Solvatomagnetic Comparison Method: A Proper Quantification of

Article pubs.acs.org/JPCB

Solvatomagnetic Comparison Method: A Proper Quantification of Solvent Hydrogen-Bond Basicity Christian Laurence,*,† Julien Legros,‡ Pierre Nicolet,† Daniela Vuluga,§ Agisilaos Chantzis,† and Denis Jacquemin†,∥ †

Université de Nantes, Laboratoire CEISAM, UMR 6230 CNRS, 2 rue de la Houssinière, 44322 Nantes, France Normandie Université, Laboratoire COBRA, UMR 6014 CNRS, 1 rue Lucien Tesnière, 76821 Mont-Saint-Aignan, France § Normandie Université, INSA de Rouen, UMR 6270 CNRS, Avenue de l’Université, 76801 Saint-Etienne du Rouvray, France ∥ Institut Universitaire de France (IUF), 103 Boulevard Saint-Michel, 75005 Paris, France ‡

S Supporting Information *

ABSTRACT: 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 4fluoroanisole 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.

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INTRODUCTION Hydrogen bonding is the most widespread specific molecular interaction governing the properties of organic solvents.1 Accordingly, solvents are often classified in terms of their behavior as hydrogen-bond acceptors (HBAs), hydrogen-bond donors (HBDs) (and conversely non-HBAs and non-HBDs), and amphiprotic solvents (having both HBA and HBD properties),2−4 and there have been several attempts to define solvent scales quantifying HBA and HBD strengths.5 This work is devoted to the measurement of the HBA strength of solvents (hereafter called HB basicity). Indeed, most solvents have HBA properties by virtue of nonbonding or π-bonding electron pairs in their electronic systems.6 Therefore, insofar as solutes have HBD properties, solvent effects on the solubility, partitioning, chemical equilibria, reaction rates, spectral properties, and electrochemical properties depend inevitably,1−4 sometimes dramatically,7,8 on the ability of solvents to accept hydrogen bonds. Today, the measurement of the HB basicity of molecules as solutes in an inert solvent (the so-called solute HB basicity) has been achieved through the construction of the βH2 ,9 pKBHX,10 and B11,12 scales. The first two scales are based on the equilibrium constants of hydrogen-bond formation between a series of HBAs and a common reference HBD measured in CCl4. Only one reference HBD, namely, 4-fluorophenol, is used in the building of the pKBHX scale,10 whereas in the construction of the βH2 scale © XXXX American Chemical Society

many reference HBDs are combined by means of a special statistical treatment.9 This data treatment anchors the scale to βH2 = 0 for non-HBA molecules and βH2 = 1 for hexamethylphosphoric triamide (HMPT).9 The measurement of the HB basicity of molecules acting as solvents (the so-called solvent HB basicity) represents a much greater challenge, since two terms are involved in the measured property: a term due to hydrogen bond formation and a term produced by nonspecific interactions. The latter contribution must be subtracted in order to isolate the hydrogen-bonding contribution. To perform this task, the most popular method used is the solvatochromic comparison method (vide infra), where either 4-nitroaniline or 4-nitrophenol is used as a probe.13 Unfortunately, for several important solvents, the solvatochromic comparison method provides divergent or unacceptable results. As shown below, (i) tri-n-butylamine is found to be more basic than HMPT with the 4-nitrophenol probe but much less basic with the 4-nitroaniline probe, (ii) water is 2 times more basic with 4-nitrophenol than with 4-nitroaniline, and (iii) a chemically unacceptable significant negative basicity is found for 1,1,1,3,3,3-hexafluoro-2-propanol (HFIP) from both probes. Received: May 11, 2014 Revised: June 11, 2014

A

dx.doi.org/10.1021/jp504630d | J. Phys. Chem. B XXXX, XXX, XXX−XXX

The Journal of Physical Chemistry B

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of less than 0.01 mol/L to avoid self-association. For 4fluoroanisole, the concentration dependence is much weaker and becomes negligible at less than 0.1 mol/L. The NMR spectra were obtained on a Jeol FX 90Q, Bruker Avance 300, or Bruker Avance 400 spectrometer. The NMR tube was maintained at 27 °C and contained an inserted capillary of 0.05 to 0.5% hexafluorobenzene in CDCl3 as an external standard. The chemical shifts are reported relative to δ 19F (CFCl3) = 0 ppm via δ 19F (C6F6) = −162.2 ppm. The ring protons were decoupled. The chemical shifts are obtained with a precision of ±0.02 ppm (See Supporting Information for details). IR Spectral Measurements. The IR spectra of 4-nitroaniline in HBA solvents were recorded with a Bruker Vector 22 FTIR spectrometer at a resolution of 1 cm−1. Infrasil quartz cells were used. The cell path length and the 4-nitroaniline concentration depend on the solubility of 4-nitroaniline and on the transparency of solvents. For instance, the concentrations were 0.015 mol/L for CCl4, saturation for NEt3, and 0.14 mol/L for MeCONMe2, and the corresponding cell path lengths were 1 cm, 0.5 mm, and 0.1 mm. A liquid film between NaCl plates was prepared to record the spectrum of neat HFIP. Quantum-Mechanical Calculations. The Gaussian 09 revision D.01 package17 was used for the density functional theory (DFT), time-dependent DFT (TD-DFT),18 and natural bond orbital (NBO)19 calculations, applying an ultrafine integration grid. For the ground-state geometry optimizations of all systems, we have used the ωB97X-D exchange-correlation functional20 and the 6-311+G(2d,p) basis set, a level of theory adequate to describe hydrogen-bonded systems. These optimizations have been performed using the tight convergence criteria and have been followed by a harmonic frequency analysis. In all cases, no imaginary frequencies were found. The NBO analyses and the TD-DFT calculations have been performed on the resulting optimized geometries using the same exchangecorrelation functional and basis set. The interaction energies were corrected for the basis set superposition error (BSSE) by using ghost atoms.21 For the definition of the monomers in the BSSE calculations, each one was considered to be a subsystem when there were only two molecules constituting the system. When there were three molecules, then the 4-fluorophenol molecule was considered to be the first subsystem, with the rest comprising the second one. For example, in the complex of 4fluorophenol with the dimer of formic acid, (HCOOH)2 was considered to be one subsystem in the BSSE computations.

In this work, the solvatochromic comparison method is revisited. We explain why the 4-nitroaniline and the 4nitrophenol probes yield conflicting HB basicity values and why such a method cannot give proper values for most amphiprotic solvents. Accordingly, a new method, called the solvatomagnetic comparison method, is proposed for constructing a solvent HB basicity scale β1 (Table 1 for symbols) reliable Table 1. Symbols for the HB Basicity Scales Discussed or Defined in This Work symbola

definition properties

β

miscellaneous

βH2

hydrogen-bonding formation constants solvatochromic shifts of 4nitroaniline solvatochromic shifts of 4nitrophenol solvatochromic shifts of 4nitrophenol 19 F NMR shifts of 4fluorophenol

β1(NH2) β1(OH) β2(OH) β1

measurement medium

type of scale

pure base and CCl4 CCl4

blend of solvent and solute solute

pure base

solvent

pure base

solvent

CCl3CH3

solute

pure base

solvent

a Solvent scales are denoted as β1, solute scales as β2, and the original scale of Kamlet and Taft13 simply as β.

for all classes of solvents. This method is based on the high sensitivity of the 19F NMR chemical shift of 4-fluorophenol to the hydrogen bonding of the OH group with HBA solvents. The new solvent HB basicity scale β1 measured by the fluorine NMR method is validated by comparison with hydrogen-bond enthalpies, 14N NMR chemical shifts, and kinetic solvent effects that all depend on the HB basicity of solvents. We also demonstrate that solute and solvent HB basicity scales are not equivalent and must not be mixed. Lastly, the calculation of secondary β1 values14 provides a comprehensive collection of 168 β1 values for use in the new linear solvation energy relationship (LSER):15,16 A = A 0 + di DI + e ES + a α1 + b β1

(1)

In this equation, A is a physicochemical property in a series of solvents and A0 is the intercept. Solvent parameters DI, ES, α1, and β1 describe, respectively, dispersion-induction, electrostatic, and hydrogen-bond (solute HBA/solvent HBD and solute HBD/solvent HBA) interactions. Regression coefficients di, e, a, and b measure the sensitivity of property A to these solvent parameters.

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RESULTS AND DISCUSSION The structures of HBD probes used in this work are given in Scheme 1. The symbols denoting the various HB basicity scales are explained in Table 1. Shortcomings of the Solvatochromic Comparison Method and of the Resulting Solvatochromic HB Basicity Scales. The β scale of HB basicity built by Kamlet and Taft (KT)13 is based on miscellaneous basicity-dependent properties. It has been averaged over four HBD probes (phenol, 4fluorophenol, 4-nitrophenol, and 4-nitroaniline), three physical properties (solvatochromic shifts, NMR shifts, and equilibrium constants), and two standard states (pure base and base diluted in CCl4). Nowadays, chemists prefer to use a single reference process and generally select the solvatochromic comparison method to determine the basicity of new solvents.22−29 The chosen probe is either 4-nitroaniline22−27 or 4-nitrophenol.26−29 They yield scales hereafter denoted as β1(NH2) and β1(OH). Unfortunately, as shown in the following text, (i) these scales are

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MATERIALS AND METHODS Chemicals. The purification of 4-fluoroanisole and 4fluorophenol (Aldrich) was achieved by vacuum distillation and sublimation, respectively. Solvents (Aldrich and Fluorochem) were of the highest purity available. Immediately before use, they were distilled and then dried via chromatography on columns of 3 or 4 Å activated molecular sieves. The final purity was checked by gas chromatography (GC) with Carbowax and Apiezon columns and found to be generally higher than 99.5%. The water content, determined by GC with a Porapack Q column, was generally reduced to below 100 ppm. To exclude moisture, the solutions were prepared and the NMR tube was filled in a glovebox under dry air. NMR Spectral Measurements. Solutions for NMR measurements were prepared at 4-fluorophenol concentrations B



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with the number of data point n = 32 (non-HBA and non-HBD solvents and gas phase), the determination coefficient r2 = 0.992, and the standard deviation of the estimate s = 96 cm−1 and where ṽ(NH2) and ṽ(NMe2) stand, respectively, for the wavenumbers of the solvatochromic band maxima of 4-nitroaniline and N,Ndimethyl-4-nitroaniline. HBA solvents fall below this line (Figure 1): a supplementary bathochromic shift is manifested by 4nitroaniline because the NH2 group is a better HBD in the excited electronic state than in the ground state. This supplementary shift for 4-nitroaniline relative to N,N-dimethyl4-nitroaniline is denoted Δṽ(NH2-NMe2) and can be calculated from the deviation of HBA solvents below the reference line via eq 3:

Scheme 1. Structures of HBD Probes Used in the Solvatochromic Comparison Method (Upper Line) and in the Solvatomagnetic and Pure Base Calorimetric Methods (Lower Line)

Δv (NH ̃ ̃ ̃ 2‐NMe 2) = [0.9752v (NMe 2) + 3278] − v (NH 2) (3)

Δṽ(NH2-NMe2) values can be scaled in a range from 0 (solvents obeying eq 2) to 1 by dividing by the 2760 cm−1 shift of HMPT. This yields the solvatochromic basicity parameter β1(NH2): not equivalent, (ii) the β1(NH2) scale is not reliable for most ethers, pyridines, and amines, and (iii) both scales are unreliable for water, alcohols, and most amphiprotic solvents. The solvatochromic comparison method consists of the comparison of solvent effects on the S0 → S1 electronic transitions of 4-nitroaniline and N,N-dimethyl-4-nitroaniline.13,30,31 Non-HBA and non-HBD solvents produce bathochromic shifts in going from the gas phase to solution because the more polar and polarizable excited electronic state is more stabilized than the ground state by solute/solvent interactions. The similarity of the two solutes explains that plotting the wavenumber of 4-nitroaniline against that of N,N-dimethyl-4nitroaniline (Figure 1) gives a reference line of eq 2 v (NH ̃ ̃ 2) = 0.9752v (NMe 2) + 3278

β1(NH 2) = Δv (NH ̃ 2‐NMe 2)/2760

(4)

The same method can be used to compare the solvatochromism of the S0 → S1 electronic transitions of 4-nitrophenol and 4-nitroanisole. This procedure provides solvatochromic scale β1(OH) by means of eqs 5−7 v (OH) = 1.0381v (OMe) − 384 ̃ ̃

(5)

Δv (OH ‐OMe) = [1.0381v (OMe) − 384] − v (OH) ̃ ̃ ̃

(6)

β1(OH) = Δv (OH ‐OMe)/2030 ̃

(7)

where ṽ(OH) and ṽ(OMe) stand, respectively, for the wavenumber of the solvatochromic band maximum of 4-nitrophenol and 4-nitroanisole in the various solvents and 2030 cm−1 is the bathochromic shift in HMPT. A list of about 150 Δṽ(NH2NMe2) and Δṽ(OH-OMe) values is given in refs 6 and 31. As shown in Table 2, β1(NH2) and β1(OH) values are the same for a number of solvents but differ significantly for others, mainly for ethers, hindered pyridines, and amines. Thermosolvatochromic studies30 show that Δṽ(NH2-NMe2) [but not Δṽ(OH-OMe)] values generally decrease when the temperature is increased from

(2)

Table 2. Comparison of Solvatochromic Basicity Parameters β1(OH) and β1(NH2) Respectively Determined with the 4Nitrophenol and 4-Nitroaniline Probes for Selected Solvents solvent

β1(OH)

Double-Bonded Oxygen Bases acetone 0.50 trimethyl phosphate 0.66 N,N-dimethylformamide 0.71 dimethyl sulfoxide 0.74 HMPT (by definition) 1.00 Ethers, Hindered Pyridine, and Amines 1,4-dioxane 0.46 tetrahydrofuran 0.59 di-tert-butyl ether 0.76 2,4,6-trimethylpyridine 0.98 N-methylcyclohexylamine 1.38 tri-n-butylamine 1.20 Amphiprotic Solvents water 0.45 HFIP −0.11

Figure 1. Solvatochromic comparison method. Wavenumber of the solvatochromic band maximum of 4-nitroaniline plotted against the same property for N,N-dimethyl-4-nitroaniline in various solvents. Circles: gas phase and non- or very weak HBA and HBD solvents (perfluoroalkanes, alkanes, polyhalogenoalkanes, and polyhalogenoarenes) fixing the reference line of eq 2. Triangles: HBA solvents; the deviation of tri-n-butylamine (blue triangles) below the reference line decreases from 1.8 to 0.44 k cm−1 when the temperature is increased from 0 to 105 °C. Diamonds: amphiprotic solvents; HFIP stands abnormally above the reference line. Data taken from refs 30 and 31. C

β1(NH2)

difference

0.51 0.66 0.74 0.73 1.00

+0.01 0.00 +0.03 −0.01

0.33 0.48 0.40 0.71 0.94 0.49

−0.13 −0.11 −0.36 −0.27 −0.44 −0.71

0.16 −0.39

−0.29 −0.28



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0 to 100 °C (representative example of NBu3 in Figure 1), which suggests an instability and an incomplete formation of the hydrogen-bonded complexes of 4-nitroaniline. Indeed, the NH2 group with two N−H bonds can form 1:1 or 1:2 complexes with HBA solvents.32−34 We have therefore studied this stoichiometry of complexation and its influence on the solvatochromic shifts. Infrared studies of the symmetric and asymmetric NH2 stretching vibrations allow the distinction of various species, since they are found at different wavenumbers when amino compounds are free, forming 1:1 or 1:2 complexes.32−34 Our infrared investigation of 4-nitroaniline in selected HBA solvents demonstrates that double-bonded oxygen bases form 1:2 complexes, whereas ethers, amines, and hindered pyridines give either a mixture of 1:1 and 1:2 complexes or only 1:1 complexes. Scheme 2 shows the NH2 stretching vibrations of 4-

bathochromic shifts than 1:2 complexation upon hydrogen bonding. Therefore, the β1(NH2) values of solvents giving (partially or totally) 1:1 complexes (mainly ethers and amines) will be smaller compared to their β1(OH) values, as shown in Table 2. Consequently, the use of the 4-nitroaniline probe should be discouraged in determining new HB basicity values. Another problem arises in determining β1(NH2) and β1(OH) values of amphiprotic solvents by the solvatochromic comparison method. In these solvents, two types of hydrogen bonds occur: type A, with NO2 as the HBA group and the solvent as the HBD, and type B, with the solvent as the HBA and OH or NH2 as the HBD group (Scheme 3).35,36 Both types produce bathochromic shifts of the S0 → S1 electronic transition of 4nitroaniline and 4-nitrophenol, and their respective contributions to the overall bathochromic shift cannot be determined experimentally since type A and type B hydrogen bonds occur simultaneously. However, TD-DFT calculations can provide individual estimates of these contributions. We have performed these calculations in vacuo on 4-nitrophenol hydrogen bonded to two alcohols, namely, methanol and trifluoromethanol (a model of HFIP). The results, provided in Scheme 3, show that the bathochromic shifts produced by type A hydrogen bonds are the most important. In particular, the most acidic and least basic alcohol, CF3OH, yields bathochromic shifts that are 6 times larger with type A than with type B hydrogen bonds. Since only type B hydrogen bonding is relevant to the solvent HB basicity, it is mandatory to partition the overall bathochromic solvent shift into the contributions of nonspecific, type A hydrogen bonding, and type B hydrogen bonding. For nonamphiprotic HBA solvents, the solvatochromic comparison method yields Δṽ(NH2-NMe2) and Δṽ(OH-OMe) values free of nonspecific solvent effects. However, for amphiprotic solvents, it is not possible to ascertain that this method is able to cancel the contribution of type A hydrogen bonding. In fact, we observe that the cancellation is not satisfactory for HFIP, an amphiprotic solvent with strong HBD properties, since the data point for this solvent stands abnormally above the reference line of the comparison method (Figure 1), giving negative β1(OH) and β1(NH2) values (Table 2) without physical significance. In the same vein, the large difference between the β1(OH) and β1(NH2) values of water (Table 2) has been attributed13 to different contributions of the bathochromic shifts produced by type A hydrogen bonding for the two couples of solvatochromic indicators (4-NO 2 C 6 H 4 NH 2 /4-NO 2 C 6 H 4 NMe 2 and 4-

Scheme 2. Wavenumbers (cm−1) of the Stretching Vibrations of the NH2 Group of 4-Nitroaniline in Various States of Hydrogen-Bond Complexation: No Complex (Free in Tetrachloromethane), 1:1 Complex (in Triethylamine), or 1:2 Complex (in N,N-Dimethylacetamide)

nitroaniline when free (in CCl4), 1:1 complexed (in triethylamine), or 1:2 complexed (in N,N-dimethylacetamide). In vacuo TD-DFT calculations of the bathochromic shifts of the S0 → S1 transition of 4-nitroaniline in going from the free form to the 1:1 and 1:2 hydrogen-bonded complexes with trimethylamine (modeling the tertiary amines used in the experimental work) yield respective values of 1405 and 2283 cm−1 . This demonstrates that 1:1 complexation produces much lower

Scheme 3. TD-DFT Calculations (ωB97X-D/6-311+G(2d,p) Level) of Wavenumbers v ̃ and Bathochromic Shifts (cm−1) of the S0 → S1 Electronic Transition of 4-Nitrophenol upon Hydrogen Bonding with Amphiprotic Molecules MeOH and CF3OH

D



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NO2C6H4OH/4-NO2C6H4-OMe). It is therefore likely that type A hydrogen bonding to the nitro group of nitroaromatics used in the solvatochromic comparison method contaminates the β1(OH) and β1(NH2) values of amphiprotic solvents. Finally, there are technical problems with the use of 4nitrophenol. The β1(OH) values of a number of bases cannot be determined either because they are not transparent (e.g., acetophenone, quinoline, pyrimidine, thioanisole, and nitrobenzene) or because they are protonated and not hydrogenbonded (e.g., 1,1,3,3-tetramethylguanidine, alkanolamines, nbutylamine, allylamine, and cyclohexylamine). As a result of these important shortcomings of the solvatochromic comparison method, it is essential to devise a more reliable and more general method for constructing a thoroughly acceptable scale of solvent HB basicity. In the following text, we present such a method that we call the solvatomagnetic comparison method. Solvatomagnetic Comparison Method and a Reliable β1 Scale of Solvent HB Basicity. In this method, the HBD probe is 4-fluorophenol and the basicity-dependent property is the 19F NMR chemical shift. When 4-fluorophenol is dissolved in various solvents, the fluorine-19 chemical shift depends on contributions from (i) the bulk magnetic susceptibility of the solvent and its molecular anisotropy, (ii) the nonspecific molecular interactions, and (iii) the hydrogen bonding of the OH group with the solvent, in the case of HBA solvents.37 The two first contributions can be subtracted by comparison with 4fluoroanisole, which resembles 4-fluorophenol in all respects except in its capability to donate hydrogen bonds. Accordingly, in a plot of the corresponding 19F chemical shifts of 4-fluorophenol versus 4-fluoroanisole, non- or very weak HBA solvents (1,2,4trichlorobenzene, carbon disulfide, tetrachloromethane, trichloromethane, dichloromethane, cyclohexane, heptane, and 1,2-dichlorobenzene) draw a reference line of eq 8 [−δ(19F)(OH)] = 1.009[−δ(19F)(OMe)] − 1.257

Figure 2. Solvatomagnetic comparison method.19F NMR chemical shift δ of 4-fluorophenol plotted against the same property for 4-fluoroanisole in various solvents. Circles: non- or very weak HBA solvents fixing the reference line of eq 8. Triangles: HBA solvents. Diamonds: amphiprotic solvents.

found for water and alcohols (vide infra for an extended comparison). Scheme 4 compares the solvent HB basicity of oxygen, sulfur, nitrogen, and carbon π bases studied by the solvatomagnetic comparison method. The ordering of β1 values mainly depends of three factors: (i) the intrinsic HB basicity of the molecule, as measured on the βH2 or pKBHX scale, (ii) the nature of the medium, and (iii) the self-association of the amphiprotic solvents caused by hydrogen bonding between their HBD and HBA functional groups. These factors are briefly described in the following text. The intrinsic HB basicity depends on the nature of the HBA atom (Et2S < Bu2O), the functionality of this atom [EtCOMe < Bu2O < MeSOMe < (Me2N)3PO], and the substituent effects (polarizability P, field-inductive I, and resonance R effects) on the functional group:

(8)

with n = 8, r = 0.9992, and s = 0.025 ppm and where δ( F)(OH) and δ(19F)(OMe) stand for the fluorine chemical shifts of 4fluorophenol and 4-fluoroanisole, respectively. The very large determination coefficient stems from the high similarity of magnetic and nonspecific effects on the two probes. In Figure 2, data points representing HBA solvents are displaced above the reference line of eq 8. These deviations are attributed to the formation of a hydrogen bond between the OH group and the HBA solvent. They correspond to the contribution of this hydrogen bond to the total chemical shift, which can therefore be calculated by eq 9: 2

19

CCl3CN(− I) < ClCH 2CN( − I) < MeCN < PrCN( + P) < tert ‐BuCN(+ P) < Me2NCN(+ R) 3‐bromopyridine( − I) < pyridine < 4‐methylpyridine(+ I + R)

Quantitative structure−HB basicity relationships have already been established10,39 and do not need to be discussed further. The nature of the medium is changing from HBA solvent to HBA solvent for bulk basicity, whereas it is constant for solute basicity (CCl4 for the pKBHX scale10). The influence of the medium can be explained by the Onsager reaction field model.40 The magnitude of this reaction field interacting with the hydrogen-bonded complex between 4-fluorophenol and the HBA molecule depends on the Onsager function of the relative permittivity εr, (εr − 1)/(2εr + 1) or, accounting for dielectric saturation, on the Block and Walker function:41

Δ1[− δ(19F)(OH‐OMe)] = [− δ(19F)(OH)] − {1.009[− δ(19F)(OMe)] − 1.257}

(9)

Δ1[−δ( F)(OH-OMe)] values can therefore be used to define a scale of solvent HB basicity. They range from 0.184 ppm for (trifluoromethyl)benzene to 3.395 ppm for piperidine. The scale can be fixed by setting Δ1[−δ(19F)(OH-OMe)] = 0 for solvents obeying reference eq 8 and, denoting the new scale as β1, β1 = 1 for HMPT that deviates from the reference line by 3.041 ppm. The β1 values calculated by eq 10 are given in Table 3 for 72 HBA and amphiprotic solvents. 19

β1 = Δ1[−δ(19F)(OH‐OMe)]/3.041

f (εr) =

(10)

In this table, the KT β values are given in parentheses for comparison. As expected, the greatest differences are generally 38

3εr ln εr 6 − −2 εr + ln εr − εr + 1 εr

The inversion of the basicity order of DMSO and tertiary amines on going from the βH2 solute scale (DMSO, 0.79; amines, 0.57 to E



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Table 3. Fluorine NMR Chemical Shifts δ (ppm) of 4-Fluorophenol and 4-Fluoroanisole with Respect to Neat CFCl3, Hydrogen-Bonding Contribution Δ1[−δ(19F)(OH-OMe)] (ppm), and β1 Scale of Solvent HB Basicitya solvents

−δ(19F)(OH)

−δ(19F)(OMe)

Non- or Very Weak HBA and HBD Solvents Fixing the Reference Line of Equation 8 carbon disulfide 121.51 121.65 1,2,4-trichlorobenzene 123.63 123.73 tetrachloromethane 123.81 123.90 1,2-dichlorobenzene 124.44 124.51 trichloromethane 124.70 124.85 cyclohexane 125.17 125.24 heptane 125.35 125.41 dichloromethane 125.67 125.80 HBA and Amphiprotic Solvents in Order of Increasing Basicity (trifluoromethyl)benzene 125.91 125.85 perfluoro-tert-butanol 124.95 124.72 3-methylphenol 123.93 123.77 1,1,1,3,3,3-hexafluoro-2-phenylpropan-2-ol 123.99 123.65 benzene 125.81 125.50 toluene 125.77 125.45 1,1,1,3,3,3-hexafluoro-2-propanol 125.18 124.74 p-xylene 125.77 125.39 1,3,5-trimethylbenzene 125.62 125.16 formic acid 126.64 126.14 2,2,2-trichloroethanol 123.36 122.84 2,2,2-trifluoroethanol 125.53 124.87 trichloroacetonitrile 125.16 124.58 1,2,3,4-tetramethylbenzene 125.59 124.98 chloroacetonitrile 126.14 125.51 2,2,3,3,3-pentafluoropropanol 125.61 124.89 1,1,1-trifluoro-2-propanol 125.40 124.58 phenylacetonitrile 125.98 125.12 diethyl sulfide 126.81 125.91 benzonitrile 126.33 125.42 acetonitrile 128.16 127.15 water 125.34 124.34 2-propyne-1-ol 126.17 125.16 acetic acid 127.49 126.45 2-chloroethanol 126.39 125.32 n-propanenitrile 128.27 127.17 n-butanenitrile 127.94 126.77 1,4-dioxane 127.46 126.24 propanoic acid 127.49 126.24 formamide 125.63 124.39 trimethylacetonitrile 127.55 126.29 ethane-1,2-diol 126.97 125.68 N,N-dimethylcyanamide 127.88 126.52 benzyl alcohol 126.10 124.70 methyl acetate 127.50 126.06 ethyl acetate 128.55 127.09 2-butanone 128.58 127.09 2-phenylethanol 126.25 124.78 methanol 129.31 127.76 allyl alcohol 127.75 126.21 cyclohexanone 127.85 126.30 diethyl chlorophosphate 128.01 126.27 3-bromopyridine 125.90 124.17 ethanol 128.75 126.97 N-methylformamide 127.27 125.49 di-n-butyl ether 127.61 125.80 1-propanol 128.13 126.28 trimethyl phosphate 128.90 127.03 1-butanol 127.87 125.96 2-propanol 128.09 126.13 F

Δ1[−δ(19F)(OH-OMe)]

0.184 0.276 0.303 0.397 0.438 0.448 0.486 0.508 0.591 0.622 0.671 0.705 0.716 0.742 0.757 0.765 0.868 0.991 1.024 1.038 1.123 1.138 1.141 1.159 1.199 1.212 1.286 1.341 1.371 1.377 1.380 1.416 1.478 1.535 1.562 1.573 1.603 1.604 1.657 1.661 1.670 1.861 1.869 1.894 1.908 1.935 1.970 1.984 2.033 2.082

β1

0.06 0.09 0.10 0.13 0.14 0.15 0.16 0.17 0.19 0.20 0.22 0.23 0.24 0.24 0.25 0.25 0.29 0.33 0.34 0.34 0.37 0.37 0.38 0.38 0.39 0.40 0.42 0.44 0.45 0.45 0.45 0.47 0.49 0.50 0.51 0.52 0.53 0.53 0.54 0.55 0.55 0.61 0.61 0.62 0.63 0.64 0.65 0.65 0.67 0.68

(0.10) (0.11)

(0.41) (0.31) (0.18)

(0.31) (0.37) (0.37)

(0.52) (0.50) (0.42) (0.45) (0.48) (0.61) (0.62) (0.53) (0.51) (0.77) (0.46)

(0.88) (0.95)



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Table 3. continued solvents

−δ(19F)(OH)

HBA and Amphiprotic Solvents in Order of Increasing Basicity N,N-dimethylformamide 127.61 di-tert-butyl ether 127.20 pyridine 127.61 1-pentanol 127.71 2-methyl-2-butanol, tert-amyl alcohol 127.23 dimethyl sulfoxide 126.93 pyrrolidine-2-one 127.69 1-hexanol 127.54 2-methyl-2-propanol, tert-butanol 127.49 cyclohexanol 127.20 N-methylacetamide 127.43 1-octanol 127.40 N,N-dimethylacetamide 127.73 1-methylpyrrolidine-2-one 127.93 4-methylpyridine 127.81 1-decanol 127.15 triethylamine 128.41 tri-n-butylamine 127.63 N,N-dimethylcyclohexylamine 127.88 hexamethylphosphoric triamide 129.09 N-methylcyclohexylamine 128.49 piperidine 129.06 a

−δ(19F)(OMe)

Δ1[−δ(19F)(OH-OMe)]

125.65 125.24 125.63 125.70 125.20 124.89 125.63 125.47 125.41 125.10 125.32 125.27 125.57 125.75 125.61 124.97 125.72 124.94 125.04 126.17 125.37 125.79

2.086 2.090 2.106 2.136 2.160 2.173 2.186 2.198 2.208 2.231 2.239 2.260 2.287 2.305 2.327 2.312 2.816 2.823 2.972 3.041 3.249 3.395

β1 0.69 0.69 0.69 0.70 0.71 0.71 0.72 0.72 0.73 0.73 0.74 0.74 0.75 0.76 0.76 0.76 0.93 0.93 0.98 1.00 1.07 1.12

(0.69) (0.64)

(0.76)

(1.01)

(0.76) (0.77) (0.67) (0.71) (0.62) (0.71) (1.05)

KT β values38 are given in parentheses.

Scheme 4. Plot of Solvent HB Basicity Scale β1 for HBA and Amphiprotic (in Red) Solventsa

reduce its electron density. Indeed, as seen in Scheme 5, a weaker interaction energy is calculated for the hydrogen bond of 4Scheme 5. ωB97X-D/6-311+G(2d,p) Interaction Energies −ΔE (kJ·mol−1) of the Hydrogen-Bonded Complexes of 4Fluorophenol (as the HBD) and (1) the Monomer of Formic Acid, (2) the Cyclic Dimer of Formic Acid, (3) the Monomer of Methanol, and (4) the Linear Dimer of Methanol (as the HBAs)

a

The single-bonded oxygen (sulfur) bases, the double-bonded oxygen bases, and the nitrogen (carbon) bases are grouped on different vertical axes. For the sake of clarity, a number of solvents in Table 3 are omitted.

fluorophenol with the cyclic dimer of formic acid than with the corresponding monomer. Consequently, the β1 value of acetic acid (0.38) is lower than that of methyl acetate (0.51). On the contrary, in the linear chains of self-associated liquid alcohols the σ-bond cooperativity44 increases the basicity of oxygen atoms. Indeed, a larger interaction energy is computed for the hydrogen bond between 4-fluorophenol and the linear dimer of methanol than for the monomer of methanol (Scheme 5). Accordingly, the β1 values of the alcohols are found to be larger than those of the corresponding ethers. For example, comparing compounds with the same carbon content, 1-octanol (β1 = 0.74) is significantly more basic than Bu2O (β1 = 0.64). Extending the comparison to nitrogen bases, most self-associated alkanols appear to be

0.70) to the β1 solvent scale (DMSO, 0.71; amines, 0.93 to 0.98) can be partially explained by the much lower reaction field strength of tertiary amines [f(εr) = 0.130 to 0.162] compared to that of DMSO [f(εr) = 0.464], since it is known that HB basicity is larger in solvents of lower reaction field strength.42,43 The self-association of amphiprotic solvents can either decrease or increase their β1 values compared to those of their monomers. In the cyclic dimers of carboxylic acids, a second lone pair on the carbonyl oxygen is still available to receive a hydrogen bond, but charge transfer from the first lone pair is likely to G



Article

stronger HBAs than pyridine (whereas the corresponding monomers are weaker).6,10 Nevertheless, polyfluoroalcohols remain weak HBAs by virtue of the field-inductive electronwithdrawing effect of fluorine atoms. However, it is satisfactory that the solvatomagnetic comparison method succeeds in providing nonzero positive β1 values for perfluoro-tert-butanol and HFIP (0.09 and 0.16, respectively). The IR spectrum of neat HFIP shows a broad band at 3428 cm−1 characteristic of the OH stretching of self-associated alcohols (the two rotamers of the free molecule absorb at 3633 and 3591 cm−1). The observation of a self-association with a significant red shift of the OH stretching (205 cm−1) constitutes direct proof of the existence of a nonzero significant HB basicity of the oxygen atom of HFIP. Therefore, the negative value found for HFIP by the solvatochromic comparison method indicates that this method is not valid for amphiprotic solvents. Similarly, the nonzero basicity of the oxygen atom of perfluoro-tert-butanol is demonstrated by the appearance, in the IR spectrum of the liquid, of the OH band of a dimer.45 Validity of Solvatomagnetic Scale β1. To test the validity of the solvatomagnetic comparison method for determining a solvent HB basicity scale, we have correlated the new parameter β1 to five physicochemical properties A, measured in a series of solvents by means of A = A 0 + b β1

Figure 3. Solvatomagnetic comparison method. Nitrogen-14 chemical shifts of pyrrole plotted against the same quantity for N-methylpyrrole in various solvents. Circles: non-HBA solvents fixing the reference line of eq 12. Triangles: HBA solvents. Diamonds: amphiprotic solvents. Data are from ref 46.

(11)

where A0 is the intercept and b is the regression coefficient. These five spectroscopic, thermodynamic, and kinetic properties depend only on the solvent’s HB basicity as described below. Property 1: 14N Chemical Shifts of Pyrrole. We have treated literature data46 dealing with the effect of 13 solvents on the 14N NMR chemical shifts of pyrrole and N-methylpyrrole by our solvatomagnetic comparison method. The similarity of the effects of non-HBA solvents (cyclohexane, tetrachloromethane, trichloromethane, and dichloromethane) on the nitrogen chemical shifts of pyrrole and N-methylpyrrole is expressed by a reference line of eq 12 (Figure 3)

Property 3: Hydrogen Bond Enthalpies of Pyrrole. Catalán et al. have determined hydrogen bond enthalpies in pure bases, using pyrrole as the reference HBD and N-methylpyrrole as a similar non-HBD probe.49 They took into account the methylation effect using benzene and toluene. An appropriate combination of the solvation enthalpies of these four compounds yields the enthalpies of hydrogen-bond formation between pyrrole and HBA solvents.49 Property 4: Rate Constants for Hydrogen Atom Abstraction from α-Tocopherol (α-TOH, Vitamin E) by tert-Butoxyl Radicals. The rate constants of reaction

[−δ(14 N)(NH)] = 0.9629[−δ(14 N)(NMe)] − 12.318

Me3CO• + α ‐TOH → Me3COH + α ‐TO•

(12)

have been measured in several solvents.50 They decline as the solvent becomes a stronger HBA. This kinetic solvent effect was attributed to hydrogen bond formation between the OH group of α-TOH and the HBA solvent.50 Property 5: Rate Constants for the Phenolic Hydrogen Atom Abstraction from Hydroxytetramethylchromanacetic Acid (HCAA, a Model of Vitamin E) by tert-Butoxyl Radicals. These rates have been measured in four basic solvents, including water.51 They also decline markedly when the HB basicity of solvents increases, by virtue of the formation of a hydrogen bond between the phenolic OH group of the solute and the HBA solvent. The results of the correlations with eq 11 are collected in Table 4. Properties 1−5 correlate well (0.912 ≤ r2 ≤ 0.996) with β1, including amphiprotic solvents. This suggests that the β1 parameter is not contaminated by the HB acidity of this class of solvents. Figure 4A shows that data points representing acetic acid and tert-butanol stand on the regression line of nonamphiprotic solvents when rate constants are correlated to the solvatomagnetic β1 scale, whereas they are displaced from this line when solvatochromic scales β1(OH) and β1(NH2) are used. Similarly, Figure 4B shows that the solvatomagnetic β1 value of water (0.37) correlates the kinetic solvent effect much better

with n = 4, r2 = 0.996, and s = 0.168 ppm and where δ(14N)(NH) and δ(14N)(NMe) stand for the nitrogen chemical shifts of pyrrole and N-methylpyrrole, respectively. For HBA solvents, the data points are displaced below the reference line of eq 12. These deviations are caused by the formation of a hydrogen bond between the NH group of pyrrole and the HBA solvent. The hydrogen-bond-induced chemical shift for an HBA solvent is calculated by Δ1[− δ(14 N)(NH‐NMe)] = [− δ(14 N)(NH)] − {0.9629[− δ(14 N)(NMe)] − 12.318}

(13)

Property 2: Hydrogen Bond Enthalpies of 4-Fluorophenol. Arnett et al. have determined the hydrogen bond enthalpies of 4fluorophenol with HBA solvents by a “pure base calorimetric method”.47,48 In this method, the heat produced when the 4fluorophenol is injected into the neat base involves two terms: that due to the formation of the hydrogen-bonded complex and that due to all other interactions. The latter contribution is subtracted by using 4-fluoroanisole. The hydrogen bond enthalpy is then determined by measuring heats of solution of 4-fluorophenol and 4-fluoroanisole in the pure base and in the reference solvent CCl4.47 H



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Table 4. Correlation of Spectroscopic, Thermodynamic, and Kinetic Properties According to Equation 11a no. 1 2 3 4 5

property A 14

HB contribution to the N chemical shift of pyrrole hydrogen-bond enthalpies of 4-fluorophenol hydrogen-bond enthalpies of pyrrole log k of Me3CO•+ vitamin E log k of Me3CO•+ vitamin E model

n

r2

9 (4) 19 (1) 19 (6) 8 (2) 4 (1)

0.912 0.970 0.917 0.955 0.996

a

n = number of HBA solvents (number of amphiprotic solvents in parentheses) and r2 = determination coefficient.

than do the solvatochromic β1(OH) and β1(NH2) values (0.45 and 0.16, respectively). Kinetic results indicate that bulk water has the same HB basicity as bulk acetonitrile. This is exactly what is found by the solvatomagnetic comparison method. The enthalpic scales ΔH°(4-fluorophenol) and ΔH°(pyrrole) determined by the pure base calorimetric method agree quite well with the β1 solvatomagnetic scale. Therefore, both the pure base calorimetric method and the solvatomagnetic comparison method can be used for a safe determination of the HB basicity of solvents. However, it is much harder to carry out measurements of heats of solution than measurements of 19F NMR chemical shifts. Therefore, the 19F solvatomagnetic comparison method is, in practice, the method of choice for extending the β1 scale to new solvents. Analysis of Solvatochromic Shifts by Means of the 19F Solvatomagnetic Shifts. The excellent correlations found between 14N hydrogen-bond shifts and hydrogen bond enthalpies as well as hydrogen-bonding kinetic effects and β1 lend confidence that the 19F solvatomagnetic shifts do indeed reflect the real HB basicity of both HBA and amphiprotic solvents. The comparison of the HB solvatochromic shifts with the 19F solvatomagnetic shifts should therefore help to analyze the causes of the problems encountered with the traditional solvatochromic comparison method. Figure 5 shows how Δṽ (OH-OMe) relates to Δ1[−δ(19F)(OH-OMe)]. It is seen that the correlation between these quantities is quite good for all nonamphiprotic HBA solvents (π bases, nitriles, ethers, double-bonded oxygen bases, and a sulfur base) except for pyridines and amines. The linear regression equation is

Figure 5. Plot of solvatochromic hydrogen-bond wavenumber shifts of 4-nitrophenol, Δṽ(OH-OMe), versus solvatomagnetic hydrogen-bond chemical shifts of 4-fluorophenol, Δ1[−δ(19F)(OH-OMe)], in various solvents. Green triangles: π bases, nitriles, ethers, double-bonded oxygen bases, and Et2S, with the corresponding green regression line of eq 14. Dark-blue triangles: pyridines. Light-blue triangles: amines. Red diamonds: amphiprotic solvents.

with n = 29, r2 = 0.981, and s = 0.065 k cm−1. Compared to this regression line, most amphiprotic hydroxylic solvents present a larger HB basicity (they deviate upward), trifluoroethanol shows the same basicity (it does not deviate), and HFIP shows a lower (and even negative) basicity (it deviates downward) on the solvatochromic scale than on the solvatomagnetic one. Bathochromic shifts produced by hydrogen bonds of type A are at the origin of these deviations. These shifts, which contaminate the bathochromic shifts produced by type B hydrogen bonds, can cancel exactly in the solvatochromic comparison method only if the photoinduced charge transfers from the OH and OMe para substituents to the nitro group (hence the variation of the type A hydrogen bond strength and consequently the bathochromic shift) are identical. These charge transfers depend on the electron-releasing power of the para substituent, which is expected to follow the orders OH··· (ROH)n > OMe, for alcohols deviating upwards OH ···(ROH)n ≈ OMe, for alcohols on or near the line of eq 14

Δv(OH ‐OMe) = 0.701Δ1[−δ(19F)(OH‐OMe)] − 0.028 ̃

OMe > OH··· (ROH)n , for alcohols deviating downward

(14)

Figure 4. Plot of log (k/M−1 s−1) for the abstraction of the phenolic hydrogen atom from (A) α-TOH and (B) HCAA by tert-butoxyl radicals (BO) in various solvents versus the β1 values for these solvents. Triangles: HBA solvents. Amphiprotic solvents appear three times marked by diamonds: once with β1 (red), once with β1(NH2) (green), and once with β1(OH) (yellow). I



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Scheme 6. TD-DFT Calculations at the ωB97X-D/6-311+G(2d,p) Level of Wavenumbers v ̃ and Type A Bathochromic Shifts (cm−1) of the S0 → S1 Electronic Transition of 4-Nitroanisole and 4-Nitrophenol upon Hydrogen Bonding with Amphiprotic Molecules MeOH and CF3OHa

a The overall bathochromic shift is 38 823 (free 4-nitrophenol) − 35 517 = 3306 cm−1, but the type A contribution is only 37 615 (type B hydrogenbonded 4-nitrophenol) − 35 517 = 2098 cm−1. bThe overall bathochromic shift is 38 823 (free 4-nitrophenol) − 34 863 = 3960 cm−1, but the type A contribution is only 38 258 (type B hydrogen-bonded 4-nitrophenol) − 34 863 = 3575 cm−1.

Table 5. Interaction Energy, ΔE (kJ·mol−1), and NBO Analysis of the n(N or O) → σ* Charge Transfer in the Hydrogen-Bonded Complexes of 4-Fluorophenol and 4Nitrophenol with Dimethyl Ether and Trimethylaminea

Indeed, the electron-donating effect of the hydrogen-bonded OH substituent depends on the degree of polarization of the OH bond by the type B hydrogen bond.52 This polarization decreases from the most basic alkanols to the less basic fluoroalcohols, which explains the different positions of alcohols relative to the line of eq 14. TD-DFT calculations reported in Schemes 3 and 6 confirm that bathochromic shifts caused by type A hydrogen bonding are larger for 4-nitrophenol (2098 cm−1) than for 4nitroanisole (2065 cm−1) hydrogen bonded with methanol, a basic alkanol, and that a reversal occurs with trifluoromethanol. Indeed, with this less-basic fluoroalkanol, type A bathochromic shifts become larger for 4-nitroanisole (3618 cm−1) than for 4nitrophenol (3575 cm−1). The position of pyridines and amines in Figure 5 shows that the families of sp2 and sp3 nitrogen bases should be distinguished from that of oxygen bases. Such family-dependent behavior can be explained by the greater ability, compared to oxygen bases, of sp2 and sp3 nitrogen atoms to transfer electron density to the σ*(OH) molecular orbital of phenols and to the greater ability of the σ*(OH) molecular orbital of 4-nitrophenol to accept this electron density, compared to 4-fluorophenol, in the hydrogenbonded complexes of phenols. This can be shown by the NBO analysis19 of Table 5 that compares the hydrogen-bonded complexes of 4-fluorophenol and 4-nitrophenol with trimethylamine (representative of sp3 nitrogen bases) and dimethyl ether (representative of oxygen bases). Indeed, the computed charge transfer and the corresponding second-order stabilization energy are larger for the complexes of 4-nitrophenol compared to those of 4-fluorophenol and for the complexes of trimethylamine compared to those of dimethyl ether. In the language of the electrostatic/covalent model of molecular interactions,6 this means that pyridines and amines are more-covalent bases than oxygen bases and that 4-nitrophenol is a more-covalent acid than 4-fluorophenol. The fact that two factors (electrostatic and covalent) are needed to describe the HB basicity of solutes6,53 precludes the construction of any quite general HB basicity scale

complex

−ΔE

QCT

E(2)n→σ*

4-F-C6H4-OH···OMe2 4-(O2N)-C6H4-OH···OMe2 4-F-C6H4-OH···NMe3 4-(O2N)-C6H4-OH···NMe3

36.9 44.0 53.6 62.8

0.030 0.037 0.059 0.070

31.71 and 36.69 27.78 and 56.19 100.21 124.64

QCT = transferred charge in a.u. and E(2)n → σ* = second-order stabilization energy in kJ·mol−1. Calculations at the ωB97X-D/6311+G(2d,p) level. a

since different scales involve different proportions of electrostatic to covalent factors. The choice of 4-fluorophenol as the reference HBD for the construction of the pKBHX scale has, however, allowed us to build a reasonably general HB solute scale10 since the electrostatic/covalent ratio of 4-fluorophenol has a value in the middle of the range of molecular HBDs.53 Therefore, 4fluorophenol also seems to be the reference HBD of choice for the construction of a reasonably general HB solvent scale. Another solvatochromic scale has been built from the HB solvatochromic shifts of 4-nitroaniline.6,31 These shifts, Δṽ(NH2NMe2), are plotted against corresponding Δ1[−δ(19F)(OHOMe)] results in Figure 6A. It is seen that these quantities are well correlated for nitriles, π bases, double-bonded oxygen bases, unhindered pyridines, and the sulfur base. The regression equation is 19 Δv(NH ̃ 2‐NMe 2) = 0.968Δ1[ − δ( F)(OH‐OMe)]

(15)

− 0.153 2

−1

with n = 29, r = 0.964, and s = 0.129 k cm . The same tendencies of amphiprotic solvents can be seen as in Figure 5. However, the behavior of ethers and amines is quite different from that in Figure 5. The data points for these solvents are J



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Figure 6. Plot of solvatochromic shifts versus solvatomagnetic hydrogen-bond chemical shifts of 4-fluorophenol, Δ1[−δ(19F)(OH-OMe)], in various solvents. (A) Solvatochromic hydrogen-bond wavenumber shifts of 4-nitroaniline, Δṽ(NH2-NMe2). Green triangles: nonamphiprotic bases with the corresponding green regression line of eq 15, except ethers (purple triangles) and amines (light-blue triangles). Red diamonds: amphiprotic solvents. (B) Solvatochromic hydrogen-bond wavenumber shifts of 5-nitroindoline, Δṽ(NH-NMe). Green triangles: all nonamphiprotic bases, with the corresponding green regression line of eq 18. Red diamonds: amphiprotic solvents.

Figure 7. Comparison of solute and solvent scales of HB basicity for various HBAs. Triangles: nonamphiprotic HBA solvents. Diamonds: alcohols. (A) Plot of the solute basicity-dependent property Δ2 versus the solvent basicity-dependent property Δ1. (B) Plot of the solute basicity scale β2(OH) versus the solvent basicity scale β1(OH).

displaced toward lower Δṽ(NH2-NMe2) values than called for by the correlation equation (eq 15). This behavior is the consequence of the inability of ethers and amines to complex the two NH bonds of 4-nitroaniline (vide supra) completely. The formation of 1:2 hydrogen-bonded complexes is all the more difficult since the solvent HBA site is more sterically hindered. Accordingly, the larger displacements toward lower Δṽ(NH2NMe2) are found for the more crowded ethers and amines, that is, tri-n-butylamine, triethylamine, and di-tert-butyl ether. Solvatochromic shifts, Δṽ(NH-NMe), can also be calculated by applying the solvatochromic comparison method to the data obtained by Catalán for the solvatochromism of 5-nitroindoline and 1-methyl-5-nitroindoline.54 The method yields a reference line of eq 16 v (NH) = 1.021v (NMe) + 0.888 ̃ ̃

solvents below this reference line in the solvatochromic comparison plot (not shown) using eq 17 Δv (NH ‐NMe) = [1.021v (NMe) + 0.888] − v (NH) ̃ ̃ ̃ (17)

These solvatochromic shifts have the advantage that, with a single NH bond, the 5-nitroindoline probe is free of the stoichiometric difficulties encountered with 4-nitroaniline. Indeed, as seen in Figure 6B, the linear relation of eq 18 Δv(NH ‐NMe) = 0.509Δ1[−δ(19F)(OH‐OMe)] − 0.152 ̃ (18)

(with n = 26, r2 = 0.954, and s = 0.102 k cm−1) is satisfactorily obeyed by all nonamphiprotic solvents (π bases, nitriles, doublebonded oxygen bases, and pyridine) as well as ethers and amines. However, the same tendencies for amphiprotic solvents can be seen as in Figures 5 and 6A. The negative Δṽ(NH-NMe) value of HFIP, and hence a negative basicity, is particularly unacceptable. In summary, the application of the solvatochromic comparison method to three pairs of nitroaromatic indicators is unable to yield reliable solvent scales of HB basicity for amphiprotic solvents because of the contaminating bathochromic shifts induced by hydrogen bonding to the nitro group of these indicators. Moreover, 4-nitrophenol yields high β1(OH) values

(16)

with n = 32 (non-HBA and non-HBD solvents), r2 = 0.998, and s = 0.050 k cm−1 and where ṽ(NH) and ṽ(NMe), respectively, stand for the wavenumber of the solvatochromic band maxima of 5-nitroindoline and 1-methyl-5-nitroindoline in the series of solvents. Hydrogen-bonding solvatochromic shifts of 5-nitroindoline can then be calculated from the deviations of HBA K



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Table 6. Secondary β1 Values14 Calculated from Solvatochromic Shifts Δv(̃ OH-OMe) of 4-Nitrophenol and Δv(̃ NH-NMe) of 5Nitroindoline (Italics)a β1

solvent Alkyne and Cycloalkenes phenylacetylene dicyclopentadiene 1,4-cyclohexadiene 1,5-cyclooctadiene trans,trans,cis-1,5,9-cyclododecatriene Haloalkanes and Haloalkyne 1-bromopropane bromoethane 1,4-dichlorobutane 3-chloro-1-propyne, propargyl chloride iodoethane 1-bromobutane 1,10-dichlorodecane 1-chlorobutane 1-chloropropane Arene diphenylmethane Pyridines, Other Heteroarenes, and Guanidine pentafluoropyridine N-methylpyrrole quinoline 2-methylpyridine N-methylimidazole 2,6-dimethylpyridine tetramethylguanidine Ethers and Thioethers thiophene furan methoxybenzene, anisole bis-2-chloroethyl ether dimethyl sulfide tetrahydrothiophene pentamethylene sulfide ethoxybenzene, phenetole di-iso-propyl sulfide di-n-butyl sulfide benzyl ether tetrahydropyran tetrahydrofuran diethyl ether di-iso-propyl ether 1,3,3-trimethyl-2-oxabicyclo[2.2.2]octane 2,2,5,5-tetramethyltetrahydrofuran Ketones 1,1,1-trichloroacetone acetone 2,2,4,4-tetramethyl-3-pentanone cyclopentanone 3-pentanone Esters ethyl trichloroacetate methyl trifluoroacetate methyl trichloroacetate ethyl propynoate, ethyl propiolate a

0.07 0.10 0.16 0.24 0.25 0.11 0.12 0.12 0.12 0.12 0.13 0.14 0.15 (0) 0.17 0.10 0.18 0.27 0.57 (0.64) 0.69 0.70 (0.82) 0.78 (0.76) 1.08 0.07 0.10 0.23 (0.22) 0.32 0.34 0.37 0.38 0.39 (0.20) 0.42 0.43 0.46 (0.41) 0.56 (0.54) 0.58 (0.55) 0.58 (0.47) 0.65 0.68 0.69 0.28 0.49 (0.48) 0.52 (0.48) 0.53 (0.52) 0.55 (0.45) 0.30 (0.25) 0.31 0.33 0.36

solvent

β1

dimethyl carbonate propylene carbonate methyl formate 1,2,3-triacetoxypropane, triacetin ethyl chloroacetate diethyl carbonate γ-butyrolactone ethyl benzoate methyl propanoate (9Z)-9-octadecenoic acid 1,2,3-propanetriyl ester methyl hexanoate, methyl caproate methyl pentanoate, methyl valerate methyl butanoate methyl octanoate, methyl caprylate methyl cis,cis-9,12-octadecadienoate methyl decanoate, methyl caprate methyl cis-9-octadecenoate, methyl oleate Tertiary Amides 1-formylpiperidine N,N-diethylformamide N,N-dimethylpropionamide N,N-diethylacetamide Tetra-N-alkyl-substituted Ureas 1,1,3,3-tetramethylurea 1,1,3,3-tetraethylurea Nitriles n-pentanenitrile, valeronitrile n-hexanenitrile Nitroalkanes nitroethane 2-nitropropane 2-methyl-2-nitropropane (30 °C) nitromethane Amines N,N-dimethylaniline morpholine N-methylmorpholine ethylenediamine 1,4-dimethylpiperazine 1-methylpiperidine tri-n-propylamine 1-methylpyrrolidine n-butylamine cyclohexylamine n-butylmethylamine pyrrolidine di-n-butylamine Phosphates triethyl phosphate tri-n-butyl phosphate Sulfoxide, Sulfone, and Sulfites tetrahydrothiophene 1,1-dioxide (30 °C) dimethyl sulfite diethyl sulfite tetramethylene sulfoxide

0.40 (0.38) 0.40 (0.40) 0.40 (0.37) 0.41 0.42 (0.35) 0.45 (0.40) 0.45 (0.49) 0.47 (0.41) 0.47 0.50 0.50 0.50 0.50 0.51 0.56 0.61 0.62 0.71 0.75 (0.69) 0.76 0.80 (0.78) 0.75 (0.80) 0.77 (0.71) 0.45 0.46 0.22 0.27 0.32 0.23 0.34 0.67 0.68 0.91 0.92 0.93 0.95 (0.56) 1.01 1.03 (0.72) 1.05 1.06 1.08 1.10 (0.70) 0.71 (0.77) 0.75 0.34 0.34 0.45 (0.45) 0.74 (0.80)

KT β values in parentheses.

for pyridines and amines because of its highly covalent character. These values are not applicable to solutes which have much lower

covalent character. Lastly, 4-nitroaniline yields too low a β1(NH2) value for single-bonded oxygen and nitrogen bases L



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solvents, pyridines, and amines) to the solvatomagnetic chemical shifts of 4-fluorophenol, secondary β1 values14 can be calculated from Δṽ(OH-OMe) values using reversed eqs 14 and 10. The results are reported in Table 6. Secondary values14 for pyridines and amines not listed in Table 3 can be calculated similarly using reversed eqs 18 and 10 and the Δṽ(NH-NMe) values determined in this work from the Catalán data. The results are given in italics in Table 6. The addition of these 96 new values to those of Table 3 gives access to a set of 168 β1 values, representing one of the most comprehensive solvent HB basicity scales so far determined for organic liquids. The KT β values38 are given in parentheses in Tables 3 and 6. There are 66 solvents for which both KT β and our β1 values are known. The determination coefficient between the two sets of values is only 0.713. The differences β1− KT β span from +0.40 (+57%) for di-n-butylamine to −0.28 (−28%) for tert-butanol. It is found that the KT β values are heavily underestimated for all amines, hindered ethers, water, and polyfluoroalcohols and overestimated for most alkanols. These solvents are precisely those for which the solvatochromic comparison method was shown to fail. We therefore discourage the use of the KT β values in further correlation analysis of solvent effects on physicochemical properties and recommend the use of our β1 values which are accurate, clearly defined, and available for a large variety of solvents. We also recommend the use of the solvatomagnetic comparison method for the determination of the HB basicity of new solvents. In the framework of green chemistry, the design of ecofriendly solvents is a topical field of research, and it is essential to determine the HB basicity of such solvents properly.

because of its inability to form 1:2 complexes with these bases. The 4-fluorophenol probe is devoid of all of these shortcomings. Therefore, as for the construction of a solute HB basicity scale,10 it appears to be the best reference HBD for the construction of a solvent HB basicity scale. Comparison of Solute and Solvent Scales of HB Basicity. There is a common belief that solute and solvent scales of HB basicity are not very different for nonassociated solvents55 and that it is possible to mix these scales in the correlation analysis of basicity-dependent properties. However, whether or not the HB basicity of molecules acting as solvents is the same as when they are in dilute solutions, acting as solutes, could not be rigorously demonstrated.56 In the following text, we make a rigorous comparison of solute and solvent scales of HB basicity by comparing the same basicity-dependent property of molecules measured in two physical states: molecules acting as solvents or molecules highly diluted in an inert solvent. Gurka and Taft have measured the difference, Δ2[−δ(19F)(OH-OMe)], between the 19F chemical shifts of free and hydrogen-bonded 4-fluorophenol.37 The measurements were performed in dilute solutions of 4-fluorophenol and HBA molecules in CCl4. There are 19 HBA molecules for which we have values for both Δ1[−δ(19F)(OH-OMe)] (solvent basicity scale) and Δ2[−δ(19F)(OH-OMe)] (solute basicity scale). The comparison is shown in Figure 7A. The determination coefficient between the two quantities is only 0.712. That means that 28.8% of the variance of the solvent scale is not explained by the solute scale. Thus, significant differences exist between solute and solvent scales. Clearly, solute scales must not be used for the correlation of solvent effects on basicity-dependent properties. This conclusion is all the more justified since the difference Δ2 − Δ1 is not statistically distributed but depends on the dielectric properties of the bulk base. They increase with the reaction field of the bulk base on the hydrogen-bonded complex, as they are correlated with the Block and Walker function41 of the relative permittivity of the bulk base (r2 = +0.852 and +0.902 when dioxane is excluded). There is another basicity-dependent property which has been measured both for bases acting as solvents and for bases diluted in a solvent. It is the enhanced bathochromic shift of 4nitrophenol relative to 4-nitroanisole57 (vide supra). For bulk bases, this property yields the β1(OH) scale. For bases diluted in 1,1,1-trichloroethane, the scale has been called βsm57 (hereafter renamed β2(OH) for the sake of homogeneity of our notations). The comparison is made in Figure 7B. For 29 data points, the determination coefficient is only 0.756. The greatest differences between β1(OH) and β2(OH) are for the three alcohols: the bulk basicity of these self-associated solvents is 30 to 60% greater than the basicity of the monomeric bases. When these amphiprotic solvents are excluded, the determination coefficient reaches 0.844, but this remains unsatisfactory since 15.6% of the variance of the solvent scale of HB basicity remains unexplained by the solute scale of HB basicity for nonamphiprotic solvents. In summary, it is essential to make a distinction between solute and solvent scales of HB basicity not only for amphiprotic but also for nonamphiprotic solvents. Comprehensive Collection of Solvent HB Basicity Parameters for Use in LSERs. More β1 values than the 72 primary values of Table 3 are needed for a comprehensive collection of solvent HB basicity parameters. Since we have recently obtained a set of ca. 150 Δṽ(OH-OMe) values6 and since we have shown above that these solvatochromic shifts of 4nitrophenol are strongly correlated (excluding amphoteric

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CONCLUSIONS Until now, the solvatochromic comparison method was the method of choice for defining experimental scales of solvent HB basicity. This work shows that these scales are flawed by a number of shortcomings. The principal drawback comes from the contamination induced by hydrogen bonding of amphiprotic solvents to the nitro group of solvatochromic indicators 4nitrophenol, 4-nitroaniline, and 5-nitroindoline. Consequently, the basicity of this important class of solvents has not yet been properly parametrized. In this work, we have therefore devised a fluorine solvatomagnetic comparison method that is simple, sensitive, and applicable to all classes of solvents (including ionic liquids and supercritical fluids). The good correlations obtained between spectroscopic, thermodynamic, and kinetic properties, that depend only on the HB basicity, and our NMR-derived basicity scale β1 validate our solvatomagnetic comparison method. Another attractive feature of the solvatomagnetic comparison method is the ease with which it can be carried out compared to the measurement of heats of solution in the pure base calorimetric method. We have finally studied the supposed equivalence of HB basicity solvent and solute scales. We have found that these two scales are not equivalent, even in the case of nonamphiprotic solvents, and that solvent and solute scales must not be mixed in the study of solvent effects on basicity-dependent properties. Finally, this work offers a simple tool for the measurement of the HB basicity of new solvents and a comprehensive collection of reliable basicity parameters for the correlation analysis of solvent effects by means of LSERs.

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ASSOCIATED CONTENT

S Supporting Information *

Optimized structures of hydrogen-bonded complexes of 4nitroaniline, 4-nitrophenol, and 4-fluorophenol. Experimental M



Article

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NMR details. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS

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REFERENCES

We thank Dr. Patrick Giraudeau (Nantes) and Prof. Hassan Oulyadi (Rouen) for helpful discussions. J.L. is grateful to Labex SynOrg (ANR-11-LABX-0029) and the Région Haute-Normandie for financial support. D.J. acknowledges the European Research Council (ERC) and the Région des Pays de la Loire for financial support in the framework of starting grants (Marches278845) and a recrutement sur poste stratégique, respectively. A C. thanks the ERC (Marches-278845) for his postdoctoral grant. This research used resources of the GENCI-CINES/IDRIS, of the CCIPL, and of a local Troy cluster.

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Solvatomagnetic Comparison Method: A Proper Quantification of

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