  Room-temperature ionic liquids (RTILs) exhibit large difference between
melting and boiling points. They are highly tunable thanks to numerous
accessible combinations of the cation and the anion. On top of that, cations
can be functionalized using methods of organic chemistry. This paper reports
gas-phase thermodynamics (enthalpy, entropy, Gibbs free energy) of the
halogenation reactions (fluorination, chlorination, bromination) involving
protonated pyrrolidine C4H10N+, protic N-ethylpyrrolidinium C4H9N(C2H5)+, and
aprotic N-ethyl-N-methylpyrrolidinium C4H8N(CH3)(C2H5)+ cations. Substitution
of all symmetrically non-equivalent hydrogen atoms was compared based of the
thermodynamic favorability. Fluorination of all sites is much more favorable
than chlorination, whereas chlorination is somewhat more favorable than
bromination. This is not trivial, since electronegative fluorine and chlorine
have to compete for the already insufficient number of electrons with other
atoms belonging to the pyrrolidinium-based cations. The difference between
different reaction sites within the cations is modest, although it often
exceeds kT at simulated temperatures. The correlation between thermodynamics
and electronic density distribution has been established, which allows new
simple prediction of the reaction pathways. The reported results inspire
further chemical modifications of the pyrrolidinium-based RTILs to achieve ever
finer tunability of physical chemical properties.
