. 65, 996 (1987). The ionization constants of the conjugate acids BH:' of two bases widely used in synthesis, 1,8-bis(dimethy1amino)naphthalene (PS), and 1,4-diazabicyclo[2.2.2]octane (DABCO), have been obtained in Me2S0 and water. The partition coefficients of both bases B and their heats of solution and protonation have also been determined. The basicity of PS is dramatically reduced when passing from water to Me2S0 while the basicity of DABCO remains unchanged, thus making DABCO the stronger base in Me2S0. The solvent effect on the base strength is analyzed in terms of free energy and enthalpy of transfer for B and H', and for BH', from water to Me2S0. The entropy-controlled much stronger solvation of PS in Me2S0 is in line with that of aromatic compounds while the increased solvation of DABCO in water is due to H-bonding of its two nitrogens. PSH', like H + , is better solvated by Me2S0 while DABCOH' does not show a definite preference for one solvent. The calculated enthalpy of solution of gaseous PSH' is abnormally low when compared to that of DABCOH+ and of related BH+ ions, thus indicating that the proton of PSH', in a strong intramolecular H-bond, does not normally H-bond to the solvent. Our results suggest that the proton sponge acid-base system could be used as a convenient reference to determine the free energy and enthalpy of transfer of H' between solvents.

. 66, 1159 (1988). The ionization constants of the conjugate acids BH+ of pyridine, 2-picoline, 2,6-lutidine, and 2,6-di-tert-butylpyridine (DTBP) have been determined in Me2S0. The partition coefficients of the bases B between Me2S0 and water, and the enthalpies of solution and protonation of B in Me2S0 have also been obtained. In contrast to its high basicity in the gas phase, DTBP is an abnormally weak base in Me2S0 (pK = 0.81). The factors responsible for this very low basicity are analyzed by considering correlations between the gas-phase, Me2S0, and aqueous basicities of B and by comparing the transfer parameters for B and BH+. The solvation of DTBP and DTBPH' in Me2S0 and in water differs. While the solvation of DTBP in Me2S0 is normal, the enthalpy of solution of DTBPH+ in Me2S0 is abnormally low and close to that of a cation whose solvation is nonspecific. This suggests a much reduced H-bonding between sterically hindered DTBPH+ and the large Me2S0 solvent molecule.ROBERT L. BENOIT, MONIQUE FRBCHETTE et DIANE LEFEBVRE. Can. J . Chem. 66, 1 159 (1988). On a dCterminC les constantes d'ionisation dans le Me2S0 des acides conjuguCs BH+ de la pyridine, de la picoline-2, de la lutidine-2,6 et de la di-tertio-butyl-2,6 pyridine (DTBP). On a aussi obtenu les coefficients de partage des bases B entre le Me2S0 Since it was first synthesized by Brown and Kanner (l), 2,6-di-tert-butylpyridine (DTBP) has attracted the interest of many researchers because of its unusually low basicity: with its two alkyl substituents, DTBP is nevertheless a weaker base than unsubstituted pyridine in aqueous solution. Brown and Kanner (1) and others (2) proposed that the abnormally low basicity of DTBP was caused by steric hindrance to hydration of DTBPH+ .Recent determinations of gas-phase proton affinities of DTBP and other alkyl-substituted pyridines showed that the basicity of DTBP in the gas phase was normal (2, 3), which confirmed that its weak basicity in water was due to solvent effects on DTBP and (or) DTBPH+. A complete analysis of the thermodynamic cycles linking the protonation processes of DTBP and other pyridines in the gas phase and in aqueous solution led Arnett and Chawla (2) to conclude that there was indeed some hindrance to the hydration of DTBPH+ as reflected in its abnormally low enthalpy of hydration. However, more recently Hopkins et al. (3), after investigating the protonation of additional tertbutylpyridines and repeating the thermodynamic determinations of Amett and Chawla (2) of DTBP, concluded from their new datathat the hydration enthalpy of DTBPH+ was normal but that the corresponding entropy was abnormal; they suggested that the rotation of the water molecule attached to DTBPH+ and of -CMe3 was restricted. These results and conclusions were in agreement with the gas phase studies of Moet-Ner and Sieck (4) on the attachment of one water molecule to a series of pyridinium cations including DTBPH+ .Since, as we have recently shown (3, contrasting solution and protonation thermodynamic data for bases in wat...