RESONANCE ENHANCED MULTIPHOTON IONIZATION (REMPI) SPECTROSCOPY

OF WEAKLY BOUND COMPLEXES

Lloyd Muzangwa

Molecular Spectroscopy and Dynamics Group

67th International symposium on molecular spectroscopy

Outline

• Brief Introduction

• REMPI-TOF Technique

• Results and Discussion

• Conclusion

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.

Noncovalent Interactions in Halobenzene clusters

Experimental apparatus (REMPI)

Pump Pump

Pump

UV laser

MCPs

M

M*

M+ + e–

GE

R

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.

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).

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

Calculated Absorption Spectra (TDDFT)

All dimer absorptions are red-shifted .

-stacked dimers show increasing red-shift with closer orientation of Cl atoms

“Free” chlorobenzene in T-shaped dimer shows smallest red-shift

What is the nature of the transition?

Transition is π to π*

What about the trimer?

Vertical absorption energies of different chlorobenzene trimers

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

Dr Scott Reid My Lab Mates National $cience Foundation

Acknowledgements