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Electronic Spectroscopy

Page history last edited by Ben Kreitz 10 years, 6 months ago

Electronic Spectroscopy

 

Introduction

 Spectroscopy, broadly defined, is the branch of science concerned with the investigation and measurement of spectra produced when matter interacts with or emits light (electromagnetic radiation). Electronic spectroscopy (stemming from “electron” spectroscopy) is a specific branch of spectroscopy that investigates how light interacts with the electrons of the atoms within a chemical sample. Electronic spectroscopy is important to chemists because it is used as another analytical method of determining the characteristics of new compounds, of identifying unknown compounds, and can even give insight into the atomic structures of materials being studied.  

 

 

 

Figure 1: An example of an electronic spectrophotometer and an example of an electronic spectrophotometer readout showing wavelength of light shined through the sample vs. absorbance reading of the sample. The right column on the picture to the right shows the readout of the spectrophotometer while the left column shows how the spectrophotometer conducted the testing (wavelength vs. intensity)

 

Basic Concepts and Underlying Scientific Principles

 

How Electronic Spectroscopy Works

 

Electronic Spectroscopy (ES) is a method used to analyze molecules by exciting their electrons to a higher energy state within the atom, measuring what wavelength of light causes this change, and then measuring what wavelength of light is emitted as the electron falls back to its ground state (see figure 2 below) . This process is very useful in imaging molecules and for seeing the changes in energy and electron transfer within the sample being tested. The other forms of spectroscopy that you may be familiar with are Nuclear Magnetic Resonance (NMR) and Infrared (IR). These three spectroscopy methods (IR, NMR, and Electron Spectroscopy) each correspond to a different part of the electromagnetic spectrum (See Figure 3 below). For example, electronic spectroscopy has to do with the UV and optical section of the spectrum. Vibrational Spectroscopy is concerned with the infra-red (IR) part of the spectrum while rotation spectroscopy is connected to the microwave section.

 

Electronic spectroscopy is largely concerned with the valence electrons of the sample (as opposed to the core electrons). In electronic spectroscopy the valence electrons are excited from the highest occupied molecular orbitals (HOMO) to the lowest unoccupied molecular orbitals (LUMO), which are also known as frontier orbitals. The energy required for transition from HOMO to LUMO differs for what molecule is being examined, but generally (pi) → (pi*) transitions are stronger than the transition between n (the nonbonding electrons) → (pi*) due to the orientation of the bonding orbitals/lone pair orbitals in relation to the antibonding orbitals.  Since electronic spectroscopy is the process where a certain intensity is directed at a sample in order to excite the electrons on the outer shell, the electrons will absorb a certain amount of light. This is described mathematically by Beer’s Law, A=ecl (A= absorbance of the molecule, l is the length the light from the spectrophotometer moves through the sample, e is the extinction coefficient, I is intensity (power/unit area) and c is the molar concentration of the sample) and I/I0 =  10-elc  which indicates that unless there is total absorbance, the intensity transmitted of the energy through the sample will be exponential related to the initial intensity.

 


 

 

Figure 2: Electron orbital diagram showing the excitation of an electron (causing absorption of light) and its return back to its normal state (causing emission of light). The diagram to the left shows the different ways light can react after hitting a solid.

 

Vision:

 The visible spectrum is from ~400nm to ~700nm (purple to red). The visible spectrum is only one part of the electromagnetic spectrum. For example, IR light has a longer wavelength than the visible spectrum; as wavelength increase, the frequency decreases because they are inversely related (c=(lamda)*v) For more specifics for other types of light, see the figure below, but our discussion will focus on the visible spectrum because it is the part of the spectrum that humans can visually process.

Figure 3: Electromagnetic Spectrum

 

 We talk about vision/visible spectrum because it is similar to electronic spectroscopy where both processes excite electrons. To talk about vision, we need to know about chromophores; they are functional groups on molecules that absorb light, and therefore, color is emitted because electrons can move and be excited within the conjugated system. The light that is not absorbed by the chromophores leads to the colors we see. This is known as a complementary phenomenon--meaning that the colors that people see are not the colors that are absorbed by the item but they are the colors that are reflected back. For example, the red apple is not red because the apple absorbs the red wavelength; instead the apple absorbs all other colors and red is reflected or transmitted through the item because it is not absorbed.

   

Quantum Mechanics Relationship

 The chromophores that absorb light are similar to the particle-in-a-box model. This is because the chromophores have electrons that move within the conjugated system at different levels of energy. The conjugated molecule represents the boundaries of a “box” and the electrons are simply the “particles”. Specifically, the particle-in-a-box model represents the energy of a particle. At the boundaries, the energy is zero. Furthermore, the potential energy within the box is zero, while it is infinity outside the bounds of the box. The picture below shows the energy levels for n=1...5 along with the probability density which is given by the integral of (wave function)2.

Figure 4: Wave function diagrams with and their corresponding probability diagrams

 

Applications

 The applications of electronic spectroscopy reach far and wide in the area of education and research. Spectroscopy gives rise to investigations of atomic, molecular, and solid-state structure. It can be used to monitor elemental composition of surfaces during processes. Chemical properties such as corrosion, stress corrosion, oxidation, and catalytic activity and mechanical properties such as fatigue, wear, adhesion, resistance to deformation processes can all be analyzed by electronic spectroscopy which makes it so useful in labs of all fields.

 

 Many undergraduate labs use Infrared (IR) or Nuclear Magnetic Resonance spectroscopy as a part of their experimental procedure to determine the purity and identity of products created in lab, and a general understanding of these topics will aid in understanding the differences between these common spectroscopic techniques and electronic spectroscopy.

 

 Nuclear Magnetic Resonance (NMR) Spectroscopy: This method deals with the nucleus of atoms and measures the physical response in which they absorb and re-emit electromagnetic radiation between nuclear spin states. There are two different types of NMR: 1H NMR (which examines the signatures of Hydrogen atoms in the sample) and 13C NMR (which gives information about the Carbon structure of the sample)

 

 Infrared (IR) Spectroscopy The method relies on the fact that molecules absorb different frequencies due to their structures, which leads to different vibrations of the bonds. These vibrations can be used to identify the functional groups on a specific molecule though the energy associated with IR issn't enough to excite electrons such as ES

 

Benefits and Limits of Electronic Spectroscopy Over Other Types of Spectroscopy:

 Electronic spectroscopy is commonly used in many professional labs for a few reasons:

  • Electronic Spectroscopy Is Very Sensitive

    • High resolution detection of valence electron emission spectra

  • Electronic Spectroscopy Is Fast

    • Only takes a few minutes to run

    • NMR takes a long time to run and involves expensive machinery

  • Electronic Spectroscopy is Non-Specific

    • Can detect any atomic signature (atomic traits specific to each atom on the periodic table) higher than Helium (He)

    • IR spectroscopy is good at detecting functional groups but not individual atoms 

 As with any detection method though, electronic spectroscopy has its limits:

  • Electronic spectroscopy does not tell us about the 3D structure of the molecule

    • IR spectroscopy can give insight into functional groups and bonding structure

    • NMR can give predictions about the 3D structure as well

  • Electronic spectroscopy can damage the sample being studied

    • Especially damaging to samples already sensitive to light 

 

Concept Questions

1) What is electronic spectroscopy?

      a) a branch of spectroscopy that investigates how light interacts with the electrons of the atoms within a chemical sample.

      b) a branch of spectroscopy that investigates how light interacts with intermolecular bonds

     c) a branch of spectroscopy that investigates how light interacts with electrical fields in a vacuum

     d) a branch of spectroscopy that investigates how light interacts with the protons of atoms helium or higher on the periodic table

 

2) What is one of the major advantages of electronic spectroscopy?

    a) it can replace NMR and IR

    b) it is quick at processing data

    c) it can reduce the number of trials needed to identify a compound

    d) it allows researchers to determine the location and momentum of an electron

 

3) What is the main theory utilized in electronic spectroscopy? (hint: lambda=h/p)

    a) the cognitive-behavioral theory

    b) the electromagnetic principle

    c) quantization of energy

    d) the solid-phase ionic principle

 

4) What is a chromophore?

    a) a protein within a plant cell that converts solar energy into ATP

    b) a substrate for a chemical technique that uses oxidation to adhere chromium (Cr ions) to a substrate

    c) a functional group on a molecule that makes them looked colored by absorbing light

    d) the molecular effect whereby a molecule turns sunlight into its components in a prism like manner

 

 

References

Chourasia, A. R., Chopra, D. R. Auger Electron Spectroscopy, Prenhall, Sept. 16, 2013 <http://www.prenhall.com/settle/chapters/ch42.pdf>

 

Delmar, Electronic Spectroscopy: Interpretation, UCDavis, Electronic Spectroscopy: Interpretation, Feb. 26, 2013, Sept. 16, 2013, <http://chemwiki.ucdavis.edu/Physical_Chemistry/Spectroscopy/Electronic_Spectroscopy/Electronic_Spectroscopy%3A_Interpretation>

 

Electronic Spectroscopy of Molecule 1-Absorption Spectroscopy, Sept. 30, 2013 <http://www.uni-konstanz.de/FuF/Bio/folding/3-Electronic%20Spectroscopy%20r.pdf>.

 

Introduction to Spectroscopy, Michigan State University, Organic Chemistry On Line, no date create, Sept. 16, 2013, <http://www2.chemistry.msu.edu/faculty/reusch/VirtTxtJml/Spectrpy/spectro.htm>

 

Multidimensional Electron Spectroscopy of Proteins, The Jimenez Lab, Multidimensional Electron Spectroscopy of Proteins2013, Sept. 16, 2013, <http://jila.colorado.edu/jimenez/research/multidimensional-electronic-spectroscopy-proteins>

 

Oxtoby, Gillis, and Campion, Introduction to Quantum Chemistry. Mason: Cengage Learning, 2012. Print.

 

Rangel, Victoria, X-ray Photoelectron Spectroscopy, X-ray Photoelectron Spectroscopy, Oct. 10, 2010, Sept. 16, 2013, <http://wiki.utep.edu/display/~vrrangel/X-ray+Photoelectron+Spectroscopy+%28XPS%29>

 

 

 

Answers

1) a

2) b

3) c

4) c

 



 

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