A new method for spin-spin coupling patterns analysis in high resolution NMR spectroscopy (original) (raw)
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1D and 2D MAS NMR Spectra of a Dipolar-Coupled Homonuclear Spin- Pair
Journal of Magnetic Resonance, 1997
An analytical approach based on the average Hamiltonian theduring the evolution of the spin system have been suggested ory is proposed for efficient calculation of the MAS NMR spectra (4, 10, 14-16). of a dipolar-coupled homonuclear spin-1 2 pair. For this purpose a The lineshapes of a coupled spin-1 2 pair in a MAS NMR superoperator formalism is developed which allows one to describe spectrum have been successfully simulated by Levitt et al. the spectra over a broad span of sample spinning rates, including (5) by means of numerical calculations in which one sample the exact rotational resonances. This formalism can also be applied rotation period was divided into a large number of small to the description of 2D polarization exchange spectra, which in steps of duration Dt , during which the Hamiltonian may many cases turns out to be useful for measuring the coupling be assumed to be time independent. A good agreement strength. The experimental MAS NMR and the 2D spectra of between the experimental spectra and the calculated spectra doubly 13 C-labeled zinc acetate were found to be in good agreement was established this way, and the analytical formula of the with the calculated spectra. ᭧ 1997 Academic Press lineshape was given for the case of vanishing CSI anisotropy. Similar results were obtained by Nakai and McDowell (7) and Schmidt and Vega (8) using Floquet theory (17). Such numerical calculations, however, need considerable 53
Stereochemical dependence of NMR geminal spin-spin coupling constants
Magnetic Resonance in Chemistry, 2009
In this work it was sought to explore the versatility of geminal spin-spin coupling constants, 2 J XY SSCCs, as probes for stereochemical studies. A set of compounds, where their experimental 2 J XY SSCCs through the X-C-Y molecular fragment are predicted to be sensitive to hyperconjugative interactions involving either bonding or antibonding orbitals containing the C carbon atom ('coupling pathway'), were analyzed. SSCC calculations were performed for some selected examples using the second order polarization propagator approximation (SOPPA) method or within the DFT-B3LYP framework. Hyperconjugative interactions were calculated within the Natural Bond Orbital (NBO) approach. Results are condensed in two qualitative rules: Rule I M -hyperconjugative interactions transferring charge into the coupling pathway yield a positive increase to the Fermi contact (FC), contribution to 2 K XY reduced spin-spin coupling constants (RSSCC), and Rule II M -hyperconjugative interactions transferring charge from the coupling pathway yield a negative increase to the FC contribution to 2 K XY RSSCC.
Journal of Magnetic Resonance, 2007
The robustness of the refocused INADEQUATE MAS NMR pulse sequence for probing through-bond connectivities has been demonstrated in a large range of solid-state applications. This pulse sequence nevertheless suffers from artifacts when applied to multispin systems, e.g. uniformly labeled 13 C solids, which distort the lineshapes and can potentially result in misleading correlation peaks. In this paper, we present a detailed account that combines product-operator analysis, numerical simulations and experiments of the behavior of a three-spin system during the refocused INADEQUATE pulse sequence. The origin of undesired anti-phase contributions to the spectral lineshapes are described, and we show that they do not interfere with the observation of long-range correlations (e.g. two-bond 13 C-13 C correlations). The suppression of undesired contributions to the refocused INADEQUATE spectra is shown to require the removal of zero-quantum coherences within a z-filter. A method is proposed to eliminate zero-quantum coherences through dephasing by heteronuclear dipolar couplings, which leads to pure in-phase spectra. 4 Abbreviations: TOBSY, total through-bond spectroscopy; INADE-QUATE, incredible natural abundance double quantum transfer experiment; CR, composite refocusing; UC2QF COSY, uniform-sign cross-peak double quantum filtered correlation spectroscopy; TQ, triplequantum; DD, dipole-dipole; CSA, chemical shift anisotropy, DQ, double quantum; ZQ, zero-quantum; CP, cross polarization; PDSD, protondriven spin diffusion.
Spinning Sidebands in Slow-Magic-Angle-Spinning NMR Spectra Arising from TightlyJ-Coupled Spin Pairs
Journal of Magnetic Resonance, 1997
Complex spinning sidebands are observed in magic-angle-spintensor is the single-crystal NMR method (5). When large ning (MAS) NMR spectra arising from isolated tightly J-coupled single crystals required for this experiment are unavailable, spin pairs under slow spinning conditions. Such spinning sidethe dipolar-chemical-shift NMR method is a useful alternabands are sensitive to the magnitude and relative orientation of tive for isolated spin-pair systems (6-8). However, successthe chemical-shift tensors, the dipolar-coupling tensor, and the ful application of this technique requires a relatively large sign of the indirect spin-spin ( J) coupling. We show that it is dipolar interaction between the two spins of the spin pair.
Chemical Physics Letters, 2008
Internuclear couplings between selected homonuclear spin pairs in a multiplylabelled spin system are determined by NMR spin echo experiments in the solid state. The spin echoes are induced by an amplitude-modulated shaped pulse. The time shift in the echo modulation curve is treated by average Hamiltonian theory and verified by numerical simulation. The J-couplings may be estimated by experiments on samples spinning at the magic angle, while the direct dipoledipole couplings may be estimated by off-magic-angle spinning. The concept is tested on a uniformly 13 C-enriched sample of L-histidine hydrochloride monohydrate.
1H and13C NMR chemical shifts and spin-spin coupling constants intrans- andcis-decalins
Magnetic Resonance in Chemistry, 2005
The NMR parameters characterizing the spectra of trans- and cis-decalins were determined from theoretical calculations and experimental spectra. The calculated values of the shielding constants are in good agreement with the measured chemical shifts, with a small but noticeable difference in accuracy for the bridgehead atoms. Of all the spin-spin coupling constants, only most of (1)J(C,C) and (1)J(C,H) values could be extracted from the spectra, and the corresponding computed values are in good agreement with experiment. It appears that the applied density functional theory (DFT) approach overestimates slightly the J(C,C) coupling and underestimates the differences between one-bond (1)J(C,H) coupling constants. For all these constants [J(C,C), J(C,H) and J(H,H)] through one to three bonds, which could not be obtained experimentally, the predicted values are in good agreement with the general rules relating spin-spin coupling to the number and spatial arrangement of the intervening bonds.
Magnetic Resonance in Chemistry, 2008
In this work 3 J CH spin-spin coupling constants (SSCCs) for the cis-and trans-conformers for α-X-acetamides (X = F, Cl, Br and CN) (1-4) were studied in detail since they were found to be notably different for both conformers. These differences are rationalized as originating in the changes of the strong negative hyperconjugative interactions that take place within the carbonyl group. Such changes are found to depend not only on conformation, but also on solvent. For the cis-conformers there is a close proximity between the X-substituent and the in-plane oxygen lone pair of pure p character, which affects notably their respective negative hyperconjugative interactions. Both the efficiency for transmitting the Fermi contact (FC) term through the coupling pathway of 3 J CH SSCCs and its potential as a probe to study the stereochemical properties of the XH 2 C group are discussed.