While a good deal of information
has accumulated concerning the
manner in which an intramolecular noncovalent bond can affect the
relative energies of various conformers, less is known about how such
bonds might affect the dynamics of interconversion between them. A
series of molecules are constructed in which symmetrically equivalent
conformers containing a noncovalent bond can be interconverted by
a bond rotation, the energy barrier to which is computed by quantum
chemical methods. The rotation of a CF3 group attached
to a phenyl ring is speeded up if a Se··F chalcogen bond
can be formed with a SeH or SeF group placed in an ortho position,
a bond that is present in and stabilizes the rotational transition
state. The analogous SnF3 group can, on the other hand,
engage in a Sn··Se tetrel bond in its global minimum. The
energetic cost of breakage of this bond is not fully compensated by
the appearance of a Se··F chalcogen bond in the rotational
transition state. Other systems were designed by placing two phenyl
rings on opposite ends of an octahedrally disposed SeF4 group. A high barrier inhibits their rotation with bulky Br atoms
in ortho positions, but this barrier is lowered if Br is replaced
by groups that can engage in either chalcogen (SeH or SeF) or pnicogen
(AsH2) bonds with the F atoms in the rotational transition
state. The barrier reduction is closely related to the strength of
these noncovalent bonds.