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NMR Spectroscopy

NMR Spectroscopy. CHEM 430. FACTORS THAT INFLUENCE PROTON SHIFTS. Local Fields Shielding by e - that surround the resonating nuclei arise from local fields They are a simple function of e - density affected by induction, resonance and hybridization effects

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NMR Spectroscopy

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  1. NMR Spectroscopy CHEM 430

  2. FACTORS THAT INFLUENCE PROTON SHIFTS Local Fields • Shielding by e- that surround the resonating nuclei arise from local fields • They are a simple function of e- density affected by induction, resonance and hybridization effects • The magnetic field at the nucleus is altered from B0 to a quantity B0(1-s) where s is called the shielding CHEM 430 – NMR Spectroscopy

  3. FACTORS THAT INFLUENCE PROTON SHIFTS Local Fields Electronegativity effects • A nearby e- withdrawing atom or group with decrease e- density moving the observed resonance downfield to higher n • Conversely, a nearby e- donating atom or group will increase e- density moving the observed resonance upfield to lower n OH CH3 B0(1 – s) sOH < sCH3 DE = hn B0 CHEM 430 – NMR Spectroscopy 3

  4. FACTORS THAT INFLUENCE PROTON SHIFTS Local Fields B0(1 – s) sO < sC DE = hn B0 CHEM 430 – NMR Spectroscopy 4

  5. FACTORS THAT INFLUENCE PROTON SHIFTS Local Fields Electronegativity effects The trend follows Pauling electronegativity (EN) scale: CHEM 430 – NMR Spectroscopy 5

  6. FACTORS THAT INFLUENCE PROTON SHIFTS Local Fields Electronegativity effects • The effect is cumulative: • The effect drops sharply with distance: CHEM 430 – NMR Spectroscopy 6

  7. FACTORS THAT INFLUENCE PROTON SHIFTS Local Fields Electronegativity effects • The effect is a useful tool in quickly deducing simple aliphatic chains: CHEM 430 – NMR Spectroscopy 7

  8. FACTORS THAT INFLUENCE PROTON SHIFTS Local Fields Resonance effects • Donation or withdrawal of e- through resonance will have shielding or deshielding effects, respectively: resonance donation shielding effect resonance withdrawal deshielding effect CHEM 430 – NMR Spectroscopy 8

  9. FACTORS THAT INFLUENCE PROTON SHIFTS Local Fields Hybridization effects • Hybridization of the carbon atom influences e- density • As proportion of s-character increases (sp3 sp2  sp) bonding electrons move toward C and away from hydrogen - deshielding • This effect however is secondary to the influence of the p-cloud of electrons from the unhybridized p-orbitals as we will see CHEM 430 – NMR Spectroscopy 9

  10. FACTORS THAT INFLUENCE PROTON SHIFTS Nonlocal Fields • Purely magnetic effects from a neighboring group can influence nuclear shielding • As we saw earlier the combined effects of local and nonlocal fields: • Nonlocal fields have a major influence on chemical shift only if the group has a non-spherical shape CHEM 430 – NMR Spectroscopy 10

  11. FACTORS THAT INFLUENCE PROTON SHIFTS Nonlocal Fields • The electrons in a spherical substituent also precess in the applied field, creating a non-local field • As the molecule tumbles the lines of magnetic force will remain lined up with the applied field, but the position of the attached nuclei will change • The effect cancels itself out leaving only the effect on the local field by the substituent CHEM 430 – NMR Spectroscopy 11

  12. FACTORS THAT INFLUENCE PROTON SHIFTS Nonlocal Fields • The electrons in a non-spherical substituent also precess in the applied field, creating a non-local field • In benzene, the 6-p-orbitals overlap to allow full circulation of electrons; as these electrons circulate in the applied magnetic field they oppose the applied magnetic field at the center: B0 CHEM 430 – NMR Spectroscopy 12

  13. FACTORS THAT INFLUENCE PROTON SHIFTS Nonlocal Fields • At the ring periphery, the effect is oppositeand the protons are deshielded : CHEM 430 – NMR Spectroscopy 13

  14. FACTORS THAT INFLUENCE PROTON SHIFTS Nonlocal Fields • The benzene system is not spherical, but an oblate ellipsoid • As the molecule tumbles in in solution there is either the effect of shielding inside the ring or deshielding outside the ring if the ring is perpendicular to the applied field OR • No effect if the ring is parallel to the applied field CHEM 430 – NMR Spectroscopy 14

  15. FACTORS THAT INFLUENCE PROTON SHIFTS Nonlocal Fields • Groups that have appreciably different currents induced by B0 resulting from different orientations in space are said to have diamagnetic anisotropy • Just as the local effect can result in shielding (electron donation) or deshielding (electron withdrawal) the nonlocal effect can result in either permutation • Regions of shielding are indicated by (+) and deshielding (-) CHEM 430 – NMR Spectroscopy 15

  16. FACTORS THAT INFLUENCE PROTON SHIFTS Nonlocal Fields • The effect was modeled quantitatively by McConnell • The following equation relates shielding to the influence of a magnetic dipole on the point in space where a proton(nuclei) resides: • At q = 54o 44’ the expression (3cos2q – 1) goes to zero • On either side of this ‘magic angle’sA changes sign sA – shielding for a proton at (r, q) cL and cT are the diamagnetic susceptibilities of the group longitudinal and transverse to B0 CHEM 430 – NMR Spectroscopy 16

  17. FACTORS THAT INFLUENCE PROTON SHIFTS Nonlocal Fields • The protons of benzene reside on the periphery of the ring within the deshielding cone • Molecules have been constructed for the purpose of confirming the shielding effect predicted by the model: -0.5 2.0 d -3.0 9.3 -1.0 d CHEM 430 – NMR Spectroscopy 17

  18. FACTORS THAT INFLUENCE PROTON SHIFTS Nonlocal Fields • 4n + 2 p-electrons (aromatic) result in the diamagnetic circulation of e-s • Pople demonstrated that 4n p-systems have the opposite or paramagnetic circulation: 5.2 10.3 CHEM 430 – NMR Spectroscopy 18

  19. FACTORS THAT INFLUENCE PROTON SHIFTS Nonlocal Fields • For a prolate ellipsoid it is sometimes not as clear as to which arrangement has the stronger induced current • The acetylene system provides a simple example CHEM 430 – NMR Spectroscopy 19

  20. FACTORS THAT INFLUENCE PROTON SHIFTS Nonlocal Fields • We say the shift for the terminal acetylene proton experiences magnetic anisotropy. Usual shifts for this H are d 1.8-3.0. For reference sp3 ethane is at 0.86 and sp2ethene at 5.28 CHEM 430 – NMR Spectroscopy 20

  21. FACTORS THAT INFLUENCE PROTON SHIFTS Nonlocal Fields • Alkanes do not possess the same degree of electron circulation as alkynes but do exert nonlocal fields on adjacent nuclei • The C—C s-bond shields a proton on its side more than its end CHEM 430 – NMR Spectroscopy 21

  22. FACTORS THAT INFLUENCE PROTON SHIFTS Nonlocal Fields • The result of which is a deshielding of equatorial protons in conformationally locked systems: • Even simple alkane systems show anisotropy: CHEM 430 – NMR Spectroscopy 22

  23. FACTORS THAT INFLUENCE PROTON SHIFTS Nonlocal Fields • The result of which is a deshielding of equatorial protons in conformationally locked systems: • Even simple alkane systems show anisotropy: CHEM 430 – NMR Spectroscopy 23

  24. FACTORS THAT INFLUENCE PROTON SHIFTS Nonlocal Fields • The highly shielded position of cyclopropane resonances may be attributed either to an aromatic-like ring current or to the anisotropy of the bond that is opposite to a group in the three- membered ring: • The effect is much larger than the indicated 1.2 ppm , because the cyclopropane carbon orbital to hydrogen ( compared with the orbital in cyclohexane) deshields the proton. • A cyclopropane ring also can shield more distant hydrogens: Heq 1.2 less than Hax CHEM 430 – NMR Spectroscopy 24

  25. FACTORS THAT INFLUENCE PROTON SHIFTS Nonlocal Fields • Most common single bonds (C-N, C-O) have shielding properties that parallel those of the bond, although the geometry is more complex than that for the C-C bond. • Lone electron pairs can have a special effect: • In N-methylpiperidine the axial lone pair shields the vicinal Hax by an n  s* interaction without any effect on Heq. As a result, Hax increases to about 1.0 ppm or more in similar systems. CHEM 430 – NMR Spectroscopy 25

  26. FACTORS THAT INFLUENCE PROTON SHIFTS Nonlocal Fields • The anisotropy of double bonds is more difficult to assess, because they have three nonequivalent axes (the McConnell equation, with only two axes, does not apply). • Protons situated over double bonds are, in general, more shielded than those in the plane both for alkenes and for carbonyl groups • The position of the methylene protons in norbornene may be explained in this fashion since the syn and endo protons, respectively, are shielded with respect to the anti and exo protons. CHEM 430 – NMR Spectroscopy 26

  27. FACTORS THAT INFLUENCE PROTON SHIFTS Nonlocal Fields • The highly deshielded position of aldehydes ( ca. 9.8) is attributed to a combination of a strong polar effect and the diamagnetic anisotropy of the carbonyl group. CHEM 430 – NMR Spectroscopy 27

  28. FACTORS THAT INFLUENCE PROTON SHIFTS Nonlocal Fields • The nonspherical array of lone pairs of e- may exhibit diamagnetic anisotropy, although, alternatively, the effect may be considered a perturbation of local currents. • A proton that is H-bonded to a lone pair is invariably deshielded. • For example: the -OH proton in ethanol • CCl4 resonates at 0.7 (dilute no H-bonding) • CD3CD2OH resonates at 5.3 (H-bonding) • Carboxylic protons resonate at extremely high frequency (11– 14), because every proton is H-bonded within a dimer or higher aggregate. CHEM 430 – NMR Spectroscopy 28

  29. FACTORS THAT INFLUENCE PROTON SHIFTS Nonlocal Fields • Lone- pair anisotropy also has been invoked to explain trends in ethyl groups : For XCH2CH3: • The resonance position of –CH2- attached to X is explained by the polar effect • The trend for the more distant –CH3 group is opposite; as X increases size, the lone pair moves closer to the –CH3 group and deshields it. CHEM 430 – NMR Spectroscopy 29

  30. FACTORS THAT INFLUENCE PROTON SHIFTS Summary • Functional group effects on proton chemical shifts are explained largely by two general effects.: • Local Effects: Electron withdrawal or donation by induction (including hybridization) or by resonance alters the electron density and hence the local field around the resonating proton. • Higher electron density shields the proton (lower n, upfield) • Low electron density deshields the proton (higher n, downfield) • Nonlocal Effects: Diamagnetic anisotropy of nonspherical substituents is largely responsible for the proton resonance positions of aromatics, acetylenes, aldehydes, cyclopropanes, cyclohexanes, alkenes, and hydrogen- bonded species. CHEM 430 – NMR Spectroscopy 30

  31. PROTON CHEMICAL SHIFTS AND STRUCTURE 3-2a Saturated Aliphatics Alkanes. CHEM 430 – NMR Spectroscopy 31

  32. PROTON CHEMICAL SHIFTS AND STRUCTURE 3-2a Saturated Aliphatics Functionalized Alkanes. Oxygen: Nitrogen: Sulfur and the Halides: CHEM 430 – NMR Spectroscopy 32

  33. PROTON CHEMICAL SHIFTS AND STRUCTURE 3-2a Saturated Aliphatics Functionalized Alkanes. CHEM 430 – NMR Spectroscopy 33

  34. PROTON CHEMICAL SHIFTS AND STRUCTURE 3-2a Saturated Aliphatics Functionalized Alkanes. • After years of collective observation of 1H (and 13C) NMR it is possible to predict chemical shift to a fair precision using Shoolery Tables • These tables use a base value for 1H (and 13C) chemical shift to which are added adjustment increments for each group on the carbon atom d= 0.23 + DX + DY + DZ • The tables work well for methyl and methylene but diverge greatly with methine due to the increased interaction between effects CHEM 430 – NMR Spectroscopy 34

  35. PROTON CHEMICAL SHIFTS AND STRUCTURE 3-2a Saturated Aliphatics Functionalized Alkanes. Example Shoolery Table - Methylene CHEM 430 – NMR Spectroscopy 35

  36. PROTON CHEMICAL SHIFTS AND STRUCTURE 3-2a Saturated Aliphatics Functionalized Alkanes. Example Shoolery Table - Methine CHEM 430 – NMR Spectroscopy 36

  37. PROTON CHEMICAL SHIFTS AND STRUCTURE 3-2a Saturated Aliphatics Functionalized Alkanes. • After years of collective observation of 1H (and 13C) NMR it is possible to predict chemical shift to a fair precision using Shoolery Tables • These tables use a base value for 1H (and 13C) chemical shift to which are added adjustment increments for each group on the carbon atom d= 0.23 + DX + DY + DZ • The tables work well for methyl and methylene but diverge greatly with methine due to the increased interaction between effects CHEM 430 – NMR Spectroscopy 37

  38. PROTON CHEMICAL SHIFTS AND STRUCTURE 3-2b Unsaturated Aliphatics Alkynes. • Anisotropy of C≡C results in a low frequency for terminal-H (1.8 – 3.0) Alkenes. • Anisotropy of C=C results in a higher frequency for alkenylic (vinyl)-H • Range is very large (4.5 – 7.7) and highly subject to other groups on C=C • Reference value used for ethene is 5.28 CHEM 430 – NMR Spectroscopy 38

  39. PROTON CHEMICAL SHIFTS AND STRUCTURE 3-2b Unsaturated Aliphatics Alkenes. • Conjugation usually increases the observed frequency • Angle strain increases s-character and therefore moves the resonance to higher frequency • The presence of a C=O group w/d electrons by both resonance and induction: CHEM 430 – NMR Spectroscopy 39

  40. PROTON CHEMICAL SHIFTS AND STRUCTURE 3-2b Unsaturated Aliphatics Alkenes. CHEM 430 – NMR Spectroscopy 40

  41. PROTON CHEMICAL SHIFTS AND STRUCTURE 3-2b Unsaturated Aliphatics Alkenes. • These effects were quantified by Tobey and of Pascual, Meier, and Simon, • Using an empirical approach they tabulated a series of geminal, cis and trans substituents relative to the proton being observed: d = 5.28 + Zgem + Zcis + Ztrans • Although the parameters incorporate inductive and resonance effects, steric effects can cause deviations from observed positions. CHEM 430 – NMR Spectroscopy 41

  42. PROTON CHEMICAL SHIFTS AND STRUCTURE 3-2b Unsaturated Aliphatics Alkenes. CHEM 430 – NMR Spectroscopy 42

  43. PROTON CHEMICAL SHIFTS AND STRUCTURE 3-2c Aromatics • Diamagnetic anisotropy from aromatic ring current shifts the protons to high frequency – benzene is used as a reference (7.27) • Rings with only aliphatic substituents tend to bunch the resonances about this frequency (toluene, ~ 7.2) • Conjugating substituents tend to spread the aromatic resonances based on contributing resonance structures and inductive effects: CHEM 430 – NMR Spectroscopy 43

  44. PROTON CHEMICAL SHIFTS AND STRUCTURE 3-2c Aromatics • The a-protons in aromatic heterocycles are shifted due to the polar effect of the heteroatom: • As with the alkane and alkene systems a systematic observation has been made of aromatic H-resonances. They can be predicted from the following: d = 7.27 + SSi CHEM 430 – NMR Spectroscopy 44

  45. PROTON CHEMICAL SHIFTS AND STRUCTURE 3-2c Aromatics CHEM 430 – NMR Spectroscopy 45

  46. PROTON CHEMICAL SHIFTS AND STRUCTURE 3-2c Aromatics Aromatics: CHEM 430 – NMR Spectroscopy 46

  47. PROTON CHEMICAL SHIFTS AND STRUCTURE 3-2c Aromatics Heteroaromatics: CHEM 430 – NMR Spectroscopy 47

  48. PROTON CHEMICAL SHIFTS AND STRUCTURE 3-2d Protons on Oxygen and Nitrogen • Chemical shifts of protons attached to highly electronegative atoms such as O or N are influenced strongly by acidity, basicity, and hydrogen-bonding Protons on Oxygen • For –OH, minute amounts of acidic or basic impurities can bring about rapid exchange • They are averaged with other exchangeable protons, either in the same molecule or in other molecules, including the solvent. • Only a single resonance is observed for all the exchangeable pro-tons at a weighted-average position and no coupling is observed. • The resonance varies from sharp to slightly broadened, depending on the exchange rate. CHEM 430 – NMR Spectroscopy 48

  49. PROTON CHEMICAL SHIFTS AND STRUCTURE 3-2d Protons on Oxygen and Nitrogen • -OH may be determined by exchange experiments described earlier (shaking the NMR sample with D2O) • At infinite dilution in (no H-bonding), the OH resonance of alcohols may be found at about 0.5. • Under more normal conditions of 5% to 20% solutions, hydrogen bonding results in resonances in the 2– 4 range. • More acidic phenols (ArOH) have resonances downfield at 4– 8. Interaction with an ortho group shifts this to 10 or higher. CHEM 430 – NMR Spectroscopy 49

  50. PROTON CHEMICAL SHIFTS AND STRUCTURE 3-2d Protons on Oxygen and Nitrogen • Most carboxylic acids exist as H-bonded dimers or oligomers, even in dilute solution. • Hydrogen bonding coupled with the strong deshielding of the carboxlate group places the acid protons resonate far downfield (11– 14) • Likewise, other highly H-bonded acidic protons also may be found in this range, such as sulfonic, phosphonic and enolic protons CHEM 430 – NMR Spectroscopy 50

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