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International Symposium on Molecular SpectroscopyInternational Symposium on Molecular Spectroscopy

6464thth Meeting - June 22-26, 2009, Meeting - June 22-26, 2009,

Ohio State University, Columbus, OHOhio State University, Columbus, OH

Presented byPresented by

Chirantha P. Rodrigo, Christian W. Müller, William H. James III, Nathan R. Pillsbury and Timothy S. Zwier

Department of Chemistry, Purdue University

West Lafayette, IN 47907

CONFORMATIONAL ISOMERIZATION OF CONFORMATIONAL ISOMERIZATION OF bisbis -(4-HYDROXYPHENYL)METHANE -(4-HYDROXYPHENYL)METHANE IN A SUPERSONIC JET EXPANSION, PART I: IN A SUPERSONIC JET EXPANSION, PART I: LOW BARRIER POTENTIAL ENERGY SURFACE IN THE SLOW BARRIER POTENTIAL ENERGY SURFACE IN THE S00 STATE. STATE.

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Conformations of Conformations of bisbis-(4-hydroxyphenyl)methane (-(4-hydroxyphenyl)methane (bb4HPM4HPM) )

Four torsional degrees of freedom.

Phenolic OH torsional barrier height is 1100-1266 cm‒1.*

what would be the phenyl torsional barrier?

• * Kudchadker, S. A. et al; J. Phys. Ref. Data, 7, 2, (1978), p417 and the references therein

• **Nathan R. Pillsbury et al: J. Chem. Phys., 129, 114301 (2008).

1100-1266 cm‒1

? cm‒1

HH

Diphenylmethane**

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Conformations of Conformations of bisbis-(4-hydroxyphenyl)methane (-(4-hydroxyphenyl)methane (bb4HPM4HPM) )

dd (C2)

0 cm‒1

uu (C2)

5 cm‒1

ud (C1)

11 cm‒1

Stationary points were calculated at DFT/B3LYP/6-311G(d,p) level of theory.

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Supersonic Jet Cooled Expansion and Spectroscopic Techniques.Supersonic Jet Cooled Expansion and Spectroscopic Techniques.

Collisional cooling to zero-point vibrational

levels

B BAC

A CC

B*B*

A*A*

A* C* C*

BAC

AAAB

AB

CC CC B

A

B

B

B

Probe20 HzUV

Δt=200 ns

Pump10 HzUV

Probe20 HzUV

Fluorescence Excitation Spectroscopy

Provides the excited state vibrational structure

UV-UV Holeburning Spectroscopy

Provides single conformer excitation spectra

Boltzmann distribution of conformers in the pre-expansion gas mixture

Probe laser

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Electronic Spectrum and UV-UV Holeburning Spectra of b4HPM.in

tens

ity /

(arb

. uni

ts)

3550035450354003535035300352503520035150

Energy / cm–1

* * * *

* incomplete subtraction

A

B (x7)

C

35185

35209

x = distance from nozzle to laser beamD = nozzle diameter

3520035180

Energy / cm-1

2 x/D) 4 (x/D) 6 (x/D) 7 (x/D) 11 (x/D) 18 (x/D)

A

BC

6

S1-00

30 4245

60 7375 117

S2-00

132

134 155

200180160140120100806040200Energy from ZPL / cm-1

**

S1-00

30 4245

60 7375 117

S2-00

132

134 155

S1-00

30 4245

60 7375 117

S2-00

132

134 155

200180160140120100806040200Energy from ZPL / cm-1

200180160140120100806040200Energy from ZPL / cm-1

**

Single Vibronic Level Fluorescence (SVLF) SpectroscopySingle Vibronic Level Fluorescence (SVLF) Spectroscopy

Conformer C :FES (HB)

S1

S0

CCD Camera

Provides information about ground state (S0) low frequency vibrational levels.

7

22

25

35

44

5267

7189

111 137

154

177181 199

S1-00

200180160140120100806040200

Energy from ZPL / cm-1

SVLF Spectroscopy andSVLF Spectroscopy andGround State Vibrational Frequencies:Ground State Vibrational Frequencies:Conformers A & CConformers A & C

C conformer and its assignments.

Conformer C :Origin SVLF

S1-00

30 4245

60 7375 117

S2-00

132

134155

200180160140120100806040200Energy from ZPL / cm-1

**

S1-00

30 4245

60 7375 117

S2-00

132

134155

S1-00

30 4245

60 7375 117

S2-00

132

134155

200180160140120100806040200Energy from ZPL / cm-1

200180160140120100806040200Energy from ZPL / cm-1

**

Conformer C :FES (HB)

S1←S0 transition

01T

02T

03T

04T

02T

04T

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Calculated Ground state vibrational frequencies. A & CCalculated Ground state vibrational frequencies. A & CThree lowest vibrational modes.Three lowest vibrational modes.

Vibration mode /Symmetry

Observed frequency/ cm–1

Calculated frequency/ cm–1* (dd/uu)

12.5 11.5/14

22 26/25.5

44 44/44.5

* Calculations were carried out at DFT/B3LYP/6-31+G(d) level of theory in GAUSSIAN03

T(A)

T(B)

(A)

T(A)T(B)

(A)

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Stimulated Emission Pumping (SEP)Stimulated Emission Pumping (SEP)

BBA

CA C

C B*B*

AA

A

C*

C*

BAC

Pump (20Hz)

Dump (10Hz)

inte

nsity

/ (a

rb. u

nits

)

3002001000

Energy above ZPL / cm –1

SVLF: Conformer A

SEP: Conformer A

Δt=0-2 ns

Dump10 HzUV

Pump20 Hz

UV

SEP gives specific information about low frequency ground state (S0) vibrational levels.

Prepare the molecule (conformer) with a known amount of energy.

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Stimulated Emission Pumping (SEP) Population Transfer Spectroscopy.*Stimulated Emission Pumping (SEP) Population Transfer Spectroscopy.*

BBA

CA C

CB*

B*A*

A*

A*

C*

C*

A

C

A A

AC

CB

A

AA

A

CCBAC

B

AC

Conformation-specific state preparation via SEP.

New conformer distribution

Collisional re-cooling to zero-point vibrational levels

Probe (20Hz)

AAAB

AB

CC C

C BA

B

B

B

Pump (20Hz)

Dump (10Hz)

S0

S1

Zero-point Level

CA

B

5. UV Probe

4. Collisional Cooling

3. UV Dump 2. UV Pump

Excited VibrationalLevel

B*

1. Initial Cooling

Fluorescence detection

SEP-PT Steps

1) Initial cooling.

2) Pump. (20 Hz)

3) Dump. (10 Hz)

4) Collisional re-cooling.

5) Probe. (20 Hz)

**Brian C. Dian, Jasper R. Clarkson, and Timothy S. ZwierScience 303, 1169 (2004).

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SEP Population Transfer Spectroscopy of Conformer A.SEP Population Transfer Spectroscopy of Conformer A.

inte

nsity

/ (a

rb. u

nits

)

300250200150100500

Energy above ZPL / cm –1

22

45

26

53

156

SEP- A

SEP-PT, A-->A

SEP-PT, A-->C

SEP-PT, A-->B

26 45

220Upperbound

Lowerbound

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SEP Population Transfer Spectroscopy of Conformer C.SEP Population Transfer Spectroscopy of Conformer C.

There is no B SEP-PTS

inte

nsity

/ (a

rb. u

nits

)

30025020015010050

Energy above ZPL / cm –1

SEP-PT, C-->C

SEP-PT, C-->A

SEP-PT, C-->B

SEP-C26

36

4549 53 156

36

36 45

45

22

Upperbound

Lowerbound

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SEP Population Transfer Spectroscopy:SEP Population Transfer Spectroscopy:Summary.Summary.

Isomerization reaction

Measured barrier(Lower and Upper

bounds) / cm‒1

A → C 26-45

A → B 0-22

C → A 36-45

C → B 36-45

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Calculated Potential Energy Surface, Ground State (SCalculated Potential Energy Surface, Ground State (S00).).

B3LYP / 6-311G(d,p)

Relative energies:

dd = 0 cm-1

uu = 5 cm-1

ud = du = 11 cm-1

TT

uu

ud

du

dd

120

120

190

190 180

180 180

180

120

120

ud

du

180

180 190

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Minimum Energy Isomerization Pathway.Minimum Energy Isomerization Pathway.

Lowest energy isomerization pathway lies along the (asymmetric torsion) coordinate.T

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Summary.Summary.

b4HPM has three conformations in the jet expansion.

All three conformers are nearly isoenergetic.

Barriers to isomerization between conformers are within 0-45 cm–1.

Theoretical calculations also indicate these unusually low barriers to isomerization.

Minimum energy pathway lies along the asymmetric torsional coordinate.

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AcknowledgementAcknowledgement

AdvisorAdvisor: : Prof. Timothy S. ZwierZwier Research GroupZwier Research Group• Dr. Christian W. Müller (Post Doctoral fellow)• William H. James III• Josh J. Newby• Josh Sebree• Evan “Duyy” Buchanan• Zachary Davis• James Redwine• Ryan Muir• Deepali Mehta

Past membersPast members• Dr. Nathan R. Pillsbury• Dr. Esteban E. Baquero• Dr. Virgil A. Shubert• Dr. Tracy A. LeGreve• Dr. Jasper R. Clarkson• Dr. Talitha Selby• Dr. Ching-Ping Liu

Collaborators:Collaborators:• Dr. David F. Plusquellic

• Prof. Lyudmila Slipchenko (PURDUE)

Computational Resources:Computational Resources:• Purdue ITap

• GridChem

Funding:Funding:• Department of Energy

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inte

nsity

(arb

. uni

ts)

368036603640

Frequency / cm-1

3659

3665

3659

Conformer A

Conformer B

Conformer C

SS00-Fluorescence Dip InfraRed (S-Fluorescence Dip InfraRed (S00-FDIR) Spectra of -FDIR) Spectra of bb4HPM.4HPM.Hydroxyl (Hydroxyl (-OH-OH) Stretch Region ) Stretch Region

*Fujimaki, E.,Fujii, A., Ebata, T., and Mikami N.; J. Chem. Phys. 2000, 112,137

Frequency calculations were carried out at DFT/M05-2X/6-31+G(d) level of theory (GAUSSIAN03 Rev E.01)

p-Cresol OH Stretch Frequency* (3658)

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uu / 5 cm-1

ud/ 11 cm-1

dd / 0.0 cm-1

Go Back

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Exciton Splitting.Exciton Splitting.

para-cresol* origin at 35338 cm–1

b4HPM origin at 35185 cm–1

* Oikawa, A.; Haruo, A.; Mikami, N.; Ito, M. J. Phys. Chem. 1984,88, 5180.

AA BB