Spectroscopy - 13C NMR Practice 1

Select the factors that affect the energy difference between two nuclear spin states.

• The type of nucleus

• Which atoms are directly bonded to the nucleus

• The electronegativity of neighbouring atoms.

• All of these.

Select the best definition for chemical environment.

• The solvent used to dissolve the sample.

• The measure of the electron withdrawing ability of an atom.

• The energy required to change an atom's nuclear spin.

• The atom or groups of atoms surrounding a nucleus.

When the local magnetic field from electrons reduces the magnetic field experienced by a nucleus. Answer is caps sensitive.

(a)  

When the electron density is pulled away from the nucleus so it experiences a greater applied magnetic field. Answer is caps sensitive.

(a)  

Identify which molecule’s carbon atom will be the most deshielded.

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Why do the use tetramethylsilane (TMS) as the reference compound in NMR spectrometry? Choose best answer.

• TMS is chemically inert, so won’t react with the compound(s) in the sample.

• Is symmetrical so there is only one chemical environment.

• It produces one peak that is located away from the peaks of interest.

• All of these.

Select the best definition for the chemical shift of an NMR signal.

• How many different chemicals the peak represents.

• The different type of bonds in the molecule.

• The number of carbon atoms in the environment.

• The position of the peak relative to the TMS peak (at 0)

A peak is said to have moved _________ when it moves to the left, i.e. has a larger chemical shift compared to TMS. Therefore, the peak located around 177 ppm is more _________ than the peak located around 20 ppm. 

(a)  

The atoms or groups of atoms that surround a NMR active nucleus.

• Chemical environment

• Deshielding

• Chemical Shift

• Downfield

A shift in electron density away from the nucleus.

• Chemical environment

• Deshielding

• Chemical Shift

• Downfield