Spectroscopy Flashcards
de Broglie relationship
λ = h / p p = momentum = mv
c =
νλ
ν = frequency (units s^-1)
ṽ =
Wavenumber
1 / λ
E =
hν = hc / λ = (kB)T = RT
For molar energies multiply by 6.02 x 10^23
Rough wave numbers in cm^-1 of electromagnetic waves
10^-4 - 10^-1 radio waves 10^-1 - 10^1 microwaves 10^1 - 10^4 infrared 10^4 - 10^4.5 visible 10^4.5 - 10^6 ultra-violet 10^6 - 10^10 x-rays 10^10 - 10^14 γ-rays
Type of electromagnetic wave used in each spectroscopy technique
NMR - radio waves
ESR - microwaves
Molecular rotation - microwaves, infrared
Molecular vibration - infrared, visible
Electronic energies - visible, UV, X-rays
Nuclear energies - γ-rays
Explain absorption and emission
Moleules in an excited state can emit light. Molecule can go from excited state to ground state and vice versa. The probability of these 2 processes happening is the same in an electromagnetic wave. So overall amount of light absorbed depends on difference in population of the 2 energy levels
What is ε, the molar extinction coefficient
Constant for a particular molecule at a certain wavelength
Has units mol^-1 dm^3 cm^-1
How to determine ε
A plot of A vs. c
Electron in a box model, larger box means…
larger charge separation –> stronger interaction with the electric field –> larger ε
Explain electronic spectroscopy
It involves the moving of electrons between molecular orbitals - changing the electronic structure
The part of the molecule that absorbs light is called the chromophore
- organic molecules, mostly involves n, π or π* orbitals
- inorganic molecules, often involves transfer of electrons between d-orbitals
Uses of electronic spectroscopy
Transitions are usually broad, so little use for identifying molecules, but can follow trends in electronic structure
Very useful for measuring the concentration of a known chromophore or following the course of a reaction
Complementary colours
violet yellow
blue orange
red green
Compounds that absorb colour on the left appear colour on the right
Vibrational spectroscopy can be modelled by…
The Simple Harmonic Oscillator
Units of force constant k
Nm^-1
ve (vibrational frequency of 2 masses connected by a spring) =
(1/2π) sqrt(k/μ)
Relationship between force constants and bond strength
In general:
triple bonds > double bonds > single bonds
stronger bond = higher k
No of vibrational modes in a molecule
If there are n-atoms, there are 3n - 6 vibrational modes
Except for linear molecules –> 3n - 5
Rough IR absorptions for functional groups
O-H broad, 3500 C-H spiky, 3000 C≡N very strong, 2400 C≡C weak, 2300 C=O very strong, 1800 C=N 1500 Aromatic 2 bands at about 1500 and 1600 C=C 1650 NO2 1500
Explain nuclear spin
Many nuclei have a quantum mechanical property known as spin, characterised by spin quantum number I
Spin gives magnetic moment μ = γsqrt(I(I+1)ħ)
ħ = h/2π, γ = magnetogyric ratio of the nucleus
In an applied magnetic field B the magnetic moment can align in 2I + 1 ways with respect to the field
Nuclear spin energy levels
In a magnetic field B, the magnetic moment can only take certain orientations with respect to B
The orientation is described by a second quantum number m(l) = -l, -l+1,….,l
The energy of these states is given by E = -γm(l)ħB
Selection rule Δm(l) = ±1
How does NMR work?
Need a big magnet
- very high field strength
- superconducting magnet
Sample placed in magnet
RF transmitter/receiver to obtain spectra
Frequency of NMR, ν(NMR) =
γB / 2π
Chemical shift 𝛿x =
(νx - νref) ν0 x 10^6 ppm
vref = ν of solvent
ν0 = operating ν
2 mass spectrum methods and what they are used for
MALDI - polymers and biological macromolecules
ESI - biomolecules, but often leads to multiply charged ions
No of rings + double bonds for CxHyNzOn =
C - 0.5H + 0.5N + 1
Ratio of carbon isotopes
12 - 98.9%
13 - 1.1%
Ratio of chlorine isotopes
35 - 75%
37 - 25%
Ratio of bromine isotopes
79 - 50%
81 - 50%
IR: molecules of greater mass…
have a lower frequency and so lower absorption
Converting g mol^-1 into kg
divide by 1000, and divide by 6.02x10^23 mol^-1
reduced mass must be in kg
