Electronic Spectroscopy of DHPH Revisited: Potential Energy Surfaces

along Different Low Frequency Coordinates

Leonardo Alvarez-Valtierra and David W. PrattDepartment of ChemistryUniversity of PittsburghPittsburgh, PA 15260

9,10-Dihydrophenathrene (DHPH)

Low-Frequency Modes in Molecules

• Low frequency modes are the major contributors to the entropy of a system.

kT• They promote vibrational energy flow in molecules.

• The density of low-frequency vibrational states is huge!

• They play an important role coupling higher electronic states in molecules.

Why are they important…?

~21.7° ~8.4°

c

b

S0 c

b

S1

Theoretical DHPH structures

MP2/6-31G** CIS/6-31G

Theoretical Study of Low Frequency Vibrations in DHPH

HF/6-31G** CIS/6-31G

Ab initio calculations have revealed the existence of the following low frequency (< 350 cm-1) vibrational modes in both S0 and S1 electronic states of DHPH.

LIF Spectrum of 9,10-Dihydrophenanthrene (DHPH)

The “a” progressionThe “b” progressionThe “c” progression

Some Rotationally Resolved Electronic Spectra of the “a” Progression.

34155.7 34157.6Frequency (cm-1)

+487 (a5)

~0.1 cm-1~0.1 cm-1

33961.2 33963.6Frequency (cm-1)

+293 (a3)

Inertial Parameters of the High Resolution Fits

Parameter +293 +487

A"/MHz 1526.3 (1) 1526.1 (1)

B"/MHz 545.5 (1) 545.5 (1)

C"/MHz 412.6 (1) 412.6 (1)

ΔI"/amu*Å2 -32.5 (1) -32.5 (1)

ΔA/MHz -35.6 (1) -35.8 (1)

ΔB/MHz 0.4 (1) 0.1 (1)

ΔC/MHz -6.4 (1) -6.2 (1)

ΔI'/amu*Å2 -20.5 (1) -21.1 (1)

Experimental Inertial Defects in S1

Excited electronic state inertial defect (ΔI')related to the ring twisting angle (φ)?

c

b φ

Transition ∆I’/amu*Å2

DHPH+0 (origin) -18.7 (1)

DHPH+98 -19.0 (1)

DHPH+196 -19.6 (1)

DHPH+293 -20.5 (1)

DHPH+390 -20.8 (1)

DHPH+487 -21.1 (1)

Theoretical Model to Predict Inertial Defect Values in both, S0 and S1

Inertial defect vs. Inversion angle (S0)

-100

-80

-60

-40

-20

0

0 10 20 30 40f (deg)

Transition ∆I”/amu*Å2

DHPH+0 (origin) -32.3 (1)

DHPH+98 -32.2 (1)

DHPH+196 -32.2 (1)

DHPH+293 -32.5 (1)

DHPH+390 -32.4 (1)

DHPH+487 -32.5 (1)

S0 (HF/6-31G**) S1 (CIS/6-31G*)

Experimental values

Inertial defect vs. Inversion angle (S1)

-25

-23

-21

-19

-17

3 5 7 9 11 13 15 17

f (deg)

Iner

tial

def

ect

(am

u Ǻ

2 )

Iner

tial

def

ect

(am

u Ǻ

2 )

Mode Assignment and Potential Energy Surfaces

S0

S1

Q2 (φ/deg)21.58.5

Highest intensitytransition

φ

“Symmetric ring twisting mode”

Theory ν = 83.7 cm-1

Experimental* v = 97.5 cm-1

Theory ν = 140.1 cm-1

Experimental** v = 104.0 cm-1

* This work.** J. M. Smith and J. L. Knee. J. Chem. Phys. 99(1), 1993, 38.

V(Q2)

Some Rotationally Resolved Electronic Spectra of the “b” Progression.

34192.6 34193.5Frequency (cm-1)

+523 (b3)

34384.3 34385.1Frequency (cm-1)

+714 (b5)

~0.1 cm-1 ~0.1 cm-1

~0.03 cm-1

Some Rotationally Resolved Electronic Spectra of the “c” Progression.

34395.5 34397.6Frequency (cm-1)

+727 (c5)

34300.0 34301.7Frequency (cm-1)

+631 (c4)

145 MHz

27 MHz ~0.03 cm-1

Transition “b” Progression “c” Progression

∆I’/amu*Å2

+427 (b2) -19.7 (1)

+523 (b3) -20.0 (1)

+535 (c3) -20.0 (1)

+619 (b4) -20.2 (1)

+631 (c4) -20.3 (1) -21.6 (1)

+714 (b5) -21.1 (1)

+727 (c5) -20.5 (1) -21.3 (1)

Important observations:

- Inertial defect values in S1 follow similar trend as in the “a” progression (but less steep).

Experimental Inertial Defects in S1

- In the “c” progression, the c3 inertial defect follows the trend of the red- shifted c4 and c5 subbands.

- On the other hand, the blue-shifted subbands in c4 and c5 manifest the opposite behavior.

Symmetric Antisymmetric

α 2

γ 5

β 3

Vib. Mode

Mode Assignments for the “b” Progression

Assignments corrected from the experimental studies performed in the ground

electronic state by Disperse Fluorescence Spectroscopy*

*Zgierski et al. J. Chem. Phys. 96(10), 1992, 7229.

α β γ

Separation (cm-1)

Mode Assignments for the “c” Progression

Vib. Mode

6

2

6 2

Potential Energy Surfaces

“b” progression “c” progression

Q3 Q5

V(Q2,Q3,Q5)

= 2650 ± 50 cm-1

P o

t e

n t

i a

l

E n

e r

g y

Conclusions

• The main “a” FC progression has been assigned to the “symmetric out-of-plane ring twisting” mode, the “b” progression to “in-plane stretching” + “in-plane bending” + “ring twisting” modes, and the “c” progression to “CH2-CH2 bridge deformation” + “ring twisting” modes.

• The c4 and c5 subband splitting is due to inversion tunneling upon the combination of the two modes involved.

• The potential barrier estimation are 2650 cm-1 (for the c4 band) and 2150 cm-1 (for the c5 band). The potential barrier decreases upon excitation of further quanta of the ring twisting mode (Q2)!

• Potential energy surfaces along different low frequency coordinates have been obtained from analyses of the experimental data for each progression of transitions.

Acknowledgements

Many thanks to:

* Dr. John Yi (WSSU) and Dr. David Borst (INTEL) for helpful contributions on the data analysis.

* To the current Pratt group members at the University of Pittsburgh.

* To the National Science Foundation (NSF) for its financial support (CHE-0615755).

* And thank YOU again, for your attention!