Understanding Electron Paramagnetic Resonance (EPR) and Molecular Symmetry
The discussion covers the relevance and applications of EPR spectroscopy in studying electron behavior, spin-orbit coupling effects, and molecular symmetry. Key concepts include anisotropy, nuclear spin coupling, and deviations in electron g-factors. It delves into how these factors impact the observed EPR signals and provide insights into electronic structure and interactions with neighboring nuclei.
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Chem 516: Day 15 EPR, Part 2 Note: any figures that look like they are from a book are from Que
Groups for Think-Pair-Share Beckley, Aidan T Bender, Noah A Lastowski, Robert J Medina Nales, Kevin Boedicker, Lauren V Chakrabarti, Sagnik Melari, Angelo J Schmit, Claire E Fatima, Saman Gray, Daniel W Wang, Jingxiang Wells, Brady Greskovich, Katherin Gullett, Kelly L Wo, Anita Zhang, Xueyong Heidari, Hirbod Hernandez Al, Erick Send your partner a chat! Holm, Annika R Hua, Qi Kelly, Kimberly A
What we saw last class for a single electron in an s orbital ms = 1/2 E E = g H ms = -1/2 H A H A H
So what good is this? For a free electron, g=2.0023 Deviation from 2.0023 is caused by spin-orbit coupling: which orbital is that unpaired electron in? Anisotropy (gx,gy,gz) gives molecular symmetry, more info on electronic structure Nuclear spins will couple to the electron spin, give hyperfine and superhyperfine coupling: which nuclei are near that unpaired electron?
Point 1: Anisotropy gz gx,gy
First question: why does this look backwards? g < g , but g is to the left of g : why? The x axis is H, and E=g H, therefore g= E/ H g and H are inversely proportional: H goes up, g dows down
But Josh, you still havent explained why g 2! Case 1: Mo(V) is 4d1, and can have several geometries 1) Compressed Octahedron 2) 8-coordinate ?2 ??,?? ?2 ?2 ?2 Rx, Ry ?? Rz ??,?? Rx, Ry Rz ?2 ?2 ?? g < g < 2 - Moving e- opposes the EPR magnet, so you need to crank up H more - Smaller energy gap: better circulation, so greater effect - g < g < 2 (remember gz=g )
If you are moving a hole, then the moving e- helps the magnet, so opposite effect NiI ?2 Rx, Ry ?2 ?2 ??,?? ?? Now ?2??,????,??, so g > 2. No rotation around z, so g = 2
This is quantitative!! ?2 ? = 2.0023 +? ? Rz, 2 ?? ?? ? is the number on the pentagon for that rotation is the spin-orbit coupling for that atom Example: CuII = -830 cm-1 Ry, 2 Ry, 2 ?2 ?2 ?? Rz, 8 CuII porphyrin, gz=2.70, gx,y=2.04 ?2 ?2 ??= 2.0023 +8 830?? 1 Rz ?2 ? Rx, Ry ??2 ?2,??= -9520 cm-1 = -1.1 eV ?? ??,?= 2.0023 +2 830?? 1 ? ??,?? ??2 ?2,??/??= -44,000 cm-1 = -5.4 eV
How the heck do they get those numbers? ??= ? ?? ?? ?? ?? ?+1= 1 15 2? ? + ?? ? ?2 2 1 2 15 2? ? ?? ? ?2 ? 1= ? + ??2 ?2 ?+2=1 15 2? 4 ? ??2 ?2 ? 2= 1 15 2? 4
Your final task for today: Suppose you measure the EPR of a Cu protein, and observe gx=2.155, gy=2.155, gz=2.0023. You suspect the Cu geometry is either an axially elongated octahedron or a trigonal bipyramid Axially elongated Oh Trigonal bipyramid Which does the EPR spectrum support?
