NMR Spectroscopy

Question 1

Suggest what information nmr spectroscopy provides about the structure of a molecule

Answer 1

positions of 13C and 1H in molecule

Question 2

State the name and structure of the chemical standard used in proton nmr

Answer 2

• tetramethylsilane (TMS)

- Si(CH3)4

Question 3

Explain why TMS is a suitable chemical standard to use in proton nmr

Answer 3

• produces single INTENSE peak since all C and H are in the same environment
• signal upfield on spectrum/found on far right of spectrum so does not interfere with other peaks
• volatile so easily removed
• non toxic
• inert

Question 4

State purpose of integration trace on proton nmr spectra

Answer 4

indicates relative numbers of 1H atoms in different environments

Question 5

Suggest why deuterated solvents (CDCl3) or CCl4 are used rather than CHCl3 in proton nmr

Answer 5

• contains no 1H atoms

- replaced with 2H deuterium which will not give a peak in same range as 1H so will not interfere with results

Question 6

State chemical shift value of TMS

Answer 6

zero by definition

Question 7

Suggest another instance where there is a singlet splitting pattern other than when there are no hydrogens on adjacent carbon atoms

Answer 7

for 1H in -OH or -NH2 bond

Question 8

Suggest what can be deduced from splitting patterns in proton nmr

Answer 8

no. of peaks = n+1

where n = no. of hydrogen atoms on adjacent carbon

Question 9

Points to include when explaining deductions from proton nmr spectrum

Answer 9

• no. of peaks refers to no. of individual H environments
• state integration ratio for H atoms based on mmr of molecule
• explain can be deduced from each chemical shift using integration ratio
• explain what splitting patterns show
• mention any equivalent groups (same environment) which indicate symmetry of molecule

Question 10

Explain why nmr spectrum shows a certain number of peaks

Answer 10

indicates number of different 13C or 1H environments

Question 11

Suggest how to determine the number of carbon or hydrogen environments in a molecule

Answer 11

based on symmetry of molecule

Question 12

Explain what can be deduced from the following: 
a) singlet peak, δ=2.18
b) singlet peak, δ=3.33
c) two triplet peaks 
d)  δ=2.6 and ∫=2
e)  δ=2.2 and ∫=3
f)   δ=1.2 and ∫=6
given that integration ratio = ratio of H atoms

Answer 12

a) RCOCH3
b) ROCH3
c) CH2CH2
d) CH2CO
e) CH3CO
f) two EQUIVALENT CH3 groups