Maryvonne Luçon

An Enthalpic Scale of Hydrogen-Bond Basicity. 4. Carbon π Bases, Oxygen Bases, and Miscellaneous Second-Row, Third-Row, and Fourth-Row Bases and a Survey of the 4-Fluorophenol Affinity Scale

The thermodynamics of the O-H...B hydrogen bond (HB) has been determined in CCl(4) by FTIR spectrometry for a wide variety of carbon pi bases, oxygen bases, and miscellaneous first- to fourth-row bases, using 4-fluorophenol as a reference hydrogen-bond donor (HBD). After inclusion of previously studied nitrogen, sulfur, and halogen bases, this 4-fluorophenol affinity scale contains 314 varied organic bases and ranges over 40 kJ mol(-1). The 4-fluorophenol affinity scale in CCl(4) is shown to be applicable to most HBDs in most media, provided a small family dependence is taken into account. The HB affinity orders are quantitatively established according to the atomic acceptor site or to its bearing functional group. A comprehensive survey of the influence of substituents on these affinity orders is then achieved, considering electronic and steric effects, as well as effects of vinylogy or iminology. Iminology is found to be more efficient than vinylogy for transmitting resonance effects. Steric effects are shown to be less important in HB affinity than in HB basicity since they mainly act on the HB entropy. The spatial proximity of two acceptor sites can favor complexation through three-center hydrogen bonds, leading to superhydrogen-bond bases on the affinity scale.

Partition coefficients and intramolecular hydrogen bonding. 2. The influence of partition solvents on the intramolecular hydrogen-bond stability of salicyclic acid derivatives

The intramolecular hydrogen bonding (chelation) of salicylaldehyde, methyl salicylate, N,N-dimethylsalicylamide and 2-hydroxyacetophenone was studied by IR spectroscopy in different phases used for partition coefficient determinations. The extent of chelation was found to be highly sensitive to the solvent and to the substituent on the carbonyl group in the orders carbon tetrachloride = chloroform ≫ octanol > water ≫ dimethyl sulfoxide and OMe ≈ Me > H ≫ NMe2. These sequences are discussed in terms of hydrogen-bond acidity of the hydroxyl group, hydrogen-bond basicity of the carbonyl group, planarity of the solute molecule and hydrogen-bond acidity/basicity properties of the solvent. Semi-empirical and ab initio calculations confirmed the substituent sequence.

Hydrogen-bond basicity of nitro-compounds

The hydrogen bond basicity scale pKHB(logarithm of the formation constant of 4-fluorophenol–base complexes in CCl4) has been measured for nitro-compounds. This thermodynamic basicity scale is correlated to a spectroscopic basicity scale. The Yukawa–Tsuno equation is obeyed in the nitroaromatics. We have shown that nitroenamines are the most basic nitro-compounds presently known and have hydrogen bond basicity comparable to tributylamine.

Resonance de Fermi et effets de substituant—II. Acétates de phényle para-substitués

Abstract The vibration frequencies ν(CO) of twelve para substituted phenyl acetates p- X C 6 H 4 OCOMe have been studied by i.r. absorption. In some cases the observed spectra are composed of two or three bands. The solvent variation method shows that this multiplicity arises from Fermi resonance. The unperturbed frequencies are calculated by means of the method described above (part I). According to the Taft procedure, it is found that substituent effects are of the σ° type in phenyl acetates, and not of the σ − type as for corresponding phenols.

Structure and molecular interactions of anti-thyroid drugs. Part 1. Dipole moments of carbimazole and methimazole, and conformation of carbimazole

The dipole moments of carbimazole and methimazole are respectively 4.30 D (in benzene) and 5.43 D (in dioxane). The 4.30 D value and the solvent variation of the two carbonyl bands of carbimazole in the IR spectrum are consistent with a Z⇌E conformational equilibrium. Polar solvents stabilize the more polar Z conformer and, whereas carbimazole exists about 90% in the E form in heptane and CCl4, it is almost entirely in the Z form in water. In CCl4 the energy difference between the Z and E isomers, determined from the ratio of the intensities of the two carbonyl bands, is found to be 9.6 kJ mol–1, and is in good agreement with 6-31G** calculations.

Polarity and basicity of solvents. Part 3. A new infrared spectroscopic indicator of hydrogen-bonding basicity

Shifts of the carbonyl stretching vibration of trichloroacetic acid are measured for a series of hydrogen-bonding solvents. A correction is made for non-hydrogen-bonding solvent effects by means of an ‘i.r. comparison method’ identical to the solvatochromic comparison method. The resulting carbonyl shift Δν(CO) is correlated with Δν(OH) of MeOH ⋯ B complexes. The correlation is family dependent (π bases, sulphides, and nitrogen and oxygen bases).

Substituent effects in infrared spectroscopy—VII. Meta and para substituted methanesulphonanilides☆

Abstract Substituent effects on the NH frequencies of the conformers of methanesulphonanilides, their cyclic dimers and their hydrogen bonded complexes with acetonitrile have been analysed by means of the Hammet equation. An electron-withdrawing substituent may either increase or decrease ν(NH) in the XC6H4NHY series according to the electronic nature of the Y group. This can be explained by the non-monotonic dependence of the NH stretching frequency on the ionic character of the NH bond.

Substituent effects in infrared spectroscopy. Part 6. meta- and para-Substituted N-ammoniobenzamidates

The i.r. spectra from 1 300 to 1 650 cm–1 of a number of meta- and para-substituted N-ammoniobenzamidates have been studied. The strong bands near 1 560 and 1 355 cm–1 in water are assigned to asymmetrical and symmetrical stretching vibrations of the CO group. The coupling of νa(CO) and ν8 ring modes is shown up by the solvent variation method. It causes the failure of the Hammett equation for νa(CO). The similarity of the mechanics of vibration of the benzamidate and benzoate groups is emphasized.

Polarity and acidity of solvents. Part 3. Polarity of non-aromatic polychloro-substituted solvents

The empirical solvent polarity parameters ET(30), (π*), and (G) have been measured for thirty-four non-aromatic polychloro-substituted solvents. The ET(30) values of these polychloro-substituted solvents, determined by means of the betaine dyes (1a) and (1b), are practically insensitive to the solvent polarizability, as previously found for the corresponding ET(30) values of hydrocarbon and aromatic solvents. The difference between the empirical solvent parameters ET(30), (π*), (G), and * for these three groups of solvents (i.e. alkanes, polychloroalkanes, and aromatics) stems above all from their different susceptibility to solvent polarizability. For those polychloroalkanes capable of solute ⋯ H–C hydrogen bonding, the empirical solvent parameters depend also on their different sensitivity to the C–H acidity of the solvent.

The influence of solvent on the inductive order of substituents from infrared measurements on 4-substituted camphors: a new model of inductive effects

4-Substituted camphors (1)–(24) have been chosen as purely inductive models of substituent effects in order to study the effect of solvent on the inductive order of substituents. The probe was the carbonyl stretching vibration, the frequency of which was measured in (i) the gas phase, (ii) non-polar solvents (heptane and carbon tetrachloride), (iii) a hydrogen-bonding acceptor solvent (pyridine), (iv) a hydrogen-bonding donor solvent (hexafluoropropan-2-ol), and (v) an amphoteric solvent (methanol). Correlation analysis shows the consistency of data for gas-phase and apolar solvents, the deviation of hydrogen-bonding donor substituents (OH, NHCO2Et, NH2, CO2H, CONH2) in pyridine, and the deviation of hydrogen-bonding acceptor substituents (all except H, alkyls, C6H5, CHCH2, halogens) in hexafluoropropan-2-ol. In methanol the two kinds of deviation are so attenuated that most substituents behave inductively, in the same way as in apolar solvents. This explains the apparent, but fallacious independence of the inductive scale of substituents (defined mainly in alcoholic or aqueous media) on the solvent.