Infrared Spectroscopy
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Why is spectroscopy important?
Helps us detect presence of compounds and confirm their structures
What are photons ?
Packets of energy which are absorbed by atoms and molecules
What does the energy absorbed by moles and atoms correspond to ?
They have a specific amount of energy which relates to region one the electromagnetic spectrum and produces a colour
What does infrared spectroscopy produce instead of electronic transitions ?
Infrared spectroscopy produces vibrational transitions instead of electronic transitions
What are the 3 types of transitions and the order from highest to low energy ?
Electronic Vibrational Rotational
Why doesn’t infrared spectroscopy produce electronic transitions instead?
They don’t have enough energy but can only cause vibrational motion
What is the relationship between frequency and photon energy?
As energy increases , frequency increases ( wavelength decreases) so more photon energy so more vibrations
What is a vibration ?
It’s a repetitive oscillation around an equilibrium point and produces a frequency due to periodic motion
What is an equilibrium point ?
It the point where the object is held
What is the equation to link frequency and photon energy ?
E = hf h = planks constant of 6.63 X 10^-28 Frequency in hertz
What is the equation to show relationship between wavelength and frequency?
F = c/ lambda
What is vibrational transitions ?
This is the molecule absorbs the energy of a photon which corresponds to a specific vibrational frequency of the bond
How do vibrational transitions work in a covelant bond such as carbon monoxide ?
The covalent bond acts like a spring and stretches and compresses Vibrations occur at the same frequency as the light absorbed This transition only happens when the light absorbed is specific and only so then the vibrations happens and matches the vibrational frequency
What are 3 factors that affect vibrational transitions?
Mass of the molecule Bond strength Symmetry effect
How does the mass of a molecule affect the vibrational transitions ?
Heavier atoms produce a low frequency vibrations whereas lighter atoms produce faster frequency vibrations
How does the bond strength affect the vibrational transitions ?
The more the bonds the higher the energy so higher the frequency For example a single covalent bond is weaker than a double and a triple covalent bond so less energy so less frequency
How does the symmetry affect the vibrational transitions ?
There is no dipole which means the electrons stay in the middle of the bond and are not partially positive or negative so they do not absorb infrared radiation so no vibrational transitions. They are called homonuclear diatomics
What are homonuclear diatomics ?
They are diatomic molecules such as Br2 ,O2 which don’t absorb IR radiation due to no dipole
What is a triatomic molecule?
Molecule with 2 or more atoms eg CO2
What happens in term of vibrations in triatomic molecules?
They produce 6 different vibrational motions instead of just stretching and compressing in diatomic molecules
What are the 6 vibrational motions in triatomic molecules?
Asymmetrical Stretching Symmetrical stretching Wagging Scissoring Rocking Twisting Some require more energy than others
Do the motion symmetrical Stretching ?
Arms going in and out
Do the motion Asymmetrical Stretching ?
Both hands in the air and one going up and down in opposite direction
Do the motion wagging ?
Both hands in the air and the body going back and forth