Molecular Dynamics Simulations and Product Vibrational Spectral Analysis for the Reactions of NO2 with 1-Ethyl-3-methylimidazolium Dicyanamide (EMIM+DCA-), 1-Butyl-3-methylimidazolium Dicyanamide (BMIM+DCA-), and 1-Allyl-3-methylimidazolium Dicyanamide (AMIM+DCA-).

Abstract

Direct dynamics trajectory simulations were carried out for the NO2 oxidation of 1-ethyl-3-methylimidazolium dicyanamide (EMIM+DCA-), which were aimed at probing the nature of the primary and secondary reactions in the system. Guided by trajectory results, reaction coordinates and potential energy diagrams were mapped out for NO2 with EMIM+DCA-, as well as with its analogues 1-butyl-3-methylimidazolium dicyanamide (BMIM+DCA-) and 1-allyl-3-methylimidazolium dicyanamide (AMIM+DCA-). Reactions of the dialkylimidazolium-dicyanamide (DCA) ionic liquids (ILs) are all initiated by proton transfer and/or alkyl abstraction between 1,3-dialkylimidazolium cations and DCA- anion, of which two exoergic pathways are particularly relevant to their oxidation activities. One pathway is the transfer of a Hβ-proton from the ethyl, butyl, or allyl group of the dialkylimidazolium cation to DCA- that results in the concomitant elimination of the corresponding alkyl as a neutral alkene, and the other pathway is the alkyl abstraction by DCA- via a second order nucleophilic substitution (SN2) mechanism. The intra-ion-pair reaction products, including [dialkylimidazolium+ - HC2+], alkylimidazole, alkene, alkyl-DCA, HDCA, and DCA-, react with NO2 and favor the formation of nitrite (-ONO) complexes over nitro (-NO2) complexes, albeit the two complex structures have similar formation energies. The exoergic intra-ion-pair reactions in the dialkylimidazolium-DCA ILs account for their significantly higher oxidation activities over the previously reported 1-methyl-4-amino-1,2,4-triazolium dicyanamide [Liu, J.; J. Phys. Chem. B 2019, 123, 2956-2970] and for the relatively higher reactivity of BMIM+DCA- vs AMIM+DCA- as BMIM+ has a higher reaction path degeneracy for intra-ion-pair Hβ-proton transfer and its Hβ-transfer is more energetically favorable. To validate and directly compare our computational results with spectral measurements in the ILs, infrared and Raman spectra of BMIM+DCA- and AMIM+DCA- and their products with NO2 were calculated using an ionic liquid solvation model. The simulated spectra reproduced all of the vibrational frequencies detected in the reactions of BMIM+DCA- and AMIM+DCA- IL droplets with NO2 (as reported by Brotton et al. [ J. Phys. Chem. A 2018, 122, 7351-7377] and Lucas et al. [ J. Phys. Chem. A 2019, 123, 400-416]).