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doi: 10.1002/(SICI)1097-458X(199605)34:5<329::AID-OMR889>3.0.CO;2-Cpmid: N/A
This review covers several high‐resolution NMR methods, including selective excitation, gradient selection and inverse spectroscopy. In this way, clean 1D spectra can be quickly obtained without extreme stability conditions because the classical difference spectroscopy is not involved. In addition, accurate NMR parameters can be easily extracted from such spectra in short measuring times, with high digital resolution and minimum data storage. For these reasons, these 1D experiments are a very useful alternative to multi‐dimensional experiments in structural and/or conformational studies of small‐ and medium‐sized molecules when a limited amount of information is desired. All these experiments are described using the shift operator formalism in order to predict a suitable gradient ratio that rephases the desired magnetization and to analyse the inherent sensitivity losses due to the gradient selection procedure compared with the conventional phase cycle procedure.
Musio, Roberta; Sciacovelli, Oronzo
doi: 10.1002/(SICI)1097-458X(199605)34:5<348::AID-OMR842>3.0.CO;2-1pmid: N/A
Theoretical calculations performed using the coupled Hartree–Fock perturbation theory (CHFPT) show that the MNDO approximation is superior to INDO and MINDO3 in computing nuclear spin coupling constants between directly bonded carbon and hydrogen in polysubstituted methanes, HCXYZ. The CHFPT–MNDO program is parametrized for the atoms of the second and third rows of the Periodic Table. Substituent effects on JHC are well reproduced except for the chlorine atom, whose effect is systematically underestimated (ca. 9 Hz).
Matasyoh, Josphat C.; Schuler, Paul; Stegmann, Hartmut B.; Lee Poyer, J.; West, Melinda; Janzen, Edward G.
doi: 10.1002/(SICI)1097-458X(199605)34:5<351::AID-OMR882>3.0.CO;2-Hpmid: N/A
A chiral spin trap, 5,5‐dimethyl‐3‐phenylpyrroline‐1‐oxide (5), and several hydroxylamines were synthesized. The structures and conformations of these compounds were investigated mainly by 1H NMR spectroscopy. This spin trap was used to trap carbon‐ and oxygen‐centered radicals. The corresponding hydrogen spin adduct was prepared by oxidizing the hydroxylamine to the nitroxide radical. By the combination of 1H NMR and EPR results, it was shown that the conformation with the phenyl group of C‐3 at the equatorial position is exclusively populated and its lifetime is long compared with the EPR time‐scale. Addition of carbon‐centered radicals to 5 leads to trans adducts whereas oxygen‐centered radicals formed cis isomers. These could be confirmed by the investigation of the monovalent oxidation products of the hydroxylamines. The preference for these conformers is explained by the competition between steric and stereoelectronic effects. Despite the racemic nature of 5 and the formation of a new chiral center(s) in the spin adducts, the presence of different diastereomers would not be observed by EPR whereas 1H NMR studies of some of the nitrones showed clear evidence of diastereomeric mixtures. However, the spin adduct of sec‐hydroxybutyl radical showed some linewidth effects which could be attributed to the presence of two groups of diastereomers that were resolved by ENDOR spectroscopy. In general, the spin adducts of 5 are closely related to those of the well known DMPO, but the presence of a phenyl substituent at the 3‐position results in a variation of the β‐H coupling constants. In contrast to DMPO, 5 can scavenge short‐lived radicals in aqueous and non‐aqueous solutions.
Cambie, Richard C.; Taylor, Michael J.; Rutledge, Peter S.; Stevenson, Ralph J.; Woodgate, Paul D.
doi: 10.1002/(SICI)1097-458X(199605)34:5<360::AID-OMR887>3.0.CO;2-4pmid: N/A
Conformational changes in N,N‐diethyl‐14‐hydroxy‐12,19‐dimethoxypodocarpa‐8,11,13‐triene‐13‐carboxamide were studied by variable‐temperature 1H and 13C NMR over the range 228–328 K using three hydrogen and six carbon indicator sites. The parameters ΔH≠ = 48.7±2.0 kJ mol−1, ΔS≠ = −23±7 J K mol−1 and ΔG298≠ = 55.6±4.3 kJ mol−1 were obtained, which were attributed to restricted rotation of the C‐CONEt2 bond. Restricted C—N rotation was also detected for which the free energy of activation at 250 K was ca. 53 kJ mol−1. Hydrogen bonding of the carboxamide to the adjacent hydroxy group can account for the low value of the C—N activation energy.
doi: 10.1002/(SICI)1097-458X(199605)34:5<365::AID-OMR880>3.0.CO;2-Apmid: N/A
The calculation of residual dipolar coupling effects between (observed) spin‐1/2 nuclei and quadrupolar neighbours in magic angle spinning NMR spectra may face complications if the nuclei involved have negative gyromagnetic ratios. A procedure is described with which these cases can be conveniently handled in order to avoid potentially confusing situations. Specific literature examples, involving (29Si, 14N) residual dipolar coupling effects in Si,N‐containing compounds, are discussed.
Peláez‐Arango, Elvira; Ferrero, Miguel; López‐Ortiz, Fernando
doi: 10.1002/(SICI)1097-458X(199605)34:5<368::AID-OMR890>3.0.CO;2-0pmid: N/A
The reaction products of the addition of 1‐(N,N‐diethylamine)prop‐1‐yne toN‐vinylcarbodiimides and ‐ketenimines have been identified. Two pyridine regioisomers can be obtained under thermal control according to a [4+2] or [2+2] cycloaddition mechanism. At temperatures above 100°C a competing electrocyclic ring closure of the heterocumulene is observed. The structures of the pyridines isolated are assigned by a combination of HMBC and NOE difference spectra.
Tedesco, Vanda; von Philipsborn, Wolfgang
doi: 10.1002/(SICI)1097-458X(199605)34:5<373::AID-OMR885>3.0.CO;2-0pmid: N/A
Rhodium‐103 and phosphorus‐31 chemical shifts and Rh,P coupling constants are reported for 59 rhodium complexes of the types RhCp*X2(PR3) (Cp* = C5Me5; X = Cl, Br, I; PR3 = PMe3, PiPr3, PnBu3, PPh3, PMePh2, PMe2Ph), RhCp*X(PR3)Ph (X = Cl, Br, I, PR3 = PMe3, PMe2Ph, PMePh2, PPh3), RhCp*Br(PR3)(aryl), and of miscellaneous mono‐ and dinuclear types. Rhodium shielding generally decreases as the steric requirement of the phosphine increases and a number of linear correlations with the Tolman cone angle of the phosphine ligand are reported. Variable para substitution on the aryl ligand has little effect on rhodium shielding.
Nishida, T.; Widmalm, G.; Sándor, P.
doi: 10.1002/(SICI)1097-458X(199605)34:5<377::AID-OMR893>3.0.CO;2-Upmid: N/A
Sources of artifacts in Hadamard spectra were investigated and identified as insufficient suppression of signals from solvent, protons attached to 12C atoms and error signals caused by non‐ideal excitation and the subsequent Hadamard transformation. Pulsed field gradients (PFG) effectively eliminate the former error signals while re‐optimized Hadamard‐shaped pulses produce practically ideal excitation and Hadamard transformation affords artifact‐free spectra. Long‐range 13C–1H coupling constants of 1,6‐anhydro‐β‐D‐galactopyranoside and 3‐acetylpyridine were determined by using a PFG pulse sequence and re‐optimized Hadamard‐8 shaped pulses.
Meltzheim, B.; Waegell, B.; Faure, R.; Germanaud, L.; Tort, F.
doi: 10.1002/(SICI)1097-458X(199605)34:5<383::AID-OMR881>3.0.CO;2-Bpmid: N/A
The reaction of phenalenone with various sulphur‐derived nucleophiles gave 1‐hydroxy‐9‐alkyl‐ or ‐aryl‐thiophenalenes. The complexity of the aromatic 1H and 13C signals does not allow straightforward assignments. These were rendered possible by the use of two‐dimensional 1H‐detected heteronuclear one‐bond (HMQC) and multiple‐bond (HMBC) correlation techniques.
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