Étude théorique (AM1) de l'affinité protonique des polyéthers cycliques (éthers couronnes) et non cycliques (glymes)

Abstract

R e p le 20 mars 199 1 M'HAMED ESSEFFAR, MOHAMED EL MOUHTADI, JosB-LUIS M. ABBOUD, JOSB ELGUERO et DANIEL LIOTARD. Can. J. Chem. 69, 1970Chem. 69, (1991.Des calculs semi-empiriques au moyen de la mCthode AM I , ont Ct C utilisCs pour discuter de l'affinitd protonique des molCcules poly-oxygCn6es (dioxane, glymes et tthers couronne). Les valeurs, calculees, bien que systCmatiquement plus faibles que les expCrimentales, sont corrC1Ces 1inCairement avec elles (r = 0,993). Etant donnC les profondes modifications confonnationnelles que subissent ces molkcules quand on les protone, le resultat est trks satisfaisant; il permet d'estimer les affinites protoniques, AP, du 9-Cther-couronne-3 (206,5 kcal mol-') et du glyme-5 (231,l kcal mol-'). En plus, ce travail justifie le fait qu'i nombre tgal d'atomes d'oxygkne, les glymes sont plus basiques que les tthers couronne. Pour ces derniers l'affinit6 protonique dCpend des tensions angulaires dans la base et des liaisons hydrogknes intramolCculaires dans le cation.Mots clis : protonation, rCactivitC, glymes, Cthers couronnes, coordination.M'HAMED ESSEFFAR, MOHAMED EL MOUHTADI, JOSE-LUIS M. ABBOUD, JOSE ELGUERO, and DANIEL LIOTARD. Can. J. Chem. 69, 1970Chem. 69, (1991.Semi-empirical calculations at the AM1 level have been used to study the proton affinity (PA) of polyether compounds (dioxane, glymes, and crown ethers). The calculated values, although systematically lower than the experimental ones, are linearly correlated with eight experimental PA's ( r = 0.993). Taking into account the considerable conformational changes that take place upon protonation, the result is highly satisfactory and provides an estimate of proton affinity for 9-ether-crown-3 (206.5 kcal mol-I) and glyme-5 (231.1 kcal mol-I). Moreover, this work provides a rationale for the fact that, for the same number of oxygen atoms, glymes are more basic than crown ethers. For the crown ethers, the proton affinity depends on angular strain in the base and on intramolecular hydrogen bonds in the cation.

Quantum Mechanics and Molecular Mechanics Studies of the Low-Energy Conformations of 9-Crown-3

“…Molecular mechanics calculations have been used to predict the low-energy conformations of 9-crown-3, 6 and semiempirical AM1 calculations of its proton affinity have been reported. 7 The only reported ab initio calculations on 9-crown-3 involve its complexes with the Li + ion 8,9 and with acetonitrile. 10 Although ab initio proton affinities of 12-crown-4, 15-crown-5, and 18-crown-6 were reported recently, 11 9-crown-3 was not included in the study.…”

“…Due to the difficult synthesis of 9-crown-3, , only its infrared spectrum, its 13 C NMR spectrum, and its interaction with Ba 2+ ions have been reported. Molecular mechanics calculations have been used to predict the low-energy conformations of 9-crown-3, and semiempirical AM1 calculations of its proton affinity have been reported . The only reported ab initio calculations on 9-crown-3 involve its complexes with the Li + ion , and with acetonitrile…”