Electronic Spectroscopy

What is electronic spectroscopy

• Study of how EM radiation interacts with a molecules to change its electronic energy 

• When observing in the UV-vis region we look at wavelength rather than wavenumber or frequency 

 

Given the three properties of electrons key for spectroscopy

• Quantized , at discrete energy levels 

• Widely Spaced, large differences in energy 

• Give electronic transitions which are in the UV visible region

What is a chromophore

A molecule or group that absorbs in the UV-visible region

• If one colour is absorbed the complementary colour will be observed 

Give the Beer-Lambert Law

A = ε c l

• Absorbance , A, no units

• Concentration, c, mol dm^-³

• Pathlength, l, cm

• Molar absorption coefficient, ε, dm³ mol^-1 cm^-1

  • For specific molecules at specific wave length

Give Absorbance equation using only pathlengths

A = log₁₀ (l₀/l)

What is an electronic transition

An electron moves from one molecular orbital to another 

What is gross selection rule for electronic transitions

Gross selection rule 

• for absorption or emission, there must be a change in dipole as the electron moves and a large dipole change will give a strong band  

What is specific selection rule for electronic transitions

Specific molecule determines the specific MOs involved 

• specific energy gaps i.e. absorption λs 

• specific band strengths i.e. ε values  

Give typical lambda max values for different electronic transitions

Give two rare transitions we may also see

• Charge transfer transitions (donor -> acceptor groups) 

• d-> d transitions (Transition metal groups) 

Give typical range of ε_max values

Why do we see many transitions within one molecule

• Several different occupied MOs 

• Several different unoccupied MOs 

• As such we can see several different transitions as there are different MOs that can transition into each other 

• We will also see several different bands in the spectrum as they correspond to the electrons jumping between the several different MOs 

draw energy level diagram for the typical transitions

Effect of conjugation on wavelength

As conjugation increases π → π* absorption band shifts to longer wavelength leading to a lower energy state 

What can we use electronic spec to measure

• Concentrations

• Chromophores

What is an Isosbestic point

• Constant absorbance at a specific λ as two species interconvert  

• occurs when two interconverting species have the same ε at a specific λ 

What is the Born-Oppenheimer approximation

• Electronic,Vibrational and Rotational energies are very different. Each can be considered independent of the others

• They can also all transition simultaneously