The first example of a cage critical point in a single ring:: A novel twisted α-helical ring topology

A type of cage critical point (CCP) that has remained so far only a 'mathematical possibility' is reported for the first time. A CCP enclosed by two ring surfaces is found in a single ring in each of three derivatives of 1,12-difluorobenzo[c]phenanthrene. The two fluorine nuclei are linked by a bond path which results in the closure of a seven-membered ring in the 'bay' region. The crowding of the fluorine atoms forces an alpha-helical (staircase) twisting of the molecule causing the ring surface to split into two giving rise to a CCP. The Poincare-Hopf relationship is satisfied in each case. (c) 2005 Elsevier B.V. All rights reserved.