Influence of Internal Noncovalent Bonds on Rotational Dynamics

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.