NMR

Made using OCR Chemistry Textbook. Pages 94- 103. Gent and Richie.

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  • Created by: M
  • Created on: 21-12-12 18:18

NMR Spectra of -OH and -NH protons

Difficult to identify -OH and -NH protons:
Why?

  • no splitting pattern
  • signals often very broad
  • peaks can appear over a wide range of chemical shift values, depending on solvent used and conc. of sample

Use of D2O:

  • Heavy water- no NMR signal as even no. of nucleons
  • Removes -OH and -NH peaks

Why?

  • the deuterium exchanges with the H present in -OH and -NH.
  • i.e. CH3CH2OH + D2O <--> CH3CH2OD + HOD. absense of -OH proton= no -OH signal.
  • Compare spectra before and after addition of D2O to identify -OH/-NH absorptions.
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Splitting from -OH and -NH protons

NMR peaks for -OH/-NH are not split, nor do they split adjacent protons- usually broad and singlet.

Why broad? difficult to get solvents that are 100% dry, traces of water form hydrogen bonds with -OH and -NH protons in the compound being analysed= broadening.

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NMR in medicine

1. NMR in analysis: Pharmaceutical firms use NMR as an analytical technique in checking the composition of organic compounds during the synthesis of drugs.

2. NMR in body scanning: MRI (magnetic resonance imaging) 

  • In MRI scan, patient placed in large, cylindrical electromagnet and radio waves sent through body. Protons align with or against the strong magnetic field and resonate in response to the pulses of radio-frequency radiation.
  • Advantages: harmless, non-invasive, in contrast to high-energy X-rays it uses low-energy radio-frequency radiation.
  • Hazards: Can't be used if patient has a ferromagnetic metal implant or heart pacemaker, due to the v. strong electromagnetic.

Many diseases change the water content within tissues and organs and the scanner detects these difference. A computer processes the information from the scan by taking slices over a period of time, this makes it possible to build up a 3D image reflecting the chemical structure of the tissues.
Less H-atoms = darker image. 

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