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Type: Artigo de periódico
Title: 2-pyridyl and 2-pyrimidyl cations: Stable o-hetarynium ions in the gas phase
Author: Gozzo, FC
Eberlin, MN
Abstract: As indicated by high-level CBS-Q ab initio calculations, extensive overlap occurs in the 2-pyridyl and 2-pyrimidyl cations between the fully occupied sp(2) orbital of nitrogen and the adjacent, coplanar, and empty spl orbital of the C2-carbon. Such effective orbital overlap results in o-aryne-like structures with substantially shorter N-C+ bond lengths and N-C+ bond orders of 1.9-2.1. Therefore. the 2-pyridyl and 2-pyrimidyl cations are best represented, and can be regarded as, o-hetarynium ions, being more stable than their positional, nonconjugated isomers by as much as 18-28 kcal/mol. The 4-pyrimidyl cation also displays characteristic o-hetarynium ion structure with substantial orbital overlap. However, the ion easily isomerizes by charge-induced ring opening, as indicated by both the calculations and the ion's lack of o-hetarynium-like reactivity. A high energy barrier of 62.8 kcal/mol hampers isomerization by H-ring walking of the 3-pyridyl cation to the far more stable 2-pyridyl cation. For the related 2-furanyl, 2-thiophenyl, and 2-pyrrolyl cations, little or none of the extra orbital overlap occurs; hence, they display energies close, and structures similar, to those of their 3-isomers. Collision-induced dissociation of collisionally quenched precursor ions performed via triple-stage QqQqQ mass spectrometric (MSS) experiments confirms the greater stability of the 2-pyridyl and 2-pyrimidyl cations.
Country: EUA
Editor: Amer Chemical Soc
Rights: fechado
Identifier DOI: 10.1021/jo980599+
Date Issue: 1999
Appears in Collections:Artigos e Materiais de Revistas Científicas - Unicamp

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