Vibrations of Polyatomic Molecules (4) Flashcards
How many fundamental molecular vibrations are possible for a nonlinear polyatomic molecule?
3n-6
How many fundamental molecular vibrations are possible for a linear polyatomic molecule?
3n-5
how many of these vibrations will be caused by bond stretching
n-1
How many vibrations are caused by bond bending motions for nonlinear molecules
2n-5
How many vibrations are caused by bond bending motions for linear molecules
2n-4
What is a normal mode of vibration
fundamental molecular vibrations; molecular motion where all atoms move in phase and at the same frequency
What is a polar bond?
unequal distribution of electrons (ex: c-cl)
what is a non polar bond?
even distribution of electrons (ex: c-c)
What is the dipole moment of a bond?
strength of polarity
how can chemical bonds and their vibrations be described in terms of molecular symmetry?
Stretching
Why are dipole moments important in IR spectroscopy?
Charge separation with magnitude and direction changes symmetry, therefore identify bond stretching
In order to absorb IR radiation, what must happen to the dipole moment during the molecular vibration?
vibration must cause permanent change in dipole moment required for IR absorption
What are degenerate vibrations
modes with equivalent energy
how is IR activity related to the permanent dipole moment of the chemical bond
activity requires permanent dipole moment
What is the IR activity of polar bonds with large values of µ?
very strong
What is the IR activity of nonpolar bonds with small values of µ?
weak or absent
What are group frequencies used for in IR
makes IR very sensitive probe of structure
What are group frequencies
certain sub molecular groups of atoms consistently produce vibrational bands in characteristic frequency regions of the vibrational spectrum
how do group frequencies originate
due to coupling oscillating dipole moments
group frequencies can be very sensitive to
configuration and conformation of local molecular groupings
Is symmetric stretching IR active
No
Sulfur dioxide is a nonlinear molecule. How many vibrational modes will the compound have? How many IR absorption bands?
3N-6 or (3x3) -6 = 3 vibrational modes. Includes symmetric and anti symmetric stretching and a bending/scissoring
Of these vibrations how many are bending motions
Linear 2N - 4, nonlinear 2N-5
