Oxygen-17 NMR and Copper EPR Linewidths in Aqueous Solutions of Copper(II) Ion and 2,2′-Dipyridine

Abstract

Between 0° and 60°C the transverse relaxation rate of oxygen-17 nuclei in water is increased in proportion to the concentration of diaquobis (2,2′-dipyridine) copper(II) which exists in equilibrium with the tris(chelate) species in solutions of tris(2,2′-dipyridine) copper(II) nitrate. The excess relaxation rate 1 / T2p due to the presence of the paramagnetic ions is controlled by the rate of relaxation of oxygen-17 nuclei in the coordination sphere of the [Cu(dipy)2(OH2)2]+ + species where the paramagnetic electron–oxygen-17 hyperfine interaction AL / h is of the order of 1.0 × 107 Hz. This value is large in comparison with the one previously reported for diaquobis (ethylenediamine) copper(II), where the two water molecules occupy positions trans with respect to each other on the long axis of the Jahn–Teller distorted complex. The X-band EPR spectrum of an ethanol–water glass where 2,2′-dipyridine/copper(II) ratio is 2.00 indicates that the cis and the trans isomers of the diaquobis (2,2′-dipyridine) copper(II) species are present in the glass. In aqueous solutions at 24°C only one disubstituted species can be observed in the EPR spectrum. It is assumed that this spectrum is predominantly due to the cis-diaquo isomer which may exist in equilibrium with small amounts of the trans isomer. Lower limits of the electron spin–lattice relaxation times and the apparent tumbling correlation times at 25°C were obtained from the X-band EPR spectra for the tetraaquo (2,2′-dipyridine), the diaquobis (2,2′-dipyridine), and the tris-(2,2′-dipyride) copper(II) species. The tumbling correlation time for the latter species is unusually low. It is proposed that the phenomenon can be explained by applying the previously discussed concept of the dynamical Jahn–Teller effect to complex ions in aqueous solutions.