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Structure and dynamics of dicyandiamide: A theoretical study
Author(s) -
Bach Robert D.,
McDouall Joseph J. W.,
Owensby Amy L.,
Schlegel H. Bernhard,
Holubka Joseph W.,
Ball James C.
Publication year - 1991
Publication title -
journal of physical organic chemistry
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.325
H-Index - 66
eISSN - 1099-1395
pISSN - 0894-3230
DOI - 10.1002/poc.610040302
Subject(s) - chemistry , protonation , isomerization , conrotatory and disrotatory , imine , ab initio , transition state , proton affinity , formamide , nitrogen inversion , computational chemistry , crystallography , stereochemistry , medicinal chemistry , nitrogen , catalysis , ring (chemistry) , organic chemistry , ion
Ab initio MO methods have been used to study the structures and energetics of dicyandiamide, [(NH 2 ) 2 CNCN], 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, (NH 2 ) 2 CNCN]; the other isomer [2, HNC(NH 2 )NHCN] 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 NH 2 ) and 40 kcal mol −1 (both NH 2 in a conrotaory or a disrotatory fashion; if the NH 2 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.