talitha m. selby, jasper r. clarkson, h. daniel lee, and timothy s. zwier isomer specific...

Talitha M. Selby, Jasper R. Clarkson, H. Daniel Lee, and Timothy S. Zwier

Isomer Specific Spectroscopy and Conformational Energetics of ortho-, meta-,

and para-Ethynylstyrenes60th Annual International Symposium on Molecular Spectroscopy

Funding by DOE

Diane Mitchell, James A. J. Fitzpatrick, and David W. Pratt

J. Phys. Chem. A, 2005, 109, 4484

Introduction: EthynylstyrenesI. Structural-Isomer Specific Spectroscopy

C4H2* +

Robinson, A. G.; Winter, P. R.; Zwier, T. S. J. Phys. Chem. A 2002, 106, 5789.

II. Conformational-Isomer Specific Spectroscopy and Dynamics

?

Ea=?

E=?

Ea=?

E=?

C10H8

cistranscistrans

ortho meta

orthometa para

Techniques

Resonance enhanced two photon ionization (R2PI)

Ultraviolet holeburning spectroscopy (UV HB)

Light Sources

UV: Nd:YAG pumped dye lasers (285-310 nm) and fourth harmonic (266 nm) of Nd:YAG

Ionization continuum

1CR2PI


S1

S0

2CR2PI

S1

S0

UV HB

S1


hb (10 Hz)probe (20 Hz) tuned

S0 (v=0)

Boltzmann distributionof the vibrational

population prior toexpansion Collisional

cooling to zero-point vibrational

level

B*

B*

B*CE

B*

C

CB

A

D

A

CA

ABC C

AAE BBB B

B D UV

C

A

Boltzmann distributionof the vibrational

population prior toexpansion Collisional

cooling to zero-point vibrational

level

B*

B*

B*CE

B*

C

CB

A

D

A

CA

ABC C

AAE BBB B

B D UV

C

A

Supersonic-jet Spectroscopy

Overview R2PI Spectra of the Ethynylstyrenes

Inte

nsity

(arb

itrar

y un

its)

3520034800344003400033600332003280032400

Wavenumbers (cm-1

)

000

2910(6b1

0)251

0(110)

3210

3110(6a1

0)

3010

2010(131

0)191

0(1310)

321000

0

3110(6a1

0)

2410(121

0)

3010(6a1

0)

2610(1

10) 201

0(1310)

2810(6b1

0)

a) pES

b) mES

c) oES

000(A)

000(B)

Inte

nsity

(arb

itrar

y un

its)

3520034800344003400033600332003280032400

Wavenumbers (cm-1

)

000

2910(6b1

0)251

0(110)

3210

3110(6a1

0)

3010

2010(131

0)191

0(1310)

321000

0

3110(6a1

0)

2410(121

0)

3010(6a1

0)

2610(1

10) 201

0(1310)

2810(6b1

0)

a) pES

b) mES

c) oES

000(A)

000(B)

1CR2PI

2CR2PI

2CR2PI

000(?)

000(?)

000

Ionization Potentials: pES below 8.29 eV | mES 8.48 eV | oES 8.53-8.93 eV

UV Holeburning Spectrum of ortho-Ethynylstyrene

Only trans-oES present

cis-oES only 5% RT population

E=1.8 kcal/mol

E=2.03 kcal/mol

E=0.0 kcal/mol

3440034200340003380033600334003320033000328003260032400

Inte

nsit

y (a

rbit

rary

uni

ts)

Wavenumbers(cm-1)

Para 00

0impurity

000

2CR2PI ortho

UVHB ortho

B3LYP/6-31+G*

c-oES has 1600 cm-1 internal energy pre-expansion

kisomerization (1600 cm-1) ~1011 s-1

kcollision early in expansion ~109-107 s-1

Rate of isomerization faster than collision rate.

UVHB Spectra of meta-Ethynylstyrenes

0.00 kcal/mol

Ea≈1200 cm-1

*B3LYP/6-31+G* level of theory

0.08 kcal/mol

B3LYP/6-31+G*

34400342003400033800336003340033200330003280032600

Wavenumbers/(cm-1)

Inte

nsit

y (a

rbit

rary

uni

ts)

A B

A(000)

B(000)

a) 2CR2PI mES

b) UVHB mES(A)

c) UVHB mES(B)

B3LYP/6-31+G*

Rotationally Resolved Fluorescence Excitation

Experimental

Simulations

32,673.232,670.5

32,672.5932,672.44Wavenumber/cm-1

Experimental

Simulations

32,673.232,670.5

32,672.5932,672.44Wavenumber/cm-1

-0.62685.2(2)982.6(2)2261(2)Experimental

′′

679.7C′′ (MHz)973.4B′′ (MHz)2252A′′ (MHz)Calc c-mESParameter

a/b hybrid423.60.3(2)-0.61679.1(2)979.4(2)2217(2)-0.1(2)

band typefwhm (MHz)OMC (MHz)I′ (amu Å2)′

C′ (MHz)B′ (MHz)A′ (MHz)I′′ (amu Å2)

653.4833.23027Calc t-mES

-0.62685.2(2)982.6(2)2261(2)Experimental

′′


a/b hybrid423.60.3(2)-0.61679.1(2)979.4(2)2217(2)-0.1(2)



653.4833.23027Calc t-mES

~90% a-type

-0.62685.2(2)982.6(2)2261(2)Experimental

′′


a/b hybrid423.60.3(2)-0.61679.1(2)979.4(2)2217(2)-0.1(2)



653.4833.23027Calc t-mES

-0.62685.2(2)982.6(2)2261(2)Experimental

′′


a/b hybrid423.60.3(2)-0.61679.1(2)979.4(2)2217(2)-0.1(2)



653.4833.23027Calc t-mES

~90% a-typea-type transitions

B3LYP/6-31+G*

red-shifted conformer of mES b

a

b

a

b

a

b

a

TDM: ~90% A type

mES (A)

Rotationally Resolved Fluorescence Excitation

Simulations

Experimental

33,408.233,405.5

33,406.6433,406.54Wavenumber/cm-1

Simulations

Experimental

33,408.233,405.5

33,406.6433,406.54Wavenumber/cm-1

a-type transitions

pES(000)

-0.96621.5(2)709.2(2)5030(2)Experimental

′′617.7C′′ (MHz)703.3B′′ (MHz)5009A′′ (MHz)CalculatedParameter

a/b hybrid424.60.5(3)-0.96615.6(2)705.7(2)4847(2)0.1(2)

band typefwhm (MHz)OMC (MHz)I′ (amu Å2)′C′ (MHz)B′ (MHz)A′ (MHz)I′′ (amu Å2)

-0.96621.5(2)709.2(2)5030(2)Experimental


a/b hybrid424.60.5(3)-0.96615.6(2)705.7(2)4847(2)0.1(2)


-0.96621.5(2)709.2(2)5030(2)Experimental


a/b hybrid424.60.5(3)-0.96615.6(2)705.7(2)4847(2)0.1(2)


*Ground state geometry at B3LYP/6-31+G* level of theory.

-0.96621.5(2)709.2(2)5030(2)Experimental


a/b hybrid424.60.5(3)-0.96615.6(2)705.7(2)4847(2)0.1(2)


-0.96621.5(2)709.2(2)5030(2)Experimental


a/b hybrid424.60.5(3)-0.96615.6(2)705.7(2)4847(2)0.1(2)


-0.96621.5(2)709.2(2)5030(2)Experimental


a/b hybrid424.60.5(3)-0.96615.6(2)705.7(2)4847(2)0.1(2)


*Ground state geometry at B3LYP/6-31+G* level of theory.

~90% a-type

a

b

a

b

TDM: ~90% A type

pES

B3LYP/6-31+G*

Stimulated Emission Pumping –Population Transfer Spectroscopy

II. UV

Pum

p, pum

p

V. U

V P

robe, probe

III. UV

Dum

p,

dump

IV. C

ollisional

Cooling,

isomerization

kisom

kcool

kcool

Dian, B. C.; Clarkson, J. R.; Zwier, T. S. Science 2004, 303, 1169.

A(v=0)

A*

B(v=0)

000(A) 00

0(B)A*

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

Population transfer spectroscopy: Fix Pump on A, Probe on B; Tune Dump: Watch population come into B from A

AB

AABB A

AAA BBB B

A B

BB B

A*A*

AA*A

BA

AB

BA

AB

BB

B

Boltzmann distribution of conformers in the

pre-expansiongas mixture

UV Pump,Dump

UV probe

SEP excites single conformation

Collisional cooling to zero-point

vibrational level

New conformer distribution

detected by UV

Initial Cooling in Expansion

B B

AB

AABB A

AAA BBB B

A B

BB B

A*A*

AA*A

BA

AB

BA

AB

BB

B

Boltzmann distribution of conformers in the

pre-expansiongas mixture

UV Pump,Dump

UV probe

SEP excites single conformation

Collisional cooling to zero-point

vibrational level

New conformer distribution

detected by UV

Initial Cooling in Expansion

B B

I. CoolingPrepare ground state A with a well defined amount of energy

1300120011001000

upper bound (1070 cm

-1)

lower bound

(990 cm-1)


-1)

lower bound

(989 cm-1)

a)

b)

c)

d)

Wavenumbers above ZPL (cm-1)

1300120011001000


-1)

lower bound

(990 cm-1)


-1)

lower bound

(989 cm-1)

1300120011001000 1300120011001000


-1)

lower bound

(990 cm-1)


-1)

lower bound

(989 cm-1)

a)

b)

c)

d)

Wavenumbers above ZPL (cm-1)

cis-ortho trans-ortho cis-meta trans-meta para cis-ortho trans-ortho cis-meta trans-meta para

a) SEP of cis-meta

d) SEP-PTS trans cis

b) SEP-PTS cis trans


SEP and SEP-PTS of mES

c) SEP of trans -meta

Near-threshold population transfer intensity determined by the competition

between isomerization, cooling

kisom(E)

kcool(E)


Harmonic RRKM estimate: At threshold, kisom(E) = 2.6X109 sec-1 and kcoll = 1.0X109 sec-1


Bounds on the barrier and relative energies of minima

Compound Relative Energy (cm-1) Exp Calc c-m-ethynylstyrene -75 -81a +29c t-m-ethynylstyrene 0a 0c Barrier to cis-trans isomerization (cm-1) Exp Calc m-ethynylstyrene 990-1070a 1237c Styrene 1070±8b 1350c

cis-ortho trans-ortho cis-meta trans-meta para cis-ortho trans-ortho cis-meta trans-meta para cis-ortho trans-ortho cis-meta trans-meta para cis-ortho trans-ortho cis-meta trans-meta para

B A

E(A→B)E(B→A)

E=E(A→B) - E(B→A)

Hollas, J. M.; Musa, H.; Ridley, T.; Turner, P. H.; Weisenberger, K. H.; Fawcett, V. J. Mol. Spectrosc. 1982, 94, 437

Comparison of Methods

•Also gives the form of the entire potential energy function along the torsional coordinate.

•Requires spectroscopic detection of the torsional energy levels

•Assumes the torsional coordinate is the only coordinate involved in isomerization

Torsional Potential Fitting


•Also gives the form of the entire potential energy function along the torsional coordinate.

•Requires spectroscopic detection of the torsional energy levels

•Assumes the torsional coordinate is the only coordinate involved in isomerization

•Not reliant on assignment of normal mode to reaction coordinate

•Relies on the spacing of the SEP transitions, but yes/no question

•Relies on isomerization occurring on a time scale that can successfully compete with collisional cooling

•Apply to cases where many conformers: Breaks into specific A→B pairs

Torsional Potential Fitting


SEP-PTS

Summary of the Ethynylstyrenes

• Only one isomer of oES was observed in the expansion. From calculated energy differences the observed conformer was assigned to the trans conformer.

• Two isomers of mES were observed. The red-shifted conformer was identified as the cis conformer from the rotationally resolved fluorescence excitation spectrum.

• The barrier to cis→trans isomerization in mES is ~1000 cm-1 and the two conformations are nearly isoenergetic, in qualitative agreement with calculations

Acknowledgments

Prof. Timothy S. Zwier The Zwier Group

-Jasper R. ClarksonH. Daniel Lee

Funding: Department of Energy

Prof. David W. PrattThe Pratt Group

-Diane Mitchell-James A. J. Fitzpatrick

UVHB spectrum of trans-ortho-Ethynylstyrene

Evidence of vibronic coupling

•Intensity of transitions

•No overtones

•False origin

Wavenumbers(cm-1)

3440034200340003380033600334003320033000328003260032400

3010(20)1

0

Inte

nsit

y (a

rbit

rary

uni

ts)

000

3110

2010

3010

3110(20)1

0

No 3120 or 302

0

Wavenumbers(cm-1)

3440034200340003380033600334003320033000328003260032400

3010(20)1

0

Inte

nsit

y (a

rbit

rary

uni

ts)

000

3110

2010

3010

3110(20)1

0

No 3120 or 302

0

S0

S1(A′)

S2(A′)

small oscillator strength

coupled by a′vibrations

large oscillator strength

S1(A′)←S0(A′)

Nature of the S0-S1 Transitions:

Transition Dipole Moment Directions

a

b

a a

a

a

aa

a

a b

styrene and phenylacetylene

divinylbenzenes

ethynylstyrenes

diethynylbenzenes

a

b

a a

a

a

aa

a

a b

styrene and phenylacetylene

divinylbenzenes

ethynylstyrenes

diethynylbenzenes

The direction of the TDM in disubstituted benzenes depends both on the nature of the substituents and their relative positions.

•All the meta disubstituted benzenes shown have the TDM along the a-axis.

•In para disubstituted benzenes it appears the nature of the substituents does matter for the TDM direction.

•In pES, the vinyl group has a larger influence on the transitionT.V. Nguyen, J.W. Ribblett, and D.W. Pratt, Chem. Phys 283,279,2002

J.A. Stearns and T.Z. Zwier, J Phys Chem. A, 107,10723,2003

Ribblett, J. W.; Borst, D. R.; Pratt, D. W. J. Chem. Phys. 1999, 111, 8454.Stearns, J. A.; Zwier, T. S. J. Phys. Chem. A 2003, 107, 107117

Wavenumbers(cm-1)

diethynylbenzenes

ethynylstyrenes

divinylbenzenes

360003500034000330003200031000

styrene

phenylacetylene

Wavenumbers(cm-1)

diethynylbenzenes

ethynylstyrenes

divinylbenzenes

360003500034000330003200031000 360003500034000330003200031000

styrene

phenylacetylene

Electronic Origin Shifts in Vinyl and Ethynyl Substituted Benzenes

•Electronic origin shifts are additive

•Trends in ortho, meta, para

• Trends in cis and trans mDVB and mES

•As substituents become closer together they further red shift

T.V. Nguyen, J.W. Ribblett, and D.W. Pratt, Chem. Phys 283,279,2002

J.A. Stearns and T.Z. Zwier, J Phys Chem. A, 107,10723,2003

J.A. Syage, F. Al Adel, and A.H. Zewail, Chem Phys Lett. 103,15,1983

K. Narayanan, G.C. Chang, K.C. Shieh, C.C. Tung, and W.B. Tzeng, Spectochim Acta A. 52,1703,1996

talitha m. selby, jasper r. clarkson, h. daniel lee, and timothy s. zwier isomer specific...

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c e c c b

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vibrational level b

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b tune dump

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