Spectroscopic probing of ultraviolet-induced degradation in elastomeric polyurea.

Aromatic polyurea has garnered assiduous research due to its excellent impact, shock, abrasion, moisture, and chemical resistance properties. Polyurea can be used in protective coating and impact mitigation applications but is inevitably exposed to harsh deployment conditions such as extended ultraviolet (UV) radiation. Fourier Transform Infrared (FTIR) spectroscopy, Terahertz-time domain spectroscopy (THz-TDS), and Excitation-Emission Matrix spectroscopy (EEMS) deciphered the effects of UV radiation on radiated polyurea samples under ambient and nitrogen-rich conditions. Samples were radiated continuously for up to 15 weeks in increments of 3 weeks. Comprehensive FTIR analyses revealed a monotonic increase in disordered hydrogen bonding as a function of exposure duration in an ambient environment. Otherwise, marginal changes were observed in UV-radiated samples under nitrogen. The hydrogen bond length exhibited significant variations in the former compared to their nitrogen atmosphere counterparts. The results infer the nitrogen shielding effect, protecting polyurea from the photodegradation and photo-oxidation observed in samples radiated under the ambient atmosphere. THz-TDS spectra affirmed the FTIR results by probing changes in the complex refractive index. Terahertz spectral peaks associated with torsional vibrations of intermolecular hydrogen bonds in polyurea were notably correlated with increased exposure duration in the ambient atmosphere. Changes in the complex index as a function of exposure duration under nitrogen are minimal. The excitation-emission spectra of polyurea samples reveal a strong fluorescent behavior in 9-week and 12-week ambient-exposed polyurea due to cluster-triggered emission mechanisms. The results synthesized based on three different spectroscopy techniques paint a holistic portrait of the adverse effects of extended ultraviolet radiation of macromolecules deployed in harsh environmental conditions.

Miniature physical sphere-in-contact models of heterogeneous catalysts and metal nanoparticles.

CONTEXT: Physical molecular models have played a fundamental role in the understanding of chemical reactions on heterogeneous catalysts and on metal nanoparticles. To date, these physical models have been based on separate models of the metal nanoparticle (NP) or surface and of the substrate and the molecular structure of reactant and product adsorbates and their intermediates. In this paper, we try to provide a new miniature physical molecular model, the sphere-in-contact model of heterogeneous catalysts and metal nanoparticles that can build inexpensive, small and efficient molecular models that can be transported or shipped easily and that depict the chemical reaction as a whole, showing reactants, intermediates, products, the metal nanoparticle bound to the substrate which can give information about a reaction mechanism. These models reveal that there are certain rules with respect to the kind of sites you observe at the metal NP interface with the support by small movement of the nanoparticle. METHODS: We have used in this study physical molecular models using the sphere-in-contact model. This is the first time such physical models are built for heterogeneous catalytic reactions and metal nanoparticles, and they are constructed out of spheres that fuse together when exposed to water.

The sphere-in-contact model of carbon materials.

A sphere-in-contact model is presented that is used to build physical models of carbon materials such as graphite, graphene, carbon nanotubes and fullerene. Unlike other molecular models, these models have correct scale and proportions because the carbon atoms are represented by their atomic radius, in contrast to the more commonly used space-fill models, where carbon atoms are represented by their van der Waals radii. Based on a survey taken among 65 undergraduate chemistry students and 28 PhD/postdoctoral students with a background in molecular modeling, we found misconceptions arising from incorrect visualization of the size and location of the electron density located in carbon materials. Based on analysis of the survey and on a conceptual basis we show that the sphere-in-contact model provides an improved molecular representation of the electron density of carbon materials compared to other molecular models commonly used in science textbooks (i.e., wire-frame, ball-and-stick, space-fill). We therefore suggest that its use in chemistry textbooks along with the ball-and-stick model would significantly enhance the visualization of molecular structures according to their electron density. Graphical Abstract A sphere-in-contact model of C60-fullerene.

The effect of glassy carbon surface oxides in non-aqueous voltammetry: the case of quinones in acetonitrile.

Glassy carbon (GC) electrodes are well-known to contain oxygenated functional groups such as phenols, carbonyls, and carboxylic acids on their surface. The effects of these groups on voltammetry in aqueous solution are well-studied, but there has been little discussion of their possible effects in nonaqueous solution. In this study, we show that the acidic functional groups, particularly phenols, are likely causes of anomalous features often seen in the voltammetry of quinones in nonaqueous solution. These features, a too small second cyclic voltammetric wave and extra current between the two waves that sometimes appears to be a small, broad third voltammetric wave, have previously been attributed to different types of dimerization. In this work, concentration-dependent voltammetry in acetonitrile rules out dimerization with a series of alkyl-benzoquinones because the anomalous features get larger as the concentration decreases. At low concentrations, solution bimolecular reactions will be relatively less important than reactions with surface groups. Addition of substoichiometric amounts of naphthol at higher quinone concentrations produces almost identical behavior as seen at low quinone concentrations with no added naphthol. This implicates hydrogen bonding and proton transfer from the surface phenolic groups as the cause of the anomalous features in quinone voltammetry at GC electrodes. This conclusion is supported by the perturbation of surface oxide coverage on GC electrodes through different electrode pretreatments.

Quantitative structure-property relationships for longitudinal, transverse, and molecular static polarizabilities in polyynes.

The present work reports for the first time quantitative structure-property relationships, derived at the benchmark CCSD(T)/cc-PVTZ level of theory that estimate the static longitudinal, transverse, and molecular polarizability in polyynes (C2nH2), as a function of their length (L). In the case of independent electron models, regardless of the form of the nuclei potential that the electrons experience, the polarizability increases strongly with system size, scaling as L(4). In contrast, the static longitudinal polarizability in polyynes have a considerably weaker length-dependence (L(1.64)). This is shown to predominantly arise from electron-electron repulsion rather than electron correlation by a systematic study of the polarizability length dependence in several simple quantum mechanical systems (e.g., particle-in-box, simple harmonic oscillator) and other molecular systems (e.g., H2, H2(+), polyynes). Decrease of the electron-electron repulsion term is suggested to be the key factor in enhancing nonlinear polarizability characteristics of linear oligomeric and polymeric materials.

Microwave spectra and ab initio studies of Ar-propane and Ne-propane complexes: structure and dynamics.

Microwave spectra in the 7-26 MHz region have been measured for the van der Waals complexes, Ar-CH3CH2CH3, Ar-(13)CH3CH2CH3, 20Ne-CH3CH2CH3, and 22Ne-CH3CH2CH3. Both a- and c-type transitions are observed for the Ar-propane complex. The c-type transitions are much stronger indicating that the small dipole moment of the propane (0.0848 D) is aligned perpendicular to the van der Waals bond axis. While the 42 transition lines observed for the primary argon complex are well fitted to a semirigid rotor Hamiltonian, the neon complexes exhibit splittings in the rotational transitions which we attribute to an internal rotation of the propane around its a inertial axis. Only c-type transitions are observed for both neon complexes, and these are found to occur between the tunneling states, indicating that internal motion involves an inversion of the dipole moment of the propane. The difference in energy between the two tunneling states within the ground vibrational state is 48.52 MHz for 20Ne-CH3CH2CH3 and 42.09 MHz for 22Ne-CH3CH2CH3. The Kraitchman substitution coordinates of the complexes show that the rare gas is oriented above the plane of the propane carbons, but shifted away from the methylene carbon, more so in Ne propane than in Ar propane. The distance between the rare gas atom and the center of mass of the propane, Rcm, is 3.823 A for Ar-propane and 3.696 A for Ne-propane. Ab initio calculations are done to map out segments of the intermolecular potential. The global minimum has the rare gas almost directly above the center of mass of the propane, and there are three local minima with the rare gas in the plane of the carbon atoms. Barriers between the minima are also calculated and support the experimental results which suggest that the tunneling path involves a rotation of the propane subunit. The path with the lowest effective barrier is through a C2v symmetric configuration in which the methyl groups are oriented toward the rare gas. Calculating the potential curve for this one-dimensional model and then calculating the energy levels for this potential roughly reproduces the spectral splittings in Ne-propane and explains the lack of splittings in Ar-propane.

Correlation of polarizabilities with Van der Waals interactions in pi-systems.

This work aims to (i) provide a semiquantitative relationship that can be used to estimate the binding energy, equilibrium separation, and potential energy surface (PES) for supermolecules consisting of benzene and small polycyclic aromatic hydrocarbons (PAHs) in parallel configuration and (ii) give a qualitative description of pi-pi interactions between PAHs. We compute the one-dimensional PES of benzene translated parallel to various PAHs within the framework of second-order Møller-Plesset (MP2) perturbation theory. For PAHs of small MW difference, we observe a linear correlation between the binding energy and the number of carbon atoms in the supermolecule. The PES of these supermolecules is fit to an (exp-6) function whose variables are subsequently used to derive a mass-centered potential energy function as a function of the number of carbon atoms in the supermolecule. The linear dependence of the binding energy in the supermolecular series examined here can be directly correlated to the average polarizability product of the supermolecule. Last, we consider the supermolecular series of benzene with n-polyacenes to study the convergence of pi-pi interactions between PAHs when their size is considerably different.