How methyl tops talk with each other Melanie Schnell Fritz-Haber-Institut der Max-Planck-Gesellschaft Faradayweg 4-6, D-14195 Berlin, Germany Jens-Uwe Grabow Gottfried-Wilhelm-Leibniz-Universität Institut für Physikalische Chemie & Elektrochemie Callinstrasse 3A, D-30167 Hannover, Germany

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

(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

+ =   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 16573.4 MHz 16574.0 MHz 7/2  5/2 9/2  7/2 11/2  9/2 13/2  11/2 F+1F K=0

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 1.17 1.22 1.44 3.0 3.1 3.6 6.91b 4.45a 1.77a 0.030b 1.8a 200a -34.8b -40.1a -35.8a ionic 18 19 26  82 81 74 35Cl: -0.085 79Br: 0.33 a Schnell et al., Angewandte Chemie Int. Ed. 45, 3465-3470 (2006). b Merke et al., J. Mol. Spectr. 216, 437-446 (2002).

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

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

The torsion-rotation spectrum of (CH3)3GeBr

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

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

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).

Tunneling Hamiltonian matrix for K=0

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

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

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

Spectroscopic results (CH3)374Ge79Bra (CH3)374Ge35Clb A / MHz 2730.92(14) 2875.1(21) B / MHz 1230.38601(47) 1989.548968(92) DJ / kHz -1.1954(48) 0.3260(14) eQq / MHz 263.368(58) -40.0703(16) V3 / cm-1 334.37(12) 372.359(47) F0 / GHz 158.0* 158.0* / ° 106.2* 106.2* N 47 168 / kHz 31.1 4.0 r(X-Cl) / Å 2.3459(21) 2.15198(97) * fixed a M. Schnell, J.-U. Grabow, Chem. Phys. (2007) accepted. b M. Schnell, J.-U. Grabow, PCCP 8, 2225-2231 (2006).

Character of the chemical bonds (CH3)374Ge79Bra (CH3)374Ge35Clb Ge-X  covalent 62.5 % 46.5 %  double 43.9 % 15.9 % ionic -6.4 % 37.6 % Ge-C  covalent 76.6 % 81 % ionic 23.4 % 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, 2225-2231 (2006).

increased ionic character Conclusions torsion-rotation spectrum recorded from 2.4-20 GHz similar torsional splitting pattern as (CH3)3GeCl barrier heights: 334 cm-1 (Br) vs. 372 cm-1 (Cl) r(Ge-Br) = 2.3459 Å compared to r(Ge-Cl) = 2.15198 Å Br NQ-hyperfine structure dominates the spectrum eQq: 263.368 MHz(Br) vs. -40.0703 MHz(Cl) no unusual additional quadrupole splitting observed like (CH3)3GeCl but unlike (CH3)3SnCl Bromine compound: increased ionic character lower barrier

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