Ultraviolet- Visible (UV-Vis) Spectroscopy Flashcards
In general, how is UV-Vis and IR spectroscopy defined?
o In general, spectroscopy can be defined as the use of electromagnetic energy to probe matter and interpret the results to characterize chemical structures
o A molecule/atom/ion absorbs energy and is then promoted to an excited state. It is this promotion to an excited state that can then be quantified through spectroscopic techniques
What is the application of UV-VIS and IR spectroscopy in forensics?
o Drugs
o Toxicology
o Fibres
o Paint
o Documents
o Dyes
Tell me about the electronic transitions that occur in spectroscopy
The absorption of UV or visible radiation corresponds to the excitation of outer electrons
There are three types of electronic transition which can be considered
1. Transitions involving p, s and n electrons
2. Transitions involved charge-transfer electrons
3. Transitions involving d and f electrons
When an atom or moleucle absorbs energy, electrons are promoted from their ground state to an excited state
Atoms can rotate and vibrate with respect to each other within a molecule
These vibrations and rotations also have discrete energy levels, which can be considered as being packed on top of each electronic level
What is the energy absorption quantified in UV-VIS spectroscopy a result of?
- The energy absorption quantified in UV-VIS spectroscopy is the result of an energetically favoured electron promotion from the Highest Occupied Molecular Orbital (HOMO) to the **Lowest Unoccupied Molecular Orbital (LUMO) **
Absorption in the UV and Vis ranges of the EM spectrum correlate with what?
Electron transitions between molecular orbitals, typically from the HOMO –> LOMO
What does a UV-VIS spectrum provide information on?
o A UV-VIS spectrum provides information on** double bonds and conjugation**
*Therefore, only compounds with Pi-electrons have a UV-Vis spectrum
* If compound has all double bond(s) and/or conjugation, UV-VIS analysis won’t act as exclusion tool for forensic chemistry, but more as preliminary analysis
What is a chromophore?
A chromophore is the part of the molecule containing the electrons involved in the molecular transitions
The chromophore is a region in the molecule i.e. carbonyl or hydroxyl, where the energy difference between the two separate molecular orbitals falls within the range of the visible spectrum
What are the different natures of shift in the spectrum what how would they each be described?
Bathochromic: Towards longer wavelength (Red shift)
Hypsochromic: Towards shorter wavelength (Blue shift)
Hyperchromic: To greater absorbance
Hypochromic: To lower absorbance
What is the Auxochrome effect?
AnAuxochrome is a group of substituents which itself does not act as a chromophore, but when bonding to a chromophore it shifts the absorption maximum towards a longer wavelength
An Auxochrome is a compound that produces a bathochromic shift
What is the conjugation effect?
Why does more conjugation lead to darker solution colour?
The presence of conjugation lowers the energy gap between the Pi orbitals
This facilitates favourable transitions, which is then quantified via UV-VIS,resulting in a bathochromic effect
The more conjugation there is, the darker the perceived colour of the solution because the wavelengths of the light absorbed is longer (as conjugation decreased the energy gap between orbitals)
longer wavelengths= lower frequency and lower energy
lowest energy transition is HOMO-LUMO
the overlap of p orbitals and change in energy is n-n*
so as conjugation increases the number of n-n* increases
Hence why, with increased conjugation the number of n-n* increases and makes the lowest transitional energy smaller which makes longer wavelengths (lower frequency and absorption) so darker colour
Why do presumptive drug tests result in a obvious change of colour if a particular drug is present?
Conjugation
Tell me about the Marquis test
- detects amphetamine or methamphetamine but have to do secondary test to know which one
- Most versatile and widely used colour test in drug analysis
- Colour produced is thought to be the result of a free-radical mechanism, although not 100% known.
- If amphetamine is present in the unknown sample, will react with Marquis reagent to emit orange-brown- red solution
* Generates carbocation; product is a dimer of original structure with more conjugation, which results in observed colour change due to decreased energy gap - Creating dimer tends to increase conjugation, which in turn facilitates interaction with visible light
- Similar dimer created when methamphetamine reacted with Marquis reagent:
General UV-Vis spectrophotometer
Example) The indicator phenolphthalein is colourless under acidic conditions and an intense pink under basic conditions. It’s often used as a presumptive test for blood. Rationalise the perceived colour change
o Under acidic conditions, two phenol groups are protonated
o Under basic conditions, these groups deprotonate and become a **donor/ acceptor pair that increases conjugation,** decreases existing energy gaps, and allows for absorption in the blue range
o This produces the observed red-pink colour (also, under basic conditions the ring opens, and another unshared pair is donated to the conjugation structure)
How does spectroscopy all help in forensics?
Forensic drug analysis: Qualitative analysis
o Can obtain UV-VIS spectra of seized drug samples to compare against spectral database as part of identification analysis
o This is 6 examples of common illicit drugs above which are stored on database
Example) As the forensic analyst tasked with analysing a seized drug sample, you find the maximum absorbance to be 260 nm, the ε260 = 8400 M-1 cm-1, and the path length is 1 cm. From your spectrophotometric analysis, you find the experimental absorbance at 260 nm to be 0.70. What is the concentration of this sample in question?
What are the pros and cons to UV-Vis spectroscopy?
