1. Charge Oscillation in C-O Stretching Vibrations: A Comparison of CO2 Anion and Carboxylate Functional Groups
Michael C. Thompson, J. Mathias Weber
72nd International Symposium on Molecular Spectroscopy
Urbana, IL
June 19-23, 2017
M.C. Thompson, J. M. Weber, Chem. Phys. Lett. 683 (2017) 586–590

2. Introduction Fundamental characteristics of vibrational transitions:
Frequency
Intensity  dependence of charge distribution on molecular coordinates !

3. Introduction
CO2 containing molecules and complexes interesting for CO2 reduction  conversion of CO2 into chemical fuels  series of experiments on IR spectroscopy of anionic metal-CO clusters [M(CO2)n]- and related species
CO stretching vibrations are important characteristic vibrations for molecules containing “CO oscillators” – fundamental information on  structure and symmetry  charge distribution and how it depends on molecular coordinates
A.D. Boese et al., JCP 122 (2005)
B.J. Knurr & JMW, JACS 134 (2012) 18804−18808
B.J. Knurr & JMW, J. Phys. Chem. A 117 (2013) 10764–10771
B.J. Knurr & JMW, J. Phys. Chem. A 118 (2014) 4056 – 4062
B.J. Knurr & JMW, J. Phys. Chem. A 118 (2014)
B.J. Knurr & JMW, J. Phys. Chem. A, 118 (2014)
B.J. Knurr & JMW, J. Phys. Chem. A, 119 (2015) 843–850
M.C. Thompson, J. Ramsay & JMW, Angew. Chem. Int. Ed., 55 (2016)
M.C. Thompson, L.G. Dodson & JMW, J. Phys. Chem. A 121 (2017)
M.C. Thompson & JMW, in preparation
L.G. Dodson, M.C. Thompson & JMW, in preparation

4. Introduction Electronic Structure of CO2: Walsh Diagram
Antisymm. CO stretch IR active
OCO bend IR active
Symm. CO stretch IR inactive

5. Introduction In CO2 anions and COO- functional groups:
Excess electron in antibonding orbital
 CO bonds are weakened  CO stretching frequencies shift to the red
CO2- is bent
symmetric CO stretching vibration becomes IR active

6. Note that OCO bond angles are similar!
Introduction
Follow frequencies and intensities of symmetric and antisymmetric stretching vibrations in CO2- and COO- functional groups!
CO HCOO AgCOO- BiCOO-
Note that OCO bond angles are similar!

7. Experimental Method: IR Photodissociation
cluster + h hot cluster fragments
CO2-·(CO2) CO2-·(CO2)6 + CO2
HCOO-·Ar HCOO- + Ar
AgCOO-·(CO2) AgCOO-·(CO2)3 + CO2
BiCOO-·(CO2) BiCOO-·(CO2)2 + CO2
J.-W. Shin, N. Hammer, M. A. Johnson, H. Schneider, A. N. Gloess, JMW, J. Phys. Chem. A 109 (2005) 3146
B.J. Knurr & JMW, J. Phys. Chem. A 117 (2013) 10764–10771
M. C. Thompson, J. Ramsay, JMW, Angew. Chem. Int. Ed., 55 (2016)

8. Experimental Setup Nd:YAG IR-OPO/OPA electron gun 600 – 4500 cm-1
mJ / 5 ns
IR-OPO/OPA
mass gate
power meter
ion
source

9. Experimental Spectra of M-COO-
CO2-·(CO2)7
M.C. Thompson, J. M. Weber, Chem. Phys. Lett. 683 (2017) 586–590

10. Experimental Spectra of M-COO-
CO2-·(CO2)7
M.C. Thompson, J. M. Weber, Chem. Phys. Lett. 683 (2017) 586–590

11. Experimental Spectra of M-COO-
CO2-·(CO2)7
M.C. Thompson, J. M. Weber, Chem. Phys. Lett. 683 (2017) 586–590

12. Experimental Spectra of M-COO-
CO2-·(CO2)7
HCOO-·Ar
AgCOO-·(CO2)4
BiCOO-·(CO2)3
M.C. Thompson, J. M. Weber, Chem. Phys. Lett. 683 (2017) 586–590

13. Experimental Spectra of M-COO-
CO2-·(CO2)7
HCOO-·Ar
AgCOO-·(CO2)4
BiCOO-·(CO2)3
M.C. Thompson, J. M. Weber, Chem. Phys. Lett. 683 (2017) 586–590

14. Experimental Spectra of M-COO-
CO2-·(CO2)7
HCOO-·Ar
AgCOO-·(CO2)4
BiCOO-·(CO2)3
M.C. Thompson, J. M. Weber, Chem. Phys. Lett. 683 (2017) 586–590

15. Experimental Spectra of M-COO-
Symmetric and antisymmetric
CO stretching modes (fundamental transitions) of target ions (and others)
Intensity ratio Is/Ias changes drastically!
CO2-·(CO2)7
HCOO-·Ar
AgCOO-·(CO2)4
BiCOO-·(CO2)3
M.C. Thompson, J. M. Weber, Chem. Phys. Lett. 683 (2017) 586–590

16. Modeling the Spectra of M-COO-
Expand in transition dipole moment between levels and of
normal mode Q in Taylor series to 1st order: =
Fixed charge model – atomic charges qj are constant, only molecular geometry varies

17. Modeling the Spectra of M-COO-
Expand in transition dipole moment between levels and of
normal mode Q in Taylor series to 1st order: =
1st term: fixed charge model – atomic charges qj are constant, only molecular geometry varies
2nd term: vibrationally mediated changes in charge distribution

18. Modeling the Spectra of M-COO-
Vibrationally mediated changes in charge distribution – charge oscillations
Enhances transition dipole
Considerable charge flow: q ≤ 0.2 e
Symmetric stretching mode: binding partner acts as “charge reservoir”
CO2-: no binding partner, only small intramolecular charge oscillations
Antisymm. CO stretching mode: independent of binding partner
q(COO-) [e] H Ag Bi

19. Modeling the Spectra of M-COO-
Vibrationally mediated changes in charge distribution – charge oscillations
Previously found in other molecular systems:
CH2X radicals (X = F, Cl; “charge sloshing”)
E.S. Whitney, F. Dong, D.J. Nesbitt, J. Chem. Phys. 125 (2006)
E.S. Whitney, T. Haeber, M.D. Schuder, A.C. Blair, D.J. Nesbitt, J. Chem. Phys. 125 (2006)
[Cu·H2O]+ complexes
P.D. Carnegie, A.B. McCoy, M.A. Duncan, J. Phys. Chem. A 113 (2009) 4849.
H3O+·X3 complexes (X = Ar, N2, CH4, H2O)
A.B. McCoy, T.L. Guasco, C.M. Leavitt, S.G. Olesen, M.A. Johnson, Phys. Chem. Chem. Phys. 14 (2012) 7205.
X-·H2O (X = OH, O, F, Cl, and Br) complexes
J.R. Roscioli, E.G. Diken, M.A. Johnson, S. Horvath, A.B. McCoy, J. Phys. Chem. A 110 (2006) 4943.

20. Modeling the Spectra of M-COO-
Expand in transition dipole moment between levels and of
normal mode Q in Taylor series to 1st order: =
1st term: fixed charge model – atomic charges qj are constant, only molecular geometry varies
2nd term: vibrationally mediated changes in charge distribution
increases with size of binding partner M and volume of M-C bond

21. Modeling the Spectra of M-COO-
Expand in transition dipole moment between levels and of
normal mode Q in Taylor series to 1st order: =
1st term: fixed charge model – atomic charges qj are constant, only molecular geometry varies
2nd term: vibrationally mediated changes in charge distribution

22. Summary
Intensity ratios between symmetric and antisymmetric CO stretching modes in M-COO- strongly depend on M
Fixed charge model is insufficient to describe this behavior.
Charge oscillations enhance transition dipole moments, effect varies strongly with M for symmetric CO stretch
M acts as a charge reservoir that can dynamically accept charge during C-O bond contraction

23. Dramatis Personae
Michael Thompson
NSF AMO
PFC

24. Thank you for your attention