Selection rules for rotational spectra. A molecule must have a transitional dipole moment that is in resonance with an electromagnetic field for rotational spectroscopy to be used. Polar molecules have a dipole moment. A transitional dipole moment not equal to zero is possible.

A pure rotation spectrum can only arise when the molecule possesses a permanent electric dipole moment. Since homonuclear molecules such as dinitrogen (N2) have no dipole moment they have no rotation spectrum.

Explanation: molecules with permanent dipoles are microwave active (the molecule must be polar), e.g. heteronuclear diatomics – HCl, CO, NO, etc. Homonuclear diatomics are microwave inactive (e.g. O2, N2, etc.) In other words, a dipole must be present in the molecule for you to get a rotational spectrum.

CH3Cl and CH2Cl2. — For rotational spectra the gross selection rule is: All molecules that do not possess a permanent dipole moment are microwave inactive. Absorption or emission of one photon can therefore increase or decrease the rotational quantum number by one unit.

Answer: homonuclear diatomic molecule like H2, O2, N2 etc. which have only stretching motion/ vibrations and no bending motion/vibrations, the dipole moment does not change during vibration. Hence these molecules do not give vibration spectra i.e. they are said to be infrared-inactive.

Due to the dipole requirement, molecules such as HF and HCl have pure rotational spectra and molecules such as H2 and N2 are rotationally inactive.

A pure rotational spectrum cannot be observed by absorption or emission spectroscopy because there is no permanent dipole moment whose rotation can be accelerated by the electric field of an incident photon.

: a vector quantum number that determines the angular momentum of a molecule rotating about an axis through its center of mass.

Rotational energy levels – diatomic molecules In this equation, J is the quantum number for total rotational angular momentum, and B is the rotational constant, which is related to the moment of inertia , I = μr2 (μ is the reduced mass and r the bond length) of the molecule.

As the molecule rotates, its rotational energy continually changes and if the change is great enough, the electron in the molecule can gain that energy and transition to a higher quantum state. Such transitions are known as molecular electron transitions and they obey the selection rules of quantum mechanics.

Rotational Transitions, Diatomic The rotational spectrum of a diatomic molecule consists of a series of equally spaced absorption lines, typically in the microwave region of the electromagnetic spectrum. The energies of the spectral lines are 2(J+1)B for the transitions J -> J+1.

radiation can cause changes in rotational energy levels within molecules, making it useful for other purposes. The rotational energy levels within a molecule correspond to the different possible ways in which a portion of a molecule can revolve around the chemical bond that binds it to the remainder of the…

Rotational Transition Selection Rules The transition ∆J = 0 (i.e. J” = 0 and J’ = 0), but where v0 = 0 and ∆v = +1, is forbidden and the pure vibrational transition is not observed in most cases. The rotational selection rule gives rise to an R-branch (when ∆J = +1) and a P-branch (when ∆J = -1).

Re: Does a monatomic atom have rotational motion? Monatomic gases don’t have any rotational energy because they have nothing around which to rotate, so yes, we only account for their translational energy since their rotational energy is zero.