W3.4_EMR & UV Spectroscopy Flashcards
What are electromagnetic radiation (EMR)? Explain the electromagnetic spectrum and the relationship between wavelength, frequency, and energy. What are the parts used in pharmaceutical analysis?
- Electromagnetic radiation (EMR): radiation that can travel through vacuum
- Classified in electromagnetic spectrum (in order of decreasing wavelength)
(radio, microwaves, infrared, visible, UV, X-rays, gamma rays) - Higher wavelength = lower frequency, lower energy
- Used in pharmaceutical analysis: 200 nm - 1000 nm (visible, some UV and IR parts)
What are light waves? Explain the equations describing the speed and energy of light. What is light and how does it generate EMR?
- Light waves: electromagnetic, moving electric (E) and magnetic (B) fields vibrating 90 degrees to each other at speed of light -> makes wave move
- c=fλ c: constant (speed of light) f: wave frequency (cycles per sec) λ: wavelength
∴ wave frequency and wavelength are negatively related to keep c constant - E=hf h: Plank’s constant E: energy
∴ high f = low λ = high E - Light: energy carrying waves that behaves like particles (photons) travelling
- Energy from each photon: E=hc/λ
- When photons hit something -> EMR waves interact like other waves
Explain the absorption and emission of radiation.
Absorption of radiation
- Selective removal of certain frequencies by transfer of energy to atoms
- Electrons promoted from lower-energy states to higher energy states (ground -> excited)
- Energy of exciting photon = energy difference between the states
Emission of radiation
- When excited particles return to lower-energy levels/ground state
What does spectroscopy measure? What does the total internal energy comprise of? Explain how UV-Vis absorption, IR absorption and fluorescence emission absorb energy.
- Measure absorption, emission, scattering of EMR by substances
- Total internal energy: sum of energy from electrons + vibrations between atoms + rotation of molecule
- UV-Vis absorption: relies on excited electrons
- IR absorption: relies on excited vibrations
- Fluorescence emission: relies on electrons returning back
Explain the change in energy level through photon hit and the excitation that comes after.
- When photons hit a molecule: energy state changes (absorption: ∆E +ve, emission: ∆E -ve)
- Each energy level is discrete but further vibrational and rotational levels are discrete too
- Excitation to higher excited states need more energy (shorter λ)
- Excitation within within vibrational levels need less energy (longer λ)
Describe the differences in σ, π, and n bonding. Explain the theory of energy conservatism regarding bonding orbital and why only some of the bondings can be used in pharmaceutical analysis.
- σ bonding: formed by electrons in 2s orbitals -> single bonds
- π bonding: formed by electrons in p orbitals -> double/triple bonds
- n bonding: non-bonding, lone pairs of electrons
- When a bonding orbital (HOMO) is created -> a corresponding unoccupied anti-bonding orbital (LUMO) is created to sustain energy conservatism (less stable state)
- σ/π/n* has a higher energy than σ/π/n
- Only π -> π* and n -> π* have low enough energy for pharmaceutical analysis, as others are in far UV where air interferes
Explain the UV-visible spectrum and how does it present in a graph. Where do the absorbance come from?
- Plot of absorbance against wavelength, where different absorbance produce peaks in graph
- Despite discrete energy levels, UV spectrum has bands not sharp lines
- ∵ vibrational/rotational levels make slight differences
- Double/triple bonds -> strong absorbances
State the types of absorption shifts (4). What causes them (4)?
- Hyperchromic, hypochromic, hypsochromic/blue, bathocrhomic/red
- Caused by substituent groups, degree of conjugation, nature of solvent, pH
Explain how conjugated structures causes redshift.
- Conjugated structures: double bonds alternating with single bonds
- Shift absorptions to longer wavelength (red shift)
- More extensive/longer conjugated system -> electrons are easily delocalised -> less separation in energy levels -> longer wavelength of absorption
- Extends if benzene ring/alternate double and single bonds/lone pairs on substituent groups with N/O/halogens are present
How does pH causes absorption shifts? Explain auxochrome and chromophore.
- pH effect: extra lone pair of auxochrome group from ionisation causes red shift and increases intensity
- Auxochrome: functional group containing lone pairs of electrons that does not absorb appreciable amount of UV/visible light but causes red shift and increase intensity
- Chromophore: part of molecule that absorbs UV/visible light (double bond/lone pair electrons)
-> In reality: difficult to interpret completely and identify a drug from UV-vis spectrum only
Describe the structure of a spectrophotometer and how it records absorbance.
- Structure: lamps (deuterium/tungsten), monochromatic, optics, detector
- Polychromatic light enters monochromator to split to different wavelength -> leaves exit slit with selected wavelength -> pass through sample -> absorb certain intensity -> detector records the absorbance
Explain the absorbance equation and Beer Lambert Law. How is the equation simplifed in BP? What are the assumptions made before recording?
- Absorbance=log(I(0)/I)
I(0): light of intensity entering -> photons absorbed -> I: light intensity leaving - Beer Lambert Law: A=εbc
A: absorbance b: path length of cell (1cm) c: concentration in mol/L
ε: molar extinction coefficient (M^-1cm^-1) at given λ for particular substance) - In BP: simplified as C=A/A(1cm,1%) (1% concentration and 1cm length of cell, concentration as g/100mL)
- Deviations: each molecule in a solution are assumed to act as independent absorbing species in solution (not true in high concentrations), in homogenous solution (not true in turbid samples), with other factors
