We develop a nonsolvatochromic comparison method for the determination of the alpha(1) scale of solvent hydrogen-bond acidity by means of F-19 NMR spectrometry. We compare the F-19 chemical shifts of 4-fluoro-2-methylpyridine (as a sensitive hydrogen-bond acceptor probe) and of 4-fluoronitrobenzene (as a chemically similar but less basic reference probe). This so-called F-19 solvatomagnetic comparison method yields the hydrogen-bonding contribution to delta (F-19)(4-fluoro-2-methylpyridine) that is well correlated to alpha(1) values obtained from the solvatochromism of Reichardt's betaine dye. Therefore, this solvatomagnetic comparison method is applied to determine the alpha(1) values of 13 choline chloride-based deep eutectic solvents.

The solvent parameters DI, ES, α1, and β1 are intended for the description of solute–solvent intermolecular forces, i.e., dispersion-induction, electrostatic, hydrogen-bond donation, and hydrogen-bond acceptance, respectively. An up-to-date collection of these parameters is presented for 380 solvents including green solvents, ionic liquids, and deep eutectic solvents. Their determination for additional solvents requires three commercial indicators, betaine dye B(30), 4-F-phenol, and 4-F-anisole (as well as the refractive index). The chemical significance of these parameters is outlined. Their use in the linear solvation energy relationship P = P° + di DI + e ES + a α 1 + b β 1 for 62 physicochemical properties P (reaction rates, equilibrium constants, and IR, UV, and NMR spectra) yields determination coefficients generally greater than 0.90 and regression coefficients whose sign and relative magnitude provide consistent information on the intermolecular forces acting on these properties.

Today, the hydrogen bonding donation (HBD) ability parameter of new solvents, α, is generally determined either by the Kamlet-Taft solvatochromic comparison of two probes, Reichardt betaine dye B(30) and 4-nitroanisole, or by the measurement of a single probe (e.g., solvatochromism of an iron coordination complex). This work highlights the shortcomings of these probes and recommends three replacement methods: (a) the theoretical comparison of the experimental and PCM-TD-DFT calculated transition energies ET(30) of B(30), (b) the semiempirical comparison of the experimental and McRae calculated ET(30), and, (c) for ionic liquids, the experimental comparison of ET(30) and ET(33) lying on the lower basicity of the betaine dye B(33) compared to B(30). These methods yield a new HBD parameter, α1, for 101 molecular solvents and 30 ionic liquids. The novelty is emblematic for water, with α1 = 1.54 instead of α (Kamlet-Taft) = 1.17. The solvent parameter α1 is not equivalent to the solute hydrogen-bond acidity parameter α2H, partly because of the self-association of HBD solvents.

The pKBHX (logarithm of complexation constant K of 4-fluorophenol with bases) hydrogen-bond basicity scale of neutral hydrogen-bond acceptors (HBAs) is extended to anionic HBAs. The scale is constructed for 26 anions through (i) the infrared measurement of K on NBu4+X- ion pairs in CCl4, (ii) the estimation of K from linear free energy relationships between measured K values and literature K values for various phenols in polar solvents, and (iii) the computation of K at the density functional theory level in CCl4. The scale extends on a 9.4 pK unit range from fluoride to tetraphenylborate. Considering a number of anions as organic functions substituted with unipolar substituents, their pKBHX values can be related to the Hammett-Taft substituent constants σ. Unipolar substituents (O- and S-) obey the same pKBHX versus σ relationships as dipolar ionic (N-N+R3) and dipolar (OH, CF3, NR2, or OR) ones for the nitrile, carbonyl, nitroso, nitro, sulfonyl, and phosphoryl functions. Like dipolar substituents, unipolar substituents at carbon and nitrogen operate by field-inductive and resonance effects, whereas substituents at sulfur and phosphorus operate only by the field-inductive effect.

A 19F solvatomagnetic comparison of 4-fluorophenol and 4-fluoroanisole yields a more reliable measurement of the hydrogen-bond acceptance of ionic liquids and green solvents than the solvatochromic comparison method.

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.

The logarithm of heterolysis rates of 4-methoxyneophyl tosylate, Et3CBr,tert-BuCl, andtert-BuBr in a set of 12 to 28 hydrogen-bond (HB) and non-HB donor solvents are correlated to four sets of solvent parameters: (a)E-T(30), (b)pi*,alpha,beta, (c)SP,SdP,SA,SB, and (d)DI,ES,alpha(1),beta(1). Disjointed results are found for Et3CBr because of an insufficient diversity of solvents. For the other solvolyses, the determination coefficientsr(2)are good to excellent, the gas-phase values are fairly predicted, and the contributions of various intermolecular forces to the global solvent effect agree satisfactorily within the four solvent sets. However, the most complete description of the solvent effect is given by the four-parameter sets because their parameters refer to a single intermolecular force, whereasE(T)(30) and pi*correspond to a fixed blend of solute/solvent interactions. For the application of theDI,ES,alpha(1), and beta(1)set to correlatetert-BuCl solvolysis,r(2)= .976, lgk(gas) = -18.6 (experimental -19.3), and the contributions of HB donation (alpha(1)), electrostatic forces (ES), and dispersion-induction (DI) to the solvent effect amounts to 59%, 21%, and 14%, respectively. The dependence of rates on the solvent HB basicity is nearly zero. The inclusion of the solvent cohesive energy density parameter delta H2into the correlation equations does not improve these correlations.

Betaine 30 is known for the extraordinary solvatochromism of its visible absorption band that goes from λ=882 nm in tetrachloromethane to λ=453 nm in water (Δλ=-429 nm). This large blueshift partly originates from a dramatic decrease of the dipole moment upon excitation. Despite several decades of research, experimental works still disagree on the exact value of the excess dipole moment, the orientation of the dipole moment of the excited-state, the role and amplitude of the change of the polarisability upon excitation as well as on the gas-phase excitation energy. In this work, we present an in-depth theoretical investigation. First, we carefully tested several levels of theory on the model system and next calculated the electric properties of betaine 30 at the CC2 level. Our best estimates are Δμ=-7 D for the excess dipole moment, that is, a significant decrease but no change of direction, a Δα value of -120 a.u. and a gas-phase vertical excitation energy of 1.127 eV. The implicit solvation models are able to reproduce the experimental trends, with large correlation coefficients for non-hydrogen-bond-donating solvents, the smallest root-mean-square deviation error being reached with the vertical excitation model (VEM). The explicit effective fragment potential method combined with time-dependent density functional theory (TD-DFT) in a QM/MM framework provides accurate estimates for hydrogen-bond-donating solvents, whereas the addition of a dispersion correction is needed to restore the correct solvatochromic direction in tetrachloromethane.

4-Nitropyridine N-oxide is a well-known molecular probe for which the experimental UV/vis absorption spectrum has been measured in a large number of solvents. Previous measurements and their analyses suggest a dominant role of the solvent hydrogen-bond donation (HBD) capability in the solvatochromic shifts measured for the absorption spectra. Herein, we analyze these solvatochromic effects using a series of complementary approaches, including empirical solvent parameters, high-level calculation of the excited-state dipole and polarizability, several flavors of the polarizable continuum model, as well as dynamics using an effective fragment potential (EFP) description of the solvent molecules. First, applying a recently proposed set of solvent parameters, we show the importance of dispersion interactions for non-HBD solvents. This statement confronts advanced coupled-cluster and multireference calculations of dipole moments and polarizabilities of both the ground and excited states in gas phase. We further address the pros and cons of implicit solvent models combined to time-dependent density functional theory (TD-DFT) in describing the solvents effects for all (HBD and non-HBD) media, the simplest linear-response approach turning out to be the most adequate. Finally, we show that the explicit TD-DFT/EFP2 models work correctly for HBD molecules and allow for restoration of the main experimental trends.

For about 300 solvents, we propose a database of new solvent parameters describing empirically solute/solvent interactions: DI for dispersion and induction, ES for electrostatic interactions between permanent multipoles, α1 for solute Lewis base/solvent Lewis acid interactions, and β1 for solute hydrogen-bond donor/solvent hydrogen-bond acceptor interactions. The main advantage over previous parametrizations is the easiness of extension of this database to newly designed solvents, since only three probes, the betaine dye 30, 4-fluorophenol, and 4-fluoroanisole are required. These parameters can be entered into the linear solvation energy relationship A = A0 + di(DI) + eES + aα1 + bβ1 to predict a large number of varied physicochemical properties A and to rationalize the multiple intermolecular forces at the origin of solvent effects through a simple examination of the sign and magnitude of regression coefficients di, e, a, and b. Such a rationalization is illustrated for conformational and tautomeric equilibria and is supported by quantum-mechanical calculations.

The hydrogen-bond-acceptor basicity of an important class of solvents, the amphiprotic solvents (water, alcohols, primary and secondary amides, and carboxylic acids), has not yet been properly parametrized. In this work, the first scale of solvent hydrogen-bond basicity applicable to amphiprotic solvents is established by means of a new method that compares the 19F NMR chemical shifts of 4-fluorophenol and 4-fluoroanisole in hydrogen-bond-acceptor solvents. This so-called solvatomagnetic comparison method is free of the shortcomings of the solvatochromic comparison method used so far and is easier to carry out than the pure base calorimetric method. The validity of the new scale is assessed by good linear correlations with spectroscopic, thermodynamic, and kinetic solute properties depending on the solvent hydrogen-bond basicity. In such correlation analysis of solvent effects on physicochemical properties, solvent and solute hydrogen-bond basicity scales must not be mixed, since it is shown here that solute and solvent scales are not equivalent. A comprehensive collection of parameters quantifying the hydrogen-bond basicity is presented for 168 solvents.

Empirical parameters of solvents describing their hydrogen-bond (HB) acidity (e.g., the Kamlet-Taft α parameter) are often difficult to determine for new solvents because they are not directly related to a single definition process. Here, we propose a simple method based on one probe, the betaine dye 30, and one reference process, the solvatochromism of this dye, measured by its first electronic transition energy, ET(30). These ET(30) values are calculated within the time-dependent density functional theory framework, using a polarizable continuum solvent model (PCM). The part of ET(30) values that is not included in the PCM calculation is taken as the HB component of the measured ET(30) values, allowing us to deduce a solvent HB acidity parameter α1. The validity of this simple model is assessed by good linear correlations between α1 and a variety of solute properties mainly depending on the solvent's HB acidity. The quality of fit observed with α1 is at least comparable with that obtained by previous solvent HB acidity scales. The simplicity of our method is illustrated by the determination of α1 and of its companion, the electrostatic solvent parameter ES, for some new green solvents derived from glycerol.

The solvent polarity parameter ET(30) is newly measured from the solvatochromism of the betaine dye 30 for 84 solvents and re‐measured for 186 additional ones. The results are organized in a database. It is shown that the validity of linear solvation energy relationships used for the determination of secondary ET(30) values is limited to non‐hydrogen‐bond donor solvents. Relationships with the chain length n are given for the determination of tertiary ET(30) values of the homologous H(CH2)nY solvent series. The parameter ET(30) is orthogonal to the function of the refractive index (n2 − 1) / (2n2 + 1). For non hydrogen‐bond donor solvents, this allows to enter ET(30) as an almost pure electrostatic parameter in a new linear solvation energy relationship. Copyright © 2014 John Wiley & Sons, Ltd.

Halogen bond (X-bond) interactions involving organic iodine have been investigated using data retrieved from the Cambridge Structural Database (CSD) and Density Functional Theory (DFT) calculations. The analysis of the mean normalised intermolecular distances involving Csp–I, Csp2–I and Csp3–I X-bond donors first shows that, for interactions with the same acceptor site, the shortest mean distance is always measured for Csp–I X-bond donors. This experimental trend is rationalised through molecular electrostatic potential calculations on the iodine surface, along the σ-hole of the iodine atom, since Csp–I donors are characterised by the strongest VS,max values, that is to say the more electron poor iodine atoms. In agreement with the trends revealed from the analysis of the I⋯N distances, the X-bond with Csp–I donors appear more linear (mean of 169.5°) than with Csp2–I and Csp3–I donors, their respective values being close to 164°. From a survey of the geometries of the X-bond contacts observed in the most extended dataset (Csp2–I: 1213), a crystallographic order of X-bond acceptor strength has been obtained through a careful consideration of the chemical functions and subfunctions to which the acceptor atom belongs. This order is in good agreement with the one observed in solution on the diiodine basicity scale pKBI2. An exception to this trend is thioureas, which show unexpected long (weak) S⋯I distances. However, these observations are rationalised from the sulfur behaviour, which acts simultaneously as a X-bond acceptor and hydrogen-bond (H-bond) acceptor in these structures. Finally, an interesting correlation between the pKBI2 and the I⋯Y normalised intermolecular distances is found for a wide and varied collection of organic bases, since we have been able to delineate 22 families of organic compounds covering more than four pK units on the pKBI2 scale.

The concept of donor number (DN), based on calorimetric measurements of adduct formation between Lewis bases and a reference Lewis acid, proved to be fruitful for the interpretation of various medium effects, and, more generally, for the understanding of Lewis basicity. Attempts to extend the DN scale (a thermodynamic basicity scale) using indirect methods are discussed. In particular, the use of δ(23Na) NMR spectrometry for the characterization of Lewis basicity of ionic liquids is examined in this Correspondence.

The influence of substituents on the properties of tri- and hexafluorinated alcohols derived from 2,2,2-trifluoroethanol (TFE) and 1,1,1,3,3,3-hexafluoro-2-propanol (HFIP) was examined. Measurements of specific solvent-solute interactions revealed that H-bond donation (HBD) of fluorinated alcohols is sensitive to the steric hindrance of the OH group, whereas their Brønsted acidity is dependent only on the number of fluorine atoms. For hexafluorinated alcohols (HFAs), their association with amines characterized by X-ray diffraction showed that the balance between HBD and acidity is influenced by their structure. Moreover, the ability of HFAs to donate H-bonds is exerted in synclinal (sc), synperiplanar (sp), and also antiperiplanar (ap) conformations along the C-O bond. Comparison of the effects of fluorinated alcohols as promoting solvents in three reactions is reported. The positive correlation between rate constants and H-bonding donation ability for sulfide oxidation and imino Diels-Alder reaction brings to light the role of this property, while acidity might have a minor influence. In the third reaction, epoxide opening by piperidine, none of these properties can clearly be put forward at this stage.

The thermodynamics and some vibrational properties of hydrogen-bonded complexes of methanol with 23 hydrogen-bond acceptors (HBAs) have been determined in CCl(4) by FTIR spectrometry. The experimental sample contains carbon, nitrogen, oxygen, sulfur, fluorine, and chlorine organic bases and covers an energetic range of 13 kJ mol(-1) in the basicity scale (-ΔG), 22 kJ mol(-1) in the affinity scale (-ΔH), and 400 cm(-1) in the spectroscopic scale (Δν((OH))) (from benzene to trimethylphosphane oxide and amines). The experimental results in CCl(4) are compared to those computed in the gas phase at various levels of theory. Ninety five percent of the variance of the red shift and 89% of the variance of the intensification of the OH stretching upon hydrogen bonding are explained by gas-phase B3LYP/6-31+G(d,p) calculations. However, this level does not satisfactorily explain the thermodynamic properties. Only 68% of the variance of the methanol affinity (-ΔH) is taken into account. MP2/aug-cc-pVTZ//B3LYP/6-31+G(d,p) affinity calculations raise the explanation to 77% for all HBAs and to 93% when three outliers (Me(2)SO, Me(3)PO, and tetrahydrothiophene) are excluded. Discrepancies are analyzed in terms of experimental errors, calculation approximations, and solvation.