W.Quapp, E.Kraka, D.Cremer,

"Finding the transition state of quasi barrier-less reactions 
 by a growing string method for Newton trajectories:
 applications to the dissociation of
 Methylenecyclopropene and Cyclopropane"



A method for finding a transition state (TS) between a reactant minimum 
and a quasi-flat, high dissociation plateau on the 
potential energy surface is described. The method is based on
the search of a growing string (GS) along reaction pathways defined by 
different Newton trajectories (NT). Searches with the GS-NT method always
make it possible to identify the TS region because monotonically increasing
NTs cross at the TS or, if not monotonically increasing, possess turning
points that are located in the TS region.
GS-NT method is applied to quasi-barrierless and truly barrierless 
chemical reactions. Examples are the dissociation of methylenecyclopropene
to acetylene and vinylidene, for which a small barrier far out in the
exit channel is found,  
and the cycloaddition of singlet methylene and ethene,  
which is barrierless for a broad reaction channel with C$_s$-symmetry 
reminding of a mountain cirque formed by a glacier.