Coherence transfer between nuclear spins in paramagnetic systems: effects of nucleus-electron dipole-dipole cross-correlation

In nuclear magnetic resonance of paramagnetic systems cross-correlations be tween the fluctuations of a nucleus-nucleus dipole-dipole coupling (IIl)-I- k and a nucleus-electron dipole coupling (IS)-S-k induces cross-relaxation and makes it possible to generate bilinear terms in the density matrix of t he type 2I(x)(k)I(z)(l) from coherence I-x(k) that can lead to 'relaxation- allowed' coherence transfer between two nuclei I-k and I-l. In this paper t hese effects are demonstrated in a complex involving a fragment of double-s tranded DNA and two chromomycin molecules complexing a paramagnetic cobalt ion. Analytical expressions are given for the cross-correlation rates in pa rticular conditions, while the extension to anisotropic g tensors or zero f ield splittings are addressed. It is shown that relaxation-allowed coherenc e transfer leads to characteristic signals in double-quantum filtered corre lation spectroscopy (DQF-COSY), but not in total correlation spectroscopy ( TOCSY). Analytical expressions are unable to reproduce the observed cross-p eak patterns. A careful numerical study reveals that in the high spin Co(II ) complex studied here, the cross-correlation dynamic shift contribution is of the same order of magnitude as the cross-correlation rate, a value much larger than what can be computed assuming isotropic Brownian motion and co mplete separation between the electron spin and the lattice.