2H Isotope Effect on 13C Chemical Shifts of Nitro-Benzo-9-Crown-3

Document Type: Research Article

Authors

1 Chemistry Department, University of Imam Hossein, Tehran, P.O. Box 16575-347, I.R. IRAN

2 Chemistry Department, University of Tehran, Tehran, I.R. IRAN

3 Chemistry Department, University of Tarbiat Modares, Tehran, I.R. IRAN

Abstract

Deuterium substitution on two ortho-substituted-OCH2-fragments in Nitro-Benzo-9-Crown-3 induces low frequency shifts, positive mnDCj, in all 13C NMR resonances which is an indication of the increased shielding in this crown ether. The magnitude of these shifts vary from 15DC7=716 to 54DC3=15 ppb for C7 and C3 CD3COCD3, and C6D6 on mnDCj values were investigated. The mnDCj values depended more on the nature of the solvent than on the concentration. The order of induced isotope shifts is: 15D, 51D>24D, 42D>34D, 43D>56D, 65D>45D, 54D. The isotope shifts observed are suggested to be a sum of contributions from low frequency shift due to inductive-type and negative hyperconjugation perturbations. The C-D bond, as a poorer electron acceptor than a C-H bond induces less positive charge on directly attached oxygens O1 and O2. This in turn, causes shielding of C1 and C2 in C1O1CD2 and C2O2CD2 fragments. The difference in 34DC1 and 43DC2 values is attributed to the conformational dependence of the negative hyperconjugation. The C1 and C2, are in fact, not equally affected by the two CD2 groups by negative hyperconjoagation because of the existence of NO2 group attached to the benzene ring.

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