Hari talk sdmc_puri_final
-
Upload
nitish-pal -
Category
Science
-
view
17 -
download
0
Transcript of Hari talk sdmc_puri_final
BHABHA ATOMIC RESEARCH CENTRE
Trombay, Mumbai
Photodissociation Dynamics of -
enones: A Laser Induced
Fluorescence Study
Hari P. Upadhyaya
Radiation & Photochemistry DivisionBhabha Atomic Research Centre
Trombay, Mumbai – 400 085
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
- enones
+C C
OC
C C
O
C
Systems Studied
1. Acrylic Acid
2. Enolic Acetyl Acetone
3. Enolic 1,2 – cyclohexanedione
1
1
2
1
2
3
3
4
4
23
4
Capacitance manometer
iris
iris
Reagent
Photodiode
Photodiode
Vacuum
Nd:YAG
DYE LASER
Electronics
Delay gen.
Boxcar
Oscilloscope
computer
TRIGGER
Excimer LASER
2
PMT
Attenuator
Attenuator
Signal
Pressure 10 m torr
EXPERIMENTAL SETUPEXPERIMENTAL SETUP
1. Collision-free Condition
2. Monophotonic Process
3. No Saturation of
Transition
Excitation Spectra of OHA-X (0,0)
Analysis of Experimental Results
Doppler Profile
Determination of partitioniningof available energy amongthe photofragments
Validation of different Theoretical models
Elucidation of Dissociation Mechanism
Detection of OHRot. 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
Acrylic Acid
s-cis, syn
Acrylic Acid at 193 nm
Results
307.5 308.0 308.5 309.00.0
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)
L
IF in
ten
sity
(a.
u.)
Excitation wavelength (nm)
Excitation SpectraA-X (0,0)
Boltzmann plot
0 200 400 600 800-21
-20
-19
-18
-17
ln
(po
pu
lati
on
)/(2
J+1)
)
Rotational energy (cm-1)
Rotational temp of OH(= 0) is 460 ± 50 K
1 2 3 4 5 6-1
0
1
2
3
4
-1-1
00
11
22
33
44
3/2/ 1/
2
N/(
N+
1)
(A)
/ (A
)
N
The statistically weighted spin-orbit ratios and -doublet ratio
-0.4 -0.2 0.0 0.2 0.4
0
1
2
3
0.07
cm
-1
0.35 cm-1
LIF
inte
nsity
(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 nm12.5 ± 3.1
kcal/molfT = 0.25
ETrans (Impulsive)
fT = 0.53
0.0 5.0x10-7 1.0x10-6 1.5x10-6 2.0x10-6 2.5x10-6
0
2
4
6
8L
aser
pro
file
L
IF in
ten
sity
(a.
u.)
pump-probe delay (s)
250 300 350 400 450 5000.0
0.2
0.4
0.6
0.8
1.0
flu
ore
scen
ce in
ten
sity
/ a.
u.
wavelength / nm
Slow formation rate for the OH product
Flouresence Spectra
Acrylic acid at 193 nm, ( * ) undergoes COH bond scission to generate OH(”, J”) radical.
Maximum Energy is partitioned into relative translation of the photoproducts.
Slow formation of OH radicalSpin-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
Acrylic Acid at 248 nm ( n - * ) Results
0 200 400 600 800-21
-20
-19
-18
-17
ln(p
op
ula
tio
n)/
(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 Doublet 193 nm Doublet 248 nm Spin-orbit Ratio 248 nm
3/2/
1/2
N/(
N+
1)
(A)
/ (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
LIF
inte
nsi
ty (
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
La
ser
pro
file
LIF
inte
nsi
ty (
a.u
.)
pump-probe delay (s)
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 nmRotational 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 -Doublet Ratios
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
Disociation Mechanism of Acrylic Acid at 193 and 248 nm
Enolic Acetylacetone
cis-cis-cis (CCC)
Enolic acetylacetone at 248 and 266
nmResults
0 1000 2000 3000 4000
-20
-18
-16
266 nm 248 nm
ln(P
J / (2
J+1)
)
Rotational energy (cm-1)
Boltzmann plots
266 nm Rotational temp. OH(= 0): 950 ±100 K
248 nm Rotational temp. OH(= 0): 1100 ±100 K
0 2 4 6 8 10 12 14-1
0
1
2
3 266 nm 248 nm
3/2/ 1/
2 N
/(N
+1)
N
0 1 2 3 4 5 6 7
0
1
2 266 nm 248 nm
(A)
/ (A
)
N
The statistically weighted spin-orbit ratios
-doublet ratio
At 266 nm and
248 nm both
spin orbit ratios
and -doublet
ratio are similar
-0.5 0.0 0.5
0.0
0.2
0.4 ex
=248nm
Q1(4)
LIF
inte
nsi
ty (
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 nm16.0 ± 2.0
kcal/mol
At 248 nm17.3 ± 2.0
kcal/mol
Different optimized structures for various excited states CIS/6-311++g**
Different optimized structures for various excited states
CONCLUSIONSCONCLUSIONS
Enolic Acetylacetone at 248 and 266 nm, ( * ) undergoes COH bond scission to generate OH(”, 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.
CASSCF (10,9) 6-31G(d,p)
Enolic 1,2 – cyclohexanedione
0.0 4.5 6.9
266 nm Rotational temps. OH(= 0) are
3100 ±100 K and 900 ± 80 K
248 nm Rotational temp of
OH(= 0) is 950 ± 80 K
Boltzmann plots
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 +(A')
At 248 nm it has no preference for either of doublets states .
The statistically weighted
spin-orbit ratios
-doublet ratio
Doppler profile of the P1 (4) line in the spectrum for
248 nm photodissociation of CHDTranslational
energy in C.M. frame
At 266 nm12.5 ± 3.0
kcal/mol
At 266 nm12.7 ± 3.0
kcal/mol
At 266 nm12.0 ± 3.0
kcal/mol
Computed MOs involved in the transition of both the conformers of enolic 1,2-
Cyclohexanedione
Different optimized
structures for various excited states of CHD
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)
CHD at 266 nm, 248 nm and 193 nm, undergoes COH bond scission to generate OH(”, 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 -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.
ETrans can be explained with an exit barrier of ~14 kcal mol-1.
CONCLUSIONSCONCLUSIONS
Upadhyaya et al. J. Phys. Chem. A 117 (2013) 2415−2426
AcknowledgmentDr. Awadhesh Kumar
Dr. P. D. Naik
Dr. D. K. Palit
Dr. B.N. Jagatap
XI - SDMC
R1(2)R2(2)
P2(2) P1(2)
Q1(2)Q2(2)
X 21/2
2
3
2
3
1
2
+-
-
++
-1.5
2.5
1.5
2.5
3.5
X 23/2
A 2
JJ
NN
-
-- 3
Partial Energy Level Diagram of the A-X System of Partial Energy Level Diagram of the A-X System of OH.OH.
0.5
2.5
1.5
2.5
3.5
–+
++
––
–+
–+
–+
––