Molecules with Microwave/Rotational spectrum

In summary, according to the textbook and Wikipedia, all molecules have a rotational spectrum. However, diatomic molecules like O2 and H2 do not have a dipole moment and therefore do not have a purely rotational spectrum. The rotation of molecules is considered relative to the center of mass, and molecules like H2, N2, and O2 can rotate but do not interact with photons in the microwave or infrared spectrum due to the lack of a net force moment.
  • #1
Tomtom
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Homework Statement


This question is about microwave (rotational) spectroscopy.
Which of the following molecules have a microwave spectrum?
H2, N2, NO, N2O, CH4, CO2, OCS, H2O, Ethene, Benzene.


The Attempt at a Solution


According to my textbook, it appears that all molecules have a rotational spectrum. However, Wikipedia, "Diatomic molecules such as dioxygen (O2), dihydrogen (H2), etc. do not have a dipole moment and hence no purely rotational spectrum."

I'm not really sure what that's supposed to mean. Do all molecules have a rotational spectrum (I don't see why not, everything with a bond should be able to rotate)?

Is the rotation considered relative to one of the atoms in the molecule, or to the centre of mass?
 
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  • #2
Have I understood it correctly if I suggest that H2, N2 and O2 all rotate, but (when not considering temporary dipoles) they should not interact with photons in the microwave (nor infrared for that matter) spectrum?
 
  • #3
That would be my understanding. Both ends of the molecule interact identically, so there is no net force moment that can change rotation.

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1. What is a molecule's microwave/rotational spectrum?

A molecule's microwave/rotational spectrum is a unique fingerprint of the molecule's rotational energy levels. It is obtained by measuring the absorption or emission of microwave radiation by the molecule. This spectrum can provide valuable information about the molecule's structure and properties.

2. What is the significance of studying a molecule's microwave/rotational spectrum?

Studying a molecule's microwave/rotational spectrum can provide important insights into its structure, size, and properties such as dipole moment, bond lengths, and rotational constants. It can also help identify different isotopes of the same molecule and aid in the identification of unknown molecules.

3. How is a molecule's microwave/rotational spectrum measured?

A molecule's microwave/rotational spectrum is measured using a microwave spectrometer. This instrument uses microwaves of different frequencies to excite the rotational energy levels of the molecule, and the resulting absorption or emission is recorded. The spectrum is then plotted as a graph, with the intensity of the signal on the y-axis and the frequency on the x-axis.

4. What factors can affect a molecule's microwave/rotational spectrum?

The shape and size of a molecule, as well as its electronic and vibrational states, can affect its microwave/rotational spectrum. Other factors such as temperature, pressure, and chemical environment can also have an impact on the spectrum. Isotopic substitution can also result in slight changes in the spectrum.

5. How is a molecule's microwave/rotational spectrum used in real-world applications?

The microwave/rotational spectrum is used in a variety of real-world applications, including in the fields of astrophysics, atmospheric science, and analytical chemistry. It is also used in the development and testing of new materials, as well as in the study of chemical reactions and intermolecular interactions. Additionally, it is an essential tool in the identification of molecules in forensic and environmental analysis.

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