Abstract
Paramagnetic contributions to the NMR chemical shifts for nitrogen and oxygen in nitroalkanes, RN0 2 (R = methyl, ethyl, propyl, isopropyl and t-butyl), and in N0 3 − NO 2 + and NO 2 − have been calculated according to the Pople MO theory of chemical shifts. INDO MO calculations were the basis for the evaluation of ΣQ, the sum of the appropriate charge density-bond order terms, and of <r −3> 2p via Slater's rules. ΔE was equated to the lowest observed singlet excitation energy. The diamagnetic shielding was evaluated in atomic terms, including contributions only from the atom in question and atoms directly bonded to it. Separate consideration was given to ΣQ AA , which depends only on the charge densities of the atom in question. Better fits to observed nitrogen and oxygen chemical shifts were obtained using ΣQ AA in the evaluation of the paramagnetic term rather than the usual ΣQ AA , which includes π-electron contributions from directly bonded atoms.
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