Large Amplitude Motions: Revealing Information Transfer Along Conjugated Systems Jens-Uwe Grabow, Hilkka Saal, Isabelle Kleiner, Angela R. Hight-Walker, Jon T. Hougen und Walther Caminati Institut für Physikalische Chemie & Elektrochemie National Institute of Standards and Technology – Gaithersburg, MD Dipartimento di Chimica “G.Ciamician” Universita’ di Bologna

Benzotrifluoride: V6 internal rotation Torsion  Rotation Centrifugal Distorsion Torsion Rotation Interaction

Benzotrifluoride: V6 internal rotation barrier with 7 digit accuracy the heavy-top case of benzotrifluride: A v=2 A E v=1 E A v=0 barrier to internal rotation V6 = 3.229948(3) cm-1 Vadim V. Ilyushin, Laura B. Favero, Walther Caminati, Jens-Uwe Grabow, ChemPhysChem, accepted.

Coaxial oriented Beam-Resonator Arrangement (COBRA) Impulse FID resonator tuning Fabry-Perot resonator FT polarization pulse: coherence between rotating molecular dipoles oscillating macroscopic dipole moment: electromagnetic field at frequencies of molecular transitions

“coaxially oriented beam resonator arrangement“ (COBRA) FTMW-Spectrometer

Information Transfer Through Conjugated bonds? para-tolualdehyde: Information Transfer Through Conjugated bonds? Top view: Side view: along internal rotation axis 3 aldehyde group introduces asymmetry: 2 H 1 2 1 C O 1 3 3 2 Will the methyl group know about the asymmetry? V6 vs. V3 barrier to internal rotation Walther Caminati, Angela R. Hight-Walker, Jon T. Hougen, Isabelle Kleiner, Hilkka Saal, Jens-Uwe Grabow, to be published.

Internal rotation: Permutation-Inversion (PI) group theoretical considerations -CH3 top: C3 permutation-inversion symmetry: C3 -C6H4-CHO frame: Cs point group symmetry: E,  Molecular symmetry group: G6 H1 C3 axis H2 H3 2 C3 group H 1 C C3 E C3² (123) (123)² = (132) A1 1  * O 3 H. C. Longuet-Higgins, Mol. Phys. 6 (1963), 445. J. T. Hougen, J. Chem. Phys. 37 (1962), 1433; J. Chem. Phys. 39 (1963), 358.

H1 H2 H3 tunnel-pathways, tunnel-Hamilton matrix, energy level split diagram K=0, vt=0 Htunneling: rotation of CH3-group H1 C3 axis E1 H2 H3 Htunneling J0J 3 (feasible) equivalent tunneling pathways in total: H11 HE HE H11 HE 2Htunneling H11 symmetric matrix, 2 eigenvalues: W1= W(A1)=H11+2HE W2=W3=W(E1)=H11-HE A1 Γ(G6)

V6/V3- torsional barrier perfect fit to torsional ground state (vt=0) spectrum archieved for different contributions to the barrier

free internal rotor – high barrier torsion gi = 2 gi = 3 E |m| = 4 E vt = 2 A |m| = 3 A vt = 1 E |m| = 2 E vt = 0 |m| = 1 A m = 0 ∞ V

“free jet“ (FJ) absorption mm-wave setup Side View Top View

“free jet“ (FJ) Absorptions-Millimeterwellen-Spektrometer

FJ-A-MMW vs. COBRA-FT-MW spectrometer

para-tolualdehyde: J = 34  33 a R-band T  50 K required broken diffusion pump heaters!

low-barrier torsional energy level diagram

reduced energy level diagram for m = +/- 3 para-tolualdehyde: reduced energy level diagram for m = +/- 3 E – (1/2)(B+C)J(J+1)

Information Transfer Through Conjugated bonds! para-tolualdehyde: Information Transfer Through Conjugated bonds! nlm CONSTANT OPERATOR VALUE STD DEV   202 A 0.166516196839980 0.000006112879081 B 0.032937322496368 0.000000063427868 C 0.027601452277977 0.000000064713936 DAB -0.002971620212815 0.000002362977471 211 RHORHO 0.030970392251857 0.000027764172184 220 F 5.490882492526767 0.003675124595046 V3 28.369747250217017 0.016741884821443 404 DJ 0.000000000766043 0.000000000002449 DJK -0.000000001773117 0.000000000020899 DK 0.000000040780968 0.000000002059929 ODELN 0.000000000152421 0.000000000001598 ODELK 0.000000002114823 0.000000000235953 413 ALV 0.000000004455893 0.000000000607588 AK1 0.000000241437257 0.000000041691112 ODELTA -0.000000067481273 0.000000015991851 C4 0.000000007123842 0.000000004163024 422 GV 0.000000119369360 0.000000034361650 AK2 0.000013649424323 0.000003574497292 DELTA -0.000009595953284 0.000001553935395 C1 0.000000455984320 0.000000016495752 FV -0.000009370236146 0.000000061416459 ODAB -0.000073087515674 0.000002201896216 431 AK3 0.000240896878198 0.000019336762283 440 V6 -5.328585545552866 0.025635841540561 AK4 -0.013457652989215 0.000793909144900 642 AMV -0.000000029064949 0.000000005471985 BK1 -0.000003052251718 0.000001041520361 C3 -0.000000161746243 0.000000005990665 DELTAB 0.000002198996436 0.000000272705423 result of global fit to ~800 transitions in 5 different torsional states at experimental accuracy

Information Transfer Through Conjugated bonds! para-tolualdehyde: Information Transfer Through Conjugated bonds! Top view: Side view: along internal rotation axis 3 2 H 1 2 1 C O 1 3 3 2 the methyl group does know about the orientation of the aldehyde group hiding in the plane - but not from sterical hindering! barrier to internal rotation: V3 = 28.3 >> V6 = 5.3 cm-1

Acknowledgement Deutsche Forschungsgemeinschaft (DFG) Land Niedersachsen Deutscher Akademischer Austauschdienst (DAAD) Università di Bologna Minister of Education, Italy