1. RESONANCE ENHANCED MULTIPHOTON IONIZATION (REMPI) SPECTROSCOPY OF WEAKLY BOUND COMPLEXES Lloyd Muzangwa Molecular Spectroscopy and Dynamics Group 67 th International symposium on molecular spectroscopy

2. Outline Brief Introduction REMPI-TOF Technique Results and Discussion Conclusion

3. Introduction  Noncovalent forces are critical in maintaining the secondary, tertiary or quaternary structure of proteins and nucleic acids, and they can also play key roles in macromolecular function.  Halobenzenes are prototypical systems where different noncovalent interactions may be operative, including π-stacking, C- H/π, and halogen bonding. 1.Yu. Zhang, Ji. Baoming, Tian. Anmin and Wang. Weizhou. J. Chem. Phys. 2012, 136, 141101. 2.Liu. Benkang, Wang. Bingxing, Wang. Yanqiu and Li. Wang. J. Chem. Phys. Letters. 2009, 477, 266-270. 3.P. Metrangolo and G. Resnati. Science. 2008, 321, 918.

4. Noncovalent Interactions in Halobenzene clusters

5. Experimental apparatus (REMPI) Pump UV laser MCPs M M* M +  + e – GERGER

6. Computational Details  Geometry optimizations and binding energies: M06-2x and MP2 Corrected for BSSE and ZPE  Electronic absorption and oscillator strength: TD-DFT (M06-2x)  Basis set: aug-cc-pVDZ  Sherrill and co-workers have shown that for aromatic clusters,the Minnesota functionals M05-2x and M06-2x provide good performance to computational cost.

7. 1+1 REMPI spectra of chlorobenzene clusters (ClBz) n where n = 1-4  All clusters show broad absorption, red shifted with respect to monomer transition.  Spectra of different clusters are similar over range we measured (n=2-4).

8. Optimized dimer structures: M06-2x/aug-cc-pVDZ

9. Calculated Absorption Spectra (TDDFT)

10. What is the nature of the transition ?  Transition is π to π *

11. What about the trimer?

12. Vertical absorption energies of different chlorobenzene trimers

13. Conclusions Red shift in cluster spectra was found to be consistent with TD-DFT calculations of the electronic absorption spectra. For the clbz dimer, five minima were found, corresponding to four π-stacked structures and one T- shaped. No halogen bonding structures were found. The calculated spectra show that these species absorb over a broad range – likely all are formed in our experiment Calculated spectra of representative trimers are similar, consistent with the similarity of spectra for different cluster sizes

14.  Dr Scott Reid  My Lab Mates  National $cience Foundation Acknowledgements