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  • A New Perspective in the Lewis Acid Catalyzed Ring Opening of Epoxides. TheoreticalStudy of Some Complexes of Methanol, Acetic Acid, Dimethyl Ether, Diethyl Ether, andEthylene Oxide with Boron Trifluoride
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  • Several 1:1, 1:2, and 2:2 complexes between BF3 and CH3OH (Met), CH3COOH (AcA), (CH3)2O (DME),(CH3CH2)2O (DEE), and (CH2)2O (EOX) have been studied using ab initio (MP2) and density functionaltheory (DFT) (PBE, B3LYP) methods and the 6-311++G(3df,2pd) basis set. Geometrical structures andvibrational frequencies are reported, in most cases, for the first time. A detailed comparison of the vibrationalfrequencies for the O···BF3 vibrational modes, as well as for the ν(OH) band in the methanol and acetic acidcomplexes with BF3, is performed, and the theoretical frequency shifts are compared with the availableexperimental information. Thermochemical properties are calculated by employing counterpoise correctionto alleviate the basis set superposition error. The DFT enthalpy of complexation of the 1:1 complexes resultsin the order of stability (AcA)2> AcA:BF3> DEE:BF3> DME:BF3> Met:BF3> EOX:BF3> (Met)2; incontrast, MP2 shows the noticeable difference that the AcA:BF3 complex is much less stable (similar toMet:BF3). The order of stability shows that, even though acetic acid prefers dimerization to complexationwith BF3, the case is exactly the opposite for methanol. In both cases, the interaction of BF3 with the dimergives rise to very stable trimers. However, in contrast to the interaction of BF3 with the methanol dimerbeing stronger than that with the monomer, the interaction of BF3 with the acetic acid dimer is weaker thanthat with the monomer. The relative strength of the complexes, discussed in the context of BF3-catalyzedring opening of epoxides, suggests that the effect of the catalyst in a nonprotogenic solvent should be moreproperly ascribed to activation of the nucleophile instead of activation of the epoxide.
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