(23) The range of 13C chemical shifts for a series of cyciobutene derivatives has been found to be small, and this indicates that this parameter may be a good indicator of ring strain, being insensitive to other effects; see AJ Jones, PJ Garratt, and KPC Volihardt, Angew. Chern., lnt. Ed. Engl., 12, 241 (1973), and unpublished resuits.