Deck 13: Nuclear Magnetic Resonance Spectroscopy

Question 1

The protons marked H_a and H_b in the molecule below are ________.

A) chemically equivalent or homotopic
B) enantiotopic
C) diastereotopic
D) endotopic
E) none of the above

Answer 1

chemically equivalent or homotopic

Question 2

What three-word term is abbreviated NMR?

Answer 2

nuclear magnetic resonance

Question 3

Predict the number of signals expected (disregarding splitting) in the ¹H NMR spectrum of the compound shown below.

Answer 3

4

Question 4

Which of the following reasons best explains why aromatic protons have a larger chemical shift than protons one carbon removed from a halogen?

A) An aromatic ring is more electron withdrawing than a halogen.
B) A halogen is more electron withdrawing than an aromatic ring.
C) Electron movement induces a magnetic field opposing the external field.
D) Electron movement induces a magnetic field reinforcing the external field.

Question 5

On a 90 MHz spectrometer, calculate the frequency at which a proton absorbs if it appears at 4.20 ppm.

Question 6

¹H nuclei located near electronegative atoms tend to be ________ relative to ¹H nuclei which are not.

A) shielded
B) deshielded
C) resonanced
D) split
E) none of the above

Question 7

How many nuclear spin states are allowed for the ¹H nucleus?

A) 1
B) 2
C) 3
D) 4
E) 10

Question 8

If a molecule contains 4 elements of unsaturation and signals in the ¹H NMR spectrum between δ 7.0 and 8.0 ppm, what structural group is likely to be present?

A) a carbonyl group
B) an aromatic ring
C) a hydroxyl group
D) a cyclohexyl ring
E) a carbon-carbon triple bond

Question 9

A nucleus with an ________ atomic number or an ________ mass number has a nuclear spin that can be observed by the NMR spectrometer.

A) even, odd
B) odd, even
C) odd, odd
D) even, even

Question 10

In the presence of an external magnetic field the approximate ratio of ¹H nuclei with α spins compared to β spins is respectively: ________.

A) 90:10
B) 25:75
C) 50:50
D) 75:25

Question 11

Predict the number of signals expected (disregarding splitting) in the ¹H NMR spectrum of the compound shown below.

Question 12

Calculate the magnetic field that corresponds to the proton resonance frequency of 300.00 MHz. The gyromagnetic ratio of the ¹H nucleus is 26,753 s^-1 gauss^-1.

Question 13

Predict the number of signals expected (disregarding splitting) in the ¹H spectrum of m-xylene (1,3-dimethylbenzene).

Question 14

The energy difference between the allowed spin states for an ¹H nucleus is ________ the strength of the external magnetic field in which it is placed.

A) independent of
B) directly proportional to
C) inversely proportional to
D) exponentially related to
E) logarithmically related to

Question 15

Predict the number of signals expected (disregarding splitting) in the ¹H spectrum of dibutyl ether.

Question 16

Predict the number of signals expected (disregarding splitting) in the ¹H spectrum of o-chlorophenol (2-chlorophenol).

Question 17

Using a 60-MHz spectrometer, the protons in dichloromethane appear at 5.30 ppm. When the same sample is placed in a 100-MHz instrument, where does the signal appear?

A) 8.33
B) 5.30
C) 3.18
D) cannot be determined from information given

Question 18

Electromagnetic radiation in the ________ region is used in ¹H NMR spectroscopy.

A) radio wave
B) ultraviolet
C) infrared
D) microwave
E) X-ray

Question 19

Which of the following functional groups has the most deshielded proton and would be predicted to show an NMR signal that is the farthest downfield?

A) benzylic - H
B) allylic - H
C) aldehyde - H
D) aromatic - H
E) acetylenic - H

Question 20

________ is commonly used as an internal reference in NMR spectroscopy; its signal is assigned d = 0 in ¹H and ¹³C NMR spectroscopy.

Question 21

A compound (C₅H₈O) shows IR absorptions at 3600 and 3300 cm^-1. Its ¹H NMR spectrum contained singlets at and 2.9 (broad) (ppm) in a ratio of 6:1:1. Name this compound.

Question 22

What ¹H NMR spectral data is expected for the compound shown?

A) 3.8 (1H, septet), 2.1 (3H, s), 1.0 (6H, d)
B) 3.8 (1H, septet), 3.3 (3H, s), 1.0 (6H, d)
C) 3.3 (3H, s), 2.6 (3H, septet), 1.0 (6H, d)
D) 2.6 (1H, septet), 2.1 (3H, s), 1.0 (6H, d)

Question 23

What compound exhibits only two signals in its ¹H NMR spectrum, a triplet and a quintet?

A) BrCH₂CH₂CH₂Br
B) BrCH₂CH₂CH₂Cl
C) (CH₃)₂CHCH(CH₃)₂
D) CH₃CH₂CH₂CH₃
E) (CH₃)₂CHOCH(CH₃)₂

Question 24

The protons marked H_a and H_b in the molecule below are ________.

A) chemically equivalent
B) enantiotopic
C) diastereotopic
D) endotopic
E) none of the above

Question 25

What is the relationships between Ha and Hb in the following structure?

A) homeotopic
B) enantiotopic
C) diastereotopic
D) none of the previous

Question 26

Use the structure below to state the relationship between the indicated protons as equivalent, enantiotopic, diastereotopic, or unrelated. H_a & H_b:
H_a & H_c:
H_b & H_c:

Question 27

What is the relative area of each peak in a quartet spin-spin splitting pattern?

A) 1:2:1
B) 1:2:2:1
C) 1:1:1:1
D) 1:4:4:1
E) 1:3:3:1

Question 28

The following splitting pattern represents one of the vinyl protons of styrene. Identify which proton is represented and list all the coupling constants (J values) for the splitting pattern.

Question 29

In the ¹H NMR spectrum of bromoethane the methylene group is split into a quartet by the α and β nuclear spins of the protons on the neighboring methyl group. If the external magnetic field, B_∘, directs upward, which sequence of nuclear spins contributes to the second farthest peak down field within the spin-spin splitting pattern?

A) ↑↓↓
B) ↑↓↑
C) ↑↑↑
D) ↓↓↓
E) ↑↑↓↓

Question 30

Describe the number of signals and their splitting in the ¹H NMR spectrum of (CH₃)₂CHOCH₃.

A) 3 signals: 2 doublets and a septet
B) 2 signals: a doublet and a septet
C) 3 signals: a doublet, a quartet, and a septet
D) 4 signals: 2 doublets, a singlet, and a septet
E) 3 signals: a singlet, a doublet, and a septet

Question 31

Predict the number of signals expected, their splitting, and their relative area in the ¹H NMR spectrum of 1,2-dichloroethane (ClCH₂CH₂Cl).

Question 32

Predict the number of signals expected (disregarding splitting) in the ¹H spectrum of 1,1-dimethylcyclobutane.

Question 33

The protons marked H_a and H_b in the molecule below are ________.

A) chemically equivalent
B) enantiotopic
C) diastereotopic
D) endotopic
E) none of the above

Question 34

Why is carbon-hydrogen splitting not a major part of ¹H NMR spectra?

Question 35

A peak split into a doublet of triplets gave the following measurements in its splitting
pattern. State the coupling constants (J values) for both the doublet and the triplet.

Question 36

Label the different ¹H environments in the structure below a, b, c.... Then complete the table for each different type of proton. The first one is done for you as an example. (Note: there may be fewer than 7 different proton environments) $\textbf{ environments }$ a b c d e f g
$\textbf{ chemical shift }$
(+/- 0.5 ppm) 1.9
$\textbf{ spin-spin splitting }$ s

Question 37

What ¹H NMR spectral data is expected for the compound shown?

A) 3.8 (1H, septet), 2.1 (3H, s), 1.0 (6H, d)
B) 3.8 (1H, septet), 3.3 (3H, s), 1.0 (6H, d)
C) 3.3 (3H, s), 2.6 (3H, septet), 1.0 (6H, d)
D) 2.6 (1H, septet), 2.1 (3H, s), 1.0 (6H, d)

Question 38

Predict the number of signals expected, their splitting, and their relative area in the ¹H NMR spectrum of (CH₃)₃CCHO.

Question 39

Predict the number of signals expected, their splitting, and their relative area in the ¹H NMR spectrum of 2-methylpropane (isobutane).

Question 40

Predict the number of signals expected, their splitting, and their relative area in the ¹H NMR spectrum of CH₃CH₂OCH₃.

Question 41

Any process faster than ________ will be recorded as an average by NMR spectroscopy.

A) 0.0005 seconds
B) 0.1 seconds
C) 0.01 seconds
D) 0.001 seconds
E) 1 minute

Question 42

Predict the number of signals expected in the proton spin decoupled ¹³C spectrum of cyclopentane.

Question 43

The ¹H NMR spectrum of ethanol is acquired and the hydroxyl signal appears as a singlet instead of a triplet. Offer an explanation.

Question 44

What type of carbon environment does not generate a signal in the DEPT-90 spectrum and gives a negative signal in the DEPT-135 spectrum?

A) quaternary
B) methine
C) methylene
D) methyl
E) carbonyl

Question 45

What is the approximate chemical shift of an alkynyl carbon in ¹³C NMR spectroscopy?

A) 10 ppm
B) 30 ppm
C) 70 ppm
D) 120 ppm
E) 200 ppm

Question 46

What type of carbon environment does not generate a signal in the DEPT-90 spectrum and gives a positive peak in the DEPT-135 spectrum?

A) quaternary
B) methine
C) methylene
D) methyl
E) carbonyl

Question 47

What compound exhibits three singlets in its proton spin decoupled ¹³C NMR spectrum?

A) BrCH₂CH₂CH₂Br
B) BrCH₂CH₂CH₂Cl
C) (CH₃)₂CHCH(CH₃)₂
D) CH₃CH₂CH₂CH₃
E) (CH₃)₂CHOCH(CH₃)₂

Question 48

How many peaks appear in the proton spin decoupled ¹³ C NMR spectrum of the compound below?

A) 1
B) 2
C) 3
D) 4
E) 5

Question 49

You have a sample and its ¹H NMR spectrum. You know your sample contains O atoms but not N atoms, and you suspect that your sample may be an alcohol. What common spectroscopic technique might you use to confirm your suspicion?

Question 50

Explain the difference between off-resonance decoupling and broadband decoupling and how each affects the C-13 NMR spectrum. In your discussion include a short description of how a chemist interprets the different spectra.

Question 51

An NMR spectrometer that operates at a frequency of 60 MHz for ¹³C NMR spectra, operates at what frequency for ¹H NMR spectra?

A) 15 MHz
B) 30 MHz
C) 60 MHz
D) 120 MHz
E) 240 MHz

Question 52

Predict the number of signals expected in the proton spin decoupled ¹³C NMR spectrum of the compound shown below.

Question 53

How might the proton spectrum of ultrapure dimethylamine, (CH₃)₂NH, differ from the spectrum of this compound to which D₂O has been added?

Question 54

What multiplicities are observed in the off-resonance decoupled ¹³C spectrum of 2,3-dimethyl-but-2-ene?

Question 55

How many peaks appear in the proton spin decoupled ¹³ C NMR spectrum of the compound below?

A) 2
B) 3
C) 4
D) 5
E) 6

Question 56

Give one reason why ¹³C NMR is less sensitive than ¹H NMR.

Question 57

Why is carbon-carbon splitting typically not seen in ¹³C NMR spectra?

Question 58

If a signal is observed in the 170 to 200 ppm range in a C-13 NMR spectrum, what is the most likely type of functional group associated with that carbon atom?

A) carbonyl carbon
B) allylic carbon
C) aromatic carbon
D) carbon/carbon triple bond
E) carbon/carbon double bond

Question 59

Which compound generates positive peaks for the carbonyl in both its DEPT-90 and DEPT-135 spectra?

A) CH₃CH₂CHO
B) CH₃CH₂COCH₃
C) CH₃CO₂CH₂CH₃
D) CH₃CH₂CONH₂
E) H₂CO

Question 60

The chair form of cyclohexane has protons in two distinct environments, axial and equatorial. When the proton NMR of cyclohexane is run on a 100-MHz instrument at 23°C, only one signal for the compound is observed. Explain this apparent contradiction.

Question 61

How many peaks appear in the proton spin decoupled ¹³C NMR spectrum of the compound below?

Question 62

Predict the number of distinct quartets expected in the off-resonance decoupled ¹³C NMR spectrum of the compound shown below.

Question 63

How many peaks appear in the proton spin decoupled ¹³C NMR spectrum of the compound below?

Question 64

How many peaks appear in the proton spin decoupled ¹³ C NMR spectrum of the compound below?

Question 65

How many peaks appear in the proton spin decoupled ¹³C NMR spectrum of the compound below?

Question 66

How many peaks appear in the proton spin decoupled ¹³C NMR spectrum of the compound below?

Question 67

How many peaks appear in the proton spin decoupled ¹³C NMR spectrum of the compound below?

Question 68

Predict the number of signals expected in the proton spin decoupled ¹³C NMR spectrum of o-diethylbenzene (1,2-diethylbenzene).

Question 69

Predict the number of signals expected in the proton spin decoupled ¹³C spectrum of p-dibromobenzene (1,4-dibromobenzene).

Question 70

Predict the number of signals expected in the proton spin decoupled ¹³C spectrum of m-dichlorobenzene (1,3-dichlorobenzene).

Question 71

Predict the number of signals expected in the proton spin decoupled ¹³C NMR spectrum of the compound shown below.

Question 72

How many peaks appear in the proton spin decoupled ¹³C NMR spectrum of the compound below?

Question 73

Predict the number of distinct quartets expected in the off-resonance decoupled ¹³C NMR spectrum of the compound shown below.

Question 74

Predict the number of signals expected in the proton spin decoupled ¹³C NMR spectrum of p-diethylbenzene (1,4-diethylbenzene).

Question 75

Predict the number of signals expected in the proton spin decoupled ¹³C spectrum of 3-hexanone, CH₃CH₂COCH₂CH₂CH₃.

Question 76

Predict the number of distinct quartets expected in the off-resonance decoupled ¹³C NMR spectrum of the compound shown below.

Question 77

Predict the number of distinct quartets expected in the off-resonance decoupled ¹³C NMR spectrum of the compound shown below.

Question 78

How many peaks appear in the proton spin decoupled ¹³C NMR spectrum of the compound below?

Question 79

Predict the number of signals expected in the proton spin decoupled ¹³C NMR spectrum of the compound shown below.

Question 80

Predict the number of signals expected in the proton spin decoupled ¹³C NMR spectrum of the compound shown below.