Abstract
The following nitroxide spin-labeled crown ethers have been synthesized: 2,2-(sym-dibenzo-16-crown-5)-4,4-dimethyloxazolidinyl-N-oxyl (1), 2,2-(sym-dibenzo-16-crown-5)-4,4,5,5-tetramethylimidazolinyl-N-oxyl (2), and 3-carboxy-2,2,5,5-tetramethyl-3-pyrrolinyl-l-oxy hydroxy-sym-dibenzo-16-crown-5 ester (3). 1 and 2 are characterized by having the nitroxide near the crown cavity and rigidly bound to a ring carbon atom of the crown. 1-3 have been studied by EPR, and the proton hyperfine splittings have been determined by second harmonic out-of-phase detection. The values obtained for the proton hyperfine splittings of 1 were compared to those determined by ENDOR and found to agree within experimental error. EPR studies of hyperfine splitting constants and Heisenberg spin exchange suggest that these spin-labeled crowns are poor complexing agents for alkali metal cations. The structure of 1 has been determined by x-ray diffraction, and the poor complexing ability of the crown is attributed to an unprecedented structural feature in which the hydrogen atoms of one methylene group of the propylene bridging linkage have turned inward with respect to the cavity of the crown. The proton hyperfine splitting observed for 1 is attributed to protons located at approximately van der Waals distances from the NO group.
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