Electronic determinants of photoacidity in cyanonaphthols

We present semiempirical AM1 calculations for the ground and excited state of 2-naphthol and some of its cyano derivatives in the gas phase. Following photoexcitation, the Mulliken electron density on the oxygen diminishes slightly for the acid and more conspicuously for the anionic conjugated base. This agrees with the measured solvatochromic parameters for 2-naphthol. In both electronic states, we find a nice correlation with the measured pK values in water. The electronic charge distribution on the distal ring of the anion agrees with the experimental acidity order in both S(0) and S(1). Upon excitation, it increases predominantly in positions 3, 5, and 8. The ring system of the anion assumes an alternate quinoidal structure in the ground state of the anion, which becomes more symmetric in the relaxed excited state. This suggests that the enhanced aromatic character of a 4n electron system in the excited state allows for better delocalization of the oxygen charge within the ring.     

Exact solution of the excited-state geminate A*+B <==> C*+D reaction with two different lifetimes and quenching

The authors obtain, in the Laplace transform space, the exact analytic solution for the Green function and survival probabilities for the excited-state diffusion-influenced reversible geminate reaction, A*+B <==> C*+D, with two different lifetimes and in the presence of an added quenching process. This extends a previous investigation by Popov and Agmon [J. Chem. Phys. 117, 5770 (2002)] of the ground-state reaction without quenching. The long-time asymptotic behavior of the survival probabilities is obtained in the time domain. It is found to be different from the equal-lifetime case. This paper also provides a useful short-time approximation for the kinetics.     

Effect of scale and surface chemistry on the mechanical properties of carbon nanotubes‐based composites

In this article, Multi‐Walled Carbon Nanotubes (MWCNTs) of varying diameters, both untreated and polycarboxylated, were dispersed at constant weight percentage in an epoxy matrix, and resulting fracture toughnesses (K_Ic) were measured in each case. We show that changing the MWCNT diameter has two effects on the composite fracture toughness: (i) a small MWCNT diameter enables larger interfacial surface for adhesion maximization, which increases toughness; (ii) at the same time, it limits the available pull‐out energy and reduces the MWCNT ability to homogeneously disperse in the matrix due to this same large active surface: this decreases toughness. Most commercially available MWCNTs have a length range of several μm, thus an optimal diameter exists which depends on MWCNT wall thickness and surface treatment. Such optimal diameter maximizes pull‐out energy and thus composite fracture toughness. © 2012 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys, 2012     

Variational Bayesian Inference for Nonlinear Hawkes Process with Gaussian Process Self-Effects

Traditionally, Hawkes processes are used to model time-continuous point processes with history dependence. Here, we propose an extended model where the self-effects are of both excitatory and inhibitory types and follow a Gaussian Process. Whereas previous work either relies on a less flexible parameterization of the model, or requires a large amount of data, our formulation allows for both a flexible model and learning when data are scarce. We continue the line of work of Bayesian inference for Hawkes processes, and derive an inference algorithm by performing inference on an aggregated sum of Gaussian Processes. Approximate Bayesian inference is achieved via data augmentation, and we describe a mean-field variational inference approach to learn the model parameters. To demonstrate the flexibility of the model we apply our methodology on data from different domains and compare it to previously reported results.     

A conformationally flexible tetrahydroxycalix[4]arene adopting the unusual 1,3-alternate conformation

The use of a chiral solvating agent enabled the determination of the NMR-silent ring-inversion process of the ketocalixarene 3. Spectroscopic and crystal data indicate that 3 adopts the unusual 1,3-alternate conformation.     

Experimental evidence for a kinetic transition in reversible reactions

We provide a first experimental verification of a theoretical prediction of a kinetic transition in a reversible binding reaction, AB right harpoon over left harpoon A+B, driven by the difference in effective lifetimes of the bound and the unbound states. We consider the kinetics of excited-state proton transfer to solvent from a photoacid whose conjugate anionic base possesses an extremely short unbound anion lifetime. Its solvent variation relative to the overall dissociation rate coefficient induces a transition in the kinetics, from power law to exponential.