Magnetic Resonance in Chemistry

doi: 10.1002/mrc.1260310101pmid: N/A

Günther, H.

doi: 10.1002/mrc.1260310102pmid: N/A

Blümel, Janet; Hofmann, Peter; Köhler, Frank H.

doi: 10.1002/mrc.1260310103pmid: N/A

The methylated metallocenes (MeCp)2M with M = V, Cr, Mn, Co, Ni (1–5) were investigated by 1H and 2H NMR spectroscopy at natural abundance. The 2H NMR signals are narrower by a factor of up to 30 compared with the corresponding 1H NMR signals, thus establishing an inexpensive method and a general improvement of the NMR spectra of paramagnetic π‐complexes. This has allowed the resolution of the signal splitting of Cp deuterons of 1 and 5 which could not be observed earlier in the 1H NMR spectra. The origin of the small (1 and 5) and large (2–4) signal splittings is discussed and related to an extended Hückel calculation. The relative magnitude of the signal splitting is reproduced, but the signal assignment had to be deduced from 13C NMR results. Primary isotope shifts of up to 4.0 ppm were found for 1, 2, 4 and 5. The much higher values for 3 (up to 16.6 ppm) reflect the combined influence of the intrinsic isotope shift and the isotope effect on the spin crossover.

Freyer, Alan J.; Nissan, Robin A.; Wilson, William S.

doi: 10.1002/mrc.1260310104pmid: N/A

Several 1‐(2′,4′,6′‐trinitrophenyl)dinitrobenzimidazoles (1‐picryldinitrobenzimidazoles) were synthesized as part of an ongoing study of heterocyclic nitro compounds as energetic materials and their structure–property relationships. The 1‐picryl‐4,6‐dinitrobenzimidazoles and their 1‐methyl analogs were examined by several NMR techniques in order to confirm their structures, and assignment strategies are discussed. Nuclear Overhauser enhancement (NOE) difference spectroscopy and 1H/13C correlation experiments were used to make definitive assignments of the 1H and 13C NMR spectra. It was found that both solvent and temperature had significant effects on the chemical shifts of the benzimidazole protons. This was especially true for the 1‐picryl derivatives.

Wazeer, Mohammed I. M.; Ali, Sk. Asrof

doi: 10.1002/mrc.1260310105pmid: N/A

Carbon‐13 NMR spectra were recorded for two series of isoxazolidines, 3 (a 5/5 fused system) and 4 (a 6/5 fused system), over the temperature range − 100 to +60°C. Whereas all the compounds in series 3 existed solely as one isomer throughout this temperature range, some compounds in series 4 showed the presence of two isomers of unequal population at low temperatures. The major isomer was shown to be the trans conformer, which is in equilibrium with the minor isomer (cis conformer) through a relatively slow nitrogen inversion. The monosubstituted isoxazolidines 4 having endo substituents at either C‐2 or C‐3 and 2,3‐di‐ and 2,2,3‐trisubstituted compounds showed the presence of only one isomer throughout the temperature range. The barriers to nitrogen inversion were determined by band shape analysis and were in the range 65.2–‐69.0 kj mol−1.

Marat, Kirk; Templeton, J. F.; Ling, Yangzhi

doi: 10.1002/mrc.1260310106pmid: N/A

Coupling constants and nuclear Overhauser effect measurements were used to establish the C‐20 configuration and the conformation about the C‐17C‐20 bond in a series of sixteen C‐20‐substituted 5β,14β‐pregn‐14‐enes. In the 14β‐pregnane series the conformation of the C‐17 side‐chain is variable, whereas in the pregnenes the side‐chain adopts a conformation in which H‐17 is anti to H‐20. The C‐17 side‐chain conformation of two 21‐nor‐5β,14β‐pregnanes was also determined. In the 21‐nor compounds a C‐20 hydroxyl adopts a conformation anti to H‐17, while a C‐20 nitro group is anti to C‐13.

Hansen, Poul Erik

doi: 10.1002/mrc.1260310107pmid: N/A

Deuterium isotope effects on 13C nuclear shielding, nΔC(OD), were investigated for a large number of substituted ortho‐substituted aromatic hydroxyacyl compounds of the type salicylaldehyde, 2‐hydroxyacetophenone, etc. The isotope effects were analysed as a function of substitution patterns. 2ΔC‐2(OD) shows a linear dependence on ΔOH, with one slope for 4‐ and 6‐substituted derivatives and another for 5‐substituted derivatives. A similar pattern is also found for the long range isotope effects. In addition, isotope effects also correlate with 13C chemical shifts. Isotope effects in o‐hydroxyacetophenones can be predicted from the δOH and nΔC(OD) values of salicylaldehyde, and vice versa. The isotope effects were analysed in terms of a vibrational term related to the intramolecular hydrogen bond and an electronic term related to the substituent influence on the transmission of the effect along the pathway between the deuterium‐substituted oxygen and the carbon in question. 2ΔC(OD) isotope effects are found to depend on the one‐bond bond order of the CC bond of the unsubstituted hydrocarbons. Long‐range isotope effects are observed to the olefinic carbons of 2‐hydroxychalcone, whereas no effects beyond C‐1′ are seen in o‐hydroxyketones or o‐hydroxybenzophenones. The tautomeric equilibrium of usnic acid is perturbed by the removal of the weak hydrogen bond between 9‐OH and the carbonyl group at C‐1.

Rasala, Danuta; Gawinecki, Ryszard

doi: 10.1002/mrc.1260310108pmid: N/A

The 13C NMR spectra of 40 vicinally substituted 2‐ and 3‐nitropyridines were recorded. The observed chemical shifts are, in general, non‐additive. These results are discussed in terms of the effects of 2‐, 3‐ and 4‐substitutents on the ring carbons and of the position of the fixed nitro group and the substituent. The chemical shifts of the para carbon, with respect to the substitutent, follow well the dual substituent parameter approach, which shows their electronic origin. However, the substituent chemical shifts of both ipso and ortho positions can be rationalized, in terms of multiple parameter substituent correlation analysis, only on a approximate level.

Wrackmeyer, Bernd; von Locquenghien, Klaus Horchler; Kupče, Ēriks; Sebald, Angelika

doi: 10.1002/mrc.1260310109pmid: N/A

Heteronuclear double and triple resonance experiments of the type 1H{X} (X = 31P, 199Hg, 207Pb), 13C{1H, 31P}, 29Si{1H, 31P}, 207Pb{1H, 31P}, pseudo‐triple resonance experiments [e.g. 13C{1H(199Hg)}, 207Pb{1H(13C)}], together with two‐dimensional (2D) 13C/1H heteronuclear shift correlations, were carried out to determine the signs of various coupling constants [1J(M13C), 2J(MC13C), 3J(MCC13C), 3J(MCC1H), 4J(MCCC1H); M = 29Si, 119Sn, 207Pb, 195Pt, 199Hg; 3J(195PtCCM); M = 29Si, 119Sn, 207Pb] in 17 alkynyl metal compounds. In all compounds studied, the sign of the reduced coupling constants nK(M13C) (n = 1, 2, 3) is positive. The same is true for 3K(MCC1H) and 3K(195PtCCM) (M = 29Si, 119Sn, 207Pb), whereas a negative sign was found for all 4K(MCCC1H) values.

Borgen, Gerd; Rise, Frode

doi: 10.1002/mrc.1260310110pmid: N/A

The conformation of ε‐caprolactam (1‐azacycloheptan‐2‐one) was studied by low‐temperature 2 D 1H and 13C NMR. Only one conformer was found in solution from the 13C NMR spectra recorded down to −155°C. This conformer was shown, by infrared spectroscopy, to be the same as that found in the crystals of ε‐caprolactam. A conformational process with a free energy of activation of 43 kJ mol−1, coalescence temperature −65°C, was interpreted as geminal 1H exchange. The assignments of the 13C and 1H shifts were made by 2 D NMR. Splitting of all averaged signals for geminal protons into axial and equatorial signals was demonstrated at −100°C.