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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)
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!