The devil in the details: A tutorial review on some undervalued aspects of density functional theory calculations

Density functional theory (DFT) has become ubiquitous for chemical applications in research and in education. The exact functional at the foundation of DFT is unfortunately unknown, and issues arise when choosing an approximation for a specific application. With this tutorial review, we tackle the selection problem and many related ones, such as the choices of a basis set and of an integration grid, that are often overlooked by occasional practitioners and by more experienced users as well. We offer a practical approach in the form of a commented notebook containing 12 experiences that can be run on a simple computer in just a few hours. We propose this review as a primary source for those who are willing to include DFT in their everyday research or teaching activities in a way that reflects the research advances of the field in the last couple of decades.     

Application of a Low Transition Temperature Mixture for the Dispersive Liquid–Liquid Microextraction of Illicit Drugs from Urine Samples

The use of psychoactive substances is a serious problem in today’s society and reliable methods of analysis are necessary to confirm their occurrence in biological matrices. In this work, a green sample preparation technique prior to HPLC-MS analysis was successfully applied to the extraction of 14 illicit drugs from urine samples. The isolation procedure was a dispersive liquid–liquid microextraction based on the use of a low transition temperature mixture (LTTM), composed of choline chloride and sesamol in a molar ratio 1:3 as the extracting solvent. This mixture was classified as LTTM after a thorough investigation carried out by FTIR and DSC, which recorded a glass transition temperature at −71 °C. The extraction procedure was optimized and validated according to the main Food and Drug Administration (FDA) guidelines for bioanalytical methods, obtaining good figures of merit for all parameters: the estimated lower limit of quantitation (LLOQ) values were between 0.01 µg L⁻¹ (bk-MMBDB) and 0.37 µg L⁻¹ (PMA); recoveries, evaluated at very low spike levels (in the ng-µg L⁻¹ range), spanned from 55% (MBDB) to 100% (bk-MMBDB and MDPV); finally, both within-run and between-run precisions were lower than 20% (LLOQ) and 15% (10xLLOQ).     

On Aronsson Equation and Deterministic Optimal Control

When Hamiltonians are nonsmooth, we define viscosity solutions of the Aronsson equation and prove that value functions of the corresponding deterministic optimal control problems are solutions if they are bilateral viscosity solutions of the Hamilton-Jacobi-Bellman equation. We characterize such a property in several ways, in particular it follows that a value function which is an absolute minimizer is a bilateral viscosity solution of the HJB equation and these two properties are often equivalent. We also determine that bilateral solutions of HJB equations are unique among absolute minimizers with prescribed boundary conditions. This research was partially supported by MIUR-Prin project “Metodi di viscosità, metrici e di teoria del controllo in equazioni alle derivate parziali nonlineari”.     

The Critical Dimension for a Fourth Order Elliptic Problem with Singular Nonlinearity

We study the regularity of the extremal solution of the semilinear biharmonic equation ${{\Delta^2} u=\frac{\lambda}{(1-u)^2}}We study the regularity of the extremal solution of the semilinear biharmonic equation D² u=\fracl(1-u)²{{\Delta^2} u=\frac{\lambda}{(1-u)^2}}, which models a simple micro-electromechanical system (MEMS) device on a ball B ì \mathbbR^N{B\subset{\mathbb{R}}^N}, under Dirichlet boundary conditions u=?_n u=0{u=\partial_\nu u=0} on ?B{\partial B}. We complete here the results of Lin and Yang [14] regarding the identification of a “pull-in voltage” λ* > 0 such that a stable classical solution u _λ with 0 < u _λ < 1 exists for l ? (0,l^*){\lambda\in (0,\lambda^*)}, while there is none of any kind when λ > λ*. Our main result asserts that the extremal solution u_l^*{u_{\lambda^*}} is regular (sup_B u_l^* < 1 ){({\rm sup}_B u_{\lambda^*} <1 )} provided N \leqq 8{N \leqq 8} while u_l^*{u_{\lambda^*}} is singular (sup_B u_l^* = 1){({\rm sup}_B u_{\lambda^*} =1)} for N \geqq 9{N \geqq 9}, in which case 1-C₀|x|^4/3 \leqq u_l^* (x) \leqq 1-|x|^4/3{1-C_0|x|^{4/3} \leqq u_{\lambda^*} (x) \leqq 1-|x|^{4/3}} on the unit ball, where C₀:=(\fracl^*[`(l)])<sup>\frac</sup>13{C_0:=\left(\frac{\lambda^*}{\overline{\lambda}}\right)^\frac{1}{3}} and [`(l)]: = \frac89(N-\frac23)(N- \frac83){\bar{\lambda}:= \frac{8}{9}\left(N-\frac{2}{3}\right)\left(N- \frac{8}{3}\right)}.     

Gas chromatographic determination of glutaraldehyde in the workplace atmosphere after derivatization with 0-(2,3,4,5,6-pentafluorobenzyl)hydroxylamine on a solid-phase microextraction fibre

Glutaraldehyde is used primarily in hospital environments for the disinfection of various instruments (e.g., endoscopes). We describe in this paper the measurement of glutaraldehyde in a hospital environment using solid-phase microextraction. The method includes, prior to sampling, the adsorption of O-(2,3,4,5,6-pentafluorobenzyl)hydroxylamine on to the fibre (with polydimethylsiloxane-divinylbenzene). The fibre is then exposed to air, after which desorption is performed in the GC injection port. This process results in the formation of a stable derivative of the glutaraldehyde that is suitable for chromatographic purposes and detectable with classical detection methods, such as flame ionisation and electron-capture detection. We demonstrate that the procedure of adsorption, thermal desorption and derivatization is robust and reproducible. We were able to detect concentrations of 60 microg/m3 (10 s sampling) or 6 microg/m3 (120 s sampling) by electron-capture detection, and 80 microg/m3 (120 s sampling) by flame ionisation detection. We compared our method to currently existing methods of glutaraldehyde measurement and highlighted several important advantages of the method.     

Lattice model for polymer hydration: collapse of poly(N-isopropylacrylamide)

Poly(N-isopropylacrylamide) (PNIPAM) in dilute aqueous solution undergoes a collapse transition from coil to globule on increasing temperature. Such coil-to-globule collapse is usually considered analogous to the cold renaturation of small globular proteins. In this paper we propose a theoretical approach that is able to reproduce, in a semi-quantitative way, the unusual behavior of PNIPAM, and the observed thermodynamic properties. The procedure is based on two main steps: (i) the characterization of single monomer hydration thermodynamics, interpreted by a balance between the removal of monomer-monomer interactions and the addition of water-monomer interactions, and (ii) a simplified analysis of a lattice self-avoiding walk (SAW) model, which allows to account for the configurational entropy in a controlled way, and hence to relate the microscopic interactions to the “macroscopic” behavior of the polymer chain. The results show that the temperature dependence and magnitude of the interaction parameters that best fit experimental data validate a recently proposed qualitative interpretation of the mechanism of collapse transition for PNIPAM. The latter result turns out to be relevant to support the analogy with the cold renaturation of small globular proteins, and to clarify some important aspects of protein thermodynamics.     

Synthesis and Structure Determination of 1, 4, 7, 11, 14, 17, 21, 24, 27-Nonaoxatriacontan-8, 10, 18, 20, 28, 30-Esaone

The title compound crystallizes in the trigonal space group R-3, with unit-cell parameters: a = 23.261(4), c = 9.537(2) Å; λ(MoKα) = 0.71073 Å, V = 4469(2) Å³, and Z = 6. The structure has been solved by direct methods using X-ray diffraction techniques. The final reliability index for the computed structure is 0.0826 for 929 observed reflections and 121 refined parameters. Crown ether adopts an almost circular shape and macrocycles are piled up in a columnar arrangement forming tubular nanochannels. The channels are filled with guest CDCl₃ molecules, characterised by rotational disorder.