1. Institut für Physikalische Chemie & Elektrochemie, Lehrgebiet A STRUCTURE OF THE BENZENE DIMER - GOVERNED BY DYNAMICS. MELANIE SCHNELL, Center for Free-Electron Laser Science, Hamburg, Germany UNDINE ERLEKAM, GERT V. HELDEN, GERARD MEIJER, Fritz-Haber-Institut, Berlin, Germany PHILIP R. BUNKER, National Research Council of Canada, Ottawa, Canada JENS-UWE GRABOW, Gottfried-Wilhelm-Leibniz-Universtät, Hannover, Germany AD VAN DER AVOIRD, Radboud University, Nijmegen, The Netherlands

2. Institut für Physikalische Chemie & Elektrochemie, Lehrgebiet A tilted T-shaped parallel displaced cap stem very floppy system - internal dynamics is not understood The benzene dimer study of intermolecular forces as benchmark for ab initio studies dispersion interaction: intermolecular (non-local) electron correlation two competing structures:

3. Institut für Physikalische Chemie & Elektrochemie, Lehrgebiet A aromatic stacking interactions in supermolecular chemistry; edge-to-face stacking (T-stacking) Interests of benzene dimer

4. Institut für Physikalische Chemie & Elektrochemie, Lehrgebiet A What is known so far... J. Chem. Phys. 63 (1975) 1419 J. Chem. Phys. 97 (1992) 2189 J. Chem. Phys. 98 (1993) 4294 symmetric-top spectrum with complex internal rotation

5. Institut für Physikalische Chemie & Elektrochemie, Lehrgebiet A Coaxially oriented Beam-Resonator Arrangement (COBRA): molecular excitation t MW pulse 1μs before excitation pulse: dipole moments cancel EbEb EaEa many two-level systems single particle, Schrödinger equation: ensemble properties, von Neumann equation wavefunction density matrix density matrix: (no) coherence: population: J.-U. Grabow, W. Stahl, Z. Naturforsch. 45a, 1043 (1990).

6. Institut für Physikalische Chemie & Elektrochemie, Lehrgebiet A Coaxially oriented Beam-Resonator Arrangement (COBRA): molecular response E t T = 100μs MW signal after excitation pulse:oscillating macroscopic dipole moment density matrix: coherence: population: Fourier Transform Frequency Domain, Spectrum Time Domain, FID U. Andresen, H. Dreizler, J.-U. Grabow, W. Stahl, Rev.Sci.Instrum. 61, 3694 (1990). J.-U. Grabow, W. Stahl, H. Dreizler, Rev.Sci.Instrum. 67, 4072 (1996).

7. Institut für Physikalische Chemie & Elektrochemie, Lehrgebiet A Production of Non-volatile Species: (Heated) Reservoire Source Resistive wire Ceramic ring Teflon ring Liquid/Solid reservoir solenoid valve / reservoir nozzle efficient benzene dimer production external reservior can be used: not heating, but cooling (~0°C) was required!

8. Institut für Physikalische Chemie & Elektrochemie, Lehrgebiet A Hyperfine structure; Quadrupole coupling constants Internal fields: quadrupole coupling Field gradients Bonding characters Internal dynamics: Tunneling processes Fine structure; Torsion-tilting-rotation interaction (Some) Aspects Rotational spectroscopy Barriers Structural information Trajectories

9. Institut für Physikalische Chemie & Elektrochemie, Lehrgebiet A Hyperfine structure; Quadrupole coupling constants Internal fields: quadrupole coupling Field gradients Bonding characters Internal dynamics: Tunneling processes Fine structure; Torsion-tilting-rotation interaction Barriers Structural information Trajectories Molecular dynamics theory Stark effect measurements (Some) Aspects Rotational spectroscopy Permutation-inversion group theory

10. Institut für Physikalische Chemie & Elektrochemie, Lehrgebiet A First FT-Microwave study: 22 quartets of lines in the 2.5 to 6 GHz region E. Arunan and H. S. Gutowsky, J. Chem. Phys. 98, 4294(1993). All 22 have a quartet structure with a 1-3-1 energy splitting observed spectral features: Spectroscopic fingerprint 22 lines fit symmetric top pattern J,K J+1,K ~180 kHz ~60 kHz Lines viewed as doubly-split (by a smaller splitting on top of a larger splitting) suggesting two motions as origin

11. Institut für Physikalische Chemie & Elektrochemie, Lehrgebiet A COBRA FT-MW transitions 112 why do we observe a symmetric-top spectrum? how can we explain exact -2:-1:1:2 quartet pattern? -125 kHz -63 kHz62 kHz 125 kHz 28 kHz Doppler- Doublets

12. Institut für Physikalische Chemie & Elektrochemie, Lehrgebiet A Deuterated Benzene Dimers different reduced masses of the internally rotating moieties spacing between the 4 tunneling components depends on site of deuteration cap stem (C 6 D 6 ) c (C 6 H 6 ) s (C 6 H 6 ) c (C 6 D 6 ) s vs.

13. Institut für Physikalische Chemie & Elektrochemie, Lehrgebiet A Mixed Dimer: Collisional interconversion Erlekam, Frankowski, von Helden, Meijer, Phys. Chem. Chem. Phys. 9 (2007) 3786 Collisional coformational conversion driven by rare gas atoms c c

14. Institut für Physikalische Chemie & Elektrochemie, Lehrgebiet A tunneling splitting changes upon isotopic labeling 8 J+1 J transitions detected more complicated due to quadrupole coupling (I(D)=1) again symmetric top spectrum; again -2:-1:1:2 pattern (reduced to about 70 %) The Mixed dimer (C 6 D 6 ) C (C 6 H 6 ) S

15. Institut für Physikalische Chemie & Elektrochemie, Lehrgebiet A Tunneling splittings: Experiment and Theory V 6 potential perfect -2:-1:1:2 splitting: fingerprint of high-barrier V 6 tunneling (of the stem) strong J and K dependence needed +2 +1 -2

16. Institut für Physikalische Chemie & Elektrochemie, Lehrgebiet A Symmetric-top character 6D calculations show: cap is nearly freely rotating about its C 6 axis group theory (G 576 ): all motions, but insignificant cap turnover and cap/stem exchange: accidental degeneracy for K cap =3n 1 K + +K -, G 7 + +G 7 -, G 1 + +G 1 - states behave as symmetric top similar findings for other benzene complexes with low barriers, such as benzene-CO Reduced-dimensionality approach and group theory: M. Schnell, U. Erlekam, P.R. Bunker, G. von Helden, J.-U. Grabow, G Meijer, A. van der Avoird, Phys. Chem. Chem. Phys. 15 (2013) 10207. 6D calculations: A. van der Avoird, R. Podeszwa, K. Szalewicz, C. Leforestier, R. van Harrevelt, P.R. Bunker, M. Schnell, G. von Helden, G. Meijer, Phys. Chem. Chem. Phys. 12 (2010) 8219. Benzene-CO: T. Brupbacher, A. Bauder, J. Chem. Phys. 99 (1993) 9394

17. Institut für Physikalische Chemie & Elektrochemie, Lehrgebiet A Reduced-dimensionality approach concerted motion of cap tilt tunneling and stem internal rotation required to describe observed splitting pattern M. Schnell, U. Erlekam, P.R. Bunker, G. von Helden, J.-U. Grabow, G Meijer, A. van der Avoird, Phys. Chem. Chem. Phys. 15 (2013) 10207.

18. Institut für Physikalische Chemie & Elektrochemie, Lehrgebiet A Resonator Reflectors as Stark Electrodes - + 15 kV max coaxially oriented beam-resonator arrangement (COBRA) 63 cm

19. Institut für Physikalische Chemie & Elektrochemie, Lehrgebiet A Circular Electrodes for Field Homogenization groundHV decreasing potential M = 1 coaxially aligned electrodes for Stark-effect applied in resonators (CAESAR) M-selection rule: M. Schnell, D. Banser, J.-U. Grabow, Rev. Sci. Instrum. 75, 2111(2004).

20. Institut für Physikalische Chemie & Elektrochemie, Lehrgebiet A Stark-effect measurements first order behavior µ exp =0.580(51) D based on 6 transitions, 30 components µ calc =0.51 D for tilted T-shape Hobza, Selzle, Schlag, J. Chem. Phys. 93 (1990) 5893

21. Institut für Physikalische Chemie & Elektrochemie, Lehrgebiet A Condition for linear Stark effect: The anti-symmetric square [ 2 ] antisym of the torsional symmetry species needs to contain the symmetry species ( f ) of the space fixed components of the dipole moment operator f Stark effect of torsional species J. K. G. Watson, J. Mol. Spectr. 50, 281 (1974). Experimental results show: Cap torsion, cap tilting, and stem torsion are feasible: MS group G 144 = G cap G stem = C 6v (M) D 6 (M) ( G 144 ): cap stem Symmetry of K cap =3n 1 levels in C 6v (M) is: ( K cap =3n 1 ) = E 1 or E 2 Symmetry of K cap =1,2,.. levels in C 6v (M) is: ( K cap =1,2,.. ) = A 1 +A 2 or B 1 +B 2 [ 2 (E 1 )] antisym = [ 2 (E 2 )] antisym = [ 2 (A 1 +A 2 )] antisym =[ 2 (B 1 +B 2 )] antisym = A 2 Symmetry of f in G 144 is: ( f ) = A 2 A 1 first order Stark effect in levels having symmetry: E 1 stem or E 2 stem levels with K cap =3n 1 but not K cap =0 will have a first order Stark effect M. Schnell, P.R. Bunker, G. von Helden, J.-U. Grabow, G Meijer, A. van der Avoird, to be published.

22. Institut für Physikalische Chemie & Elektrochemie, Lehrgebiet A Summary group theory: dynamics leads to symmetric-top spectrum (with cap internal rotation) and linear Stark effect concerted motion of cap tilt tunneling and stem internal rotation required to describe observed splitting pattern important as benchmark system transferable to other molecular systems involving C-H -- -bonding M. Schnell, U. Erlekam, P.R. Bunker, G. von Helden, J.-U. Grabow, G Meijer, A. van der Avoird, Phys. Chem. Chem.Phys. 2013, DOI: 10.1039/C3CP51181B, in print. Benzene-CO: T. Brupbacher, A. Bauder, J. Chem. Phys. 99 (1993) 9394

23. Institut für Physikalische Chemie & Elektrochemie, Lehrgebiet A Intramolecular dynamics The benzene dimer: Structure governed by dynamics Angew. Chem. Int. Ed. 52 (2013) 5180-5183.

24. Institut für Physikalische Chemie & Elektrochemie, Lehrgebiet A Thanks! Wolfgang Rogge, electronic shop of the PCI Mechanical shop of the PCI Fonds der Chemischen Industrie Deutsche Forschungsgemeinschaft Land Niedersachsen Deutscher Akademischer Austauschdienst