Stereoelectronic Properties of Tetrahedral Species Derived from Carbonyl Groups. Ab initio study of aminodihydroxymethane, CH(OH)₂NH₂, a model tetrahedral intermediate in amide hydrolysis

An ab initio theoretical study of all fifteen fully staggered conformations of aminodihydroxymethane, CH(OH)₂NH₂ has been performed. Optimization of the CO and CN bond lengths, population analyses and orbital localisation reveal the presence of marked conformation dependent stereoelectronic effects which influence bond lengths and overlap populations. These effects may be parametrized as a function of number and nature of antiperiplanar (app) oriented electronic lone pairs (1p) and polar bonds. In a YCX fragment an app orientation between a lone pair on Y and the CX bond increases the length and weakens the CX bond, shortens and strengthens the CY bond. Thus a CX (X  O, N) bond of CH(OH)₂NH₂ is longest and weakest when: (i) it is app to two vicinal lp's; (ii) the X 1p's are not app to a vicinal polar bond; (iii) the conformation of the molecule has as many axially oriented lp's as possible. Results (i) and (ii) agree with a simple hyperconjugation picture involving interaction between an electronic 1p and an app oriented antibonding bond orbital σ* (CX). Bond properties, relative energies and effects on reactivity of the tetrahedral species are discussed, as well as their relations to experimental results on the cleavage of tetrahedral intermediates and to enzyme catalysis. Copyright © 1978 Verlag GmbH & Co. KGaA, Weinheim