Relative Signs of the Nuclear Spin Coupling Constants in Propylene Oxide and Indene Oxide

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Abstract
The 60-Mc/sec high-resolution NMR spectrum of propylene oxide was analyzed as an ABCD3 nuclear spin system in the conventional iterative manner utilizing in addition double resonance to decouple the D3 (methyl group) protons from the ABC protons. A fit was obtained by two sets of parameters whose main difference was that either both long-range couplings JBD and JCD are positive with respect to the other couplings or only JCD is negative. By use of a weak perturbing rf field to irradiate a particular resonance transition it was determined that JCD has a different sign than the other couplings. The 60-Mc/sec spectrum of the protons of the five-membered ring of indene oxide (II) was analyzed as an ABCD system with the aid of triple-irradiation experiments in which the long-range couplings of the A, C, and D protons with the aromatic ring protons were destroyed by a large rf irradiation applied at the resonance position of the aromatic protons while a second weaker perturbing irradiation was applied at particular lines in the ABCD spectrum. The results of these experiments determined the energy level scheme and suggested that if this spectrum is truly first order then all the coupling constants in the four spin system are positive except JCD. However, complete iterative analysis using the Swalen and Reilly program showed the spectrum of II was definitely not first order and that JAD is almost zero and JBC very small. The following sets of parameters were found for I and II. JABJACJADJBCJBDJCDδAδBδCδDI+3.88+2.57+5.15+5.37+0.52−0.362.8492.5892.2791.214II+2.90+1.32−0.01+0.12+3.11−18.224.1393.9833.1422.879 The spin—spin couplings are in cycles per second and chemical shifts in parts per million downfield from tetramethylsilane. Stereochemically the H–C–C–C–H couplings in II and the positive H–C–C–C–H coupling in I are similar, i.e., the protons on the oxirane ring are related in what can be considered a ``trans'' manner to the carbon—carbon bond joining the methyl or methylene group to the oxirane ring. The results from the rigid molecule II suggest that this particular long-range coupling involving four saturated bonds is positive when the protons are in an ``anti—trans'' relation or furthest away and near zero when the protons are in a ``syn—trans'' relation or nearer. The magnitudes and signs of the H–C–C–C–H couplings found in I and II are discussed in some detail and compared with other reported similar long-range couplings. The description of the field-frequency lock spectrometer used in this work is given. With this system it was possible to detect excursions of the field-frequency setting of the order of 1–2 parts in 1010.

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