1. CHIRPED-PULSE FOURIER-TRANSFORM MICROWAVE SPECTROSCOPY OF THE PROTOTYPICAL C-H…π INTERACTION: THE BENZENE…ACETYLENE WEAKLY BOUND DIMER Nathan W. Ulrich, a Nathan A. Seifert, b Rachel E. Dorris, a Ashley M. Anderton, a Rebecca A. Peebles, a Brooks H. Pate, b Sean A. Peebles a a Department of Chemistry, Eastern Illinois University, 600 Lincoln Ave., Charleston, IL 61920 b University of Virginia, Department of Chemistry and Biochemistry, McCormick Rd., PO Box 400319, Charlottesville, VA 22904

2. Introduction CH…  (aromatic) interactions CH…  (aromatic) interactions – First suggested by Tamres (1952) 1 HCCH pK a ~ 25 HCCH pK a ~ 25 – CH 4 ~ 48, HCF 3 ~ 25.5, HF ~ 3.17 Few experimental results Few experimental results – Resonance enhanced multiphoton ionization 2 – Co-crystal 3 2 1 M. Tamres, J. Am. Chem. Soc. 74 (1952) 3375. 2 E. Carrasquillo M., T. S. Zwier, D. H. Levy, J. Chem. Phys. 83 (1985) 4990. 3 R. Boese, T. Clark, A. Gavezzotti, Helv. Chim. Acta, 86 (2003) 1085. 2.447 Å at 123 K 2.462 Å at 201 K

3. Previous Work 3 FBZ…HCl:M.E. Sanz,et al., J. Chem. Phys. 118 (2003) 9278.(BZ) 2 : M. Schnell, et al., Angew. Chem. Int. Ed., 52 (2013) 1. FBZ…HCCH: N. W. Ulrich, et al., Phys. Chem. Chem. Phys. 15 (2013) 18148.BZ…CF 3 H: J. C. López, et al., Angew. Chem. Int. Ed. Eng. 45 (2006) 290. FC 6 H 5 …HCl (C 6 H 6 ) 2 FC 6 H 5 …HCCH C 6 H 6 …HCF 3

4. Ab Initio Calculations MP2/6-311++G(2d,2p) MP2/6-311++G(2d,2p) Fluorobenzene-HCCH Fluorobenzene-HCCH – ~0.3 D induced dipole observed – Ab initio dipole components in good agreement with experiment Benzene-HCCH Benzene-HCCH – C 6v, ~0.5 D dipole 4 Gaussian 09, Revision C.01, M. J. Frisch, et al., Gaussian, Inc., Wallingford CT, 2010. 2.37 Å 2.38 Å

5. Experimental 0.1% benzene, 0.2% HCCH in Ne, ~1.7 atm 0.1% benzene, 0.2% HCCH in Ne, ~1.7 atm – 5 nozzles @ 3.3 Hz, 8 FIDs/gas pulse, 2  s chirp – Average of 520,000 FIDs – 13 C 12 C 5 H 6 in natural abundance Second broadband scan using C 6 H 5 D Second broadband scan using C 6 H 5 D – 400,000 total FIDs, ~3.0 atm H 13 C 12 CH and H 12 C 13 CH on cavity FTMW H 13 C 12 CH and H 12 C 13 CH on cavity FTMW – 0.5% benzene, 0.5% H 13 C 12 CH in He/Ne, ~2.8 atm – 1 nozzle @ 10 Hz, 1 FID/gas pulse 5 UVa CP-FTMW, Spring 2013

6. Spectroscopic Parameters 6

7. Structure 7   r 0 : 2.4921(1) Å r s : 2.4717(7) Å r e : 2.3694 Å R cm R H…  1 M. D. Harmony, et al., J. Phys. Chem. Ref. Data 8 (1979) 619. 2 J. Pliva, et al., J. Mol. Spectrosc., 140 (1990) 214.

8. Dipole Moments/Induction 8

9. Comparison… 9 BZ…HCCH co-crystal: 2.447 Å at 127 K

10. Binding Energy Calculations 10

11. Excited States 11 Three sets of transitions Three sets of transitions Each has a different pattern Each has a different pattern All lower than parent All lower than parent Scale and have similar rotational constant to parent Scale and have similar rotational constant to parent Could be intermolecular vibrational modes Could be intermolecular vibrational modes J = 4  3

12. 12 Frequencies: M. Böning, et al., ChemPhysChem 14 (2013) 837. Intermolecular Vibrational Modes? J = 5  4

13. Summary, Conclusions, What’s Next? Trimer(s)? – still unassigned lines with some interesting patterns… Trimer(s)? – still unassigned lines with some interesting patterns… Need to understand excited states Need to understand excited states A reliable frequency calculation would be nice A reliable frequency calculation would be nice 13 Trimers: A. Fujii, et al., J. Phys. Chem. A 108 (2004) 2652.

14. Acknowledgements RUI grant CHE-1214070 (EIU) and MRI-R2 grant CHE-0960074 (UVa) RUI grant CHE-1214070 (EIU) and MRI-R2 grant CHE-0960074 (UVa) Pate lab Pate lab – Brooks Pate – Nate Seifert EIU Students EIU Students – Anu Akmeemana – Cori Christenholz – Lena Elmuti 14

15. 15 [BZ…HF] F.A. Baiocchi, et al, J. Phys. Chem., 87, (1983) 2079. [BZ…HCl] W.G. Read, et al, J. Chem. Phys., 78, (1983) 3501. [BZ…HBr] S.A. Cooke, et al, Chem. Phys. Lett., 272, (1997) 61. [BZ…HCF 3 ] J.C. Lopez, et al, Angew. Chem., Int. Ed., 45, (2006) 290. [BZ…HCN] H. S. Gutowsky, et al, J. Chem. Phys., 103, (1995) 3917. [FBZ…HCl] M.E. Sanz, et al, J. Chem. Phys., 118, (2003) 9278. [FBZ…HCCH] N. W. Ulrich, et al, Phys. Chem. Chem. Phys., 15, (2013) 18148. [BZ…HCCH] N. W. Ulrich, et al, Phys. Chem. Chem. Phys., 16, (2014) 8886. [BZ…BZ] E. Arunan, et al, J. Chem. Phys., 98, (199), 4294; M. Schnell, et al, Angew. Chem. Int. Ed., 52, (2013), 5180.

16. 16

17. DFT Calculation for XH…  Interactions Usual method is MP2/6-311++G(2d,2p) Usual method is MP2/6-311++G(2d,2p) – Need a faster but equally accurate computational approach G09 incorporates new DFT functionals designed for dispersion G09 incorporates new DFT functionals designed for dispersion 17

18. 18 MP2  B97XD M062X

19. 19 J K ' ← J K "|M||M| Number of electric fields 3 0 ← 2 0 16 23 3 1 ← 2 1 06 4 0 ← 3 0 34 RMS:5.8 kHz μ:0.438(11) D Estimates of the H ⋯ π distance range from 2.20 to 2.61 Å,35, 36 with high level CCSD(T)/aug-cc-pVTZ calculations predicting a distance of ~2.50 Å M06-2X/6-311+G(d,p) calculation giving an H ⋯ π distance of 2.393 Å and a binding energy of 12.2 kJ mol–1.39