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Unformatted text preview: Chem 216 W11 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 1 H nucleus, more the external magnetic energy is needed for the excitation of that 1 H nucleus as the electron shields the nucleus. Where each 1 H peak appears in the 1 H NMR spectrum is the reflection of what kind of a chemical environment each 1 H nucleus is in,* thus the name chemical shift. *This is manifested in the electron density surrounding each 1 H nucleus. NMR spectra are obtained usually in CDCl 3 (99.8% D and 0.2% 1 H) with (CH 3 ) 4 Si [tetramethylsilane or TMS] as internal reference in a certain operating magnetic field such as 200 MHz, 400 MHz, etc. H 3 C CH 3 O (acetone) in CDCl 3 on a 200 MHz magnet NMR spectrometer: TMS acetone residual CH Cl 3 in CD Cl 3 lower field or down-field higher field or up-field 440 Hz "The peak of acetone 1 H's appears as one peak at 440 Hz down-filed from TMS on a 200 MHz NMR spectrometer" H 3 C CH 3 O (acetone) in CDCl 3 on a 400 MHz magnet NMR spectrometer: TMS acetone residual CH Cl 3 in CD Cl 3 lower field or down-field higher field or up-field 880 Hz "The peak of acetone 1 H's appears as one peak at 880 Hz down-filed from TMS on a 200 MHz NMR spectrometer" note: MHz = 10 6 Hz MHz (mega Hertz); Hz = cps Since these are too lengthy descriptions of 1 H 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. (H 3 C) 2 C=O on a 200 MHz NMR spectrometer: 440 Hz / 200 x 10 6 Hz = 2.2 x 10-6 2 .2 ppm (H 3 C) 2 C=O on a 400 MHz NMR spectrometer: 880 Hz / 400 x 10 6 Hz = 2.2 x 10-6 2 .2 ppm Now, these are independent of the strength of an operating magnetic field. Chem 216 W11 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 10 6 Hz = 200 Hz and 1 ppm on a 400 MHz NMR spectrometer corresponds to: 1 x 10-6 x 400 x 10 6 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 1 H 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) Hybridization C H sp 3 typically = 0.5 ~ 1.5 ppm C H sp 2 alkenes typically = 5 ~ 6 ppm However,-C C - H sp = ~ 2.2 ppm! note: H typically = 7 ~ 8 ppm aromatic H's (2) Electron Density on each 1 H H effective = H 0 (1 ) H : applied magnetic field = shielding constant, 10-2 ~ 10-5 (reflects chemical environments of a specific 1 H) increasing electron density on a 1 H more shielding (larger ) more external H o to reach H effective 1 H peak at a higher field a smaller value for the peak....
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