Amidine N−C(N)−N Skeleton:  Its Structure in Isolated and Hydrogen-Bonded Guanidines fromab InitioCalculations

Abstract

Geometries of guanidine and eight of its N-imino derivatives (CH 3 , SiH 3 , OH, CN, F, Cl, CHO, and NO 2 ) were calculated using ab initio molecular orbital techniques. MP2, MP4, and CISD geometries indicate that the guanidine molecule is pyramidal at amino groups and π-conjugation through the amidine skeleton is modest. MP2 structures of the eight N-imino guanidines reveal that substituting the hydrogen atom of the imino nitrogen by a functional group leads to a concerted variation of the CN bond distances. Topological electron density analyses indicate that the structural changes calculated upon N-imino substitution are largely due to changes of conjugation onto the amidine π-skeleton. Intermolecular hydrogen bonding involving the imino and amino groups is also found to affect the amidine geometry. Special attention has been paid in the work of 2-cyanoguanidine. The centrosymmetric dimer and a more extended hydrogen-bonded complex have been proposed as simulation of the crystal environment and the respective ab initio geometries (HF/ 4-31G(d) and MP2/6-31G(d)) are discussed. The structural changes due to self-association have been rationalized supposing that the contribution of polar canonical forms increases upon hydrogen-bonding formation.

Unusual Oxidative Chemistry ofN ω-Hydroxyarginine and N-Hydroxyguanidine Catalyzed at an Engineered Cavity in a Heme Peroxidase

“…This leaves two of the R48A cavity waters (W394 and W514) in place, but displaces R48A cavity water W423, as well as two of the waters that are observed in WT CCP (W300 and W313). Unambiguous orientation of the NHG ligand was not possible due to the apparent symmetry of the electron density feature; the orientation chosen places the hydroxyl group at ϳ1.8 Å from the heme iron, serving as a coordinating ligand as is consistent with the high to low spin conversion upon binding, and also accounting for the angle between the ligand and heme planes (41,42 Catalytic Oxidation of N-Hydroxyguanidine and N -Hydroxyarginine-R48A catalyzed the peroxide-induced oxidation of NHA and NHG, but not of arginine or guanidine. Initial experiments compared the ability of these four potential substrates to react with the peroxide-induced ferryl heme state.…”

Solid‐State Structure of Free Base Guanidine Achieved at Last

“…If we compare the experimentally determined molecular geometry with the most recently calculated structure of guanidine in the gas phase, [14] both structures resemble each other quite well. The calculated structures also exhibit a nonplanar geometry with strongly pyramidal amino groups (NH 2 ) and anti-conformation.…”

“…The intense interest in guanidine is demonstrated by the fact that its molecular structure was predicted by many quantum-chemical studies over several decades; it is all too obvious that the molecular structure is the source of its unique properties. [8][9][10][11][12][13][14] Theoretical investigations reveal that the equilibrium structure of guanidine should be nonplanar with strongly pyramidal NH 2 groups. [12][13][14] Only recently, more information concerning the guanidine molecule was gathered when the crystal structures of the 1:1 and the 2:1 co-crystals of neutral guanidine and the complex molecule 2-amino-4,6-dimethyl-1,3,5-triazine (C 5 H 8 N 4 ) were determined by using single-crystal X-ray diffraction.…”

Diprotonation of Guanidine in Superacidic Solutions

“…The guanidinium(1+) cation was discussed controversially regarding its stability. On the one hand, the stability of the guanidinium(1+) ion may be a result of resonance stabilization by six π‐electrons and Y‐aromaticity and, on the other hand, the stability can be explained by the formation of numerous hydrogen bonds …”

“…[10] The guanidinium(1 +)c ation was discussed controversially regarding its stability.O nt he one hand, the stabilityo ft he guanidinium(1 +)i on may be ar esult of resonance stabilization by six p-electrons and Y-aromaticitya nd, on the other hand, the stability can be explained by the formation of numerous hydrogen bonds. [11][12][13][14][15][16][17][18][19][20] The protonation of guanidine has alreadyb een studied by Olah et al in 1968 by means of 1 HNMR spectroscopic analysis in "Magic Acid" (HSO 3 F/SbF 5 ), in whicht he 1 HNMR shifts indicated diprotonationo fg uanidine. [21] Attempts to isolate guanidinium(2 +)s alts have not been reported, which prompted us to investigatet he protonation of guanidine in superacidic media.…”