Interaction between closed-shell and open-shell molecules. 1H relaxation studies of the hydrogen bond between proton-donor molecules and a nitroxide radical

Abstract

Pulsed and cw NMR methods have been used to investigate the 1H relaxation of proton donor molecules hydrogen bonded with the DTBN radical. Frequency dependences of proton relaxation times T 1 and T 2 were studied with a DTBN solution of CHCl 3 by the pulsed NMR method. It is shown that the relaxation mechanism is determined chiefly by the nucleus—electron dipole—dipole interaction (τ c ≲ 10 −10 s), but partially characterized by the exchange coupling. Using the relation (1/ T 1) DD ≈ (1/ T 2) DD which holds at low magnetic fields, the lifetime of the chemical exchange associated with the CHCl 3···DTBN hydrogen bond was evaluated as 3.6 × 10 −8 s from the difference between 1/ T 2 and 1/ T 1 measured at 15 MHz.