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Posted: May 22, 2012

Monitoring of liquid-phase organic reactions by photoelectron spectroscopy

(Nanowerk News) X-ray photoelectron spectroscopy (XPS) under ultra high vacuum conditions enables the monitoring of organic reactions in solution when carried out in mixtures of reactive ionic liquids (ILs), as demonstrated by EAM scientists for a thermally activated alkylation of a nucleophilic amine by a chloroalkyl species.
Ionic liquids (ILs) are a new class of materials, which have attracted vast scientific and industrial interest over the last decade. They are special salt melts exhibiting unusual physico-chemical properties (e.g., melting points even below room temperature, extremely low vapour pressure, unusual solvation and miscibility properties, and a wide electrochemical window). Possible application areas range from green chemistry, catalysis, and electrochemistry to analytics and separation technologies.
X-ray photoelectron spectroscopy
Classic organic reactions in solution are accessible by X-ray photoelectron spectroscopy under ultra-high vacuum conditions when carried out in mixtures of reactive ionic liquids (ILs) as it is demonstrated for a thermally activated alkylation of a nucleo-philic amine linked to an IL cation by a chloroalkyl species attached to an IL anion.
The main motivation for the collaborative work of the research teams of Prof. Hans-Peter Steinrück (Chair of Physical Chemistry II) and of Prof. Peter Wasserscheid (Chair of Chemical Reaction Engineering), conducted within EAM Research Area D "Catalytic Materials", is related to "Supported Ionic Liquid Phase" catalysis. In this context, the fundamental issue of in-situ monitoring of reactions in the near-surface IL region plays a crucial role. Due to the high vapor pressure of conventional liquids, ultra high vacuum-based surface science experiments are extremely difficult or even impossible. In contrast, the very low volatility of ionic liquids (ILs) allows for addressing fundamental mechanistic questions by surface science methods that were originally developed for solids, such as angle-resolved X-ray photoelectron spectroscopy (ARXPS).
The EAM team has recently demonstrated that even classical organic reactions can be monitored in the near-surface region of the liquid phase under ultra high vacuum conditions when anchoring the otherwise volatile reactive groups to ionic liquid molecules. This novel concept is successfully applied to nucleophilic substitution reactions between alkylamine and alkyl chloride species, which are bound to cations and anions of ionic liquid molecules, respectively. For the first time, this type of reaction – belonging to the fundamental organic reaction mechanisms since the pioneering work of C. K. Ingold in the 1920s – have been thoroughly studied by ARXPS, which enables the monitoring of the fate of all elements present in the reaction in a quantitative and oxidation state-sensitive manner on the molecular level, in one experiment (see papers in Angewandte Chemie: "Monitoring of Liquid-Phase Organic Reactions by Photoelectron Spectroscopy" and ChemPhysChem: "Organic Reactions in Ionic Liquids Studied by in Situ XPS").
Source: Cluster of Excellence Engineering of Advanced Materials
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