Diffusion of Dioxygen in Alkanes and Cycloalkanes

Representation of Multiphase OH Oxidation of Amorphous Organic Aerosol for Tropospheric Conditions

Organic aerosol (OA) is ubiquitous in the atmosphere and, during transport, can experience chemical transformation with consequences for air quality and climate. Prediction of the chemical evolution of OA depends on its reactivity with atmospheric oxidants such as the OH radical. OA particles undergo amorphous phase transitions from liquid to solid (glassy) states in response to temperature changes, which, in turn, will impact its reactivity toward OH oxidation. To improve the predictability of OA reactivity toward OH oxidation, the reactive uptake coefficients (γ) of OH radicals reacting with triacontane and squalane serving as amorphous OA surrogates were measured at temperatures from 213-293 K. γ increases strongest with temperature when the organic species is in the liquid phase, compared to when being in the semisolid or solid phase. The resistor model is applied, accounting for the amorphous phase state changes using the organic species' glass transition temperature and fragility, to evaluate the physicochemical parameters of the temperature dependent OH uptake process. This allows for the derivation of a semiempirical formula, applicable to models, to predict the degree of oxidation and chemical lifetime of the condensed-phase organic species for typical tropospheric temperature and humidity when OA particle viscosity is known.

Molecular Dynamics Simulations of Membrane Permeability

This Review illustrates the evaluation of permeability of lipid membranes from molecular dynamics (MD) simulation primarily using water and oxygen as examples. Membrane entrance, translocation, and exit of these simple permeants (one hydrophilic and one hydrophobic) can be simulated by conventional MD, and permeabilities can be evaluated directly by Fick's First Law, transition rates, and a global Bayesian analysis of the inhomogeneous solubility-diffusion model. The assorted results, many of which are applicable to simulations of nonbiological membranes, highlight the limitations of the homogeneous solubility diffusion model; support the utility of inhomogeneous solubility diffusion and compartmental models; underscore the need for comparison with experiment for both simple solvent systems (such as water/hexadecane) and well-characterized membranes; and demonstrate the need for microsecond simulations for even simple permeants like water and oxygen. Undulations, subdiffusion, fractional viscosity dependence, periodic boundary conditions, and recent developments in the field are also discussed. Last, while enhanced sampling methods and increasingly sophisticated treatments of diffusion add substantially to the repertoire of simulation-based approaches, they do not address directly the critical need for force fields with polarizability and multipoles, and constant pH methods.

Impact of cyclic topology: odd–even glass transition temperatures and fluorescence quantum yields in molecularly-defined macrocycles

The topological structure of cyclic-TPE_n+1 (n = 1–6) induces odd–even effects on the T_g and AIE behavior, arising from the alternation of intermolecular interactions.

Estimation of the fluorescence lifetime for optically inaccessible exciplexes in nonpolar solutions under ionizing irradiation

X-irradiation of nonpolar solutions likely provides a possibility to create exciplexes for any donor-acceptor pair that would energetically and sterically allow this. Thorough study and characterization of X-radiation generated exciplexes usually cannot be carried out with conventional methods because of the complex and non-exponential formation and decay dynamics of these species. In this paper, we present a simple and universal experimental approach for the estimation of fluorescence lifetimes (τF) of X-radiation generated exciplexes. The suggested procedure is based on the comparison of quenching of the exciplex emission band and the emission band from a standard luminophore with a known excited state lifetime by dissolved oxygen. Using this approach we report the τF values for two systems with optically inaccessible exciplexes, diphenylacetylene-N,N-dimethylaniline (DMA) and p-terphenyl-DMA, and for two typical exciplex forming systems, naphthalene-DMA and anthracene-DMA. All the found τF values for the X-radiation generated exciplexes lie in the range of 50-70 ns. The accuracy of this approach was checked by time-resolved measurements under X- or near-UV irradiation for those pairs, whose properties make this feasible. The proposed method gives a possibility to avoid a complex numerical evaluation of the non-exponential kinetics of recombination luminescence, and can be used to estimate the characteristic τF values for luminophores and excited complexes formed under X-radiation.

Electron paramagnetic resonance line shifts and line shape changes due to heisenberg spin exchange and dipole-dipole interactions of nitroxide free radicals in liquids 8. Further experimental and theoretical efforts to separate the effects of the two interactions

The work in part 6 of this series (J. Phys. Chem. A 2009, 113, 4930), addressing the task of separating the effects of Heisenberg spin exchange (HSE) and dipole-dipole interactions (DD) on electron paramagnetic resonance (EPR) spectra of nitroxide spin probes in solution, is extended experimentally and theoretically. Comprehensive measurements of perdeuterated 2,2,6,6-tetramethyl-4-oxopiperidine-1-oxyl (pDT) in squalane, a viscous alkane, paying special attention to lower temperatures and lower concentrations, were carried out in an attempt to focus on DD, the lesser understood of the two interactions. Theoretically, the analysis has been extended to include the recent comprehensive treatment by Salikhov (Appl. Magn. Reson. 2010, 38, 237). In dilute solutions, both interactions (1) introduce a dispersion component, (2) broaden the lines, and (3) shift the lines. DD introduces a dispersion component proportional to the concentration and of opposite sign to that of HSE. Equations relating the EPR spectral parameters to the rate constants due to HSE and DD have been derived. By employing nonlinear least-squares fitting of theoretical spectra to a simple analytical function and the proposed equations, the contributions of the two interactions to items 1-3 may be quantified and compared with the same parameters obtained by fitting experimental spectra. This comparison supports the theory in its broad predictions; however, at low temperatures, the DD contribution to the experimental dispersion amplitude does not increase linearly with concentration. We are unable to deduce whether this discrepancy is due to inadequate analysis of the experimental data or an incomplete theory. A new key aspect of the more comprehensive theory is that there is enough information in the experimental spectra to find items 1-3 due to both interactions; however, in principle, appeal must be made to a model of molecular diffusion to separate the two. The permanent diffusion model is used to illustrate the separation in this work. In practice, because the effects of DD are dominated by HSE, negligible error is incurred by using the model-independent extreme DD limit of the spectral density functions, which means that DD and HSE may be separated without appealing to a particular model.

Nitroxide spin exchange due to re-encounter collisions in a series of n-alkanes

Bimolecular collisions between perdeuterated 2,2,6,6-tetramethyl-4-oxopiperidine-l-oxyl molecules in three alkanes have been studied by measuring the electron paramagnetic resonance (EPR) spectral changes induced by spin exchange. We define an "encounter" to be a first-time collision followed by a series of re-encounters prior to the diffusing pair's escaping each other's presence. The present work stems from a recent proposal [B. L. Bales et al., J. Phys. Chem. A 107, 9086 (2003)] that an unexpected linear dependence of the spin-exchange-induced EPR line shifts on spin-exchange frequency can be explained by re-encounters of the same probe pair during one encounter. By employing nonlinear least-squares fitting, full use of the information available from the spectral changes allows us to study encounters and re-encounters separately. The encounter rate constants appear to be dominated by hydrodynamic forces, forming a common curve for hexane, decane, and hexadecane when plotted against T/eta, where eta is the shear viscosity. Unexpectedly, encounters are not dependent on the ratio mu = a/a(s), where a and a(s) are the van der Waals radii of the nitroxide probe and the solvent, respectively. It is argued that the near coincidence of the resulting encounter rate constant with the hydrodynamic prediction is likely due to a near cancellation of terms in the general diffusion coefficient. Thus, the semblance of hydrodynamic behavior is coincidental rather than intrinsic. In contrast, the mean times between re-encounters do depend on the relative sizes of probe and solvent. For hexane at lower temperatures, the Stokes-Einstein equation apparently describes re-encounters well; however, at higher temperatures and for decane and hexadecane, departures from the hydrodynamic prediction become larger as mu becomes smaller. This is in qualitative agreement with the theory of microscopic diffusion of Hynes et al. [J. Chem. Phys. 70, 1456 (1979)]. These departures are well correlated with the free volume available in the solvent; thus, the mean times between re-encounters form a common curve when plotted versus the free volume. Because free volume is manifested macroscopically by the isothermal compressibility, it is expected and observed that the re-encounter rate also forms a common curve across all three solvents when plotted with respect to compressibility. The existence of a common curve for alkanes raises the prospect of using EPR to determine the compressibility of substances such as fossil fuels and biological membranes.