The Synthesis of 4,6,8,14tetradehydro[15]annulenone, an Aromatic Macrocyclic Ketone. The Demonstration of a Diamagnetic Ring Current in a [4 + 3]Annulenone Derivative1
The low-abundance isotope of nitrogen, 15N, is now accessible to study by the latest high resolution nuclear magnetic resonance techniques. Structure and motion in polypeptides of moderate size may now be usefully investigated in this way.
Proton magnetic resonance spectra of N-deuterated Co(en)33+ and Co(/?-pn)33+ in D20 at various temperatures and phosphate concentrations were obtained at 251 MHz with spin decoupling of cobalt-59. From the coupling constants obtained by analysis of the Co(en)33+ AA'BB spectra, the fraction of time spent by an ethylenediamine ring in each conformational state (i.e., with carbon-carbon bonds perpendicular or parallel to the C3 axis) is determined. The molecular species having one carbon-carbon bond perpendicular and two carbon-carbon bonds parallel to the C3 axis (e.g., -) is more abundant than the species having all three carbon-carbon bonds parallel to the C3 axis (e.g.,). However, the population ofexceeds that ofat high phosphate concentrations. For efficient cobalt-59 decoupling, the extent of N-deuteration must be extremely high because of large deuterium isotope effects on cobalt-59 chemical shifts, as shown by cobalt-59 magnetic resonance spectra.Nonequivalent methyl resonances and nonequivalent methylene resonances were resolved in the 250-MHz pmr spectra of Co(i?-pn)33+ (cis-trans mixture), and approximate chemical shifts and coupling constants were obtained by first-order analysis.
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