STRUCTURE AND DYNAMICS OF DICYANDIAMIDE - A THEORETICAL-STUDY

Ab initio MO methods have been used to study the structures and energetics of dicyandiamide, [(NH2)2C=N-C=N], its isomers, protonated species, radical anions, transition structures for internal conformational change and transition structures for isomerization. Structures were optimized at the HF/STO-3G, HF/3-21G and HF/6-31G* levels; selected barrier heights for smaller analogues were also computed at the MP4SDTQ/6-31G* level. The most stable isomer of dicyandiamide has the cyano group on the imine nitrogen [1, (NH2)2C=NC=N]; the other isomer [2, HN=C(NH2)NH-C=N] lies 12.8 kcal mol-1 higher. Inversion at the imino nitrogen proceeds by a linear, in plane process with a barrier of 32.5 kcal mol-1. The amino rotation barriers are 19 kcal mol-1 (single NH2) and 40 kcal mol-1 (both NH2 in a conrotaory or a disrotatory fashion; if the NH2 groups are allowed to pyramidalize the disrotatory barrier drops to 20 kcal mol)-1. Protonation occurs preferentially on the imine nitrogen (PA = 219.7 kcal mol-1 for 1); the proton affinities PA of the amino nitrogens are 25-30 kcal mol-1 lower. Isomerization between 2 and 1 would go via a 1,3-sigmatropic hydrogen shift, but the barrier is high (48.3 kcal mol-1); protonation reduces the hydrogen shift barrier by ca 15 kcal mol-1. However, the most likely mechanism for isomerization involves protonation of the imine nitrogen in 2 followed by deprotonation of the cyano-substituted nitrogen to form 1, circumventing the energetically costly 1,3-sigmatropic hydrogen shift. When an electron is transferred to dicyandiamide, a sizeable fraction of the resonance stabilization of the guanidine moiety is lost.