Transcript Columbus-2013_FB10.ppt

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
Institut für Physikalische Chemie & Elektrochemie, Lehrgebiet A
The benzene dimer
•study
of intermolecular forces as benchmark for ab initio studies
dispersion interaction: intermolecular (non-local) electron correlation
two competing structures:
tilted T-shaped
parallel displaced
cap
stem
very floppy system - internal dynamics is not understood
Institut für Physikalische Chemie & Elektrochemie, Lehrgebiet A
Interests of benzene dimer
aromatic stacking interactions in supermolecular chemistry;
edge-to-face stacking (T-stacking)
What is known so far...
Institut für Physikalische Chemie & Elektrochemie, Lehrgebiet A
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
Coaxially oriented Beam-Resonator Arrangement (COBRA):
molecular excitation
Institut für Physikalische Chemie & Elektrochemie, Lehrgebiet A
many two-level systems
before excitation pulse:
Eb
Ea
dipole moments cancel

1μs
t
MW pulse
single particle, Schrödinger equation:

wavefunction
H  i

H,    i 
density matrix
ensemble properties, von Neumann equation
density matrix:
  (t )  (t ) 
ab

   aa
  (t )  (t ) 
bb
 ba

population:
 aa  bb  0
(no) coherence:     0
ab
J.-U. Grabow, W. Stahl, Z. Naturforsch. 45a, 1043 (1990).
ba
Coaxially oriented Beam-Resonator Arrangement (COBRA):
molecular response
Institut für Physikalische Chemie & Elektrochemie, Lehrgebiet A
after excitation pulse:
oscillating macroscopic dipole moment
E
T = 100μs
MW signal
t
density matrix:
  (t )  (t ) 
ab

   aa
  (t )  (t ) 
bb
 ba

population:  aa   bb  0
coherence:  ab ,  ba  0
Time Domain, FID
Fourier Transform
Frequency Domain, Spectrum
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).
Institut für Physikalische Chemie & Elektrochemie, Lehrgebiet A
Production of Non-volatile Species:
(Heated) Reservoire Source
Resistive wire
Ceramic ring
solenoid valve / reservoir nozzle
Teflon ring
Liquid/Solid reservoir
efficient benzene dimer production external reservior can be used:
not heating, but cooling (~0°C) was required!
Institut für Physikalische Chemie & Elektrochemie, Lehrgebiet A
(Some) Aspects Rotational spectroscopy
Internal dynamics:
Tunneling processes
Internal fields:
quadrupole coupling
Fine structure;
Torsion-tilting-rotation
interaction
Hyperfine structure;
Quadrupole coupling
constants
Barriers
Structural information
Trajectories
Field gradients
Bonding characters
Institut für Physikalische Chemie & Elektrochemie, Lehrgebiet A
(Some) Aspects Rotational spectroscopy
Internal dynamics:
Tunneling processes
Internal fields:
quadrupole coupling
Fine structure;
Torsion-tilting-rotation
interaction
Hyperfine structure;
Quadrupole coupling
constants
Barriers
Structural information
Trajectories
Field gradients
Bonding characters
Permutation-inversion
group theory
Molecular dynamics
theory
Stark effect measurements
Institut für Physikalische Chemie & Elektrochemie, Lehrgebiet A
Spectroscopic fingerprint
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).
observed spectral features:
~180 kHz
~60 kHz ~60 kHz
All 22 have a
quartet structure
with a “1-3-1” energy
splitting
Lines viewed as doubly-split
(by a smaller splitting on top
of a larger splitting) suggesting
two motions as origin
22 lines fit symmetric top
pattern J,K → J+1,K
COBRA FT-MW transitions
-63 kHz
62 kHz
Institut für Physikalische Chemie & Elektrochemie, Lehrgebiet A
125 kHz
-125 kHz
28 kHz
DopplerDoublets
•why
do we observe a symmetric-top spectrum?
1 exact -2:-1:1:2
2
1 pattern?
how can we explain
quartet
Institut für Physikalische Chemie & Elektrochemie, Lehrgebiet A
Deuterated Benzene Dimers
cap
stem
(C6D6)c(C6H6)s
vs.
(C6H6)c(C6D6)s
different reduced masses of the internally rotating moieties
spacing between the 4 tunneling components depends on
site of deuteration
Institut für Physikalische Chemie & Elektrochemie, Lehrgebiet A
Mixed Dimer:
Collisional interconversion
c
c
Collisional coformational conversion driven by rare gas atoms
Erlekam, Frankowski, von Helden, Meijer, Phys. Chem. Chem. Phys. 9 (2007) 3786
Institut für Physikalische Chemie & Elektrochemie, Lehrgebiet A
The Mixed dimer (C6D6)C(C6H6)S
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 %)
Tunneling splittings:
Institut für Physikalische Chemie & Elektrochemie, Lehrgebiet A
V6 potential
Experiment and Theory
+2
+1
-1
-2
“perfect” -2:-1:1:2 splitting:
fingerprint of high-barrier
V6 tunneling (of the stem)
strong J and K
dependence needed
Institut für Physikalische Chemie & Elektrochemie, Lehrgebiet A
Symmetric-top character
6D calculations show: cap is nearly freely rotating about its C6
axis
group theory (G576): all motions, but insignificant cap turnover
and cap/stem exchange: accidental degeneracy for Kcap=3n1
 K++K-, G7++G7-, G1++G1- 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
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.
Resonator Reflectors as Stark Electrodes
Institut für Physikalische Chemie & Elektrochemie, Lehrgebiet A
-
+
15 kV max

E MW

E Stark
coaxially oriented beam-resonator arrangement (COBRA)
63 cm
Circular Electrodes for Field Homogenization
coaxially aligned electrodes for Stark-effect applied in resonators (CAESAR)
Institut für Physikalische Chemie & Elektrochemie, Lehrgebiet A
decreasing potential
ground
HV

E MW

E Stark
M-selection rule:
M = 1
M. Schnell, D. Banser, J.-U. Grabow, Rev. Sci. Instrum. 75, 2111(2004).
Stark-effect measurements
Institut für Physikalische Chemie & Elektrochemie, Lehrgebiet A
first order behavior
µexp=0.580(51) D based on 6 transitions, 30 components
µcalc=0.51 D for tilted T-shape Hobza, Selzle, Schlag, J. Chem. Phys. 93 (1990) 5893
Stark effect of torsional species
Institut für Physikalische Chemie & Elektrochemie, Lehrgebiet A
Condition for linear Stark effect:
 
 f   J     2
antisym
J. K. G. Watson, J. Mol. Spectr. 50, 281 (1974).
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
Experimental results show:
Cap torsion, cap tilting, and stem torsion are feasible:
 MS group G144 = Gcap  Gstem = C6v(M)  D6(M)
 (G144): cap  stem
Symmetry of Kcap=3n1 levels in C6v(M) is: (Kcap=3n1) = E1or E2
Symmetry of Kcap=1,2,.. levels in C6v(M) is: (Kcap=1,2,..) = A1+A2 or B1+B2
[2(E1)]antisym = [2(E2)]antisym = [2(A1+A2)]antisym =[2(B1+B2)]antisym = A2
Symmetry of f in G144 is: (f) = A2  A1
first order Stark effect in levels having symmetry: E1  stem or E2  stem
levels with Kcap=3n1 but not Kcap=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.
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
Institut für Physikalische Chemie & Elektrochemie, Lehrgebiet A
Intramolecular dynamics
The benzene dimer:
Structure governed by dynamics
Angew. Chem. Int. Ed. 52 (2013) 5180-5183.
Institut für Physikalische Chemie & Elektrochemie, Lehrgebiet A
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