NMR - Unit 4 AOS 2

Question 1

principals of NMR

Answer 1

• Nuclei with an odd number of nucleons can be detected by NMR
• two nuclei with an odd number of nucleons are 1H and 13C
• Radio waves are provided to change the spin state or ‘flip’ the nuclei from the low-energy (with the field) to the high-energy (against the field) alignment.
• When a nucleus moves back to the low-energy alignment it releases the specific energy difference between the two states, which can be detected

Question 2

shielding and chemical environments

Answer 2

• Electrons surrounding a nucleus will somewhat shield it from the applied magnetic field of an NMR spectrometer.
• nuclei in different chemical environments will experience different levels of shielding
• The energy difference between these two spin states depends on the strength of the external magnetic field that is ‘felt’ by the nucleus.
• the more shielded the nucleus is, the, lower the magnetic field will be to flip it towards the field
• electrons next to electronegative atoms will be more exposed.
• Nuclei that are connected to the same atoms are in the same chemical environment and will produce one NMR signal.

Question 3

chemical shift

Answer 3

• All NMR signals produced by a sample are compared to that produced by a standard,
  tetramethylsilane (TMS).
• The value for the TMS signal is set to zero and the relative position of the signal generated by nuclei in a sample is known as the chemical shift
• This represents the difference in energy required to flip a nucleus in a sample compared to TMS and
  is specific for each chemical environment

Question 4

low resolution carbon 13 NMR

Answer 4

• a technique which provides information about the non-equivalent carbon environments in an organic compound
• the area under the peak indicated the ratio of atoms responsible for a peak
• the chemical shift indicates the specific carbon environment
• The number of non-equivalent carbon environments is simply equal to the number of peaks (aside from the TMS peak) shown on the spectrum

Question 5

low resolution proton NMR

Answer 5

• Low resolution 1H-NMR spectroscopy is used to obtain the following information about the the number of non-equivalent proton environments
• the peal area is the the ratio of the number of protons in each of these environments
• the chemical shift indicates the possible types of proton environments present.

Question 6

high resolution proton NMR

Answer 6

• high resolution 1H-NMR is used to obtain information about the number of non equivalent proton environments as well as the number of protons in neighbouring environments
• the chemical shift indicates the specific proton environment

Question 7

high resolution proton NMR

Answer 7

• splitting provides information about the number of protons in neighbouring environments
• the number of splits will be equal to the number of proton environments plus 1

Question 8

peak names

Answer 8

1 peak: singlet - no protons in neighbouring environment

2 peaks: doublet - 1 protons in neighbouring environment

3 peaks: triplet - 2 protons in neighbouring environment

4 peaks: quartet - 3 protons in neighbouring environment