Unit 8 Flashcards
Spectroscopy
What is the frequency and wavelength of low energy? eg radio waves
Low frequency high wavelength
What is the frequency and wavelength of high energy? eg gamma waves
High frequency low wavelength
What energy change and part of the spectrum does electronic spectroscopy occur?
It shows electron movement between orbitals and takes place in the visible, ultra violet and X-ray region
What energy change and part of the spectrum does infrared/vibrational spectroscopy occur?
Vibrational changes takes place in the infrared spectrum
What energy change and part of the spectrum does rotational spectroscopy occur?
Rotational changes in microwave region
Equation for speed?
speed= Frequency X wavelength
Equation for energy?
Energy= Frequency X Plancks
Equation for energy using wavelength
Energy= Plancks X speed of light/ wavelength
What is an electromagnetic wave made out of?
A magnetic field and an electric field at 90 degree angles to each other
How does the electric field of a electromagnetic wave affect electrons?
if it matches the energy difference between energy levels it can give the electron kinetic energy to promote the electron (dE=hf)
what path does electric field make electrons take?
it makes electrons oscillate and go from positive to negative of the field.
What transitions aren’t allowed?
s-s s-d p-p d-d
in visible light spectroscopy what causes the colours we see?
Some of the light is absorbed and used in the molecule for vibrations, the light that isn’t absorbed passes through and is visible. If none of the light is absorbed it will just pass through as white light
why is copper nitrate blue in water?
Because it forms a complex which will absorb the red parts of light and the blue part will be transmitted which we can see
What happens that allows D-d transitions?
The vibrations can split and distort the orbitals symmetry and allow transitions, this is why many transition metal complexes are coloured.
What happens when you attach ligands
The electric fields can cause repulsions in the d orbitals and cause them to split into different energy levels
Why can’t d-d transitions happen if they have 10 electrons?
All orbitals are filled so there is no empty higher orbital for them to move into
Why do d orbitals with 10 electrons in not have a colour?
Because they are fully filled the electrons cant be promoted, therefore they are not absorbing the energy so it just gets released
Why do Alkenes and aromatic compounds absorb in the UV spectrum?
Double bonds contain pi electrons which can use energy from the uv region to promote from bonding orbitals to antibonding
What region of the magnetic spectrum does electronic spectroscopy take place in/
ultra violet, visible and x-rays
What region of the magnetic spectrum does Vibrational spectroscopy take place in?
Infra-red
What do multiple conjugated bonds give rise to?
Electron delocalisation
What do conjugated bonds do?
Give rise to electron delocalisation, this means electrons require lower energy for bonding to antibonding orbitals
What energies do bonding and anti bonding orbitals have compared to the atomic orbitals that made them?
Bonding orbitals have a lower energy than the ones that made them
Anti bonding a higher energy
When electrons relax to ground state what is the excess energy used for?
It can be used to increase vibrations in the molecule
Why would an absorption band occur in the visible region?
If it has conjugated bonds which give rise to electron delocalisation it wont need as much energy for pi to pi* transitions
What allows d-d orbital transition?
Vibrations which cause orbital mixing and distortion which breaks the symmetry
can molecules that are colourless still absorb in the uv region?
yes
Why is an antibonding orbital not as stable as a bonding?
Bonding orbitals have attractions between the nuclei and electrons in antibonding there are only repulsions
What is a conjugated bond?
When multiple p orbitals overlap leading to electron delocalisation.
How can you recognise conjugated bonds?
If the bond is shown to be alternating
How is a sigma bond formed?
end to end overlap of atomic orbitals
