Electronic Spectroscopy

Question 1

What arises from electronic transitions in atoms?

Answer 1

Atomic Absorption Spectroscopy

/ Atomic Emission Spectrometry arise from electronic transitions in atoms

Question 2

What does the quantised energy levels within molecules give rise to?

Answer 2

Energy levels within molecules are also
quantised - transitions between electronic
energy levels in molecules also give rise to spectra

Question 3

Important information about electronic spectra?

Answer 3

Higher energy process than vibration or rotation
Usually around UV / visible wavelengths
wavelength ~ 100-400 nm (UV), 400-700 nm (Vis)
E (blue photon) ~ 5.68 x 10 -19 J
Probing larger change in E in energy level diagram

Question 4

What does the intensity of the electronic spectroscopy peak depend on?

Answer 4

The intensity of a peak in the spectrum depends on how many photons are absorbed
Amount of material (path length, concentration)
Probability of the absorption (absorption coefficient)
Transmittance and absorbance used

Question 5

Beer lambert law?

Answer 5

Absorbance is related to concentration through the Beer-Lambert Law
A = E x c x l

Question 6

Limits of absorbance measurements?

Answer 6

Minimum absorbance accurately measurable is A approximately 0.01
Very highly coloured compounds have
E = 250 m2 mol–1
A = 0.01 corresponds to c = 4 x 10–6 moldm-3 for a 1 cm cell, used an estimate of the practical detection limit

Question 7

What about weak absorbers?

Answer 7

Low intrinsic E values, Can add a complexing agent - forming a coloured complex

Question 8

Practical application of Beer Lamber law?

Answer 8

Sources of light at ~650nm (red) and ~ 910nm (IR) and two light collecting sensors, measure the intensity that passes through the tissues
Oxygenated blood absorbs light more at 660nm, deoxygenated blood more at 910nm. The light is partly absorbed by oxyhemoglobin (HbO) and deoxyhemoglobin. By measuring absorption at the two wavelengths the Oximeter can compute the proportion of haemoglobin which is oxygenated

Question 9

Why is potassium permanganate purple?

Answer 9

Observed colour is due to the light not absorbed, the energy of absorption corresponds to differences in electronic energy levels in the molecular orbitals of KMnO4

Question 10

How can electrons in a box be used for molecular energy levels?

Answer 10

If we have a particle in a box of length l, quantum mechanics tells us that the only states it is allowed to have must be associated with an integral number of half-wavelengths, the allowed wavelengths have a corresponding energy, imagine a particle of mass m that can only move back and forth on one axis this is described as motion in a one dimensional box the length of the box is l and the potential energy EPE is zero inside the box and infinite outside, this mean the particle can move alone the x axis between x = 0 and x = l but can never get out of the box due to the infinite energy barrier

Question 11

Boundary conditions for electron in a box model?

Answer 11

Two approaches
De Broglie with the equation (less mathematical limited applicability)
Schrödinger (more mathematical, more widely used)

Question 12

Is any energy available in the box model?

Answer 12

Remember that particles also can be described as a wave, the ‘wave’ is only stable if it exactly fits inside the box
for the first energy level allowed exactly half a wave fits into the box so l - 1/2lambda or lambda = 2l
In the next allowed energy level a whole wave fits into the box so l = lambda
The next energy levels three half waves for into the box so l = 3/2 lambda or
lambda = 2/3l
An equation can be used to describe this

Question 13

Schrödinger, de Broglie and electron in a box all agree?

Answer 13

The calculation using the Schrödinger wavefunction approach (with the energy Hamiltonian, H) gives the same answer, this can be used to estimate the energy levels for linear (or extended) molecules with delocalised electrons, these can be considered to be moving in a one- dimensional “box” – the length of the molecule

Question 14

Electron distribution in conjugated hydrocarbons?

Answer 14

We can draw the wavefunctions representing the electrons, The electron density is given by wave function squared and as a density map in conjugated hydrocarbons delocalised pi electrons free to move along the chain but can’t move off the carbon framework, only consider pi electrons on per carbon atom

Question 15

What does UV vis/electronic spectroscopy involve?

Answer 15

Electronic spectroscopy involves the excitation of electrons by absorption of UV or visible radiation

Question 16

What is the colour of substances due to?

Answer 16

The colour of substances is due to the absorption of some wavelengths of visible light by the molecules –observed colour is due to the light not absorbed

Question 17

What is the absorption of radiation described by?

Answer 17

Beer-Lambert Law

Question 18

What does the molar absorption coefficient characterise?

Answer 18

The amount of light a species absorbs

Question 19

What is the absorbance of a solution proportional to?

Answer 19

The absorbance of a solution is proportional to its concentration – used widely in Analytical Chemistry. The absorbances of several species in solution are additive

Question 20

How can transitions be deduced?

Answer 20

Electronic energy levels can be calculated and hence transitions deduced: possible for atoms and for simple molecules

Question 21

How can approximations be used for certain types of molecule?

Answer 21

Also possible using approximations for certain types of molecule – e.g. conjugated molecules using particle-in-a-box

Question 22

What are the consequences of the wave function squared being related to the probability of finding the particle in electron in a box model?

Answer 22

Since EPE is infinite outside the particle cannot leave the box, as a result the probability of finding the particle outside the box is zero so here wave function squared and hence the wave function is 0
Because wave function is related to the probability of finding the particle its value cannot change discontinuously wave function is zero just outside the box so it must also be zero at the sides of the box that is wave function is 0 and x = 0 and x = l

Question 23

Electron in a box when E=0?

Answer 23

Lambda is infinite and the wave function is a straight line, since wave function = 0 at the edges of the box when E = 0 both wave function and wave function squared must be zero everywhere within the box so the probability of finding the particle with zero kinetic energy is also zero thus this energy is not allowed and n cannot take a value of 0, the zero point energy of the particle its energy at T = 0K when it occupies the lowest energy level cannot be zero, the lowest energy level is that with n = 1. The occurrence of a zero point energy is quite common in quantum systems although it does not arise in classical mechanism, all forms of linear motion have zero point energy

Question 24

What do the results from particle in a box show?

Answer 24

In the same way that the wave particle behaviour of electrons give rise to the quantisation at electronic energy in atoms the wave particle behaviour of molecules gives rise to quantisation of molecular energy, this arises from the requirement that the wave function has to be zero at the edges of the box called the boundary condition which means that only whole or half waves can fit in the box

Question 25

What do transitions involve in molecules?

Answer 25

The transitions involve the promotion of an electron from a lower energy occupied molecular orbital to a higher energy unoccupied or partially occupied molecular orbital

Question 26

Electronic states?

Answer 26

Each electronic state has its own set of vibrational energy levels, promotion of an electron to a higher energy molecular obtain often changes the geometry of the molecules, when the Morse curves for both the group and excited electronic states of a molecule are represented on the same diagram the curves show how the potential energy in each electronic state changes with the distance between the atomic, the vibrational energy levels in each electronic state are shown as horizontal lines, the spacing of the vibration levels in the upper electronic state is similar than that in the group state because the bond is weaker

Question 27

Applying Boltzmann distribution?

Answer 27

Practically all molecules are in their electronic ground states around room temperature and the vast majority are in their vibrational group state, transitions can occur into one of a number of vibrational levels of the electronically excited state, this means that there will be a number of absorptions in the spectrum, if these absorption’s are closely spaces they merge to form a broad band in the spectrum sometimes extending over a wide range of energies

Question 28

High resolution spectrophotometers?

Answer 28

Additional information on the vibrational energy levels can be optioned, there are rotational states associated with the vibrational states and transitions between these also occur however this rotational fine structure requires very high resolution spectra and can usually only be seen in the gasp phase.

Question 29

Franck Condon principle?

Answer 29

The excitation of an electron by the absorption of a photon occurs on a much shorter time scale than that for nuclear motion

Question 30

Why does the Franck Condon principle occur?

Answer 30

Because nuclei have much greater masses than electrons and so move more slowly, electronic excitation happens on a timescale of 10^-14 to 10^-15 seconds, the photon is therefore absorbed to form the excited state much more quickly than the atom can move, the absorption first changes the electron distribution around the atoms which then respond by moving to new equilibrium positions, in some cases the absorption of a new photon may add enough energy to exceed the bond dissociation energy in the excited state, the bond breaks and a photochemical reaction may result

Question 31

What is a chromophore?

Answer 31

The part of the molecule that is responsible for the absorption of a photo of UV/VIS radiation

Question 32

What is generally a more popular chromophore?

Answer 32

A conjugated system in which the electrons are delocalised and the electronic transitions involve pi orbitals, the longer the conjugated system the longer the wavelength of lambda max and the larger the value of E mac (the stronger the absorption). A higher wavelength correspond to a lower energy