Ab Initio Studies on Amides: Cyanamide, Dicyanamide and Tricyanamide

Abstract

Optimization of the geometry of cyanamide with the 3-21G basis set leads to a planar (C_2v) structure, whereas the STO-3G, 6-31G* and 6- 31G** basis sets lead to a pyramidal structure at the amido -nitrogen atom. The 6-31G* and 6-31G** geometries are reasonably close to the experimental geometry, and the calculated barriers to inversion (4.5 and 3.4 kJ mol^-1, respectively) are also close to experimental values (5.4-5.6 kJ mol^-1), but the STO-3G value (13.9 kJ mol^-1) is much too high. Dicyanamide is far from planar at the STO-3G level but planar at the more definitive 6-31G* level, whereas tricyanamide is planar even at the STO-3G level. The prediction is that, in contrast to cyanamide itself, dicyanamide and tricyanamide are planar molecules. Calculated increases in N-C bond-lengths and decreases in C≡N bond-lengths for dicyanamide and tricyanamide as compared with the planar form of cyanamide are used to predict experimental lengths for these bonds in di - and tri- cyanamide.