THE STRUCTURE OF THE TRIMETHYLENEMETHANE DIANION AND THE QUESTION OF Y-AROMATICITY

Neither delocalization nor “Y‐aromaticity” explains the stability of the trimethylenemethane dianion 1, but rather the optimal separation of the lone pairs on the terminal C atoms. This was shown by quantum‐chemical calculations, according to which the planar form of 1 (1a) corresponds to a higher‐order saddle point, whereas the lower‐energy structure 1 b displays strongly pyramidalized CH2 groups and only two of the three lone pairs are situated on the same side of the molecular plane. (Figure Presented.) Copyright © 1991 by VCH Verlagsgesellschaft mbH, Germany