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Guanidinium‐Type resonance stabilization and its biological implications. 2. The doubly‐extended‐guanidine series
Author(s) -
Jordan Meredith J.,
Gready Jill E.
Publication year - 1989
Publication title -
journal of computational chemistry
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.907
H-Index - 188
eISSN - 1096-987X
pISSN - 0192-8651
DOI - 10.1002/jcc.540100206
Subject(s) - chemistry , delocalized electron , tautomer , computational chemistry , protonation , steric effects , natural bond orbital , conformational isomerism , lone pair , intramolecular force , molecule , stereochemistry , crystallography , density functional theory , organic chemistry , ion
π‐Electron delocalization in neutral and protonated “doubly‐extended‐guanidine,” (H 2 N) 2 CNCHN—CHNH, has been studied by ab initio methods at the self‐consistent field (SCF) STO ‐3G and 3‐21G levels for a large number of tautomeric, rotameric, pseudocyclic, and monocyclic (disubstituted triazine) forms. These π systems have been characterized in terms of a number of structural and energetic parameters: degree of single/bond character from bond lengths and π bond orders, electron distributions, and tautomer, rotamer, and protonation energies. The acyclic neutral forms exhibit largely alternant single–double bond patterns as predicted by classical bonding structures but with, however, significant deviations due to conjugation. The acyclic protonated forms exhibit bond patterns consistent with resonance delocalized structures extending over the whole molecule (“doubly‐extended guanidinium”) or part of the molecule (“extended‐guanidinium”) or guanidinium . All systems showed alternant charge distributions with electron‐deficient carbons. The energy results have been analyzed in terms of possible contributions from steric interactions, lone‐pair repulsions, purportive electrostatic interactions in pseudocyclic forms, overall π‐system conformation (extended, kinked, or folded), and specific through‐space π‐overlap interactions in some pseudocyclic forms. It was found that these other interactions usually dominate the specifically π effects so that the general concept of preferential π delocalization in straight lines does not hold for the acyclic systems. Some interesting examples of pseudocyclic forms exhibiting strongly stabilizing intramolecular interactions attributed to π through‐space coupling are identified. These systems with incipient‐ring characteristics present intermediate bonding models between the acyclic and closed‐ring π systems. The extent of stabilization of the guanidinium‐type cations by resonance delocalization in cyclic systems depended on whether it reinforced or interfered with the overall ring delocalization.