Revision Questions #4

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1.	The 1H NMR spectrum of isovaleraldehyde, (CH3)2CHCH2CHO, is given below. The spectrum consists of 4 distinct resonances : d 9.2 (1H, narrow triplet), d 2.2 (2H, doublet of doublets), d 1.6 (1H, multiplet) and d 0.9 (6H, doublet). Assign the spectrum then sketch and clearly describe the spectra which would be obtained while applying a strong Rf field at :
	(i) d 9.2 ppm.
	(ii) d 2.2 ppm.
	(iii) d 1.6 ppm.
	(iv) d 0.9 ppm.
	Answer

2.	The 13C (proton coupled) and 1H NMR spectra of trimethyl phosphate [(CH30)3P=O] are given below.

i)	Rationalise the multiplicity of signals in each spectrum.
ii)	What would happen to the appearance of the 13C spectrum if the 1H spectrum was irradiated with strong broad-band irradiation?
iii)	What would happen to the appearance of the 1H spectrum if the 31P spectrum was irradiated with strong broad-band irradiation?
iv)	What would happen to the appearance of the 13C spectrum if the 31P spectrum was irradiated with strong broad-band irradiation?
v)	What would happen to the appearance of the 1H spectrum if the 13C spectrum was irradiated with strong broad-band irradiation?
Answer

3. (hardish)	The element lead (Pb) has several naturally occuring isotopes including 204Pb (1.5%), 206Pb (24%), 207Pb (22%) and 208Pb (52%). Of these isotopes only 207Pb has a non-zero nuclear spin (207Pb, I=1/2). What would you expect to observe for the 1H NMR spectrum of tetramethyllead [(CH3)4Pb] given that 2J207Pb-H= 60Hz ? Answer
4.	Below are the 1H and 13C NMR spectra of 2-hexanone (CH3COCH2CH2CH2CH3). Explain carefully how, using homonuclear and heteronuclear decoupling experiments, you could assign the each of the resonances in the 1H and 13C spectra to which nuclei give rise to them. Answer

¹H NMR spectra of 2-hexanone




¹³C NMR spectra of 2-hexanone




Supplementary Exercise.

Computer Simulation of Complex NMR Spectra.

This is an exercise which uses a computer to calculate and display the NMR spectrum of simple and complex spin systems. Please see Dr. Ian Luck (room 106) to arrange a time to use one of the NMR computers on the 1st floor of the Chemistry Building. Dr. Luck will show you how to start the computer and access the program 'PANIC' [Parameter Adjustment for NMR by Iterative Calculation].

1. A simple AX spin system.

Set up a 2 spin system with chemical shifts :
W(1) = 100Hz
W(2) = 150Hz.

and coupling constant :
J₁₂ = 25.0Hz

This should give you a typical 4-line spectrum (2 doublets with a common splitting). The frequencies of the predicted transitions can be listed out with the LT command.

2nd Order Effects

Adjust the chemical shift of nucleus 2 by using the CS command to enter a new value and recalculate the spectrum. Note what happens to the appearance of the spectrum as the chemical shift difference [W(1)-W(2)] becomes comparable to the value of the coupling constant.

2. An AMX spin system.

Set up a 3 spin system with chemical shifts :
W(1) = 100Hz.
W(2) = 250Hz.
W(3) = 400Hz.

and coupling constants :
J₁₂ = 10.0Hz
J₁₃ = 4.0Hz
J₂₃ = 16.0Hz

This should give you a typical 12-line spectrum (3 doublets of doublets). The frequencies of the predicted transitions can be listed out with the LT command.

3. An AA'XX' spin system.

This is the spin system typical of the protons on a para-substituted benzene ring.

Set up a 4 spin system with chemical shifts :
W(1) = 100Hz.
W(2) = 100Hz.
W(3) = 200Hz.
W(4) = 200Hz.

and coupling constants :
J₁₂ = 1.5Hz
J₁₃ = 8.0Hz
J₁₄ = 0.5Hz
J₂₃ = 0.5Hz
J₂₄ = 8.0Hz
J₃₄ = 1.6Hz

Note what happens to the appearance of the spectrum as the chemical shift difference between the A protons and the X protons is reduced by changing the chemical shift of W(3) and W(4) to values closer to the shift of W(1) and W(2) using the CS 1 2 instruction then recalculating the spectrum.

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