Thermal Decomposition Mechanisms of Ionic Liquids by Direct Dynamics Simulations and Vacuum Ultraviolet Photoionization Mass Spectrometry

Due to the unusually high heats of vaporization of room-temperature ionic liquids (RTILs), volatilization of RTILs through thermal decomposition and vaporization of the decomposition products can be significant. In complex molecules like RTILs, use of chemical intuition to predict reaction pathways can prove unreliable, especially when the internal energy content is high. There may be concerted reactions that are difficult to predict, and once energy stored in the molecule begins to release, the system does not necessarily follow the minimum energy reaction path, i.e., the subsequent behavior is controlled by dynamics. A useful approach to treating such a system is quasi-classical, direct dynamics trajectory simulations, where the motion of the molecule is followed, allowing the molecule to show us what the preferred reaction pathways are. The direct dynamics method dispenses with the potential energy surface. Instead, it calculates the energies, force constants, and Hessian on the fly using quantum chemistry methods. This method becomes computationally attractive when the dimensionality of the system increases, particularly for RTILs, which typically contain 10 or more heavy atoms. Dynamics simulations allows the partitioning of the energy generated by exothermic reactions into vibrational, rotational, and translational degrees of freedom, thereby increasing the chance of locating new reaction pathways in the system. In addition, by following the variation of the potential energy during the trajectory rather than relying on intuition, we can identify better geometries for TS searching. Results obtained from these direct dynamics simulations are compared to and consistent with the gaseous products detected experimentally via tunable vacuum ultraviolet photoionization mass spectrometry performed at the Chemical Dynamics Beamline 9.0.2 at the Advanced Light Source. The likely reaction mechanisms in the thermal decomposition of RTILs are discussed in this work.
The original document contains color images.