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NMR N uclear M agnetic R esonance

NMR N uclear M agnetic R esonance. NMR for Organometallic compounds. Index. NMR-basics. H-NMR. NMR-Symmetry. Heteronuclear-NMR. Dynamic-NMR. NMR and Organometallic compounds. NMR in Organometallic compounds spins 1/2 nuclei.

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NMR N uclear M agnetic R esonance

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  1. NMRNuclear Magnetic Resonance NMR for Organometallic compounds Index NMR-basics H-NMR NMR-Symmetry Heteronuclear-NMR Dynamic-NMR NMR and Organometallic compounds

  2. NMR in Organometallic compoundsspins 1/2 nuclei For small molecules having nuclei I=1/2 : Sharp lines are expected W1/2 (line width at half height) = 0-10 Hz If the nuclei has very weak interactions with the environment, Long relaxation time occur (109Ag => T1 up to 1000 s !!!) This makes the detection quite difficult!

  3. Isotope Nat. Abun-dance % () 107 rad T-1 s-1 Frequency (MHz) Rel. Receptivity 1H 99.985 26.7519 100.0 1.00 3H - 28.535 106.7 -- 3He 0.00013 -20.380 76.2 5.8 * 10-7 13C 1.11 6.7283 25.1 1.8 * 10-4 15N 0.37 -2.712 10.1 3.9 * 10-6 19F 100.0 25.181 94.1 8.3 * 10-1 29Si 4.7 -5.3188 19.9 3.7 * 10-4 31P 100.0 10.841 40.5 6.6 * 10-2 57Fe 2.2 0.8661 3.2 7.4 * 10-7 77Se 7.6 5.12 19.1 5.3 * 10-4 89Y 100.0 -1.3155 4.9 1.2 * 10-4 103Rh 100.0 -0.846 3.2 3.2 * 10-5 107Ag 51.8 -1.087 4.0 3.5 * 10-5 109Ag 48.2 -1.250 4.7 4.9 * 10-5 111Cd 12.8 -5.6926 21.2 1.2 * 10-3 113Cd 12.3 -5.9550 22.2 1.3 * 10-3 NMR in Organometallic compoundsNMR properties of some spins 1/2 nuclei Index

  4. Isotope 117Sn Nat. Abundance % 7.6 Magnetogyric ratio () 107 rad T-1 s-1 -9.578 35.6 Relative NMR frequency (MHz) Rel. Receptivity 3.5 * 10-3 119Sn 8.6 -10.021 37.3 4.5 * 10-3 125Te 7.0 -8.498 31.5 2.2 * 10-3 129Xe 26.4 -7.441 27.8 5.7 * 10-3 169Tm 100.0 -2.21 8.3 5.7 * 10-4 171Yb 14.3 4.712 17.6 7.8 * 10-4 183W 14.4 1.120 4.2 1.1 * 10-5 187Os 1.6 0.616 2.3 2.0 * 10-7 195Pt 33.8 5.768 21.4 3.4 * 10-3 199Hg 16.8 4.8154 17.9 9.8 * 10-4 203Tl 29.5 15.436 57.1 5.7 * 10-2 205Tl 70.5 15.589 57.6 1.4 * 10-1 207Pb 22.6 5.540 20.9 2.0 * 10-3 Spin 1/2

  5. Q = quadrupole moment qzz = electric field gradient tc = correlation time I = spin quantum number (2I + 3) Q2 q2zztc W1/2 ~ I2 (2I -1) NMR in Organometallic compoundsspins > 1/2 nuclei These nuclei possess a quadrupole moment (deviation from spherical charge distribution) which cause extremely short relaxation time and extremely large linewidth W1/2 (up to 50 KHz) Narrow lines can be obtained for low molecular weight (small tc) and if nuclei are embedded in ligand field of cubic (tetrahedral, octahedral) symmetry (qzz blocked)

  6. Isotope 2H Spin 1 Abun-dance % 0.015 () 107 rad T-1 s-1 4.1066 Freq. (MHz) 15.4 Rel. Recep-tivity 1.5 * 10-6 Quadrupole moment10-28m2 2.8 * 10-3 6Li 1 7.4 3.9371 14.7 6.3 * 10-4 -8 * 10-4 7Li 3/2 92.6 10.3975 38.9 2.7 * 10-1 -4 * 10-2 9Be 3/2 100.0 -3.7596 14.1 1.4 * 10-2 5 * 10-2 10B 3 19.6 2.8746 10.7 3.9 * 10-3 8.5 * 10-2 11B 3/2 80.4 8.5843 32.1 1.3 * 10-1 4.1 * 10-2 14N 1 99.6 1.9338 7.2 1.0 * 10-3 1 * 10-2 17O 5/2 0.037 -3.6279 13.6 1.1 * 10-5 -2.6 * 10-2 23Na 3/2 100.0 7.0801 26.5 9.3 * 10-2 1 * 10-1 25Mg 5/2 10.1 -1.639 6.1 2.7 * 10-4 2.2 * 10-1 27Al 5/2 100.0 6.9760 26.1 2.1 * 10-1 1.5 * 10-1 33S 3/2 0.76 2.055 7.7 1.7 * 10-5 -5.5 * 10-2 35Cl 3/2 75.5 2.6240 9.8 3.6 * 10-3 -1 * 10-1 37Cl 3/2 24.5 2.1842 8.2 6.7 * 10-4 -7.9 * 10-2 39K 3/2 93.1 1.2498 4.7 4.8 * 10-4 4.9 * 10-2 47Ti 5/2 7.3 -1.5105 5.6 1.5 * 10-4 2.9 * 10-1 49Ti 7/2 5.5 -1.5109 5.6 2.1 * 10-4 2.4 * 10-1 51V 7/2 99.8 7.0453 26.3 3.8 * 10-1 -5 * 10-2 55Mn 5/2 100.0 6.608 24.7 1.8 * 10-1 4 * 10-1 NMR properties of some spins quadrupolar nuclei

  7. Chemical shift for organometallic In molecules, the nuclei are screened by the electrons. So the effective field at the nucleus is: Beff = B0(1-) Where  is the shielding constant. The shielding constant has 2 terms: d (diamagnetic) and p (paramagnetic) d - depends on electron distribution in the ground state p - depends on excited state as well. It is zero for electrons in s-orbital. This is why the proton shift is dominated by the diamagnetic term. But heavier nuclei are dominated by the paramagnetic term. Index

  8. Symmetry Non-equivalent nuclei could "by accident" have the same shift and this could cause confusion. Some Non-equivalent group might also become equivalent due to some averaging process that is fast on NMR time scale. (rate of exchange is greater than the chemical shift difference) e.g. PF5: Fluorine are equivalent at room temperature (equatorial and axial positions are exchanging by pseudorotation) Index

  9. Symmetry in Boron compounds

  10. M = C M = Si M = Ge MH4 0.1 3.2 3.1 MH3I 2.0 3.4 3.5 MH3Br 2.5 4.2 4.5 MH3Cl 2.8 4.6 5.1 (MH3)2O 3.2 4.6 5.3 MH3F 4.1 4.8 5.7 Proton - NMR Increasing the 1 s orbital density increases the shielding Shift to low field when the metal is heavier (SnH4 -  = 3.9 ppm) Index

  11. Proton – NMR : Chemical shift • Further contribution to shielding / deshielding is the anisotropic magnetic susceptibility from neighboring groups (e.g. Alkenes, Aromatic rings -> deshielding in the plane of the bound) • In transition metal complexes there are often low-lying excited electronic states. When magnetic field is applied, it has the effect of mixing these to some extent with the ground state. • Therefore the paramagnetic term is important for those nuclei themselves => large high frequency shifts (low field). The protons bound to these will be shielded ( => 0 to -40 ppm) (these resonances are good diagnostic. ) • For transition metal hydride this range should be extended to 70 ppm! • If paramagnetic species are to be included, the range can go to 1000 ppm!! Index

  12. Proton NMR and other nuclei • The usual range for proton NMR is quite small if we compare to other nuclei: • 13C => 400 ppm • 19F => 900 ppm • 195Pt => 13,000 ppm !!! • Advantage of proton NMR : Solvent effects are relatively small • Disadvantage: peak overlap Index

  13. Chemical shifts of other element There is no room to discuss all chemical shifts for all elements in the periodical table. The discussion will be limited to 13C, 19F, 31P *as these are so widely used. Alkali Organometallics (lithium) will be briefly discuss For heavier non-metal element we will discuss 77Se and 125Te. For transition metal, we will discuss 55Mn and 195Pt Index

  14. Alkali organometallics: Organolithium For Lithium: we have the choice between 2 nuclei: 6Li : Q=8.0*10-4 a=7.4% I=1 7Li : Q=4.5*10-2 a=92.6% I=3/2 6Li : Higher resolution7Li : Higher sensitivity 7Li NMR : larger diversity of bonding compare to Na-Cs (ionic) • Solvent effects are important (solvating power affects the polarity of Li-C bond and govern degree of association • d covers a small range: 10 ppm • Covalent compound appear at low field (2 ppm range) • Coupling 1JC-Li between carbon and Lithium indicate covalent bond

  15. Organolithium

  16. Boron NMR For Boron: we have the choice between 2 nuclei: 10B : Q= 8.5 * 10-2 a=19.6% I=3 11B : Q= 4.1 * 10-2 a=80.4% I=3/2 11B : Higher sensitivity

  17. Boron NMR

  18. Boron NMR

  19. Symmetry in Boron compounds Index

  20. JF-10B n10B = JF-11B n11B 11B coupling with Fluorine: 19F-NMR 10B : Q= 8.5 * 10-2 a=19.6% n=10.7 I=3 2nI+1 = 7 11B : Q= 4.1 * 10-2 a=80.4% n=32.1 I=3/2 2nI+1 = 4 Boron can couple to other nuclei as shown here on 19F-NMR Isotopic shift 19F-NMR 11BF4 NaBF4 / D2O 10BF4 JBF=0.5 Hz JBF=1.4 Hz

  21. C13 shifts • Saturated Carbon appear between 0-100 ppm with electronegative substituents increasing the shifts. • CH3-X : directly related to the electronegativity of X. • The effects are non-additive: CH2XY cannot be easily predicted • Shifts for aromatic compounds appear between 110-170 ppm • -bonded metal alkene may be shifted up to 100 ppm: shift depends on the mode of coordination • one extreme shift is CI4 = -293 ppm !!! • Metal carbonyls are found between 170-290 ppm. (very long relaxation time make their detection very difficult) • Metal carbene have resonances between 250-370 ppm Index

  22. F-19 shifts Wide range: 900 ppm! And are not easy to interpret. The accepted reference is now: CCl3F. With literature chemical shift, care must be taken to ensure they referenced their shifts properly. Sensitive to: • electronegativity • Oxidation state of neighbor • Stereochemistry • Effect of more distant group Index

  23. F-19 shifts The wide shift scale allow to observe all the products in the reaction of : WF6 + WCl6 --> WFnCln-6 (n=1-6) Index

  24. Sn shifts

  25. Dynamic NMR p261

  26. C13

  27. Cycloheptatriene

  28. Dynamic NMR

  29. 1H-NMR

  30. P-31 Shifts The range of shifts is ± 250 ppm from H3PO4 Extremes: • - 460 ppm for P4 • +1,362 ppm phosphinidene complexe: tBuP[Cr(CO)5]2 • Interpretation of the shifts is not easy : there seems to be many contributing factors • PIII covers the whole normal range: strongly substituent dependant • PV narrower range:  - 50 to  + 100. • Unknown can be predicted by extrapolation or interpolation • PX2Y or PY3 can be predicted from those for PX3 and PXY2 • The best is to compare with literature values. Index

  31. P-31 Shifts Index

  32. Other nuclei: Selenium, Telurium There are many analogies between Phosphorus and Selenium chemistry. There are also analogies between the chemical shifts of 31P and 77Se but the effect are much larger in Selenium! For example: Se(SiH3)2 and P(SiH3)3 are very close to the low frequency limit (high field) The shifts in the series SeR2 and PR3 increase in the order R= Me < Et < Pri < But There is also a remarkable correlation between 77Se and125Te. (see picture next slide) Index

  33. Correlation between Tellurium and Selenium Shifts Index

  34. Manganese-55 • Manganese-55 can be easily observed in NMR but due to it’s large quadrupole moment it produces broad lines • 10 Hz for symmetrical environment e.g. MnO4- • 10,000 Hz for some carbonyl compounds. • It’s shift range is => 3,000 ppm • As with other metals, there is a relationship between the oxidation state and chemical shielding • Reference: MnVII : d = 0 ppm (MnO4-) • MnI : d –1000 to –1500 • Mn-I : d –1500 to -3000 • 55Mn chemical shifts seems to reflect the total electron density on the metal atom Index

  35. Pt-195 : coupling with protons CSA relaxation on 195Pt can have unexpected influence on proton satellites. CSA relaxation increases with the square of the field. If the relaxation (time necessary for the spins to changes their spin state) is fast compare to the coupling, the coupling can even disapear! 1H-NMR CH2=CH2

  36. Pt-195 Shifts Platinum is a heavy transition element. It has wide chemical shift scale: 13,000 ppm! The shifts depends strongly on the donor atom but vary little with long range. For example: PtCl2(PR3)2 have very similar shifts with different R Many platinum complexes have been studied by 1H, 13C and 31P NMR. But products not involving those nuclei can be missed : PtCl42- Major part of Pt NMR studies deals with phosphine ligands as these can be easily studied with P-31 NMR. Index

  37. Pople Notation Spin > ½ are generally omitted. Index

  38. Effect of Coupling with exotic nuclei in NMR Natural abundance 100% 1H, 19F, 31P, 103Rh : all have 100% natural abundance. When these nuclei are present in a molecule, scalar coupling must be present. Giving rise to multiplets of n+1 lines. One bond coupling can have hundreds or thousands of Hz. They are an order of magnitude smaller per extra bound between the nuclei involved. Usually coupling occur up to 3-4 bounds. Example: P(SiH3)3 + LiMe -> Product : P-31 NMRshows septet ===> product is then P(SiH3)2- Index

  39. P-31 Spectrum of PF2H(NH2)2 labeled with 15N 2 x 3 x 3 x 5 = 90 lines ! coupling with H (largest coupling : Doublet) then we see triplet with large coupling with fluorine With further Coupling to 2 N produce triplets, further coupled to 4protons => quintets

  40. Effect of Coupling with exotic nuclei in NMR Low abundance nuclei of spin 1/2 13C, 29Si, 117Sn, 119Sn, 183W : should show scalar coupling => satellite signals around the major isotope. • For example: WF6 as 183W has 14% abundance, the fluorine spectra should show satellite signals separated by the coupling constant between fluorine and tungsten. The central signal has 86% intensity and the satellites have 14%. This will produce 1:12:1 pattern Index

  41. Si-29 coupling • 29Si has 5% abundance. • For H3Si-SiH3 , the chance of finding • H3-28Si--29Si-H3 is 10%. Interestingly we can see that the two kind of protons are no longer equivalent so homonuclear coupling become observable! The molecule with 2 Si-29 is present with 0.25% intensity and is difficult to observe. • The second group gives smaller coupling Index

  42. Coupling with Platinum 195Pt the abundance is 33%. Platinum specie will give rise to satellite signal with a relative ratio of1 : 4 : 1. This intensity pattern is diagnostic for the presence of platinum. If the atom is coupled to 2 Pt, the situation is more complex: 2/3 x 2/3 => no Pt spin (central resonance) 1/3 x 1/3 => two Pt with spin 1/2 => triplet remaining molecule has 2x (1/3 x 2/3) = 4/9 => one Pt with spin 1/2 => doublet  Adding the various components together we now have 1:8:18:8:1 pattern. The weak outer lines are often missed, leaving what appear to be a triplet 1:2:1 !!! Index

  43. Carbon-13 in organometallic NMR 13C is extremely useful to organometallic NMR • For example: • Palladium complexe has: • 4 non-equivalent Methyls • 2 methylenes • Allyl :1methylene, 2 methynyl • Phenyl: 4 C: mono-subst. Index

  44. 29Si-NMR • Polymeric siloxanes are easily studied by NMR: These have • terminal R3SiO- • Chain R2Si (O-)2 • Branch R-Si(O-)3 • Quaternary Si(O-)4 All these Silicon have different shifts making it possible to study the degree of polymerization and cross-linking Index

  45. Coupling with Quadrupolar Nuclei (I>1/2) • 2nI+ 1 lines • The observation of such coupling depends on the relaxation rate of the quadrupolar nuclei (respect to coupling constant) Index

  46. Coupling with Quadrupolar Nuclei (I>1/2)

  47. Factors contributing to Coupling constant • Magnetic Moment of one nuclei interact with the field produced by orbital motion of the electrons – which in turn interact with the second nuclei. • There is a dipole interaction involving the electron spin magnetic moment • There is also a contribution from spins of electrons which have non-zero probability of being at the nucleus =>Fermi contact Index

  48. 1-bound coupling • Depends on s-orbital character of the bound • Hybridization of the nuclei involved1JCH => 125 (sp3), 160 (sp2), 250 (sp) • Electronegativity is another factor: increase the coupling • CCl3H => 1JCH = 209 Hz • Coupling can be used to determine coordination number of PF , PH compounds, and to distinguish axial, equatorial orientation of Fluorines. • 1JPH = 180 (3 coordinate) , 1JPH = 400 (4 coordinate) • Coupling can also be used to distinguish single double bond • E.g. Index

  49. 2-bound coupling • 2J can give structural information: There is a relationship between 2J and Bond angle • => coupling range passes through zero. Therefore the sign of the coupling must be determined Index

  50. 3-bound coupling • Depends on Dihedral angle3JXY = A cos 2f + B cos f + CA, B, C : empirical constants Index

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