1. How methyl tops talk with each other
Melanie Schnell
Fritz-Haber-Institut der Max-Planck-Gesellschaft
Faradayweg 4-6, D Berlin, Germany
Jens-Uwe Grabow
Gottfried-Wilhelm-Leibniz-Universität
Institut für Physikalische Chemie & Elektrochemie
Callinstrasse 3A, D Hannover, Germany

2. Motivation
- learn more about the internal rotation behavior in C3v-symmetric
molecules with three C3v-symmetric tops
- studied so far: (CH3)3SiCl, (CH3)3GeCl, (CH3)3SnCl, (CH3)3SiI,
(CH3)3GeCCH …
- large influence of the central atom on the chemical bond character
and thus on the torsional behavior:
top-top communication
- enlarge the series to gain more information

3. (CH3)3GeBr ! … unusual K=0 quadrupole patterns
- observed for (CH3)3SnCl, but not for (CH3)3SiCl and (CH3)3GeCl.
- hypothesis: low torsional barrier and thus large splitting
strong rotorsional interaction K=±1 mixing
Idea
probe this effect using another molecule with different
quadrupole coupling and barrier to internal rotation: !
(CH3)3GeBr
- intermediate torsional coupling
- large quadrupole coupling

4. + =   K=0 K=1 K=0 F+1F 7/2  5/2 9/2  7/2 11/2  9/2 13/2  11/2
9/27/2
11/2  9/2
7/2  5/2
13/2  11/2
K=1
MHz
MHz
7/2  5/2
9/2  7/2
11/2  9/2
13/2  11/2
F+1F
K=0

5. Motivation
decrease in steric hinderance
increase in ionic contribution
A recent studya of the series (CH3)3XCl, X = Si, Ge, Sn revealed
the major influence of the central atom.
covalent radii / Å
smallest r(H---H) between
2 CH3 groups / Å
torsional barrier (exp.) / kJ/mol
tors. Splitting (J: 21, K=0) / MHz
eQq(35Cl) /MHz
X-C bond character / %
(Townes-Dailey model)
Quadrupole Moment / 1024 cm2
X = Si Ge Sn
6.91b 4.45a 1.77a
0.030b 1.8a 200a
-34.8b -40.1a -35.8a
ionic 
35Cl: Br: 0.33
a Schnell et al., Angewandte Chemie Int. Ed. 45, (2006).
b Merke et al., J. Mol. Spectr. 216, (2002).

6. COBRA-FT microwave spectroscopy
The apparatus
COBRA-FT microwave spectroscopy
excitation
1μs
Random rotation
of the molecules
before excitation:
Dipole moments cancel
MW pulse
detection
T = 100μs
Coherence
after excitation:
Oscillating macroscopic
dipole moment
MW signal

7. The torsion-rotation spectrum of (CH3)3GeBr
(J+1 J = 7 6-transition)
Br-NQ-HFS
torsional pattern
81Br
79Br

8. The torsion-rotation spectrum of (CH3)3GeBr

9. The torsional pattern of (CH3)3GeBr
(J+1 J = 4 3-transition)
2 closely lying
hyperfine
components

10. Multidimensional high-barrier tunneling formalism
(MS-group G162)
27 different possibilities of orienting the CH3-groups
with respect to each other (33) 
27 frameworks
5 topologically inequivalent tunneling pathways:
HR: rotation of 1 single CH3-group
HA: anti-geared rotation of 2 CH3-groups
HG: geared rotation of 2 CH3-groups
HL: rotation of 3 CH3-groups with the same sense
HE: rotation of 3 CH3-groups with different sense

11. C3-overall molecular rotation
MS-group G162
 = (1, 2, 3),  = (4, 5, 6),  = (7, 8, 9); D = (a, b, c)(1, 4, 7)(2, 5, 8)(3, 6, 9); R = (b, c)(2, 3)(4, 7)(5, 9)(6, 8)*
torsions
C3-overall molecular rotation
Permution-Inversion
 reflection
K. D. Möller, H. G. Andresen, J. Chem. Phys. 39, 17 (1963).

12. Tunneling Hamiltonian matrix for K=0

13. G162 Eigenvalues W1(A1) = H11 +6HR+6HE+2HL+6HA+6HG
W2(E2) = H HR-3HE+2HL-3HA+6HG
W3(I1) = H HE-HL-3HA
W4(I2) = H HR-3HE-HL
W5(I3) = H HR-HL+3HA
W6(I4) = H HL-3HG
rigid rotor
energy level
tunneling shifts
lam
energy
levels
torsional symmetry species tors

14. Torsional species in G162 tors K=0 K=1,2 K=3 Jeven Jodd
A1 A1 A2 E1 A1+A2
I I2 I2 2I2 2I2
I I1 I1 2I1 2I1
I I4 I5 I4+I5 I4+I5
I I3 I3 2I3 2I3
E2 E2 E2 E3+E4 2E2

15. The predicted torsional splitting pattern
60th International Symposium on Molecular Spectroscopy, Columbus (OH)
No unusual K=0 quadrupole splitting observed!

16. Spectroscopic results
(CH3)374Ge79Bra (CH3)374Ge35Clb
A / MHz (14) (21)
B / MHz (47) (92)
DJ / kHz (48) (14)
eQq / MHz (58) (16)
V3 / cm (12) (47)
F0 / GHz * *
/ ° * * N
/ kHz
r(X-Cl) / Å (21) (97)
* fixed
a M. Schnell, J.-U. Grabow, Chem. Phys. (2007) accepted.
b M. Schnell, J.-U. Grabow, PCCP 8, (2006).

17. Character of the chemical bonds
(CH3)374Ge79Bra (CH3)374Ge35Clb
Ge-X
 covalent % %
 double % %
ionic % %
Ge-C
 covalent % 81 %
ionic % 19 %
backbonding of the
Br towards the Ge
exceeds polarization
of the -bond:
positive charge results
on the coupling halogen
-backbonding is not possible
a M. Schnell, J.-U. Grabow, Chem. Phys. (2007) accepted.
b M. Schnell, J.-U. Grabow, PCCP 8, (2006).

18. increased ionic character
Conclusions
torsion-rotation spectrum recorded from GHz
similar torsional splitting pattern as (CH3)3GeCl
barrier heights: 334 cm-1 (Br) vs. 372 cm-1 (Cl)
r(Ge-Br) = Å compared to r(Ge-Cl) = Å
Br NQ-hyperfine structure dominates the spectrum
eQq: MHz(Br) vs MHz(Cl)
no unusual additional quadrupole splitting observed
like (CH3)3GeCl but unlike (CH3)3SnCl
Bromine compound:
increased ionic character
lower barrier

19. Thanks! Wolfgang Rogge, electronic shop PCI, GWLU Hannover
Mechanical shop PCI, GWLU Hannover
Fonds der Chemischen Industrie
Deutsche Forschungsgemeinschaft
Land Niedersachsen