HCl yield and chemical kinetics study of the reaction of Cl atoms with CH3I at the 298 K temperature using the infra-red tunable diode laser absorption spectroscopy

Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 128 (2014) 176–182

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HCl yield and chemical kinetics study of the reaction of Cl atoms with CH3I at the 298 K temperature using the infra-red tunable diode laser absorption spectroscopy R.C. Sharma ⇑, M. Blitz, R. Wada, P.W. Seakins Lasers Laboratory, Department of Chemistry, The University of Leeds, Leeds LS 2 9JT, United Kingdom

h i g h l i g h t s

g r a p h i c a l a b s t r a c t

 ArF pulsed excimer photolysis laser 0.25

Cl+CH3I Cl+C4H10 [CH3I] and [C4H10]=5.14E14 molecule cm-3

0.20

HCl (a u)

and IR tunable diode probe laser.  The modelling of the reactions CH3 + CH3ICl and CH3I + CH3ICl.  HCl yield is nonlinear decreasing with increasing the [CH3I].  HCl yield is nonlinear decreasing with increasing the [CH3] radical.

0.15

0.10

0.05

0.00 -0.001

0.000

0.001

0.002

0.003

0.004

0.005

Time (sec)

a r t i c l e

i n f o

Article history: Received 7 November 2013 Received in revised form 7 February 2014 Accepted 14 February 2014 Available online 6 March 2014 Keywords: IR absorption spectroscopy IR tunable diode laser UV photo dissociation Reaction kinetics HCl yield Herriott type multiple absorption cell

a b s t r a c t Pulsed ArF excimer laser (193 nm) – CW infrared(IR) tunable diode laser Herriott type absorption spectroscopic technique has been made for the detection of product hydrochloric acid HCl. Absorption spectroscopic technique is used in the reaction chlorine atoms with methyl iodide (Cl + CH3I) to the study of kinetics on reaction Cl + CH3I and the yield of (HCl). The reaction of Cl + CH3I has been studied with the support of the reaction Cl + C4H10 (100% HCl) at temperature 298 K. In the reaction Cl + CH3I, the total pressure of He between 20 and 125 Torr at the constant concentration of [CH3I] 7.0  1014 molecule cm3. In the present work, we estimated adduct formation is very important in the reaction Cl + CH3I and reversible processes as well and CH3I molecule photo-dissociated in the methyl [CH3] radical. The secondary chemistry has been studied as CH3 + CH3ICl = product, and CH3I + CH3ICl = product2. The system has been modeled theoretically for secondary chemistry in the present work. The calculated and experimentally HCl yield nearly 65% at the concentration 1.00  1014 molecule cm3 of [CH3I] and 24% at the concentration 4.0  1015 molecule cm3 of [CH3I], at constant concentration 4.85  1012 molecule cm3 of [CH3], and at 7.3  1012 molecule cm3 of [Cl]. The pressure dependent also studied product of HCl at the constant [CH3], [Cl] and [CH3I]. The experimental results are also very good matching with the modelling work at the reaction CH3 + CH3ICl = product (k = (2.75 ± 0.35)  1010 s1) and CH3I + CH3 ICl = product2 (k = 1.90 ± 0.15)  1012 s1. The rate coefficients of the reaction CH3 + CH3ICl and CH3I + CH3ICl has been made in the present work. The experimental results has been studied by two method (1) phase locked and (2) burst mode. Ó 2014 Elsevier B.V. All rights reserved.

⇑ Corresponding author. Present address: Laser Science and Technology Centre DRDO, Delhi-110054. Tel.: +91 11 23907 539. E-mail address: [email protected] (R.C. Sharma). http://dx.doi.org/10.1016/j.saa.2014.02.065 1386-1425/Ó 2014 Elsevier B.V. All rights reserved.

R.C. Sharma et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 128 (2014) 176–182

Introduction The atmospheric science of iodine containing compounds is a topic of current interest [1]. Iodinated compound are easily photo-dissociated by the near ultra violet wavelength (UV) region [2]. Alkyl iodides have been observed in the marine boundary layer (MBL) [3,4] and when liberated by reaction or photolysis, the iodine contained plays an important role in particle formation [5,6]. Whilst photolysis is the dominant removal process, radical reactions play a significant role and under certain conditions the reaction with Cl atoms can effectively compete with other removal processes [5]. Methyl iodide is the most abundant iodine-containing species in the air and the main source of active iodine-compounds [3,4]. The potential importance of the Cl atoms reaction with alkyl iodides has led to a number of kinetic and product [7–14] studies. Such studies have confirmed that under elevated [Cl] (>105) molecule cm3) Cl reactions will compete with photolytic removal, but more interestingly have shown that the reaction mechanism is complex; at ambient temperatures Cl atoms are removed by reversible formation of an adduct or by abstraction:

Cl þ RI $ RI- -Cl

ð1aÞ

Cl þ RI ! HCl þ RI

ð1bÞ

The reaction of Cl atoms with CH3I proceeds via two channels; adduct formation and hydrogen atom abstraction [9,12].

Cl þ CH3 I ! CH2 I þ HCl

ð2aÞ

Cl þ CH3 I þ M $ CH3 ICl þ M

ð2bÞ

At temperature and pressures relevant for atmospheric chemistry considerations, the rate coefficient for addition is much faster then the coefficient for hydrogen abstraction [13,14]. Direct observation of the adduct, CH3I- -Cl, has been reported in absorption method by Enami et al. [12] and for CH3I- - - -Cl by laser induced fluorescence by Gravestock et al. [13]. Enami et al. studied a range of adducts via cavity ring down spectroscopy (CRDS) following the reaction of Cl atoms with alkyl halides [12,14]. Aim of the present work to study HCl yield and kinetics of the reaction Cl + CH3I at room temp 298 K. ArF excimer photolysis laser (193 nm)/infrared (IR) tunable diode laser absorption spectroscopic technique has been made. The product of HCl yield in the reaction (Cl + CH3I) has been normalized with the support of reaction Cl + C4H10 at the same condition of experiment. The secondary chemistry has been studied in the reactions CH3 + CH3ICl and CH3I + CH3ICl and theoretically modeled in the this work. Experimental ArF excimer photolysis laser/infrared (IR) tunable diode laser absorption spectroscopic technique in the reaction Cl + CH3I has been made. Pulsed ArF excimer laser model (Lambda Physik: LEX TRA) has been operated at the UV wavelength 193 nm, pulse duration 10 ns, repetition rate 1 Hz, and energy 8–10 mJ/p. ArF laser beam is deflected along the centre of a reaction cell using a dichroic mirror (DM). The diameter of the photolysis beam can be adjusted using an iris and a typical diameter is 16 mm. The Cl atoms have been generated from a 3% (COCl)2 mix in helium (He) gas photolysis by ArF excimer laser 193 nm. The Cl atoms is given in Eq. (3). The absorption cross section of (COCl)2 at 193 nm is (3.83 ± 0.09)  1018 cm2 [15].

ðCOClÞ2 þ 193 nm ¼ 2Cl þ 2CO

ð3Þ

Cl atoms react with CH3I (20%) in helium and C4H10 (20%) in helium. The products of the reaction are; as given in Eq. (1a and b) at room

177

temp. In the experiment, total pressure has been used from 20 Torr to 135 Torr in helium. The concentration of [CH3I] and [C4H10] were between 2.0  1014 and 3.0  1015 molecule cm3. Flows of all reagents and He buffer gas were measured by calibrated mass flow controller. The experiment has been done Cl + CH3I and Cl + C4H10 at nearly 20 Torr at 298 K with the [CH3I] and [C4H10] between 2.0  1014 and 3  1015 molecule cm3 to measure the HCl yield. The concentration of [Cl]0 is always kept constant nearly 6.0  1012 molecule cm3 with using the calibrated mass flow control (MFC) of 3% (COCl)2 vapour in helium gas. The beam of the liquid N2 cooled infrared diode laser (Laser Components: model L5830 TDL controller) is split into two parts by a CaF2 beam splitter (90:10). The main portion (90%) of diode laser beam travels 27 times inside the reaction cell, by means of a Herriott – type multiple reflection configuration [14]. IR diode laser beam was overlapped with excimer laser beam 4.85 m in absorption length. IR diode laser beam has been aligned with the help to He–Ne laser beam (632.8 nm). He–Ne laser beam inside the cell with 27 pass, 26 spots of the He–Ne laser on the gold coated highly reflected (99%) both mirrors. Initially He–Ne laser beam very tightly overlapped with IR tunable diode laser beam via the three pin hole and three mirrors before the reaction cell and very good aligned with output side of the reaction cell to the detector (D1). IR diode laser beam to the Hg–Cd–Te (MCT) (model EG & G Judson, J15012) liquid N2 cooled detector (EG & G optoelectronics) via the optics as iris, mirror and lens. The detector signal is amplified with a gain of 500 and sent to a LeCroy (model 9310A) oscilloscope. The narrow bandwidth of the cw diode laser allows tuning to the R(2) absorption line (2944.91377 cm1) of HCl. The transient absorption signals are collected on the LeCroy, averaged and stored on a personal computer for subsequent HCl yield and kinetics analysis. A minor portion (10%) of the diode laser beam is passed through an HCl reference cell to a second Liquid N2 cooled MCT detector (D2). The detector signal was processed to the second pre-amplifier (optoelectronics model PA-101). Amplified reference signal processed to the lock-in amplifier (EG & G model 9503SC) and second Lecroy oscilloscope (model LT262). The tunable diode laser frequency has been locked by the instruments lock-in amplifier, function generator and stabilized frequency electronics box at 4.0 kHz external trigger to IR tunable diode laser with synchronized excimer laser at 1 Hz repetition rate. The tunable diode laser is also cooled by liquid N2; operating at a temp nearly 96 K for HCl strong absorption. The diode laser frequency has been locked for the absorption experiment of HCl molecule via the lock-in amplifier. The cell is stainless-steel and divided into three parts. The central part is 70 cm long with a 40 mm internal diameter. The end sections have a slightly greater diameter and have two-gas inlet/ outlet ports mounted on axial translators. Two concave spherical gold coated mirrors, 50 mm in diameter, with an identical radius of curvature of 50 cm, are held by three adjustable screws inside the end sections. The separation of the two mirrors is about 1.0 m. In order to achieve different path lengths and compensate the expansion of the central tube at high temperature, the end sections are connected to the central tube by a pair of 15 mm long bellows. The cell is sealed by two CaF2 windows and evacuated by a mechanical pump. Each mirror has a 25 mm diameter. This arrangement allows the photolysis and probe laser beams to have a better overlap and also allows the control of the effective absorption path length by adjusting the diameter of the photolysis beam. The pulsed ArF excimer laser photolysis/IR diode laser system is shown schematically in Fig. 1. In the experiment has been used second method burst mode to measure the HCl yield in the reaction Cl + CH3I with the calibration reaction Cl + C4H10. The advantage of the method can be applied for unstable molecule/radical

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IR Tunable Diode Laser

Osc

Detector

Computer

L BS

Iris2

Gold coated Mirrors R=99%

M5

M2 M4 M1 Ref HCl Cell M3 To mechanical pump

Sample line

ArF Excimer Laser, 193nm

Function Generator

Osc Frequency stabilizer Fig. 1. Experimental setup ArF photolysis exciter laser – IR tunable diode laser Herriotte type absorption spectroscopy.

like HO2, IO etc. This method can be made without reference cell and frequency stabilization box. Excimer laser directly trigger to function generator, and function generator trigger to external mode of diode laser controller at the 1 Hz repetition rate of excimer laser. Function generator has been operated at the 500 kHz. The layout of the burst mode is given in Fig. 2.

Results and discussion The reactions of Cl atoms with CH3I have been studied at temperatures 298 K, where adduct CH3ICl and abstraction HCl + CH2I formation are significant. CH3I molecule is also photo-dissociated in CH3 radicals and iodine atoms by the 193 nm. The secondary chemistry is very important role in this reaction. The rate coefficients are determined in the reactions CH3 + CH3ICl and CH3I + CH3ICl using the kinetic modelling. The optical absorption cross

y¼

P1 :P2 ½expðP 3 :xÞ  expðP2 :xÞ þ P4 P2  P3

ð4Þ

Here, P1, P2, P3 and P4 are maximum amplitude of HCl signal, exponential rising, exponential decay and base of absorption profile respectively. In Fig. 4a and b, parameter P1 has been fitted as maximum amplitude of HCl signal. These parameters are as phase lock and burst mode method. The actual HCl signal has normalized using the absorption of IR radiation and absorption of UV by CH3I. The normalized HCl signal in the reaction Cl + CH3I and Cl + C4H10 are given in the Fig. 3a and b at 298 K temperature and 20 Torr pressure. Rate coefficients (2.0 ± 0.09)  1012 and (2.00 ± 0.06)  1010 of both reactions Cl + CH3I, and Cl + C4H10 at 298 K and 20 Torr pressure are given in Fig. 5a and b. These rate coefficients are tabulated in Table 1.

From Excimer Sync 0.25

Ext. Trigger

Cl+CH3I Cl+C4H10 [CH3I] and [C4H10]=5.14E14 molecule cm-3

0.20

Function Generator

Oscilloscope CH1

Mode in

signal

Ext

HCl (au)

D1 Lock-in

Iris 1

section of CH3I at 193 nm is (1.18 ± 0.04)  1018 cm2 [15]. The HCl yield from reaction (2) has been studied at 298 K temperature using the reaction of Cl atoms with the support of n-butane as a calibration reaction. Although it is possible to calculate absolute HCl concentrations from absorption measurements, uncertainties in the initial chlorine atom concentration and in the effective path length result in significant uncertainties in the HCl yield. An alternative method is to use a calibration reaction. In this method the amount of HCl produced from a reaction with an excess of a reagent (n-butane) with well determined product yield (100%) is measured and then the calibrant is replaced with an excess of the test substrate and the experiment repeated with identical initial chlorine concentrations and path length. In practice corrections need to be made for any absorbance of the probe laser by the substrate or calibrant. These small correction factors can be calculated by observing the modulation of a chopped probe beam in the presence of varying concentrations of substrate or calibrant. In the reactions Cl + CH3I and Cl + C4H10, IR diode laser absorption in real time normalized signal of HCl are given in Fig. 3a and b. HCl yield of the reaction Cl + CH3I is given in Fig. 3(a). Fig. 3b signal shows of Cl + C4H10, with 100% HCl. The signal of the reaction Cl + CH3I is fitted in Fig. 4, using Eq. (4) [16–18].

0.15

0.10

Mixer D1

Detector 0.05

L5830 TDL Controller

To diode laser head Fig. 2. Burst mode setup.

Reaction Cell

To L5830

0.00 -0.001

0.000

0.001

0.002

0.003

0.004

0.005

Time (sec) Fig. 3. Normalized HCl signal (a.u.) vs time (sec) in the reaction (a) Cl + CH3I reaction (black curve) and (b) Cl + C4H10 reaction (Red curve) at 20 Torr pressure and 296 K. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

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R.C. Sharma et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 128 (2014) 176–182

0.06

(a)

Equation: y=((P1*P2)/(P2-P3)*(exp(-P3*x)-exp(-P2*x)))+P4 Weighting: y 2

Chi /DoF R2 P1 P2 P3 P4

= 3.0197E-6

= 0.97038 0.06007 1933.25624 47.12892 -0.03723

4000

±0.00024 ±18.21246 ±1.49825 ±0

3000

2000

3

0.03

HCl signal (a u)

5000

No weighting

k' ( cm molecule -1 sec- 1

(a)

0.00

Reaction: Cl + CH3I -12

1000

-0.03

0.000

0.001

0.002

0.003

0.004

k' cm3 molecule -1 s -1

HCl absorption signal (a u)

(b)

Equation: y=((P1*P2)/(P2-P3)*(exp(-P3*x)-exp(-P2*x)))+P4

0.06 0.04

Weighting: y No weighting

Chi2/DoF = 0.00001 R2 = 0.95082 P1 P2 P3 P4

4E+14

8E+14

1.2E+15

1.6E+15

2E+15

0.005

Time (sec)

0.08

-1

-3

0

(b)

-1

CH3l Molecule cm

0 -0.001

3

Rate Constant k=(2.0 ±0.09) x 10 cm molecule sec At Temp 298 K, P= 20 Torr

0.07885±0.00094 1147.08475 ±33.273 65.5908±5.24648 -0.04551 ±0.00056

Reaction Cl + C4 H10

400000

-10

3

-1

k= (2.0±0.06) x 10 cm molecule s At 298K, P= 20 Torr

-1

200000

0.02 -3

[C4H10 ] molecule cm

0.00

0 0

5E+14

1E+15

1.5E+15

-0.02 Fig. 5. (a) Rate coefficients in the reaction Cl + CH3I and (b) rate coefficient in the reaction Cl + C4H10.

Burst Mode Method -0.04 -0.06 -0.001

0.000

0.001

0.002

0.003

0.004

0.005

Time (Sec) Fig. 4. (a) Best fitted parameters for the HCl absorption signal (phase mode) and (b) best fitted parameters for the HCl absorption signal (burst mode).

The aim of the experiment to measure the HCl yield in the reaction Cl + CH3I with respect to the reaction Cl + C4H10 = HCl (100%). The measured absorption cross section of CH3I (=1.02 ± 0.04)  1019 cm2 and C4H10 (=3.86 ± 0.11)  1019 cm2 using the modulated IR laser beam by the mechanical chopper for normalized the HCl signal. In the experiment the IR diode laser beam is absorbed by the CH3I and C4H10. The measured IR absorption for the molecule CH3I over the concentration range of [CH3I] between 2.0  1014 and 4.0  1015 molecule cm3 at a pressure 20 Torr, and also have measured IR absorption with varying the total pressure of He at the range 20–125 Torr at constant concentration 1.07  1014 molecule cm3 of CH3I. Similar at identical conditions experiment has done for C4H10 for the normalization. The UV 193 nm ArF laser light is also absorbed by the molecule CH3I but not C4H10 molecule. Similarly, have measured absorption for the molecule CH3I over the concentration range of [CH3I] between 2.0  1014 and 4.0  1015 molecule cm3 at a constant pressure 20 Torr, and measured the absorption with varying the total pressure of He between the range 20 and 125 Torr at constant concentration 1.07  1014 molecule cm3 of CH3I. HCl is normalized by IR and UV absorption for the Cl + CH3I and normalized by IR absorption for C4H10.

The experimentally HCl yield has been obtained using the Eq. (5)

HClðyieldÞ ¼

Normalized HCl in the reactionðCl þ CH3 IÞ Normalized HCl in the reactionðCl þ C4 H10 Þ

ð5Þ

Fig. 3a shows for reaction Cl + CH3I, here is not 100% HCl yield. In the case, (1) the adduction formation is here and (2) CH3I molecule is also reacting with the other. Here are the possibilities CH3 + CH3I = product, CH3ICl + CH3I = product2, and may be some effect of CH3 + CH3 = C2H6. The rate coefficient of the reaction CH3 + CH3 = C2H6 at room temp is, k = (5.0 ± 0.5)  1011 [19]. So that, we can say in the experiment the reaction mechanism at 298 K and 20 Torr are given in Eq. (6). The rate coefficients are given [9,12,19].

Cl þ CH3 I ¼ HCl þ CH2 I ðk ¼ 8:0  1013 Þ

ð6aÞ

Cl þ CH3 I þ M ¼ CH3 ICl þ M ðk ¼ 2:50  1012 Þ

ð6bÞ

CH3 I  Cl þ M ¼ CH3 I þ Cl þ M ðk ¼ 1:03  103 Þ

ð6cÞ

CH3 þ CH3 ¼ C2 H6 ðk ¼ 5:0  1011 Þ

ð6dÞ

CH3 þ CH3 ICl ¼ product ðkÞ

ð6eÞ

CH3 I þ CH3 ICl ¼ product2 ðkÞ

ð6fÞ

At the pressure 20 Torr and Temp 298 K, in the reaction Cl + CH3I and Cl + C4H10, the concentration of CH3I and C4H10 are between 2.0  1014 and 4.0  1015 molecule cm3. The HCl (Cl + CH3I = HCl) has normalized with the (100% HCl of Cl + C4H10). Facsimile

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Table 1 Rate coefficients of the reaction Cl + CH3I and Cl + C4H10 at 298 K. T (K)

P (Torr)

k1a (1013 s1)

k1b (1012 s1)

K1b (103 s1)

k1c (1011 s1)

k1d (1010 s1)

k1e (1012 s1)

k1f (1012 s1)

k1g (1010 s1)

298 298 298

20 70 125

8.00 8.00 8.00

2.50 4.50 9.85

1.03 1.85 4.50

5.00 5.00 5.00

2.75 ± 0.35 2.75 ± 0.35 2.75 ± 0.35

1.90 ± 0.15 1.90 ± 0.15 1.90 ± 0.15

2.0 ± 0.09

2.0 ± 0.06

Rate constants in the reactions: k1a = (Cl + CH3I = HCl + CH2I), k1b = (Cl + CH3I = CH3ICl), k1b = (CH3ICl = CH3I + Cl), k1c = (CH3 + CH3 = C2H6), k1d = (CH3 + CH3ICl = product), k1e = (CH3I + CH3ICl = product2), k1f = (Cl + CH3I = HCl + CH2I), k1g = (Cl + C4H10 = HCl + C4H9).

program modelling in the case for Eq. (6) at the maximum [Cl] concentration 7.3  1012 molecule cm3, HCl, product, and adduct vs Time is given in Fig. 6. The [CH3] is generated by the UV 193 nm between 1.0  1012 and 9.0  1012 molecule cm3 with varying the CH3I molecule between 2.0  1014 and 4.0  1015 molecule cm3 at 20 Torr. The HCl yield is given in Fig. 7. The theoretically HCl yield has calculated using the Facsimile Software Package [20] from the Eq. (7)

HClðyieldÞ ¼

HCl in the reaction Cl þ CH3 IðcalculatedÞ

ð7Þ

3

7:3  1012 molecule cm ð100%HClÞ

The obtained HCl yield experimentally as well as modelling is nearly 65% at the concentration of 1.0  1014 molecule cm3 and 25% at the concentration 4.0  1015 molecule cm3 of CH3I is given in Fig. 7. In the modelling of the reaction CH3I + CH3ICl = product2 [k = (1.90 ± 0.15)  1012 s1] and CH3 + CH3ICl = product

0.8

8.00E+12

P=20 Torr, 298k Constant [CH3 ]= 4.8E+12 molecule cm-3

0.7

constant [Cl] = 7.30 E+12 molecule cm -3 Cl + CH 3I = HCl + CH2 I (8.0 ) E-13 sec -1

[Cl]

6.00E+12

-1 Cl+ CH 3 I = CH 3 ICl (2.5E-12) sec CH3+ CH 3 ICl =product [k=(2.75 ± 0.35) E-10] CH 3ICl = CH 3 I + Cl (1.03 E+03 sec-1) CH3+ CH 3 =C 2 H 6 (k= 5.0 E-11)

0.6 PRODUCT

0.5

CH 3 I + CH 3 ICl = product2(1.90±0.15) E-12sec

0.4 [HCl]

0.3

2.00E+12

0.2

[ADDUCT]

0.00E+00

-1

Absolute HCl yield (a u)

4.00E+12

0.1 0

0.001

0.002

0.003

0.004

0.005

Time (sec)

0 0.00E+00

[CH 3 I] molecule cm

1.00E+15

2.00E+15

-3

3.00E+15

4.00E+15

Fig. 6. Modelling results in the reaction Cl + CH3I at the 298 K. Fig. 8. HCl yield vs [CH3I] at constant [Cl] and constant [CH3] at 20 Torr, 298 K.

0.7

0.8 P=20 Torr, T=298K -3 At varying radical [CH 3 ]=9.0E11 to 2.0E13 molecule cm

P= 70 Torr, 298K, At constant [Cl]=7.3E12 molecule cm-3 At constant [CH3 ] = 4.85E12 molecule cm-3 CH 3I + Cl = HCl + CH 2 I (k=8.00E-13) Cl + CH3 I = CH 3 ICl (k=4.50 E-12) CH 3 ICl = CH 3 + Cl (k= 1.85E+03) CH 3+CH 3 = C2 H6(k=5.0E-11)

0.6

At constant [Cl]= 7.30E12 molecule cm-3 -1 Cl+CH 3I= HCl + CH 2 I (k=8.0E-13) sec

0.6

-1

Cl+CH3 I= CH3 ICl (k=2.50E-12) sec

0.5

CH 3 ICl= CH3 I + Cl (k=1.03E+03) sec-1 -1 -1

0.4

CH3 I + CH3 ICl = Product2 [k=1.90±0.15)E-12] sec -1

0.3

0.2

Absolute HCl yield (a u)

0.2

Absolute HCl yield (a.u.)

0.4

CH 3+ CH 3 ICl = product [k=(2.75± 0.35) E-10] CH3 I + CH 3 ICl = product2 [k=(1.90± 0.15) E-12]

CH 3+CH3 = C2H 6 (k=5.0E-11)E-11 sec

CH3+ CH3 ICl = Product [k=(2.75±0.35)E-10] sec

0.1 -3

0 0.00E+00

[CH 3 I] molecule cm

1.00E+15

2.00E+15

-3

[CH3 I] molecule cm

3.00E+15

4.00E+15

0

0.00E+00 Fig. 7. HCl yield vs [CH3I] at constant [Cl] but varying the [CH3] radical and [CH3I] at pressure 20 Torr, temperature 298 K.

1.00E+15

2.00E+15

3.00E+15

4.00E+15

Fig. 9. HCl yield vs CH3I at constant [CH3] and constant [Cl] and at 70 Torr, 298 K.

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(a)

0.3

[CH3 ] molecule cm-3

0.2 0.00E+00

(b) 0.4 P=125 Torr, T=298K Cl + CH 3 I = HCl + CH2 I (8.00 E-13) Cl + CH 3 I = CH 3 ICl (9.85E-12) CH 3 ICl= Cl + CH 3 I (4.05E+03) CH 3 + CH3 = C2 H 6 (k=5.0E-11) CH 3 I + CH3 ICl= product k=(1.90 ± 0.15) E-12 CH 3+ CH3 ICl = product2 k=(2.75 ± 0.35 E-10)

0.5

0.4

0.2

0.1

Absolute HCl yield (a u)

0.3

0.3

[Cl] = 7.3 E+12 molecule cm -3 [CH3 ] = 4.8E+12 molecule cm-3

Absolute HCl yield (a u)

0.7

0.6

P=20 Torr, 298K -3 constant [Cl] =7.3E12 molecule cm constant [CH3 I] =8.85 E14 molecule cm-3 CH3 + CH 3 ICl = Product [k=(2.75±0.35)E-10] CH3 I + CH3ICl = Product2 [k=(1.90±0.15)E-12] Cl + CH3 I = HCl + CH2 I (k= 8.0E-13) Cl + CH3 I = CH3 ICl (k= 2.50E-12) CH 3 ICl = CH3 I + Cl (k=1.03 E+03) CH3 + CH 3 = C2 H 6 (k=5.0E-11)

0.4

Absolute HCl yield (a u)

[k = (2.75 ± 0.35)  1010 s1] with best model value of the rate coefficient has been made first in the experiment. At the constant [Cl] (=7.3  10+12) molecule cm3 and constant generated [CH3] radical ([CH3] = 4.85  1012) molecule cm3 with varying the [CH3I] between 2.0  1014 and 4.0  1015 molecule cm3 at 20 Torr, 70 Torr and 125 Torr. Absolute HCl yield is nonlinear decreasing with increasing the CH3I molecule. Experimentally HCl yield with best modelling results are given in Figs. 8–10. The experiment at constant CH3I (=1.06  1015 molecule cm3) and constant Cl (=7.3  1012 molecule cm3) and at constant ([CH3] = 4.85  1012) molecule cm3 but only varying the total pressure of the reaction cell between 20 Torr and 125 Torr. The pressure dependent rate coefficients has been used [9–12] for Facsimile program modelling. The obtained HCl yield 34% at the 20 Torr and 18% at the 125 Torr is given in Fig. 11 at best value of CH3 + CH3ICl = product [k = 2.75 ± 0.35)  1010] and CH3I + CH3ICl = product2[k = (1.90 ± 0.15)  1012]. The experiment at constant [Cl] (=7.3  1012 molecule cm3), constant [CH3I] (=1.05  1015 molecule cm3) but varying the

3.00E+12

6.00E+12

9.00E+12

1.20E+13

At constant [Cl]=5.0E12 molecule cm -3 At constant [CH3 I] = 7.30E14 molecule cm-3 Cl + CH 3 I = HCl + CH2 I (k=8.00E-13) Cl + CH3 I = CH3 ICl (k=2.50E-12) CH3 ICl = Cl + CH 3(1.03 E+03) CH3+CH 3 = C 2 H 6( k=5.0E-11) CH 3+CH3 ICl =product [k=(2.75 ±0.35) E-10] CH3I + CH3 ICl = product2 [k=(1.90± 0.15) E-12]

[CH3 ] molecule cm-3

0.2 0.00E+00 -3

[CH 3 I] molecule cm

0 0.00E+00

1.00E+15

2.00E+15

2.00E+12 4.00E+12 6.00E+12 8.00E+12 1.00E+13 1.20E+13

Fig. 12. (a) Absolute HCl yield vs [CH3] radical at constant [Cl] and constant [CH3I]. (b) Absolute HCl yield vs [CH3] radical at constant [Cl] and constant [CH3I].

3.00E+15

4.00E+15

Fig. 10. HCl yield vs CH3I, at P = 125 Torr, 298 K, at constant CH3 radical and [Cl].

0.4

[CH3] radical between 1.0  1012 and 1.20  1013 molecule cm3, using the 193 nm excimer laser energy. The absolute HCl yield is between nearly 34% and 29% with best modelling results is given in Fig. 12a and b. HCl yield nonlinear decreasing with increasing CH3 radicals at constant [Cl] atom and [CH3I] molecule. Conclusion

HCl yield (a.u.)

0.3

0.2

At 298K -3 Constant [CH3 ]= 4.85E12 molecule cm -3

constant[CH 3 I]= 1.07 E15 molecule cm constant [Cl]= 7.3 E12 molecule cm-3 CH 3+CH3 =C2 H 6 (k= 5.0E-11 sec

-1

)

CH 3 +CH3 ICl=product [k=(2.75±0.35)E-10 sec-1 ] CH 3 I + CH 3 ICl = product2 [k=(1.90±0.15)E-12 sec ]

0.1

-1

Pressure (Torr) 0 0

25

50

75

100

125

Fig. 11. HCl yield vs total pressure at the constant [CH3] radical, constant [CH3I] and constant [Cl].

ArF pulsed excimer photolysis laser – IR tunable diode laser absorption spectroscopic technique has been employed to study the quantum yield of HCl in the reaction Cl + CH3I as a function of the concentration of [CH3I] in the range between 2.0  1014 and 4.0  1015 molecule cm3 at room temperature and methyle radical [CH3] = 4.85  1012 molecule cm3 at the pressure range between 20 and 125 Torr at the Temp 298 K. The reactions CH3 + CH3ICl = product, and CH3I + CH3ICl = product2 is very important to analyzed with the experimental results. In the complex reaction Cl + CH3I is a dominant adduct formation at the 298 K with pressure dependent and the reaction Cl + CH3I = HCl + CH2I, is pressure independent at the 298 K. The CH3I molecule is photolysis by the 193 nm in the CH3 radical. Here, the effect of the reaction is also CH3 + CH3 = C2H6. Experimental results are excellent matching with calculated results using the Facsimile

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program. According the modelling, has been determined, best value the rate coefficients of the reaction and CH3 + CH3ICl = product (k = 2.75 ± 0.35)  1010 s1 and CH3I + CH3ICl = product2 are (1.90 ± 0.15)  1012 s1 for the first time. HCl yield is nonlinear decreasing with increasing the CH3I, and HCl yield is also nonlinear decreasing with increasing the CH3 radical. As per atmospheric relevance, HCl signal was obtained until total pressure of 200 Torr in the reaction Cl + CH3I at the 298 K. The total pressure was in He gas. In the experiment HCl yield and chemical kinetics has been studied identically by both method (1) phase locked and (2) burst mode. The advantage of the burst mode is only for study of unstable molecules and radicals and also no need the instruments as lock-in amplifier and frequency stabilizer for the burst mode method. Acknowledgement This work was funded by an EPSRC grant GR/T28560/01. The authors appreciate discussions with Professors Mike Pilling and Dwayne Heard. The authors also acknowledge the helpful suggestions of the referees. References [1] L. Carpenter, J. Chem. Rev. 49 (2003) 103.

[2] G.P. Brasseur, J.J. Orlando, G.S. Tyndall, Atmospheric Chemistry and Global Change, Oxford University Press, New York (NY), 1999. [3] Y. Yokouchi, H. Mukai, H. Yamamoto, A. Otsuki, C. Saitoh, Y. Nojiri, J. Geophys. Res. Atmos. 102 (1997) 8805. [4] L.J. Carpenter, W.T. Sturges, S.A. Penkett, P.S. Liss, B. Alicke, K. Hebestreit, U. Platt, J. Geophys. Res. Atmos. 104 (1999) 1679. [5] S. Pechtl, E.R. Lovejoy, J.B. Burkholder, R. von Glasow, Atmos. Chem. Phys. 6 (2006) 505. [6] A. Saiz-Lopez, J.M.C. Plane, G. McFiggans, P.I. Williams, S.M. Ball, M. Bitter, R.L. Jones, C. Hongwei, T. Hoffmann, Atmos. Chem. Phys. 6 (2006) 883. [7] E.S.N. Cotter, N.J. Booth, C.E. Canosa-Mas, D.J. Gray, D.E. Shallcross, R.P. Wayne, Phys. Chem. Chem. Phys. 3 (2001) 402. [8] J.J. Orlando, C.A. Piety, J.M. Nicovich, M.L. McKee, P.H. Wine, J. Phys. Chem. A 109 (2005) 6659. [9] Y.V. Ayhens, J.M. Nicovich, M.L. McKee, P.H. Wine, J. Phys. Chem. A 101 (1997) 9382. [10] M. Bilde, T.J. Wallington, J. Phys. Chem. A 102 (1998) 1550. [11] E.S.N. Cotter, N.J. Booth, C.E. Canosa-Mas, R.P. Wayne, Atmos. Environ. 35 (2001) 2169. [12] S. Enami, S. Hashimoto, M. Kawasaki, Y. Nakano, T. Ishiwata, K. Tonokura, T.J. Wallington, J. Phys. Chem. A 109 (2005) 1587. [13] T.J. Gravestock, M.A. Blitz, D.E. Heard, J. Phys. Chem. A 112 (2008) 9544. [14] S. Enami, T. Yamanaka, S. Hashimoto, M. Kawasaki, K. Tonokura, J. Phys. Chem. A 109 (2005) 6066. [15] V. Baklanov Alexey, N. Krasnoperov Lev, J. Phys. Chem. A 105 (2001) 97. [16] A. Fahr, A.K. Nayak, M.J. Kurylo, Chem. Phys. 197 (1995) 203. [17] S.A. Carr, M.T. Baeza Romero, M. Blitz, M.J. Pilling, D.E. Heard, P.W. Seakins, Chem. Phys. Lett. 445 (2007) 108. [18] M.T. Baeza Romero, M.A. Blitz, D.E. Heard, M.J. Pilling, B. Price, P.W. Seakins, Phys. Chem. Chem. Phy. 9 (2007) 4114. [19] S.J. Klippenstein, L.B. Harding, J. Phys. Chem. A 104 (2000) 2351. [20] Facsimile. MCPA Software: Harwell, 2003.