Why do H2 and H2+ molecules have different vibrational frequencies?

  • Thread starter terp.asessed
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In summary, the conversation discusses the vibrational frequencies of H2 and H2+ molecules and why they differ. The equation v = ω/(2πc) is mentioned as a possible explanation, and it is suggested that the difference in vibrational frequency could be due to the number of electrons responsible for the bond. It is also noted that cm-1 is a unit for describing wavelength in spectroscopy and that wavenumber is a more convenient unit for measuring energy.
  • #1
terp.asessed
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Homework Statement


It's just that I am curious from the vibrational frequency table in the textbook--could someone tell/explain to me why H2 molecule's vib. frequency is about 4400cm-1, whereas H2+ is 2300cm-1? It just seems bizarre to me that such similar molecule should have different vibrational frequency.

Homework Equations


maybe v = ω/(2πc)? I am not sure, but I think the equation is related to my question above.

The Attempt at a Solution


What I am sure of now is that H2's vib. frequency is about x2 that of H2+...so I wonder if this problem arises from H2+ having less electron, thus more delocalization between two H nuclei?
 
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  • #2
There are two electrons, to some extent each is responsible for half energy of the bond. When you remove one, there is only "half bond" left.
 
  • #3
Thank you for the reply! Aside, so, since H2+ has only one electron responsible for the bond, b/c it has to take responsiblity of ALL the bond energy, one electron taking the role of two at the same time, its vibrational frequency is twice the H2? So, it has nothing to do with angular frequency?
 
  • #4
Vibrational vs rotational, are you sure you are not confusing them?
 
  • #5
What kind of frequency has the units cm^(-1)?
 
  • #6
It is not a frequency. cm-1 is a unit of describing the wavelength in spectroscopy (in a way similar to Hz describing frequency), and wavelength and frequency are related by [itex]c=\nu\lambda[/itex]. A little bit obscure when you are not in the field, but should be clear for every chemist :)
 
  • #7
Thank goodness I'm not a chemist!

It must be extremely confusing to be required to call everything by the wrong name. Why not call it wave length and be done with it?
 
  • #8
I don't think it is strictly limited to chemistry, more like to spectroscopy (being a chemist I am a little bit skewed). Wavenumber is more convenient than the wavelength as it is directly proportional to energy (which is particularly convenient in the case of the initial question - 2300 cm-1 is almost exactly half of the 4400 cm-1. Bingo!
 

Related to Why do H2 and H2+ molecules have different vibrational frequencies?

1. What is vibrational frequency?

Vibrational frequency is a measure of the rate at which particles in a substance vibrate or oscillate. It is a physical property that can be used to describe the energy of a system.

2. How is vibrational frequency measured?

Vibrational frequency is typically measured in units of hertz (Hz), which represents the number of vibrations per second. This can be measured using specialized equipment such as an oscilloscope or spectrometer.

3. What factors affect the vibrational frequency of a substance?

The vibrational frequency of a substance can be affected by factors such as the chemical bonds between particles, the temperature of the substance, and the presence of external forces like pressure or electric fields.

4. Why is vibrational frequency important in science?

Vibrational frequency is important in science because it can provide information about the physical and chemical properties of a substance. It is also used in various fields such as chemistry, physics, and engineering to study and manipulate materials at the atomic level.

5. How does vibrational frequency relate to sound and light waves?

Vibrational frequency is the basis for sound and light waves. In sound, it refers to the frequency at which air particles vibrate to create sound waves. In light, it refers to the frequency of electromagnetic radiation which determines the color of light. Both sound and light waves can be described in terms of their vibrational frequency.

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