Vibrational spectroscopy Flashcards
What is a vibrational transition?
Transition between vibrational energy levels
What is the value of r at the bottom of the well equal to?
Equilibrium bond distance
What must the mode have for a specific vibrational mode to absorb or emit infared radiation?
The mode must have a dipole moment that changes during the period of the molecular vibration
What type of molecules do not usually absorb/emit infared radiation?
Homonuclear diatomics
What type of molecule have observable infared spectra?
Heteronuclear diatomics
What are the three different ways an atom can move?
Rotation
Translation
Vibration
How many different movements do diatomic atoms have?
3 translations
2 rotations
1 vibrations
What do you need to make sure for the movements of diatomics?
That there is either a translation, rotation or vibration but not a mixture of them
Why don’t you want a mix of movements for diatomics?
A mixture wont give us the simplest, more reducibly outcome in terms of describing a system
what happens if you can change the dipole moment as a result of the vibrational moment?
It will absorb IR radiation
What happens if you cant change the dipole moment as a result of the vibrational moment?
It wont absorb the IR radiation
What are the 4 modes of CO2?
Symmetric stretch
Bending motion
Asymmetric stretch
Bending motion
Which modes of CO2 are IR active?
Bending
Asymmetric
What are the features of a symmetric vibration?
No dipole moment change
IR inactive
What is the equation to work out the number of vibrational nodes in a linear molecule?
3N-5
N is the number of atoms
What is the equation to workout the number of vibrational nodes for non-linear molecules?
3N-6
N is the number of atoms
What do translation and rotation modes add to?
5
What do vibrational modes of H2O all lead to?
A change in dipole moment
What are the three modes of H2O?
Bending
Symmetric
Antisymmetric
Are the H2O nodes IR active or inactive?
Active
How many modes of vibration do translations have?
3
How many modes of vibrations do rotations have?
3
What are the 6 molecular motions?
Symmetrical stretching Antisymmetrical stretching Scissoring Rocking Wagging Twisting
Which molecular motions are found at higher frequencies?
symmetric stretch
Asymmetric stretch
