Group theory + symmetry Flashcards
What is Raman spectra due to vs IR
Scattering of radiation due to vibration as opposed to absorbtion of radiation (IR)
What are the 3 degrees of freedom
Translational, rotational and vibrational motion
How to work out the number of vibrational degrees of freedom of a compound + an exception
3N - 6 and N is number of atoms in compound. -6 becasue 3 directions of translation (x, y, z) and 3 rotational axis. Exception: 3N - 5 for LINEAR molecules - no rotation about molecular axis
How does IR and number of vibrational degrees of freedom corrolate
If 3 degrees -> 3 peaks in IR. But there are expections if molecs have high symmetry
Difference between peaks of H2O and D2O in IR spectra
D2O has lower frequency because harder to vibrate heaver atoms
What does improper rotation eg S3 do?
Rotate by C3 then reflect in σh
For improper rotation, what does S3^3 (S 3 times) and S3^6 do
S33 = σh and S3^6 is E aka identity
What is the character of a square matrix
Sum of diagonal elements (top left to bottom right), symbol: χ
What is gamma
Used to describe translations
10 steps to do gamma transfomations?
1) Assign point group. 2) See character table (has A1, A2, E) 3) Count unshifted atoms for every symmetry opetation. 4) Find contribution of eahc unshifted atoms = from character of sq matrix for each operation. 5) Derive reducible representation of Gamma 3N. 6) Use reduction formula. 7) gives irreducible rep of gamma 3N. 8) Derive gamma trans and gamma rotational motion. 9) Gives gamma vibrations. 10) Check answer with 3N - 6
When will Xe form bonds/react
With very electronegative reactant s
Lewisstructure if the central atom is bonded to an oxygen and has no charge
Double bond to O, single bond causes one -ve charge
What does σd symmetry mean
Mirror planes between bonds - diagonal
What does 3N-6 show
There are 3 translations, 3 rotations and 3N-6 vibrations
Why is there 2 C4 symmetries
Should be 4xC4 but C4 x 4 times is E. C4 x 2 is C2 so there are only 2 C4s left so it says 2C4 in character table
When calculating degrees of freedom from A1, A4, B1 … how much is E worth
2 because it is doubly degenerate
How is translations sometimes labelled in character table
x, y, z instead of Tx, Ty, Tz
Difference between working out Γ3N and Γstr, ipb and oopb
Γ3N uses contribution of atoms from character in matrix. Γ for the rest does not - don’t multiply with anything
Why does Γstr + Γipb + Γoopb equal more than Γvib
Some are redundant. Compare each ones to see if its A1 or whatever that is redundant. Can’t increase all angles at the same time - one has to get smaller
What does the A/B/E/T Mulliken symbols represent
A/B= 1x1 rep, E = 2 (double degenerate), T = 3, triply degen
How to work out ipb and oopb for a tetrahedral
No plane so work out all of them together as bends
How Raman works, very brief
Laser at sample cell, photons bounce off gas molecules and collected in detector. Monochromator reads it
3 types of scattering in Raman spectroscopy
Stokes: Molecule absorbes energy (photon loses some), Rayleigh: no change in energy. Anti-stokes: molec absorbs energy from photon
Which scattering is looked at in Raman
Stokes lines
How is Raleigh scattering related to wavelength
Raleigh is inversely prop to fourth power of wavelength - eg short blue light will scatter more than longer. (sky looks blue)
How to know if vibration appears in Raman/IR
Look at character table. x, y, z = IR, anything squared eg x^2, xy.. is Raman
Whats the only vibration in Raman that gives (pol) = polarised light
Totally symmetric eg A1
Do stretches or vibrations occur at higher energy in Raman + IR
Stretches are at higher
What type of vibration is normally under 600 in IR
Bend
How to tell if a line is B1 or B2
Difficult to know, just have to guess kinda
Does ipb or oopb tend to have a lower energy in IR and Raman
oopb normally lower E
