intervals across all derived groups are 1.4 kcal mol(-1) for the enthalpy, 1.3 cal mol(-1) K(-1) for the entropy, and 1.0 cal/mol K for the heat capacity. The presented group values will be useful when employing automatic reaction mechanism generation tools to examine the role of fuel-bound or molecular nitrogen in energy-related or atmospheric processes.     

QEPAS methane sensor performance for humidified gases

A trace gas sensor based on quartz enhanced photoacoustic spectroscopy (QEPAS) was evaluated using humidified nitrogen samples and ambient air. Relaxation processes following vibrational excitation of 2ν₃ state of CH₄ were investigated. Sensor performance at different gas pressures could be predicted based on a developed kinetic model. The experimentally determined normalized detection sensitivity for CH₄ in humid gas is 1.0×10^-8 cm^-1 W/Hz^1/2. PACS  82.80.Kq; 42.62.Fi     

Sorption isotherms, sorption enthalpies, diffusion coefficients and permeabilities of water in a multilayer PEO/PAA polymer film using the quartz crystal microbalance/heat conduction calorimeter

Sorption isotherms, sorption enthalpies, and diffusion coefficients for water in an 11 μm thick PEO/PAA multi-layer film have been measured at 30, 40, and 60 °C for relative humidities between 0 and 70%. All quantities were measured on the same film using the quartz crystal microbalance/heat conduction calorimeter. Water diffusion coefficients in the film are several orders of magnitude lower than in the separate components. Sorption isotherms are of type III at 30 and 40 °C and linear at 60 °C. Water vapor permeabilities are calculated as the product of Henry's law solubility and diffusion coefficient. The permeability of the PEO/PAA multilayer film is exceedingly low compared to other polymer films used as membranes. The enthalpy of water sorption determined from the sorption isotherms using the van’t Hoff relation is 32.9 ± 0.3 kJ/mol. Calorimetric enthalpies of water sorption range from 42 to 34 kJ/mol at 30 and 40 °C over the humidity range studied. The change in motional resistance, a quantity proportion to the loss compliance of the film, has also been recorded at all three temperatures, and a common trend is an increase in loss compliance with increasing relative humidity, indicating plasticization of the film by water.     

Using domain decomposition to find graph bisectors

In this paper we introduce a three-step approach to find a vertex bisector of a graph. The first step finds adomain decomposition of the graph, consisting of a set of domains and a multisector. Eachdomain is a connected subgraph, and themultisector contains the remaining vertices that separate the domains from each other. The second step uses a block variant of the Kernighan-Lin scheme to find a bisector that is a subset of the multisector. The third step improves the bisector by bipartite graph matching. Experimental results show this domain decomposition method finds graph partitions that compare favorably with a state-of-the-art multilevel partitioning scheme in both quality and execution time. This research was supported in part by the ARPA Contract DABT63-95-C-0122, and in part by the Natural Sciences and Engineering Research Council of Canada under grant A5509.     

Reflection of plane waves from the boundary of a pre-stressed compressible elastic half-space

The effect of pre-stress on the propagation and reflection ofplane waves in an incompressible isotropic elastic half-spacehas been examined recently by the authors (Ogden & Sotiropoulos,1997). In the present paper the corresponding analysis for compressiblematerials is detailed. In the two-dimensional context consideredfor incompressible materials the (homogeneous) plane waves werenecessarily shear waves. By contrast, in the compressible contextpure shear waves can propagate only in specific directions inthe considered principal plane and, in a general direction,a quasi-shear wave may be accompanied by a quasi-longitudinalwave, as is the case in the anisotropic linear theory. The dependenceof the (in-plane) slowness section on the pre-stress (and finitedeformation) and on the choice of constitutive law is elucidated.This information is used to determine the reflection coefficientsfor reflection of either a (quasi-) shear wave or a (quasi-)longitudinal wave from the boundary of the half-space and tocharacterize the different cases which arise depending on thegeometry of the slowness section. The theoretical results are illustrated by numerical calculationsfor the range of possible types of behaviour with referenceforms of strain-energy function and different states of finitedeformation and to the question of stability of the half-space.     

A Molecular Strategy to Lock-in the Conformation of a Perylene Bisimide-Derived Supramolecular Polymer

Locking-in the conformation of supramolecular assemblies provides a new avenue to regulate the (opto)electronic properties of robust nanoscale objects. In the present contribution, we show that the covalent tethering of a perylene bisimide (PBI)-derived supramolecular polymer with a molecular locker enables the formation of a locked superstructure equipped with emergent structure–function relationships. Experiments that exploit variable-temperature ground-state electronic absorption spectroscopy unambiguously demonstrate that the excitonic coupling between nearest neighboring units in the tethered superstructure is preserved at a temperature (371 K) where the pristine, non-covalent assembly exists exclusively in a molecularly dissolved state. A close examination of the solid-state morphologies reveals that the locked superstructure engenders the formation of hierarchical 1D materials which are not achievable by unlocked assemblies. To complement these structural attributes, we further demonstrate that covalently tethering a supramolecular polymer built from PBI subunits enables the emergence of electronic properties not evidenced in non-covalent assemblies. Using cyclic voltammetry experiments, the elucidation of the potentiometric properties of the locked superstructure reveals a 100-mV stabilization of the conduction band energy when compared to that recorded for the non-covalent assembly.