Density Functional Theory Analysis of the Impact of Steric Interaction on the Function of Switchable Polarity Solvents

A density functional theory (DFT)
analysis has been performed to
explore the impact of steric interactions on the function of switchable
polarity solvents (SPS) and their implications on a quantitative structure–activity
relationship (QSAR) model previously proposed for SPS. An X-ray crystal
structure of the N,N-dimethylcyclohexylammonium
bicarbonate (Hdmcha) salt has been solved as an asymmetric unit containing
two cation/anion pairs, with a hydrogen bonding interaction observed
between the bicarbonate anions, as well as between the cation and
anion in each pair. DFT calculations provide an optimized structure
of Hdmcha that closely resembles experimental data and reproduces
the cation/anion interaction with the inclusion of a dielectric field.
Relaxed potential energy surface (PES) scans have been performed on
Hdmcha-based computational model compounds, differing in the size
of functional group bonded to the nitrogen center, to assess the steric
impact of the group on the relative energy and structural properties
of the compound. Results suggest that both the length and amount of
branching associated with the substituent impact the energetic limitations
on rotation of the group along the N–R bond and NC–R
bond, and disrupt the energy minimized position of the hydrogen bonded
bicarbonate group. The largest interaction resulted from functional
groups that featured five bonds between the ammonium proton and a
proton on a functional group with the freedom of rotation to form
a pseudo six membered ring which included both protons.