What are the main features of molecular spectra?

The specific features of molecular spectra are mainly due to the partition of the energy of the molecule into electronic, vibrational and rotational parts, each of the latter two being small compared with the previous one.

What do you mean by rotational spectra?

Definition of rotational spectrum : the part of a molecular spectrum in which the bands arise from quantized changes in the energy of molecular rotation — compare vibrational spectrum.

What is the basic principle of microwave spectroscopy?

It is based on the principle that microwave radiation (see microwaves) causes changes in the rotational energy levels of molecules and absorption consequently occurs at characteristic frequencies.

What are the selection rules for rotational spectrum?

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.

What do you understand by molecular spectra What are qualitative features of there spectra?

Definition of molecular spectrum : a spectrum of radiation due to electron transitions and other quantum energy changes within molecules and consisting of series of characteristic spectrum bands which are found upon high dispersion to be made up of very fine lines.

What molecules give rotational spectra?

Thus polar molecules like H2O, NO, N2O etc. give pure rotational spectral.

Why is rotational spectra given only by gaseous molecules?

Rotational Spectroscopy, Theory In solids, molecules are usually not free to rotate, and in liquids collisions normally render absorption featureless; we therefore consider only the rotational spectroscopy of gaseous molecules.

Which molecules will show a microwave rotational spectrum?

Gross Selection Rule: 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.

What type of molecules give rotational spectra?

What are selection rules explain?

A selection rule describes how the probability of transitioning from one level to another cannot be zero. It has two sub-pieces: a gross selection rule and a specific selection rule. Selection rules specify the possible transitions among quantum levels due to absorption or emission of electromagnetic radiation.

What is the principle behind molecular absorption spectroscopy?

The basis of molecular spectroscopy is the excitation of atoms and molecules by photons. Atoms and molecules excited from the ground state undergo either resonant vibrations or electronic transitions, depending on the nature of the induced quantum mechanical changes.

What is the difference between pure rotation and vibrational spectra?

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.

What is an example of rotational spectroscopy?

Explore examples of rotational spectroscopy of simple molecules. A rotational spectrum would have the following appearence. Each line corresponds to a transition between energy levels, as shown. Notice that there are no lines for, for example, J = 0 to J = 2 etc.

What is pure rotation spectroscopy?

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.

What is the rotational spectrum of a spherical top molecule?

Unlike other molecules, spherical top molecules have no net dipole moment, and hence they do not exhibit a pure rotational spectrum. The spectrum for these molecules usually involves many lines due to three different rotational modes and their combinations.