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How strong is hydrogen bonding in ionic liquids? Combined X-ray crystallographic, infrared/Raman spectroscopic, and density functional theory study.
J Phys Chem B ; 117(30): 9094-105, 2013 Aug 01.
Artigo em Inglês | MEDLINE | ID: mdl-23822750
Hydrogen bonding in ionic liquids based on the 1-(2'-hydroxylethyl)-3-methylimidazolium cation ([C2OHmim](+)) and various anions ([A](-)) of differing H-bond acceptor strength, viz. hexafluorophosphate [PF6](-), tetrafluoroborate [BF4](-), bis(trifluoromethanesulfonimide) [Tf2N](-), trifluoromethylsulfonate [OTf](-), and trifluoroacetate [TFA](-), was studied by a range of spectroscopic and computational techniques and, in the case of [C2OHmim][PF6], by single crystal X-ray diffraction. The first quantitative estimates of the energy (E(HB)) and the enthalpy (-ΔH(HB)) of H-bonds in bulk ILs were obtained from a theoretical analysis of the solid-state electron-density map of crystalline [C2OHmim][PF6] and an analysis of the IR spectra in crystal and liquid samples. E(HB) for OH···[PF6](-) H-bonds amounts to ~3.4-3.8 kcal·mol(-1), whereas weaker H-bonds (2.8-3.1 kcal·mol(-1)) are formed between aromatic C2H group of imidazolium ring and the [PF6](-) anion. The enthalpy of the OH···[A](-) H-bonds follows the order [PF6] (2.4 kcal·mol(-1)) < [BF4] (3.3 kcal·mol(-1)) < [Tf2N] (3.4 kcal·mol(-1)) < [OTf] (4.7 kcal·mol(-1)l) < [TFA] (6.2 kcal·mol(-1)). The formation of aggregates of self-associated [C2OHmim](+) cations is present in liquid [C2OHmim][PF6], [C2OHmim][BF4], and [C2OHmim][Tf2N], with the energy of the OH···OH H-bonds amounting to ~6 kcal·mol(-1). Multiple secondary interactions in the bulk ILs influence their structure, vibrational spectra, and H-bond strength. In particular, these interactions can blue-shift the stretching frequencies of the CH groups of the imidazolium ring in spite of red-shifting CH···[A](-) H-bonds. They also weaken the H-bonding in the IL relative to the isolated ion pairs, with these anticooperative effects amounting to ca. 50% of the E(HB) value.





Texto completo: Disponível Coleções: Bases de dados internacionais Base de dados: MEDLINE Idioma: Inglês Revista: J Phys Chem B Assunto da revista: Química Ano de publicação: 2013 Tipo de documento: Artigo País de afiliação: Federação Russa