The nascent OH detection in photodissociation of 2-(bromomethyl)hexafluoro- 2-propanol at 193 nm: Laser-induced fluorescence study

Article abstract of DOI:10.1016/j.jphotochem.2011.01.026

Photodissociation of 2-(bromomethyl)hexafluoro-2-propanol (BMHFP) and 3-bromo-1-propanol (BP), involving σC-Brn_Br transition at 193 nm, has been investigated by measuring laser-induced fluorescence spectra of the expected OH product. The OH channel is a minor dissociation pathway with a quantum yield of 0.17 ± 0.05 in BMHFP, whereas it was not observed in BP. Partitioning of the available energy into translation, rotation, and vibration of the photoproducts has been measured by state selective detection of the nascent OH product in BMHFP. OH is produced mostly in the ground vibrational level (v″ = 0), with a rotational distribution being characterized by a temperature of 465 ± 25 K. But, a significant fraction of the available energy of 30.2 kcal mol^-1 is partitioned into translation of OH (14.6 kcal mol^-1). The OH(v″ = 0, J″) populations in the spin-orbit states as well as in the Λ-doublet states are statistical. A plausible mechanism of OH formation on excitation of BMHFP at 193 nm is suggested, with the primary reaction channel being elimination of Br atom by direct C-Br bond dissociation from a repulsive surface. The Br radical is detected using (2 + 1) resonance-enhanced multiphoton ionization (REMPI) at ～234 nm. It is produced in both the ground (²P_3/2) and the excited (²P_1/2) spin-orbit states with the relative quantum yield of the latter to be 0.36. The co-fragment of Br undergoes secondary C-O bond dissociation to produce OH and F₃C-C(CH ₂)-CF₃, with the reaction having a barrier located in the exit channel. In this two-step three-body dissociation process, a major fraction of the available energy is released into translation (〈f_T〉 ～ 0.75), resulting from an impulsive C-Br bond dissociation in the primary step and presence of an exit barrier in the secondary process. Experimental results combined with theoretical calculations provide a clear picture of the dynamics of OH formation from BMHFP at 193 nm. In addition, the energetics of another channel, competing with OH, have been calculated from the primary product F₃C-C(CH₂)(OH)-CF₃. In contrast to BMHFP, the OH product could not be observed from the photolysis of 3-bromo-1-propanol (another saturated halogenated propanol) at 193 nm under the detection limit of the present experimental condition, although it has a higher absorption cross-section at 193 nm.

Full text of DOI:10.1016/j.jphotochem.2011.01.026

Evaluation Only. Created with Aspose.PDF. Copyright 2002-2021 Aspose Pty Ltd.
Journal of Photochemistry and Photobiology A: Chemistry 222 (2011) 210–219
Contents lists available at ScienceDirect
Journal of Photochemistry and Photobiology A:
Chemistry
journal homepage: www.elsevier.com/locate/jphotochem
The nascent OH detection in photodissociation of
2-(bromomethyl)hexafluoro-2-propanol at 193 nm: Laser-induced fluorescence
study
Yogesh N. Indulkara,b, Hari P. Upadhyayaa, Awadhesh Kumara, Suresh B. Waghmodeb, Prakash D. Naika,∗
a Radiation & Photochemistry Division, Bhabha Atomic Research Centre, Trombay, Mumbai-400 085, India
b Department of Chemistry, University of Pune, Ganeshkhind, Pune-411 007, India
a r t i c l e i n f o
a b s t r a c t
Article history:
Photodissociation of 2-(bromomethyl)hexafluoro-2-propanol (BMHFP) and 3-bromo-1-propanol (BP),
∗
Received 24 September 2010
Received in revised form
20 December 2010
Accepted 24 January 2011
Available online 12 June 2011
involving ␴
← nBr transition at 193 nm, has been investigated by measuring laser-induced fluores-
C–Br
cence spectra of the expected OH product. The OH channel is a minor dissociation pathway with a
quantum yield of 0.17 0.05 in BMHFP, whereas it was not observed in BP. Partitioning of the avail-
able energy into translation, rotation, and vibration of the photoproducts has been measured by state
selective detection of the nascent OH product in BMHFP. OH is produced mostly in the ground vibra-
tional level (vꢀꢀ = 0), with a rotational distribution being characterized by a temperature of 465 25 K.
But, a significant fraction of the available energy of 30.2 kcal mol−1 is partitioned into translation of OH
(14.6 kcal mol−1). The OH(vꢀꢀ = 0, Jꢀꢀ) populations in the spin-orbit states as well as in the ꢀ-doublet states
are statistical. A plausible mechanism of OH formation on excitation of BMHFP at 193 nm is suggested,
with the primary reaction channel being elimination of Br atom by direct C–Br bond dissociation from
a repulsive surface. The Br radical is detected using (2 + 1) resonance-enhanced multiphoton ionization
(REMPI) at ∼234 nm. It is produced in both the ground (2P3/2) and the excited (2P1/2) spin-orbit states
with the relative quantum yield of the latter to be 0.36. The co-fragment of Br undergoes secondary C–O
bond dissociation to produce OH and F3C–C( CH2)–CF3, with the reaction having a barrier located in the
exit channel. In this two-step three-body dissociation process, a major fraction of the available energy is
released into translation (ꢁfTꢂ ∼ 0.75), resulting from an impulsive C–Br bond dissociation in the primary
step and presence of an exit barrier in the secondary process. Experimental results combined with the-
oretical calculations provide a clear picture of the dynamics of OH formation from BMHFP at 193 nm. In
addition, the energetics of another channel, competing with OH, have been calculated from the primary
product F3C–C(CH2)(OH)–CF3. In contrast to BMHFP, the OH product could not be observed from the pho-
tolysis of 3-bromo-1-propanol (another saturated halogenated propanol) at 193 nm under the detection
limit of the present experimental condition, although it has a higher absorption cross-section at 193 nm.
Keywords:
Fire extinguisher
Laser photolysis
Dynamics
Halogenated alcohol
OH formation
Energy partitioning
© 2011 Elsevier B.V. All rights reserved.
1. Introduction
2-propanol (BTFP) is a suggested fire extinguisher with its cup
burner value of 4.1 and the extinguishing concentration against
Photodissociation dynamics of fluoroalcohols has attracted
considerable attention in the past two decades, following the
suggestion that fluoroalcohols can be potential fire extinguishing
agents as an alternative to Halon 1301. 3-Bromo-1,1,1-trifluoro-
hydrogen and hydrocarbon flame of 5.7 and 4.0, respectively [1].
Like BTFP, its derivative 2-(bromomethyl)hexafluoro-2-propanol
(BMHFP) with the H atom on C (attached to OH) replaced with a
CF3 group, can be a potential fire extinguishing agent. However,
no reports on photochemistry and kinetics of BMHFP are avail-
able in the literature. The removal pathways of these compounds
from the atmosphere are not known. Several processes, such as
reaction with OH, photolysis, wet deposition and dry deposition,
can remove these compounds. Solar photolysis should not con-
tribute significantly in the troposphere, but expected to be a viable
loss mechanism in the stratosphere, because of the availability
∗
Corresponding author. Tel.: +91 2225595398; fax: +91 2225505151.
E-mail addresses: yogesh unichem@yahoo.co.in (Y.N. Indulkar),
haripu@barc.gov.in (H.P. Upadhyaya), awadesh@barc.gov.in (A. Kumar),
suresh@chem.unipune.ernet.in (S.B. Waghmode), pdnaik@barc.gov.in,
pd1959@rediffmail.com (P.D. Naik).
1010-6030/$ – see front matter © 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.jphotochem.2011.01.026

Evaluation Only. Created with Aspose.PDF. Copyright 2002-2021 Aspose Pty Ltd.
Y.N. Indulkar et al. / Journal of Photochemistry and Photobiology A: Chemistry 222 (2011) 210–219
211
of shorter wavelengths in the solar spectrum. To understand the
photolytic behavior of BTFP, in our previous work we investigated
its photodissociation dynamics at 193 nm in the gas phase [2],
and detected the OH radical as a transient product among other
products. Combined with experimental and theoretical studies, a
plausible mechanism for this reaction channel was reported. Sim-
ilar to BTFP, BMHFP photodissociation is also expected to be a
potential source of bromine atom and hydroxyl in the atmosphere.
To investigate the nature of photodissociation dynamics of BMHFP,
a study on its photodissociation is necessary.
which is described in detail in a previous paper [9]. Briefly, the
glass reactor had crossed arms at right angles with windows, allow-
ing the photolysis and the probe laser beams to intersect at the
centre of the chamber. The photolysis laser used is an excimer
laser (Lambda Physik, Model Complex-102, Fluorine version) and
the probe laser is a Quantel dye laser with frequency doubling
and mixing modules (TDL90) pumped by a seeded neodymium
doped yttrium aluminium garnet (Nd:YAG) laser (Quantel model
YAG980 E-20). The detector is attached to the bottom window of
the glass reactor to view the intersection volume of the photol-
ysis and probe lasers. It consists of a lens of focal length 50 mm
(diameter = 38 mm), to collect the fluorescence, photomultiplier
tube (Hamamatsu, model R-928P) to detect it, and a band pass fil-
ter (ꢁcentre = 310 nm, FWHM = 10 nm, %T310 nm = 10) placed between
the lens and the PMT to cut off the scattering from the photolysis
laser. Both laser beams were unfocused and attenuated to prevent
any saturation effect or multiphoton event. The photomultiplier
output was fed into a boxcar for gate integration and digitization.
The scanning of the dye laser, and data acquisition were controlled
by a Pentium II personal computer.
The OH fragment was probed state selectively by exciting the
A2ꢂ ← X2ꢃ (0,0) transition of OH (306–309 nm) and monitoring
the subsequent A → X fluorescence. A digital delay generator was
used to vary the time delay between the photolysis and the probe
beams, and all the LIF excitation spectra were measured at the time
delay of ∼50 ns. The compound in the form of vapor was flowed
through the glass reactor with the static pressure ∼50 mTorr at a
flow velocity of ∼10 cm/s, and it was photolyzed by ArF laser at
193 nm. The sample pressure in the cell was measured with a capac-
itance manometer. The intensities of the photolysis and the probe
lasers were separately measured using photodiodes, and measured
fluorescence excitation spectra were normalized with the mea-
sured lasers intensities. The LIF intensity was found to be linearly
proportional to both the photolysis and probe laser intensities.
Absorption cross-section of BMHFP at 193 nm was measured by
filling a known pressure of the sample into a 51 cm long cylindrical
absorption cell, fitted with MgF2 windows for passing the excimer
laser beam. A beam splitter was used for dividing the original laser
beam into two parts, one for passing through the sample and the
other as a reference. Two photodiodes were used for measuring
the intensities of the sample beam, after exit from the sample cell,
and the reference beam. By measuring ratio of the intensities of the
two beams, the fraction of the intensity absorbed by the sample
was determined. This fraction was plotted against the number of
molecules in the cell in a semilog plot.
Br is expected to be a photoproduct on UV photodissociation
of BMHFP, and an experimental arrangement was made to detect
this product. The photodissociation dynamics experiments were
performed in a molecular beam condition by combining resonance
enhanced multiphoton ionization (REMPI) and time of flight (TOF)
mass spectrometer to state-selectively monitor the photoproducts
Br (4P 2P3/2) and Br (4P 2P1/2) atoms, which are referred to as Br and
Br*, respectively. Experimental details are similar to that employed
in our earlier work [10]. The (2 + 1) REMPI transitions of Br and
Br* atoms, in the wavelength region of 230–235 nm, were used
to probe Br and Br* atoms. The laser pulses were generated from
a Quantel dye laser, TDL 90, using rhodamine 101 (LC 6400) dye
solution in methanol, pumped by a Quantel seeded Nd:YAG laser,
YG-981-C-20. The fundamental dye laser output was frequency-
doubled in a KDP crystal, and mixed with the fundamental output
of the Nd:YAG laser, to obtain an output in the range 230–235 nm.
The above laser output was separated from the rest of laser beams,
using a set of four Pellin-Broca prisms. In all the REMPI experiments,
the same laser beam was employed as a pump and a probe, i.e., for
both photodissociation of the parent molecule and ionization of the
photoproducts Br and Br* atoms. The laser beam was focused by a
In photodissociation of saturated alcohols, such as methanol [3],
∗
ethanol [4], 1-propanol and 2-propanol [5], after the ␴
← n(O)
(O–H)
excitation, the O–H bond breaking is the predominant pathway,
and this fast dissociation process occurs on the repulsive excited
state potential energy surface (PES) along the O–H coordinate with
a large translational energy release. In general, in all these satu-
rated alcohols the C–OH bond cleavage leading to OH formation is
a minor channel, and many times not observed, even with a sen-
sitive laser-induced fluorescence (LIF) technique, on excitation of
these alcohols at 193 nm. The OH channel is not observed mainly
because of presence of a barrier resulting from an avoided curve
crossing in this dissociation channel. In contrast, photo-excitation
of unsaturated alcohols, such as propargyl [6] and allyl [6,7] alco-
hols at 193 nm, which involves ␲* ← ␲ electronic transition, leads
to the formation of OH, as detected by LIF. In addition to intro-
duction of unsaturation centre, other structural modifications in
saturated alcohols, such as halogenations, can alter the nature of the
electronic excitation at 193 nm, and this can facilitate operation of
the OH channel from saturated alcohol involving a different reac-
tion mechanism. The photodissociation of halogenated saturated
alcohols, 2-bromoethanol and 2-chloroethanol, at 193 nm has been
studied experimentally [8], and only one primary channel, halo-
gen atom formation after C–X (X = Br, Cl) bond scission, has been
observed. In case of 2-bromoethanol, the co-product C2H4OH was
reported to undergo secondary dissociation to produce C2H4 and
OH. BTFP [2] is a saturated halogenated alcohol and un∗dergoes pho-
todissociation on excitation at 193 nm, involving ␴
← n(Br)
(C–Br)
transition, to produce OH as a secondary product from the primary
product F3C–CH(OH)–CH2, formed by direct C–Br bond scission
from a repulsive surface. Thus, photodissociation of these halo-
genated saturated alcohols at 193 nm involves ␴* ← n transition,
and generates OH in the secondary reaction channel. A question
arises; do all the halogenated alcohols generate OH on excitation at
193 nm? To address this question, we have investigated photodis-
sociation dynamics of BMHFP, and another brominated saturated
alcohol, 3-bromo-1-propanol (BP).
In the present study, we have investigated the photodissociation
dynamics of BMHFP and BP at 193 nm under collision-free condi-
tions, and detected the nascent OH photoproduct only from BMHFP,
by employing the LIF technique. We have also carried out ab initio
theoretical calculations to investigate a probable channel that leads
to formation of OH and to characterize the nature of the transi-
tion state. Partitioning of the available energy among translational,
rotational, and vibrational degrees of freedom of the photoproduct,
the ꢀ-doublet distribution, and the spin-orbit population ratio of
the OH fragments have been measured. We have also character-
ized some of the stable photoproducts of BMHFP, employing FTIR
absorption spectroscopy, and proposed mechanisms of their for-
mation with the help of both experimental results and molecular
orbital (MO) calculations.
2. Experimental
To study the photodissociation dynamics of BMHFP, laser
photolysis-laser induced fluorescence (LP-LIF) technique was used,

Evaluation Only. Created with Aspose.PDF. Copyright 2002-2021 Aspose Pty Ltd.
212
Y.N. Indulkar et al. / Journal of Photochemistry and Photobiology A: Chemistry 222 (2011) 210–219
employing density functional theory (DFT) at the B3LYP level, using
6-311+G(d,p) basis sets. Besides the B3LYP energy, the energies of
the optimized structures were calculated also at the second-order
Moller–Plesset (MP2) level of theory, using the same basis sets. The
harmonic vibrational frequencies and force constants were calcu-
lated to ensure that the stationary points on the potential energy
surfaces are true local minima. All transition state structures were
characterized by only one imaginary frequency and one negative
eigenvalue of the force constant matrix.
We also carried out the excited electronic state calculations
at the configuration interaction with the single electronic excita-
tion (CIS) level to understand the nature of electronic excitation of
BMHFP at 193 nm.
0.4
0.3
0.2
0.1
0.0
4. Results and analysis
4.1. Detection of OH
307.5
308.0
308.5
309.0
309.5
Photolysis of BMHFP at 193 nm produces the OH radical, which
was detected employing the LP-LIF technique. However, under sim-
ilar experimental conditions, OH could not be detected from BP. We
probed the quantum state distribution of the nascent OH radical by
measuring the fluorescence generated from its A–X system after
exciting the same system by a tunable dye laser. The measured
LIF signal was found to be linearly dependent on the photolysis
and probe laser intensities, indicating that the photodissociation
of BMHFP was a single photon process and that the OH molecular
transition was not saturated. Rotationally resolved lines could be
measured for the (0,0) vibronic band of OH. However, no LIF signal
could be measured for the (1,1) band, indicating that the OH radical
is generated mostly in the ground vibrational level i.e., ꢄꢀꢀ = 0. The
excitation spectra of OH, with several rotational lines for the (0,0)
transition, were measured. A typical LIF excitation spectrum of OH,
at 50 mTorr pressure and 50 ns time delay between the photolysis
and probe lasers, is shown in Fig. 1. We extracted the information on
the dynamics of OH formation from the profiles of these rotational
lines of the excitation spectrum.
Wavelength (nm)
Fig. 1. LIF excitation spectrum of the (0,0) band of the A2ꢂ+ ← X2ꢃ system of the
nascent OH radical formed in photodissociation of BMHFP (50 mTorr) at 193 nm.
lens of 280 mm focal length. The relative quantum yields of Br and
Br* were extracted from the relative integrated signal intensities in
the TOF spectrum at 233.70 nm and 234.04 nm, respectively, cor-
4
2
responding to two photon transitions, 6p P3/2 ← 4p P3/2 and 6p
2S1/2 ← 4p 2P1/2
.
Fourier transform infra-red (FTIR) spectrophotometer (Bruker,
model IFS 66v/S) and a glass cell fitted with a pair of ZnSe win-
dows, filled with a known amount of BMHFP (∼5 Torr pressure),
were used to measure the absorption spectra. To identify the sta-
ble photoproducts, FTIR absorption spectra were measured after
irradiation at 193 nm for 500 pulses (energy fluence = 40 mJ cm−2).
2-(Bromomethyl)hexafluoro-2-propanol (97%, SynQuest Labo-
ratories, Inc.) and 3-bromo-1-propanol (97%, Aldrich) were used
after several freeze-pump-thaw cycles in this work.
3. Theoretical methods
4.1.1. Average rotational energy of OH
The rotational lines of the (0,0) band of the A2ꢂ+–X2ꢃ system
of the OH radical were assigned based on its reported spectral data
[12]. Using integrated areas of these measured lines, after normal-
ization with respect to the photolysis and probe laser intensities,
pressure of the sample in the cell, and Einstein’s absorption coef-
ficients Bij (taken from Ref. [13]), the relative population of each
For a better understanding of the dynamics and the energet-
ics of the experimentally observed photodissociation channels
of BMHFP, ab initio molecular orbital (MO) calculations were
performed, using Gaussian 03 program [11]. The structures of
molecular species along with transition states were optimized
-16
-17
-18
-19
-20
0
200
400
600
800
1000
1200
Rotational energy (cm-1)
Fig. 2. A Boltzmann plot of rotational state population against energy of rotational states of OH (vꢀꢀ = 0) generated in dissociation of BMHFP at 193 nm laser excitation.

Evaluation Only. Created with Aspose.PDF. Copyright 2002-2021 Aspose Pty Ltd.
Y.N. Indulkar et al. / Journal of Photochemistry and Photobiology A: Chemistry 222 (2011) 210–219
213
0.12
0.10
0.08
0.06
0.04
0.02
0.00
2.0
1.0
0.0
0.42 0.04cm-1
0.25 0.02cm-1
-1.0
-0.5
0.0
Doppler shift (cm-1)
0.5
1.0
0
1
2
3
4
5
6
7
8
Rotational quantum number (N")
Fig. 3. Doppler profile of P1(2) line of the A2ꢂ+ ← X2ꢃ (0,0) system of the OH radical
produced in dissociation of BMHFP with 193 nm laser. The dotted line represents
the laser spectral profile.
Fig. 4. A plot of ꢀ-doublet population ratio against rotational quantum number Nꢀꢀ
for the nascent OH formed in laser-induced photodissociation of BMHFP at 193 nm.
The plot indicates no preference for any particular ꢀ-doublet state.
rotational level was calculated. A Boltzmann plot, Fig. 2, was con-
structed by plotting the corrected populations P(Jꢀꢀ) of the nascent
OH fragment against energy of the rotational levels (ε), using the
equation,
against the corresponding rotational quantum number (Nꢀꢀ), shown
in Fig. 4, and these values are close to unity. The observed statistical
distribution in the ꢀ-doublet states indicates that the OH product
has no preference for any particular ꢀ-doublet state.
P(Jꢀꢀ)
(2Jꢀꢀ + 1)
−εhc
The ground electronic state of OH is 2ꢃ, which splits into two
states 2ꢃ3/2 and 2ꢃ1/2 due to spin-orbit coupling. The population
of the ꢃ3/2 level of the OH radical is measured by P1(Nꢀꢀ) or Q1(Nꢀꢀ) or
R1(Nꢀꢀ) lines, and that of the ꢃ1/2 level by the intensity of P2(Nꢀꢀ) or
Q2(Nꢀꢀ) or R2(Nꢀꢀ) lines. A plot of the ratios of corrected intensities
of P1(Nꢀꢀ) and P2(Nꢀꢀ) against Nꢀꢀ shows these values to be almost
unity and invariant up to Nꢀꢀ = 7 (Fig. 5), implying that the levels
ln
=
+ constant.
(1)
kTR
The rotational temperature (TR) corresponding to the rotational
population distribution of OH(vꢀꢀ = 0), produced on photoexcitation
of BMHFP at 193 nm, was calculated from the slope of the plot, to
be 465 25 K, which implies 0.93 0.05 kcal mol−1 of the average
rotational energy into the OH fragment.
are equally populated for all probed rotational levels, and the X 2
ꢃ
spin-orbit components (2ꢃ3/2 and ꢃ1/2) of the OH product are
in equilibrium. It suggests that BMHFP dissociates to produce OH
from a PES, which does not interact with nearby triplet states, since
the singlet–triplet interaction can contribute to a preference for the
2ꢃ3/2 state.
2
4.1.2. Average translational energy release in OH
The average translational energy release in the photoproduct
OH can be evaluated using widths of Doppler broadened rotational
lines, since the Doppler broadening of the LIF lines is due to molec-
ular velocity of the fragments. A typical Doppler profile of the Q1(6)
line of OH from BMHFP is depicted, after de-convolution, in Fig. 3.
Doppler profiles of ten each of P1(2) and Q1(6) lines were measured,
and after de-convoluting these observed profiles with the instru-
mental functions, the average Doppler width of 0.42 0.04 cm−1 is
obtained. The Doppler width ꢅꢄD is related to the temperature (TT)
by the equation [14],
4.1.4. Formation time of OH
Time evolution of OH was measured by scanning the time delay
between the photolysis and the probe lasers to understand how fast
ꢀ
TT(OH)
mOH
ꢅvD = 7.16 × 10−7v0
,
(2)
2.0
1.0
0.0
where v0 and mOH are the centre frequency of the rotational line and
the mass of OH, respectively. The measured average Doppler width
corresponds to the average translational energy in OH, ET(OH) of
14.6 2.8 kcal mol−1
.
4.1.3. ꢀ-doublets and spin-orbit states distributions
An interaction between the rotation of the nuclei and the elec-
tronic orbital angular momentum is found to produce a splitting
into two components for each J value in the state with ꢀ =/ 0,
which are otherwise doubly degenerate without rotation [15]. The
ꢀ doublets in a 2ꢃ state differ in the relative orientation of the
␲ molecular orbital and the angular momentum J. In the classi-
cal (high J) limit, 2ꢃ+(Aꢀ) and 2ꢃ−(Aꢀꢀ) refer to the ␲ lobes being
respectively parallel and perpendicular to the rotational plane. The
population in the ꢃ+ state is probed by the P or R rotational lines,
and that in the ꢃ− state by the Q lines of the excitation spectrum.
The ratio of the corrected intensities of P to Q lines was plotted
1
2
3
4
5
6
7
8
Rotational quantum number (N")
Fig. 5. A plot of the spin-orbit state population distribution ratio against rotational
quantum number Nꢀꢀ for the OH radical produced in photodissociation of BMHFP
with 193 nm laser. Both the spin-orbit populations are statistically produced.