Deck 14: Nuclear Magnetic Resonance Spectroscopy

Question 1

Which letter corresponds to the set of protons that experience the greatest magnetic field when a sample of the compound is placed in the probe of an NMR spectrometer?

A) a
B) b
C) c

Answer 1

C

Question 2

Consider the labeled protons in the compound.
The resonance of protons c would appear in the NMR spectrum as

A) a singlet.
B) a doublet.
C) a triplet.
D) a quartet.
E) an octet (an eight-line splitting pattern).f.
some other splitting pattern.

Answer 2

A

Question 3

Of the protons that are labeled, which set experiences the greatest magnetic field in an NMR spectrometer?

A) a
B) b
C) c
D) d
E) e

Answer 3

B

Question 4

The resonance for the methyl group in 1-bromo-1-chloroethane (CH₃-CHBrCl) is

A) a singlet.
B) a doublet.
C) a triplet.
D) two doublets because the protons are diastereotopic.
E) none of these choices.

Question 5

In an NMR spectrometer operating at ν₀ = 300 MHz, a resonance that has a chemical shift of 2.0 ppm (i.e.,  2) occurs at a frequency that is

A) 600 Hz greater than the absorption frequency of the standard TMS.
B) 600 × 106 Hz greater than the absorption frequency of TMS
C) 150 Hz greater than the absorption frequency of TMS.
D) 150,000,000 Hz greater than the absorption frequency of TMS.
E) 302 Hz greater than the absorption frequency of TMS.f.
less than the absorption frequency of TMS.

Question 6

Consider the compound.
a. How many separate singlets (unsplit NMR resonances) are there in the NMR spectrum of this compound?
b. The resonance that is at the highest frequency (farthest to the left) is
1. a.
2. b.
3. c.
4. d and/or e.
5. f.
c. The resonance of protons c appears as
1. a singlet.
2. a doublet.
3. a triplet.
4. a very complex pattern because the protons are diastereotopic.

Question 7

7. Consider the NMR spectrum of the compound:

-The protons labeled ________________.

A) should have the smallest chemical shift in the spectrum
B) should have the greatest chemical shift in the spectrum
C) should have neither the smallest nor the greatest chemical shift in the spectrum

Question 8

7. Consider the NMR spectrum of the compound:

-The resonance for protons (a) is ________________.

A) a singlet
B) a doublet
C) a triplet
D) a quartet
E) more complex

Question 9

Which compound contains two three-proton singlets in its NMR spectrum?

A) compound a
B) compound b
C) compound c
D) compound d

Question 10

What radiation frequency (in sec^-1, or Hz) is required to bring a set of protons into resonance if they experience a magnetic field of 50,000 gauss? Show your work.

Question 11

Consider the bolded protons in 1-propanol.
$$\mathrm { H } _ { 3 } \mathrm { C } - \mathrm { CH } _ { 2 } - \mathrm { CH } _ { 2 } - \mathrm { OH }$$ Following a "D₂O shake," the splitting of the 1-propanol protons shown in bold type should be observed as

A) a singlet.
B) a doublet.
C) a triplet.
D) a quartet.
E) some other splitting.

Question 12

11. Consider the ether:

-How many resonances should there be in the NMR spectrum of the compound? (A split resonance counts as one resonance.)

Question 13

11. Consider the ether:

-The resonance for the CH₂ hydrogens in the structure .is: (circle ONE)

A) two doublets.
B) two triplets.
C) one triplet.
D) one singlet.
E) some other pattern.

Question 14

A compound X has a formula C₇H₁₆O₄ and the following NMR spectrum, given as chemical shift (integral, splitting, coupling constant).
 1.93 (2H, t, J = 6 Hz);  3.35 (12H, s);  4.49 (2H, t, J = 6 Hz)
Which structure is consistent with this NMR spectrum?

A) compound A
B) compound B
C) compound C
D) compound D
E) compound E
F) compound F
G) compound G

Question 15

Consider how the NMR spectrum of protons a in the compound changes on the addition of a trace of acid (assume that J_ab ≠ J_ac):

A) no change
B) from a doublet of doublets to a simple doublet
C) from a triplet to a singlet
D) from a doublet of doublets to a triplet
E) none of these choices

Question 16

Deduce a structure that corresponds to the molecular formula and NMR spectra. Explain how you arrived at this structure.
​
Molecular formula: C₅H₁₂O.
¹H NMR:  1.18 (3H, t, J = 8 Hz);  1.13 (6H, d, J = 6.8 Hz);  3.53 (2H, q, J = 8.0 Hz);  3.65 (1H, septet, J = 6.8 Hz).

Question 17

How many signals would you expect in a ¹³C NMR spectrum for this compound? Identify the carbon with the highest chemical shift.

Question 18

Explain how you could use proton decoupled ¹³C NMR to differentiate between these two constitutional isomers?

Question 19

Deduce the structure of a compound with a molecular formula C₈H₁₀ that gives the proton decoupled ¹³C NMR data:  21.3, 128.9, 135.4 ppm.

Question 20

Deduce the structure of an unknown compound with a molecular formula of C₄H₈O given the following ¹³C NMR - DEPT spectrum. The numbers in parentheses indicate the number of attached hydrogens.
Proton decoupled ¹³C NMR:  23.3 (3), 68.5 (1), 116.4 (2), 135.7 (1)

Question 21

a. Identify compound X with the formula C₅H₁₀O that has the IR and NMR spectra shown.
b. Compound X is subjected to catalytic hydrogenation (H₂, Pd/C) to give compound Y. Give the structure of compound Y and describe the most significant difference between the NMR spectra of X and Y.

Question 22

Deduce the structure of the unknown compound C₆H₁₂O, whose IR and NMR spectra are shown. Explain how you arrived at your structure.
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Question 23

A compound X with the formula C₄H₁₀O₂ has the NMR spectrum shown. The integrals are given over their respective resonances, and the J values are coupling constants. Deduce the structure of compound X and explain how you came to this conclusion. Hints: (1) The only chemical shift information you need is that protons on carbons  to an oxygen have chemical shifts in the  3.2- 4.0 range. (2) You do not need to interpret the complex splittings at  3.22,  3.37, and  3.9 to deduce the structure, but you will need to interpret the splittings in the gray boxes.

Question 24

Deduce the structure of a compound C₆H₁₄O that corresponds to the proton NMR spectrum. The integral numbers are relative numbers of hydrogens. Assign each peak. There are no absorptions in the 3200-3600 cm^-1 region of the IR.

Question 25

The NMR spectrum of the missing compound is given below the reaction. Draw the structure of the missing starting material.

Question 26

The spectra were recorded for a compound with a molecular formula C₅H₁₀O₂. Deduce the structure of this compound and explain how you arrive at the correct structure of the compound. (Hint: There are diastereotopic protons in this compound.) NOTE: In the NMR, after a D₂O shake, the triplet at  3.27 disappears, and the doublet at  3.7 becomes a singlet.

Question 27

An unknown compound has a molecular formula of C₅H₈. Deduce the structure by using the proton and carbon NMR spectra provided.