Question

Which of the following molecules would show rotational spectrum: Br2, CO,
N2, HBr and CS2? Why? Among them which are IR active? Calculate their
total mode of vibrations

Answer 1

Answer:

HBr and CO show rotational spectrum.

Br₂ , N₂ and CS₂ are IR inactive while CO and HBr are IR active.

Number of modes of vibration for Br₂, N₂ ,CO and HBr is equal to one while four for  CS₂.

Explanation:

• Molecules which do not have permanent dipole moment, cannot show rotational spectrum such as homodiatomic molecules.
• The mandatory condition for a molecule to be IR active is that the molecule should have permanent electric dipole moment.
• Homo-diatomic molecules are always IR inactive because they have zero dipole moment. Therefore, Br₂ and N₂ are IR inactive.
• Similarly, CS₂ has linear structure and both dipoles $C=S$ cancel each other. Therefore it is also IR inactive.
• While HBr and CO have non-zero diploe moment. Therefore they are IR active.
• Number of modes of vibrations for linear molecules $=3N-5$

1. For  Br₂, N₂ ,CO and HBr , the value of N=2. Therefore, number of modes of vibrations $= 3(2)-5=1$
2. For  CS₂, number of modes of vibrations $=3(3)-5=9-5=4$

Answer 2

Answer:

Concept:

Measurement of the energy of transitions between quantized rotational states of molecules in the gas phase is the focus of rotational spectroscopy. Microwave or far infrared spectroscopy can be used to measure the absorption or emission spectra of polar compounds. By contrast, Raman spectroscopy allows for the observation and measurement of the rotational spectra of non-polar molecules, which cannot be done by the other methods.

Given:

Which of the following molecules: Br2, CO, N2, HBr, and CS2 would exhibit a rotating spectrum? Why? which of them have IR activity? Determine their overall vibrational mode.

Find:

find the answer for the given question

Answer:

the region of a chemical spectrum where the bands are caused by quantized variations in the energy of rotating molecules, as opposed to vibrational spectrum.

A particle or system's permanent electric dipole moment (EDM), which results from elementary particle interactions that flagrantly break parity and time-reversal symmetry, is a separation of charge along the direction of the system's total angular momentum.

HBr and CO are rotational spectrum

Br₂, N₂, CO, and HBr all have one mode of vibration, whereas CS₂ has four.

While each atom has its own motion, a molecule as a whole has translational and rotational motion. Raman or IR active vibrational modes are both possible. Changes in the permanent dipole are necessary for the observation of a mode in the IR spectrum (i.e. not diatomic molecules). In contrast to IR spectra, diatomic molecules can be seen in Raman spectra. Diatomic molecules only contain one band and no permanent dipole, leading to a single vibration as a result.

modes of vibration

$=3(2)-5\\=6-5\\=1$

For  CS₂

$=3(3)-5\\=9-5\\=4$

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