Electronic Donation or Steric Contraction: A Spectroscopic and Structural Analysis of Medium-Sized Constrained Rings for Potential Long-Range Hyperconjugation.

Herein, we report the 1JCH analyses, natural bond orbital analyses, and X-ray crystal structures of a number of C, O, and N constrained tricyclic cycles. These experiments provide access into the nature of the apparent Perlin effect previously reported in constrained tricyclic cycles, as well as evidence suggesting both steric contraction and long-range hyperconjugation account for the observed 1JCH perturbations. We report a true Perlin effect of 10.9 Hz in an azocane and large steric effect resulting in Δ1JC-H = 10.9 Hz in a cyclooctane.