Ab initio studies on amides: triformamide

Abstract

Complete optimization with the STO-3G basis set of the geometries of the planar ground-state conformers of the (unknown) triformamide molecule predicts that the E,E,E conformer will be more stable than the E,Z,Z conformer by 12.6 kJ mol^-1 (or 42.3 kJ mol^-1 by energy evaluations with the 4-31G basis set). These differences make direct observation of the E,Z,Z-conformer unlikely. Corresponding optimization of a model transition state for the conversion of the E,Z,Z into the E,E,E conformer suggests that the barrier to internal rotation is low with a calculated height of 33 kJ mol^-1 (24 kJ mol^-1 when evaluated with the 4-31G basis set). Successive formyl substitution in ammonia, represented by the series of molecules formamide, diformamide (formimide) and triformamide, leads to decreasing double-bond character in the N-C bonds as reflected in increasing N-C bond lengths, decreasing Mulliken π-overlap populations and decreasing barriers to rotation. Extension of the results for these three molecules suggests that the (known) triacetamide molecule has a non-planar E,E,E-type structure as is consistent with the observation of a single signal in its ¹H n.m.r. spectrum.