Transcript Ohio State 2005 ppene.ppt

CONFORMATION-SPECIFIC ELECTRONIC
SPECTROSCOPY OF JET-COOLED 5-PHENYL-1-PENTENE
NATHAN R. PILLSBURY, TALITHA M. SELBY, AND TIMOTHY S. ZWIER,
Department of Chemistry, Purdue University, West Lafayette, IN 47907
Motivation for Studying 5-phenyl-1-pentene
5-phenyl-1-pentene
5-phenyl-1-pentyne
What differences arise from replacing the ethynyl group with a vinyl group?
Barriers to exciplex formation from
different starting structures could be
reflected in different lifetimes as a
function of energy above the origin
Ho, C. D.; Morrison, H. J. Am. Chem. Soc. 2005, 127, 2114-2124.
Schematic Diagram of TOF Mass Spectrometer
pulsed valve
ground plate
2 stage ion
acceleration
Einzel
lens
manual
gate
steering valve
plates
mass gate
pulser
laser port
pneumatic
gate valve
to
roughing
pump
cryocooler
diffusion pump
microchannel
plate detector
to
roughing
pump
Resonant Two-Photon Ionization Spectroscopy (R2PI)
5-phenyl-1-pentene + + e-
5-phenyl-1-pentene * (S1)
5-phenyl-1-pentene (S0)
•Molecules are cooled to zero
point vibrational levels in the free
jet expansion
• Mass selection gives
confirmation that the spectrum is
due to the molecule of interest
Ion Signal
R2PI of 5-phenyl-1-pentene
37500
38000
38500
-1
Wavenumbers (cm )
39000
UV-UV Hole-burning spectroscopy
Records the UV spectrum of a single conformation
free from interference from others present in the expansion
UV Hole-burn laser fixed:
Provides l selectivity
UV probe laser tuned
Conformer A
Conformer B
5-phenyl-1-pentene + + eUV
C A C A B*
B B*C B*
A
C B*
5-phenyl-1-pentene * (S1)
Hole-burn
Collisional cooling
to zero-point
vibrational level
50-500
nsec
5-phenyl-1-pentene (S0)
UV
UV
Hole-burn probe
Laser Timing
Probe
UV
Hole-burn
Boltzmann distribution
of conformers in the
pre-expansion
Probe
B AB
C
CA
C B B BB
B AA A
B
UV-UV Hole-burning Spectra
000
1
60
1
1210 & 180
R2PI
Ion Signal
A
B
C
D
E
37500
38000
38500
-1
Wavenumbers (cm )
39000
Calculated Structures and Relative Energies
0.0 kcal/mole
Cε
0.41 kcal/mole
0.68 kcal/mole
anti-gauche-eH’
gauche-anti-eH
Cδ
Cγ
H
H’
Cα
Cβ
C(1)
anti-anti-eH
0.77 kcal/mole
0.80 kcal/mole
0.99 kcal/mole
1.64 kcal/mole
gauche-anti-eH’
anti-anti-eC
anti-gauche-eH
gauche-anti-eC
Dihedral Angle Definitions
Dihedral Labels
t2 = C(1)-Cα-Cβ-Cγ
t2 (t3) = 1800 = anti
t2 (t3) = ±600 = gauche
t4 = 00 = eC (eclipsed with Cβ)
t4 = 1200 = eH
t4 = -1200 = eH’
t3 = Cα-Cβ-Cγ-Cδ
t4 = Cβ-Cγ-Cδ-Cε
Origin Region of 5-phenyl-1-pentene
A
D
E
B
Ion Signal
gauche (t2)
anti (t2)
C
Vib A/B
Vib C
37520
37540
37560
-1
Wavenumbers (cm )
37580
Transition Dipole Moment Sensitivity
The TDM in monosubstituted benzenes has been found to be very
sensitive to the nature and orientation of the substituent
According to Pratt and Simons*, the TDM in gauche conformations
swings about 30 degrees from the anti (trans) conformations
Surprisingly, CIS calculations correctly predict the transition
moment direction in these gauche structures
* Kroemer, R. T. L., K. R; Dickinson, J. A.; Robertson, E. G.; Simons, J. P.; Borst, D. R.; Pratt, D.W. J. Am.
Chem. Soc. 1998, 120, 12573.
Rotational Band Contours of Origins A-E
A
Ion Signal
B
C
D
E
3
-60x10
-20
0
MHz
20
40
60
Rotational Band Contour Fits
%A:%B:%C
Experimental
Best Fit
67:0:33
A
Ion Signal
0:36:64
B
0:12:88
C
28:16:56
D
16:48:36
E
3
-60x10
-40
-20
0
MHz
20
40
60
Structural Assignments
A
aa(eH)
ga(eC)
ag(eH’) D
ga(eH)
E
B
Ion Signal
ag(eH)
C
ga(eH’)
Vib A/B
Vib C
37520
37540
37560
-1
Wavenumbers (cm )
37580
Comparison of Electronic Frequency Shifts
5-phenyl-1-pentyne (37601 cm-1)
5-phenyl-1-pentene (37580 cm-1)
ag
0 cm-1
aa(eH)
0 cm-1
gg
-23
ag(eH)(eH’)
-3
ga
-63
ga(eH’)
-54
ga(eH)
-62
ga(eC)
-68
Vinyl group red shifts the spectrum by a about 20 cm-1 to the red and adds
two more conformations; however the shifts between ag and ga are held
roughly constant
Lifetime Study of 5-phenyl-1-pentene
R2PI
A
Ion Signal
62ns
56ns
13ns
14ns
14ns
83ns
B
73ns
96ns
51ns
C
75ns
11ns
67ns
88ns
12ns
12ns
D
14ns
35ns
83ns
87ns
E
85ns
80ns
37500
7ns
10ns
76ns
38000
38500
-1
Wavenumbers (cm )
14ns
39000
10ns
7ns
Possible Reasons for Lack of Conformation Specificity
• Barrier to exciplex structures is too high and therefore not probed in the FC
region
• Lifetime shortening may be determined by something other than exciplex
formation (e.g. internal conversion or intersystem crossing)
• IVR may be fast relative to isomerization
S1 Lifetimes of 5-phenyl-1-pentyne (nsec)
~80
~50
82
76
75
78
70
72
70 67
80
56
37600
37800
38000
38200
38400
38600
38800
39000
39200
39400
39600
Future Work
Dispersed Fluorescence
Shows where strong SEP transitions are located
May show conformation-specific IVR effects
Probe isomerization in the S1 state (may see a difference between gauche vs. anti)
SEP-Population Transfer Spectroscopy
Measures the barriers to isomerization experimentally
Jasper Clarkson
Talitha Selby
Acknowledgements
• The Zwier Group
• Dr. Timothy Zwier
• Department of Energy (DOE)
Optically active ring modes of mono-substituted alkylbenzenes
Hopkins, J. B.; Powers, D. E.; Smalley, R. E. J. Chem. Phys. 1980, 72, 5039.
SEP-Population Transfer Spectroscopy
II. UV Dump, l2
A*
III. Collisional
cooling,
isomerization
IV. UV Probe, l3
I. UV Pump, l1
S1
S0
B
A
C
Excited
vibrational
Level
Zero-point
level
Resonant Ion-Dip Infrared Spectroscopy (RIDIRS)
Records IR spectrum of single species free from interference
from others present in the expansion
S0 RIDIRS
Hydrocarbon+ + e-
S1 RIDIRS
Hydrocarbon+ + e-
Hydrocarbon *(S1)
Hydrocarbon *(S1)
Hydrocarbon (A)
(S0)
Hydrocarbon (A)
(S0)
S0 RIDIRS Spectra of A-E
A
Ion Signal
B
C
D
E
2800
2850
2900
2950
3000
Wavenumbers
3050
3100
3150
S1 RIDIRS Spectra of A-E
A
Ion Signal
B
C
D
E
2800
2850
2900
2950
3000
Wavenumbers
3050
3100
3150