tropylium ions

https://doi.org/10.1351/goldbook.T06523
The delocalized @C00812@, cycloheptatrienylium, C7H7+, derived formally by @D01625@ of one @H02904@ ion from the CH2 group of cyclohepta-1,3,5-triene and substitution derivatives thereof.
Source:
PAC, 1995, 67, 1307. (Glossary of class names of organic compounds and reactivity intermediates based on structure (IUPAC Recommendations 1995)) on page 1373 [Terms] [Paper]