Transcript t1.ppt

Isomer Specific Spectroscopy and
Conformational Energetics of ortho-, meta-,
and para-Ethynylstyrenes
J. Phys. Chem. A, 2005, 109, 4484
60th Annual International Symposium on Molecular Spectroscopy
Talitha M. Selby, Jasper R. Clarkson, H. Daniel Lee, and Timothy S. Zwier
Diane Mitchell, James A. J. Fitzpatrick, and David W. Pratt
Funding by DOE
Introduction: Ethynylstyrenes
I.
Structural-Isomer Specific Spectroscopy
C10H8
?
C H* +
4 2
ortho
II.
meta
para
Conformational-Isomer Specific Spectroscopy and Dynamics
ortho
trans
meta
cis
trans
cis
Ea=?
Ea=?
D E=?
D E=?
Robinson, A. G.; Winter, P. R.; Zwier, T. S. J. Phys. Chem. A 2002, 106, 5789.
Supersonic-jet Spectroscopy
B AD
C
CA
C E B BB
A
B A A
B
UV
C D C E B*
B B*C B*
A
A
C B*
Boltzmann distribution
of the vibrational
population prior to
expansion
Collisional
cooling
to zero-point
vibrational
level
1CR2PI
2CR2PI
UV HB
Ionization
continuum
Ionization
continuum
S1
S1
S1
S0
S0
Ionization
continuum
S0 (v=0)
lhb (10 Hz)
lprobe (20 Hz) tuned
Light Sources
Techniques
UV: Nd:YAG pumped dye lasers
(285-310 nm) and fourth harmonic
(266 nm) of Nd:YAG
Resonance enhanced two photon ionization
(R2PI)
Ultraviolet holeburning spectroscopy (UV HB)
Overview R2PI Spectra of the Ethynylstyrenes
a) pES
000
Intensity (arbitrary units)
1CR2PI
2510(110)
1910(1310)
2910(6b10)
2010(1310)
3010
3110(6a10)
3210
b) mES
(B)
000000(?)
00000 (A)
0
2CR2PI
c) oES
1
31100(6a
0 (?)0)
0
000
32400
3010(6a10)
2CR2PI
1
1
2810(6b10) 26 0(1 0)
3210
32800
2010(1310)
2410(1210)
33200
33600
34000
34400
34800
35200
-1
Wavenumbers (cm )
Ionization Potentials: pES below 8.29 eV | mES 8.48 eV | oES 8.53-8.93 eV
UV Holeburning Spectrum of ortho-Ethynylstyrene
Intensity (arbitrary units)
2CR2PI ortho
Para
000impurity
Only trans-oES present
000
cis-oES only 5% RT
population
UVHB ortho
32400
32600
32800
33000
33200
33400
33600
33800
34000
34200
34400
Wavenumbers(cm-1)
D E=2.03 kcal/mol
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.
E=1.8 kcal/mol
B3LYP/6-31+G*
E=0.0 kcal/mol
UVHB Spectra of meta-Ethynylstyrenes
Intensity (arbitrary units)
a) 2CR2PI mES
B
A
b) UVHB mES(A)
A(000)
c) UVHB mES(B)
B(000)
32600
32800
33000
33200
33400
33600
33800
34000
Wavenumbers/(cm-1)
B3LYP/6-31+G*
Ea≈1200 cm-1
0.08 kcal/mol
0.00 kcal/mol
*B3LYP/6-31+G* level of theory
B3LYP/6-31+G*
34200
34400
Rotationally Resolved Fluorescence Excitation
b
red-shifted conformer of mES
mES (A)
a
TDM: ~90% A type
32,670.5
32,673.2
Experimental
Simulations
a-type
transitions
32,672.44
32,672.59
Wavenumber/cm-1
Parameter
A′′ (MHz)
B′′ (MHz)
C′′ (MHz)
′′
DI′′ (amu Å2)
A′ (MHz)
B′ (MHz)
C′ (MHz)
′
DI′ (amu Å2)
OMC (MHz)
fwhm (MHz)
band type
Experimental Calc c-mES
2261(2)
2252
982.6(2)
973.4
685.2(2)
679.7
-0.62
-0.1(2)
2217(2)
979.4(2)
679.1(2)
-0.61
0.3(2)
3.6
42
~90%
a-type
a/b
hybrid
B3LYP/6-31+G*
Calc t-mES
3027
833.2
653.4
Rotationally Resolved Fluorescence Excitation
a
pES(00
0)
pES
b
33,405.5
33,408.2
Experimental
Simulations
a-type transitions
33,406.54
33,406.64
Wavenumber/cm-1
TDM: ~90% A type
Parameter
A′′ (MHz)
B′′ (MHz)
C′′ (MHz)
′′
DI′′ (amu Å2)
A′ (MHz)
B′ (MHz)
C′ (MHz)
′
DI′ (amu Å2)
OMC (MHz)
fwhm (MHz)
band type
Experimental
5030(2)
709.2(2)
621.5(2)
-0.96
0.1(2)
4847(2)
705.7(2)
615.6(2)
-0.96
0.5(3)
4.6
42
~90%
a-type
a/b
hybrid
Calculated
5009
703.3
617.7
*Ground state geometry at B3LYP/6-31+G* level of theory.
Stimulated Emission Pumping –Population Transfer
Spectroscopy
Expt’l protocol:
1.
Cool
Prepare ground
2.
Pump
state A with a well
3.
Dump
defined amount of
4. Re-cool
energy
5.
Probe
I. Cooling
Initial
Cooling in Expansion
Collisional cooling
UV Pump, to zero-point
A AB
Dump
vibrational level
B AA
B B
B AA
B
B ABA
B
A
kisom
A*
A(v=0)
kcool
kcool
Dian, B. C.; Clarkson, J. R.; Zwier, T. S. Science
2004, 303, 1169.
000(B)
V. UV Probe, lprobe
Population transfer spectroscopy:
Fix Pump on A, Probe on B;
Tune Dump:
Watch population come
into B from A
A*
000(A)
II. UV Pump, lpump
New conformer
distribution
detected by UV
BB
AA
B B
B A
A B
B
III. UV Dump, ldump
single conformation
UV probe
IV. Collisional
Cooling,
isomerization
Boltzmann distribution
of conformers in the
pre-expansion
gas mixture SEP excites
B A*
A*
B B
B
A*
A
B(v=0)
SEP and SEP-PTS of mES
a) SEP of cis-meta
cis-meta
b)
trans-meta
b) SEP-PTS cis
para
trans
c) SEP of trans -meta
trans-meta
d) SEP-PTS trans
d)
para
cis
1000
upper bound
(1065 cm -1)
cis-meta
c)
lower bound
(989 cm -1)
ns-ortho
upper bound
(1070 cm -1)
ns-ortho
lower bound
(990 cm -1)
a)
1100
1200
1300
Wavenumbers above ZPL (cm-1)
Near-threshold population transfer
intensity determined by the competition
between isomerization, cooling
kisom(E)
kcool(E
)
trans-ortho
cis-meta
cis-ortho
trans-meta
trans-ortho
para
cis-meta
trans-meta
Harmonic RRKM estimate:
At threshold, kisom(E) = 2.6X109 sec-1 and kcoll = 1.0X109 sec-1
para
Bounds on the barrier and relative energies of minima
E(A→B)
E(B→A)
A
B
DE=E(A→B) - E(B→A)
cis-ortho
trans-ortho
cis-meta
trans-meta
cis-ortho
Compound
c-m-ethynylstyrene
t-m-ethynylstyrene
m-ethynylstyrene
Styrene
para
trans-ortho
cis-meta
trans-meta
para
Relative Energy (cm-1)
Exp
Calc
a
-75 -81
+29c
0a
0c
Barrier to cis-trans isomerization (cm-1)
Exp
Calc
a
990-1070
1237c
1070±8b
1350c
Hollas, J. M.; Musa, H.; Ridley, T.; Turner, P. H.; Weisenberger, K. H.; Fawcett, V. J. Mol. Spectrosc. 1982, 94, 437
Comparison of Methods
Comparison of Methods
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
Comparison of Methods
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
SEP-PTS
•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
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. Clarkson
H. Daniel Lee
Prof. David W. Pratt
The Pratt Group
-Diane Mitchell
-James A. J. Fitzpatrick
Funding: Department of Energy
Intensity (arbitrary units)
UVHB spectrum of trans-ortho-Ethynylstyrene
30
3110
1
S1(A′)←S0(A′)
0
No 3120 or 3020
3010(20)10
2010
00 0
3110(20)10
32400
32600
32800
33000
33200
33400
33600
33800
34000
34200
34400
Wavenumbers(cm-1)
large oscillator
strength
Evidence of vibronic coupling
•Intensity of transitions
S2(A′)
S1(A′)
•No overtones
•False origin
coupled by a′
vibrations
small oscillator
strength
S0
Nature of the S0-S1
Transitions:
Transition Dipole
Moment Directions
a
b
styrene and phenylacetylene
a
The direction of the TDM in disubstituted
benzenes depends both on the nature of
the substituents and their relative
positions.
divinylbenzenes
•All the meta disubstituted benzenes
shown have the TDM along the a-axis.
a
•In para disubstituted benzenes it appears
the nature of the substituents does matter
for the TDM direction.
ethynylstyrenes
a
•In pES, the vinyl group has a larger
influence on the transition
T.V. Nguyen,
J.W.
Ribblett,
D.W.
Pratt, D.
Chem.
283,279,2002
Ribblett,
J. W.;
Borst,
D.andR.;
Pratt,
W.Phys
J. Chem.
Phys.
J.A.
Stearns
and
T.Z.
Zwier,
J
Phys
Chem.
A,
107,10723,2003
1999, 111, 8454.
a
a
a
a
a
diethynylbenzenes
b
Electronic Origin Shifts in Vinyl and Ethynyl Substituted Benzenes
phenylacetylene
styrene
•Trends in ortho, meta,
para
diethynylbenzenes
divinylbenzenes
32000
33000
34000
35000
• Trends in cis and trans
mDVB and mES
•As substituents become
closer together they
further red shift
ethynylstyrenes
31000
•Electronic origin shifts
are additive
36000
Wavenumbers(cm-1)
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
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