Influence of remote intramolecular hydrogen bonds on the stabilities of phenoxyl radicals and benzyl cations. (Articolo in rivista)

Type
Label
  • Influence of remote intramolecular hydrogen bonds on the stabilities of phenoxyl radicals and benzyl cations. (Articolo in rivista) (literal)
Anno
  • 2010-01-01T00:00:00+01:00 (literal)
Http://www.cnr.it/ontology/cnr/pubblicazioni.owl#doi
  • 10.1021/jo100491a (literal)
Alternative label
  • Foti MC; Amorati R; Pedulli GF; Daquino C; Pratt DA; Ingold KU (2010)
    Influence of remote intramolecular hydrogen bonds on the stabilities of phenoxyl radicals and benzyl cations.
    in Journal of organic chemistry
    (literal)
Http://www.cnr.it/ontology/cnr/pubblicazioni.owl#autori
  • Foti MC; Amorati R; Pedulli GF; Daquino C; Pratt DA; Ingold KU (literal)
Pagina inizio
  • 4434 (literal)
Pagina fine
  • 4440 (literal)
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  • http://pubs.acs.org/doi/pdf/10.1021/jo100491a (literal)
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  • 75 (literal)
Rivista
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  • 13 (literal)
Note
  • Scopu (literal)
  • ISI Web of Science (WOS) (literal)
Http://www.cnr.it/ontology/cnr/pubblicazioni.owl#affiliazioni
  • Istituto di Chimica Biomolecolare del CNR, Via Paolo Gaifami 18, I-95126 Catania, Italy, Dipartimento di Chimica Organica \"A. Mangini\", Via San Giacomo 11, Università di Bologna, Bologna, Italy, Department of Chemistry, Queen's University, 90 Bader Lane, Kingston, ON, Canada K7L 3N6, and National Research Council of Canada, 100 Sussex Drive, Ottawa, ON, Canada K1A 0R6 (literal)
Titolo
  • Influence of remote intramolecular hydrogen bonds on the stabilities of phenoxyl radicals and benzyl cations. (literal)
Abstract
  • Remote intramolecular hydrogen bonds (HBs) in phenols and benzylammonium cations influence the dissociation enthalpies of their O-H and C-N bonds, respectively. The direction of these intramolecular HBs, parafmeta or metafpara, determines the sign of the variation with respect to molecules lacking remote intramolecular HBs. For example, the O-H bond dissociation enthalpy of 3-methoxy-4-hydroxyphenol, 4, is about 2.5 kcal/mol lower than that of its isomer 3-hydroxy-4-methoxyphenol, 5, although group additivity rules would predict nearly identical values. In the case of 3-methoxy-4-hydroxybenzylammonium and 3-hydroxy-4-methoxybenzylammonium ions, the CBS-QB3 level calculated C-N eterolytic dissociation enthalpy is about 3.7 kcal/mol lower in the former ion. These effects are caused by the strong electron-withdrawing character of the -Oo and -CH2+ groups in the phenoxyl radical and benzyl cation, respectively, which modulates the strength of the HB. An O-H group in the para position of ArOo or ArCH2+ becomes more acidic than in the parent molecules and hence forms stronger HBs with hydrogen bond acceptors (HBAs) in the meta position. Conversely, HBAs, such as OCH3, in the para position become weaker HBAs in phenoxyl radicals and benzyl cations than in the parent molecules. These product thermochemistries are reflected in the transition states for, and hence in the kinetics of, hydrogen atom abstraction from phenols by free radicals (dppho and ROOo). For example, the 298 K rate constant for the 4 þ dppho reaction is 22 times greater than that for the 5 þ dppho reaction. Fragmentation of ring-substituted benzylammonium ions, generated by ESI-MS, to form the benzyl cations reflects similar remote intramolecular HB effects. (literal)
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