1. Infrared Spectra of Chloride- Fluorobenzene Complexes in the Gas Phase: Electrostatics versus Hydrogen Bonding Holger Schneider OSU International Symposium on Molecular Spectroscopy June 20 th

2. Motivation: Anion Recognition

3. Hydrogen bonds ElectrostaticsLewis acids Hydrophobic effects Beer et al, Angew. Chem. Int. Ed., 40, 486, 2001 Motivation: Anion Recognition

4. Hydrogen bonds ElectrostaticsLewis acids Hydrophobic effects Beer et al, Angew. Chem. Int. Ed., 40, 486, 2001 Motivation: Anion Recognition

5. Questions: binding motifs? competition between different binding sites? Beer et al, Angew. Chem. Int. Ed., 40, 486, 2001 Motivation: Anion Recognition Hydrogen bonds ElectrostaticsLewis acids Hydrophobic effects

6. Questions: binding motifs? competition between different binding sites? Needed:structural information Beer et al, Angew. Chem. Int. Ed., 40, 486, 2001 Motivation: Anion Recognition Hydrogen bonds ElectrostaticsLewis acids Hydrophobic effects

7. Questions: binding motifs? competition between different binding sites? Needed:structural information Possible Tool:infrared spectroscopy Beer et al, Angew. Chem. Int. Ed., 40, 486, 2001 Motivation: Anion Recognition Hydrogen bonds ElectrostaticsLewis acids Hydrophobic effects

8. Questions: binding motifs? competition between different binding sites? Needed:structural information Possible Tool:infrared spectroscopy Model system in this study:Cl - ·C 6 F n H 6-n (n = 1-5) Beer et al, Angew. Chem. Int. Ed., 40, 486, 2001 Motivation: Anion Recognition Hydrogen bonds ElectrostaticsLewis acids Hydrophobic effects

9. Experimental Realization: Infrared Predissociation Spectroscopy Step 1: Ion formation and mass selection [A - ·B n ]

10. Step 1: Ion formation and mass selection Step 2: Exposure to infrared light [A - ·B n ] * Experimental Realization: Infrared Predissociation Spectroscopy [A - ·B n ]

11. Step 1: Ion formation and mass selection Step 2: Exposure to infrared light [A - ·B n-m ] + B m Step 3: Registration of fragment ions Experimental Realization: Infrared Predissociation Spectroscopy [A - ·B n ] [A - ·B n ] *

12. Mechanism: one-photon-dissociation only if E B [A - ·B n ] < h [A - ·B n ] h [A - ·B n ] * [A - ·B n-m ] + m·B

13. Mechanism: [A - ·B n ] h [A - ·B n ] * [A - ·B n-m ] + m·B[A - ·B n ·Ar m ] h [A - ·B n ·Ar m ] * [A - ·B n ] + m·Ar one-photon-dissociation only if E B [A - ·B n ] < h Attachment of a weakly bound “messenger” atom (e.g. Ar) Alternative: Advantages: - one-photon-dissociation - production of cold clusters Disadvantage: - difficult to produce

14. Experimental Setup Entrainment Source: W.H. Robertson, M.A. Kelley, M.A. Johnson, Rev. Sci. Inst., 71, 4431, 2000 Weber J.M., Schneider H., J. Chem. Phys., 120, 10056, 2004

15. General Considerations: Cl - ·C 6 F n H 6-n

16. 1.EA Cl : 3.61 eV (1) >> EA C 6 F 6 : 0.53 eV (2)  negative charge will be predominantly localized on chloride  fluorobenzenes will serve as ligands with planar ring structures General Considerations: Cl - ·C 6 F n H 6-n (1) : Berzinsh, U. et al, Phys. Rev. A, 1995, 51, 231 (2) : Miller, T.M et al, Int. J. Mass Spectrom., 2004, 233, 67-73

17. 2.Consider interaction of aromatic charge distribution with anion: Quinonero et al., Angew. Chem. Int. Ed., 41, 2001 Hiraoka, K. et al, J. Phys. Chem., 91, 529,1987 Loh et al, J. Chem. Phys., 119, 9559, 2003 General Considerations: Cl - ·C 6 F n H 6-n 1.EA Cl : 3.61 eV (1) >> EA C 6 F 6 : 0.53 eV (2)  negative charge will be predominantly localized on chloride  fluorobenzenes will serve as ligands with planar ring structures

18. - anion binds to hydrogen atoms at the periphery of the ring (Loh et al, J. Chem. Phys., 119, 9559, 2003) - red shift of hydrogen bonding CH oscillators - anion binds to positive partial charge in the ring (Quinonero et al., Angew. Chem. Int. Ed., 41, 2001; Hiraoka, K. et al, J. Phys. Chem., 91, 529, 1987) - minor influence on bonds of the aromatic ring General Considerations: Cl - ·C 6 F n H 6-n

19. at what degree of fluorination does the binding motif switch? how will different distributions of the fluorine atoms around the ring influence the binding motif? Questions: General Considerations: Cl - ·C 6 F n H 6-n

20. Consider Simplest Case: Cl - ·C 6 H 6 … Two possible binding motifs: LinearBifurcated Loh et al, J. Chem. Phys., 119, 9559, 2003

21. Two possible binding motifs: LinearBifurcated Loh et al, J. Chem. Phys., 119, 9559, 2003 Consider Simplest Case: Cl - ·C 6 H 6 …

23. 1 2 3 1: overlap of 20 and 7 (Wilson numbering of benzene) 2: 2  mode 3: combination band (Fermi interaction) Consider Simplest Case: Cl - ·C 6 H 6 …

24. spectra complicated due to interaction of several CH stretch modes (Fermi resonances) large intensity only due to H bonding of CH groups to the chloride anion

25. ... the consequences for Cl - ·C 6 F n H 6-n : spectra complicated (e.g. by combination bands / Fermi resonances) intense features centered around H bonded groups: allows structural interpretation! Consider Simplest Case: Cl - ·C 6 H 6 … spectra complicated due to interaction of several CH stretch modes (Fermi resonances) large intensity only due to H bonding of CH groups to the chloride anion

26. … and increase the level of fluorination! n = 0 Cl - ·C 6 F n H 6-n

27. n = 0 Cl - ·C 6 F n H 6-n … and increase the level of fluorination! n = 1

28. n = 0 n = 1 n = 2 Cl - ·C 6 F n H 6-n … and increase the level of fluorination!

29. n = 0 n = 1 n = 2 n = 3 Cl - ·C 6 F n H 6-n … and increase the level of fluorination!

30. n = 0 n = 1 n = 2 n = 3 Dramatic change in red shift! Cl - ·C 6 F n H 6-n … and increase the level of fluorination!

31. n = 0 n = 1 n = 2 n = 3 n = 4 Cl - ·C 6 F n H 6-n … and increase the level of fluorination! Dramatic change in red shift!

32. n = 0 n = 1 n = 2 n = 3 n = 4 n = 5Red shifted CH stretch vibration  Still hydrogen bonding! Cl - ·C 6 F n H 6-n … and increase the level of fluorination! Dramatic change in red shift!

33. IR Spectra of Cl - ·C 6 F n H 6-n n = 0 n = 1 n = 2 n = 3 n = 4 n = 5 Cl - ·C 6 F n H 6-n

34. n = 0 n = 1 n = 2 n = 3 n = 4 n = 5 Absorption ~ 3000 cm -1 Absorption ~ 2600 cm -1 Cl - ·C 6 F n H 6-n IR Spectra of Cl - ·C 6 F n H 6-n

35. only linear binding motif possible linear and bifurcated binding motif possible

36. IR Spectra of Cl - ·C 6 H 6 (top trace) vs. Cl - ·C 6 F 5 H (bottom trace)

37. Calculated potential energy surface along the CH stretching coordinate in [C 6 F 5 ···H···Cl] - (MP2/TZVP)

38. Calculations (DFT, B3-LYP, TZVP, scaled harmonic frequencies): CH = 2544 cm -1 CH, bound = 2586 cm -1 CH, free = 3091 cm -1 CH, asymm = 2999 cm -1 CH, symm = 3006 cm -1

39. ν [cm -1 ] E rel [meV] 2982 0 3036 3088 ν [cm -1 ] E rel [meV] 2656 ~170 3066 3079 Calculations (DFT, B3-LYP, TZVP, scaled harmonic frequencies):

40. ν [cm -1 ] E rel [meV] 2982 0 3036 3088 ν [cm -1 ] E rel [meV] 2656 ~170 3066 3079 intense Calculations (DFT, B3-LYP, TZVP, scaled harmonic frequencies):

41. ν [cm -1 ] E rel [meV] 2982 0 3036 3088 ν [cm -1 ] E rel [meV] 2656 ~170 3066 3079 intense Calculations (DFT, B3-LYP, TZVP, scaled harmonic frequencies):

42. Comparison to Experimental Spectrum ν [cm -1 ] E rel [meV] 2982 0 3036 3088 ν [cm -1 ] E rel [meV] 2656 ~170 3066 3079

43. Comparison to Experimental Spectrum ν [cm -1 ] E rel [meV] 2982 0 3036 3088 ν [cm -1 ] E rel [meV] 2656 ~170 3066 3079

44. Comparison to Experimental Spectrum ν [cm -1 ] E rel [meV] 2982 0 3036 3088 ν [cm -1 ] E rel [meV] 2656 ~170 3066 3079

45. So How About The Ring Bound Isomer After All? doesn’t show up in any of the recorded infrared spectra but is a stable structure according to our calculations, for Cl - ·C 6 F 5 H it lies ~ 300 meV above the ground state (DFT, B3-LYP, TZVP)

46. Summary / Interpretation no change in binding motif until perfluorobenzene (Even for highest fluorination degree under study hydrogen bonding is preferred to binding to the top of the ring!) hydrogen atoms become more and more acidic with increasing fluorination (red shift of CH bands increases) calculations support these assignments

47. Thanks to... Prof. J.M. Weber Kristen Vogelhuber Financial support by - NSF (JILA Physics Frontier Center) - Petroleum Research Fund... all of you for your attention!!!

48. CH Stretch region of benzene Benzene Fermi tetrad Page et al., J. Chem. Phys., 88, 4621 (1988) Page et al., J. Chem. Phys., 88, 5362 (1988)

49. Normal modes of benzene (Wilson‘s numbering) Page et al., J. Chem. Phys., 88, 5362 (1988)

50. Quinonero et al., Angew. Chem. Int. Ed., 41, 3389 (2002)

51. Bryantsev et al., Org. Lett., 7, 5031 (2006)

52. 1 6 5 4 3 2 7 12 10 9 11 8 1 6 5 4 3 2 7 12 10 9 11 8 1: -0.10 7: 0.18 2: 0.19 8: -0.17 3: 0.09 9: -0.19 4: 0.11 10: -0.19 5: 0.09 11: -0.19 6: 0.19 12: -0.17 Cl - : -0.83 1: -0.21 7: 0.18 2: 0.23 8: -0.17 3: 0.11 9: -0.16 4: 0.15 10: -0.15 5: 0.11 11: -0.16 6: 0.23 12: -0.17 Mulliken population analysis of Cl - ·C 6 F 5 H and neutral C 6 F 5 H

53. ν [cm -1 ] 3103 3159 3213 ν [cm -1 ] 2764 3190 3204

54. Illustration of electrostatic potential around C 6 H 6 (left) and C 6 F 6 (right) (calculated with Gaussian03W, HF/3-21G* level) The color coding from blue to red represents positive to negative electrostatic potentials.