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Questions and Answers
What is the main purpose of Ultraviolet (UV) spectroscopy?
What is the main purpose of Ultraviolet (UV) spectroscopy?
What is the result of the absorption of ultraviolet or visible radiation in a molecule?
What is the result of the absorption of ultraviolet or visible radiation in a molecule?
In molecular absorption spectroscopy, what happens to the electrons in the bonds within the molecule when radiation is passed through a solution?
In molecular absorption spectroscopy, what happens to the electrons in the bonds within the molecule when radiation is passed through a solution?
What is the relationship between the wavelength of radiation absorbed and the energy of the radiation?
What is the relationship between the wavelength of radiation absorbed and the energy of the radiation?
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What is the wavelength range of radiation used in Ultraviolet (UV) spectroscopy?
What is the wavelength range of radiation used in Ultraviolet (UV) spectroscopy?
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What is the main application of Nuclear Magnetic Resonance (NMR) spectroscopy?
What is the main application of Nuclear Magnetic Resonance (NMR) spectroscopy?
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What is the type of spectroscopy that fragments the molecule and measures the masses?
What is the type of spectroscopy that fragments the molecule and measures the masses?
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What is the type of spectroscopy that measures the bond vibration frequencies?
What is the type of spectroscopy that measures the bond vibration frequencies?
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What is the correlation between the wavelengths of absorption peaks and the types of bonds in the species under study?
What is the correlation between the wavelengths of absorption peaks and the types of bonds in the species under study?
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What is the study of the absorption and emission of light and other radiation by matter?
What is the study of the absorption and emission of light and other radiation by matter?
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Study Notes
Red Shift and Conjugation
- Increasing conjugation in compounds leads to a red shift of λ max.
- Example compounds:
- CH2=CH-CH=CH-CH=CH2: λ max = 185 nm
- CH2=CH-CH=CH2: λ max = 217 nm
Red Shift and Ring Structure
- More rings in a molecule also result in a red shift of λ max.
- Example compounds:
- Benzene: λ max = 204 nm
- Naphthalene: λ max = 286 nm
Influence of Solvents on UV Spectrum
- Solvents significantly affect UV spectra; their peaks can overlap with compound peaks.
- Optimal solvents should not absorb in the UV region of interest.
- Transparency in the region under investigation is crucial.
- Common solvents with their lower λ limit:
- Water: 180 nm
- Ethanol: 220 nm
- n-Hexane: 200 nm
- Cyclohexane: 200 nm
- Diethyl ether: 210 nm
- Acetone: 330 nm
Components of UV/Visible Spectrophotometer
- Light sources initiate light dispersion.
- Monochromator disperses light into constituent wavelengths and selects them via a slit.
- Optical systems may allow for double-beam configuration to correct readings using a blank solution.
Absorption Spectrometer Range
- Operates within 200 nm (near UV) to 700 nm.
- Key electronic transitions:
- π to π* transition
- n to π* transition
- n to σ* transition
Energy Transition Notes
- The energy for σ to σ* transitions is significant, requiring radiation of higher frequencies than UV.
- Example: Methane, with only C-H bonds, shows no UV absorption due to lack of π bonds.
Chromophores and Auxochromes
- Chromophores are parts of molecules that absorb UV-visible light, commonly including benzene rings.
- Auxochromes (e.g., -OH, -NH2) are substituents that modify the absorption spectrum.
Terminology for Absorption Shifts
- Bathochromic shift: Shift to longer wavelengths (red shift).
- Hypsochromic shift: Shift to shorter wavelengths (blue shift).
- Hyperchromic effect: Increase in absorption.
- Hypochromic effect: Decrease in absorption.
Conjugation Effect
- Structures with multiple conjugated double bonds absorb at longer wavelengths with increased intensity.
Overview of Spectroscopy
- Spectroscopy studies light absorption and emission by matter.
- Types include:
- UV spectroscopy: Focuses on electron transitions for quantification.
- Mass spectrometry (MS): Analyzes molecule fragments by mass.
- Infrared (IR) spectroscopy: Measures bond vibration frequencies.
- Nuclear magnetic resonance (NMR) spectroscopy: Used for structural analysis.
Molecular Absorption Principles
- Radiation (200-700 nm) excites electrons in molecular bonds, leading to higher quantum states.
- More loosely held electrons result in lower energy, longer wavelength absorption.
- UV (200-400 nm) and visible radiation typically excite bonding electrons, correlating absorption peaks with bond types.
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Description
Understand the effects of conjugation and solvents on UV-Visible spectra. Learn how increasing conjugation and the number of rings affect λ max, and how solvents can obscure compound peaks. Test your knowledge of UV-Vis spectroscopy principles.
