Question

H2o,hcl,co2,n2o which will not give rotational spectra

Answer 1

Answer:

Explanation:

Molecular rotations require little energy to excite them. Pure rotation spectra occur in the microwave region of the spectrum (~1 - 200 cm-1). It is important to note that a molecule cannot rotate about some arbitrary axis - the principle of conservation of angular momentum dictates that only a few rotations are possible. In general, rotation must be about the centre of mass of a molecule, and the axis must allow for conservation of angular momentum. In simple cases, this can often be recognised intuitively through symmetry - such as with the water molecule.

A pure rotation spectrum can only arise when the molecule possesses a permanent electric dipole moment. Like with vibrational spectroscopy, the physical effect that couples to photons is a changing dipole moment. Since molecular bond lengths remain constant in pure rotation, the magnitude of a molecule's dipole cannot change. However, since electric dipole is a vector quantity (it has both size and direction) rotation can cause a permanent dipole to change direction, and hence we observe its spectra.  Since homonuclear molecules such as dinitrogen (N2) have no dipole moment they have no rotation spectrum.  Highly symmetric polyatomic molecules, such as carbon dioxide, also have no net dipole moment - the dipoles along the C-O bonds are always equal and opposite and cancel each other out. It is important to recognise also that if a molecule has a permanent dipole, but this dipole lies along the main rotation axis, then the molecule will not have a rotational spectrum - such as for a water molecule.

In pure rotational spectroscopy for a simple diatomic molecule, the energy levels - as displayed below - are given by EJ = BJ(J+1), where J is the rotational quantum number, B is the rotational constant for the particular molecule given by B = h2 / 8π2I with the unit of Joules, where I is the moment of inertia, given by I = μr2 - where r is the bond length of this particular diatomic molecule and μ is the reduced mass, given by μ = m1m2 / m1 + m2.

Most energy level transitions in spectroscopy come with selection rules. These rules restrict certain transitions from occuring - though often they can be broken. In pure rotational spectroscopy, the selection rule is ΔJ = ±1.

Note: After selecting a molecule, select the energy level again to observe its rotation. Changing the molecule's orientation will cause it to display an incorrect rotation.

 

Answer 2

$CO_{2}$ will not give rotational spectra.

• Homonuclear compounds like dinitrogen (N2) don't have rotation spectra because they lack a dipole moment.
• Carbon dioxide is an example of a highly symmetric polyatomic molecule that has no net dipole moment because the dipoles along the C-O bonds are always equal, opposing, and cancel each other out.
• the polar molecules provide the pure rotational spectrum. Therefore, symmetrical linear molecules like CO2 (or symmetric molecules like C6 H6) and homonuclear diatomic molecules like H2 and N2 do not produce rotational spectra.
• polar molecules like H2O, NO, N2O etc. give pure rotational spectra.

Application

• Utilizing radio telescopes to investigate the chemical make-up of the interstellar medium is a significant use of rotational spectroscopy.

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