B - Spectroscopy - IR Flashcards
What is a vibrational mode?
Atoms vibrate around their equilibrium position in well-defined patterns = vibrational modes
What is a harmonic oscillator?
The model for the vibration of a diatomic molecule -> like a spring
What is Hooke’s Law?
The force needed to extend or compress a spring by some distance is proportional to that distance:
F = -kx
where k = force constant, which is related to the bond strength
units Nm-1, usually around 100 - 1000 Nm-1
What do r and v0 stand for?
r is the bond length; at rest r = r0, but during the vibration r fluctuates periodically. v0 is the vibrational frequency; atoms oscillate about r0 with a characteristic frequency.
What is the equation for the reduced mass, ν?
ν = (m1 x m2) / (m1 + m2)
What is the purpose of calculating the reduced mass?
It removes translations and rotations; only stretched and compressions will be considered.
How do you convert from atomic mass units to kg?
Multiply by the mass of a proton
1.67 x 10-27 kg
What is the equation for v0 in s-1 (harmonic oscillator equation)?
v0 = 1/2π x (k/ν)1/2
How is v0 proportional to k and ν?
v0 increases when k increases
v0 decreases when ν increases -> heavier atoms
How do you convert to frequency in s-1 to wavenumber, cm-1 ?
cm-1 = v0 / speed of light (3 x 1010 cm/s )
What is the equation for v0 in cm-1 ?
v0 = 130 x (k/ν)1/2
What is the effect of isotopes on reduced mass, vibrational frequencies and k?
Isotopes give rise to different reduced masses, so this will change the v0; heavier isotopes reduce the vibrational frequency. k, however, stays the same.
What is the isotope substitution formula?
v0(heavy) / v0(light) = (νlight/νheavy)1/2
What kind of vibrational modes are there?
Symmetric and asymmetric stretching, symmetric and asymmetric bending, twisting.
What is the fingerprint region?
It is the IR absorption specific to that molecule, it can be matched to a known database of spectra given that the sample is pure. It is mostly used for functional group analysis
What is the equation for turning IR transmission into absorbance?
A = -log(T%/100) = -log(It/Io) = log(Io/It)
What are IR invisible compounds?
Molecules that don’t have a dipole moment, so vibrating does not change the electric dipole moment, ν0 -> eg diatomic molecules. these are suitable materials to use as a medium
Are all molecules without a dipole moment IR invisible?
No, because some vibrations cause a change in ν0, and these will be IR active.
Which bends or stretches are IR active?
Ones that cause a change in ν0 eg in CO2 symmetric stretching is IR inactive, but bending and asymmetric stretching are both IR active.
Where do O-H stretches occur?
3700 - 3600 cm-1 lowered to 3200 - 2500 cm-1 by H bonding
Where do N-H stretches occur?
3500 - 3300 cm-1 less affected by H-bonding than O-H
Where do C-H stretches occur?
saturated = 2960 - 2800 cm-1 where the intensity is related to the number of C-H bonds present
unsaturated = 3050 - 3020 cm-1 -> can be weak or broad
Where do double bond stretches occur?
1800 - 1600 cm-1
Where do triple bond stretches occur?
2300 - 2100 cm-1
Where does the fingerprint region occur?
1600 - 400 cm-1 This region contains C-C stretching, bending, torsion etc
What can a peak at 1670cm-1 mean?
Either a C=O stretching or O-H bending. To distinguish between them, we must take into account that the C=O peak is usually very sharp and strong.
What does an IR peak at 3200cm-1 correspond to?
H-bonded OH or NH.
Why does H-bonding lower the IR vibration frequency of OH groups?
Forming H-bonds reduces the strength of the bond. This reduces k, which is directly proportional to frequency. Frequency also decreases because the second atom the H is attached to starts to move, causing the reduced mass to increase slightly.
How does IR work?
The atoms in molecules are not static: they vibrate around their equilibrium positions. The vibrational frequencies correspond to light energies in the IR region of the spectrum. Some of the vibration cause a change in the electric dipole moment of the molecule. This results in a fluctuating molecular electric field associated with the vibration. This ‘resonates’ with the fluctuating electric field of the light wave, causing absorption of the light energy at each vibrational frequency which results in a peak at that frequency in the IR spectrum.
How can IR be used for functional group analysis?
Functional groups will always have approximately the same IR peaks in different molecules, so they can be used to work out features of the structure of an ‘unknown’ molecule.
What is the IR frequency of a bond proportional and inversely proportional to?
Proportional to the electronegativity and the strength of the bond Inversely proportional to the weight of the atoms
Which gives a higher wavenumber (lower wavelength), stretching or bending vibrations?
Stretching vibrations
What type of molecule can IR not identify?
Homonuclear diatoms because there is no dipole moment.
Which of these has the highest and lowest absorption frequencies?
Symmetric stretching
Bending
Asymmetric stretching
Highest = asymmetric stretching
Symmetric stretching
Lowest = bending
