Hydrogen-bonding 8. Possible equivalence of solute and solvent scales of hydrogen-bond basicity of non-associated compounds

The surface site interaction point approach to non-covalent interactions

The functional properties of molecular systems are generally determined by the sum of many weak non-covalent interactions, and therefore methods for predicting the relative magnitudes of these interactions is fundamental to understanding the relationship between function and structure in chemistry, biology and materials science. This review focuses on the Surface Site Interaction Point (SSIP) approach which describes molecules as a set of points that capture the properties of all possible non-covalent interactions that the molecule might make with another molecule. The first half of the review focuses on the empirical non-covalent interaction parameters, α and β, and provides simple rules of thumb to estimate free energy changes for interactions between different types of functional group. These parameters have been used to have been used to establish a quantitative understanding of the role of solvent in solution phase equilibria, and to describe non-covalent interactions at the interface between macroscopic surfaces as well as in the solid state. The second half of the review focuses on a computational approach for obtaining SSIPs and applications in multi-component systems where many different interactions compete. Ab initio calculation of the Molecular Electrostatic Potential (MEP) surface is used to derive an SSIP description of a molecule, where each SSIP is assigned a value equivalent to the corresponding empirical parameter, α or β. By considering the free energies of all possible pairing interactions between all SSIPs in a molecular ensemble, it is possible to calculate the speciation of all intermolecular interactions and hence predict thermodynamic properties using the SSIMPLE algorithm. SSIPs have been used to describe both the solution phase and the solid state and provide accurate predictions of partition coefficients, solvent effects on association constants for formation of intermolecular complexes, and the probability of cocrystal formation. SSIPs represent a simple and intuitive tool for describing the relationship between chemical structure and non-covalent interactions with sufficient accuracy to understand and predict the properties of complex molecular ensembles without the need for computationally expensive simulations.

Hydrogen-Bond Acceptance of Solvents: A 19F Solvatomagnetic β1 Database to Replace Solvatochromic and Solvatovibrational Scales

A variety of physicochemical properties and several hydrogen-bond donors have been used to define methods and to build scales aiming at measuring the hydrogen-bond acceptance of solvents. There is a great deal of confusion in these scales and methods. Solvatochromic, solvatocalorimetric, solvatovibrational, and ¹⁹F solvatomagnetic comparison methods are critically reviewed. Only two methods, the solvatomagnetic and the solvatocalorimetric ones, are able to yield reliable solvent hydrogen-bond acceptance scales. The solvatomagnetic β₁ scale defined from the ¹⁹F chemical shift of 4-fluorophenol is extended to many solvents including ionic liquids and green solvents. The results for about 240 hydrogen-bond acceptor solvents are organized in a numerical β₁ database. The comparison of β₁ with solvatochromic scales highlights their shortcomings, in particular for the important class of amphiprotic solvents. Therefore, the use of the ¹⁹F solvatomagnetic comparison method and of the solvatomagnetic β₁ scale is recommended in solvent effect studies.

Polarisation effects on the solvation properties of alcohols

Alcohol solvents are significantly more polar than expected based on the measured H-bonding properties of monomeric alcohols in dilute solution. Self-association of alcohols leads to formation of cyclic aggregates and linear polymeric chains that have a different polarity from the alcohol monomer. Cyclic aggregates are less polar than the monomer, and the chain ends of linear polymers are more polar. The solvation properties of alcohols therefore depend on the interplay of these self-association equilibria and the equilibria involving interactions with solutes. Twenty-one different molecular recognition probes of varying polarity were used to probe the solvation properties of alkane-alcohol mixtures across a wide range of different solvent compositions. The results allow dissection of the complex equilibria present in these systems. Formation of a H-bond between two alcohol molecules leads to polarisation of the hydroxyl groups, resulting in an increase in binding affinity for subsequent interactions with the unbound donor and acceptor sites. The H-bond donor parameter (α) for these sites increases from 2.7 to 3.5, and the H-bond acceptor parameter (β) increases from 5.3 to 6.9. Polarisation is a short range effect limited to the first H-bond in a chain, and formation of subsequent H-bonds in longer chains does not further increase the polarity of chain ends. H-bond donor sites involved in a H-bond are unavailable for further interactions, because the formation of a bifurcated three-centre H-bond is three orders of magnitude less favourable than formation of a conventional two-centre H-bond. These findings are reproduced by quantum chemical calculations of the molecular electrostatic potential surfaces of alcohol aggregates. Thus, the overall solvation properties of alcohols depend on the speciation of different aggregates, the polarities of these species and the polarities of the solutes. At low alcohol concentrations, polar solutes are solvated by alcohol monomers, and at higher alcohol concentrations, solutes are solvated by the more polar chain ends of linear polymers. The less polar cyclic aggregates are less important for interactions with solutes. Similar behavior was found for ten different alcohol solvents. Tertiary alcohols are marginally less polar solvents than primary alcohols, due to steric interactions that destabilises the formation of polymeric aggregates leading to lower concentrations of polar chain ends. One alcohol with an electron-withdrawing substituent was studied, and this solvent showed slightly different behavior, because the H-bond donor and acceptor properties are different.

Kamlet-Taft and Catalán solvatochromism of some π-expanded phenanthrimidazole derivatives - DFT analysis

Some π-expanded phenanthrimidazole derivatives have been synthesised and characterised by different spectral techniques. Kamlet-Taft and Catalán solvatochromism of synthesised phenanthrimidazole derivatives have been analysed. Non-linear optical (NLO) and natural bond orbital (NBO) analysis have been made by ab initio method to show intramolecular interactions. The energies of the HOMO and LUMO levels, the molecular electrostatic potential (MEP) energy surface, NBO studies have exploited the existence of intramolecular charge transfer (ICT) within the molecule.

Lifetime time measurements, Kamlet-Taft and Catalan solvatochromism of some 2-aryl benzimidazole derivatives

Some 2-aryl benzimidazole derivatives (1-6) have been prepared and characterized by different spectral techniques. Fluorescence lifetime of synthesized 2-aryl benzimidazole derivatives was calculated. Kamlet-Taft and Catalan solvatochromism of synthesized 2-aryl benzimidazole derivatives have been discussed. Crystal structure of 1-(4-methylbenzyl)-2-p-tolyl-1H-benzo[d]imidazole has been studied.

Molecular probes of solvation phenomena

The properties of the molecules present in any chemical or biological system are dependent on interactions with the environment, and a quantitative understanding of solvation phenomena remains a major challenge. Molecular recognition probes provide a new approach to quantitatively measure the properties of solvents. Traditionally, solvent polarity scales have been based on spectroscopic probes that provide insight into the nature of solvent-solute interactions. This review compares the solvent polarity parameters obtained from the wavelengths of UV/Visible absorption maxima with solute H-bond parameters obtained from the free energies of solution equilibria. The similarity of the solvent and solute H-bond scales leads to a general H-bond scale that uses the same parameters to describe both solvent and solute. The general H-bond scale provides a framework for understanding the relationship between local intermolecular interactions and the properties of the bulk medium. Intermolecular interactions are sensitive to solvation equilibria, so molecular recognition probes provide fundamentally different information from spectroscopic probes that are sensitive to the populations of different solvation states of the solute. Studies of mixed solvents demonstrate the potential of molecular recognition probes for providing new insights into solvation phenomena.

Measuring the micro-polarity and hydrogen-bond donor/acceptor ability of thermoresponsive N-isopropylacrylamide/N-tert-butylacrylamide copolymer films using solvatochromic indicators

Thin polymer films are important in many areas of biomaterials research, biomedical devices, and biological sensors. The accurate in situ measurement of multiple physicochemical properties of thin polymer films is critical in understanding biocompatibility, polymer function, and performance. In this work we demonstrate a facile spectroscopic methodology for accurately measuring the micro-polarity and hydrogen-bond donor/acceptor ability for a series of relatively hydrophilic thermoresponsive copolymers. The micro-polarity of the N-isopropylacrylamide (NIPAM) and N-tert-butylacrylamide (NtBA) co-polymers was evaluated by means of the E(T)(30), alpha, beta, and pi empirical solvatochromic polarity parameters. The data shows that increasing the NtBA fraction in the dry copolymer film reduces polarity and hydrogen-bonding ability. Within the Kamlet-Taft polarity framework, the NIPAM/NtBA copolymer films are strong hydrogen-bond acceptors, strongly dipolar/polarizable, and rather moderate hydrogen-bond donors. This characterization provides a more comprehensive physicochemical description of polymers, which aids the interpretation of film performance. Comparison of the measured E(T)(30) values with literature data for other water-soluble polymers show that dry NIPAM/NtBA copolymers are slightly more polar than poly(ethylene oxide), less polar than polyvinylalcohol, and approximately the same polarity as poly(N-vinyl-2-pyrrolidone). These findings indicate that this spectroscopic method is a facile, rapid, and nondestructive methodology for measuring polymer properties in situ, suitable for most biomaterials research laboratories.