1. CONFORMATION-SPECIFIC ELECTRONIC AND VIBRATIONAL SPECTROSCOPY OF DIBENZO-15-CROWN-5 ETHER IN A SUPERSONIC JET.
Evan Buchanan, Chirantha P. Rodrigo, William H. James, Josh J. Newby, and Timothy S. Zwier
Department of Chemistry, Purdue University
West Lafayette, Indiana

2. Dibenzo-15-crown-5 (DB15C)
15-Crown-5 Ethers
Motivation:
Crown ethers provide a chemical means to preferentially sequester substrates, especially cations.
Model enzyme-substrate interactions
Conformational consequence of adding a second benzene ring. How flexible is the macrocycle?
Where is the second excited state?
Investigate the electronic energy transfer between the two chromophores..
Dibenzo-15-crown-5 (DB15C)
Benzo-15-crown-5 (B15C)
J.-M. Lehn. Pure & Appl. Chem., 1979, 51, 979
R. Kusaka, Y. Inokuchi, T. Ebata. Phys. Chem. Phys., 2007, 9, 4452

3. Supersonic Expansion
MCP
Large, flexible molecule with many conformational isomers is entrained in a high pressure backing gas.
Gas expands through a pinhole into a vacuum.
Molecules collisionally cool with the buffer gas
Pulsed valve
Expansion reduces the number of transitions observed in the spectra
Rotational temperature of a few Kelvin
Vibrational temperature of Kelvin.

4. Experimental Methods Resonant Two-Photon Ionization UV-UV Holeburning
Ionization Continuum
Molecule + + e-
Ionization Continuum Sn S0 A B
Molecule + + e-
Spatially overlapped Sn
Δt = 200 ns S0 A
10 Hz HB
20 Hz Probe B
R2PI: Electronic Spectrum
UVHB: Conformation specific electronic spectrum

5. LIF and UVHB Spectra of B15C
Three conformers present in the jet expansion.
One conformer blue shifted by over 500 cm-1.
S0 – S1 origin serve as a sensitive probe of the local environment about the phenyl ring.
DMB
DEB
LIF A * B * C *
B15C5-H2O
V. A. Shubert et al. J. Phys. Chem. In press.
R. Kusaka, Y. Inokuchi, T. Ebata. Phys. Chem. Chem. Phys., 2007, 9, 4452

6. Something Simpler: 1,2-diethoxybenzeneγ
Carbon Position β γ
Relative Origin (cm-1)*
in/in
in/out
-480
out/in
343
out/out
-800 to -900
38 to 53
1286 β
* TDDFT predicted S0 – S1 origins
B15C conclusions:
Conformers A, and B: in/in; in/in
Conformer C: out/in; in/in

7. B15C Assigned Structures
1.5 KJ/mol
0.0 KJ/mol
B15CA): in/in; in/in
B15C(C): out/in; in/in
B3LYP/6-31+G(d)
Can we use the same technique to assign the local environment for DB1C?
7.1 KJ/mol
B15C(B): in/in; in/in

8. R2PI and UVHB Spectra of DB15C* B
(S1 vibronic levels) * C * D *
B15C
Three conformers indicative of in/in; in/in structures for ring 1.
Blue shifted conformer indicative of an out/in; in/in structure for ring 1.
Position of S2 in A, B, and C is not clear.

9. Resonant Ion-dip Infrared Spectroscopy
Δt = ns
IR spatially overlapped
Ionization Continuum
B+ + e- Sn
10 Hz IR
20 Hz UV S0
UV only
IR and UV A
Active Baseline Subtraction B
RIDIRS: Conformation specific IR S0 spectrum

10. Alkyl C-H Stretch Spectra
C(x3) B D
Spectral congestion due to 14 C-H stretch fundamentals.
Transitions above 2930 cm-1 indicative of CH…O interactions and the degree of buckling of the crown.

11. Where is the S2 Origin? B 109 S1 DB15C B15C All planar region
Unusually large transition occurring at +109.
Possible S2 origin?

12. Relative Origin (cm-1)*
DFL Conformer B
B origin
Carbon Position β γ
Relative Origin (cm-1)*
in/in
in/out
-480
out/in
343
out/out
-800 to -900
38 to 53
1286
109
Internal Mixing 00 S2
{S1(ν)}

13. Where is the S2 origin? D 24 72 48 DB15C B15C
+500 cm-1 from conf. A,B, and C.
Similar vibronic structure to B15C.
Extra transitions could be the S2 origin.
Out/in; in/in structure for both rings.

14. DFL Conformer D D origin + 25 + 48 + 72
Two overlapping transitions under holeburn transition
Two stable excited state minima for each ring.

15. Future work
Acquire more DFL to identify the S0 – S2 origin and probe electronic coupling between states.
Water clusters: B15C distorts the crown to accommodate
hydrogen bonding with water. Does DB15C behave in a
similar manner?
SEP-PT: measure barriers to isomerization.
How flexible is the crown?
How does it depend on one vs. two rings?

16. Acknowledgements Prof. Timothy S. Zwier
Current Group Members: Past Group Members:
Dr. Christian W. Müller Dr. Tracy LeGreve
William H. James Dr. Nathan Pillsbury
Josh J. Newby Dr. Alvin Shubert
Chirantha P. Rodrigo Dr. Esteban Baquero
Josh A. Sebree
Zachary Davis
James Redwine
Ryan Muir
Deepali Mehta
Computational Resources:
Information Technology at Purdue (ITaP)
Funding:
National Science Foundation