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

–+

++

––

–+

–+

–+

––