FTIR SPECTROSCOPY

In the following compound, rank the highlighted bonds in terms of increasing wavenumber:

• A < B < C

• C < A < B

• C < B < A

• A < C < B

Answer explanation

The correct ranking is C < A < B because the wavenumber is inversely proportional to bond strength, with triple bonds having the highest wavenumber.

In what region of the spectrum does infrared radiation occurs?

• Between the visible and ultraviolet regions

• Between the visible and microwave regions

• Between the visible and x-ray regions

• Between ultraviolet and gamma regions

Answer explanation

Infrared radiation occurs between the visible and microwave regions of the spectrum.

Which of the following molecules can’t be analysed by IR

• FeCl3

• H2O

• Cl2

• CH3OH

Answer explanation

Cl2 is a diatomic molecule composed of two chlorine atoms, which do not have any functional groups that can be analyzed by IR spectroscopy.

Which of the following bending vibration takes place in different planes.

• Asymmetric stretching

• Rocking

• Scissoring

• Twisting

Answer explanation

The bending vibration that takes place in different planes is twisting, as it involves movement in multiple directions simultaneously.

According to Hook’s law which of the following linkages will have more force constant?

• P

• Q

• R

• P and R will have equal and greater force constant

Answer explanation

According to Hook’s law, the linkage with more force constant will have a stiffer spring, which is linkage R.

The infrared spectrum was obtained from a compound and showed absorptions at 2100 cm^-1,
1700 cm^-1, and 1200 cm^-1.  Identify the compound.

• CH₃COOCH₃

• C₆H₅COOH

• CH₂=CHCH₂OH

• CH≡CCH₂CO₂CH₃

Answer explanation

The compound is identified based on the absorptions in the infrared spectrum: 2100 cm^-1 (alkyne C≡C), 1700 cm^-1 (ester C=O), and 1200 cm^-1 (ester C-O). The correct choice is CH≡CCH2CO2CH3.

A molecule absorbs IR at a wavenumber of 1720 cm^-1.  Which functional group could account for this absorption.

• Aldehydes

• Carboxylic Acids

• Ethers

• Alcohols

Answer explanation

The absorption at 1720 cm^-1 corresponds to the C=O stretching frequency of aldehydes, indicating the presence of an aldehyde functional group.

Which of the following statements about infrared spectroscopy is correct?

• Infrared spectroscopy can be used to determine the size and shape of a compound’s carbon skeleton.

• An IR spectrometer illuminates a compound with infrared light and records the positions where the light is blocked by the compound. This results in the peaks of the spectrum.

• The fingerprint region of the spectrum can be used to identify functional groups.

• When the frequency of infrared light matches the frequency of bond vibration in a molecule, a peak appears on the spectrum.

Answer explanation

When the frequency of infrared light matches the frequency of bond vibration in a molecule, a peak appears on the spectrum.

Which is belongs to fingerprint region?

• 2500-4000

• 2000 - 2500

• 1500 - 2000

• 500 - 1500

Answer explanation

The correct choice for the fingerprint region is 500 - 1500.

What type of transitions mainly occur in infrared spectroscopy?

• Electronic transitions only

• Rotational and vibrational transitions

• Rotational transitions only

• All the electronic, rotational, vibrational transitions

Answer explanation

Infrared spectroscopy mainly involves rotational and vibrational transitions, not just electronic transitions.