This document summarizes a laser-induced fluorescence study of the photodissociation dynamics of α,β-enones. Three systems were studied: acrylic acid, enolic acetylacetone, and enolic 1,2-cyclohexanedione. The experiments were conducted under collision-free conditions using a dye laser excitation source. Product state distributions, kinetic parameters, and dissociation mechanisms were determined. For all systems, maximum available energy was partitioned into relative translation of the photofragments. Spin-orbit ratios and Λ-doublet ratios provided insight into dissociation pathways. A hybrid model involving both impulsive and exit-barrier dynamics best explained the experimental energy distributions. Substituent effects and excited state

1. BHABHA ATOMIC RESEARCH CENTRE
Trombay, Mumbai

2. Photodissociation Dynamics of α, β -
enones: A Laser Induced
Fluorescence Study
Hari P. Upadhyaya
Radiation & Photochemistry Division
Bhabha Atomic Research Centre
Trombay, Mumbai – 400 085

3. Motivation
 Available energy distribution among the
photoproducts: A complete picture
 Dynamics of dissociation of various VOC
containing OH moiety
 Mechanism of generation of OH radical
and its kinetics in atmosphere
 Effect of various substituents and
Hydrogen bonding in the
Photodissociation process
 Validation of various models, theoretical
data

4. α, β - enones
+C C
OC
C C
O
C

5. Systems Studied
1. Acrylic Acid
2. Enolic Acetyl Acetone
3. Enolic 1,2 – cyclohexanedione
1
1
2
1
2
3
3
4
4
2
3
4

6.
Capacitance
manometer
iris
iris
Reagent
Photodiode
Photodiode
Vacuum
Nd:YAG
DYE LASER
Electronics
Delay gen.
Boxcar
Oscilloscope
computer
T
R
I
G
G
E
R
Excimer
LASER
ν 2 ν
PM
T
Attenuator
Attenuator
Signal
Pressure ≈ 10 m torr
EXPERIMENTAL SETUPEXPERIMENTAL SETUP

7. 1. Collision-free Condition
2. Monophotonic Process
3. No Saturation of
Transition
Excitation Spectra of OH
A-X (0,0)

8. Analysis of Experimental
Results
Doppler Profile
Determination of partitionining
of available energy among
the photofragments
Validation of different
Theoretical models
Elucidation of Dissociation Mechanism
Detection of OH
Rot. population
distribution in
diff. vibrational
levels
Rot. and vib. energy
of the products
Translational energy
of the photoproducts
Spin-orbit and
Lambda Doublet
ratios
Computational Methods

9. Acrylic Acid
s-cis, syn

10. Acrylic Acid at 193 nm
Results
307.5 308.0 308.5 309.0
0.0
0.5
1.0
1.5
2.0
2.5
3.0
Q21
(2)
Q21
(1)
Q1
(6)
P1
(2)
Q1
(5)
R2
(1)
Q1
(4)
P1
(1)
Q1
(3)
R2
(2)
Q1
(2)
Q1
(1)
R2
(3)
R2
(4)
LIFintensity(a.u.)
Excitation wavelength (nm)
Excitation Spectra
A-X (0,0)

11. Boltzmann plot
0 200 400 600 800
-21
-20
-19
-18
-17
ln(population)/(2J+1))
Rotational energy (cm
-1
)
Rotational temp of
OH(ν″= 0) is 460 ± 50 K
1 2 3 4 5 6
-1
0
1
2
3
4
-1-1
00
11
22
33
44
Π3/2
/Π1/2
×N/(N+1)
Π(A″)/Π(A′)
N
The statistically weighted
spin-orbit ratios and Λ-
doublet ratio

12. -0.4 -0.2 0.0 0.2 0.4
0
1
2
3
0.07cm-1
0.35 cm
-1
LIFintensity(a.u.)
Doppler shift (cm
-1
)
Doppler profile of the Q1 (4) line in the
spectrum for
193 nm photodissociation of Acrylic Acid
Translational
energy in
C.M. frame
At 193 nm
12.5 ± 3.1
kcal/mol
fT = 0.25
ETrans (Impulsive)
fT = 0.53

13. 0.0 5.0x10
-7
1.0x10
-6
1.5x10
-6
2.0x10
-6
2.5x10
-6
0
2
4
6
8
Laserprofile
LIFintensity(a.u.)
pump-probe delay (s)
250 300 350 400 450 500
0.0
0.2
0.4
0.6
0.8
1.0
fluorescenceintensity/a.u.
wavelength / nm
Slow formation rate for the
OH product
Flouresence Spectra

14. Acrylic acid at 193 nm, ( π − π * ) undergoes
C−OH bond scission to generate OH(ν”, J”)
radical.
Maximum Energy is partitioned into relative
translation of the photoproducts.
 Slow formation of OH radical
Spin-orbit ratio indicates the participation
of triplet state in the dissociation process
 Fluorescence from the excited state of the
parent molecule.
 Dissociation has an exit barrier therefore
bond cleavage takes place from excited
electronic state ( but not from S2 state)
Summarizing Results

15. Acrylic Acid at 248 nm ( n - π * )
Results
0 200 400 600 800
-21
-20
-19
-18
-17
ln(population)/(2J+1))
Rotational energy (cm
-1
)
1 2 3 4 5 6
-1
0
1
2
3
4
-1-1
00
11
22
33
44
Spin-orbit Ratio 193 nm
Λ Doublet 193 nm
Λ Doublet 248 nm
Spin-orbit Ratio 248 nm
Π3/2
/Π1/2
×N/(N+1)
Π(A″)/Π(A′)
N
-0.4 -0.2 0.0 0.2 0.4
0
1
2
3 193 nm
248 nm
C
0.07 cm
-1
0.30 cm
-1
0.35 cm
-1
LIFintensity(a.u.)
Doppler shift (cm
-1
)
0.0 5.0x10
-7
1.0x10
-6
1.5x10
-6
2.0x10
-6
2.5x10
-6
0
2
4
6
8
248 nm
193 nm
Laserprofile
LIFintensity(a.u.)
pump-probe delay (s)

16. Comparison
Rotational Temperature of
460 ± 50 K
Slow formation of OH
Fluorescence from the
parent compound
ET (OH) = 12.5 ± 3.1
kcal/mol, fT = 0.25
Did not Obey Impulsive
model
248 nm193 nm
Rotational Temperature of
360 ± 50 K
Fast formation of OH
No Fluorescence from the
parent compound
ET (OH) = 10.2 ± 2.8
kcal/mol, fT = 0.54
Obey Impulsive model
Similar trend for Spin-orbit and Λ -Doublet
Ratios

17. Hybrid Model for ETrans
Dissociation having an Exit barrier
Barrier Impulsive Model
ETrans (Total) = Eimp + EStat
Experimental
Etrans modeled with
an exit Barrier of
~16 kcal/mol

18. Disociation Mechanism of
Acrylic Acid at 193 and 248 nm

19. Enolic Acetylacetone
cis-cis-cis (CCC)

20. Enolic acetylacetone at 248 and 266
nm
Results
0 1000 2000 3000 4000
-20
-18
-16
266 nm
248 nm
ln(PJ
/(2J+1))
Rotational energy (cm
-1
)
Boltzmann plots
266 nm
Rotational temp.
OH(ν″= 0): 950 ±100
K
248 nm
Rotational temp.
OH(ν″= 0): 1100
±100 K

21. 0 2 4 6 8 10 12 14
-1
0
1
2
3
266 nm
248 nm
Π3/2
/Π1/2
N/(N+1)
N
0 1 2 3 4 5 6 7
0
1
2
266 nm
248 nm
Π(A″)/Π(A′)
N
The statistically weighted
spin-orbit ratios
Λ-doublet ratio
At 266 nm and
248 nm both spin
orbit ratios and
Λ-doublet ratio
are similar

22. -0.5 0.0 0.5
0.0
0.2
0.4 λex
=248nm
Q1
(4)
LIFintensity(a.u.)
Doppler shift (cm
-1
)
Doppler profile of the Q1 (4) line in the
spectrum for
248 nm photodissociation of enolic
Acetylacetone
Translational
energy in
C.M. frame
At 266 nm
16.0 ± 2.0
kcal/mol
At 248 nm
17.3 ± 2.0
kcal/mol

23. Different optimized structures for various excited states
CIS/6-311++g**

24. Different optimized structures for various excited states

25. CONCLUSIONSCONCLUSIONS
Enolic Acetylacetone at 248 and 266 nm,
( π − π * ) undergoes C−OH bond scission to
generate OH(ν”, J”) radical predominantly.
Maximum Energy is partitioned into relative
translation of the photoproducts.
 Fast formation of OH radical
No effect of H-bonding the dissociation
process generating OH
Dissociation has an exit barrier
The hybrid model to explain the
experimental energy distribution
ETrans explained with an exit barrier of ~19
kcal mol-1
.

27. CASSCF (10,9)
6-31G(d,p)

28. Enolic 1,2 – cyclohexanedione
0.0 4.5 6.9

29. 266 nm
Rotational temps. OH(ν″= 0) are
3100 ±100 K and 900 ± 80 K
248 nm
Rotational temp of
OH(ν″= 0) is 950 ± 80 K
Boltzmann plots

30.  At 266 nm CHD has preference
for the spin orbit states .
 At 248 nm it has almost
statistically distribution.
 At 266 it has preference for Π+
(A')
 At 248 nm it has no preference
for either of doublets
states .
The statistically
weighted
spin-orbit ratios
Λ-doublet ratio

31. Doppler profile of the P1 (4) line in the
spectrum for
248 nm photodissociation of CHD
Translational
energy in
C.M. frame
At 266 nm
12.5 ± 3.0
kcal/mol
At 266 nm
12.7 ± 3.0
kcal/mol
At 266 nm

32. Computed MOs involved in the transition of
both the conformers of enolic 1,2-
Cyclohexanedione

33. Different
optimized
structures for
various excited
states of CHD

34. Potential energy curves for various excited states of
H–bonded and non–H–bonded conformer calculated as
a function of the C2–O2 bond (TD-DFT method)

35. CHD at 266 nm, 248 nm and 193 nm, undergoes
C−OH bond scission to generate OH(ν”, J”)
radical.
Maximum Energy is partitioned into relative
translation of the photoproducts.
Two types of Rotational Distribution at 266 nm.
Difference in spin orbit and Λ-doublet states at
different wavelength, namely 266 and 248 nm.
The hybrid model to explain the experimental
energy distribution.
Dissociation has an exit barrier therefore bond
cleavage takes place from excited electronic
state
 Involvement of H-bonded and non-H bonded CHD
conformers.
CONCLUSIONSCONCLUSIONS
Upadhyaya et al. J. Phys. Chem. A 117 (2013) 2415−2426

36. Acknowledgment
Dr. Awadhesh Kumar
Dr. P. D. Naik
Dr. D. K. Palit
Dr. B.N. Jagatap
XI - SDMC

38. R1(2)R2(2)
P2(2) P1(2)
Q1(2)Q2(2)
X 2
Π1/2
2
3
2
3
1
2
+
-
-
+
+
-
1.5
2.5
1.5
2.5
3.5
X 2
Π3/2
A 2
Σ+
J
J
N
N
-
-
- 3
Partial Energy Level Diagram of the A-X System of OH.Partial Energy Level Diagram of the A-X System of OH.
0.5
2.5
1.5
2.5
3.5
–
+
+
+
–
–
–
+
–
+
–
+
–
–