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)

Limitations

• 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

Absorption

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

Beer-Lambert

• 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

Chromophores

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

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

Conjugation

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

Analysis

• 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