uv vis spec Flashcards
What is spectroscopy?
The study of the interaction of matter and electromagnetic radiation
What is electromagnetic radiation (EMR)?
• EMR is radiant energy that displays the properties of both particles and waves • There are different types of electromagnetic waves that make up the electromagnetic spectrum (EMS)
An example of electromagnetic radiation
Visible light
The electromagnetic spectrum
gamma x uv visi ir radio
Gamma rays
are emitted from the nuclei of some radioactive elements and because of their high energy can severely damage biological organisms.
X-rays
are less harmful than gamma except in high doses.
UV light
is responsible for sunburn. Repeated exposure leading to skin cancer by damaging DNA molecules in skin cells.
Visible light
electromagnetic radiation that we see.
Infrared radiation
We feel IR as heat
Radio waves
have the lowest energy (frequency) Radio, TV. Radio waves are also used in NMR and in magnetic resonance imaging MRI
•Electromagnetic radiation has 2 components:
an electric field and a magnetic field which are in planes at right angles to each other.
A KEY Difference between electromagnetic waves and other types of waves,
(e.g. sound, water and seismic waves which exist only by virtue of the medium in which they move), EM waves can travel through a vacuum
Frequency,
n = the number of crests of the wave that pass a point per second. The SI unit of frequency, n, is the reciprocal second, s-1, which is also called the hertz, Hz.
Wavelength,
lama = distance between two adjacent crests or troughs of the wave. The SI unit of wavelength, , is metres, m, though multiples such as nm
(10-9 m) are frequently used.
Velocity,
in a vacuum all electromagnetic waves travel with the same velocity: c 8 -1
= 2.998 x 10 ms .
speed of light eqn
c=lambda x v
photon
A particle of electromagnetic radiation is called a
The energy, E, of a photon is given by
E = hv (h is Planck’s constant, h = 6.626 x 10-34 J s)
Energy of the photon (wave) is directly proportional to
FREQUENCY
Energy of the photon (wave) is inversely proportional to
WAVELENGTH
The SI unit of energy
is the joule, J (although kJ mol-1 is often quoted)
wavenumber
E = hc (1/lambda) = hc v
The SI unit of v (vibrational frequency)
is m-1
c speed of light
2.998 x 10^8 m s^-1
energy kj to kj/mol
multiply by Avogadro’s number 6.023 x 10^23 mol^-1
how does uv/visible spec work
UV/Vis Light source to monochromator to cell ( Io incident light) to I (light not absorbed by sample) to the detector
monochromator
The monochromator selects a single wavelength from the wide range provided by the light source.
cell
The cell (cuvette) can be glass or plastic for visible light but must be quartz for UV light (glass and plastic absorb UV).
What factors control the amount of absorbance?
- The thicker the sample, the more absorption takes place.
- More absorption is seen for more concentrated samples (more molecules present to absorb the light).
- Some molecules are better than others at absorbing a particular wavelength; it is an inherent property of the molecule.
The Beer-Lambert law - at a single wavelength
A= Ecl
Absorbance, A, of a sample depends on:
- concentration, c, of the absorbing species. mol L^-1
- length of light path, l, through the cell cm
- molar absorption coefficient, E. L mol^-1 cm^-1
Molar absorption coefficient, E:
some molecules are better than
others at absorbing a particular wavelength; it is an inherent property
of the molecule. e is also known as the molar absorptivity and the extinction coefficient in older textbooks.
transmitted light is equal to
I = Io 10^(-elc)
Absorbance is equal
log(Io/I)
Transmission, T
Absorption data can also be reported as transmission
T= Io/I
A=-log(T)
Visible light
400nm (violet) to 750nm (red), UV 200-400 nm
two types of transition
n to π* and π to π*
transition
A change between energy levels is called a transition
vibrationally excited
The lowest energy transition moves an electron between the HOMO and LUMO orbitals. Generally the excited state will also be vibrationally excited.
Excited states last for
only last for very short periods of time (1 to 10 nanoseconds), because the higher energy state is energetically unstable.
relaxation processes
The extra energy is lost through relaxation processes such as emission of light or heat.
n to π* is ________ energy than π to π*
n to π* is less energy than π to π*
Effects of conjugation
- Conjugation decreases the energy gap between HOMO and LUMO.
- Hence less energy is required for electronic transitions.
- Therefore transitions occur at longer wavelengths.
- If a compound has enough double bonds it will absorb visible light and the compound will be coloured, e.g. beta-carotene which is orange and is found
in carrots and tomatoes has lamda max= 455nm
chromophore
A molecule or part of a molecule that can be excited by absorption is called a chromophore.
