conformation-specific electronic spectroscopy of jet-cooled 5-phenyl-1-pentene
DESCRIPTION
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. 5-phenyl-1-pentene. 5-phenyl-1-pentyne. - PowerPoint PPT PresentationTRANSCRIPT
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
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.
What differences arise from replacing the ethynyl group with a vinyl group?
Schematic Diagram of TOF Mass Spectrometer
pulsed valve
laser port
to roughing
pump
diffusion pump
cryocoolerto
roughing pump
2 stage ion acceleration
pneumatic gate valve
Einzel lens
steering plates
manual gate
valve mass gate pulser
ground plate
microchannel plate detector
5-phenyl-1-pentene * (S1)
5-phenyl-1-pentene (S0)
5-phenyl-1-pentene + + e-
Resonant Two-Photon Ionization Spectroscopy (R2PI)
•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
Sig
nal
39000385003800037500Wavenumbers (cm-1)
R2PI of 5-phenyl-1-pentene
Records the UV spectrum of a single conformationfree from interference from others present in the expansion
Laser Timing
50-500nsec
UVHole-burn
UVprobe
UV-UV Hole-burning spectroscopy
UV Hole-burn laser fixed: Provides selectivity UV probe laser tuned
Boltzmann distributionof conformers in the pre-expansion
Collisional cooling to zero-point vibrational level
B*
B*B*C
AB*
C
CB A
A
AC
AABC C
AAB BBB B
B B
UV
UV
C
5-phenyl-1-pentene * (S1)
5-phenyl-1-pentene (S0)
5-phenyl-1-pentene + + e-
Hol
e-bu
rn
Pro
be
Conformer A Conformer B
Hol
e-bu
rn
Pro
be
Ion
Sig
nal
39000385003800037500Wavenumbers (cm-1)
A
B
C
D
E
R2PI
000 60
1 120 & 1801 1
UV-UV Hole-burning Spectra
0.77 kcal/mole 1.64 kcal/mole
gauche-anti-eCgauche-anti-eH’
0.99 kcal/mole
anti-gauche-eH
Dihedral Angle Definitions
2 = C(1)-Cα-Cβ-Cγ
3 = Cα-Cβ-Cγ-Cδ
4 = Cβ-Cγ-Cδ-Cε
0.0 kcal/mole 0.41 kcal/mole 0.68 kcal/mole
anti-anti-eH
Cα Cβ
Cγ
C(1)
Cδ
Calculated Structures and Relative Energies
0.80 kcal/mole
anti-anti-eC
Cε
2 (3) = 1800 = anti2 (3) = ±600 = gauche4 = 00 = eC (eclipsed with Cβ) 4 = 1200 = eH4 = -1200 = eH’
Dihedral Labels
gauche-anti-eH
HH’
anti-gauche-eH’
Ion
Sig
nal
37580375603754037520Wavenumbers (cm-1)
A
B
C
Vib A/B
Vib C
D
E
Origin Region of 5-phenyl-1-pentene
gauche ( anti (
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.
Ion
Sign
al
-60x103 -20 0 20 40 60MHz
A
B
C
D
E
Rotational Band Contours of Origins A-E
Ion
Sign
al
-60x103 -40 -20 0 20 40 60MHz
A
B
C
D
E
Rotational Band Contour Fits
ExperimentalBest Fit
0:36:64
28:16:56
0:12:88
67:0:33
16:48:36
%A:%B:%C
Structural Assignments
B
C
D
E
Vib A/B
Vib C
ga(eC)
ga(eH’)
ag(eH)
aa(eH)
ga(eH)
A
ag(eH’)
Ion
Sig
nal
37580375603754037520Wavenumbers (cm-1)
ag 0 cm-1
gg -23
ga -63
5-phenyl-1-pentyne (37601 cm-1)
aa(eH) 0 cm-1
ag(eH)(eH’) -3
ga(eH’) -54
ga(eH) -62
ga(eC) -68
5-phenyl-1-pentene (37580 cm-1)
Comparison of Electronic Frequency Shifts
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
Ion
Sig
nal
39000385003800037500Wavenumbers (cm-1)
A
B
C
D
E
R2PI
62ns
51ns
75ns
83ns
80ns
83ns
96ns
88ns
87ns
85ns
56ns
73ns
67ns
35ns
76ns
13ns
14ns
14ns
14ns
11ns
12ns
10ns
10ns
14ns
12ns 7ns
7ns
Lifetime Study of 5-phenyl-1-pentene
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
4020040000398003960039400392003900038800386003840038200380003780037600
78
72
80
70
70
75
82
76
67
56
S1 Lifetimes of 5-phenyl-1-pentyne (nsec)~80 ~50
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
Hopkins, J. B.; Powers, D. E.; Smalley, R. E. J. Chem. Phys. 1980, 72, 5039.
Optically active ring modes of mono-substituted alkylbenzenes
S0
S1
Zero-point levelCB A
IV. UV Probe, 3II. UV Dump, 2 I. UV Pump, 1
Excited vibrational
LevelA*
III. Collisionalcooling,isomerization
SEP-Population Transfer Spectroscopy
Resonant Ion-Dip Infrared Spectroscopy (RIDIRS)
Hydrocarbon *(S1)
Hydrocarbon+ + e-
Hydrocarbon (A) (S0)
Records IR spectrum of single species free from interference from others present in the expansion
Hydrocarbon *(S1)
Hydrocarbon+ + e-
Hydrocarbon (A) (S0)
S0 RIDIRS S1 RIDIRS
S0 RIDIRS Spectra of A-EIo
n Si
gnal
31503100305030002950290028502800Wavenumbers
A
B
C
D
E
S1 RIDIRS Spectra of A-EIo
n Si
gnal
31503100305030002950290028502800Wavenumbers
A
B
C
D
E