1,721,022 research outputs found
Selective J-resolved spectra: A double pulsed field gradient spin-echo approach
No full textA simple method to obtain selective J-resolved spectra is presented, which relies on the refocusing properties of double pulsed field gradient spin-echoes and provides unambiguous assignment of the measured coupling constants. The proposed examples show how this method is of general applicability, and requires no more than a simple optimization strategy to produce artifact-free spectra. Examples of application include the determination of a small, long-range coupling constant (0.7 Hz) in trans-retinal and H-alpha couplings in a tripeptide
Characterization of Paramagnetic Reactive Intermediates: Predicting the NMR Spectra of Iron(IV)-Oxo Complexes by DFT
No full textThe relative energies of spin states of several iron(IV)–oxo complexes and related species have been calculated
with DFT methods by employing the B3LYP* functional. We show that such calculations can predict the correct
ground spin state of FeIV complexes and can then be used to determine the 1H NMR spectra of all spin states; the
spectral features are remarkably different, hence calculated paramagnetic 1H NMR spectra can be used to support the
structure elucidation of numerous paramagnetic complexes. Applications to a number of stable and reactive iron(IV)–oxo
species are described
Detecting Intermolecular NOEs by Means of a Novel DPFGSE Pulse Sequence. Application to the Solvation of Carboydrates in Binary Mixtures
No full textWe present a pulse sequence based on solute-to-solvent NOE enhancement and aimed at the detection of intermolecular NOE's. Thus, a W3 pulse cluster is used to selectively filter the solvent signals in a DPFGSE sequence. The sequence has been tested on a sample of glucose dissolved in two binary aqueous mixtures (water-acetonitrile and water-DMSO). We show how the resulting enhancements may derive from intermolecular cross-relaxation or, in the water-DMSO sample, also from chemical exchange. In each case, a quantitative interpretation of the data is also supplied, both in terms of local enrichment in one specific solvent (preferential solvation), and by means of a kinetic model for a two-site chemical exchange
Dynamic covalent capture of hydrazides by a phosphonate-target immobilized on resin
Full text linkA protocol is described that permits the self-selection of hydrazides from a small library by a phosphonate-target immobilized on resin. Hydrazides are captured by a neighbouring aldehyde group through reversible hydrazone bond formation. Stabilizing intramolecular interactions between the phosphonate-target and functional groups of the hydrazides drive the selection process. The phosphonate-target is introduced onto commercially available Tentagel resin through straightforward synthetic steps. The functionalized resin could be conveniently characterized by HR-MAS NMR spectroscopy using a recently developed transverse relaxation filter that eliminates the strong phase defects commonly observed with CPMG sequences. In addition, a protocol was developed to quantitatively remove the captured hydrazides from resin in order to analyse their composition by LC/MS. Kinetic experiments were used to study hydrazone formation and exchange on resin yielding similar results to those obtained previously in solution. Competition experiments showed that the system reaches thermodynamic equilibrium if multiple hydrazides are added to the resin. Finally, competition experiments showed that the immobilized phosphonate-target indeed amplifies the capture of those hydrazides able to develop stabilizing interactions with the target. Importantly, the obtained amplification profile was nearly identical to the ones obtained previously in solution studies. Notably, the observed amplification factors for the self-selected hydrazides were higher, which was attributed to steric effects imposed by the resin
Predicting C-13 NMR spectra by DFT calculations
No full text13C chemical shifts and nJCH coupling constants have been determined both experimentally (by means of J-resolved NMR spectroscopy) and theoretically (by DFT calculations) for a series of organic molecules. With the exception of halogen-bonded carbon nuclei, a good correlation is observed between experimental and calculated data. The magnitude of the most important contributions to the spin-spin coupling constant (Fermi-contact, diamagnetic and paramagnetic spin-orbit contributions) have been determined. The spin-orbit terms are negligible or cancel out (1JCH and 3JCH), thus leaving the contact term as the only relevant contribution, but become important for 2JCH in aromatic (but not in aliphatic) compounds. Relativistic effects on the 13C chemical shift of carbon bonded to a fairly heavy atom (bromine) have also been investigated. Finally, conformational effects on the long-range nJCH coupling constants has been investigated in a model alkane derivative (n-butyl chloride). The implications to structure prediction and determination by NMR are discussed
Aggregation Behavior of Octyl Viologen Di[bis(trifluoromethanesulfonyl)amide] in Nonpolar Solvents
No full textThe aggregation behavior in nonpolar solvents of the octyl viologen (OV) salt with the hydrophobic anion bis(trifluoromethanesulfonyl)amide (Tf2N−) has been investigated. 1H and 19F NMR, ESI-MS and DFT calculations suggest that large aggregates are formed in toluene, benzene and chloroform, where the salt is highly soluble. The lifetime of the aggregates is long enough to be detected as independent species by 1H and 19F NMR spectroscopy, together with the smaller neutral OV(Tf2N)2 cluster. This behavior is quite at variance with usual NMR detected equilibria where only average signals are generally observed. Large aggregates are also observed in ESI-MS spectra of toluene and chloroform solutions despite the well-known low-coordinating ability of Tf2N−. It is suggested that the structure of the large aggregates mimics the thermotropic smectic phase that this system exhibits near room temperature
Predicting the 1H and 13C NMR Spectra of Paramagnetic Ru(III) Complexes by DFT
No full textNuclear shieldings, including the Fermi-contact and pseudocontact terms, have been calculated with DFT (non-relativistic and relativistic) methods in several Ru(III) complexes, thereby predicting 1H and 13C paramagnetic shifts. A fair agreement with experimental values is observed. Structural, magnetic and dynamic parameters have also been input to the Solomon-Bloembergen equation in order to predict signal lineshapes. It is shown that DFT-predicted paramagnetic shifts can greatly aid in obtaining and understanding NMR spectra of paramagnetic Ru(III) complexes
Predicting the NMR Spectra of Paramagnetic Molecules by DFT: Application to Organic Free Radicals and Transition-Metal Complexes
No full textNuclear shieldings, including the Fermi-contact and pseudocontact terms, have been calculated with DFT methods in a variety of open-shell molecules (nitroxides, aryloxyl and various transition-metal complexes), thereby predicting 1H and 13C chemical shifts. In general, when experimental data are reliable a good agreement with experimental values is observed, thus demonstrating the predictive power of DFT also in this context. However, the general accuracy is lower than for closed-shell species. A few inconsistencies in literature values are reconciled by re-assigning some shifts. Structural, magnetic and dynamic parameters have also been input to the Solomon-Bloembergen equation in order to predict signal lineshapes, in particular those of signals that are difficult to locate or undetectable. Guidelines are provided to predict the order of magnitude of relaxation rates. It is shown that DFT-predicted paramagnetic shifts can greatly aid in obtaining and understanding NMR spectra of paramagnetic molecules, which generally require different experimental strategies and exhibit problems in detection and assignment
Toward the complete prediction of the H-1 and C-13 NMR spectra of complex organic molecules by DFT methods: Application to natural substances
No full textThe NMR parameters (1H and 13C chemical shifts and coupling constants) for a series of naturally occurring molecules have been calculated mostly with DFT methods, and their spectra compared with available experimental ones. The comparison includes strychnine as a test case, as well as some examples of recently isolated natural products (corianlactone, daphnipaxinin, boletunone B) featuring unusual and/or crowded structures and, for boletunone B, being the subject of a recent revision. Whenever experimental spectra were obtained in polar solvents, the calculation of NMR parameters was also carried out with the IEF-PCM continuum method. The computed results generally show a good agreement with experiment, as judged not only by statistical parameters but by visual comparison of line spectra as well. The origin of remaining discrepancies is traced to the incomplete modeling of conformational and specific solvent effects
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