IR Flashcards
How do atoms move
Atoms in a molecule do not maintain fixed positions - they vibrate back and forth.
The bond length is
is an average value.
A diatomic molecule motion
A diatomic molecule, e.g. H-Cl can only
undergo a STRETCHING vibration.
More complex molecules motion
More complex molecules exhibit both STRETCHING and BENDING vibrations.
Vibrational motion and infrared light
Vibrational motion is excited within a single electronic state by infrared light.
Vibrations of bonds involving HYDROGEN are
Vibrations of bonds involving HYDROGEN are very significant as atoms of low mass move a lot more in comparison to atoms of higher mass.
diatomic molecule
Treat a diatomic molecule as two masses joined by a spring
Hooke’s Law
The force to compress or extend a spring is: F(r) = – k (r - re) k spring constant r actual length re equilibrium length
Units of force constant are:
Units of force constant are: N m-1
Strong spring - _______ force constant
Strong spring - large force constant
Weak spring - _________ force constant
Weak spring - small force constant
Strong bonds have ________ force constants.
Strong bonds have large force constants.
Potential energy in the spring, V(r)
V (r) = ½ k(r–re)^2
The vibrational frequency is given by:
v = 1/2(pi) radical (k/μ)
Hz
SI Units
• Hz (or s-1) for v
• N m-1 for k
• kg (per molecule) for μ
μ is the reduced mass
μ= (m1m2) / m1 + m2.
kg
We frequently use masses
We frequently use masses in g mol-1 (e.g. H=1, C=12, O=16 …)
so
μ= ((m1m2) / m1 + m2)* u
Where u is the unified mass constant u = 1.66 x 10^-27 kg
The SI unit of wavenumber is
The SI unit of wavenumber is m^-1
But cm^-1 is more common.
wavenumber
v = 1/lambda
wavenumber
cm^-1
v = 1 k
——– x radical ( ____)
200(pi) c μ
The frequency is:
The frequency is:
- directly proportional to the force constant
- inversely proportional to the reduced mass
Energy levels of a harmonic oscillator
classically:
quantum mechanically:
Classically the harmonic oscillator can have any energy.
Quantum mechanically it only has discrete equally spaced energy levels given by:
Ev = (v + ½) hV
V ( vib. freq)
v = 0, 1, 2,……. energy level
Selection rule
Absorption of light only occurs for ∆v = ± 1
This means that each transition has the same energy change.
what determines whether a molecule will absorb the infrared radiation?
Interaction with light
Oscillating dipole
Interaction with light
- Light has an oscillating electric field.
- The molecule must also possess an oscillating electric field (at
the same frequency). - Photon energy must equal the difference between energy levels.
Oscillating dipole
- Heteronuclear diatomics have a dipole due to the uneven electron distribution around each atom.
- The dipole is the product of the charge distance apart.
- Bond vibrations change the distance and hence the dipole
oscillates. This creates an oscillating electric field.
Infrared inactive
- Homonuclear diatomics (H2, N2, Cl2 etc.) do not have a dipole and cannot absorb light in the IR to excite bond vibrations. (The vibrational motion of homonuclear diatomics can be probed using RAMAN spectroscopy).
- In many molecules, some vibrations are IR inactive, some are IR active. The requirement is a net oscillating dipole, e.g. CO2
Characteristic Vibrations of H2O - all IR active
symmetric stretch n = 3657 cm-1
bend n = 1595 cm-1
asymmetric stretch n = 3756 cm-1
Number of vibrational modes
Linear molecule with n atoms - number modes
= 3n–5
e.g. CO2 n = 3 so number vib modes = 3x3–5 = 4
Non-linear molecule with n atoms - number modes
= 3n–6
e.g. CH4 n = 5 so number vib modes = 3x5–6 = 9
IR spectra are complex
The number of vibrational modes increases very rapidly with n, PLUS many of these vibrations may occur at the same frequency and so not all of the possible bands are seen as independent absorptions.
Stretching vibrations can be:
Stretching vibrations can be: (i) symmetric
(ii) asymetric
Bending vibrations can be:
Bending vibrations can be:
i) scissoring (in-plane
(ii) rocking (in-plane)
(iii) wagging (out-of-plane) (iv) twisting out of plane
Infrared INACTIVE vibrations
Infrared INACTIVE vibrations
• Not all vibrations result in absorption bands.
• For a vibration to cause IR absorption the dipole moment of the molecule must change when the vibration occurs.
IR spectra of Organic/Biological molecules
- Each stretching and bending vibration of a bond is associated with radiation whose frequency exactly matches the frequency of the vibration of the bond.
- Different functional groups, e.g. C–C, C=C, C=O, N–H … will have different characteristic frequencies
- Therefore, by determining the frequencies of light (IR) that are absorbed by a particular compound we can determine what kind of bonds it has.
IR spectra
IR spectra are often shown as % Transmittance vs Wavenumber.
fingerprint region
The fingerprint region is an area of complex overlaps where it is impossible to assign all (any?) of the bands. It can act as a unique ‘fingerprint’ for a given molecule.
Middle IR spectrum can also be roughly divided into:
- FUNCTIONAL GROUP REGION, v = 4000 - 1200 cm-1
2. FINGERPRINT REGION, v = 1200 - 400 cm-1
stretching vs bending
It takes more energy to stretch a bond than bend it
Therefore stretching bands are found in the
4000-1200 cm-1 (functional group region)
Bends are typically observed in the fingerprint region
functional group frequency
to be done later
