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UV-Vis Spectra

UV-Vis modes

  • Utilises the excitation of molecular orbital electrons rather than atomic orbital electrons
  • Each transition is accompanied by a small vibrational transition as well
  • The bands tend to be broad and undefined, due to unresolved vibrational contributions
  • The precision of the readings is much lower (10 - 100 nm) than atomic spectra (0.01 nm)


  • It is primarily quantitative, as there is far less ability to characterise functionality based on behaviour
  • Local maxima will help to identify bond types
  • Absorption at higher energies has the potential to ionise the molecule completely so it’s useful to start at lower energy and sweep up to the higher energy
  • Air tends to absorb at wavelengths < 200 nm


Is a two stage process,

\[ \begin{align} M + h\nu &\ce{->} M^∗\\ M^∗ &\ce{->} M + \text{heat} \end{align} \]

The small amounts of heat released are negligible to the system temperature

  • The relaxations can also occur via photodecomposition, fluorescence, and phosphorescence

Absorption will occur on \(\sigma\), \(\pi\) and \(n\) electrons

  • Typical transitions are:
    • \(n \ce{->} \pi^*\)
    • \(\pi \ce{->} \pi^*\)
  • Though any transitions are possible

\(\sigma \ce{->} \sigma^*\)

  • Very high energy transitions
  • Single bonds, such as in alkanes
  • \(\ce{C\bond{-}H}\) bonds are higher energy (~125 nm) than \(\ce{C\bond{-}C}\) bonds (~135 nm)
  • Not particularly useful for analysis since it’s beyond our 200 nm limit

\(n \ce{->} \sigma^*\)

  • Nonbonding electrons are specific to species with lone pair electrons, such as oxygen and nitrogen
  • Seen at 150 - 250 nm
  • Primary amine LPE (215 nm)
  • Ether LPE (184 nm)

\(n \ce{->} \pi^*/\pi \ce{->} \pi^*\)

  • Most active and most important transitions
  • Seen at 200 - 700 nm


  • To use the beer lambert law, it’s important to consider how ϵ is influenced by different bonding electron types
  • It can also be influenced by the solvent used

Solvent influences

Transition Decrease polarity Increase polarity \(\varepsilon\)
\(\ce{\sigma -> \sigma^*}\) Not measurable
\(\ce{n -> \sigma^*}\) Decrease energy Increase energy 100 - 3000
\(\ce{n -> \pi^*}\) Decrease energy Increase energy 10 - 100
\(\ce{\pi -> \pi^*}\) Increase energy Decrease energy 1000 - 10,000
  • A shift towards a shorter wavelength (higher energy) is called a blueshift - hypsochromic
  • A shift towards a longer wavelength (higher energy) is called a redshift - bathochromic


Are unsaturated functional groups that absorb longer wavelength UV/Vis radiation (closer to vis)

  • Multiple chromophores won’t effect \(\lambda\) max, but will increase \(\varepsilon\)


Shifts the absorbed radiation further towards vis (decreases energy) but increases \(\varepsilon\)

  • The conjugation causes delocalisation of \(\pi\) electrons, causing \(\pi^∗\) to be stabilised
  • The more conjugated, the more stabilised the molecule, hence \(\beta\) carotene’s blue absorbance

Aromatic species

Have three sets of excitation bands arising from the three \(n \ce{->} \pi^*\) transitions

Excitation \(\lambda\) (nm) \(\varepsilon\)
1 184 60,000
2 204 7,900
3 356 200

Each band has fine structure from the vibrational transitions that disappear in polar solvents


  • UV-Vis is typically not used for qualitative analysis since \(\lambda\) max is highly dependent on the solvent
  • Polar solvents also destroy any fine structure caused by vibrational transitions

Solvatochromic shifts

Are shifts that occur from the solvent’s interaction with the molecule Simply put:

  • Increased H-Bonds blueshifts (increases the energy required)
  • Increased dispersion forces redshifts (decreases the energy required)

Quantitative Analysis

UV-Vis is useful because they are:

  1. Applicable to many organic compounds
  2. Decently sensitive, working on the \(\mu\)M to nM scale
  3. Decently selective
  4. Accurate
  5. Simple to use for data collection and analysis
  • \(\lambda\) max is a safe place to start analysis from, as it will typically behave linearly with the Beer-Lambert law
  • Sensistivity is also greatest at \(\lambda\) max
  • Variables that will influence the spectra are:
    • Solvent
    • pH
    • Temperature
    • Sample matrix