Chemical shift nucleus, shielding

Since the short relaxation times associated with a quadrupolar nucleus drastically reduce the time delay to be applied in an NMR experiment between two pulses, measuring times are short or, in other words, distinct NMR signals can often be detected with a limited time spent down to micromolar concentrations. Along with this apparent advantage, quadrupolar nuclei provide information in addition to the classical parameters chemical shift (or shielding) and nuclear spin-spin coupling constants. Variations in linewidths for quadrupolar nuclei are another sensitive quantity allowing for the evaluation of the electronic and the steric situation in the first coordination sphere of a vanadium compound, its periphery, its (local) symmetry and its interaction with the matrix, i.e. counter-ions, solvent molecules and other constituents present in solution. 

Shielding tensor The electron motion around the nucleus induced by the applied magnetic field generates an internal magnetic field. The induced field is said to shield or screen the nucleus from the applied field and is observed as the chemical shift. The shielding tensor defines the principal values and orientation of the chemical shift in the molecular frame. The maximum difference between the principal values defines the CSA. 

Chemical shift (Section 13.4) A measure of how shielded the nucleus of a particular atom is. Nuclei of different atoms have different chemical shifts, and nuclei of the same atom have chemical shifts that are sensitive to their molecular environment. In proton and carbon-13 NMR, chemical shifts are cited as 8, or parts per million (ppm), from the hydrogens or carbons, respectively, of tetramethylsilane. 

Each electronically distinct 1H or 13C nucleus in a molecule comes into resonance at a slightly different value of the applied field, thereby producing a unique absorption signal. The exact position of each peak is called the chemical shift. Chemical shifts are caused by electrons setting up tiny local magnetic fields that shield a nearby nucleus from the applied field. 

The chemical shifts, Sp, of substituted arylphosphonic acids (9) have been found to be linearly related to the Hammett a and Taft ojt and cti parameters. The shielding of the phosphorus nucleus increases with the electron-withdrawing properties of the substituents, which is analogous... 

The difference in resonance NMR frequency of a chemically shielded nucleus measured relative to that of a suitable reference compound is termed chemical shift [164,165], and is a measure of the immediate electromagnetic environment of a nucleus. While the chemical shift depends on the Bo field, J does not. Chemical shifts, which cover a range of about 10 ppm for protons (i.e. 600 Hz in case of a 14.1 kG magnetic field) and 250 ppm for 13C, are the substance of NMR. 

Assignment of an anti configuration to a [2](2,5)furano[2](l,4)naph-thalenophane (42) synthesized by Wasserman and Keehn 65> followed from a comparison of its 1H—NMR spectrum with that of [2]paracyclo-[2](2,5)furanophane (41 a) 66>. The absorption band assigned to the /3-furanoid proton Ha in the spectrum of 42 (r=4.38) appears in the same region as the corresponding band for 41 a. In the case of syn-42, a chemical shift would be expected due to the transannular shielding effect of the naphthalene nucleus. 

Besides, information on intermolecular interactions has been derived in these studies from complexation-induced shifts (CIS). The chemical shift is an indicator for the shielding of a nucleus and thus for the electronic state of a specific proton. Since the electronic environment may change on complexation, CIS can be used to monitor where host-guest contacts may take place. If these interactions occur stereoselectively, the CIS will be different for the two guest enantiomers (AS distinct from 0) giving possibly some insight into the chiral recognition mechanism. 

The effective field experienced by a nucleus in a chemical compound is generally different from the applied field due to the shielding by the distribution of electrons around the nucleus. A shift in NMR frequency, known as the chemical shift, proportional to applied field results. For closed... 

Examination of the C-NMR spectra of roseadine (23) (Table XI) through comparison with vindoline (3) and leurosine (11) permitted the assignment of all carbons of the dihydroindole unit. The carbons of the indole nucleus were assigned by comparison with vinblastine (1), and the presence of three deshielded carbons, a methine carbon at 8 142.9 and two quaternary carbons at 8 133.2 and 169.2, were observed. The latter was assigned to the methoxycarbonyl carbon, which is shielded somewhat from its characteristic chemical shift of 8 174 1 ppm in the vinblastine series by attachment of an olefinic unit. The other two deshielded carbons at 8 133.2 and 142.9 could be assigned as C-18 and C-17, respec-... 

Each ifi nucleus is shielded or screened by the electrons that surround it. Consequently each nucleus feels the influence of the main magnetic field to a different extent, depending on the efficiency with which it is screened. Each nucleus with a different chemical environment has a slightly different shielding and hence a different chemical shift in the H NMR spectrum. Conversely, the number of different signals in the iff NMR spectrum reflects the number of chemically distinct environments for iff in the molecule. Unless two iff environments are precisely identical (by symmetry) their chemical shifts must be different. When two nuclei have identical molecular environments and hence the same chemical shift, they are termed chemically equivalent or isochronous nuclei. Non-equivalent nuclei that fortuitously have chemical shifts that are so close that their signals are indistinguishable are termed accidentally equivalent nuclei. 

Any effect which alters the density or spatial distribution of electrons around a nucleus will alter the degree of shielding and hence its chemical shift. H chemical shifts are sensitive to both the hybridisation of the atom to which the H nucleus is attached sp, sp etc.) and to electronic effects (the presence of neighbouring electronegative/electropositive groups). 

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