UV-VIS Spectrophotometry Flashcards
What is UV-VIS spectrophotometry?
*instrument for irradiating a sample of photons of UV or vis light particular wavelength/s
*liquid sample dispensed into a cuvette (UV- quartz, Vis - plastic, polystyrene)
*measure either:
-absorption (light not passing through sample)
-transmission (light passing through sample)
What is the molecular orbital (MO)?
the region in which there is the highest probability of finding an electron of a molecule
What is the highest occupied molecular orbital (HOMO)?
highest energy MO with any electrons in it
What is the lowest unoccupied molecular orbital (LUMO)?
the next highest energy orbital. It will be empty and so is the lowest energy orbital in which to place an electron or excite one into.
Describe the UV absorption spectroscopy
- wavelengths of UV light absorbed by a molecule determined by the electronic energy differences between HOMO and LUMO
*absorption only possible if the molecule contains two molecular levels separated by energy (transition from bonding to anti bonding orbitals)
*molecule will now be in an excited state - energy will dissipate rapidly and will return to the ground state
What is beers law equation + meanings of the parts of equation
A = ecl
A = absorbance
e = molar absorption coefficient M-1cm-1
c = molar concentration M
l = optical path length = cm
What is beer’s law plot?
*if beers law is obeyed, a straight line will be produced
* the gradient = the molar absorption coefficient
interpreting UV-Vis spectra?
- wavelength(s) of maximum absorbance
*molar absorptivity e at each maximum
*aromatic compounds, highly conjugated, absorb strongly in UV
What is a chromophore? (UV-Vis active compound)
structural features of a molecule that absorb specific wavelengths for example conjugation
What is a auxochromes?
auxiliary groups attached to a chromophore, unable to absorb, but cause often substantial shifts.
How do we see colour?
*white object reflects all light
coloured objects absorbs some wavelengths and reflects the rest of the spectrum
colour we see = complete visible spectrum - absorbed portion
What is the application of UV-Vis?
*used to determine pka of a compound if either the acidic or basic form of the compound absorbs UV or visible light
