Valence electronic structure of [EMIM][B(CN)₄]: ion-pair vs. bulk description

Area of Science:

• Physical Chemistry
• Computational Chemistry
• Materials Science

Background:

• Ionic liquids (ILs) are salts that are liquid at room temperature, offering unique solvent properties.
• Understanding the electronic structure of ILs is crucial for predicting their behavior and applications.
• The electronic structure of [EMIM][B(CN)4] ionic liquid requires detailed investigation.

Purpose of the Study:

• To experimentally determine the ultraviolet photoelectron spectrum of [EMIM][B(CN)4].
• To computationally investigate the electronic structure of [EMIM][B(CN)4] using various ab initio methods.
• To assess the accuracy of different theoretical approaches in modeling the electronic properties of ionic liquids.

Main Methods:

• Ultraviolet photoelectron spectroscopy (UPS) was used to record the experimental spectrum.
• Ab initio calculations, including bulk calculations on a proposed crystal structure, were performed.
• Various density functional theory (DFT) functionals (PBE-D3BJ, B97-D, BLYP-D3(BJ), LDA, optB88-vdW, CX-vdW) were employed and compared.

Main Results:

• The ion-pair approximation was insufficient for describing the liquid's electronic structure.
• Bulk ab initio calculations on a crystal structure successfully modeled the experimental electronic spectrum.
• Dispersion-corrected PBE (PBE-D3BJ) functional showed excellent agreement with the experimental UPS data.
• B97-D and BLYP-D3(BJ) functionals also provided good agreement, while LDA offered only qualitative results.

Conclusions:

• Accurate modeling of the [EMIM][B(CN)4] ionic liquid's electronic structure requires sophisticated computational methods beyond simple approximations.
• Dispersion-corrected DFT functionals, particularly PBE-D3BJ, are effective for reproducing experimental electronic spectra of ionic liquids.
• Specific computational requirements must be met for precise electronic structure characterization of ionic liquids.