Effect of Chlorine Substitution on Sulfide Reactivity with OH ... - DTIC

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EFFECT OF CHLORINE SUBSTITUTION ON SULFIDE REACTIVITY WITH OH RADICALS John D. Hearn Applied Research Associates P.O. Box 41028 Tyndall Air Force Base, FL 32403

Michael V. Henley Air Force Research Laboratory

Marshall G. Cory, Joseph L. Vasey and Douglas S. Burns ENSCO, Inc. 4849 N. Wickham Road Melbourne, FL 32940

SEPTEMBER 2008 Distribution Statement A: Approved for public release; distribution unlimited. This work was presented at the 2008 Chemical and Biological Defense Physical Science and Technology (CBD PS&T) Conference in New Orleans LA, 17-21 Nov 08. At least one of the authors is a U.S. Government employee; therefore, the U.S. Government is joint owner of the work. If published in the conference proceedings, copyright may be asserted. If so, the United States has for itself and others acting on its behalf an unlimited, nonexclusive, irrevocable, paid-up, royalty-free worldwide license to use for its purposes.

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Effect of Chlorine Substitution on Sulfide Reactivity with OH Radicals

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*Hearn, John D.; **Henley, Michael V.; +Cory, Marshall G.; +Vasey, Joseph L.; +Burns, Douglas S.;

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Ref AFRL/RXQ Public Affairs Case # 08-171. Presented at the 2008 Chemical and Biological Defense Physical Science and Technology (CBD PS&T) Conference in New Orleans LA, 17-21 Nov 08. 14. ABSTRACT

Predicting plume migration and evolution of hazardous chemical species (toxic industrial compounds, chemical warfare agents, pesticides, etc.) is challenging because current models cannot accurately predict reaction rate constants with atmospheric oxidants. Predictive models based on structure activity relationships have not been developed to calculate rate constants for phosphorous or halogen containing compounds (elements that are often present in hazardous chemicals). In order to have a comprehensive approach for modeling plume migration and evolution of hazardous chemicals, rate constants need to be accurately predicted so that chemical degradation can be included. We have studied the effects that chlorine substitution has on the reaction between OH radicals and sulfides to determine the effect halogen substitution has on reactivity. The results show appreciable reduction in the rate constant and a dramatic decrease in the activation energy of the chlorine-substituted organics relative to their unmodified analogs. This implies that the chlorine decreases the stability of the intermediate complex that is formed with the sulfur during the reaction. Atmospheric implications are discussed.

15. SUBJECT TERMS

Hydroxyl radical, dimethyl sulfide, chloromethyl methyl sulfide, atmospheric chemistry 16. SECURITY CLASSIFICATION OF: a. REPORT b. ABSTRACT c. THIS PAGE

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EFFECT OF CHLORINE SUBSTITUTION ON SULFIDE REACTIVITY WITH OH RADICALS John D. Hearn1 and Michael V. Henley2 1 Applied Research Associates, Inc. 2 Air Force Research Laboratory 139 Barnes Dr. Suite 2 Tyndall AFB, FL 32403 Marshall G. Cory, Joseph L. Vasey and Douglas S. Burns ENSCO Inc. 4849 N Wickham Rd Melbourne, FL 32940 ABSTRACT Predicting plume migration and evolution of hazardous chemical species (toxic industrial compounds, pesticides, etc.) is challenging because current models cannot reliably predict reaction rate constants of these (often) complex organic molecules with atmospheric oxidants. In order to have a comprehensive approach for modeling plume migration and evolution of hazardous chemicals, rate constants need to be accurately predicted so that chemical degradation can be included. Here we present experimental and theoretical results of the reaction between OH radicals and organic sulfides to examine the effect of chlorine substitution. The results show appreciable reduction in the overall rate constant of the chlorine-substituted organics relative to their unmodified analogs. Both the theoretical calculations and experimental results show that the abstraction channel is similar for both compounds investigated suggesting that the addition channel is slower for the chlorine substituted sulfide. INTRODUCTION Reduced sulfur compounds have both natural and anthropogenic sources,1 and many toxic chemicals contain sulfur and halogen atoms. In order to predict the atmospheric reactivity of sulfur and halogen containing compounds, we must understand their mechanisms of degradation. This is best accomplished by starting with small representative molecules and incrementally increasing complexity. Many researchers have measured the reactivity of dimethyl sulfide (DMS) with atmospheric oxidants (OH, NO3, and O3) the most important of which is the OH radical during daylight. A convenient approach to measuring the effect of halogen substitution on reactivity is to measure the reaction rates of the substituted and unsubstituted analogs relative to one another. Relative rate experiments circumvent many experimental problems such as the need to accurately measure OH concentration or reaction time. Moreover, since the rate constant for DMS reacting with OH is known, the experimentally derived relative rate of chloromethyl methyl sulfide (CMMS) can be placed on an absolute scale.

METHODS Gas-phase reactions of OH + DMS/CMMS were carried out in a Teflon bag (80 L) mounted in a light box. UVB irradiation of methyl nitirite was used to provide the source of OH radicals. The organics were sampled from the chamber through a Teflon transfer line at 25 cm3/min and 100 cm3 was collected on a cryogenically cooled trap at -90ºC. The sample was then warmed to 50ºC and transferred for analysis by GC-FID. Theoretical structures were optimized at CCSD level of theory with the 6-31++G** basis set. Rate constants were then calculated using a semiclassical flux-flux autocorrelation function approach.2

-LN([CMMS]/[CMMS]0)

RESULTS Figure 1 shows a relative rate plot 0.5 of CMMS and DMS reacting with OH where CMMS reacts at 0.225 times the rate of DMS. The reaction of 0.4 reduced sulfur compounds with OH can proceed through either H0.3 abstraction or addition to the sulfur. The addition channel is dependent on 0.2 the oxygen concentration, so the experiment was repeated with 2% O2 to examine the relative dependence on 0.1 this channel. The lower O2 concentration favors the H-abstraction 0.0 channel. As shown in Figure 1, the 0.0 0.5 1.0 1.5 relative rate of CMMS vs. DMS with -LN([DMS][DMS]0) low O2 increases to 1.07 suggesting that the abstraction channel for Figure 1. Relative rate plot of the reaction of CMMS CMMS is approximately the same as and DMS with OH at 20.8% (■) and 2.0% (○) O2. that for DMS. These relative rates are placed on an absolute scale in Table 1 by using the recommended DMS rate constants for these two conditions. TABLE 1. OH rate constants determined in this work (× 1012 cm3/molecule/sec.) at 20ºC. Rate constants in parentheses are taken from the indicated reference. Abstraction rate constants for primary and secondary hydrogens are per hydrogen. ND = not determined. Compound DMS CMMS

Experiment 20.8% O2 2% O2 (6.82)3 1.53

(4.97)3 5.32

Abstraction Primary Secondary 0.65 N/A 0.28 1.55

Theory Total 3.9 3.9

Addition

Total

2.1 ND

6.0 3.9

These reactions were also examined theoretically with the results shown in Table 1. The theoretically determined rate constant for DMS at 20ºC is in excellent agreement with the recommended value (6.0 and 6.82 × 10-12 cm3/molecule/sec, respectively). The total theoretical

abstraction rate constants for DMS and CMMS are the same, corroborating the experimental results at low O2 pressure. However, the primary hydrogens of CMMS react more slowly than those for DMS, and it is unclear what causes this. The secondary hydrogens in CMMS react much faster because the C-H bond is weakened by the electron-withdrawing chlorine atom. The calculated transition states for OH attack on DMS and CMMS are shown in Figure 2 where the chlorine atom in CMMS causes the OH radical to partially align with the C-Cl bond. O O

C

S

C

Cl

C

S

C

Figure 2. Calculated transition states for CMMS-OH (left) and DMS-OH (right). Unlabeled atoms are hydrogens. The theoretical rate constant prediction is bracketed by the 20.8% and 2.0% O2 measurements for CMMS. It is unclear what causes the increase in the rate constant at the lower O2 concentration for CMMS. One possible explanation is the interference of NOx on the degradation of DMS or CMMS or both. NOx has been a problem with previous relative rate measurements of DMS, so these experiments need to be repeated with a NOx free OH source (H2O2 or O3). The rate constant for the CMMS + OH reaction has been determined at 2 Torr in argon (k = 2.5 × 10-12 cm3/molecule/sec.) using an absolute technique.4 Under these conditions, H-abstraction is expected to dominate the reaction and this compares well with the theoretical prediction and the relative measurement at low O2 pressure (favoring H-abstraction). CONCLUSIONS Chlorine substitution on a reduced sulfur compound reduces the overall rate of reaction with the OH radical. The abstraction channel is shown to be the same for DMS and CMMS, so the addition channel may be different for DMS and CMMS. Additional work needs to be done to quantify the contribution of the OH addition channel with CMMS in order to determine what effect the chlorine substitution has on it. REFERENCES 1. Finlayson-Pitts, B. J.; Pitts, J. N. J. Chemistry of the Upper and Lower Atmosphere; Academic Press: San Diego, CA, 2000. 2. Runge, K.; Cory, M. G.; Bartlett, R. J. The Journal of Chemical Physics 2001, 114, 5141. 3. Atkinson, R.; Baulch, D. L.; Cox, R. A.; Crowley, J. N.; Hampson, R. F.; Hynes, R. G.; Jenkin, M. E.; Rossi, M. J.; Troe, J. Atmospheric Chemistry and Physics 2004, 4, 1461. 4. Shallcross, D. E.; Vaughan, S.; Trease, D. R.; Canosa-Mas, C. E.; Ghosh, M. V.; Dyke, J. M.; Wayne, R. P. Atmospheric Environment 2006, 40, 6899.

EFFECT OF CHLORINE SUBSTITUTION ON SULFIDE REACTIVITY WITH OH RADICALS John D. Hearn, Applied Research Associates, Inc. and Michael V. Henley, Air Force Research Laboratory/RXQL Marshall G. Cory, Joseph L. Vasey and Douglas S. Burns, ENSCO Inc.

Overview

CMMS and DMS  have similar  abstraction rates.

Results 0.5

Parent structures DMS

0.1

0.5

• Rate law: • Solution:

1.0

CMMS

1.5

-LN([DMS][DMS]0)

• Experimental:

Compound

OH

SAR

• OH source: photolysis of methyl nitrite

[OH]t  cancels out! l t!

• Detection: cryotrap, GC-FID

Rate Constant [cm3 m molec-1 sec-1]

Shallcross (2006)

• Relative:

• Computational

Total SAR

DMS CMMS

SCFFAF (TOTAL)

Abstraction Only SAR

3.0E-12

(6.82) 1 53 1.53

(4.97) 5 32 5.32

Theory Abstraction Primary Secondary 0.65 N/A 0 28 0.28 1 55 1.55

Total 3.9 39 3.9

Addition

Total

2.1

6.0 33.99

DMS rate constants in parentheses are from Atkinson, R.; et al. Atmospheric Chemistry and Physics 2004, 4, 1461.

Negative EA

Theory predicts equivalent abstraction rates.

1.0E-12

260

270

280

290

300

310

320

330

340

350

Temperature [K]

Shallcross, D. E. et al. Atmospheric Environment 2006, 40, 6899.

kkOHfor DMS for+ OH DMS OH

1.2E-11

 DMS and CMMS have similar abstraction rate constants  CMMS has a slower addition rate constant  CMMS has a negative activation energy

8.0E-12

-1

-1

Conclusions

Expt (O2) Expt (Inert) SAR SCFFAF (Abstraction) SCFFAF (Addition) MP2/MED SCFFAF (Addition) MP2/LRG SCFFAF (Total kOH)

1.0E-11

kOH [cm molec sec ]

6.0E-12

3

• Structures optimized p at CCSD/6-31++G** • Activation energy • Single point energy: QCISD(T)/6-311G** (QCI) • Single point energy: MP2/6-311G** (SML) • Single point energy: MP2/6-311+G(3df,2p) (LRG) • Zero Point Energy from a vibrational frequency analysis: MP2/6-31++G** (ZPE) • Extrapolated energy = E(QCI) + E(LARG) – E(SML) + ZPE • Characterize the TS • Use a three-point fit methodology – fit a harmonic potential to three CCSD single point energies (one at the TS and one on either “side” of the TS) • Minimum Energy Path • Use a reparameterized Hamiltonian (a variant of the AM1 NDDO Hamiltonian) to accurately reproduce coupled cluster theory to compute the reaction path • Dynamics Modeling • Semi-Classical Flux-Flux Autocorrelation Function (SCFFAF) K Runge, MG Cory, and RJ Bartlett, J. Chem. Phys. 2001, 114, 5141.

SCFFAF (TS4, primary) SCFFAF (TS6, secondary)

4.0E-12

0.0E+00 250

Experiment 20.8% O2 2% O2

AFRL (2008)

5.0E-12

2.0E-12

1.0883 Ǻ

Rate constants (x 1012 cm3/molecule/sec.)

kOH for k forCMMS CMMS

6.0E-12

1.178 Ǻ 1.8091 Ǻ 1.7885 Ǻ

1.0894 Ǻ 1.8137 Ǻ 1.8163 Ǻ 1.0906 Ǻ

CMMS has a slower  CMMS has a slower addition rate.

[OH] difficult to  measure

1.8136 Ǻ 1.7777 Ǻ

Secondary hydrogens react  much faster than primary ones.

20.8% O2 kCMMS/kDMS = 0.224

0.2

0.0

1.0893 Ǻ

1.0907 Ǻ

0.0

Methods

1.8118 Ǻ

2% O2 kCMMS/kDMS = 1.07

0.3

Transition states 1.187 Ǻ

0.4

-LN([CMMS]/[C CMMS]0)

Predicting plume migration and evolution of hazardous chemical species (toxic industrial compounds, pesticides, etc.) is challenging because current models cannot reliably predict reaction rate constants of these (often) complex organic molecules with atmospheric oxidants. In order to have a comprehensive approach for modeling plume migration and evolution of hazardous chemicals, rate constants need to be accurately predicted so that chemical degradation can be included. Here we present experimental and theoretical results of the reaction between OH radicals and organic sulfides to examine the effect of chlorine substitution. The results show appreciable reduction in the overall rate constant of chloromethyl methyl sulfide (CMMS) relative to dimethyl sulfide (DMS). Both the theoretical calculations and experimental results show that the abstraction channel is similar for both compounds investigated suggesting that the chlorine substituted sulfide has a slower addition channel.

4.0E-12

Need to…

2.0E-12

0.0E+00 225

250

275

300

325

350

375

Temperature [K]

400

425

450

475

• Determine theoretical addition rate for CMMS Determine theoretical addition rate for CMMS • Measure temperature dependence of CMMS abstraction channel • Measure relative rates with O3 photolysis as OH source (NOx free!)

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