Question

Which of the following statements regarding IR spectroscopy is wrong? a) Infrared radiation is higher in energy than UV radiation. b) Infrared spectra record the transmission of IR radiation. c) Molecular vibrations are due to periodic motions of atoms in molecules, and include bond stretching, torsional changes, and bond angle changes. d) Infrared spectra give information about bonding features and functional groups in molecules

Answer 1

c) Molecular vibrations are due to periodic motions of atoms in molecules , and include bond stretching, torsional changes , and bond angle changes .

Answer 2

Solution:

(a) Infrared radiation is higher in energy than UV radiation.

• In the electromagnetic spectrum, the wavelength of UV radiation is shorter (10 nm to 380 nm) than wavelength of Infrared red radiation (750 nm to 1 mm).

• Relation between energy and wavelength is given by following expression:

         $E = \frac{hc}{\lambda}$

         $E \propto \frac{1}{\lambda}$

• This means energy is inversely proportional to wavelength.
• Smaller is the wavelength of a radiation, greater will be its energy.

• UV radiation is higher in energy than Infrared radiation.
• The given statement is wrong.

(b) Infrared spectra record the transmission of IR radiation.

• The infrared spectrum of a sample is recorded by passing a beam of infrared light through the sample.
• When the frequency of the IR is the same as the vibrational frequency of a bond or collection of bonds, absorption occurs.
• The given statement is correct.

(c) Molecular vibrations are due to periodic motions of atoms in molecules, and include bond stretching, torsional changes, and bond angle changes.

• A molecular vibration is a periodic motion of the atoms of a molecule relative to each other, such that the center of mass of the molecule remains unchanged.
• The given statement is correct.

(d) Infrared spectra give information about bonding features and functional groups in molecules.

• The IR spectra tell us what types of vibrational modes (motion) the molecule responds with after it absorbs that light.
• When we figure out which peaks correspond to which motions, one can figure out what functional groups the molecule has and (almost) what the molecule is.
• The given statement is correct.