Subject: Summary Is "Vibrational Spectroscopy" generally equals to "IR spectroscopy"?

From: "Zhenhua Li" <[email-masked]>
Date: Mon, 21 Jan 2002 09:23:11 +0800

Thanks for everyone. Here is my summary. In my original question, I made a
mistake typing "absorption" to "adsorption".

It seems most listers agree that Vibrational Spectroscopy does not equal to
IR spectroscopy. It is more general than the later.

Original question:

Hi, Dear listers,
Today several staffs have a discussion about what is the implied or
generally accepted meaning of "vibrational spectroscopy". One guy insisted
that it is generally accepted that when we talked about vibrational
spectroscopy, without specify IR or Raman before it, we must say that it is
an "adsorption spectroscopy".  But I insisted that any spectroscopy that
originated from vibrational adsorption or emission can be called
"vibrational spectroscopy". Thus, when we talked about "vibrational
spectroscopy", we do not really means it is an IR spectroscopy.
This is important since the meaning of this term has appeared in an exam
paper. It is important to ensure not to give students wrong concept.
Thanks a lot.

Li Zhenhua

Comments:

1. Kimberley Cousins
Vibrational spectroscopy means any method that caused vibrations
(stretches bends). Some molecular stretching and bending occurs outside
the "normal" mid range IR region, hence could not fully and correctly be
called IR spectroscopy.

2. Gary Strahan
You are correct, "vibrational spectroscopy" refers to any method
of spectroscopy which involves the vibrational modes or energy
levels of a molecule.

Note that "absorption spectroscopy" does not need to involve
vibrations, but can also involve rotational energy levels and the
link. Hence, "absorption spectroscopy" it includes many other
spectroscopic techniques besides besides UV, IR and Raman.

Indeed, Raman and fluorescence are not strictly "absorption"
spectroscopies as they also involve scattering and emmission
processes, respectively.

In other words, the terms "absorption" and "vibrational" describe
different things.  Some techniques fall into only one of the two
categories, while other techniques fall into both categories.

So, to be completely correct, one should use terms such as,
"vibrational IR absorption," "vibrational Raman spectroscopy,"
"rotational fluorescence emmission."  More general terms might
be "vibrational absorption spectroscopy," "rotational absorption
spectroscopy," "vibrational Raman scattering," etc.

Note that you used the term "adsorption" below, instead of
"absorption."  These are also quite different things.  Adsorption
is the phenomenon of one molecule sticking to the surface of something
else, usually a chromatographic column.

3. Steve Cabaniss
	In my opinion, the term 'vibrational spectroscopy' refers to both IR
absorption and to Raman scattering.  This is how we use the term in our
analytical spectroscopy classes.  Graybeal (Molecular Spectroscopy 1988)
discusses Raman in his chapter on vibration, and McHale (Molecular
Spectroscopy 1999) discusses Raman in her chapters on vibrational
spectroscopy.  This appears to be a generally accepted useage.

4. David Shobe

My own opinion only:

In practice "vibrational spectroscopy" is generally synonymous with "IR
spectroscopy."  However, there is a reason for having the 2 terms, since
they are not *precisely* synonymous, and different concepts are involved in
the 2 terms even if in practice the spectra being discussed are the same.

"Vibrational" spectroscopy refers to the method in which the substance gains
energy as a result of absorption: the energy goes largely into nuclear
motions which involve changing of bond lengths & angles.  "IR" (=
"infrared") spectroscopy refers to the frequencies of electromagnetic energy
being absorbed.

As for "absorption" spectroscopy (I expect "adsorption" is a mistake, since
I still get those words mixed up on occasion myself :-), this is an entirely
separate issue.  Absorption spectroscopy means that material is gaining
energy at the expense of the electromagnetic field; emission spectroscopy
means that the material is emitting energy as electromagnetic radiation.

5. Steve Williams
As one whose graduate training was in one of more unusual vibrational
spectroscopies, I would suggest that a vibrational spectroscopy is one of
any methods that probes differences between vibrational energy
levels.  This would include IR (the usual absorbance method as well as
emission) as well as a whole host of inelastic scattering methods:
Raman spectroscopy for which the selection rules are well understood;
Inelastic Neutron Scattering
Inelastic Electron Tunneling Spectroscopy (IETS)
Low energy Electron Energy Loss Spectroscopy (LEELS)
There are probably more such techniques now.  I think that the selection
rules for the later scattering methods are not as well understood as IR and
Raman.
In addition to the direct vibrational methods, there is often a significant
amount of information about ground and excited vibrational levels in the
electronic spectrum of a molecule as well.

6. Stefan Grimme
In my opinion, vibrational spectroscopy invloves
transitions between (ro)vibrational wave functions
(i.e. depending mainly on nuclear degrees of freedom)
and is independent how these transitions are induced
and how the selection rules look like (think e.g.
of vibrational CD)

7. Roy Jensen

In my opinion, INFRARED SPECTROSCOPY is any method that probes the
vibrational levels in a single electronic state of a molecule.

Similar definitions can be applied to ROTATIONAL and ELECTRONIC
spectroscopy. In all cases, all lower energy transitions are coupled to
the given transtion. They may not be resolved, but they account for
features. For example, the rotational transitions associated with a given
infrared transition generate the PQR bands common in conventional
infrared spectroscopy.

8. Ida N. L. Ma
        Vibrational spectroscopy describes the physical origin of the
absorption/emission of the energy transition.  IR is just at the correct
wavelength for this to happen.