In a 300-MHz NMR spectrometer, absorption of radiation at 1500 Hz corresponds to

Question

In a 300-MHz NMR spectrometer, absorption of radiation at 1500 Hz corresponds to a chemical shift ___ of ppm. Show calculation. (b) The frequencies in (a) are in the __ frequency range of the electromagnetic spectrum. (c) The chemical shifts () are caused by deshielding of the ______ compared to TMS (tetramethyl silane). Define deshielding and tell how the amount of deshielding affects. 6. Arrange these methy-group hydrogens from smallest to largest chemical shift. R is any alkyl group. Write values for each one that is available. CH_3Cl CH_3CH_2C1 CH_3 CCH_3 CH_3OH R-CH_3

Explanation / Answer

5a)

The chemical shift in absolute terms is defined by the frequency of the resonance expressed with reference to a standard compound which is defined to be at 0 ppm.

chemical shift = (frequency of signal – frequency of reference)/) spectrometer frequency)

                             = 1500Hz – 0Hz/300 x 10^6 Hz

= 5 x 1/10^6 (1ppm = 1/10^6)

= 5 ppm

b) The frequencies in (a) are in the radio frequency range of Electro Magnetic Spectrum.            

              (<3x10^7 hz frequencies are in radio region.)

c)

The by chemical sift is caused deshielding of a nucleus compared to TMS.

Deshielding:

If the electron density around a nucleus decreases, the opposing magnetic field becomes small and therefore, the nucleus feels more the external magnetic field B0, and it is said to be deshielded. The proton experiences higher external magnetic field, so it needs a higher frequency to achieve resonance, and therefore, the chemical shift shifts downfield (higher ppm).

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