AC 8 - Uv 0vis spectroscopy in pharmacy

What is UV–Vis spectroscopy?

 A technique that measures absorption of ultraviolet (200–400 nm) and visible (400–800 nm) light by molecules due to electronic transitions.

What is the definition of a chromophore ?

Chromophore: The group of atoms responsible for absorption of light in the UV-visible region.

What is an auxochrome?

 A group with little UV–Vis absorption on its own that modifies the absorption of a chromophore when attached.

What is a bathochromic shift?

A shift of absorption to a longer wavelength (red shift).

What is a hypsochromic shift?

A shift of absorption to a shorter wavelength (blue shift).

What is the vacuum region?

The "vacuum region" in UV-Visible spectroscopy refers to the vacuum-ultraviolet (VUV) range, specifically wavelengths below 200 nm.

This region is so named because air strongly absorbs this high-energy radiation, making it necessary to perform experiments in a vacuum environment to prevent atmospheric interference.

What are bonding and antibonding orbitals ?

Bonding and antibonding orbitals are molecular orbitals that describe the behaviour of electrons in a molecule.

Bonding orbitals have lower energy and are formed when atomic orbitals combine in phase, leading to a higher electron density between the nuclei, which stabilizes the molecule. – take part in covalent binding

Antibonding orbitals are higher in energy and have a node between the nuclei; electrons in these orbitals are concentrated outside the bonding region, creating a repulsive force that destabilizes the molecule.  -  lone pairs = antibonding = repel

what causes uv-vis absorption ?

•Transition of electrons between energy levels – between atomic or molecular orbitals

•Addition of energy allows promotion of an electron from an occupied to an unoccupied (or partially occupied) level

•This HOMO to LUMO change is associated with an exact amount of energy

•Often corresponds to the energy of the UV-visible range (200-800 nm)

●One electrons moves up  , through addition of electron

●Humo- highest occupied molecular orbital  lumo- lowest unoccupied molecular orbital

 Promotion of electrons from occupied to unoccupied molecular orbitals.

What are the transitions of sigma bonds ?

• high energy Transitions between the sigma bonding and anti- bonding orbitals of a sigma bond require a large amount of energy

• Transitions between the bonding and anti - bonding orbitals of a sigma bond require a large amount of energy

• sigma bonds are strong so require a lot of energy

Where do these sigma transitions occur

 In the vacuum UV region (~135 nm), so they are not normally observed.

what transitions are in pi bonds?

• This transitions p* ← p depends on the number of double bonds and the conjugation

• bonding and anti-bonding orbitals are close in energy so transition are seen

Why are the Pi transitions observed more easily than sigma transitions ?

Because the energy gap between π and π* orbitals is smaller.

What happens as conjugation increases ?

Increasing the conjugation:

•Increases the l_max

•Increases the absorption coefficient e- beer lambert equation

more conjugated system = more likely it is going to absorb light

note- chromophore tends to be conjugated

What are the standard rules for predicting wavelength of absorption?

Standard rules

1.For each additional conjugated double bond – add 36 nm (pi)

2.For each additional single bond – add 6 nm (sigma)

Other auxochromes have an effect :

Note – the effect of auxochromes on spec –

Atoms that have a lone pair can increase the conjugation and extend the chromophore:

• OR  6 nm  red shift

• SR  30 nm  red shift

 NR₂  60 nm  red shift

less absorption - in cis arrangements

How is absorption is enhanced by conjugation and substituents ?

• it is enhanced by conjugation and substituents

Non bonding orbitals (those of the lone pairs) are of higher energy than bonding

  " longer wavelength (less energy change)

But forbidden transition- electron won’t/can’t  move  between states ( e g n. to sigma rather go somewhere else  )

  " lower absorption coefficient= less absorption lower peak

which compounds ar einvolved in carbonyl UV absorption?

π, π*, and non-bonding (n) orbitals.

What transitions are seen in carbonyls ?

π → π* and n → π* transitions.

Which transitions occur at longer wavelength ?

n → π* (lower energy change).

Why the is n → π* absorption weak?

It is a partially forbidden transition, giving a low absorption coefficient.

What two electronic transitions occur in azo dyes?

 π → π* and n → π* transitions.

Why are azo dyes often coloured?

The n → π* transition can occur in the visible region.

What kind of UV–Vis spectra do aromatic compounds give?

Sharp, well-defined spectra.

How do sigma electron donors affect absorption?

 Reduce energy gaps and shift absorption to longer wavelengths.

Which groups extend π-conjugation in aromatics?

 NH₂, OH, NO₂, CO₂H.

 What is an ideal substitution pattern in aromatics?

 c

How does protonation affect aromatic UV absorption?

Reduces lone pair availability and decreases conjugation.

How does deprotonation affect aromatic UV absorption?

 Increases electron availability and conjugation.

What wavelength range is visible light?

400–800 nm.

Why do highly conjugated polyenes appear coloured?

They absorb light in the visible region.

What is the λmax of β-carotene?

~455 nm.

What is the λmax of lycopene?

~505 nm.

What are the UV regions?

UV-A: 315–400 nm,

UV-B: 290–315 nm,

UV-C: 100–290 nm.

 Which UV region is most dangerous to skin?

UV-B.

What does SPF measure?

Protection against UV-B radiation only.

What does SPF 15 block?

~95% of UV-B radiation.

What do star ratings measure?

 Protection against UV-A radiation.

What are chemical sunscreens?

Compounds that absorb UV radiation and dissipate the energy.

What are physical sunscreens?

Agents that reflect and scatter UV light (e.g. ZnO, TiO₂).

 Why are combinations used in sunscreens?

To provide broad UV-A and UV-B protection.

Why is UV–Vis spectroscopy important in pharmacy?

 It is essential for compound detection, LC detectors, pigment analysis, sunscreens, and structure–function relationships.