B - Spectroscopy - UV-vis Flashcards
Why are spectroscopic techniques useful for biology and medicine?
They: • are non-destructive • can be used in-situ • mean the sample can be retrieved • allow small or large samples to be used
How do electric and magnetic fields travel in space?
They fluctuate phase: both in time and in the propagation direction. These are perpendicular to the direction of travel
What is the speed of light in a vacuum?
c = λ x ν = 2.998 x 10^8 m/s
What is the range of the visible spectrum in nm?
740nm (red) to 390nm (purple)
What is the order of the EM spectrum?
Radio waves, microwaves, infrared, visible light, UV, X-rays, Gamma rays
What is the equation for the energy of one photon?
E = h x ν = h x (c/λ) where h = Planck’s constant, 6.626 x 10^-34 J s )
What kind of electrons does UV-vis absorption correspond to?
Valence electrons
What happens when a molecule absorbs light in the UV-vis range?
It causes an electron to be promoted to an anti-bonding orbital
What type of molecules absorb in the visible range?
Unsaturated organic compounds ie. chromophores • conjugated polyunsaturated systems • polyaromatic systems • systems with C=O, C=S, C-P etc • molecules with transition metal ions
Where do π to π* and π to σ* transitions lie?
- UV (200nm - 300nm)
* lie at too high an energy to be detected in UV-vis
What is the effect of increasing the number of π bonds in a conjugated system?
The gap between bonding and anti-bonding levels decreases. A lower energy and therefore longer wavelength is needed for the transition, hence it is called a red shift. Required wavelength is increased by 30nm for each additional C=C bond
What kind of transitions occur in aromatic ring systems, and what is their λmax?
π to π* = 203nm
Additional groups change λmax to 203nm - 295nm
What wavelengths do polycyclic aromatics (such as chlorophyll) absorb?
400 - 450nm (blue - violet) and 600 - 700nm (red)
This results in the reflection of green - yellow light -> plant leaves
What is the Beer-Lambert law?
Absorbance, A = log (Io/It) = ε c l where ε = molar absorptivity, dm^3 mol^-1 cm^-1 c = concentration, mol dm^-3 l = sample thickness, cm A has no units
If there is no UV-vis spectrum, what kind of molecule is it?
There are no: • multiple bonds • aromatics • O, N or S containing groups • transition metal ions
-> likely a fully saturated hydrocarbon
Why are small, unsaturated hydrocarbons not coloured?
They have their absorptions entirely in the UV range, lambda
What are spikes on the peaks of UV-vis spectra due to?
Vibrational transitions occurring alongside electronic transitions
Where does absorption occur for heteroatoms, and how strong is it?
200 - 400nm (n to π*)
Usually 10 - 100 times weaker than π to π* transitions
How is fluorescence imaging different to UV-vis?
UV-vis detects the absorption of incident light. In fluorescence imaging, after radiation, the excited molecule re-emits light as the electron goes back to its ground state.
How is phosphorescence different to fluorescence?
Fluorescence emits light for a short period of time. Phosphorescence emits light for a long period of time, usually because the electron drops down several intermediate levels.
What is the de Broglie relationship?
It expresses the particle-wave duality of light. Each ‘particle’ has a pilot wave, termed a photon, with a wavelength related to the mass and particle velocity.
λ = h / p = h / mv
Which electronic transitions involve valence electrons, tightly bound electrons and weakly bound electrons?
valence = UV-vis
tightly bound = X-Ray transitions
weakly bound = IR transitions
What is UV-vis useful for studying?
π - π* transitions in aromatic organic molecules
determining chain length in conjugated hydrocarbons
studying C=C vs C=O unsaturated species
determining the presence of chromophores
determinging the presence of organometallic, transition metal compounds
