Solvent Basicity, A Study of Kamlet–Taft β and Gutmann DN Values Using Computationally Derived Molecular Properties

A Study of Abraham’s Effective Hydrogen Bond Acidity and Polarity/Polarizability Parameters, A and S, Using Computationally Derived Molecular Properties

Abraham’s hydrogen bond donor and polarity/polarizability parameters, A and S, respectively, are correlated with molecular properties derived from computational chemistry. The analyses show that A, like Kamlet and Taaft’s α, correlates only with the charge on the most positive hydrogen atom of the molecule but that there are marked steric effects. In contrast, S correlates with the molecular dipole moment, the partial charge on the most negative atom in the molecule and with the polarizability of single ring aromatic compounds but not those of aliphatic compounds. These results are compared with those for corresponding Kamlet and Taft parameters, α and π* and Reichardt’s ET(30) and discussed in terms of the experimental methods used to determine the parameters.

Twisted Intramolecular Charge Transfer (TICT) Controlled by Dimerization: An Overlooked Piece of the TICT Puzzle

Organic dyes have shown high efficiencies in solar cells, which is mainly attributed to the push–pull strategy present in such dyes upon attaching to the semiconductor surfaces. We deeply studied the fundamental photophysical properties of cyanoacrylic dyes, mostly the L1 dye, and found unique emission properties that depend on many factors such as the solvent polarity and the concentration of the dye and could present a complete emission picture about this family of dyes. The L1 dye shows an intramolecular charge transfer (ICT) emission state at low concentrations (approximately nanomolar scale) and shows a twisted intramolecular charge transfer (TICT) emission state in specific solvents upon increasing the concentration to the micromolar scale. Moreover, the associated emission lifetimes of the ICT and TICT states of the L1 dye depend on solvent basicity, highlighting the role of hydrogen bond formation on controlling such states. Density functional theory calculations are performed to gain insight into the photophysical properties of the dye and revealed that H-bonding between the carboxylic groups triggers the dimerization at low concentrations. Using femtosecond transient absorption, we assigned the rate of TICT formation to be in the range (160–650 fs)⁻¹, depending on the size of the studied cyanoacrylic dye. Therefore, we add herein a new dimension for controlling the formation of the TICT state, in addition to the solvent polarity and acceptor strength parameters. These findings are not limited to the studied dyes, and we expect that numerous organic carboxylic acids dyes show similar properties.

Using computational chemistry to explore experimental solvent parameters – solvent basicity, acidity and polarity/polarizability

Solvent basicity and polarity/polarizability parameters are analysed using molecular properties of solvents derived from computational chemistry. The results show that Kamlet and Taft’s measure of hydrogen bond basicity, β, is essentially identical to Gutmann’s donor number, a measure of Lewis basicity, both being determined by the charge on the most negative atom of the solvent molecule and the energy of the electron donor orbital. It is also found that, for both parameters, the calculated values for alcohols and N–H containing bases deviate systematically from those for aprotic solvents. This mirrors Kamlet and Taft’s earlier observation that different solvatochromic probes yield different β values in amphiprotic solvents. Reichardt’s E T (30) and Kamlet, Abboud and Taft’s π * both show direct dependences on the dipole moments and quadrupolar amplitudes of the solvent molecules and, surprisingly, an inverse dependence on the molecular polarizability. Additionally, E T (30) has a strong dependence on the charge on the most positive hydrogen atom of the solvent molecule, reflecting its sensitivity to hydrogen bonding. Unexpectedly, π * shows a dependence on the energy of the electron donor orbital. Kammet and Taaft’s hydrogen bond acidity parameter, α, is discussed in light of the results for π * and E T (30).

A Method of Calculating the Kamlet-Abboud-Taft Solvatochromic Parameters Using COSMO-RS

There is demand for safer and bio-based solvents, brought on by legislation and sustainability objectives. The prediction of physical properties is highly desirable to help design new molecules. Here we present an in silico approach to obtain calculated Kamlet-Abboud-Taft solvatochromic parameters using virtual experiments. The tautomerisation equilibrium of methyl acetoacetate and dimedone was calculated in different solvents with COSMO-RS theory and converted into estimates of solvent dipolarity and hydrogen bond accepting ability, respectively. Hydrogen bond donating ability was calculated as a function of the electron deficient surface area on protic solvents. These polarity descriptors correlate with rate constants and equilibria, and so ability of calculated Kamlet-Abboud-Taft solvatochromic parameters to recreate experimental free energy relationships was tested with sixteen case studies taken from the literature. The accuracy of the calculated parameters was also satisfactory for solvent selection, as demonstrated with a 1,4-addition reaction and a multicomponent heterocycle synthesis.