Assignment of carbon‐13 NMR spectra and sign determination of long range carbon‐13—proton coupling constants with SPI

Abstract

Abstract-The application of selective population inversion (SPI) to the assignment of 13C resonances and to the determination of the relative sign of W-lH coupling constants is demonstrated for l-phenyl-l,2-dibromo-2-nitroethane. A negative sign for the geminal C-243-1 coupling constant has been determined from the I3C spectra. The relationship between SPI and TSI is discussed.A PULSED double resonance method, selective population inversion (SPI), which should give the same information as INDORl and which can easily be performed on modern FT NMR spectrometers, has recently been described.2 In 13C-(1H) SPI a selective r.f. 7~ pulse applied at a proton transition inverts the populations of the upper and lower energy levels. A strong nonselective r.f. 7r/2 pulse is immediately applied in the usual way to the 13C nuclei and Fourier transformation of the resulting free induction decay gives the frequency spectrum. This communication describes how the method has been used to assign I3C resonances and to determine the relative sign of a long range l3C--IH coupling constant in I-phenyl-l,2-dibromo-2-nitroethane (1).C,H,CHBr-CHBrNO,(1) RESULTS lH NMR spectrumThe 100 MHz lH spectrum of 1 in deuterochloroform shows a five proton phenyl resonance at 6 = 7.38 and an AM spin system for the aliphatic protons with JAM = 10.9 Hz. Irradiation of the phenyl resonance sharpened the low frequency AM doublet giving the assignment d a .~ = 5-50 and 6~~ = 6.37. L i 13C NMR spectrumThe 25.2 MHz single resonance FT 13C spectrum of 1 in deuterochloroform (Fig. 1) shows a complex pattern for the aromatic carbons, but the aliphatic resonances are simpler, exhibiting clearly the directly bonded and long range 13C--lH couplings. The chemical shifts of the aliphatic carbons (C-1 and C-2) and the relevant 13C--lH coupling constants for the assignment proved below are given in Table 1. Decoupling of the aromatic protons removed the fine structure from the C-1 doublet and left the C-2 quartet unchanged. C-2, therefore, shows a geminal coupling to H-1, but no similar geminal interaction between C-1 and H-2 is observed. 13C-(1R} SPI EXPERIMENTS 13C AssignmentThe I3C resonances of the aliphatic carbon atoms are basically X parts of AMX spin systems. A stick spectrum of the C-1 isotopomer AMX system (JMx > JAM > 0; JAx c11 0) is shown in Fig. 2 with the transitions numbered according to the AMX energy level diagram (Fig. 3). A T pulse at v M -&JMx with sufficient power will invert the populations of the energy levels E2 and E,, as well as the populations of E6 and E3, because both M1 and M3 are irradiated. At maximum, the intensities of the progressively connected transitions X4 and X2 will increase fivefold, while the regressive transitions X3 and X1 will appear as negative peaks with three times their normal intensity. The result of the experiment is shown in the insert of Fig. 1 and provides

High Resolution 13C Spectrum and Long-Range 13C,H Couplings in 2-Pyrone

“…An SPI irradiation in the region of a proton resonance 6(Hi) will usually cause sufficient perturbation of the 13C lines for assignment of all long-range J(C,Hi) provided there is no serious overlap of the Hi lines with those of other protons. This convenient procedure has been demonstrated in a previous communication (7). Irradiations at the other 6(H) regions will enable all the J(C,H) to be assigned and hence theoretical 'H spectra for every 13C isotopomer can be constructed.…”

“…The double-resonance method used here, 13C-{'HI SPI (6), has been employed previously in studies on three-spin systems (7,8) and so the five-spin systems of 2-pyrone (1) provide a more exacting test of the method.…”

Comparison and Use of Vector and Quantum Representations of J-Coupled Spin Evolution in an IS Spin System during RF Irradiation of One Spin

“…In the absence of an RF field, B e ϭ ⌬H, and all the terms in [15] are zero except Phase cycling a single rectangular pulse can be analyzed by changing ␤ to ␤ Ϯ n*90Њ in matrix [7] and summing or subtracting the corresponding f ij terms in the resulting matrices depending on the sign of the receiver phase. This eliminates some of the f ij terms in the final 3 ϫ 3 matrix and thus some orthogonal components of magnetization, and it demonstrates how the results of pulse sequences can be changed by phase cycling single pulses-examples are given in Ref.…”

Some stereochemical aspects of two-bond 13C–1H spin–spin coupling. Sign determination in natural abundance 13C nuclear magnetic resonance spectra

“…The sign of 2Jc-, was determined by the pulsed double resonance selective population transfer (spt) technique (10)(11)(12). In the spt experiment, a selective pulse applied to a given 'H transition alters the equilibrium distribution of spins in the upper and lower energy levels.…”