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Titolo:
Proton delocalization and hyperacidity in some phenolic resins
Autore:
Yan, ZL; Yeh, TF; Schmidling, DG; Reiser, A; Mirau, PA; Sroubkova, L; Zahradnik, R;
Indirizzi:
Polytech Univ, Inst Imaging Sci, Metrotech Ctr 6, Brooklyn, NY 11201 USA Polytech Univ Brooklyn NY USA 11201 trotech Ctr 6, Brooklyn, NY 11201 USA Lucent Technol, Bell Labs, Murray Hill, NJ 07974 USA Lucent Technol Murray Hill NJ USA 07974 l Labs, Murray Hill, NJ 07974 USA Acad Sci, Heyrovsky Inst, Prague 8, Czech Republic Acad Sci Prague CzechRepublic 8 eyrovsky Inst, Prague 8, Czech Republic
Titolo Testata:
MACROMOLECULES
fascicolo: 9, volume: 34, anno: 2001,
pagine: 2901 - 2907
SICI:
0024-9297(20010424)34:9<2901:PDAHIS>2.0.ZU;2-Z
Fonte:
ISI
Lingua:
ENG
Soggetto:
DISSOLUTION;
Tipo documento:
Article
Natura:
Periodico
Settore Disciplinare:
Physical, Chemical & Earth Sciences
Citazioni:
17
Recensione:
Indirizzi per estratti:
Indirizzo: Reiser, A Polytech Univ, Inst Imaging Sci, Metrotech Ctr 6, Brooklyn, NY 11201 USA Polytech Univ Brooklyn NY USA 11201 r 6, Brooklyn, NY 11201 USA
Citazione:
Z.L. Yan et al., "Proton delocalization and hyperacidity in some phenolic resins", MACROMOLEC, 34(9), 2001, pp. 2901-2907

Abstract

Interaction between OH groups can produce large acidity increases in phenolic systems. We report here on the behavior of a group of oligomers of p-cresol, where the cresol units are linked to each other by -CH2- bridges. In this series the acid dissociation constant increases consistently with the length of the oligomer chain. The effect is very significant; between the monomeric cresol and the pentamer the acid dissociation constants as measured in organic solvents span 5-8 orders of magnitude. In phenols, part of theacid dissociation energy is the electrostatic work required to separate the incipient ions, and that work depends on the charge located at the oxygenof the corresponding phenolate anion. It appears that with increasing length of the oligomer chain the charge on the anion site decreases. We have found that this is caused by proton delocalization between the oxygen centersof the oligomeric anions. Proton delocalization makes possible the redistribution of charge among all the oxygen centers of the oligomer, thereby lowering the charge on any one of them and increasing the acidity of the monoanions. The reality of proton delocalization is evidenced by the C-13 NMR spectra of the monoanions. The two carbon atoms of the dimer anion are indistinguishable and produce a single line in the NMR spectrum and so do the terminal carbons of the trimer anion. These observations are in line with the results of an ab initio calculation indicating an unexpectedly low energy barrier (3.5-0.3 kcal/mol) separating the two equilibrium positions of the proton. It appears that the proton oscillates almost freely between the two positions and is effectively delocalized over both oxygen centers.

ASDD Area Sistemi Dipartimentali e Documentali, Università di Bologna, Catalogo delle riviste ed altri periodici
Documento generato il 11/07/20 alle ore 10:40:39