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CHEM205: Spectroscopy

Unit 2: Ultraviolet-Visible Spectroscopy   Ultraviolet-visible spectroscopy (UV-Vis) refers to absorption spectroscopy in the ultraviolet-visibleregion of the electromagnetic spectrum.  In this region of the electromagnetic spectrum, molecules undergo electronic transitions, where electrons move from a lower energy orbital (ground state) to one of higher energy (excited state).  The nature of the electronic transition determines the nature of the spectra produced.

This unit begins with a discussion of electromagnetic radiation and spectroscopy, followed by the theory of UV-Vis spectroscopy.  The course continues with a discussion of types of electronic transitions and a brief review of how UV-Vis is used for analytical methods.  The unit concludes with methods of empirically determining absorption wavelengths based on structural implications.     

Unit 2 Time Advisory
This unit will take approximately 11 hours to complete.

☐    Subunit 2.1: 2.0 hours

☐    Subunit 2.2 1.0 hours

☐    Subunit 2.3: 3.0 hours

☐    Subunit 2.4: 5.0 hours

Unit2 Learning Outcomes
Upon successful completion of this unit, the student will be able to:

  • Identify the components of an unltraviolet-visible spectrophotometer.
  • Discuss the electronic transitions measured in UV-Vis spectroscopy.
  • Explain how UV-Vis spectroscopy is used as an analytical technique.
  • Predict UV-Vis absorption wavelengths based on molecular structure.

2.1 General Introduction to Spectroscopy   - Reading: William Reusch’s “Virtual Text of Organic Chemistry: Visible and Ultraviolet Spectroscopy” Link: William Reusch’s “Virtual Text of Organic Chemistry: Visible and Ultraviolet Spectroscopy” (HTML)
 
Instructions: Please read the entire webpage.  Be sure to follow the links for supplemental information required for successful completion of this course.  This material covers Subunits 2.1, 2.4, 2.5, and 2.6. 
 
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2.1.1 The Electromagnetic Spectrum   - Reading: Central Connecticut State University: Dr. Neil Glagovich’s “IR Spectroscopy: Background” Link: Central Connecticut State University: Dr. Neil Glagovich’s “IR Spectroscopy: Background” (HTML)
 
Instructions: Click on the “IR” button on the left-hand side of the webpage, then click the link called “Electromagnetic Radiation” (Section 1, A). Please read the entire webpage.  Please note that, although the material is located under the IR spectroscopy section of this webpage, it is applicable to all spectroscopic methods because it is the interaction of the molecules with various wavelengths of light from the electromagnetic spectrum that give rise to the spectra.
 
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2.1.2 Ultraviolet and Visible (UV-Vis) Theory   - Lecture: OCHeM.com: Thomas Poon’s “Spectroscopy, Part 1 of 4” Link: OCHem.com: Thomas Poon’s “Spectroscopy, Part 1 of 4” (QuickTime)
 
Instructions: Scroll down the list of “PreLectures” to find a link entitled “Spectroscopy, Part 1 of 4”.  Click on the link; it will launch a QuickTime application in a new window.  Watch the video (runtime = 11:09 minutes), which gives an overview of spectroscopic methods.  The energy transfer between the ground and excited states during UV-Visible spectroscopy is explained.  Infrared and nuclear magnetic resonance spectroscopy are also briefly covered.
 
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2.2 UV-Vis Spectroscopy and the Excited State   - Reading: Central Connecticut State University: Dr. Neil Glagovich’s “Ultraviolet/Visible Spectroscopy: Types of Electronic Transitions” Link: Central Connecticut State University: Dr. Neil Glagovich’s “Ultraviolet/Visible Spectroscopy: Types of Electronic Transitions” (HTML)
 
Instructions: Click on the “UV/Vis” button on the left-hand side of the webpage, then follow the links (A–D) below “Section II: Types of Electronic Transitions.”  Please read all of the information presented in each section.  This material explains the allowed electronic transitions observed in UV-Vis spectroscopy.  
 
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2.3 UV-Vis Spectroscopy as an Analytical Tool   - Reading: Central Connecticut State University: Dr. Neil Glagovich’s “Ultraviolet/Visible Spectroscopy: Background Information” Link: Central Connecticut State University: Dr. Neil Glagovich’s “Ultraviolet/Visible Spectroscopy: Background Information” (HTML)
 
Instructions: Click on the “UV/Vis” button on the left-hand side of the webpage, then follow links (A–G) below “Section I: Background Information.”  Please read all of the information presented in each section.  This material gives theory and experimental design and analysis for the use of UV-Vis spectroscopy as an analytical method.  
 
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  • Reading: Connexions: Brittany L. Oliva and Andrew R. Barron’s “Basics of UV-Visible Spectroscopy” Link: Connexions: Brittany L. Oliva and Andrew R. Barron’s “Basics of UV-Visible Spectroscopy” (HTML or PDF)
     
    Instructions: Please read through the entire webpage.  This material is a review of concepts from Analytical Chemistry (CHEM108). You can also see this material in PDF format from the “Download” tab on the top right.
     
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2.4 Empirical Determination of UV/Vis Absorption Wavelengths   - Reading: Central Connecticut State University: Dr. Neil Glagovich’s “Ultraviolet/Visible Spectroscopy: Typical Absorptions for Simple Isolated Chromophores” Link: Central Connecticut State University: Dr. Neil Glagovich’s “Ultraviolet/Visible Spectroscopy: Typical Absorptions for Simple Isolated Chromophores” (HTML)
 
Instructions: Click on the “UV/Vis” button on the left-hand side of the webpage, then click on “Section III: Table of Typical Absorptions of Simple Isolated Chromophores.”  This table lists the types of transitions, approximate absorption wavelengths, and relative absorptivities of various types of compounds.  Please note the influence of increased conjugation on the absorption wavelengths.  Conjugation is the overlap of one p-orbital with another across an intervening sigma bond.  A conjugated system has a region of alternating single and double bonds.  The pi electrons do not belong to a single bond or atom, but rather to a group of atoms.  The system allows a delocalization of electrons across all the adjacent aligned p-orbitals, which in general may lower the overall energy (increasing the absorption wavelength) of the molecule and increase stability.
 
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2.4.1 The Woodward?Fieser Rules for Dienes   - Reading: Central Connecticut State University: Dr. Neil Glagovich’s “Ultraviolet/Visible Spectroscopy: Empirical Rules for Calculating Uv/Vis Absorptions” Link: Central Connecticut State University: Dr. Neil Glagovich’s “Ultraviolet/Visible Spectroscopy: Empirical Rules for Calculating Uv/Vis Absorptions” (HTML)
 
Instructions: Click on the “UV/Vis” button on the left-hand side of the webpage, then click on the link “Woodward-Fieser Rules for Dienes” (Section IV, A).  This material gives base absorption wavelength for cis- and trans- dienes and subsequent wavelength (energy) shifts based on additional substituents.  Please work through the examples given. 
 
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2.4.2 Woodward's Rules for Conjugated Carbonyl Compounds   - Reading: Central Connecticut State University: Dr. Neil Glagovich’s “Ultraviolet/Visible Spectroscopy: Empirical Rules for Calculating Uv/Vis Absorptions” Link: Central Connecticut State University: Dr. Neil Glagovich’s “Ultraviolet/Visible Spectroscopy: Empirical Rules for Calculating Uv/Vis Absorptions” (HTML)
 
Instructions: Click on the “UV/Vis” button on the left-hand side of the webpage, then click on the link “Woodward's Rules for Conjugated Carbonyl Compounds” (Section IV, B).  This material gives a base absorption wavelength for conjugated carbonyl compounds and subsequent wavelength (energy) shifts based on additional substituents.  Please work through the examples given. 
 
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2.4.3 Substituted Benzene and Benzoyl Derivatives   - Reading: Central Connecticut State University: Dr. Neil Glagovich’s “Ultraviolet/Visible Spectroscopy: Empirical Rules for Calculating Uv/Vis Absorptions” Link: Central Connecticut State University: Dr. Neil Glagovich’s “Ultraviolet/Visible Spectroscopy: Empirical Rules for Calculating Uv/Vis Absorptions” (HTML)
 
Instructions: Click on the “UV/Vis” button on the left-hand side of the webpage, then click on links C–E in Section IV. This material gives base absorption wavelengths for mono- and di-substituted benzene and benzoyl derivatives and subsequent wavelength (energy) shifts based on additional substituents.  Please work through the examples given.
 
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  • Interactive Lab: Jean-Claude Bradley and Andrew Lang’s “Spectral Game” Link: Jean-Claude Bradley and Andrew Lang’s “Spectral Game” (HTML)
     
    Instructions: Please enter a username in the required field.  Note: You do not need to register and this does not need to be your actual name (i.e., student1).  Choose the format you prefer for your viewer and select UV-Vis from the “Type” drop-down box.  A general video tutorial from the creators of the game can be found here (YouTube). Please spend an ample amount of time using this game as a learning tool to increase your spectral analysis proficiency.
     
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