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X-ray crystal structures (top: top view; bottom: side view) of (a) 5a, (b) 5b, and (c) 5c with thermal ellipsoids at 50% probability; all hydrogen atoms are omitted for clarity. Visualized local strains in (d) 5a, (e) 5b, and (f) 5c were calculated at the B3LYP/6-31G(d) level.
