WEAK CH…F BRIDGES AND INTERNAL DYNAMICS IN THE CH3F-CHF3 MOLECULAR COMPLEX Walther Caminati, Università di Bologna, Italy Juan C. Lopez, Jose L. Alonso, Universidad de Valladolid, Spain Jens-Uwe Grabow, Universität Hannover, Germany

Hydrogen Bond Interactions A) Normal Hydrogen Bond (Strong, ≈ 25 kJ/mol) B) Improper Hydrogen Bond (Weak, << 25 kJ/mol) The weak hydrogen bond in structural chemistry and biology, IUCr Monographs on crystallography, Vol. IX (G.R. Desiraju, T. Steiner eds.), Oxford University Press (2001). T. Steiner, Angew. Chem. 114 (2002) 50; Angew. Chem. Int. Ed. 41 (2002) 48. E. Kryachko, S. Scheiner, J. Phys. Chem. A 108 (2004) 2527. S. N. Delanoye, W.A. Herrebout, B.J. Van der Veken, J. Am. Chem. Soc. 124 (2002) 11854. C. G. Cole, A. C. Legon, Chem. Phys. Lett. 396 (2003) 31. data are available from X-ray diffraction, theoretical calculations, IR absorption in rare gas solutions, and rotationally resolved spectroscopy. C) Anti Hydrogen Bond (Weak, << 25 kJ/mol)

FT-MW spectroscopy of species with weak hydrogen bonds (WHB) Supersonic Jet Expansion Molecular Clusters Hydrogen Bonding Conformational Equilibria Molecular Dynamics Large Amplitude Motions Internal Rotation Standard ab-initio and DFT calculations: Gaussian or Gamess. Economy calculations: Distributed polarizability model. Conformations and potential energy surfaces of molecular adducts. Effective Hamiltonian fits: Watson, coupled, … Analysis withobs.-calc. deviations down to a few kHz (< 10-7 cm-1). Effective potential function fits: flexible model Inclusion of structural relaxations.

Bologna supersonic jet FT-MW Spectrometer:

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

Plausible Conformations of CH3F…CHF3   I II III IV V MP2/6-311++G(2df,2p): E/cm-1 I: 713.8 II: 0.0 III: 29.7 IV: 113.9 V: 119.7 I + II: minima. Three non-linear week hydrogen-bonds (WHB) different dipole-dipole interaction. III + V: saddle points.

Permutation inversion group theory CH3 group & CF3 group: C3(1)C3(2) = G9 Cs point group symmetry: E,   Molecular symmetry group: G18 X1 C3 axis X2 X3 C3 group C3 E C3² (123) (123)² = (132) A1 1  * H. C. Longuet-Higgins, Mol. Phys. 6 (1963), 445. J. T. Hougen, J. Chem. Phys. 37 (1962), 1433; J. Chem. Phys. 39 (1963), 358.

Tunneling pathways and Tunneling Hamiltonian Matrix HH: rotation of CH3-group HF: rotation of CF3-group HA: anti-geared rotation of CX3-groups HG: geared rotation of CX3-groups H11 HH HH HF HG HA HF HA HG H11 HH HA HF HG HG HF HA H11 HG HA HF HA HG HF symmetric matrix, 5 eigenvalues: W1= W(A1)=H11+2HH+2HF+2HA+2HG W2=W3=W(E1)=H11+2HH-HF-HA-HG W4=W5=W(E2)=H11-HH+2HF-HA-HG W6=W7=W(E3)=H11-HH-HF+2HA-HG W8=W9=W(E4)=H11-HH-HF-HA+2HG H11 HH HH HF HG HA H11 HH HA HF HG H11 HG HA HF H11 HH HH H11 HH H11

Energy level splitting diagram for K = 0 Δ'AE ΔAE E3 2HH 2HF 2HA 2HG -CH3 -CF3 E EE EA AE A AA J0J HH >> HF > HA ≈ -HG Γ(G18)

Internal Rotation Fine Structure J,Ka,Kc ← J,Ka,Kc = 31,3←21,2 CH3 top & CF3 top Only one splitting observed Effective moment of inertia Iα = 86.0(3) uÅ2 for A1-E1 splitting of CH3F-CHF3 only slightly smaller than Iα = 89.23(2) uÅ2 for isolated CHF3 A1 E1

CH3F-CHF3 Spectrocopic Constants and CF3-Top Internal Rotation [a]: Parameter fixed to value of normal species. [b]: Smaller than sum of values of CH3F and CHF3.

Oxirane-Difluoromethane: Planar Moment   Pii = 1/2 (-Iii + Ijj + Ikk) , i,j,k = a,b,c S. Blanco, J.C. Lopez, A. Lesarri, W. Caminati, J.L. Alonso, ChemPhysChem 5 (2004) 1779.

CH3-Top Internal Rotation   Pbb of the complex should amount to Pbb(CHF3) + Pbb(CH3F) = 1.55 uÅ2 + 44.62 uÅ2 = 46.17 uÅ2. Experimental Pbb of the complex is Pbb(CHF3…CH3F) = 44.69 uÅ2, i.e. 1.48 uÅ2 smaller. Planar moment of inertia Pbb = (h/16π2)(-1/B + 1/A + 1/C) is related to V3 barrier of internal rotor by: A00 = Ar + W00(2) F ρa2 B00 = Br C00 = Cr+ W00(2) F ρc2 Experimental Pbb is reproduced for V3 (CH3) = 0.36 kJ/mol (= 30 cm-1).

Barriers to Internal Rotation: CH3 and CF3 Tops MP2/6-311++G(2df,2p): EXPERIMENTAL: V3 (CH3) = 0.36 kJ/mol from planar moments Pbb V’3 (CF3) = 0.840(5) kJ/mol from A1-E1 splittings

CF3 and CH3 Internal Rotors: Energy Spacing   Barrier & Inertia Ratio  106 [a]: Energy spacing between A1 and E1 or E2 sublevels. [b]: Number of torsional levels below the V3 barrier.

CF3 and CH3 LAM Potential Surface   VIP Molecular Complexes Weak CH···F Bridges and Internal Dynamics in the CH3F·CHF3 Molecular Complex** Walther Caminati,* Juan C. Lòpez, José L. Alonso, and Jens-Uwe Grabow Angew. Chem. Int. Ed. 2005, 44, 3840-3844

CH3F…CHF3 Complex: Force Constant and Dissociation Energy   Stabilized by three weak CHF hydrogen bonds (WHB) and electrostatic dipole-dipole interaction. ks = 5.2 Nm-1 ED = 5.3 kJ·mol-1 1.8 kJ·mol-1/[CHF] (<< 25kJ·mol-1) DJ = DJ(eff) – [-1/2 (ρb4 + ρc4) W00(4) F] (small contribution) ks = 16π4 (μ RCM)2 [4B4 + 4C4 - (B-C)2 (B+C)2] / (h DJ) ED(Lennard-Jones) = 1/72 ks RCM2 V’3 (CF3) = 0.84 kJ·mol-1 V3 (CH3) = 0.36 kJ·mol-1

Structural Parameters [a] fitted parameter [b] derived parameter [c] from rs coordinates CCHF3: |a| = 84(6) pm |b| = 0* |c| = 43(4) pm CCH3F: |a| = 261.9(7) pm |c| = 57(3) pm *set to zero because imaginary α β r

Acknowledgement Deutsche Forschungsgemeinschaft (DFG) Land Niedersachsen Deutscher Akademischer Austauschdienst (DAAD) Directión General de Investigación – Ministerio de Ciencia y Technología the Junta de Castilla y León Minister of Education, Spain Università di Bologna Minister of Education, Italy