Deck 9: Analytical Chemistry: Spectroscopy

Question 1

What type of compound corresponds to the IR spectrum shown here?

A) ketone
B) carboxylic acid
C) amine
D) alcohol
E) alkane

Answer 1

carboxylic acid

Question 2

Which set of m/z values is most likely to be observed in the mass spectrum of the ketone below?

A) 100,57,43
B) 100,86,72
C) 100,85,29
D) 57,43,30
E) 102,100,85

Answer 2

100,57,43

Question 3

As conjugation increases across a series of molecules, which of the following statements is true?
I. The HOMO-LUMO gap becomes smaller.
II. The number of possible UV transitions increases.
III. Molecules absorb light at shorter wavelengths.
IV. Molecules absorb light of higher energy.

A) I
B) I, II
C) I, III
D) I, II, IV
E) I, II, III, IV

Answer 3

I, II

Question 4

Which of the following compounds corresponds to the IR spectrum shown here?

A)
B)
C)
D)
E)

Question 5

Which of the following statements is true?

A) All nuclei are NMR active.
B) Only nuclei with nonzero spins are NMR active.
C) Only nuclei with spins of 1/2 are NMR active.
D) A nucleus with a spin of 3/2 , such as ¹¹B , is not NMR active.
E) The spin value of a nucleus is not related to whether it will be NMR active.

Question 6

At a field strength of 90 MHz , the resonance frequency of a particular proton is 216 Hz . What is the resonance frequency of this proton at a field strength of 600 MHz ?

A) 2.4 Hz
B) 216 Hz
C) 600 Hz
D) 1440 Hz
E) 33 Hz

Question 7

Which of the following is the stationary phase in gas chromatography?

A)helium or hydrogen gas
B)a high-molecular-weight polymer
C)an organic solvent
D)the sample
E)either a or b

Question 8

Which statement about shielding is false?

A)Hydrogens that are deshielded experience a net magnetic field that is greater than the applied magnetic field.
B)Hydrogens that are shielded experience a net magnetic field that is less than the applied magnetic field.
C)The net magnetic field experienced by the hydrogens in a given molecule is uniform throughout the molecule.
D)As an external magnetic field is applied, electrons in a molecule circulate and create a magnetic field that opposes the external field.
E)Electron-withdrawing groups exert deshielding effects on nearby hydrogens.

Question 9

Which of these is not an observed transition when a ground state ethylene molecule absorbs energy?

A)
B)
C)
D)
E)

Question 10

Which m/z value(s) most likely indicates an alkyl chloride?

A) an M peak at 98
B) a base peak at 85 and an M peak at 142
C) a base peak that is 15 mass units lower than the parent ion
D) an M peak at 106 and a peak at 108 that is three times as intense
E) an M peak at 150 and a peak of roughly equal intensity at 152

Question 11

Which of the following statements is false?

A) Protons generate their own tiny magnetic fields.
B) When no external magnetic field is applied, the two spin states for a proton are equal in energy.
C) In the NMR experiment, radio frequency energy is applied to change the orientation of nuclear spin ("flip" the spin).
D) The presence of an external magnetic field causes a random orientation of the nuclear spins in the atoms in a molecule.
E) Alignment of nuclear spin with an external magnetic field is more favorable than alignment against the field.

Question 12

Which of the following compounds corresponds to the IR spectrum shown here?

A)
B)
C)
D)
E) none of these compounds

Question 13

Which of the following statements about IR spectroscopy is true?

A) All molecular vibrations can be detected.
B) Absorption of infrared light is associated with both electronic transitions and molecular vibrations.
C) Only vibrations of multiple bonds are observed.
D) For a vibration to be IR active, it must produce a net change in the molecule's dipole moment.
E) Symmetrical vibrations are observed in the IR spectrum, but unsymmetrical vibrations are not.

Question 14

Which of the following compounds corresponds to the IR spectrum shown here?

A)
B)
C)
D)
E)

Question 15

Which of the following compounds corresponds to the IR spectrum shown here?

A)
B)
C)
D)
E)

Question 16

Why are peaks for ¹³C not observed in a spectrum for ¹H ?

A) .¹³C nuclei are not NMR active.
B) The gyromagnetic ratios for ¹³C and ¹H are similar, so signals generated by these two nuclei are separated from each other.
C) The gyromagnetic ratios for ¹³C and ¹H are sufficiently different that signals generated by these two nuclei are separated from one another.
D) The field strength of the magnet used differs for ¹³C NMR vs. ¹H NMR.
E) The reference substance used in a ¹³C NMR experiment is different than that used in ¹H NMR experiment.

Question 17

Which of these molecules do you expect to have the largest value of ?

A) ethylene
B) 1,3 -butadiene
C) a dodecene (12 double bonds) consisting of isolated alkenes
D) a dodecene (12 double bonds) consisting of fully conjugated alkenes
E) benzene

Question 18

Which of the following statements is true?

A) In a transition, a smaller HOMO-LUMO gap corresponds to lower energy and
longer wavelength.
B) In a transition, a larger HOMO-LUMO gap corresponds to lower energy and
shorter wavelength.
C) In a transition, a smaller HOMO-LUMO gap corresponds to higher energy and
shorter wavelength.
D) In a transition, a larger HOMO-LUMO gap corresponds to higher energy and
longer wavelength.
E) The HOMO-LUMO gap is related only to energy and not to wavelength.

Question 19

Chemical shift is

A)the area under a given signal.
B)the total number of neighbors that a given hydrogen has.
C)the location of a signal along the x-axis, reported in ppm.
D)the number of peaks into which a signal is split.
E)the distance between the individual "lines" in a signal, reported in Hz.

Question 20

Which of the following compounds corresponds to the IR spectrum shown here?

A)
B)
C)
D)
E)

Question 21

Identify the mobile and stationary phases in gas chromatography and explain how a mixture of
compounds is separated in this experiment.

Question 22

A compound with the following constitution was subjected to mass spectrometry.

Suggest structures for the ions that correspond to m/z , and 70. Does this spectrum represent the bromo compound or the chloro compound? Explain your reasoning.

Question 23

Which molecule might plausibly have a signal around 124 ppm in a ¹³C spectrum?

A)
B)
C)
D)
E)

Question 24

What do you expect to be the relative integration value of the signal generated by H_A protons in the ¹H NMR spectrum for this molecule?

A) 0
B) 1
C) 2
D) 3
E) 4

Question 25

The compound shown here was subjected to mass spectrometry. What structure gives rise the peak in its mass spectrum with m / z= 92 ?

Question 26

To which structure does this ¹H NMR spectrum correspond?

A)
B)
C)
D)
E)

Question 27

Which of the following statements is true?

A)Only hydrogens that are attached to the same carbon atom are said to be "coupled."
B)Only hydrogens that are attached to adjacent carbon atoms are said to be "coupled."
C)The coupling constant, J, is a measure of the chemical shift for a particular hydrogen or group of hydrogen atoms.
D)The magnitude of the coupling constant, J, depends on the dihedral angle between two hydrogens on adjacent carbons.
E)The coupling constant, J, is independent of the dihedral angle between hydrogens on adjacent carbons.

Question 28

To which structure does this 1H NMR spectrum correspond?

A)
B)
C)
D)
E)

Question 29

To which structure does this ¹H NMR spectrum correspond?

A)
B)
C)
D)
E)

Question 30

Which molecule might plausibly have a signal around 197 ppm in a ¹³C spectrum?

A)
B)
C)
D)
E)

Question 31

A common fragmentation pattern of cyclohexene derivatives is a reverse Diels-Alder reaction, where the charged fragment that results is often the diene.Indicate at least two peaks (with m/z values) you would expect to see in the mass spectrum for the compound shown here, and the
structures that correspond to these peaks.

Question 32

Draw the products of McLafferty rearrangement of the following compound.

Question 33

Which hydrogen would have a signal farthest downfield in the ¹H NMR spectrum for this molecule?

A) A
B) B
C) C
D) D
E) E

Question 34

Explain why there is always an M + 1 peak in a mass spectrum.

Question 35

Which hydrogen would have a signal farthest upfield in the ¹H NMR spectrum for this molecule?

A) A
B) B
C) C
D) D
E) E

Question 36

What would be the effect on the ¹H NMR spectrum of adding a drop of D₂O to a sample of the molecule shown here?

A) Adding the D₂O will cause more hydrogen bonding and will change the chemical shift of the hydroxyl proton.
B) Adding the D₂O will cause less hydrogen bonding and will change the chemical shift of the hydroxyl proton.
C) A new signal for D₂O will appear in the ¹H NMR spectrum.
D) A deuterium atom will exchange for the hydroxyl proton and the signal in the ¹H NMR spectrum will disappear.
E) Adding D₂O will have no effect on the ¹H NMR spectrum.

Question 37

How many "lines" do you expect in the signal for H_A?

A)2
B)3
C)6
D)7
E)There is insufficient information to answer the question.

Question 38

Which of the following compounds corresponds to the ¹H NMR spectrum shown here?

A)
B)
C)
D)
E)

Question 39

The molecule shown here was analyzed by mass spectrometry. What structure gives rise to the peak in the mass spectrum with m/z=81 ?

Question 40

Which structure matches the ¹H NMR spectrum shown here?

A)
B)
C)
D)
E)

Question 41

Are the two hydrogens indicated homotopic, enantiotopic, or diastereotopic?

Question 42

The compound shown here exhibits a peak at m/z =70 . Explain why.

Question 43

Are the two hydrogens indicated homotopic, enantiotopic, or diastereotopic?

Question 44

Predict the splitting pattern of the indicated hydrogen in the following molecule.

Question 45

Which of the following compounds corresponds to this ¹H NMR spectrum?

Question 46

Which carbon in the molecule below will give a singlet even in the off resonance decoupled ¹³C spectrum?

Question 47

Predict the splitting pattern of the indicated hydrogen in the following structure.

Question 48

Match the three compounds shown here to the three IR spectra.

Question 49

Match each of these ¹H NMR spectra to the correct compound. Then assign each signal in both spectra to the corresponding hydrogen(s).

Question 50

Explain why chemical shifts are reported in parts per million (ppm).

Question 51

β-carotene absorbs light at 453 nm and 483 nm. Calculate the energy, in kcal/mol , corresponding to each of these wavelengths of light. Recall that
E=Nhc/, where N=6.022X10²³molecules/mol, h=1.583x10^-37 kcal•s/molecule, and c=3.00x 10¹⁷nm/s.

Question 52

Which carbon in this molecule will have a ¹³C signal around 205 \ppm ?

Question 53

If a solution with an absorbance of 0.350 is diluted in half, what will be the absorbance of the new solution, assuming the same cell is used?

Question 54

The indole side chain of tryptophan (the amino acid blamed for postfeast sleepiness at Thanksgiving) has a molar absorbtivity of about 5,500 M^-1cm^-1 at 280 nm . What absorbance do you expect for a 100 micromolar solution of tryptophan if measured in a 1cm cell?

Question 55

A compound with four degrees of unsaturation has the molecular formula C₁₀H₁₄O . Its IR and ¹H NMR spectra are shown. Draw the structure of the compound.
Hint: The chemical shift for an OH signal can vary considerably.

Question 56

Which of the following compounds corresponds to this ¹H NMR spectrum?

Question 57

Are the two hydrogens indicated homotopic, enantiotopic, or diastereotopic?

Question 58

Two constitutional isomers of phenolphthalein are shown below.One of them is colorless, while
the other is responsible for the characteristic pinkish hue desperately sought by beginning students
learning titration techniques.Identify which is the pink isomer, and explain how you know.

Question 59

Which carbon in this molecule will have a ¹³C signal around 73/ppm ?

Question 60

The IR, ¹H NMR, and mass spectra for a compound with molecular formula are shown. The molecular ion is at and the base peak is at Determine the structure of the compound.

Question 61

A compound with one degree of unsaturation has the molecular formula C₄H₈O₃. Its IR and ¹H NMR spectra are shown. Draw the structure of the compound.

Question 62

A compound with three degrees of unsaturation has the molecular formula C₇H₁₀O. Its IR and ¹H NMR spectra are shown. Draw the structure of the compound.

Question 63

A compound with zero degrees of unsaturation has the molecular formula C₆H₁₄O. Its IR and ¹H NMR spectra are shown. Draw the structure of the compound.

Question 64

A compound with four degrees of unsaturation has the molecular formula C₁₀H₁₅NO.
Its IR and ¹H NMR spectra are shown. Draw the structure of the compound.
Hint: The chemical shift for an OH signal can vary considerably.

Question 65

A compound with five degrees of unsaturation has the molecular formula C₈H₉NO.
Its IR and ¹H NMR spectra are shown. Draw the structure of the compound.