CONTACT SHIFT and SHIFT REAGENTS
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The document discusses shift reagents, first introduced by Hinckley in 1969, which are used to manipulate NMR spectra without altering the magnetic field strength. It highlights the types of shift reagents, including lanthanide complexes and chiral shift reagents, alongside their effects on spectral simplification and resonance shifts. Additionally, it covers the advantages and disadvantages of using these reagents in organic chemistry analysis.
CONTACT SHIFT and SHIFT REAGENTS
- 1. SHIFT REAGENTS Presented By: K. Muthu Kannan II M.Sc.,Chemistry S.B.K.College Aruppukottai.
- 2. Complex spectra Shift reagents History Paramagnetic materials Contact shift Pseudo-contact shift Chiral shift reagents Advantages Disadvantages 2 CONTENTS
- 6. Shift reagents were first introduced by Hinckley in 1969. Shift reagents provide a useful method for spreading out normal NMR absorption pattens without increasing the strength of the applied magnetic field. The shift reagents are usually enolic ß-dicarbonyl complexes of a rare earth (lanthanide) metals and these complexes are mild Lewis acids. Europium is probably the most commonly used metal for shift reagents. Two of its widely used complexes are Eu(dpm)3 and Eu(fod)3. 6 HISTORY
- 7. 7 MOSTLY USED SHIFT REAGENTS
- 8. 8 OTHER SHIFT REAGENTS Pirkle’s alcohol TRISPHAT – tris(tetrachlorocatecholato)phosphate(-1)
- 9. 9 RARELY USED SHIFT REAGENTS
- 10. Complexes of europium, erbium, thulium and ytterbium shift resonances to down field (larger δ). Complexes of cerium, praseodymium, neodymium, samarium, terbium and holmium generally shift resonances to up field (lower δ). Olefins and arenes do not show Lanthanide shift reagents induced shifts since they cannot form complexes with the lanthanide ion. These lanthanide complexes produce spectral simplifications in the NMR spectrum of any compound with a relatively basic pair of electrons (an unshared pair) which can coordinate with Eu3+. Typically aldehydes, ketones, alcohols, thiols, ethers and amines all interact. The fully deuteriated derivative of Eu(fod)3 is also commercially available, and this eliminates the signals from ligand protons. 10 IMPORTANTS OF SHIFT REAGENTS
- 11. 11 EFFECT OF LANTHANIDE SHIFT REAGENT
- 12. Paramagnetic materials by shortening relaxation times cause line broadening.They also cause a shift in signals. Co and Ni cause several line broadening thus cannot be used. Lanthanide ions are found to cause line broadening. Europium and praseodymium are an extraordinary exception giving a very low shift broadening. The NMR spectrum of paramagnetic compounds cannot be obtained because the unpaired electrons broaden the spectrum by both electron spin - nuclear spin coupling mechanism. 12 PARAMAGNETIC MATERIALS
- 13. Contact shifts result from spin polarization conveyed through the molecular orbitals of the molecule. The origin of contact shift can be understood if we consider coupling between the electron and the nucleus, which would give a doublet in the NMR spectrum under conditions of slow electronic spin-lattice relaxation, but with a large (millions of Hz) coupling constant. With such a large coupling, the intensities of the two resonances are not equal, so their weighted mean position is not mid way between them. 13 CONTACT SHIFT
- 14. 14 Under fast relaxation conditions the doublet collapses back to a singlet at the weighted mean position, and so may fall some thousands of Hz away from the expected position. Contact shifts give a measure of unpaired spin density at the resonating nucleus, and so are particularly useful in studying the spin distribution in radical species, or in ligands in organometallic compound. CONTACT SHIFT
- 15. 15 PSEUDO-CONTACT SHIFT There is a through-space dipolar interaction between the magnetic moments of the electron and the resonating nucleus, which gives a dipolar shift, also some times known as a pseudo-contact shift. The chemical shift is caused by the dipolar coupling between the magnetic moments of the nucleus and of the unpaired electron. This situation is some what similar to the case of j-coupling where we would expect the NMR peak to split into a doublet due to the coupling with the magnetic moment of the unpaired electron.
- 16. 16 Lanthanide complexes give shifts primarily by the pseudo-contact mechanism. The principal factor is the distance between the metal ion and the proton; the shorter the distance, the greater the shift obtained. On the other hand, the direction of the shift depends on the lanthanide complex used. PSEUDO-CONTACT SHIFT
- 17. 17 EFFECT OF LANTHANIDE SHIFT REAGENT
- 18. 18 EFFECT OF LANTHANIDE SHIFT REAGENT
- 19. 19 A chiral shift reagents is a reagent used in analytical chemistry for determining the optical purity of a sample. Compounds must contain Lewis bases. Some of the more effective reagents developed are Eu(facam)3 (tris(3- trifluoroacetyl-d-camphorato)europium(III)) and Eu(hfc)3 (tris(3- heptafluoropropylhydroxymethylene)-d-camphorato)europium(III). CHIRAL SHIFT REAGENTS
- 20. 20 Often sufficient separation between the R and S enantiomers can be obtained so that the enantiomeric purity can be determined directly by NMR integration. CHIRAL SHIFT REAGENTS
- 21. 21 EFFECT OF CHIRAL SHIFT REAGENTS
- 22. Give spectra which are much easier to interpret. More easily obtained, don’t require high frequency instruments. Identifying enantiomeric mixtures in solution. No chemical manipulation of the sample required with the use of shift reagents. Shift reagents cause a small amount of line broadening. 22 ADVANTAGES DISADVANTAGES
- 23. Organic spectroscopy by L.D.S.Yadav. Introduction to spectroscopy by Pavia 4th edition. Organic spectroscopy by Willam kemp 3rd edition. https://www.researchgate.net/figure/Commonly-used-shift-reagents-top-and-first- generation-commercial-contrast-agents-for_fig1_37428740 https://inis.iaea.org/collection/NCLCollectionStore/_Public/05/125/5125697.pdf https://organicchemistrydata.org/hansreich/resources/nmr/?page=08-tech-07- lis%2F 23 REFERENCES
Editor's Notes
- #6: The action of the first type of shift reagent is shown by the fact that using an aromatic solvent instead of a nonaromatic one, yields a slightly different NMR pattern for the compound dissolved in it,. This phenomenon is called ASIS (aromatic solvent induced shifts). is usually very small (0.1 - 0.5 ppm), and therefore no really dramatic effects in NMR spectra will occur. As regards the diamagnetic shift reagents, the B(C,HL)# ion [ 35-37 ] and silicon, germanium, and iron phthalocyanines I 38-43 ] and germanium porphyrins [ 43-44 ] were found to be effective. Shifts as high as 11.4 ppra [43 ] were induced on NMR resonances of molecules bound to these reagents, vhile linebroadening was negligible. Unfortunately, their applicability is rather limited, since the B(C,H,.), ion is only effective with onium centers, while the other reagents must be complexed to the molecule of interest with a covalent bond, by means of a chemical reaction. Although paramagnetic shifts are usually much larger than shifts due to ring current shielding effects, they usually are accompanied with severe broadening of the NMR signals. This is illustrated by a number of transition metal complexes [45-60 j , such as Ni(II) and Co(II) acetylacetonates. When shifts induced by these complexes become substantial, linebroadening becomes serious. In this aspect, the radical anion shift reagents, Tp'.Na , Cor'.Na and Cor',Li are much more effective, since large induced shifts, together with little linebroadening are observed (see chapter V ) . Unfortunately, as already pointed out in the introduction, their applicability is probably restricted to the glymes.
- #9: TRISPHAT (full name tris(tetrachlorocatecholato)phosphate(1−)) is an inorganic anion with the formula P(O2C6Cl4)-3often prepared as the tributylammonium ((C4H9)3NH+) or tetrabutylammonium ((C4H9)4N+ salt. The anion features phosphorus(V) bonded to three tetrachlorocatecholate (C6Cl4O22-) ligands. This anion can be resolved into the axially chiral enantiomers, which are optically stable (the picture shows the Δ enantiomer). The TRISPHAT anion has been used as a chiral shift reagent for cations.[1] It improves the resolution of 1H NMR spectra by forming diastereomeric ion pairs. Preparation; The anion is prepared by treatment of phosphorus pentachloride with tetrachlorocatechol followed by a tertiary amine gives the anion: PCl5 + 3 C6Cl4(OH)2 → H[P(O2C6Cl4)3] + 5 HClH[P(O2C6Cl4)3] + Bu3N → Bu3NH+ [P(O2C6Cl4)3]−Using a chiral amine, the anion can be readily resolved. Pirkle's alcohol is an off-white, crystalline solid that is stable at room temperature when protected from light and oxygen. This chiral molecule is typically used, in nonracemic form, as a chiral shift reagent in nuclear magnetic resonance spectroscopy, in order to simultaneously determine absolute configuration and enantiomeric purity of other chiral molecules. The molecule is named after William H. Pirkle, Professor of Chemistry at the University of Illinois whose group reported its synthesis and its application as a chiral shift reagent.
- #11: Arenes – Aromatic hydrocarbons which contain benzene ring. Cn H2n – 6 , n is greater than or equal to 6.
- #13: Line broadening reasons: Heisenberg uncertainty principle ∆E x ∆t = h/2∏ Collision broadening Doppler broadening. Relaxation time; The spectral line width is inversely proportional to the lifetime of the excited state (i.e. higher energy state). The shorter the lifetime of the excited state greater is line width. An efficient relaxation process involves shorter time T1 and results in broadening of the absorption peak. Paramagnetic nuclear magnetic resonance spectroscopy refers to nuclear magnetic resonance (NMR) spectroscopy of paramagnetic compounds. ➢ Although most NMR measurements are conducted on diamagnetic compounds, paramagnetic samples are also amenable to analysis and give rise to special effects indicated by a wide chemical shift range and broadened signals. ➢ Paramagnetism diminishes the resolution of an NMR spectrum to the extent that coupling is rarely resolved. ➢ Nonetheless spectra of paramagnetic compounds provide insight into the bonding and structure of the sample. ➢ For example, the broadening of signals is compensated in part by the wide chemical shift range (often 200 ppm). Since paramagnetism leads to shorter relaxation times (T1), the rate of spectral acquisition can be high. Inorganic – any metal that has unpaired e- will cause chemical shift range to be extremely large Proteins – many proteins contain paramagnetic ions (often Fe+3) in their active site. But one can also substitute paramagnetics (e.g., Co+2 for Zn+2) into the protein to spread out the chemical shifts near the active site
- #14: Scalar coupling - Through covalent bond the interaction between two NMR active nuclei each other is called spin – spin coupling / j –coupling / Covalent bond mediated coupling.