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

Are Bond Energies “ Real ” ? Bond Dissociation Energies

BondDissn Energies best values as of 2003

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

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

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

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 F–F– 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 !

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 [ ± 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

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.

EA(CH 3 O) = ( ) = ± eV no extra vibration  eV Engelking, Ellison, Lineberger, J. Chem. Phys. 69, 1826 (1978) CH 3 O — +   0  CH 3 O + e — Electron Kinetic Energy/eV Photoelectron counts

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 ) = ± 0.5 kcal mol -1 (Bierbaum) IE(H) = eV or (at 298K) kcal mol -1 EA( CH 3 O ) = ± eV or 36.3 ± 0.5 kcal mol -1 (Ellison et al.) DH 298 (CH 3 O-H) = ± 0.6 kcal mol -1

c) Acidity/Negative Ion Cycle Problems ? Can’t 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 – can’t measure  acid H 298 (H-CH 2 OH) & can’t measure EA(CH 2 OH)

c) Acidity/Negative Ion Cycle Problems ? Can’t 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 – can’t measure  acid H 298 (H-CH 2 OH) & can’t 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 ) = ± 0.03 kcal mol -1 DH 298 (H-CH 2 OH) = 96.1 ± 0.2 kcal mol -1

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 

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) = ± 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,  f H 298 (CH 3 ) = ± 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

Ellison I

Ellison II

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 ) = ± 0.5 kcal mol -1 (Bierbaum) IE(H) = eV or (at 298K) kcal mol -1 EA( CH 3 O ) = ± eV or 36.3 ± 0.5 kcal mol -1 (Ellison et al.) DH 298 (CH 3 O-H) = ± 0.6 kcal mol -1

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

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

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

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

What is a bond strength? Consider methane: CH 4  C + 4 H  atomization H 298 (CH 4 ) = 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 ±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 BED’s is ± 0.6 kcal mol st Law really works!

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

EA(CH 3 O) = ( ) = ± eV no extra vibration  eV Engelking, Ellison, Lineberger, J. Chem. Phys. 69, 1826 (1978) CH 3 O — +   0  CH 3 O + e — Electron Kinetic Energy/eV Photoelectron counts ?

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.

EA(CH 3 O) = ( ) = ± eV no extra vibration  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)

End of Lecture 69 April 15, 2011 Copyright © G. B. Ellison 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