Molecular spectroscopy and structure: Basics Flashcards
What are the two components of electromagnetic radiation?
the electric field component and the magnetic field component
What is the relationship between the two components?
they are perpendicular to one another and in phase
How is the electric field component denoted?
E
How is the magnetic field component denoted?
B
What is the orientation of the planes?
random unless polarised circularly clockwise or anticlockwise
How is the direction of motion denoted?
Z
How is wavelength denoted?
λ
What are the units of wavelength?
nm
To what wavelength of light is the eye most sensitive and why?
~500 nm
This is the frequency absorbed by 11-cis retinal, which forms half of rhodopsin. When the 11-cis retinal chromophore absorbs a photon, it isomerises from the 11-cis state to the all-trans state. This triggers a response from nerve impulses, facilitating sight.
How is frequency denoted?
ν or f
What are the units of frequency?
Hz (s^-1)
How is speed denoted?
c
What are the units of speed?
m.s^-1
How are speed, frequency and wavelength related to one another?
speed = frequency x wavelength c = v λ
What is the wavenumber of a wave?
The reciprocal of the wavelength
σ = 1/λ
How is the wavenumber denoted?
σ
What are the units of the wavenumber?
cm^-1
How is the refractive index calculated?
n = c/speed of light in material
What does ‘strength’ refer to?
the intensity (or amplitude) of the wave
What dominates the interaction between electromagnetic radiation and matter?
The electric field component, since this effect is of the order of 10^5 stronger than the coupling to the magnetic field.
How is the electric component of a light wave represented mathematically?
E(t,z) = E0cos(ωt-kz) where E(t) is the real part of the electric field vector at position z and time t; E0 is the amplitude of the wave, which we call the electric field strength
How is the angular frequency defined?
ω=2πv
How is the wavefactor defined?
k=2π/λ
What are the units of the electric field strength?
N.C^-1 or V.m^-1
What does the energy density express?
the energy ‘carried’ by a classical light wave
How is the energy density denoted?
U
How is the intensity of the wave denoted?
I
What are the units of irradiance?
W.m^-2 or J.s^-1.m^-2
What are the units of energy density?
J.m^-3
How is the energy of a photon calculated?
E = h v
How is the photon density calculated?
np/V (number of photons/volume)
How is energy density of a photon calculated?
U = hv (np/V)
What is the effect of increased energy on frequency and wavelength?
increased frequency
decreased wavelength
When you put a particle with electric charge q moving into an electric field E, what force will the particle feel?
F = qE (+ q v B/c)
What is the result of the interaction between the electric field and the charged particles?
some of the electromagnetic energy can be absorbed by the matter
What does it mean that the rotational, vibrational and electronic energies of a molecule are quantised?
The energy levels are discrete. The system cannot have an electronic energy in the range E(i)
What is the ground state?
the state of a system corresponding to the lowest energy level
When is a molecule in its true ground state?
When it is simultaneously in the lowest vibrational, rotational, electronic, etc. levels.
What does electronic excitation involve?
the absorption of UV/visible photons to transfer electrons from one electronic energy level to a higher energy electronic level
What is the resonance condition?
An atom or molecule can only absorb a photon when the energy of that photon corresponds precisely to the energy separation between two quantum states of that molecule.
For a system at ‘room’ temperature, are all molecules in their ground rotational, vibrational and electronic states?
No. At equilibrium, the molecules in a system will be spread between different energy levels depending on the amount of ‘thermal’ energy available to them. Obviously the low energy levels will be more populated than higher energy levels; however, at finite temperature, we must at least consider the possibility that there is sufficient thermal energy to populate excited states.
How are molecules distributed over the available energy levels?
according to the Boltzmann distribution
What can be said about NMR levels?
NMR levels are almost equally populated, even at very low temperatures, reflecting the fact that the energy spacing between levels is very small.
What can be said about rotational levels?
Excited rotational levels are well populated; at room temperature, one would expect to see many rotational levels populated.
What can be said about vibrational levels?
Excited vibrational levels are only populated at high temperature. At room temperature, the vast majority of molecules will be in the lowest energy vibrational state.
What does absorption spectroscopy involve?
Absorption spectroscopy involves shining a light source through a sample and recording the attenuation of the signal (how much the intensity of the light drops) as a function of wavelength.
Why is there broadening of absorption and emission peaks?
due to quantum uncertainty (related to the lifetime of the excited state) and collisions between molecules, as well as due to instrument resolution
What is emission spectroscopy?
The sample is excited by illuminating it with a short pulse of electromagnetic radiation and then any subsequent luminescence (emission of light from the sample) is recorded.
Describe the light used in emission spectroscopy
monochromatic (single wavelength)
What is the advantage of emission spectroscopy?
the lack of background signal
When are absorption and emission spectra mirror images?
- the molecule has similar geometries in the ground and excited electronic states (similar vibrational energy spacing)
- excess vibrational energy in the upper electronic state can be efficiently quenched (vibrational relaxation) so that the emission is from the lowest vibrational level
How is the energy of absorption calculated?
h v = ΔE(electronic) + n ΔE(vibrational)
How is the energy of emission after electronic relaxation calculated?
h v = ΔE(electronic) - n ΔE(vibrational)
