10.07.2011 - Lecture 14 Wednesday, October 7, 2011...

Info iconThis preview shows page 1. Sign up to view the full content.

View Full Document Right Arrow Icon
This is the end of the preview. Sign up to access the rest of the document.

Unformatted text preview: Lecture 14 Wednesday, October 7, 2011 Today in Chemistry 260/261 •  Electronic spectroscopy •  Applications •  Review of rotation/vibration/electronic spectroscopy Coming up in Chemistry 260/261 • Monday: Quantum Connections (no reading, bring clickers!) • Problem set 5 due Monday, October 10 by 3:00 pm • MIDTERM 1: Tuesday, October 11, 8 ­10 pm, Chem 1800 Electronic Spectroscopy Electronic Spectroscopy of Organic Molecules Electronic Spectroscopy of Organic Molecules Organic molecules that absorb visible light have conjugation Organic molecules that absorb visible light have conjugation Polymerize with catalyst NP 2000 Trans-polyacetylene (tPA) 1 Graphene: Nobel Prize 2010 Graphene: Nobel Prize 2010 How to win a Nobel Prize with Scotch Tape How to win a Nobel Prize with Scotch Tape Graphite: layers of carbon sheets Graphene: one carbon sheet Graphite is a semiconductor (see experiment) Strongest known material (per weight) Very high conductance Very high absorption (2.3%) Ω Wikipedia: Graphite Wikipedia: Graphene Graphene: Nobel Prize 2010 Graphene: Nobel Prize 2010 How to win a Nobel Prize with Scotch Tape How to win a Nobel Prize with Scotch Tape Graphene: one carbon sheet Graphene: one carbon sheet Strongest known material (per weight) Strongest known material (per weight) Very high conductance Very high conductance Very high absorption (2.3%) Very high absorption (2.3%) Mass of the hammock: 0.77 mg (less than a cat s whisker) UC Berkeley: Zettl Group Royal Swedish Academy of Sciences 2 Molecular Electronics Electronic Spectroscopy of Organic Molecules Organic molecules that absorb visible light have conjugation # π electrons 4 8.5 8 1.3Å 2 pz 5.7 11.3 0.8 Å 1.5Å 0.8 Å Length (Å) 6 1.3Å 2 pz 2 pz 2 pz h"p://www.uam.es/personal_pdi/ciencias/jcuevas/Molecular%20electronics.html En = PIAB is a good model for conjugated molecules 1.3Å 0.8 Å 8.5 8 n2 , n = 1, 2, 3, ... L2 11.3 0.8 Å 1.5Å 1.3Å En = PIAB is a good model for conjugated molecules h2n2 , n = 1, 2, 3, ... 8 mL2 E0 = Organic molecules that absorb visible light have conjugation 5.7 6 En = E0 Length (Å) 4 h2 8m E0 = Organic molecules that absorb visible light have conjugation # π electrons h2n2 , n = 1, 2, 3, ... 8 mL2 En = E0 h2 8m n2 , n = 1, 2, 3, ... L2 # π electrons 1.3Å 0.8 Å Length (Å) 4 5.7 n=5 6 8.5 n=4 8 11.3 0.8 Å 1.5Å 1.3Å n=4 n=3 n=3 n=2 n=2 n=1 n=1 1,3-butadiene 1,3,5-hexatriene lowest E transition is from n=2 to n=3 2 !E3"2 = E0 (3) # (2) (5.7 Å) 2 2 = E0 5 (5.7 Å) 2 lowest E transition is from n=3 to n=4 2 = 9.3 $ 10 #19 J !E3"2 = E0 Wavelength = 213 nm n=1 n=2 ( 4) # (3) ( 8.5 Å ) 2 2 = E0 7 (8.5Å) 2 = 5.8 $ 10 #19 J Wavelength = 342 nm n=1 n=2 3 En = PIAB is a good model for conjugated molecules h2 E0 = 8m n2 En = E0 2 , n = 1, 2, 3, ... L Organic molecules that absorb visible light have conjugation # π electrons 1.3Å h2n2 , n = 1, 2, 3, ... 8 mL2 Length (Å) 0.8 Å 1.5Å 4 n=5 8.5 11.3 Organic molecules that absorb visible light have conjugation wavelength experimental # π electrons length (Å) 4 5.7 213 nm 217 nm 6 8.5 342 nm 288 nm 8 1.3Å 11.3 467 nm 303 nm 0.8 Å 1.5Å n=4 8 0.8 Å 5.7 6 1.3Å PIAB is a good model for conjugated molecules 0.8 Å 1.3Å n=3 n=2 n=1 1,3,5,7-octatetraene lowest E transition is from n=4 to n=5 2 !E3"2 = E0 n=1 (5) # ( 4) (11.3Å) 2 2 = E0 9 (11.3Å) 2 Wavelength = 467 nm n=2 Why might there be discrepancies? = 4.25 $ 10 #19 J n=1 Size effects are general Real Molecular Orbitals In general, size is the determining factor in any kind of spectroscopy n=2 dinitrogen, N2 CdSe dots Dyes of various size and degree of conjugation Exciting the lowest energy transition, what happens to the bond order? Ground state: (6-0)/2 = 3 Excited state: (5-1)/2 = 2 Bond should get weaker! In vibrational and rotational spectroscopy, the bigger the motion the smaller the energy difference between levels A S1 Vibrational levels on the excited state are closer together kex < kgr S0 B Frequency: 1655 cm-1 1.33 Å A 10% bond length change leads to 500 cm-1 shift (that s a lot) B 1.20 Å One reason molecules break in sunlight Frequency: 2120 cm-1 ground excited x 4 Real Molecular Orbitals dinitrogen vs. dioxygen How do we know these pictures are reasonable? O2 starts absorbing around 200 nm N2 absorbs between 150-200 nm HOW DO THESE TRANSITIONS REPRESENT A CHANGE IN DIPOLE MOMENT? 5 ...
View Full Document

This note was uploaded on 01/19/2012 for the course CHEM 260 taught by Professor Staff during the Fall '08 term at University of Michigan.

Ask a homework question - tutors are online