The Interaction of Paramagnetic Relaxation Reagents with Intra- and Intermolecular Hydrogen Bonded Phenols

摘要

Intermolecular electron-nuclear 13-C relaxation times (T(1)sup e's) from solutions containing the paramagnetic relaxation reagent (PARR), Cr(acac)3, used in conjunction with 13-C T(1)'s in diamagnetic solutions (intramolecular 13-C - (1)H dipolar T(1)'s) provide a significant increase of information in studies of hydrogen-bonded liquids. Analysis of the association process with these data shows great promise in testing association models. It is also possible to determine the geometry of Cr(acac)3-substrate complexation using the inverse sixth power distance dependence of (T(1) sup e's). Several models for free and hindered internal rotation are tested for hydroxyl containing organic substrates (phenols, borneol) which are rigid at the point of attachment to the paramagnetic relaxation reagent (PARR). For symmetrically substituted phenols the observed electron-nuclear relaxation rates cannot be accounted for by a 'static' model with a point locus for the PARR. Better agreement requires models with free internal rotation about the C(1)-0 bond for phenol. For ortho-substituted phenols hindered rotation models appear to be superior to the 'static' model. The Cr-O distances calculated evidence hydrogen bonding between Cr(acac)3 and phenols. The average distance is ca. 4.9 A for 4-chloro- and 3,5-dichlorophenol while for the rest of the phenols tested the distance is ca. 4.1 A. Calculations indicate that there may be two different types of PARR-phenol complexes. For 4-chlorophenol and 3,5-dichlorophenol solutions the average composition of the solvation sphere of Cr(acac)3 appears to be quite different from the solvation spheres for the rest of the phenols studied.