Q: Which equation correctly relates the energy of a photon to its frequency and wavelength?

$$E = h \nu = \frac{hc}{\lambda}$$

Question 1

Which of the following statements best describes the relationship between wavelength, frequency, and energy for electromagnetic radiation?

Accepted Answer

As wavelength increases, frequency and energy both decrease.

Question 2

Which equation correctly relates the energy of a photon to its frequency and wavelength?

Accepted Answer

$$E = h 
u = \frac{hc}{\lambda}$$

Question 3

A molecule absorbs IR light when the frequency of the IR matches the frequency of which molecular property?

Accepted Answer

Its vibrational mode

Question 4

Which region of the IR spectrum is most useful for identifying functional groups in organic molecules?

Accepted Answer

The functional group region (above 1500 cm$$^{-1})$$

Question 5

According to Hooke’s Law, which bond will vibrate at the highest frequency?

Accepted Answer

A triple bond between two light atoms

Question 6

Which of the following bonds would NOT be IR active?

Accepted Answer

C≡C in 2-butyne

Question 7

A compound shows a strong, broad absorption at 3400 cm$$^{-1}$$ and a strong absorption at 1700 cm$$^{-1}. $$Which functional groups are most likely present?

Accepted Answer

Carboxylic acid

Question 8

Which of the following IR absorptions is characteristic of a nitrile group (C≡N)?

Accepted Answer

Around 2250 cm$$^{-1}$$

Question 9

How does conjugation with a C=C bond affect the IR absorption of a carbonyl (C=O) group?

Accepted Answer

It shifts the absorption to a lower wavenumber by about 30 cm$$^{-1}.$$

Question 10

Which of the following best explains why the O–H stretch in alcohols appears as a broad, strong peak in the IR spectrum?

Accepted Answer

Hydrogen bonding causes a range of vibrational energies.

Question 11

A student analyzes an unknown compound and observes strong IR absorptions at 3300 cm$$^{-1}$$ and 2100 cm$$^{-1}. $$Which compound is most likely present?

Accepted Answer

Hex-1-yne

Question 12

Which of the following statements about IR spectra is TRUE?

Accepted Answer

Each peak corresponds to a specific type of bond vibration.

Question 13

Which of the following compounds would show two strong absorptions above 1500 cm$$^{-1}$$ due to N–H and C=O groups?

Accepted Answer

Propanamide

Question 14

Which factor will increase the wavenumber of a bond’s IR absorption?

Accepted Answer

Increasing the percent s-character of the bond

Question 15

A molecule with a strong absorption at 1700 cm$$^{-1}$$ and no significant absorption above 3000 cm$$^{-1} is $$most likely a:

Accepted Answer

Ketone

Question 16

Which of the following best describes why a symmetrical alkyne (e.g., 2-butyne) is IR inactive?

Accepted Answer

It does not have a dipole moment change during vibration.

Question 17

Which of the following functional groups would show a strong, sharp absorption near 2250 cm$$^{-1} in $$its IR spectrum?

Accepted Answer

Nitrile

Question 18

A compound shows IR absorptions at 3300 cm$$^{-1} ($$medium), 2250 cm$$^{-1} ($$medium), and 3000–2850 cm$$^{-1} ($$strong). Which compound is most consistent with this data?

Accepted Answer

Hex-1-yne

Question 19

Which of the following best explains why the IR absorption for the C=O group in N,N-dimethylacetamide appears at a lower wavenumber than in ethyl acetate?

Accepted Answer

The nitrogen atom is more basic and donates electron density, increasing resonance and lowering the absorption frequency.

Infrared (IR) Spectroscopy in Organic Chemistry: Principles and Applications

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