Advanced Level Organic Chemistry: 15.3.2 Uses and applications of 13C NMR

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Doc Brown's Advanced Organic Chemistry: PART 15.3.2 USES of 1H NMR

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15.3.1 The theory of H-1 NMR spectroscopy and spectrometer - low and high resolution spectra

15.3.3 Index of H-1 (proton) NMR spectra of organic compounds

15.3.4 Some simple NMR-IR problem solving questions

15.3.2 The uses and applications of 1H (proton) NMR Spectroscopy

Sub-index for this page

(a) Indentifying molecules

(b) Determination of molecular structure

(c) Magnetic resonance imaging (MRI scanning)

(d) Chemical kinetics - experimental studies of the speed of a chemical reaction

(a) Identifying molecules

You can match 1H NMR resonance signals from an 'unknown' substance with a database of NMR spectra of known well characterised compounds.

A computing data base can match many signals (chemical shift, δ in ppm) to suggest possible structures, ideally a unique 'fingerprint' NMR signal.

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(b) Determination of molecular structure

1H NMR spectroscopy is a very powerful technique for the structural determination of molecules.

From the different chemical shifts you can build up a picture of parts of a molecule's structure.

NMR spectroscopy is so accurate that the structure of quite complex organic molecules can be worked out when combined with other data such as molecular mass and % C, H and O etc.

In a sense you are working backwards from the point of view of interpreting high resolution 1H NMR spectra explained in ...

The theory of H-1 NMR spectroscopy - interpreting high resolution spectra

NMR analysis can used in many situations including identifying molecules from natural materials e.g. molecules in a plant fragrance, molecules involved in some complex biosynthesis.

It is a non-destructive method of investigating molecular structure, which is a particularly useful for complex biological molecules e.g. nucleic acids, including RNA and DNA, or proteins, can be studied using nuclear magnetic resonance for weeks or months before using destructive biochemical experiments.

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(c) Magnetic resonance imaging (MRI scanning)

An important scanning technique in clinical medicine.

Collating the 1H NMR resonance signals from sections of the human body is an important diagnostic tool e.g. for medical problems associated with soft tissue and collagen in bones.

An extremely powerful magnet is used to generate a very strong magnetic field and the patient lies in between the poles of this magnet and is subjected to pulses of radio waves of around 40 MHz.

The hydrogen nuclei will re-emit the absorbed the NMR frequencies.

The hydrogen nuclei in the soft tissue have a different chemical shift to those in harder tissue, giving contrasting contours of different emissions and a computer can generate pictures of 'slices' of the parts of the body being investigated.

Potential or actual problems can be detected and diagnosed and the usually patient suffers no ill effects from the radio waves and is a safe non-invasive medical procedure.

MRI scanning also has the advantage of generating images of the body which CT scans, ultrasound and X-rays cannot.

An MRI scanner

(Image credit: <a href='http://www.shutterstock.com/pic.mhtml?id=98759036&src=id'>MRI scan</a> via Shutterstock)

 (Image credit: Courtesy FONAR Corporation)

An MRI scan can reveals the gross anatomical structure of the human brain.

The brain, spinal cord and nerves, as well as muscles, ligaments, and tendons are seen much more clearly with MRI than with regular x-rays and CT and for this reason MRI is often used to image knee and shoulder injuries.

A special kind of MRI scanning is functional Magnetic Resonance Imaging (fMRI.) in which brain structures can be observed and determine which areas of the brain “activate” (consume more oxygen) during various cognitive tasks. It is used to advance the understanding of brain function and organization and offers an insight into assessing neurological status and neurosurgical risk.

Because of the strong magnetic fields involved you have to be care if any magnetic object is associated with the patient e.g.
People with implants, particularly those containing iron e.g. pacemakers, vagus nerve stimulators, implantable cardioverter- defibrillators, loop recorders, insulin pumps, cochlear implants, deep brain stimulators, and capsules from capsule endoscopy - non of these should not enter an MRI machine.

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(d) Chemical kinetics - experimental studies of the speed of a chemical reaction

If the reaction involves a change in functional group, both involving a proton, you can follow the change in intensity of chemical shifts that are specific to each of the functional groups.

You can also following changes in conformation, that is changes in the 3D orientation of a molecule.

key words and phrases: What can you use NMR for? What is MRI How does MRI work? How are molecules identified with 1H NMR? How is 1H NMR used to investigate molecular structure?

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