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.
The thermodynamic hydrogen-bond basicity scale pK HB (logarithm of the formation constant of 4-fluorophenol-aldehyde or ketone complexes in CCl 4 at 298 K) has been determined for aldehydes, aliphatic ketones, cycloalkanones, diketones and quinones, halogenated ketones, pyrones and related compounds, acetophenones, benzophenones and various other conjugated ketones. The relationship between pK HB and a spectroscopic scale of basicity is obscured by the presence of two stereoisomeric complexes. In the R 1 COMe series the electronic and steric effects of the alkyl R 1 almost cancel out, whereas steric effects prevail in R 1 COR 2 . Among alkyl substituents the 1-adamantyl is the most electrondonating. In cycloalkanones the basicity sequence with ring size is 4 < 11 ~12 ~15 < 5 < 6 < 7 < 8. Quantitative structure-basicity relationships have been established in the aromatic 3-and 4-XC 6 H 4 COMe and the aliphatic XCOMe series. Intramolecular hydrogen bonding causes a basicity decrease in acetylacetone. Hydrogen bonding sites are discussed.
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