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The electronic and geometric structures of paramagnetic iron dinitrosyl complexes have been investigated using electron spin resonance, infrared spectroscopy, and X-ray crystallography. It is concluded that these compounds are best described as 17 electron complexes with a d 9 configuration rather than the d 7 configuration assumed by most previous investigators. The anisotropy of the g values, determined from the electron spin resonance spectra of frozen solutions, varies considerably from complex to complex. The results are consistent with the supposition that all of the complexes have a distorted tetrahedral geometry, but the nature of the distortion changes as the ligands are varied. As a result of this variation there are changes in the nature of the spin-containing d orbital. Ligands containing hard, nonpolarizable donor atoms such as oxygen or fluorine produce a distortion towards a planar geometry, placing the odd electron in a predominantly d, 2-, 2 orbital, while those containing softer donor atoms such as phosphorus or sulfur give complexes with a different type of distortion, leading to placement of the odd electron in a predominantly dz2 orbital. Nitrogen and halide donor ligands produce smaller distortions, leading to spin-containing molecular orbitals with contributions from a mixture of d orbitals. In accordance with this model, the crystal structure of [Fe(NO),I,]-has been found to be only slightly distorted from regular tetrahedral coordination about the iron atom. TRACI R. BRYAR et DONALD R. EATON. Can. J. Chem. 70, 1917Chem. 70, (1992. Faisant appel 2 la resonance paramagnetique Clectronique (rpe), a la spectroscopie infrarouge et i la diffraction des rayons X, on a CtudiC les structures Clectroniques et geometriques des complexes paramagnetiques du dinitrosyl fer. On en conclut que la meilleure f a~o n de dCcrire ces composCs est de les considerer comme des complexes 2 17 klectrons possidant une configuration d 9 plut6t que la configuration d7 utilisee par la plupart des auteurs qui en ont trait6 anterieurement. L'anisotropie des valeurs g, determinee par les spectres rpe de solutions congelees, varie beaucoup d'un complexe 2 l'autre. Les resultats sont en accord avec l'hypothkse selon laquelle tous les complexes possedent une gkomktrie tCtraCdrique dCformCe, mais que la nature de la distorsion varie avec la nature des ligands. Un rksultat de cette variation amkne des changements dans la nature de l'orbitale contenant le spin. Les ligands contenant des atomes donneurs durs et non-polarisables, comme l'oxygkne ou le fluor, provoquent une distorsion vers une gComCtrie plane forqant 1'Clectron celibataire 2 1 occuper une orbitale principalement dX2-y2 alors que ceux qui contient des atomes donneurs plus mous, comme le phosphore ou le soufre, donnent des complexes~avec un type de distorsion different, conduisant . [Traduit par la rkdaction]

[1] Laboratory proton nuclear magnetic resonance (NMR) relaxation measurements were made on fluid-saturated soil samples under reducing and oxidizing conditions. We used well-characterized kaolinite and sand samples with known concentrations of Fe(II) and Fe(III) compounds as the representative soils. Changes in dissolved oxygen concentration in the water in the soils caused changes in relaxation time that are too small to be reliably detected by field NMR measurements. In contrast, NMR relaxation measurements were shown to be very sensitive to small changes in the concentration of Fe(III) species in soil, as a consequence of changing redox conditions. Oxidation of as little as 0.030 mg/g of Fe(II) species to Fe(III) oxyhydroxides resulted in a 30-50% decrease in relaxation time. These results have important implications for the potential use of NMR field instruments to monitor changing redox conditions using the Fe(III)-Fe(II) redox couple as an indicator. INDEX TERMS:3929 Mineral Physics: NMR, Mossbauer spectroscopy, and other magnetic techniques; 1094 Geochemistry: Instruments and techniques; 1806 Hydrology: Chemistry of fresh water. Citation: Bryar, T. R., and R. J. Knight, Sensitivity of nuclear magnetic resonance relaxation measurements to changing soil redox conditions, Geophys.

Proton NMR (nuclear magnetic resonance) measurements were made of T1 and T2 relaxation times of water in saturated sands containing varying amounts of sorbed oil on the grain surfaces. The porosity, surface area, and grain density of the sands and the relaxation times of the extracted pore water were also determined experimentally. Sorption of oil changed the relaxation time of water in the saturated sands through changes in surface area and surface relaxivity, the parameter used to quantify the ability of the surface of the pore space to reduce NMR relaxation times. In some cases the addition of oil to the surfaces decreased the surface area, an observation that suggested the oil was coating the surface in a way to reduce surface roughness. When larger amounts of oil were added to the surface, surface area increased. The changes in surface relaxivity with the amount of sorbed oil were governed by the relaxivity of the clean, oil‐free surfaces. In the Wedron sand, with a surface relaxivity typical of naturally occurring sands, the relaxivity decreased with the addition of oil to the surface of the sand grains. In the A–A sand, a clean, pure quartz sand, the relaxivity increased from a very low value for the oil‐free sample to a higher value, interpreted to be that of the oil surface.

An ability to detect residual (sorbed) concentrations of hydrocarbons in a porous medium using nonintrusive geophysical methods is of interest in the remediation of hydrocarbon-contaminated sites. Proton NMR is an noninvasive analytical technique that can be applied to characterize several rock properties in the lab and the field. In this study, we perform a series of laboratory experiments to quantify the effect of sorbed crude oil on the NMR relaxation time (T 1 ) of water-saturated silica gels, both in the presence and absence of iron oxyhydroxide. The experimental data indicate that in the absence of paramagnetic iron impurities, T 1 remains constant in the presence of sorbed hydrocarbons. T 1 increases with increasing hydrocarbon concentration only if the silica surface is coated with a paramagnetic substance such as iron oxyhydroxide. These results suggest that increases in T 1 previously observed for water in natural materials coated with oil are caused by the shielding of paramagnetic surface sites by the sorbed hydrocarbons. The major implication of this study is that proton NMR is a potential means of detecting sorbed or residual hydrocarbons in natural environments.