JASPER R. CLARKSON AND TIMOTHY S. ZWIER

Funding: National Science Foundation Petroleum Research Fund, Type AC Purdue Research Foundation

CONFORMATIONAL ISOMERIZATION BARRIERS AND DYNAMICS OF PHENOL-ETHANOLAMINE CLUSTERS.

How does solvent bind at a specific site affect the barriers to conformational isomerization of the flexible molecule?

Studying Clusters

Shuttling a water molecule between remote H-bonding sites on the same

‘solute’ molecule

The effect of H-bonded water on an intramolecular isomerization barrier

Measuring the barrier to moving from one lone pair to the other in an

OH--OH H-bondTI11 TODAY, 4:36 PM 1015 McPHERSON

JCP 122, 214312 (2005)

Pulsed valve

To Roots pump

PMT

Carrier gas

y x

z Lens

Mirror

Expansion

CCD

SPEX Monochromator0.75 m grating

S0

S1

Zero-point LevelCA B

IV. UV Probe

III. Collisional Cooling

II. UV Dump

I. UV Pump

Excited Vibrational

LevelB*

Expt’l protocol:1. Cool2. Pump3. Dump4. Re-cool5. Probe

AC

AABC C

AB BB

B B

B

BBC

A*B*

C

CB

A*

A

AA

AC

CA

AC

CB

B

SEP excitessingle conformation C

AC

AABC C

AB BB

B B

B

BBC

A*B*

C

CB

A*

A

AA

AC

CA

AC

CB

B

C

Experimental Setup

1.

2-3.

4.

5.

Macleod and Simons PCCP, 2003, 5, 1123-1129

Complex A0.00 kcal/mol

Complex B0.51 kcal/mol

Complex C0.73 kcal/mol

Phenol-(2-aminoethanol)1

Complex B Complex C~200 cm-1

H

OR..

..

C ↔ B isomerization pathway:The barrier between lone pairs on the acceptor oxygen

Ion

inte

nsity

(ar

bitr

ary

units

)

359003580035700356003550035400 (cm

-1)

Complex C origin

Complex B originComplex A origin

2nd order cluster (D)

LIF Phenol-(2-Aminoethanol)1 Clusters

Wavenumbers(cm-1)

Inte

nsi

ty (

arb

. u

nit

s)

10008006004002000

Energy above ZPL (cm-1)

next slide Gains = 20 + 10 Hz fluorescenceDips = SEP

SEP C

SEP-PT C-->B

SEP-PT C-->C

0-10-20 Hz Holeburning

Inte

nsi

ty (

arb

. u

nit

s)

Overview SEP-PT Spectra Phenol-(2-Aminoethanol)1 C

inte

nsi

ty (

arbi

trar

y un

its)

?

Upper bound206 cm -1Lower bound

136cm -1

400350300250200150100Energy above ZPL (cm-1)

SEP C

SEP-PT C-->B

SEP-PT C-->C

0-10-20 Hz Holeburning

SEP-PT Spectra Phenol-(2-Aminoethanol)1 C BIn

ten

sity

(ar

b.

un

its)

SEP-PT Spectra Phenol-(2-Aminoethanol)1 B C

Inte

nsi

ty (

arb

. u

nit

s)

5004003002001000 Energy above ZPL (cm-1)

SEP-PT B C

Upper Bound520 cm-1

180 cm-1Lower Bound140 cm-1

SEP-PT B B

SEP-PT Spectra Phenol-(2-Aminoethanol)1 C A

1700160015001400130012001100 Energy above ZPL (cm

-1)

SEP C

SEP-PT CA

SEP-PT CB

Inte

nsi

ty (

arb

. u

nit

s)

160015001400130012001100 Energy above ZPL (cm-1)

1700

• Why can’t we pump into and out of A?

136-206 cm-1

Bounds on the barriers to isomerization

• Computed Barriers

A

140-520 cm-1

???? ??

??

B C320 cm-1

260 cm-1

1850 cm-132

2 cm

-1

134

cm-1

1790 cm-1

A

B C

A B C

150010005000

Wavenumbers (cm-1

)

Black – DF Complex ARed – DF tuned out of gas pulse

•Short S1 lifetime•Broad, red-shifted fluorescence

Population transfer out of A: Competition with H-atom transfer in the S1 state?

Conclusions:

• Measuring the barrier to moving from one lone pair to the other in an OH…OH H-bond: ~200 cm-1

• Measuring the relative energies between Complex C and B

• Locating minimum energy pathways

• Observing possible H-atom transfer

B3LYP/6-31+G* full optimizations of minima and optimization of TS using QST3 G03. TS verified with imaginary frequency.

707 cm-1

IPA(A)-gauche1 IPA(B)-gauche1

425 cm-1

IPA(A)-gauche1 IPA(A)-gauche2

238 cm-1

IPA(A)-gauche1 IPA(A)-anti

Intramolecular isomerization:Higher barrier

Solvent isomerization:Lower barrier,

energy flow fromindole through H-bonds

to ethanol torsion

‘Solute’ isomerization in competitionwith ‘solvent’ isomerization: IPA-ethanol

Acknowledgements

Collaborators:Professor Ken JordanProfessor David WalesDr. Dave Evans

Funding:National Science FoundationPetroleum Research FoundationPurdue Research Foundation

• Professor Tim Zwier• Zwier Group

Esteban BaqueroAlvin ShubertJaime StearnsTalitha SelbyTracy LeGreveNathan PillsburyBill JamesJosh Newby