. 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.

Molar excess free energies of the systems dimethylsulfoxide-water and propylene carbonate -water have been calculated from static vapor pressure measurements at 25 "C. Enthalpies of mixing at low water concentrations have also been determined. Possible association interactions in these systems are discussed.

. Can. J. Chem. 63, 3053 (1985). The protonation equilibria of three hydroxypurines, hypoxanthine, guanine, and xanthine, and of related caffeine have been studied by ' H and ',c nmr and uv spectroscopies in aqueous sulfuric or perchloric acids. The results have been interpreted on the basis of the excess acidity method. The pKBH::+ values and the protonation sites are discussed and comparisons are made with results of recent theoretical calculations.ROBERT L. BENOIT et MONIQUE FRECHETTE. Can. J. Chem. 63, 3053 (1985). Les tquilibres de protonation de trois hydroxypurines, I'hypoxanthine, la guanine, la xanthine, et du composC voisin la cafkine ont CtC CtudiCs par les spectroscopies rmn du 'H et du "C et uv dans les acides sulfurique et perchlorique aqueux. On a interprCt6 les resultats sur la base de la methode dite "d'acidite en excks". Les valeurs des PKDH::+ et les sites de protonation sont examints et des comparaisons sont faites avec les resultats de calculs theoriques rtcents.In addition to their importance in biological systems, purines offer a challenging thermodynamic and structural problem specifically in the determination of the reactivity of their many basic sites. Establishing correlations between the reactivity of the purines basic sites with the proton and other electrophiles such as metal ions also presents an interesting challenge. We recently reported results of a study of the first protonation step of some purines and of the second and third protonation steps of purine and adenine in more acidic media (1). We have now extended our study to the protonation of 3 hydroxypurines,OH* NH2 guanine (Gua) OH* OH* xanthine (Xan) hypoxanthine (6-hydroxypurine), guanine (2-amino-6-hydroxypurine), and xanthine (2,6-dihydroxypurine), and of related caffeine (1,3,7-trimethylxanthine). For this work we abandoned the calorimetric method because a strong medium effect at high acidity interfered with the interpretation of the results, and we used, instead, uv spectroscopy. Adding uv spectroscopy to "C and 'H nmr spectroscopies makes it also possible to compare data at widely different concentrations (lo-, M vs. 0.2 MI. The known pKBH+ values for the first protonation step of hypoxanthine and guanine, which occur in dilute acid media, are respectively 1.9 (2) and 3.3 (2), while for xanthine the values quoted are 0.8 (3) and 1.2 (4) and for caffeine 0.5 (4). As for pKBHi+, the only value reported is -1.05 for guanine (5); ~u a~f ' has also been identified in both trifluoroacetic acid (TFA) and HS03F (6).We present here the results of our study of the protonation reactions of the three hydroxypurines and caffeine between 0.05 and 18 M H2S04. Our pK values as well as the protonation sites are discussed, and comparisons are made with results of recent theoretical calculations. ExperimentaIHypoxanthine (L. Light), guanine (L. Light), xanthine (L. Light and Aldrich), and caffeine (Baker) were used as received. Our uv spectra of HypH+ and GuaH+ were in agreement with those published (7). The spectra of Xan for both ...

A L F O~S OM u c c~, ROBERT DOMAIN, and ROBERT L. BENOIT. Can. J. Chem. 58,953 (1980). Thermodynamic data were obtained at 25'C for the solution and protonation of six bases B: ammonia, three methylamines. and monoethyl and n-propylamines in dimethylsulfoxide (Me,SO) and propylene carbonate (PC). Vapour pressure, potentiometric. and calorimetric determinations were carried out. The solvent effect on the basicity order and protonation of the amines is analysed by using gas phase data and comparing the solution thermodynamics of gaseous Band BH+. The heat of solution of B in both solvents is close to its heat of condensation. The solution data for BH' are discussed in terms of two recent hydration data interpretations. Chem. 58.953 (1980). On a obtenu des donnees thermodynamiques sur la dissolution et la protonation a 25°C de six bases B: I'ammoniac. les trois methylamines et les mono&thyl et n-propylamines dans le dim&thylsulfoxyde (Me2SO) et le carbonate de propylene (PC). On a effectue des determinations de pression de vapeur, potentiometrie et calorimetrie. L'effet de solvant sur l'ordre de basicite et la protonation des amines est analyse en utilisant des donnees en phase gazeuse et en comparant la thermodynamique de solution de B et BH+ gazeux. La chaleur de solution de B dans les deux solvants est voisine de sa chaleur de condensation. Les donnees sur la wlution de BH+ sont examinees en relation avec deux interpretations recentes de donnees sur l'hydratation.Trends in the reactivity of chemical species be-the other hand, Aue et al. ( 2 ) , in their recent longing to related series, when referred to con-analysis, accounted for the changes in AH:. (BH+) densed phases, may differ widely from the more (after suitable corrections) in terms of the simple regular trends observed in the gas phase. For in-classical Born electrostatic theory. stance, it is well known that the gas phase basicity Given the complex nature of the interactions of NH, and the methylamines B as deduced from between both solutes B and BH+ and solvent water, proton affinities varies according to NH, < MeNH, a thermodynamic study of the protonation of a < Me,NH < Me,N, while in aqueous solution the series of amines in selected non-aqueous solvents order based on enthalpy of protonation is NH, < should be of interest since it might provide a test of MeNH, > Me,NH > Me3N (1). The apparently the previously proposed models and should give anomalous order in water has been shown to result further insight into the solvent effect on an imporfrom two opposite trends, one for B where its solu-tant proton transfer reaction. We are presenting tion exothermicity increases in the order NH, < here the results of such a study in which the solMeNH, < Me2NH --Me3N, while for BH+ the vents dimethylsulfoxide (Me,SO) and propylene trend reverses with -AH:, (BH+) decreasing ac-carbonate (PC) were used. These solvents, which cording to NH,+ > MeNH,+ > Me,NH,+ > belong to the dipolar aprotic class, were selected Me3NH+.for several reasons. First, solute-solvent inte...

. Can. J. Chem. 59, 1501Chem. 59, (1981.The ionization constants of the conjugated acids BH+ of aniline, N-methyl-and N,N-dimethylaniline, and pyridine have been determined potentiometrically in MeZSO. The heats of neutralization of the amines B and their heats of solution have been obtained by calorimetry. The pK's and enthalpies of ionization of these BH+ ions and alkylammonium ions are linearly related. Comparisons with gas phase data show that the abnormal basicity order of the amines B in Me,SO is mainly due to the decreasing exothermicity of the heat of solution of the BH+ ions with the number of N-H protons available for H-bonding to Me,SO. The large increase in basicity difference between equally substituted aromatic and alkylamines, when passing from the gas phase to Me,SO, is linked to the larger heat of vaporization of the aromatic amines. While the acid-base properties of compounds (+NMe2), and pyridine (Py). Potentiometric and belonging to related series often follow a rational calorimetric determinations were carried out to sequence in the gas phase, the sequence can be provide the necessary data in Me2S0 and some complex when the compounds are present as sol-supplementary data in water and CCl,. utes. The modifications in the basicity of alExperimental kylamines B according to the nature and number of Materials a l k~l substituents is a case in point 9 2).Me,SO (BDH) was reagent grade. It was dried (water content example, in the gas phase the basicity order is less than 0.02%) and stored over 4A molecular sieves. Aniline (Anachemia), N,N-dimethylaniline (Fisher Scientific), and Me3N > > n-PrNH2> EtNH2 > MeNH2 > NH3 pyridine (BDH, 99.5% purity) were reagent grade and used while in the solvent dimethylsulfoxide ( M~~s o ) the without further purification. N-Methylaniline (Eastman Kodak practical grade) was distilled under reduced pressure and the order becomes middle fraction used. Trifluoromethanesulfonic acid (3M MeNH, -E~NH, > n-PrNH2 > NH, -Me2NH > Me,N Chemical) was used as received, as were tetraethylammonium iodide (Baker reagent grade) after drying 12 hat 50°C and tetrathus NH3 is more basic than Me3N by 2.0 pK units. ethylammonium perchlorate (Eastman reagent) after similar Then in water, the sequence changes to drying.Potentiometric titrations of Me,SO solutions of the amines ( 2 SO that NH3 is now a weaker base than Me3N with a x lo-) to 1 x 10-2M) with standardized solutionsof HCF,SO, in 0.6 pK difference. Comparison of amines protons-Me2S0 were carried out in a glove box under a stream of dry nitrogen. The solution in the cell was maintained at 25.0°C by data in and in the gas-phase provides a means of a jacket through which temperature-controlled water llnderstanding the effect and circulated. The electrodes used were a glass electrode some interesting Variations in the ~0lVati0n of both (Radiometer G202B) which was calibrated twice with standard B and BH+. We have recently reported such HCF,S03 to give a 62.4 f 0.3 mV slope (intercept: 85.76 f a comparison (2) and we present here th...

Vapour–liquid equilibrium data for dilute sulfur dioxide solutions in sixteen solvents were derived from total vapour pressure measurements at 25 °C. The SO2 enthalpies of solution at infinite dilution ΔH0 were determined at 25 °C by direct calorimetry. The solvents used belong to the aprotic class. The ΔH0 values (kcal mol−1) are for the non-polar solvents, isooctane (−3.5), cyclohexane (−3.5), n-heptane (−3.8), benzene (−5.8), and for the polar solvents, 1,2-dichloroethane (−5.5), nitromethane (−6.2), nitrobenzene (−6.3), acetonitrile (−6.7), ethyl acetate (−7.1), sulfolane (−7.3), propylene carbonate (−7.5), trimethyl phosphate (−8.9), tetrahydrofuran (−9.4), dimethylformamide (−10.9), pyridine (−11.6), dimethylsulfoxide (−13.0). Out of three correlation methods which we tested to account for our data in non-polar solvents, the Hildebrand solubility parameter treatment gives the best results. The SO2 enthalpies of solution, ΔH0, in the polar solvents are discussed in terms of solvent basicity. There is a good correlation between the ΔH0 values, which relate to the basicity of the bulk solvent, and the solvent 'donor number' which is a molecular basicity parameter.

. 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...

A. Motztrinl ( Q~r i . ) . Caticlda H3C 3'v'l ROBERT L. BENOIT et MONIQUE FRECHETTE. Can. J . Chcm. 62. 995 (1984).Faisant suite a une valeur proposde pour I'cnthalpie de seconde protonation dc I'adeninc, on a CtudiC par ealorimktrie et spectroscopie rmn la protonation des bascs B biologiqucmcnt importantes: addnine, purine, addnosinc, et incidemmcnt, ccllc dc I'imidazolc et de I'amino-3 pyridine. Lcs chalcurs dc dissolution dcs bascs solidcs ct Ics ddplaccmcnts chirniqucs dc leurs protons ct carboncs-13 ont CtC ddterminCs cn fonction dcs conccntrations dcs acidcs HClO, ct H,SO,. Nos resultats ddmontrent quc contrairement i i une utilisation antericure, un effet dc milicu important rcnd impossible I'crnploi dcs donnCes calorimCtriques pour calculer les fonctions thcrmodynamiqucs des reactions de protonation. D'un autrc cBtC, Ics donnCes de carbone-I3 ont CtC interprCtCes avec succks cn utilisant la mCthode dc Cox et Yatcs dite "cxccss acidity". Lcs valcurs obtcnues pour pKBbII?+ ct pKsH33+ sont rcspectivemcnt: adCninc -0.43, -4,23, purinc -1.66. < -6, adknosinc -1,4 avcc hydrolyse du glucoside. Les sitcs de protonation ont dtC confirmds comme &ant essentiellcmcnt sur Ic cyclc imidazolc pour la scconde protonation et sur le cycle pyrimidinc (N-3) pour la troisikme.ROBERT L. BENOIT and MONIQUE FRECHETTE. Can. J . Chcm. 62. 995 ( 1984).In view of a previously proposcd cnthalpy valuc for the second protonation of adeninc. a calorimctric and nmr spectroscopic study has been made of thc protonation of biologically important bases B: adcninc, purinc, adcnosinc and, incidcntally, that of imidazole and 3-amino pyridine. Heats of solution of the solid bascs. and their carbon-13 and proton chemical shifts were determined as a function of HClO, and HZS02 conccntration. Our results demonstratc that, contrary to previous usagc, a strong medium effect precludes the use of thc calorimetric rcsults to calculate the thermodynamic functions for the protonation reactions. On thc other hand, the carbon-13 nmr data arc successfully intcrprctcd on thc basis of thc Cox and Yates excess acidity method. The valucs obtaincd for pKB!l2?+ and pKB,,,l+ are, respectively: adcninc -0.43, -4.23, purine -1.66, < -6, adenosine -1.4 with hydrolytic cleavage of thc glycosidic bond. Thc protonation sitcs arc confirmcd as bcing essentially on the imidazole ring for the second protonation and on the pyrimidine ring (N-3) for thc third one.