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Unformatted text preview: 215-216 HH W10 Notes- Dr. Masato Koreeda Date: January 6, 2010 Topic: _NMR-II___ page 1 of 10. IV. Chemical Shifts- 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 shieldsthe nucleus. Where each 1H peak appears in the 1H NMR spectrum is the reflection of what kind of a chemical environmenteach 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:TMSacetoneresidualCHCl3in 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:TMSacetoneresidualCHCl3in 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 HzMHz (mega Hertz); Hz = cpsSince 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 106Hz = 2.2 x 10-62.2ppm(H3C)2C=O on a 400 MHz NMR spectrometer: 880 Hz / 400 x 106Hz = 2.2 x 10-62.2ppm Now, these are independent of the strength of an operating magnetic field.215-216 HH W10 Notes- Dr. Masato Koreeda Date: January 6, 2010 Topic: _NMR-II___ page 2 of 10. Conversely, 1 ppm on a 200 MHz NMR spectrometer corresponds to: 1 x 10-6x 200 x 106 Hz = 200Hz and 1 ppm on a 400 MHz NMR spectrometer corresponds to: 1 x 10-6x 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 -scale. The 1H peaks of most of the organic compounds fall in between = 0 ~ 10 ppm (see Table 10.3 on p. 361 of Eges book). The Factors That Affect Chemical Shifts(1) HybridizationCHsp3typically = 0.5 ~ 1.5 ppmCHsp2alkenestypically = 5 ~ 6 ppmHowever,-CC - Hsp= ~ 2.2 ppm!note:Htypically = 7 ~ 8 ppmaromatic H's(2) Electron Density on each 1H Heffective = H0 (1 ) H: applied magnetic field = shielding constant, 10-2 ~ 10-5 (reflects chemical environments of a specific 1H) increasing electron density on a 1H more shielding (larger ) more external Ho to reach Heffective1H peak at a higher field a smaller value for the peak....
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