Shapes and Structures 1 - Infrared Spectroscopy Flashcards
(42 cards)
What information do IR spectra give?
Types of bonds present
What is frequency plotted as?
Wavenumbers (cm^-1)
Peaks over 1500 cm^-1 correspond to bonds doing what?
Stretching
For a diatomic molecule, what can vibrations be modelled as?
A harmonic oscillator
What is frequency proportional to for vibrations?
v ∝ sqrt(Kf/m)
v = frequency
Kf = force constant
m = mass
How is the frequency equation adapted for stretching in diatomic molecules?
wavenumber = (1/2πc) sqrt(K(f)/μ)
K(f) is bond strength in N m^-1
μ is reduced mass in kg/molecule
c in cm s^-1 (so 3.00 x 10^10)
For a diatomic molecule, what is the equation for reduced mass, μ?
μ = (m1m2)/(m1 + m2)
For a diatomic molecule, what is the equation for reduced mass when m1»_space; m2?
μ ≈ m1m2/m1
so μ ≈ m2 (lighter mass)
How does the inverse relationship of μ and frequency explain the X-H stretch region being 2500-4000 cm^-1?
Smaller μ means higher frequency
μ(X-H) ≈ H so very small μ
Aside from the X-H region, what do the other regions in IR spectra correspond to?
Different bond strengths
triple > double > single
The frequency range for X-H?
2500-4000 cm^-1
The frequency range for triple bonds?
2000-2500 cm^-1
The frequency range for double bonds?
1500-2000 cm^-1
The frequency range for the fingerprint region?
500-1500 cm^-1
What is contained in the fingerprint region of an IR spectrum?
X-Y single bond stretches
Other vibrational modes such as bending
How big the peak in IR spectra depends on what?
Give examples
The dipole moment of the bond
C=O, strong dipole moment, strong absorption
Symmetric C=C, no dipole, no absorption
Asymmetric C=C, weak dipole, weak absorption
How can symmetrical vibrations with no change in dipole moment be determined?
Raman spectroscopy
How does IR spectroscopy differ from Raman spectroscopy?
IR looks at frequencies of light absorbed by a sample
Raman looks at frequencies of light scattered by a sample
For homonuclear diatomics such as O2 and N2, when will they be active?
Raman
For heteronuclear diatomics such as HCl and CO, when will they be active?
IR and Raman
Where are C-H stretches usually?
How does it change for tetrahedral and trigonal carbons?
2900 - 3200 cm^-1
Just less than 3000 cm^-1 for tetrahedral
Just over 3000 cm^-1 for trigonal
Most C-H stretches are not mentioned as they feature in all organic compounds, what is the exception?
C≡C-H
Strong, sharp absorption around 3300 cm^-1
Where are N-H stretches usually?
Sharp absorption around 3300 cm^-1
NH2 groups often show two absorptions where?
Symmetric stretch: around 3300 cm^-1
Asymmetric stretch: around 3400 cm^-1