IR spectroscopy Flashcards
What is a wavenumber? (v)
Number of wavelengths in 1cm
v = 1/ λ
Can IR energy cause electronic transitions?
NO, unlike UV-visible light the IR energy is too low to cause electronic transitions. It is just enough to cause vibrational and rotational excitations which have much smaller energy gaps
Define vibrational state
Molecules are excited to a higher energy state
Explain absorption
The absorption is quantized -energy has to match energy gap between the vibrational states
Only certain frequencies corresponding to natural vibrational frequencies of bonds are absorbed
What is each vibrational level associated with?
Several rotational levels so broad band of absorption appears rather than single lines
How are IR spectra traditionally displayed?
% transmittance vs Wavenumber (4000-650 cm^-1)
How is absorption of IR radiation represented on the spectra?
By a trough in the curve i.e. 0% T = 100% absorption of light at that wavelength.
How do you calculate the frequency of vibration at a v = 4000 cm^-1 ?
λ = 1/4000 = 2.5 x 10-4 frequency = c/λ = 3 x 1010 / 2.5 x 10-4 = 123 x 1012 vibrations/sec
What is the frequency at a given bond vibrate?
CONSTANT
Why can certain bonds only absorb certain frequencies?
Vibrations are discrete.
Every bond will have DIFFERENT frequencies (CONSTANT). So no two molecules will have exactly the same IR spectrum
What levels can you go to in the mid IR region?
In the mid IR region you can only go from V0 to V1, V1 to V2 but not V0 to V2 directly.
What are the 2 types of vibrations?
Stretching and Bending
What is stretching?
Rhythmic movement along the bond axis. There is symmetrical stretch and asymmetrical stretch. In both cases, interatomic distances (C-H) change during stretch, Increases or decreases
What requires more energy, stretching or bending?
STRETCHING
Asymmetric > symmetric > bending
How many ways of bending are there?
FOUR
i) In plane bending
a) scissoring - more
narrow
b) rocking
ii) Out of plane bending
c) wagging
d) twistingWhat would a bond require to be able to interact with the electric component of EMR?
Charge separation (dipole moment)
What vibrations give rise to IR bands?
Vibrations that result in a net change in polarity (dipole moment). This is the only way the photon from IR radiation can transfer its energy to the molecule
What do vibrations that result in bigger dipole movement change have?
Stronger absorption e.g. O-H bond absorbs stronger than C-H
To have a change in energy what does there have to be?
A net change in polarity (dipole-dipole)
What type of bonds give no IR peaks at all?
Symmetrical bonds (N2, O2 , H2, Cl2 ) with no dipole moment - therefore no absorption
Do symmetrical stretches give peaks?
Symmetrical stretches (e.g. CH2, CO2 ) – NO peaks but asymmetrical stretches and bending modes that gives rise to dipole moment change gives peaks
