Use of a stable free radical as a NMR spin probe for studying intermolecular interactions. XIV. A proton relaxation study of the hydrogen bond involving a stable free radical

Abstract

Pulsed NMR methods have been used to investigate the proton spin relaxation for the solutions containing a DTBN radical. The present study on frequency and temperature dependences of proton relaxation times affords the information on the distance between solvent molecules and the DTBN radical, and on the dynamical aspects of the solvent–radical interaction. The observed ¹H relaxation rates were analyzed on the basis of the Hubbard and Solomon–Bloembergen theories. The ¹H relaxation of aprotic solvent molecules [CH₃OD, C₆H₆ and (CH₃)₂CO] in the presence of DTBN is explained by a pure dipole–dipole interaction governed by translational random motions, while the proton relaxation of proton–donor molecules (CD₃OH and CHCl₃) in the presence of DTBN radical is dominated by the dipolar correlation time for the hydrogen bonding. The correlation times, the activation energies, and the intermolecular distances were evaluated. The closest distance between the proton and the paramagnetic center of the radical in the aprotic solvent was found to be close to the distance estimated from van der Waals radii. The hydrogen bond lengths for CD₃OH– and CHCl₃–DTBN bimolecular interaction systems were evaluated as r_OH⋅⋅⋅O?1.9 Å and r_CH⋅⋅⋅O ?2.1 Å from the effective separation between the proton and the odd electron center. The lifetime of the hydrogen bond complex was also obtained.