Unformatted text preview: termine the symmetries of all of the acetylene
vibrations; remember that rotation around the axis (i.e. the molecular axis) is trivial and not
therefore a meaningful degree of freedom.
, the translations are
gives and the two nontrivial rotations are
. . Removing b. (4 points) How many normal modes are potentially active in an IR spectrum? In a Raman
and axes transform as
and , so there are three potentially IR active
modes, should be degenerate and give rise to only
vibrations (though two of them, the
one IR band). The second order functions (e.g. ) transform as , , and , so there
are four potentially Raman active modes (but once again, the
modes would be
degenerate and give rise to a single band).
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