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Isomers of C20: An energy profile II

Semi‐empirical calculations, at the PM3 level provided within the Winmopac v2.0 software package, are used to geometrically optimize and determine the absolute energies (heats of formation) of a variety of C20 isomers that are predicted to exist in and around the bowl and cage isomers. Using the optimized Cartesian coordinates for the bowl and the cage isomers, a saddle‐point calculation was performed. The output file generated, containing energy, distance, and geometry information, is then organized into a graphical format. The resulting graph, which plots the energy of the 20‐atom system as a function of the distance from the geometric midpoint, is a two‐dimensional energy profile. This profile illustrates an estimation of the contours on the potential energy surface, showing energy minima and maxima that are encountered as the bowl evolves into the cage structure, or vice‐versa. To expand the surface into three dimensions, geometry optimizations were performed on the sets of Cartesian coordinates that correspond to energy minima in the bowl‐cage profile. Based on these optimizations, eight additional isomers of C20 have been identified and are predicted to be energetically stable. These additional isomers were subsequently subjected to saddle‐point calculations in order to identify those isomers that lie adjacent to one another on the three‐dimensional surface. Two isomers that are adjacent to each other will exhibit an energy profile that progresses smoothly from the potential well of each isomer up to the saddle point separating them. Consequently, these adjacent pairs of isomers establish a step‐wise transformation between the bowl and the cage. This process, which extends out over the three‐dimensional surface, is predicted to require less energy than that of the direct, two‐dimensional transformation predicted in the bowl‐cage profile. © 2003 Wiley Periodicals, Inc. J Comput Chem 24: 1287–1290, 2003