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

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Discussion Overview

The discussion revolves around the differences in vibrational frequencies of H2 and H2+ molecules, specifically questioning why H2 has a vibrational frequency of approximately 4400 cm-1 while H2+ has a frequency of about 2300 cm-1. The scope includes theoretical considerations and conceptual clarifications related to molecular vibrations and spectroscopy.

Discussion Character

  • Exploratory
  • Technical explanation
  • Conceptual clarification
  • Debate/contested

Main Points Raised

  • One participant notes the significant difference in vibrational frequencies and speculates that it may be due to H2+ having one less electron, leading to more delocalization between the hydrogen nuclei.
  • Another participant suggests that with only one electron in H2+, it is responsible for the entire bond energy, potentially affecting the vibrational frequency.
  • A participant questions whether there is confusion between vibrational and rotational frequencies.
  • There is a discussion about the units cm-1, with one participant clarifying that it is a unit related to wavelength in spectroscopy, not frequency.
  • Another participant expresses confusion about the terminology used in chemistry and spectroscopy, suggesting that it complicates understanding.
  • A chemist in the discussion argues that wavenumber is more convenient than wavelength as it is directly proportional to energy, relating this back to the initial question about vibrational frequencies.

Areas of Agreement / Disagreement

Participants express differing views on the implications of electron count on vibrational frequency and the terminology used in spectroscopy. The discussion remains unresolved regarding the exact reasons for the differences in vibrational frequencies.

Contextual Notes

There are limitations in the discussion, including assumptions about the relationship between electron count and bond energy, as well as the potential confusion surrounding the terminology of frequency and wavenumber in spectroscopy.

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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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.
 
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?
 
Vibrational vs rotational, are you sure you are not confusing them?
 
What kind of frequency has the units cm^(-1)?
 
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 c=\nu\lambda. A little bit obscure when you are not in the field, but should be clear for every chemist :)
 
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?
 
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!
 

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