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

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

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-CO2 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) 154301 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) 8753-8757 B.J. Knurr & JMW, J. Phys. Chem. A, 118 (2014) 10246-10251 B.J. Knurr & JMW, J. Phys. Chem. A, 119 (2015) 843–850 M.C. Thompson, J. Ramsay & JMW, Angew. Chem. Int. Ed., 55 (2016) 15171-15174 M.C. Thompson, L.G. Dodson & JMW, J. Phys. Chem. A 121 (2017) 4132-4138 M.C. Thompson & JMW, in preparation L.G. Dodson, M.C. Thompson & JMW, in preparation

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

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

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

Experimental Method: IR Photodissociation cluster + h hot cluster fragments CO2-·(CO2)7 CO2-·(CO2)6 + CO2 HCOO-·Ar HCOO- + Ar AgCOO-·(CO2)4 AgCOO-·(CO2)3 + CO2 BiCOO-·(CO2)3 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) 15171-15174

Experimental Setup Nd:YAG IR-OPO/OPA electron gun 600 – 4500 cm-1 0.1-10 mJ / 5 ns IR-OPO/OPA mass gate power meter ion source

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

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

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

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

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

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

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

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

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

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

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) 054304. E.S. Whitney, T. Haeber, M.D. Schuder, A.C. Blair, D.J. Nesbitt, J. Chem. Phys. 125 (2006) 054303. [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.

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

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

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

Dramatis Personae Michael Thompson NSF AMO PFC

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