Deck 10: Molecular Spectroscopy

Question 1

Calculate the transmittance of a solution of benzene in cyclohexane of concentration 0.020 mol dm^-3 and path length 0.20 cm at a wavelength of 255 nm, if the molar absorption coefficient at this wavelength is 210 (mol dm^-3) ^-1 cm^-1.

A) 2.32
B) 6.92
C) 0.43
D) 0.14

Answer 1

D

Question 2

When radiation of wavelength 256 nm is incident on a solution of a dye of concentration 0.0125 mol dm^-3, and path length 1.0 cm, 27% of the intensity of the radiation is absorbed. Calculate the molar absorption coefficient.

A) 25.2
B) 10.9
C) 45.5
D) 105

Answer 2

B

Question 3

The molar absorption coefficient of potassium permanganate, KMnO₄, is 200 m² mol^-1 at a wavelength of 200 nm. Determine the concentration of a solution that would absorb 75% of the intensity of radiation of this wavelength for a path length of 1.0 cm.

A) 120 mol dm^-3
B) 0.69 × 10^-3 mol dm^-3
C) 0.30 × 10^-3 mol dm^-3
D) 6.2 × 10^-5 mol dm^-3

Answer 3

C

Question 4

The two singly degenerate different spin levels of a ¹H nucleus in a 600 MHz NMR are separated by an energy of 3.9800 *10^-25 J. Determine the relative populations of the two levels at a temperature of 298 K.

A) n_upper = 1.00000 n_lower
B) n_upper = 0.98885 n_lower
C) n_upper = 0.99990 n_lower
D) n_upper = 1.00010 n_lower

Question 5

The transition between the ground and first excited vibrational levels of the ICl molecule is observed at a wavenumber of 381 cm^-1. Given that the vibrational energy levels of a diatomic molecule such as ICl are singly degenerate, determine the relative populations of the ground and first excited vibrational levels at temperature of 400 K.

A) n_upper =0.014 n_lower
B) n_upper =1.000 n_lower
C) n_upper =0.042 n_lower
D) n_upper =0.004 n_lower

Question 6

The rotational constant of the ¹⁴N¹⁶O molecule is 50.84 GHz. Determine the bond length.

A) 0.132 nm
B) 0.115 nm
C) 0.101 nm
D) 0.169 nm

Question 7

Calculate the rotational constant of a ¹H₂ molecule, given that the bond length is 0.0740 nm.

A) 925 GHz
B) 463 GHz
C) 1.85 THz
D) 3.70 THz

Question 8

Which of the following molecules is expected to show a rotational spectrum: benzene, C₆H₆; water, H₂O; boron trifluoride, BF₃; monodeuterated acetylene, C₂HD?

A) C₆H₆, C₂HD
B) H₂O only
C) H₂O, BF₃
D) H₂O, C₂HD

Question 9

Calculate the frequency of the J = 6 $\rightarrow$ J = 7 transition in hydrogen fluoride, HF, for which the rotational constant is 616.20 GHz.

A) 616.2 GHz
B) 3697 GHz
C) 4313 GHz
D) 8627 GHz

Question 10

Transitions in the rotational spectrum of the sodium chloride molecule, ²³Na³⁵Cl, which is present in the interstellar medium, are observed at frequencies of 247.2, 260.2, 273.2, 286.2 and 299.2 GHz. Determine the bond length of a sodium chloride molecule.

A) 0.237 nm
B) 0.562 nm
C) 0.109 nm
D) 0.201 nm

Question 11

The force constant of the bond in ClO radical is 472 N m^-1. Determine the frequency of vibration of the ³⁵Cl¹⁶O isotopomer.

A) 16.1 * 10¹³ s^-1
B) 2.56 *10¹³ s^-1
C) 41.1 * 10¹³ s^-1
D) 1.79 * 10¹³ s^-1

Question 12

In the infrared spectrum of sulfur monoxide, SO, the transition corresponding to vibrational excitation of the bond in the most common isotopomer, ³²S¹⁶O, is observed at a wavenumber of 1138 cm^-1. Estimate the force constant of the bond.

A) 277 N m^-1
B) 122 N m^-1
C) 766 N m^-1
D) 149 N m^-1

Question 13

Calculate the number of normal vibrational modes for phenol, C₆H₅OH.

A) 33
B) 39
C) 34
D) 6

Question 14

Predict the number of infrared active vibrational normal modes for nitrogen dioxide, NO₂, and nitrous oxide, N₂O.

A) NO₂: 3; N₂O: 4
B) NO₂: 3; N₂O: 3
C) NO₂: 4; N₂O: 4
D) NO₂: 4; N₂O: 3

Question 15

Use the particle-in-a-box model to estimate the wavelength of the lowest energy transition in $\beta$ carotene, whose structure is shown in Figure 10.36. Assume that the average carbon-carbon distance is 0.15 nm.

A) 8970 nm
B) 1495 nm
C) 748 nm
D) 690 nm

Question 16

Calculate the magnetic field of a ¹H-NMR spectrometer with an operating frequency of 400 MHz. The magnetogyric ratio for ¹H nuclei is $\gamma$ = 26.75 *10⁷ T-¹ s-¹.

A) 2.24 T
B) 14.09 T
C) 7.05 T
D) 4.48 T

Question 17

A resonance is observed at a chemical shift of 4.2 in a ¹H-NMR spectrum. Calculate the difference in the resonance frequency of the ¹H nucleus from the TMS reference in a 600 MHz spectrometer.

A) 4.2 kHz
B) 7.0 kHz
C) 143 kHz
D) 2.52 kHz

Question 18

The ¹H-NMR spectrum of one of the isomers of dibromobenzene, C₆H₄Br₂, appears as just a single peak. With which isomer is this pattern consistent?

A) 1,2-dibromobenzene
B) 1,3-dibromobenzene
C) 1,4-dibromobenzene
D) None

Question 19

Predict the form of the ¹H-NMR spectrum of chloroethene, CHClCH₂.

A) Two groups of resonances, each consisting of a pair of doublets
B) Three groups of resonances, each consisting of a doublet of doublets
C) Three single peaks
D) Three groups of resonances, each consisting of a triplet of triplets

Question 20

Predict the coupling pattern expected for the ¹⁹F-NMR spectrum of sulfur hexafluoride, SF₆.

A) Two groups of resonances, the first appearing as a triplet and the second as a quintet
B) One group of resonances, appearing as a septet
C) One group of resonances, appearing as a sextet
D) One single peak

Question 21

Predict the first three rotational lines in a microwave spectrum of ¹⁴N¹⁶O if the bond length of NO is 121 pm.

A) 1.54, 3.08, 6.16 cm⁻¹
B) 3.08, 6.16, 9.24 cm⁻¹
C) 3.08, 6.16, 18.48 cm⁻¹
D) 0, 3.08, 9.24 cm^-1

Question 22

The energy difference $\Delta$ E between the J = 1 and J = 2 rotational levels of NO is 1.24 × 10⁻²⁴ J. Given that the degeneracy of the rotational levels is 2J + 1, calculate the ratio of populations of these levels at 298 K.

A) 1.64
B) 1
C) 0.60
D) 2

Question 23

Considering the energy transitions that occur in Raman spectroscopy, predict which of the following series of peaks, in order of increasing energy, best matches what is typically observed in a Raman spectrum.

A) Three peaks of equal intensity
B) A low intensity peak, a very high intensity peak, a very low intensity peak
C) A very high intensity peak, a low intensity peak, a low intensity peak
D) A very low intensity peak, a very high intensity peak, a low intensity peak

Question 24

VSEPR theory suggests that the sulfur pentafluoride ion SF₅⁻ adopts a trigonal bipyramidal structure. Predict the coupling pattern expected for the ¹⁹F-NMR spectrum of SF₅⁻ obtained at very low temperatures.

A) One single peak
B) One group of resonances, appearing as a doublet
C) One group of resonances, appearing as a sextet
D) Two groups of resonances, one appearing as a triplet and the second as a quartet

Question 25

The ¹H NMR spectrum of 1-bromopropane would have resonances (with integral intensities) best described by:

A) A doublet (2H), a doublet (2H), and a triplet (3H)
B) A triplet (2H), a sextet (2H), and a triplet (2H) Incorrect
C) A quartet (2H), a triplet (3H), and a triplet (2H) Incorrect
D) A singlet (7H)