IR Flashcards
what is IR spectroscopy
IR radiation is shone through a sample. certain frequencies are absorbed/ ( a photon is absorbed).this causes the sample to vibrate at a higher level.
what does 100% transmission mean in IR
No energy is absorbed.
how to prepare a liquid for IR Spec
it is applied as a thin film between two nacl platyes
how to prepare a solid for IR Spec
grind it with nujol to create mull. it is applied as a thin film between 2 nacl plates
how to prepare a gas for IR Spec
a glass cell if filled with sample, the window is made with inorganic salt as glass absorbs IR.
what happens when A molecule absorb IR photon
They gain vibrational energy. the bonds vibrate more.
names of vibration energy levels
v= 0 , v = 1 ground state or fundemental
v = 2 first overtone
v =3 second overtone
size of intensities of energy levels
v= 0,1 is greater than v=2 , v=2 is greater than v= 3.
It mimics the spacing of vibrational levels.
How to model vibration of diatomic molecules
two balls on a spring.
what can bonds do
compress and stretch
what happens when the bonds stretch and compress
the energy of the system rises to go back to the equilibrium length
harmonic oscillator
a model use to describe small oscillations
frequency of oscillations
v = 1/2 pi * sqrt Kf / m
formula for u (mew )
u = m1 m2 / m1 + m2
what is mew
reduced mass
how to find wave number
v/ = 1/2pi c * sqrt kf/ mew
why is does molecules with stronger bonds give higher vibration energy
they have a higher force constant.
what happens during normal mode for molecules
the centre of the mass remains fixed and the atoms vibrate the same frequency but possibly at different amplitudes
how to calculate normal mode
linear 3N - 5
‘non linear 3N - 6
Ways molecules can bend as stretch
asymmetrical stretch
symmetrical stretch
bending
Strength of IR absorption
the strength is the intensity which it is absorbed. the bigger the trough the greater the intensity.
This is caused by dipole dipole moments changing in vibration.
homonuclear
Homonuclear diatomics do not have permanent dipole moments. This means no absorption from IR
from x-h on ir
4000-2500
Triple bonds
2500-2000
double bonds
2000-1500
Other single bonds
1500-500
why does hybridization increase bond stretching frequency
Greater bond strengths. greater KF
why do smaller rings have greater bond frequency
harder to compress smaller rings
in symetrical shapes bonds / carbons can be
degenerate
do aldehydes or ketones have greater bond frequency
aldehydes
why do aldehydes have greater bond frequency
less conguation
one set of alpha hydrogen
