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Questions and Answers
What is the main purpose of Ultraviolet (UV) spectroscopy?
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?
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?
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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?
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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?
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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.
