Linked Life Data

20527908

Source:http://linkedlifedata.com/resource/pubmed/id/20527908

Statements in which the resource exists as a subject.
PredicateObject
rdf:type
lifeskim:mentions
pubmed:issue
13
pubmed:dateCreated
2010-6-25
pubmed:abstractText
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, para --> meta or meta --> para, 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 -O(*) and -CH(2)(+) 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 ArO(*) or ArCH(2)(+) 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 OCH(3), 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 (dpph(*) and ROO(*)). For example, the 298 K rate constant for the 4 + dpph(*) reaction is 22 times greater than that for the 5 + dpph(*) reaction. Fragmentation of ring-substituted benzylammonium ions, generated by ESI-MS, to form the benzyl cations reflects similar remote intramolecular HB effects.
pubmed:language
eng
pubmed:journal
pubmed:citationSubset
IM
pubmed:chemical
pubmed:status
MEDLINE
pubmed:month
Jul
pubmed:issn
1520-6904
pubmed:author
pubmed:issnType
Electronic
pubmed:day
2
pubmed:volume
75
pubmed:owner
NLM
pubmed:authorsComplete
Y
pubmed:pagination
4434-40
pubmed:meshHeading
pubmed:year
2010
pubmed:articleTitle
Influence of "remote" intramolecular hydrogen bonds on the stabilities of phenoxyl radicals and benzyl cations.
pubmed:affiliation
Istituto di Chimica Biomolecolare del CNR, Via Paolo Gaifami 18, I-95126 Catania, Italy. mario.foti@icb.cnr.it
pubmed:publicationType
Journal Article, Research Support, Non-U.S. Gov't