Dear Varathu,
Rahul is right. Only if the molecule is assymetric (heteronuclear) will
rotational spectrum be observed, since if it is homonuclear there will be
no dipole component change during the rotation and hence no interaction
with radiation. But you can't understand what the selection rules and
dipole terms are until unless you have completed your first year M.Sc.
So here the anwer goes in a classical manner (not in QM al)
The basic requirement for the emission or absorption of radiation by a
transition between rotational energy states is that the molecule must have
a permanent dipole moment. This is what the classical electrodynamics
says. Accor. to this a rotating molecule can lead to emission of radiation
only if a changing dipole moment is associated with it. Heteronuclear
diatomic molecules with unlike atoms, for which the centers of +ive and
-ive charges don't coincide, hence have a dipole moment. During the
rotation of the molecule the component of the dipole moment in a fixed
direction changes periodically with the frequency of rotation,v , of the
molecule so that, classically, the radiation of frequency v should be
emmited. Homonuclear molecules have no dipole moment and hence there is no
emmision of IR radiation.

I think this answers your question.


Best wishes,
Vandana


Vandana Sharma
Laboratory Astrophysics,
LAB- 570
Space and Planetary Atmosphere Division,
Physical Research Laboratory,
Navrangpura,
Ahmedabad- 380 009
Gujarat, India

Phone No : (91)-079-6302129-Ext-4570/4567(lab)
Email : vandanas@...
van_res@...
URL : http://www.prl.ernet.in/~vandanas

On Sat, 3 Jul 2004, [iso-8859-1] varathu rajan wrote:

> Dear Madam,
> My question is why a molecule should have permanent dipole moment to
absorb the Micro Wave radiation.
> In other words why molecules such as Hydrogen does not give rotational
spectra?
> Similarly why symmetrical stretching of CO2 is Infrred inactive?
>
> Friendly,
> Varathu.
>
> Vandana Sharma <vandanas@...> wrote:
> Dear Varathu,
>
> Could you please elaborate your question.
>
> Regards,
> Vandana
>
>
> Vandana Sharma
> Research Scholar
> Laboratory Astrophysics,
> LAB- 570
> Space and Planetary Atmosphere Division,
> Physical Research Laboratory,
> Navrangpura,
> Ahmedabad- 380 009
> Gujarat, India
>
> Phone No : (91)-079-6302129-Ext-4570/4567(lab)
> Email : vandanas@...
> van_res@...
> URL : http://www.prl.ernet.in/~vandanas
>
> On Thu, 24 Jun 2004, [iso-8859-1] varathu rajan wrote:
>
> > Dear Friends,
> >
> > I have a dobt in the fundamentals of spectrosopy.
> > My doubt is
> > 1.For a molecule to be MW active,why it should have permanent Dipole moment
&.
> > 2.To be IR active, why a molecule should undergo net change in its dipole
moment?
> >
> > Plz clear my doubts
> >
> > Friendly,
> > Varathu
> >
> >
> >
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