A Benchmark Study of DFT-Computed p-Block Element Lewis Pair Formation Enthalpies against Experimental Calorimetric Data.

The quantification of Lewis acidity is of fundamental and applied importance in chemistry. While the computed fluoride ion affinity (FIA) is the most widely accepted thermodynamic metric, only sparse experimental values exist. Accordingly, a benchmark of methods for computing Lewis pair formation enthalpies, also with a broader set of Lewis bases against experimental data, is missing. Herein, we evaluate different density functionals against a set of 112 experimentally determined Lewis acid/base binding enthalpies and gauge influences such as solvation correction in structure optimization. From that, we can recommend r2SCAN-3c for robust quantification of this omnipresent interaction.

Imidazole-Based Ionic Liquid Engineering for Perovskite Solar Cells with High Efficiency and Excellent Stability.

Despite the remarkable progress of perovskite solar cells (PSCs), the substantial inherent defects within perovskites restrict the achievement of higher efficiency and better long-term stability. Herein, we introduced a novel multifunctional imidazole analogue, namely, 1-benzyl-3-methylimidazolium bromide (BzMIMBr), into perovskite precursors to reduce bulk defects and inhibit ion migration in inverted PSCs. The electron-rich environment of -N- in the BzMIMBr structure, which is attributed to the electron-rich adjacent benzene ring-conjugated structure, effectively passivates the uncoordinated Pb2+ cations. Moreover, the interaction between the BzMIMBr additive and perovskite can effectively hinder the deprotonation of formamidinium iodide/methylammonium iodide (FAI/MAI), extending the crystallization time and improving the quality of the perovskite precursors and films. This interaction also effectively inhibits ion migration to subsequent deposited films, leading to a noteworthy decrease in trap states. Various characterization studies show that the BzMIMBr-doped films exhibit superior film morphology and surface uniformity and reduced nonradiative carrier recombination, consequently enhancing crystallinity by reducing bulk/surface defects. The PSCs fabricated on the BzMIMBr-doped perovskite thin film exhibit a power conversion efficiency of 23.37%, surpassing that of the pristine perovskite device (20.71%). Additionally, the added BzMIMBr substantially increased the hydrophobicity of perovskite, as unencapsulated devices still retained 93% of the initial efficiency after 1800 h of exposure to air (45% relative humidity).

Reduced Nucleophilicities ИB of Lewis Bases B: Is ИB Independent of Whether B is Involved in a Hydrogen Bond or a Halogen Bond?

Reduced nucleophilicities ИB of axially symmetric molecules B were determined from , where De is the equilibrium dissociation energy of the complexes B⋅⋅⋅XY, NB is the nucleophilicity of B, EXY is the electrophilicity of the halogen-bond donor XY and σ min ${{\sigma }_{{\rm { min}}}{\rm \ }}$ is the minimum electrostatic surface potential of B. The series B⋅⋅⋅ClY, B⋅⋅⋅BrY, B⋅⋅⋅IY (Y=F, Cl, Br, I, CN, and CCH) as well as (B⋅⋅⋅XY, XY=F2 , Cl2 , Br2 ,and BrCl) of complexes were investigated. Molecules B were grouped so that the terminal atom involved in the halogen bond was fixed within the group. Groups having N as the terminal atom were RCN (R=CH3 , H, and F) or RN (R=N and P), those with C as the terminal atom were RNC (R=H and F) and RC (R=O, S and Se), and those with a terminal O atom were R=C=O (R=O or S). Graphs of D e ${{D}_{{\rm { e}}}}$ versus EXY for each group were straight lines through the origin, with generally different gradients, hence implying different NB . By contrast, when D e / σ min ${{D}_{{\rm { e}}}/{\sigma }_{{\rm { min}}}}$ was the ordinate the lines conflated to give a single straight line, which then defines a common (reduced) nucleophilicity ИB for that group of B. Hence it was concluded that ИB is an intrinsic property of the terminal atom, independent of the remainder of B, and only weakly dependent on the type (C, N or O) of the terminal atom. Moreover, ИB for each B was the same as determined previously from the hydrogen-bonded series B⋅⋅⋅HX, (X=F, Cl, Br, I, CN, CCH, and CP).

Reactivity of a Cationic Bismuth Amide towards Unsymmetric Heterocumulenes.

The reactivity of a literature-known, ring-strained bismuth amide cation towards a range of unsymmetric heterocumulene substrates has been investigated. Reactions with ketenes R2 C=C=O (R=Me, Ph), isocyanates R'N=C=O, and isothiocyanates R'N=C=S (R'=Ph, 4-CF3 -C6 H4 ) proceed via facile insertion of the heterocumulene in the Bi-N bond of the cationic bismuth amide. Unexpectedly pronounced differences in the regioselectivity of these insertion reactions have been observed, yielding a rich variety of heterocycle motifs (BiC2 NC2 , BiC2 NCO, BiC2 NCS, BiC2 NCN), some of which are unprecedented. Parameters that control the regioselectivity of the insertion reactions have been identified and are discussed based on experimental and theoretical investigations. Analytical techniques applied in this work include heteronuclear and two-dimensional NMR spectroscopy, IR spectroscopy, elemental analysis, single-crystal X-ray diffraction analyses, and DFT calculations.

In vivo neurophysiological assessment of in silico predictions of neurotoxicity: Citronellal, 3,4-dichloro-1-butene, and benzyl bromoacetate.

Neurotoxicants may be widespread in the environment and can produce serious health impacts in the human population. Screening programs that use in vitro methods have generated data for thousands of chemicals. However, these methods often do not evaluate repeated or prolonged exposures, which are required for many neurotoxic outcomes. Additionally, the data produced by such screening methods may not include mechanisms which play critical biological roles necessary for in vivo neurotoxicity. The Hard and Soft Acids and Bases (HSAB) in silico model focuses on chemical structure and electrophilic properties which are important to the formation of protein adducts. A group of structurally diverse chemicals have been evaluated with an in silico screening approach incorporating HSAB parameters. However, the predictions from the expanded chemical space have not been evaluated using in vivo methods. Three chemicals predicted to be cumulative toxicants were selected for in vivo neurotoxicological testing. Adult male Long-Evans rats were treated orally with citronellal (CIT), 3,4-dichloro-1-butene (DCB), or benzyl bromoacetate (BBA) for 8 weeks. Behavioral observations were recorded weekly to assess motor function. Peripheral neurophysiological measurements were derived from nerve excitability (NE) tests which involved compound muscle action potentials (CMAPs) in the tail and foot, and mixed nerve action potentials (MNAPs) in the tail. Compound nerve action potentials (CNAPs) and nerve conduction velocity (NCV) in the tail were also quantified. Peripheral inputs into the central nervous system were examined using somatosensory evoked potentials recorded from the cortex (SEPCTX) and cerebellum (SEPCEREB). CIT or BBA did not result in significant alterations to peripheral nerve or somatosensory function. DCB reduced grip-strength and altered peripheral nerve function. The MNAPs required less current to reach 50% amplitude and had a lower calculated rheobase, suggesting increased excitability. Increased CNAP amplitudes and greater NCV were also observed. Novel changes were found in the SEPCTX with an abnormal peak forming in the early portion of the waveforms of treated rats, and decreased latencies and increased amplitudes were observed in SEPCEREB recordings. These data contribute to testing an expanded chemical space from an in silico HSAB model for predicting cumulative neurotoxicity and may assist with prioritizing chemicals to protect human health.

Developing the relationship between metal ionic characters and ecological risk assessment screening values using QICAR.

Metals are widely released and distributed in soil and may have a negative impact on terrestrial organisms. Over the past years, a series of criteria or standards for assessing the ecological risks and toxicity of metals have been published in many countries; however, few studies have investigated their metal ionic properties and toxicity. In the present study, the ecological risk assessment screening values (ERASV) recommended by the Oregon Department of Environmental Quality were selected to investigate the correlation between metal toxicity and their ionic characters based on the hard and soft acids and bases (HSAB) concept. The results showed that more ionic characters were significantly correlated with ERASV using the HSAB theory, while only one metal ionic characteristic was correlated with ERASV in organisms. For borderline metal ions, maximum complex stability constants (log ßn) and the softness (δp) of borderline ions were correlated with ERASV, while log ßn and electronegativity (Xm) were significantly related to ERASV for borderline plus hard ions, and the boiling point (BP) and electron density (AR/AW) (AR indicates atomic radius and AW is atomic mass) were significantly related to ERASV for borderline plus soft ions. These results indicated that different metal ion characteristics play different roles in different types of metal toxicity in organisms and the mechanisms of toxicity are different. Based on these relationships, a set of quantitative ion characteristic parameter-activity relationship (QICAR) was developed. The QICAR predicted ERASV for metals that were reasonably consistent with those recommended by the Oregon Department of Environmental Quality, with differences between them generally < 2.0 orders of magnitude. However, there were discrepancies between the recommended and predicted values, and these discrepancies may be related to terrestrial geochemical properties. These soil properties should be further considered when developing QICAR models in future studies, such as soil type, organic matter, and pH. Overall, the QICAR models were able to determine the relationships between metal ionic properties and their toxicity and will be useful for assessing toxicity data on unknown toxic metals and will provide a basis for ecological assessment.

From pKa to the pH of weak base solutions.

Undergraduate biochemistry students frequently find the quantitative treatment of weak acids and bases troublesome. Given the pKa of a weak acid HA, for instance, many students struggle to calculate the pH of a solution of the conjugate base A- at concentration C, pH(A- , C). The traditional method involves calculating the base dissociation constant Kb and the artificial quantity pOH before reaching pH, but these steps increase the risk of mistakes and provide little insight into acid-base equilibria. The alternative method presented here allows students to calculate the pH of a weak base solution from the pKa of its conjugate acid without calculating Kb and pOH, using a memorable relationship: pH(HA, C) + pH(A- , C) = pKa + 7.

The Reactivity of Ambident Nucleophiles: Marcus Theory or Hard and Soft Acids and Bases Principle?

The model reactions CH3 X + (NH-CH=O)M ➔ CH3 -NH-NH═O or NH═CH-O-CH3 + MX (M = none, Li, Na, K, Ag, Cu; X = F, Cl, Br) are investigated to demonstrate the feasibility of Marcus theory and the hard and soft acids and bases (HSAB) principle in predicting the reactivity of ambident nucleophiles. The delocalization indices (DI) are defined in the framework of the quantum theory of atoms in molecules (QT-AIM), and are used as the scale of softness in the HSAB principle. To react with the ambident nucleophile NH═CH-O- , the carbocation H3 C+ from CH3 X (F, Cl, Br) is actually a borderline acid according to the DI values of the forming C…N and C…O bonds in the transition states (between 0.25 and 0.49), while the counter ions are divided into three groups according to the DI values of weak interactions involving M (M…X, M…N, and M…O): group I (M = none, and Me4 N) basically show zero DI values; group II species (M = Li, Na, and K) have noticeable DI values but the magnitudes are usually less than 0.15; and group III species (M = Ag and Cu(I)) have significant DI values (0.30-0.61). On a relative basis, H3 C+ is a soft acid with respect to group I and group II counter ions, and a hard acid with respect to group III counter ions. Therefore, N-regioselectivity is found in the presence of group I and group II counter ions (M = Me4 N, Li, Na, K), while O-regioselectivity is observed in the presence of the group III counter ions (M = Ag, and Cu(I)). The hardness of atoms, groups, and molecules is also calculated with new functions that depend on ionization potential (I) and electron affinity (A) and use the atomic charges obtained from localization indices (LI), so that the regioselectivity is explained by the atomic hardness of reactive nitrogen atoms in the transition states according to the maximum hardness principle (MHP). The exact Marcus equation is derived from the simple harmonic potential energy parabola, so that the concepts of activation free energy, intrinsic activation barrier, and reaction energy are completely connected. The required intrinsic activation barriers can be either estimated from ab initio calculations on reactant, transition state, and product of the model reactions, or calculated from identity reactions. The counter ions stabilize the reactant through bridging N- and O-site of reactant of identity reactions, so that the intrinsic barriers for the salts are higher than those for free ambident anions, which is explained by the increased reorganization parameter Δr. The proper application of Marcus theory should quantitatively consider all three terms of Marcus equation, and reliably represent the results with potential energy parabolas for reactants and all products. For the model reactions, both Marcus theory and HSAB principle/MHP principle predict the N-regioselectivity when M = none, Me4 N, Li, Na, K, and the O-regioselectivity when M = Ag and Cu(I). © 2019 Wiley Periodicals, Inc.

Quantifying correlations of metal ionic characters with ecological soil screening levels (Eco-SSLs) of metals using QICAR models.

Soil pollution by heavy metals is a major challenge for soil ecosystems; therefore, many countries have published thresholds or standards for protecting soil organisms based on toxicity testing. However, there have been few studies on the mechanism of metal toxicity on organisms in soils, especially relationships between metal's ionic properties and its toxicity. Herein, we selected environmental soil screening levels (Eco-SSLs), which are internationally recognized ecotoxicity values recommended by the United States Environmental Protection Agency (USEPA), and investigated relationships between Eco-SSLs and metal ionic characteristics. The results showed that several ionic characteristics were significantly correlated with Eco-SSL using a classification of metal ions according to hard and soft acids and bases. Electrochemical potential, atomic ionization potential, the first hydrolysis constant, the maximum complex stability constant, a polarization force parameter and covalent radius showed significant correlations with Eco-SSLs for borderline plus hard ions, while the soft index exhibited significant fitting for borderline plus soft ions, suggesting that ionic bonding and covalent bonding played important roles in metal toxicity on borderline plus hard ions and soft ions, respectively. Then, we chose characteristics that had the strongest correlations with Eco-SSLs, and developed quantitative ion character-activity relationship (QICAR) for soil organisms. The QICARs predicted Eco-SSLs for metals that were reasonably consistent with those recommended by USEPA, with differences between them generally <0.5 orders of magnitude. Overall, QICAR provide a basis for ecological risk assessment and could be useful to interpret relationships between metal's ionic properties and its toxicity.

Application of nanoparticle tracking analysis for characterising the fate of engineered nanoparticles in sediment-water systems.

Novel applications of nanotechnology may lead to the release of engineered nanoparticles (ENPs), which result in concerns over their potential environmental hazardous impact. It is essential for the research workers to be able to quantitatively characterise ENPs in the environment and subsequently to assist the risk assessment of the ENPs. This study hence explored the application of nanoparticle tracking system (NTA) to quantitatively describe the behaviour of the ENPs in natural sediment-water systems. The NTA allows the measurement of both particle number concentration (PNC) and particle size distribution (PSD) of the ENPs. The developed NTA method was applied to a range of gold and magnetite ENPs with a selection of surface properties. The results showed that the positively-charged ENPs interacted more strongly with the sediment than neutral and negatively-charged ENPs. It was also found that the citrate coated Au ENPs had a higher distribution percentage (53%) than 11-mercaptoundecanoic acid coated Au ENPs (20%) and citrate coated magnetite ENPs (21%). The principles of the electrostatic interactions between hard (and soft) acids and bases (HSAB) are used to explain such behaviours; the hard base coating (i.e. citrate ions) will interact more strongly with hard acid (i.e. magnetite) than soft acid (i.e. gold). The results indicate that NTA is a complementary method to existing approaches to characterise the fate and behaviour of ENPs in natural sediment.

Diacetyl and related flavorant α-Diketones: Biotransformation, cellular interactions, and respiratory-tract toxicity.

Exposure to diacetyl and related α-diketones causes respiratory-tract damage in humans and experimental animals. Chemical toxicity is often associated with covalent modification of cellular nucleophiles by electrophilic chemicals. Electrophilic α-diketones may covalently modify nucleophilic arginine residues in critical proteins and, thereby, produce the observed respiratory-tract pathology. The major pathway for the biotransformation of α-diketones is reduction to α-hydroxyketones (acyloins), which is catalyzed by NAD(P)H-dependent enzymes of the short-chain dehydrogenase/reductase (SDR) and the aldo-keto reductase (AKR) superfamilies. Reduction of α-diketones to the less electrophilic acyloins is a detoxication pathway for α-diketones. The pyruvate dehydrogenase complex may play a significant role in the biotransformation of diacetyl to CO2. The interaction of toxic electrophilic chemicals with cellular nucleophiles can be predicted by the hard and soft, acids and bases (HSAB) principle. Application of the HSAB principle to the interactions of electrophilic α-diketones with cellular nucleophiles shows that α-diketones react preferentially with arginine residues. Furthermore, the respiratory-tract toxicity and the quantum-chemical reactivity parameters of diacetyl and replacement flavorant α-diketones are similar. Hence, the identified replacement flavorant α-diketones may pose a risk of flavorant-induced respiratory-tract toxicity. The calculated indices for the reaction of α-diketones with arginine support the hypothesis that modification of protein-bound arginine residues is a critical event in α-diketone-induced respiratory-tract toxicity.

Acid-Responsive Absorption and Emission of 5-N-Arylaminothiazoles: Emission of White Light from a Single Fluorescent Dye and a Lewis Acid.

Solutions of 5-N-arylaminothiazoles containing pyridyl groups exhibited clear halochromism and halofluorism upon addition of Brønsted and Lewis acids. The addition of triflic acid to solutions of 5-N-arylaminothiazoles in Et2O induced bathochromic shifts of the absorption and emission bands. DFT calculations suggested that the spectral changes arise from the protonation of the pyridyl group of the thiazoles in Et2O. Single-crystal X-ray diffraction analysis of a thiazole and its protonated form revealed the change of the conformation around the thiazole ring. The emission of white light was accomplished from a single fluorescent dye by adjusting the ratio of dye and B(C6F5)3, whereby the International Commission on Illumination coordinates showed a linear change from blue to orange.

Theoretical Analysis of Drug Dissolution: I. Solubility and Intrinsic Dissolution Rate.

The first-principles approach presented in this work combines surface kinetics and convective diffusion modeling applied to compounds with pH-dependent solubility and in different dissolution media. This analysis is based on experimental data available for approximately 100 compounds of pharmaceutical interest. Overall, there is a linear relationship between the drug solubility and intrinsic dissolution rate expressed through the total kinetic coefficient of dissolution and dimensionless numbers defining the mass transfer regime. The contribution of surface kinetics appears to be significant constituting on average ∼20% resistance to the dissolution flux in the compendial rotating disk apparatus at 100 rpm. The surface kinetics contribution becomes more dominant under conditions of fast laminar or turbulent flows or in cases when the surface kinetic coefficient may decrease as a function of solution composition or pH. Limitations of the well-known convective diffusion equation for rotating disk by Levich are examined using direct computational modeling with simultaneous dissociation and acid-base reactions in which intrinsic dissolution rate is strongly dependent on pH profile and solution ionic strength. It is shown that concept of diffusion boundary layer does not strictly apply for reacting/interacting species and that thin-film diffusion models cannot be used quantitatively in general case.

Catalyst design for biorefining.

The quest for sustainable resources to meet the demands of a rapidly rising global population while mitigating the risks of rising CO2 emissions and associated climate change, represents a grand challenge for humanity. Biomass offers the most readily implemented and low-cost solution for sustainable transportation fuels, and the only non-petroleum route to organic molecules for the manufacture of bulk, fine and speciality chemicals and polymers. To be considered truly sustainable, biomass must be derived from resources which do not compete with agricultural land use for food production, or compromise the environment (e.g. via deforestation). Potential feedstocks include waste lignocellulosic or oil-based materials derived from plant or aquatic sources, with the so-called biorefinery concept offering the co-production of biofuels, platform chemicals and energy; analogous to today's petroleum refineries which deliver both high-volume/low-value (e.g. fuels and commodity chemicals) and low-volume/high-value (e.g. fine/speciality chemicals) products, thereby maximizing biomass valorization. This article addresses the challenges to catalytic biomass processing and highlights recent successes in the rational design of heterogeneous catalysts facilitated by advances in nanotechnology and the synthesis of templated porous materials, as well as the use of tailored catalyst surfaces to generate bifunctional solid acid/base materials or tune hydrophobicity.

Lewis acid behavior of SF4 : synthesis, characterization, and computational study of adducts of SF4 with pyridine and pyridine derivatives.

Sulfur tetrafluoride was shown to act as a Lewis acid towards organic nitrogen bases, such as pyridine, 2,6-dimethylpyridine, 4-methylpyridine, and 4-dimethylaminopyridine. The SF4 ⋅NC5 H5 , SF4 ⋅2,6-NC5 H3 (CH3 )2 , SF4 ⋅4-NC5 H4 (CH3 ), and SF4 ⋅4-NC5 H4 N(CH3 )2 adducts can be isolated as solids that are stable below -45 °C. The Lewis acid-base adducts were characterized by low-temperature Raman spectroscopy and the vibrational bands were fully assigned with the aid of density functional theory (DFT) calculations. The electronic structures obtained from the DFT calculations were analyzed by the quantum theory of atoms in molecules (QTAIM). The crystal structures of SF4 ⋅NC5 H5 , SF4 ⋅4-NC5 H4 (CH3 ), and SF4 ⋅4-NC5 H4 N(CH3 )2 revealed weak SN dative bonds with nitrogen coordinating in the equatorial position of SF4 . Based on the QTAIM analysis, the non-bonded valence shell charge concentration on sulfur, which represents the lone pair, is only slightly distorted by the weak dative SN bond. No evidence for adducts between quinoline or isoquinoline with SF4 was found by low-temperature Raman spectroscopy.

Spectral characterisation of Z-isomers of lycopene formed during heat treatment and solvent effects on the E/Z isomerisation process.

The geometric isomerisation of (all-E)-lycopene, purified from tomato paste, was investigated in various organic solvents. Isomerisation ratios to the Z-isomers of lycopene in CH2Cl2 and CHCl3 over 24h were calculated to be 19.7% and 11.4% at 4°C and 77.8% and 48.4% at 50°C, respectively. In CH2Br2, more than 60% was attained in the first several hours, independent of temperature. The predominant Z-isomers obtained thermally, (9Z)-lycopene and (13Z)-lycopene, were purified and their absorption maxima and molar extinction coefficients in hexane were determined for the first time. Absorption values at 460 nm were also measured for both Z-isomers along with (all-E)-lycopene to accurately evaluate their concentrations by HPLC analysis. This approach successfully revealed that (13Z)-lycopene formed predominantly in benzene or CHCl3 at 50°C; in contrast, the 5Z-isomer was preferentially obtained in CH2Cl2 or CH2Br2.

Moderately Strong Photoacid Dissociates in Alcohols with High Transient Concentration of the Proton-Transfer Contact Pair.

Proton transfer from strong photoacids to hydroxylic solvents is much under debate. Experimentally, the main issue stems from relaxation and diffusion processes that are concomitant with ultrafast proton transfer and blur population dynamics. To overcome this, we propose a fast photodissociation reaction that, however, proceeds slower than solvent relaxation. Fluorescence spectroscopy of the cationic photoacid 2-(1'-hydroxy-2'-naphtyl)benzimidazolium reveals a two-stage mechanism: (a) reversible elementary proton transfer inside the solvent shell and (b) irreversible contact-pair splitting. The time evolution of the fluorescence signal is complex, yet this is explained quantitatively by simultaneous, spectrally overlapping emission of the acid, the conjugate base, and the contact proton-transfer pair. The latter attains high transient concentration in linear alcohols. Microscopic rate constants of dissociation are determined.