# 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