Chem 215-216 HH W11 Notes - Dr. Masato Koreeda Date: January 5, 2011 Topic: _NMR-II___ page 1 of 10. IV. Chemical Shifts - δ unit Each H nucleus in a molecule has a different degree of electron surrounding it. Higher the electron density is found surrounding the 1H nucleus, more the external magnetic energy is needed for the excitation of that 1H nucleus as the electron shields the nucleus. Where each 1H peak appears in the 1H NMR spectrum is the reflection of what kind of a chemical environment each 1H nucleus is in,* thus the name “chemical shift.” *This is manifested in the electron density surrounding each 1H nucleus. NMR spectra are obtained usually in CDCl3 (99.8% D and 0.2% 1H) with (CH3)4Si [tetramethylsilane or TMS] as internal reference in a certain operating magnetic field such as 200 MHz, 400 MHz, etc. H3CCH3O(acetone)in CDCl3 on a 200 MHz magnet NMR spectrometer:TMSacetoneresidualCHCl3 in CDCl3lower fieldor down-fieldhigher fieldor up-field440 Hz"The peak of acetone 1H's appears as one peak at 440 Hz down-filed from TMS on a 200 MHz NMR spectrometer"H3CCH3O(acetone)in CDCl3 on a 400 MHz magnet NMR spectrometer:TMSacetoneresidualCHCl3 in CDCl3lower fieldor down-fieldhigher fieldor up-field880 Hz"The peak of acetone 1H's appears as one peak at 880 Hz down-filed from TMS on a 200 MHz NMR spectrometer"note: MHz = 106 Hz MHz (mega Hertz); Hz = cps Since these are too lengthy descriptions of 1H NMR data and the acetone peak appears at a down field position from TMS proportionally to the strength of an operating magnetic field, the following dimension less ppm unit has been introduced. (H3C)2C=O on a 200 MHz NMR spectrometer: 440 Hz / 200 x 106 Hz = 2.2 x 10-6 ≡ 2.2 ppm (H3C)2C=O on a 400 MHz NMR spectrometer: 880 Hz / 400 x 106 Hz = 2.2 x 10-6 ≡ 2.2 ppm !!! Now, these are independent of the strength of an operating magnetic field.Chem 215-216 HH W11 Notes - Dr. Masato Koreeda Date: January 5, 2011 Topic: _NMR-II___ page 2 of 10. Conversely, 1 ppm on a 200 MHz NMR spectrometer corresponds to: 1 x 10-6 x 200 x 106 Hz = 200 Hz and 1 ppm on a 400 MHz NMR spectrometer corresponds to: 1 x 10-6 x 400 x 106 Hz = 400 Hz This ppm scale relative to TMS and the increasing value to the lower magnetic field (i.e., to the left from TMS) is called “the δ-scale.” The 1H peaks of most of the organic compounds fall in between δ = 0 ~ 10 ppm (see Table 10.3 on p. 361 of Ege’s book). The Factors That Affect Chemical Shifts (1) Hybridization C Hsp3typically δ = 0.5 ~ 1.5 ppmCHsp2alkenestypically δ = 5 ~ 6 ppmHowever, -C ≡ C - Hspδ = ~ 2.2 ppm!note:Htypically δ = 7 ~ 8 ppmaromatic H's (2) Electron Density on each 1H Heffective = H0 (1 – σ) H0 : applied magnetic field σ = shielding constant, 10-2 ~ 10-5 (reflects chemical environments of a specific 1H) • increasing electron density on a 1H → more shielding (larger σ) → more external Ho to reach Heffective → 1H peak at a higher field → a smaller δ value for the peak. Examples: (i) tetramethylsilane (TMS): H3CCH3CH3CHHHelectronegativity: 2.1e.n.: 2.5e.n.: 1.8SiConsequently, the electron density of the 1H's of the CH3 increases--> 1H's of the CH become highly shielded --> requires more external magnetic field energy. Therefore, (H3C)4Si H’s appear at a significantly higher field than H’s in most of the organic compounds; one of the reasons why TMS is used as the internal reference compound. (ii) Halomethanes [H3C-X] HHHelectronegativity: 4.0e.n.: 2.5CFδ 4.30HHHe.n.: 3.0e.n.: 2.5CClδ 3.05HHHe.n.: 2.5CBrδ 2.70e.n.: 2.8HHHe.n.: 2.5C Iδ 2.10e.n.: 2.5least shielded;most deshieldedChem 215-216 HH W11 Notes - Dr. Masato Koreeda Date: January 5, 2011 Topic: _NMR-II___ page 3 of 10. (iii) H3C---OR H3C---NR2 H3C---SR δ 3.5 e.n.: 3.5 δ 2.5-3.0 e.n.: 3.0 δ 2.2-2.5 e.n.: 2.5 (iv) alkenes vs α,β-unsaturated ketones HHHHOδ 5.225.56.5HHOHHO+HO+--Hdeshieldedαβ (3) Magnetic Anisotropy ----- A through-space effect (i) Aromatic Ring-Current Effect See: Günther, H. NMR Spectroscopy; Wiley: New York, 1998; p. 85. For a recent dispute on the origin of the aromatic ring-current effect, see: Wannere, C. S.; von Ragué Schleyer, P. Org. Lett. 2003, 5, 605. Haromatic H'sHHδ 5.22HHHHH7.22|Δδ| = ca. 2 ppm HHHHHHInduced magnetic fieldInduced magnetic fieldHoThese aromatic delocalized 6 π electrons start circulatinginto one direction in an external magnetic field (Ho). Thiscirculation of the 6 π-electrons results in the induction of a secondary magnetic field following the Fleming's rule.flow of the 6 π-electrons in the ringinduced magneticfield opposed tothe applied magneticfieldinduced magnetic fieldreinforcing the appliedmagnetic fieldHoThis type of differenct effect in magnetic properties at a different point in space is referred to as "magnetic anisotropy."Hcirculatingπ-electronsChem 215-216 HH W11 Notes - Dr. Masato Koreeda Date: January 5, 2011 Topic: _NMR-II___ page 4 of 10. (3) (i) Aromatic Ring-Current Effect (continued) Hca. 55 °H's located inside this imaginarycone requires more external magnetic field power for their excitation --> high- field shifted; smaller δ valuesH's located outside the cone, including the H's on the benzene ring, requires less externalmagnetic field power (i.e., energy) for their excitation --> down-field shift; larger δ valuesCH2CH2CH2H2CH2CH2CH2CH2CCH2H2Cshieldingzonedeshielding zoneδ 2.631.551.080.700.51Example:Strongly deshieldedStrongly shielded by the ringcurrent effect of the benzene6π-electronsThe closer the H is to the center of the benzene ring, the stronger the extent of the ring current effect is. (ii) Acetylene groups. C C HRRHIn a magnetic field, theseπ-electrons induce a secondarymagnetic fieldC C HRshielding zonedeshielding zoneAs this H is quite close to the center of the C ≡ C bond,an extremely strong shielding effect is observed for this H.δ 2.2 V. Integration The peak intensity is proportional to the number of H’s belonging to each peak in the case of 1H NMR. H3COCH2OCH3TMS3.55.0 0 ppm12 36These integrated areas are expressed in arbitrary numbers. Only their numerical ratio is