A theoretical study of α‐substituted isopropyl and cyclopropyl anions

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Abstract
Ab initio molecular orbital theory has been used to probe the effect of the substituent X on the structures, strain energies, stabilization energies, inversion barriers, and proton affinities of carbanions CH3CX CH and cis‐C3H4X−, where X = H, F, CN, and NC. All geometries have been optimized with a 3‐21G basis set, and the parent anions (X = H) were also optimized with the same basis set with a diffuse function added (i.e. the 3‐21 + G basis set). The anions, with the exception of the α‐cyanoisopropyl anion, are pyramidal. The out‐of‐plane angle, α, for the pyramidal anions decreases in the order F > H ≈ NC > CN, and the barriers to inversion follow the same order with the cyclopropyl anions consistently having higher barriers than the isopropyl anions. The substituents strongly stabilize the anions with the stabilization energy following the order CN > NC > F. The cyano group slightly reduces the strain energy of cyclopropane, but the isocyano and fluoro substituents are weakly and strongly destabilizing, respectively. The pyramidal cyclopropyl anions are less strained than the cyclopropanes except when the substituent is a cyano group where the strain energies are reversed but are very similar. The planar anions all have higher strain energies than the cyclopropanes.

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