https://www.pnas.org/content/pnas/90/5/1635.full.pdf

Supramolecular chemistry.

We propose and discuss an efficient scheme for the in silico sampling for parts of the molecular chemical space by semiempirical tight-binding methods combined with a meta-dynamics driven search algorithm. The focus of this work is set on the generation of proper thermodynamic ensembles at a quantum chemical level for conformers, but similar procedures for protonation states, tautomerism and non-covalent complex geometries are also discussed. The conformational ensembles consisting of all significantly populated minimum energy structures normally form the basis of further, mostly DFT computational work, such as the calculation of spectra or macroscopic properties. By using basic quantum chemical methods, electronic effects or possible bond breaking/formation are accounted for and a very reasonable initial energetic ranking of the candidate structures is obtained. Due to the huge computational speedup gained by the fast low-cost quantum chemical methods, overall short computation times even for systems with hundreds of atoms (typically drug-sized molecules) are achieved. Furthermore, specialized applications, such as sampling with implicit solvation models or constrained conformational sampling for transition-states, metal-, surface-, or noncovalently bound complexes are discussed, opening many possible applications in modern computational chemistry and drug discovery. The procedures have been implemented in a freely available computer code called CREST, that makes use of the fast and reliable GFNn-xTB methods.

Automated exploration of the low-energy chemical space with fast quantum chemical methods

Tautomeric equilibria in relation to pi-electron delocalization.

On a evalue les effets de solvant sur un certain nombre de processus et on discute des resultats de l'application de l'analyse de regression lineaire multiple. Les processus etudies comportent des exemples de solubilite des gaz ou des vapeurs, des coefficients de distribution des solutes entre l'eau et une serie de solvants ainsi que des effets de solvant sur l'equilibre conformationel, sur la tautomerie ceto-enolique et sur les vitesses de reaction. On demontre que deux equations particulieres, celle de Koppel et Palm qui a ete etendue par Makitra et Pirig ainsi que celle d'Abraham, Kamlet et Taft, peuvent rendre compte d'une facon satisfaisante des effets de solvant sur ces divers processus. On note que l'interpretation des parametres obtenus a l'aide de ces equations qui impliquent des quantites macroscopiques comme le ΔG ou le ΔG 0 ne sont pas simples; des modeles sont necessaires pour interpreter ces quantites macroscopiques en fonction de quantites microscopiques qui peuvent caracteriser, par exemple, les interactions solute-solvant

Solvent effects in organic chemistry: recent developments

Despite recent attempts using metal–organic frameworks (MOFs) directly as electrocatalysts, the electrochemical stability of MOFs and the role of in situ-formed species during electrochemistry are ...

Electrochemical Instability of Metal-Organic Frameworks: In Situ Spectroelectrochemical Investigation of the Real Active Sites

The enol imine ⇄ enaminone tautomerization constants, KT, and thermodynamic parameters, ΔHT and ΔST, of 1-hydroxy-2-naphthaldehyde Schiff bases are determined by UV/vis spectroscopy. Polar solvents shift the equilibrium toward the quinone form (for the unsubstituted derivative 1c, KT = 0.20 (cyclohexane) and KT = 1.49 (ethanol)). Both donor (MeO, NMe2) and acceptor (CN, NO2) substituents lead to a decreased KT independent of solvent polarity. In apolar solvents, for all derivatives 1a−1e, the enol imine ⇄ enaminone equilibria are endergonic but exothermic. Linear solvation energy relationships allow extrapolation of ΔGT to the gas phase. Density functional theory calculations (B3LYP/6-311+G**) yield good agreement with these extrapolated ΔGT values. Solvent effects on 1c are also successfully reproduced by the calculations. Geometric (O···N distance) and energetic criteria (conformer energy differences, homodesmotic reactions) establish the importance of intramolecular hydrogen bonding for the tautomerism...

Tautomerism in Hydroxynaphthaldehyde Anils and Azo Analogues: a Combined Experimental and Computational Study

A UV–Vis spectroscopic study based on the recently developed chemometric approach for quantitative analysis of undefined mixtures is performed on a series of donor and acceptor substituted Schiff bases of 2-hydroxynaphthaldehydes. In CCl4 solution all compounds preferentially exist as the phenol tautomer independent of the nature of the respective substituent. With increasing polarity the tautomeric equilibrium is shifted towards the quinone form. In CHCl3 and, especially, ethanol a clear distinction between the effect of donors (stabilization of the quinone form) and acceptors (stabilization of the phenol tautomer) is evident. Ab initio calculations including solvent effects via the polarized continuum model of solvation as well as the supermolecule approach are used to rationalize the experimental findings.

Tautomerism of 2-hydroxynaphthaldehyde Schiff bases

Electronic transitions appear in the spectrum as individual
bands, described by three basic parameters (position, intensity and width),
which can be used for estimation of the fundamental transition
characteristics as well as for quantitative analysis. The main problems
concerning the mathematical resolution of overlapped individual bands in a
complicated spectrum (estimation of the number of overlapping bands, the
noise problem, artificial baseline and efficiency of the computing
procedure) are discussed and a principal scheme and some solutions are
suggested. The use of the resolution of overlapping bands technique for
quantitative analysis is given step-by-step in its historical development
on real spectral problems. Although the described methods for quantitative
analysis are discussed only in terms of UV–Vis spectroscopy they can
be easily used in all branches of molecular spectroscopy.

Resolution of overlapping UV–Vis absorption bandsand quantitative analysis

Herein, we demonstrate a conceptual idea for a tautomeric switch based on implementation of a flexible piperidine unit in 4-(phenyldiazenyl)naphthalen-1-ol. The results show that a directed shift in the position of the tautomeric equilibrium can be achieved through protonation/deprotonation in a number of solvents. The developed molecular switch, in spite of the simple host–guest system, has shown acceptable complexation ability towards small alkaliand alkalineearth-metal ions and can be a promising basis for further development of effective molecular sensors through implementation of azacrown ethers. Organic molecular materials are increasingly recognized as suitable molecular-level elements (such as switching, signaling, and memory elements) for molecular devices, because the wide range of molecular characteristics can be combined with the versatility of synthetic chemistry to alter and optimize molecular structure in the direction of desired properties. Virtually every molecule changes its behavior when acted upon by external fields or other stimuli. True molecular switches undergo reversible structural changes, caused by a number of influences, which give a variety of possibilities for control. Several classes of photoresponsive molecular switches are already known; these operate through processes such as bond formation and bond breaking, cis– trans isomerization, and photoinduced electron transfer upon complexation. A conceptual scheme of a molecular switch based on molecular recognition is shown in Scheme 1. The host–guest system represents, for instance, a crown ether that can bind ions or a cyclodextrin that can bind other small molecules. It is bound to a signal converter. The complexation behavior is monitored by the state of the signal converter, and in turn its optical or electronic properties are determined by the complexation state of the host–guest system. The main requirement in the design of new molecular switches is to provide fast and clean interconversion between structurally different molecular states (on and off). Tautomerism could be a possibility, because change in the tautomeric state can be accomplished by a fast proton transfer reaction between two or more structures, each of them with clear and different molecular properties. Therefore, our aim herein is to show how tautomerism can be exploited for signal conversion. The conceptual idea of such a device is presented in Scheme 1. In this structure, a change in tautomeric state, labeled A and B, is linked to changes in the complexation abilities of the host–guest system by modulating the propensity of the system to hydrogen bond to the antenna. At the same time, engagement of this antenna causes a change in the tautomeric state. The sensitivity of the electronic ground and excited states of the tautomeric forms to environment stimuli (light, pH value, temperature, solvent) and to the presence of a variety of substituents or to hydrogen bonding can be exploited in the design of flexible tools for control. Obviously, such a device should be based on a tautomeric structure with easy proton exchange between the tautomers, which means that they must coexist in solution. At the same time, a main feature of systems of tautomers coexisting in solution is that the overall optical response is a mixture of the optical responses of the individual tautomers. Consequently, in the design of tautomeric switches, conditions for obtaining pure end tautomer in the corresponding off and on states must be provided. Herein we report the properties of two tautomeric switches, namely 3 and 4 (Scheme 2), based on 4-(phenyldiazenyl)phenol (1) and 4-(phenyldiazenyl)naphthalen-1-ol (2). The parent compound 2 is the first dye that was shown to tautomerize by Zincke and Bindewald in 1884. It has been the object of many spectral and theoretical studies because its tautomeric forms coexist in solution and the equilibrium Scheme 1. Molecular switch based on molecular recognition (left) and conceptual idea for a tautomerism-based molecular switch (right).

Daniela Nedeltcheva

Papers

Tautomerism in Hydroxynaphthaldehyde Anils and Azo Analogues: a Combined Experimental and Computational Study

Tautomerism of 2-hydroxynaphthaldehyde Schiff bases

Resolution of overlapping UV–Vis absorption bandsand quantitative analysis

Exploiting Tautomerism for Switching and Signaling

Chemometric Models For Quantitative Analysis of Tautomeric Schiff Bases and Azo Dyes