13C nuclear magnetic resonance spin–lattice relaxation times and conformation of 2-chlorodibenzo-p-dioxine

Abstract

The carbon-13 spin–lattice relaxation times for the hydrogen-bearing carbon atoms in 2-chlorodibenzo-p-dioxine have been measured for a saturated solution in DMSO + CD₃CN (2 : 1) by the inversion–recovery–pulse sequence. The relaxation times were found to be in the range 4.2–6.3 s. Equations based on the extended relaxation equation of Huntress have been developed which allowed the calculation of relaxation times for 2-chlorodibenzo-p-dioxine under the assumption of a rigid model of anisotropic reorientation. These calculations, which were carried out for the conformations with angles of fold of 180, 164 and 150°, suggest that the experimental relaxation times are compatible with a planar (or almost planar) conformation with no internal ‘butterfly’ motion on the n.m.r. timescale. The method applied to 2-chlorodibenzo-p-dioxine may help to elucidate conformational dynamics.