6.3.2 Spectroscopy

Question 1

What is the chemical shift based on?

Answer 1

• the frequency absorbed depends on the environment of the carbon atom (what is it bonded to)

Question 2

What is the standard used?

Answer 2

• tetramethylsilane (TMS) is used as the standard
• the peak has a value of 0, all other peaks are compared to it and given a chemical shift in ppm

Question 3

Why is TMS used as the standard?

Answer 3

• it produces a single peak as all H are in the same environment
• it is non-toxic, volatile and inert

Question 4

What is the problem with common solvents?

Answer 4

• contain C and H and so will produce peaks in the spectrum
• these peaks will swamp the spectrum so that the sample investigated cannot be seen

Question 5

What solvents are used?

Answer 5

• use deuterated solvents
• deuterium is an isotope if hydrogen that will not give a peak in the NMR spectrum

Question 6

What do the number of peaks show?

Answer 6

• number of different proton environments

Question 7

What does the relative peak area show?

Answer 7

• the area under the peaks is proportional to the number of protons responsible for that peak

Question 8

Why are OH, NH or NH2 peak singlets?

Answer 8

• there is no splitting through O or N
• their chemical shifts vary greatly

Question 9

How to confirm the presence of an OH or NH proton?

Answer 9

• add D2O shake vigorously and run the spectrum again
• if the peak disappears then it is an OH or NH proton
• the deuterium in the D2O exchanges with the H in the OH or NH and deuterium doesn’t show up on the NMR spectrum

Question 10

What are the splitting patterns?

Answer 10

• n protons on neighbouring carbon = n+1 peaks