A Solid-State Nitrogen-15 Nuclear Magnetic Resonance Spectroscopic and Quantum Chemical Investigation of Nitrosoarene−Metal Interactions in Model Systems and in Heme Proteins

Abstract

We have obtained the solid-state 15N nuclear magnetic resonance isotropic chemical shifts and/or shielding tensor elements for a range of nitrosoarene complexes: p-[15N]nitroso-N,N-dimethylaniline, p-[15N]nitroso-N,N-dimethylaniline hydrochloride monohydrate, PdCl2(p-[15N]nitroso-N,N-dimethylaniline)2, ZnCl2(p-[15N]nitroso-N,N-dimethylaniline)2, SnCl2(CH3)2(p-[15N]nitroso-N,N-dimethylaniline)2, PdCl2([15N]nitrosobenzene)2, [Fe(CO)3([15N]nitrosobenzene)]2, and the [15N]nitrosobenzene adducts of horse heart myoglobin and adult human hemoglobin. The isotropic chemical shifts range from 171 to 802 ppm downfield from NH3(ext,l). Using a density functional method, we have computed the isotropic shifts, the shielding tensor elements, and the absolute shieldings, for each of these compounds. There is excellent accord between theory and experiment. In addition, the orientations of the tensors have been calculated, and for the dimer of PhNO, cis-dioxyazodibenzene, there is good accord with an experimental determination of the shielding tensor. Our results indicate that the shielding patterns observed from compound to compound are overwhelmingly dominated by the behavior of σ11, the least shielded element of the shielding tensor, which is oriented close to the N−O bond vector (perpendicular to the PhNO π orbital). We also find an excellent correlation between σ11 and the N−O Mayer bond order, with hemoglobin, myoglobin and all model compounds fitting the correlation well (R2 = 0.963). The nitrosoarenes have among the largest known 15N shielding tensor widths, but by using density functional methods, it is possible to accurately compute them, even when they are bonded to transition metals. Overall, these results thus represent the first comprehensive NMR and quantum chemical study of RNO bonding to heme proteins and model systems, and should form the basis for future comparative studies of the biologically important isoelectronic species, dioxygen.