NMR paramagnetic relaxation enhancement: Test of the controlling influenceof ZFS rhombicity for S=1

Prior theoretical work has predicted that the NMR paramagnetic relaxation e nhancement (NMR-PRE) produced by electron spin S = 1 ions is highly sensiti ve to orthorhombic terms in the static zero held splitting (zfs) tensor. Zf s orthorhombicity (which implies chemical inequivalence of the three princi pal directions of the zfs-principal axis system and is described by the zfs E-parameter) is predicted to suppress the NMR-PRE profoundly relative to t he reference cylindrical zfs-limit situation. This expectation was tested e xperimentally by a comparison of the zfs-limit NMR-PRE produced by [Ni(II)( en)(3)](2+) (en = ethylenediamine), a trigonal complex which lacks zfs-rhom bicity, with the zfs-limit NMR-PRE produced by two orthorhombic complexes, [Ni(II)(en)(2)(H2O)(2)](2+) and [Ni(II)(en)(H2O)(4)](2+). As predicted, the zfs-limit NMR-PRE produced by the orthorhombic complexes in the proton res onance of a dioxane probe species in the solvent was strongly suppressed (b y factors of approximately 5 and 7, respectively) relative to the comparabl e measurement on the trigonal complex. The suppression of the NMR-PRE due t o the orthorhombic zfs terms is counteracted by an applied Zeeman held, lea ding to a predicted rise in the NMR-PRE with increasing Zeeman field streng th; this rise occurs when the Zeeman energy is comparable to the orthorhomb ic zfs splitting, 2E. This second prediction of theory was likewise confirm ed: the expected rhombicity-induced magnetic field dependence in the NMR-PR E was observed for the orthorhombic complexes but not for the trigonal comp lex. (C) 2001 Academic Press.