38 Questions
What is the definition of a chromophore?
A chemical group in a molecule that is responsible for its color
What is the purpose of an auxochrome in a chromophore?
To shift the wavelength of absorption to a longer value
What is the definition of a 'hypsochromic shift'?
A shift of the absorption spectrum to lower wavelengths (blue shift)
What is the effect of a 'hypochromic effect' on a compound's absorption spectrum?
It decreases the intensity of the absorption spectrum
What is the relationship between the Beer-Lambert law and the absorption spectrum?
The Beer-Lambert law describes how the absorbance of a compound is related to its concentration and path length
How does the presence of a conjugated system in a molecule affect its UV-Vis absorption spectrum?
It results in a bathochromic (red) shift of the absorption spectrum
What is the purpose of using a dilute solution when measuring the UV-Vis absorption spectrum of a compound?
All of the above
Which of the following statements about auxochromes is correct?
Auxochromes are atoms with lone pairs of electrons, such as -OH, -OR, -NR2, and halogens.
Which of the following transitions involves the excitation of an electron from a non-bonding orbital to an anti-bonding sigma orbital?
n-σ*
What is the effect of a red shift or bathochromic shift on the absorption of a chromophore?
The absorption maximum shifts towards longer wavelengths.
Which of the following is an auxochrome that can cause a bathochromic (red) shift in the absorption spectrum of a chromophore?
-NH₂ group
Which of the following statements is true about the hypochromic effect?
It causes a decrease in the intensity of the absorption band.
Which of the following statements about a blue shift or hypsochromic shift is correct?
It is caused by the removal of conjugation in a molecule.
What is the effect of a hypochromic shift on the absorption of a chromophore?
It decreases the absorption intensity.
In the π-π* transition, which of the following orbitals is the electron excited from?
Bonding pi orbital (π)
Which of the following statements is true about the solvent effect on the π-π* transition?
The excited state π* is more polar than the ground state π, so the energy of π* is lowered more than π in polar solvents.
Which of the following statements about a hyperchromic effect is correct?
It is an effect that increases the absorption intensity.
What is the main utility of UV-Vis spectroscopy in pharmaceutical analysis?
Understanding unsaturation and resonance in compounds
Which of the following groups is NOT considered an auxochrome?
-CH3
Which of the following transitions is associated with the absorption of light in the visible region (400-750 nm)?
π-π*
In pharmaceutical analysis, what is the function of FTIR spectroscopy?
Identifying the functional group
What is the primary role of a chromophore in a molecule?
To provide the molecule with color by absorbing visible light.
Which of the following statements is true about chromophores?
They are groups that absorb light in a specific region of the electromagnetic spectrum.
What does NMR spectroscopy primarily identify in compounds?
Environments of protons and carbons
What does Mass spectrometry help determine in compounds?
Molecular weight
Which method provides information on the percentage of sulfur, carbon, nitrogen, and hydrogen in a compound?
SCHN element analysis
What are the five components that every spectrometer must consist of?
Source, holder, monochromator, detector, recorder
Electromagnetic waves are characterized by which three main elements?
Wavelength, frequency, amplitude
Which type of shift is observed when a molecule absorbs light and the absorption wavelength increases?
Red shift
What is the role of auxochromes in UV-Vis absorption?
Enhancing the color of the compound
When a molecule experiences a decrease in absorbance due to structural modification, what effect is this known as?
Hypochromic effect
What is the primary function of chromophores in UV-Vis spectroscopy?
Absorb specific wavelengths of light
Which type of shift is observed when there is a decrease in the absorption wavelength of a molecule?
Blue shift
What governs which electronic transitions are allowed in a molecule during UV-Vis absorption?
Symmetry rules
In UV-Vis spectroscopy, what does Beer-Lambert's law primarily help in determining?
Concentration of compounds
Which property determines the 'allowedness' or otherwise, of an electronic transition during UV-Vis absorption?
'Forbiddenness' effect
'Allowed transitions' during UV-Vis absorption are mainly dependent on what factor?
'Chromophoric properties'
'Hyperchromic effect' refers to what change in a molecule's absorbance spectrum?
Increase in intensity
Study Notes
Auxochromes and Chromophores
- Auxochromes are groups that have significant effects on the absorption of a chromophore, attached to a molecule responsible for the intensity of color in a compound.
- Auxochromes are typically atoms with one or more lone pairs of electrons, e.g., -OH, -OR, -NR2, halogen.
Types of Electronic Transitions
- δ-δ*: High energy transition, occurs in saturated hydrocarbons, and requires vacuum UV.
- π-π*: Occurs in unsaturated compounds (containing π bonds), e.g., alkene, alkyne.
- n-π*: Occurs in compounds containing N, O, S, and P atoms with π bonds, e.g., ketone, nitrile.
- n-δ*: Occurs in compounds containing N, O, S, and P atoms with no π bonds, e.g., amines, alcohols (saturated compounds).
Sigma and Pi Bonds
- Sigma bonds are very stable, and electrons in sigma bonds are usually unaffected by UV wavelengths of light above 200 nm.
- Pi bonds have electrons that are more easily excited into higher energy orbitals.
Solvent Effect
- The excited state π* is more polar than the ground state π, resulting in dipole-dipole interactions with solvent molecules, which lower the energy of π* more than π.
Spectroscopic Methods
- Spectroscopic methods depend on the interaction between matter and electromagnetic radiation.
- UV-Vis: used to know the unsaturation and resonance in compounds.
- FTIR: used to identify functional groups.
- NMR: identifies the environments of protons and carbons in compounds.
- Mass: identifies the molecular weight of compounds.
- SCHN: element analysis gives the percentage of the presence of sulfur, carbon, nitrogen, and hydrogen in compounds.
Spectrometer Components
- Every spectrometer must consist of 5 parts:
- Electromagnetic radiation source
- Sample holder
- Monochromator
- Detector for signals
- Recorder for output signal data
Electromagnetic Waves
- Electromagnetic waves are characterized by wavelength, frequency, and amplitude.
- Wavelength (λ) is the distance between two successive wave maxima.
- Amplitude is the height of the wave measured from the center.
UV-Vis Spectroscopy
- The wavelengths of UV light absorbed by a molecule are determined by the electronic energy differences between orbitals in the molecule.
- The range of vacuum ultraviolet (vacuum UV) is below 200 nm.
- The range of UV radiation is between 200-380 nm.
- The range of visible radiation is between 380-800 nm.
- UV-Vis can be used in qualitative and quantitative analysis.
Beer-Lambert Law
- The Beer-Lambert law is widely used in UV-visible spectroscopy for quantitative analysis.
- A = ε l c, where A is the absorbance, ε is the molar absorption coefficient, l is the sample length, and c is the concentration.
Measurement of the Spectrum
- The ultraviolet or visible spectrum is usually taken using a dilute solution.
- The excitation of electrons is accompanied by changes in the vibrational and rotational quantum numbers, resulting in a broad peak containing all the vibrational and rotational fine structure.
Spectrum of UV-Vis
- The spectrum is drawn between A (absorbance) and wavelength λ.
- The wavelength at which the absorbance will be the highest (peak) is called λmax.
Test your knowledge on auxochromes, which are groups that affect the absorption of a chromophore, and the bathochromic effect, which is a shift towards longer wavelengths. Learn about the impact of these concepts on the intensity and color of compounds.
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