Charge Distribution of Phosphonium Ionic Liquids: Phosphonium versus Phosphate
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TRUCTURE OF MATTER AND QUANTUM CHEMISTRY
Charge Distribution of Phosphonium Ionic Liquids: Phosphonium versus Phosphate Yan Hea and Shuang Mena,* a
School of Material Science and Engineering, Shenyang Ligong University, Shenyang, 110159 China * e-mail: [email protected] Received October 2, 2019; revised February 15, 2020; accepted March 10, 2020
Abstract—X-ray photoelectron spectroscopy is used to study six trihexyltetradecyl phosphonium ionic liquids. The charge distribution in both the phosphonium cation and the phosphate anion is discussed in detail. We can conclude that the P 2p binding energy for the cationic phosphorus centre is lower than that of the anion-based phosphorus centre. The electronic environment of the anionic fluorine, nitrogen and sulfur is compared. The effect of the anion on the measured cationic P 2p binding energy is investigated employing commonly used anions. It is found that the cationic P 2p binding energy is inversely proportional to the basicity of the anion. Keywords: phosphonium ionic liquids, charge distribution, X-ray photoelectron spectroscopy DOI: 10.1134/S0036024420100131
INTRODUCTION Ionic liquids, which are composed of entirely ions, exhibit unique properties and therefore become excellent alternatives towards traditional organic solvents [1–3]. They have found applications in many research areas, such as catalysis [4], phase separation [5], and CO2 capture [6]. To date, the investigation of phosphonium-based ionic liquids has usually been neglected due to the low stability, compared to their nitrogen-based counterparts. However, it has been concluded that by replacement of central cationic nitrogen atom to phosphorus atom, it becomes possible to remarkably tune the physico-chemical properties of ionic liquids [7]. Over the past decade, X-ray photoelectron spectroscopy (XPS) has been successfully employed to study ionic liquid-based systems [5, 6, 9, 10]. In specific for the cation–anion interactions in ionic liquids, Binding energy has been used as a sensitive indicator reflecting the subtle change in electronic environment for either a cation- or an anion-based component [8– 11]. As a result, it is expected that XPS can provide key information on the electronic environment of both cations and anions when changing from nitrogenbased cations to phosphorus-based cations. In this study, six trihexyltetradecylphosphonium ([P6,6,6,14]+) ionic liquids are analyzed by XPS. A fitting model for the C 1s spectrum is developed. The charge distribution on the phosphonium cation and the phosphate anion is discussed. The impact of anion basicity on the electronic environment of the cationic phosphorus centre is probed by XPS data. The measured
P 2p3/2 binding energy shows the trend which is found inversely proportional to the anion basicity. The electronic environment of each anion-based component is also compared, including F 1s, N 1s, and S 2p. The results show that binding energy can be a sensitive probe of the electronic environment of either the cation or the anion of ionic li
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