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

Acknowledgements

• The Zwier Group• Dr. Timothy Zwier• Department of Energy (DOE)

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