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