1. Electronic and vibronic spectroscopy of crown ether water complexes: benzo-15-crown-5 (B15C) and 4’-aminobenzo- 15-crown (ABC) V. Alvin Shubert and Timothy S. Zwier Purdue University, Department of Chemistry, West Lafayette, IN 47907 ABC-(H 2 O) n B15C-(H 2 O) n

2. Crown ethers long noted for ability to selectively bind substrates, especially cations Much work has focused on structure and binding energy of crown-cation complex in solution However, oxygen-rich pocket is ideally suited to binding other types of substrates, including water Motivations We can study the binding of water to crown ethers in the absence of ions using jet-cooled gas phase spectroscopy As a first step, we present the IR and UV spectra of water clusters of 4’-aminobenzo-15-crown-5 and benzo-15-crown-5 ethers

3. Resonant 2 photon ionization (R2PI): Records spectra in mass selective fashion Experimental Methods R2PI: Electronic spectrum Biomolecule * (S 1 ) Biomolecule + + e - Biomolecule (S 0 ) UV-UV Hole-burning: Conformation specific electronic spectrum Hole-burn Probe Conformer A Conformer B Hole-burn Probe UV source (20 Hz) tuned UV hole-burn (10 Hz) Laser Timing 200 ns Resonant ion dip infrared spectroscopy (RIDIRS) UV Source fixed IR Source tuned Laser Timing 200 ns Also used analogous laser induced fluorescence (LIF) methods. A C A A B C C A A B B B B B B B Boltzmann distribution of conformers in the pre-expansion Collisional cooling to zero-point vibrational levels Laser(s) B* C A C C B A A C

4. Computational Methods Build water clusters from optimized monomers (see monomer presentation, FD07) Place water in position such that it can form two H-bonds to crown oxygens Optimize with DFT B3LYP/6-31+G(d), ultrafine grid and tight convergence options using the GAUSSIAN03 suite of programs For uniquely optimized structures, perform frequency calculations

5. Results: ABC-(H 2 O) n=1,2 R2PI and UV-UV HB ABC-(H 2 O) 1 – 1 conformation ABC-(H 2 O) 2 – 1 conformation 325003200032100322003230032400 Frequency (cm -1 ) Ion intensity/depletion (arb. units) R2PI taken in ABC-(H 2 O) 1 mass channel ABC-(H 2 O) 1 ABC-(H 2 O) 2 Vertical dashed lines show monomer origins, 4 th of which is ~1200 cm -1 blue of these (see FD07)

6. Results: ABC-(H 2 O) n=1,2 RIDIRS in NH and OH stretch regions ABC-(H 2 O) 1 – NH 2 stretches most similar to ABC-C monomer (see FD07) Both OH groups in H-bonds to crown oxygens and observe 3 AS OH stretches! Ion depletion (arb. units) 34003460352035803640 Frequency (cm -1 ) ABC-(H 2 O) 1 ABC-(H 2 O) 2 -NH 2 symmetric stretch (3401.9) -NH 2 anti-symmetric stretch (3484.3) H 2 O symmetric stretch (3568.7) H 2 O asymmetric stretch (3633.2, 3634.7, 3636.6)

7. Results: ABC-(H 2 O) 1 RIDIRS: OH stretches No obvious shoulders on SS stretch band Could multiple conformations with overlapping origins and SS OH stretch frequencies be contributing to triplet? 3560359036203650 Frequency (cm -1 ) Ion depletion (arb. units) 3633.2 3634.7 3636.6 3568.7 dimethoxy benzene – only a single H-bonded OH and only an AS OH stretch singlets 3585.9 3724.8

8. IR-UV HB attributes triplet to a single conformation Could triplet be due to water motion (e.g. rotation) or a tunneling splitting? Results: ABC-(H 2 O) 1 IR-UV HB on AS OH stretch bands 320003202032040320603208032100 Frequency (cm -1 ) Ion intensity/depletion (arb. units) R2PI in ABC-(H 2 O) 1 mass channel IR-UV HB (3633.2) IR-UV HB (3634.7) IR-UV HB (3636.6)

9. Results: ABC-(HDO) 1 RIDIRS in OH and OD stretch regions 2620264026602680270035603580360036203640 Frequency (cm -1 ) Ion depletion (arb. units) 2633.8 2660.7 3583.1 3619.7 * * 2 OH and 2 OD stretches No triplet! Asterisks label ABC-(H 2 O) 1 bands IR-UV HB on 3583.1 (green) and 3619.7 (blue) demonstrated the UV-spectra are too closely overlapped to obtain clean conformer specific RIDIR spectra

10. Laser Timing 3.5 μs200 ns UV Source fixed (10 Hz) Provides  selectivity IR 2 tuned (10 Hz) IR 1 fixed, hole-burn laser (5 Hz) Conformer AConformer B B (S 0 ) B* (S 1 ) A (S 0 ) A* (S 1 ) In ABC-HDO, two OH and two OD stretches were seen. IR-UV hole-burning indicated that this was due to two different ABC-HDO species, IR-RIDIRS confirmed this and showed which pairs belonged together. IR-IR-UV Hole-burning: Conformation specific IR spectrum when electronic spectra overlap

11. Results: ABC-(HDO) 1 : IR-IR-UV HB, 2 conformations 26202640266026802700 2600 Frequency (cm -1 ) Ion depletion (arb. units) 2633.8 2660.7 35603580360036203640 3583.1 3619.7 IR-IR-UV HB proves OD and OH stretches are due to two conformations H-bonds of unequal strength – A: OD stronger, OH weaker B: OH stronger, OD weaker Raises possibility that triplet in AS OH stretch could be do to multiple conformations – need to do IR-IR-UV HB to check ABC-(HDO)-B ABC-(HDO)-A

12. Results: B15C-(H 2 O) 1 : UV-UV HB, 2 conformations Unlike ABC-H 2 O, UV-UV HB resolves 2 B15C-(H 2 O) 1 conformations For more on B15C monomer, see FD07 358003590036000 35700 Frequency (cm -1 ) B15C-(H 2 O) 1 -A B15C-(H 2 O) 1 -B B15C-A B15C-B Fluorescence depletion (arb. units)

13. Results: B15C-(H 2 O) 1 : RIDIRS in OH stretch region Observe a doublet in B15C-(H 2 O) 1 -A AS OH stretch Singlet in B15C-(H 2 O) 1 -B AS OH stretch Why doublet? Same possibilities as for ABC-(H 2 O) 1 AS OH stretch triplet 360036503700 3550 Frequency (cm -1 ) B15C-(H 2 O) 1 -A B15C-(H 2 O) 1 -B Fluorescence depletion (arb. units) 3568.5 3635.0 3638.1 3583.9 3639.7

14. Results: RIDIRS in alkyl CH stretch region B15C-(H 2 O) 1 -A and ABC- (H 2 O) 1 have almost identical alkyl CH stretch spectra B15C-(H 2 O) 1 -B is also very similar Data is evidence that all three share the same crown conformation but differing in water-binding site 285029002950 2800 Frequency (cm -1 ) B15C-(H 2 O) 1 -A B15C-(H 2 O) 1 -B Fluorescence/ion depletion (arb. units) ABC-(H 2 O) 1

15. Results: RIDIRS in alkyl CH stretch region Furthermore, the crown conformation for the water clusters may be different from those seen in monomer (see FD07). Assumes water does not significantly perturb CH stretch frequencies. However, in tryptamine with H 2 O bound to -NH 2 group, alkyl CH stretches were perturbed. Would offer insight into water binding site If crown conformation of water clusters is different from monomer, demonstrates crown is flexible and adjusts to accommodate the water 285029002950 2800 Frequency (cm -1 ) B15C-(H 2 O) 1 -A B15C-(H 2 O) 1 -B Fluorescence/ion depletion (arb. units) ABC-(H 2 O) 1 B15C-C B15C-B B15C-A

16. Conclusions ABC-(H 2 O) 1 : 1 conformer; B15C-(H 2 O) 1 : 2 conformers Crown conformation same in all three water clusters (alkyl CH stretch) Triplet/doublet observed in AS OH stretch of ABC-(H 2 O) 1 /B15C-(H 2 O) 1 In ABC-(H 2 O) 1, disappears upon substitution with HDO Two conformers associated with two different strength H-bonds Tunneling splitting? – observed intensity ratio is reverse of that predicted by spin statistics Water motion, rotation? Multiple overlapped conformations? Future Work Perform IR-IR-UV HB on AS OH stretch peaks (for both ABC and B15C-A) Measure water binding energies Study higher order water clusters

17. Acknowledgements Zwier group: Prof. Timothy S. Zwier Jasper R. Clarkson Esteban E. Baquero Tracy Legreve Nathan Pillsbury Josh Newby William H. James III Chirantha Rodrigo Ching Ping Liu Christian Müller Josh Sebree Funding: National Science Foundation Computing Resources Information and Technology at Purdue (ITaP), Rosenbaum Computing Center (RCC)