The solubility of hydrogen gas in the dimethyl sulfoxide (DMSO) -water system has been measured as a function of temperature from 25 to 80 "C. A distinct minimum occurs in the solubility, as expressed on the molar scale, in the region 25-35 mol% DMSO at 25 ^C. As the temperature is raised towards 80 ^C the minimum becomes less pronounced. The maximum deviation from calculated "ideal" behavior is found at 32 i 5,35 ? 5, and 32 i 5 mol% DMSO for 25,50, and 80 "C, respectively.A maximum is observed for both the partial molal entropy and heat of solution of hydrogen in the same solvent composition region. These results are discussed in terms of a changing entropy and energy of hole formation in the solvent with composition, which in turn is related to DMSO-water interaction in solution.A temperature-dependent equilibrium between D M S 0 . 2 H 2 0 and DMS0.3H2O complexes in solution is proposed in an attempt to correlate the considerable data which have been reported in the literature for the DMSO-water system. The second complex is thought to be present in significant quantity only at temperatures well below 25 'C.On a mesure la solubilitC de I'hydrogene gazeux dans le systeme dimethyl sulfoxyde (DMSO) -eau en fonction de la temperature, de 25 a 80 'C. Un minimum distinct de solubilite se produit dans la region de 25 a 35 mol% de DMSO a 25 "C, tel qu'exprime sur l'echelle molaire. Lorsqu'on eleve la temperature aux environs de 80 "C, le minimum devient moins prononce. On trouve la dCviation maximale au comportement "ideal" calcule a des compositions de 32 i 5, 35 i 5 et 32 i-5 mol% de DMSO a 25, 50 et 80 "C, respectivement.On observe un maximum, a la fois pour I'entropie molaire partielle et pour la chaleur de dissolution de I'hydrogene dans la mCme region de composition de solvant. On discute de ces risultats en termes de changement d'entropie et d'energie de formation de trous dans le solvant avec la composition, ce qui est relie a I'interaction de I'eau et du DMSO dans la solution.On propose un equilibre dependant de la temperat~~re entre les complexes en solution D M S 0 . 2 H 2 0 et D M S 0 . 3 H 2 0 dans le but de faire la correlation entre les differents resultats qui ont etC rapportts en nombre considerable dans la litttrature pour le systeme DMSO-eau. On croit que le second complexe est present en quantite appreciable a des temperatures bien au-dessous de 25 "C.
. Can. J. Chen~. 56, 1470 (1978).Proton nmr and infrared studies of neat N,N1-dimethylformamidine (DMFA) have shown that the cis isomer 1 is present exclusively and that it undergoes tautomerisni. At 25 C, k,,,,, = 8.2 x 102s-', E, = 41.5 + 1.5 kJ mol-I, AH+ = 39.1 + 1.5 kJ mol-l, and AS' = -57.5 5.0 J mol-' K-I. The tautomerism is proposed to occur through a hydrogen-bonded cyclic dimer. The effects of solvent on the tautonieric rate in DMFA have also been investigated for CD,NH2 and CDCI3 solutions.Potass~uni N,Nf-dimethylformamidide (PDMFA) in CD3NH2 does not exhibit hindered rotation about the single C-N bond on the nmr tlme scale over the temperature range -56 to 25°C. Howevcr, protonated DMFA undergoes hindered rotation about this bond in highly In this series Part I (1) dealt with the thermal decomposition of both potassium methylamide in methylamine solutions (which yielded potassium N,N1-dimethylformamidide (PDMFA)) and the neat free base, N,N'-dimethylformamidine (DMFA). Part I1 (2) discussed the hydrolysis of the D M F A as a function of pH. In the course of these studies several unusual features associated with isomerism (geometricaI, rotational) and tautomerism (3) The solvents CD3NH2 (>98 at.% D ) and CDC13 (> 99.8 at.% D) (Merck, Sharp, and Dohme), the former dried over l i t h i~~m metal, were distilled under vacuum before use. CHC13 (Fisher Spectranalyzed) was used for the infrared measurements. and hir;de;ed rotation in the title compound and its Although there is a reaction between DMFA and C D~N H Z , cationic and anionic forms as functions of solvent eq. [[I (4a), that produces the i s o t o~i c a l l~ labeled mono-a n d di-CD3 DMFA species, it is sufficiently slow that measurements and temperature.of the tautomerisnl in DMFA can be made without appreExperimental ciable deuterium isotope effects interfering.Proton nmr spectra were recorded o n a Bruker HX90 nnlr All nmr san~ples were handled using dry box (He atmos-SPectronIeter operating at 90 MHz ill a field of 2.114 T.Variable temperature studies were performed using a Bruker variable temperature controller, B-ST 100/700 (+0.5"C For personal use only.
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