Slide 1

Chemistry 125: Lecture 69 April 14, 2011 Measuring Bond Energies This For copyright notice see final page of this file

Slide 2

Are Bond Energies Real ? Bond Dissociation Energies

Slide 3

BondDissn Energies 99 90 113 89 105 111 89 115 111 123 136.2 127 84 85 91 97 74 122857254 5946 51 67 56 58 57 72 74 73 84 63 92 94 best values as of 2003

Slide 4

Presentation by Prof. G. Barney Ellison University of Colorado, Boulder

Slide 5

D 0 (RH) = Bond Dissociation Energy Definition of D 0 100 kcal mol -1 or 4 eV

Slide 6

How could specific bond energies be measured? Consider methyl alcohol CH 3 O-H CH 3 O + H or H-CH 2 OH H + CH 2 OH or CH 3 -OH CH 3 + OH

Slide 7

One way to measure BDE: Acidity/Negative Ion Cycle CH 3 OH + F CH 3 O + HF K eq k1k1 k -1 He Monitor time Experiment of Veronica Bierbaum HF FF CH 3 OH Add CH 3 OH to a flowing stream of He containing F and see how much CH 3 O - has formed at various times later. This gives k 1. growth of CH 3 O m/z 31 CH 3 O CH 3 ON=O HF growth of F m/z 19 Add HF to a flowing stream of He containing CH 3 O and see how much F - has appeared downstream at various times later. This gives k -1. CH 2 OH never !

Slide 8

c) Acidity/Negative Ion Cycle CH 3 OH + F CH 3 O + HF acid H 298 (CH 3 O-H) = DH 298 (CH 3 O-H) + IE(H) EA(CH 3 O) K eq k1k1 k -1 known ? Bierbaum The equilibrium constant K eq = k 1 /k -1 gives the difference in acidity between CH 3 OH and HF. Since the acidity of HF is known [370.424 0.003 kcal mol -1 ], this experiment determines the energy required for acid dissociation of CH 3 OH. CH 3 OH CH 3 O + H + Acid which can be thought of as: CH 3 OH CH 3 O + H -Electron Affinity Ionization Energy Dissn to find

Slide 9

Anion Photoelectron Spectrum Measures Electron Affinity as Electron Binding Energy: CH 3 O + 0 CH 3 O + e (KE) CH 3 O (no extra vibration) CH 3 O (no extra vibration) laser light energy Measured Kinetic Energy of ejected photoelectron Electron Binding Energy If the product radical is vibrating, the photo-electron kinetic energy will be smaller and the measured electron binding energy will be larger.

Slide 10

EA(CH 3 O) = (2.540 - 0.968) = 1.572 0.004 eV no extra vibration 0.968 eV Engelking, Ellison, Lineberger, J. Chem. Phys. 69, 1826 (1978) CH 3 O + 0 CH 3 O + e Electron Kinetic Energy/eV Photoelectron counts

Slide 11

c) Acidity/Negative Ion Cycle acid H 298 (CH 3 OH) = DH 298 (CH 3 O-H) + IE(H) EA(CH 3 O) acid H 298 ( CH 3 OH ) = 381.9 0.5 kcal mol -1 (Bierbaum) IE(H) = 13.59844 eV or (at 298K) 313.6 kcal mol -1 EA( CH 3 O ) = 1.572 0.004 eV or 36.3 0.5 kcal mol -1 (Ellison et al.) DH 298 (CH 3 O-H) = 104.6 0.6 kcal mol -1

Slide 12

c) Acidity/Negative Ion Cycle Problems ? Cant apply to H-CH 2 OH any base you can think of always gets most acidic proton CH 3 OH CH 3 O + H + electron on O atom (good) CH 2 OH + H + electron on C atom (bad) acid H 298 (H-CH 2 OH) = DH 298 (H- CH 2 OH ) + IE(H) EA( CH 2 OH ) no gas-phase [CH 2 OH] CH 2 OH + e cant measure acid H 298 (H-CH 2 OH) & cant measure EA(CH 2 OH)

Slide 13

c) Acidity/Negative Ion Cycle Problems ? Cant apply to H-CH 2 OH any base you can think of always gets most acidic proton CH 3 OH [CH 3 O] + H + [CH 2 OH] + H + acid H 298 (H-CH 2 OH) = DH 298 (H- CH 2 OH ) + IE(H) EA( CH 2 OH ) no gas-phase [CH 2 OH] CH 2 OH + e cant measure acid H 298 (H-CH 2 OH) & cant measure EA(CH 2 OH) However: CH 3 OH + C CH 2 OH + HC measure K equi via k 1 and k -1 to extract rxn H 298 DH 298 (H-CH 2 OH) - DH 298 (HC ) DH 298 (HC ) = 103.15 0.03 kcal mol -1 DH 298 (H-CH 2 OH) = 96.1 0.2 kcal mol -1

Slide 14

Uses of heats of formation, f H 298 (R) ? What is the C-O bond in methanol? CH 3 -OH CH 3 OH What is ? or

Slide 15

Uses of heats of formation, f H 298 (R) ? What is the C-O bond in methanol? CH 3 -OH CH 3 OH DH 298 (CH 3 -H) = f H 298 (CH 3 ) + f H 298 (H) - f H 298 (CH 4 ) Radical kinetics/PIMS studies DH 298 (CH 3 -H) = 104.99 0.03 kcal -1 & f H 298 (H) is known from D 0 (H 2 ) Classical thermochemistry finds f H 298 (CH 4 ) J. B. Pedley, R. D. Naylor, and S. P. Kirby, Thermochemistry of Organic Compounds; 2 nd ed.; Chapman and Hall: New York, 1986. f H 298 (CH 3 ) = 35.05 0.07 kcal mol -1 BDE(H 2 O) f H 298 (OH) = 8.91 0.07 kcal mol -1 Pedley et al provides f H 298 (CH 3 OH) DH 298 (CH 3 -OH) = f H 298 (CH 3 ) + f H 298 (OH) - f H 298 (CH 3 OH) DH 298 (CH 3 -OH) = 92.1 0.1 kcal mol -1

Slide 16

Ellison I

Slide 17

Ellison II

Slide 18

c) Acidity/Negative Ion Cycle acid H 298 (CH 3 OH) = DH 298 (CH 3 O-H) + IE(H) EA(CH 3 O) acid H 298 ( CH 3 OH ) = 381.9 0.5 kcal mol -1 (Bierbaum) IE(H) = 13.59844 eV or (at 298K) 313.6 kcal mol -1 EA( CH 3 O ) = 1.572 0.004 eV or 36.3 0.5 kcal mol -1 (Ellison et al.) DH 298 (CH 3 O-H) = 104.6 0.6 kcal mol -1

Slide 19

Bond Energies of Alcohols/kcal mol -1 DH 298 (CH 3 O-H) = 104.6 0.7 CH 3 O + H DH 298 (CH 3 CH 2 O-H) = 104.7 0.8 DH 298 ((CH 3 ) 2 CHO-H) = 104.6 0.7 DH 298 ((CH 3 ) 3 CO-H) = 106.3 0.7

Slide 20

Bond Energies of Alcohols/kcal mol -1 DH 298 (CH 3 O-H) = 104.6 0.7 CH 3 O + H DH 298 (CH 3 CH 2 O-H) = 104.7 0.8 DH 298 ((CH 3 ) 2 CHO-H) = 104.6 0.7 DH 298 ((CH 3 ) 3 CO-H) = 106.3 0.7 DH 298 (C 6 H 5 O-H) = 85.8 0.1

Slide 21

Bond Energies of Alcohols/kcal mol -1 DH 298 (CH 3 O-H) = 104.6 0.7 CH 3 O + H DH 298 (CH 3 CH 2 O-H) = 104.7 0.8 DH 298 ((CH 3 ) 2 CHO-H) = 104.6 0.7 DH 298 ((CH 3 ) 3 CO-H) = 106.3 0.7 DH 298 (C 6 H 5 O-H) = 85.8 0.1 DH 298 (HOO-H) = 87.8 0.5 HOO + H DH 298 (CH 3 OO-H) = 88 1 DH 298 (CH 3 CH 2 OO-H) = 85 2 DH 298 (CH 3 ) 3 COO-H) = 84 2

Slide 22

Bond Energies of Alcohols/kcal mol -1 DH 298 (CH 3 O-H) = 104.6 0.7 DH 298 (CH 3 CH 2 O-H) = 104.7 0.8 DH 298 ((CH 3 ) 2 CHO-H) = 104.6 0.7 DH 298 ((CH 3 ) 3 CO-H) = 106.3 0.7 DH 298 (C 6 H 5 O-H) = 85.8 0.1 DH 298 (HO-H) = 118.82 0.07 HO + H

Slide 23

What is a bond strength? Consider methane: CH 4 C + 4 H atomization H 298 (CH 4 ) = 397.5 kcal mol -1 D avg H 298 (CH 4 ) = 99.4 kcal mol -1 SpeciesDH 298 /kcal mol -1 Heat of Formation CH 3 -H 104.99 0.03 f H 298 [CH 3 ] CH 2 -H110.4 0.2 f H 298 [CH 2 ] CH-H101.3 0.3 f H 298 [CH] C-H80.9 0.2 f H 298 [C] a) No bond equals the average C-H bond careful. b) The sum of the BEDs is 397.5 0.6 kcal mol -1. 1 st Law really works!

Slide 24

Boulder Ion Gang, 1980 Leone Bierbaum Herman Ellison DePuy Ferguson Lineberger

Slide 25

EA(CH 3 O) = (2.540 - 0.968) = 1.572 0.004 eV no extra vibration 0.968 eV Engelking, Ellison, Lineberger, J. Chem. Phys. 69, 1826 (1978) CH 3 O + 0 CH 3 O + e Electron Kinetic Energy/eV Photoelectron counts ?

Slide 26

Anion Photoelectron Spectrum Measures Electron Affinity as Electron Binding Energy: CH 3 O + 0 CH 3 O + e (KE) CH 3 O (no extra vibration) CH 3 O (no extra vibration) laser light energy Measured Kinetic Energy of ejected photoelectron Electron Binding Energy If the product radical is vibrating, the photo-electron kinetic energy will be smaller and the measured electron binding energy will be larger. Vibrationally excited CH 3 O - gives hot band leaving more of 0 energy for ejected electron.

Slide 27

EA(CH 3 O) = (2.540 - 0.968) = 1.572 0.004 eV no extra vibration 0.968 eV Engelking, Ellison, Lineberger, J. Chem. Phys. 69, 1826 (1978) CH 3 O + 0 CH 3 O + e Electron Kinetic Energy/eV Photoelectron counts transitions to vibrationally excited states of CH 3 O radical transition from vibrationally excited state of CH 3 O - anion (weaker vibration)

Slide 28

Slide 29

Slide 30

End of Lecture 69 April 15, 2011 Copyright G. B. Ellison 2011. Some rights reserved. Except for cited third-party materials, all content is licensed under a Creative Commons License (Attribution-NonCommercial-ShareAlike 3.0). Creative Commons License (Attribution-NonCommercial-ShareAlike 3.0) Use of this content constitutes your acceptance of the noted license and the terms and conditions of use. Third party materials may be subject to additional intellectual property notices, information, or restrictions. The following attribution may be used when reusing material that is not identified as third-party content: G. B. Ellison, Chem 125. License: Creative Commons BY-NC-SA 3.0