[ACS Symposium Series] Halon Replacements Volume 611 (Technology and Science) || Theoretical...

Chapter 27

Theoretical Prediction of Thermochemical and Kinetic Properties of Fluorocarbons

M. R. Zachariah1, P. R. Westmoreland2, D. R. F. Burgess, Jr. 1, Wing Tsang1, and C. F. Melius3

1Chemical Sciences and Technology Laboratory, Chemical Kinetics and Thermodynamics Division, National Institute of Standards

and Technology, Gaithersburg, MD 20899-0001 2Department of Chemical Engineering, University of Massachusetts,

Amherst, MA 01003-3110 3Sandia National Laboratories, P.O. Box 969, Livermore, CA 94551-0969

An ab-initio quantum chemistry procedure has been applied to the development of a database for thermochemistry and kinetics of C/H/F/O species. This information has been used to construct a chemical kinetic mechanism for the prediction of the behavior of fluorocarbons as flame suppressants. Bond-additivity corrected (BAC) Mollet-Plesset many-body perturbation theory (MP4) calculations have been performed to obtain a large body of thermochemical data on both closed-and-open shell fluorocarbon species. In addition, data on transition state structures for reactions have also been generated and rate constants based on RRKM analysis have been derived. Comparisons between theory and experiment for both thermochemistry and kinetics show excellent agreement. Calculated bond dissociation energies have been correlated to Mulliken charge distribution and have been used to understand bond energy trends in terms of electrostatic effects and molecular conformation.

CF3Br is a highly effective agent for the suppression of flames, whose activity is generally considered to be derived by bromine atom's activity in catalytically removing H atoms. The nature of CFsBr's (Halon 1301) environmental impact (ozone depletion potential), however, has prompted a search for alternative agents for flame suppression. The most promising replacement candidates seem to be fluorocarbons and hydrofluorcarbons, which have recently been evaluated in a critical study conducted at NIST under the auspices of the Air Force and other agencies [1]. As an aid to the testing and subsequent selection procedure, a theoretical model based on the application of detailed chemical kinetics has been developed [2-4]. Because the available thermochemical and kinetic data were not sufficient to the task, we have undertaken to calculate thermochemical data for a large set of stable and radical species along with a critical evaluation against experiment. In addition, for selected reactions deemed to be important, transition states were determined and used to calculate rate constants based on reaction rate theory (RRKM/master equation) methods.

This chapter not subject to U.S. copyright Published 1995 American Chemical Society

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In Halon Replacements; Miziolek, A., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1997.

27. ZACHARIAH ET AL. Thermochemical and Kinetic Properties of FCs 359

Calculation Methodology A l l calculations were performed using the BAC-MP4 procedure outlined by Melius [5]. This procedure involves ab initio molecular orbital calculation using the Gaussian series of programs [6], followed by application of a bond additivity correction (BAC) procedure to the ab initio calculated energy. The essence of the B A C procedure is to enable one to calculate energies at accuracies sufficient for chemical applications, without the need to resort to large basis sets or configuration interaction terms. This is a particularly important issue when the goal is the generation of a sufficiently complete data set for detailed chemical modeling.

Equilibrium geometries, vibrational frequencies and zero point energies were calculated at the HF/6-3 lG(d) level. Single point energies were calculated at the MP4/6-31G(d,p) level, to which the B A C procedure was applied. In the B A C method, errors in the electronic energy of a molecule are treated as bond-wise additive and depend on bonding partner and distance. The energy per bond is corrected by calibration at a given level of theory against molecules of known energy as listed in Table 1.

Melius [5 ] has shown that for any molecule Ak-Aj-Aj -Ai , the error in calculating the electronic energy can be estimated through a bond correction of the form.

and Aij and ay are calibration constants that depend on bond type and ry is the bond length at the Hartree-Fock level.

E B A C (Ai-Aj) = fij g k i j g^ (1)

where fy= Ay exp(-0Cjj ry) (2)

and gkij = (1 - h i k hy) (3)

is the second-nearest neighbor correction

where h i k = B k exp ( -a i k (r i k - 1.4 A) (4)

T A B L E 1: Bond Additivity Correction Parameters

Bond C-H C-C O-H C-O H-F C-F H-H

HF C F 4

H 2

C H 4

C2H6, C2H2

H 2 0 CH3OH, C H 2 0

MP4/6-3 lG(d,p)//HF/6-3 lG(d) Ai i &jj Atom Type 38.61 2.0 H

1444.1 3.8 C 72.45 2.0 O 175.6 2.14 F 84.21 2.0 143.29 2.1 18.98 2.0

0 0.31 0.225 0.33

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360 HALON REPLACEMENTS

For open shell molecules, an additional correction is needed due to contamination from higher spin states. This error is estimated using an approach developed by Schlegel in which the spin energy correction is obtained from [7]:

ESpin = E(UMP3) - E(PUMP3) (2)

For closed-shell species having a UHF instability

E s p i n = K S (S+l) where K = 10.0 (kcal/mol) (3)

The transition state for a reaction was obtained in the usual way, by searching a geometry with one negative eigenvalue (saddle point on the potential energy surface), followed by steepest-descent reaction path analysis to ensure the calculated transition state corresponds to the appropriate reactants and products. B A C corrections are assigned in the same manner as with the equilibrium structures. Where needed, RRKM/master equation analysis was employed using the calculated transition state to obtain reaction rate constants.

Results

Equil ibrium Thermochemistry

Heats of formation for C\ and C2 fluorocarbons have been calculated and, where possible, compared with available experimental data or other calculations. Table 2 summarizes the species for which calculations have been performed and their associated heats of formation. Of the over 90 species calculated to date, 44 were compared with available literature data, resulting in an average deviation of 6.5 kJ/mol (1.6 kcal/mol).

One of the key issues arising in this work turned out to be the heat of formation of carbonyl difluoride. Of the over 90 species calculated, carbonyl difluoride gave by far the largest deviation of 37.2 kJ/mol in the heat of formation at standard state. The previously accepted JANAF value is -635 kJ/mol, as compared to our calculated value of -598.2 kJ/mol.

During the course of this work, large basis sets and a limited number of G2 calculations were used to find a possible error in smaller basis sets, electron correlation or the B A C corrections. However, two independent calculations cast doubt on the validity of the accepted JANAF number. Schnieder and Wallington [8] have recently completed a study of the thermochemistry of CF2O and related compounds using QCI-based calculations and have concluded that the discrepancies they observe can only be explained by experimental error. They recommended a value of -607.3 ± 7 kJ/mol, which is consistent with our -598.2 kJ/mol value. Montgomery et al. [9] have independently come to the same conclusion based on calculations using the CBS method and determined a value for the heat of formation at 298 K of -608.6 kJ/mol. On the basis of these independent calculations, we proceed on the assumption that while one cannot definitely conclude that the experimental number is wrong, it is unlikely that ab-initio calculations using different approaches that have demonstrated high accuracy for other fluorocarbons, should produce an error of the magnitude necessary for the JANAF assignment to be correct. In general, however, the agreement with experiment (where available and appropriate) was excellent.

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General Chemical Features

The bond dissociation energies for C-F, C-H and C-C are shown in the figures below. In addition, we have plotted the Mulliken charge difference for comparison purposes. The Mulliken charge analysis is a procedure for assigning the relative local charge to an atom. As such, it can be used as an indicator of covalent versus ionic character.

Beginning with the fluoromethanes, the C-H and C-F bond dissociation energy (BDE) is plotted (Figure la,b) versus number of fluorine atoms and the absolute value of the associated difference in Mulliken charges A = IS i - 8jl between the bonding atoms. For C-H BDE's , addition of the first fluorine will decrease the bond dissociation energy, but upon subsequent substitution of fluorines the BDE begins to increase again. This correlates very well with the Mulliken population analysis. In methane, carbon is an electron acceptor and is slightly ionic. Addition of one fluorine decreases ionic character and so also the BDE. Further addition of fluorine changes the character of carbon from an electron acceptor to an electron donor returning the C-H bond to a more ionic behavior and therefore a stronger bond. The C-F bond by contrast shows a monotonic increase in BDE with fluorine substitution which again correlates well with the Mulliken population analysis. In both cases, our calculations compare very favorably with experiment as shown in the figures.

Figure 2a,b shows the BDE for the C-C bond in substituted ethanes as well as the Mulliken analysis. As is clear, the C-C BDE increases upon successive addition of fluorine to the same carbon. The molecule with the highest BDE CH3-CF3, also has the largest difference in Mulliken charges between the carbons, in keeping with the increased ionic character.

One intriguing point to note is that the BDE for C2F6 > C2H6 ! One's intuition might suggest the opposite. The explanation comes from the fact that as defined, the BDE is really a measure of the relative stability between radical and parent and not the intrinsic bond strength. The explanation for the anomalous behavior between C2F6, C2H6 and for the other symmetrically substitute fluoroethanes is that progressing from C H 3 to C F 3 , the radical goes from planar (sp2) to pyramidal (sp3). As such when the CF3 radical is formed from a bond breaking event, it is already at its equilibrium conformation. By contrast, the methyl radical goes from an sp3 when bonded in ethane to sp2» and must undergo a conformational rearrangement to lower energy. We have calculated the energy of these conformational relaxations to be about 80 kJ/mol in ethane (40 kJ/mol per methyl fragment). The C H F 2 and CH2F fragments were calculated to have conformational energies of 6 and 22 kJ/mol, respectively. If one adds back this conformational energy to the BDE we can define an intrinsic bond energy which for ethane is in fact larger than the perfluoroethane, in keeping with one's expectations.

While not shown here, we have used this analysis to calculate the relative contribution of the ionic and covalent components to the C-C bond energy. By first adjusting for the conformational correction, we can obtain an absolute measure of the ionic component by subtracting the BDE energy, between the symmetrically (no ionic character) and asymmetrically substituted fluoroethanes. This difference correlates linearly with the Mulliken charge difference. This analysis indicated that indeed the intrinsic C-C bond strength in C2H6 > C2F6, even though the BDE shows the opposite to be true.

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Table 2. BAC-MP4 Enthalpy of Formation kJ/mol

SPECIES BAC-MP4 EXPT REF CHjF -233.9 -237.8 A CHJFJ -451.0 -452.9 A CHF3 -699.6 -693.3 A CF4 -934.3 -933.1 A

•CHjF -31.4 -32.6 B •CHF2 -247.3 -247.7 B •CF3 -471.5 -464.8 B :CHF 131.7 125.5 C :CF2 -203.3 -182.0 C •CF 236.4 255.2 C CHF=0 -395.0 -376.6 C CF2=0 -598.3 -638.9 C •CF=0 -182.8 -171.5 C

-194.6 CHFjO -405.8 CF30* -628.4 -655.6 D CHjFOH(JE) -412.1 CHjFOU(Z) -420.9 CHjFOH(G) -430.1 CHFpH(G) -672.0 CHF/)H(£) -684.5 CF3OH -918.8 CHjOF -91.9 CHjFOF -260.1 CHFjOF -520.9 CFjOF -749.4 •CHFOH -239.4 •CFpH(£) -456.5 •CFjOHCG) -463.2 •CHjOF -42.3 •CHFOF -308.8

CF3OOH -807.5 CHjFOO -172.8 CHFjOO* -401.2 CFjOO -627.6

CF(0)OH -615.0 CF^OH), -903.0 CFj(0)OH -620.7 FCOj -336.5

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Table 2; continued SPECIES BAC-MP4 EXPT REF CHF=CH*(Z) 1243

CHF=CH«(E) 123.0 CH 2=CF« 109.2 CHF=CF«(Z) -41.0

CHF=CF-(E) -42.7 CF 2=CH» -67.8 CF a=CF» -2163

CjHF 118.0 125.5 c 31.8 20.9 c

454.0

CHF=C=0 -147.3 CF a=C=0 -290.4 -CF=C=OCE) 69.0

CHjF-CHCKZ) -322.6 CHjF-CHOCE) -328.9 CHF2-CHO(E) -525.1 CHF2-CHOCZ) -538.9 CF,-CHO -774.5

CHjF-CO(Z) -169.9 CHjF-COKE) -172.8 CHF2-CO<Z) -377.4 CF,-CCKZ) -610.0 OyCCKE) -6113

C H J - C H J F -272.4 -263.2 E

C H J F - C H J F -446.9 -431.0 F CHj-CHF, -505.4 -500.8 E

CHJF-CHFJI -671.5 -643.5 G

CHj-CF, -755.2 -745.6 E

C H F J - C H F J -890.4 -860.6 H CHjF-CF, -913.4 -895.8 E

CHF 2-CF, -1124.2 -1104.6 E

CFj-CF, -1357.0 -1342.6 E

CHjF-CH^ -56.2 -48.1 H CHj-CHF» -75.6 -78.2 J CHjF-CHF* -2473 -235.6 H CHFj-CH,* -280.7 -285.8 H CH,-CF,« -300.0 -302.5 I CByF-CFj* -460.1 -438.9 H CHFj-CHF* -459.8 -451.4 H CF,-CHj» -527.2 -517.1 I CHF2-CF,« -673.2 CF,-CHF» -702.8 -680.7 J

CF 3-CFj- -907-5 -391.2 L

CHj=CHF -1393 -138.9 K CHF=CHF(Z) -301.2 CHF=CHF(£) -302.1 CH 2=CF 2 -340.2 -336.8 L CHF=CF, -485.8 CF Z=CF 2 -653.5 •658.6 c

NOTE: Complete Reference Information available on page 373.

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i • - • • i • - • • " i "T—•—•—i—]

J • I • • • • l • • , . L

0 1 2 * 3 Number of Fluorine Rtoms

FIG. la. Calculated C-H BDE, experimental data and absolute value of difference of Mulliken charges between C-H as a function of fluorine substitution in fluoromethanes.

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s a CH5-CH5

B 2 4 6

Number of Fluorine Rtoms

FIG 2a. C-C BDE as a function of number of fluorine atoms

i . . . , , . •-, CH3CF3 n

i . . . ,

1.5 / \ -

arg

e

CH3CHF2 J \ JC u

/ \ \ CH3CHF2 e 1.B - / \ A \

\ U CH3CH2F J \ / \ CHF2CF3 I

LJ <

B.5 / \ CH2FCHF2/K

B.B C 2 H ^ CH2FCH2fV / CHF2CHF2

- • • i i

\ C2F6-

i i

B 2 4 6

Number of Fluorine Atoms

FIG 2b. Absolute value of difference in Mulliken charges between carbons in substituted ethanes.

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B 1 2 3 4 5

Number of fluorine Atoms

FIG 3a. C-H BDE for substitute ethanes as a function of number of fluorines

1 , i , . | CF3CH2-H

C H F 2 C H 2 - j ^ g CH2FCH2-H

" * \

•""""^ \

C2H5-H * •

* \ * V * \

xCH3CF2^H * \ • ^ ^ H 2 F C F ^ - H C H F 2 C F 2 - H

\ N V \ x \

-V » V \ v \

CF3CHF-H

CH3CHF-H ET"

i i

CHF2CHF-H CH2FCHF-H

i i i i B 1 2 3 4 5


FIG 3b. Absolute value of difference in Mulliken charges between C-H in substituted ethanes.

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The C-H and C-F BDE's are presented in Figures 3 and 4. As was the case for the fluoromethanes, the Mulliken charges generally correlate well with the BDE's. The basic trend observed are that fluorine substitution on the a carbon decreases the C-H BDE and increases the C-F BDE. On the other hand fluorine substitutions effect on the P carbon is to increase the C-H B D E and has virtually no effect on the C-F BDE. Substituted ethylenes showed similar trends to those discussed for the ethanes.

- 8

AddF A B D E =0

B D E

AddF

- 5 + 8 X F

+ 5 small

Transition States

The energetic properties for transition states are tabulated in Table 3. Transition state energies are presented as heats of formation in an analogous fashion to the equilibrium properties of molecules.

Reactions involving substituted methanes are summarized in Figure 5 as a function of number of fluorines. The most favored unimolecular decomposition process is HF elimination, followed by H 2 elimination, with F 2 elimination highly unlikely. In general, the more highly fluorine-substituted, the lower the activation barrier to decomposition. The effect can be quite significant, particularly for the HF elimination case where the activation barrier decreases by over 50 kJ/mol in going from CH3F (367 kJ/mol) to CHF3 (314 kJ/mol). Comparison with experimental data where available is quite good and follows the calculations in both qualitative trends and quantitative results.

Attack by H atoms favors abstraction of H, with activation barriers (in the 40 -50 kJ/mole range ) that are relatively insensitive to fluorine substitution. In contrast, fluorine abstraction by H requires a barrier about two and a half times as large as H abstraction and clearly indicates an increase in barrier height with increased fluorine substitution.

There are numerous studies on the thermal decomposition of CF3H using the shock tube technique [10-14]. It is clear that even at the highest pressures the reactions are still in the pressure-dependent regime. Schug et al. [10] have carried out an extrapolation of their results to obtain the limiting high-pressure rate expression for HF elimination from C F 3 H (1.2x1014 exp (-36300/T) s-i. An Arrhenius plot of the results can be found in Figure 6 along with our calculation based on transition state theory. In a

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o E

~ 520

u 500

o .2 S480

o QQ

460

a CH3CF2-F P / C I I2FCF2f

CH3CHF-F

i—«—1—1—1—i—«—•—1—1—i— CHF2CF2-F C2F5-F

C2H5-F

CF3CHF-F

CH2FCH2-F

1 2 3 4 5 6


FIG 4a. C-F BDE for substituted ethanes as a function of number of fluorines

1.4

EM.2 re n u

£ 1.8

* 8 . 8 i

< 8.6

8.4

C H 3 C F 2 - F B ^ C H 2 F C F 2 - F C H F 2 C F 2 - F

CH3CHF-F

C2F5-F.

CHF2CHF-F/

/ CH2FCHF-F7

C2H5-F CH2FCH2-F

CHF2CH2-F CF3CH2-F

1 2 3 4 5 6

Number of Fluorine Atoms

FIG 4b. Absolute value of difference in Mulliken charges between C-F in substituted ethanes.

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Table 3. BAC-MP4 Transition State Calculations kl/mol

T R A N S I T I O N S T A T E A H ; * 1 CH,F - » CH2 + HF 120.8 354.5 -CHF + H 2 182.2 4163 CHF + HF -121.8 329.2 -CF2 + H 2 -51.9 465.8

— CHj + F 2 346.0 797.3 CHF, -CF2 + HF -3883 311.1

CHF + F 2 52.2 751.9 CF4 -* CF2+F2 -310.0 624.0 CF3OH - CF2=0 + HF -741.8 177.6

-•CH=0 + HF -109.6 84.6 CHjFO- - CF2=0 + F -19.2 175.1 CHF/> -••CF=0 + HF -248.6 157.0 CHF/> -» CF2=0 + H -328.9 76.6 •CFPH(E) •CF=0 + HF -300.0 163.0

-» CF2=0 + H -315.2 147.9 —» CHFjO- -301.9 161.0

CHrCHjF CH2=CH2 + HF -4.6 267.7 CHF r

CHF2

CHA +:CF 2

-375.5 507.8

CH4 + :CF2 CHs-CHF, -97.2 181.0 CH4 + :CHF CH3-CHjF 129.5 72.7 | CHjF + H —» •CHjF + H 2 29.2 45.1 |

•CH, + HF 111.0 127.2 1 CHJF, + H —» •CHF, + H 2 -189.3 43.9 1 -* •CHjF + HF -87.0

146.4 1 CHF, + H -•CF, + H 2 -431.0 50.5 1 -•CHF2 + HF -317.6

164.2 1 CF4 + H -* •CF, + HF -545.2 171.2 1 CF2=0 + H -•CF=0 + HF -229.7 150.8 I

-* •CFjOH -314.7 65.3 -CHFjO* -328.5 51.4 CF2=0 + HjO -FC(0)OH + HF -718.9 121.2J

CF̂ OH), -714.4 125.8 CF20 + OH F2CO(OH) -547.5 1133 | F2CO(OH) FCO, + HF -473.5 147.9 | FCO(OH) CO, + HF -489.0 125.8 | CH,-CHF« CH^CH* + HF 216.7 2923 | -CH^CHF + H 983 W j 1

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o E 3

o CO

800

600

400

200

Substituted Methanes - B - HF Elimination

H2 Elimination F2 Elimination F Rbstraction by H H abstract ion by H Exp

1. Schug & UJagner, 1973 2. Schug et al., 1979 3. Kochuubei and Moin, 1971 4. Ulestenberg and Dehaas, 1975 5. Ridley, et al., 1972 6. Arthur & Bell, 1978

2 3 Number of Fluorine Rtoms

NOTE: Complete Reference Information available on page 373.

FIG 5. Activation barriers for unimolecular decomposition and H atom attack of fluoromethanes as a function of number of fluorine atoms.

similar fashion, we compare the extrapolated experimental rate constants from Schug and Wagner [14] for the thermal decomposition of C H 3 F to C H 2 + HF with those derived on the basis of transition state theory based on BAC-MP4 calculations. Once again there is excellent agreement in the rate constants within the error limits of the extrapolated high-pressure rate constants. From these results we conclude that for organic fluorine dehydrofluorination, BAC-MP4 calculations of the transition state leads to unimolecular rate constants that are probably within a factor of 3 of the true values.

For carbene insertions ( 1 C H 2 , !CHF, ICF2 ) we use the singlet state of C H 2

for comparison because the ground state for the fluorocarbenes are singlet. The results are summarized in Figure 7. Singlet C H 2 is well know to insert into virtually any molecule. Insertion by CHF has moderate barriers of up to 45 kJ/mol, while insertion of CF2 involves much higher activation barriers of between 85 kJ/mol for insertion into HF to 280 kJ/mol for insertion into CH2F2. The implications are that highly fluorinated compounds would produce longer lived CF2, which, rather than insert as would be the case for methylene, would preferentially be oxidized.

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8.4 B.5 8.6 180B/T(K)

FIG 6. Comparison of experiment and theoretical calculation of limiting high pressure rate constants for HF elimination from fluoromethanes.

— T i T 1 1 1 1 1 1 1

Fluorocarbene Insertion Reactions $+ CH 2 F 2

CH2_KFH + R fl = CH 2 F 2 ; CH 4; H 2; HF; F 2

t _ i I i J

8 1 2 Number of Fluorine Rtoms (x) in methylene Carbon

FIG 7. Activation barriers for insertions of !CH2, ! C H 2 , and 1 CF2-

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We have also undertaken an extensive study of the reactions of CF2O + H and H2O to be published elsewhere [15], which indicated that the H atom reaction should be the most important under typical flame conditions. The analysis has lead to a predictive rate constant for the H atom attack leading to CFO + HF, in excellent agreement with both the seminal work of Biordi [16] and more recent data obtained by Richter etal. [17].

Conclusions

A bond additivity correction procedure has been applied to a large body of ab initio molecular orbital computations on fluorocarbon molecules. Where available, the computations have been compared with literature values and show overall excellent agreement. Transition state computations have also been used to obtain barrier heights for reaction and have been subsequently used to obtain reaction rate constants from RRKM/master equation analysis. The results of the work suggest that heavy reliance on computational chemistry methods can under appropriate circumstances lead both to chemical insight and to thermochemical and kinetic data with requisite chemical accuracy, which could otherwise be unattainable by experimental methods, given time and resource constraints. The results presented here bode well for the wider use of these methodologies for a wider range of chemical systems of environmental interest.

References

1. Nyden M.D, Linteris, G.T., Burgess Jr, D., Westmoreland, P.R., Tsang, W., M.R.,Zachariah, M.R., "Flame Inhibition Chemistry and the Search for Additional Fire Fighting Chemicals" in Evaluation of Alternative In-Flight Fire Suppressants for Full-Scale Testing in Simulated Aircraft Engine Nacelles and Dry Bays , Ed. W. Grosshandler, R. Gann, W. Pitts, Pages 467-641, Report # NIST SP 861 (1994)

2. Burgess Jr., D., Tsang, W., Zachariah,M.R., and Westmoreland, P.R., "Fluorinated Hydrocarbon Flame Supression Chemistry" to appear in ACS Symposium on Fuel Chemistry (1994)

3. Westmoreland, P.R., Burgess Jr., D.R.F., Tsang, W., and Zachariah,M.R.,"Kinetics of Fluoromethanes in Flames" to appear in the 25 t h

Symposium (International) on Combustion (1994) 4. Burgess Jr., D.R.F., Tsang, W., Zachariah, M.R., and Westmoreland, P.R.,

"Kinetics of Fluorine-Inhibited Hydrocarbon Flames", Proceedings of the Halon Options Technical Working Confrence , 489-501 (1994)

5. Melius, C.F., and Binkley, J.S.,Twenty-First Symposium (International) on Combustion, 1953 (1986)

6. M. J. Frish, M. Head-Gordon, G. W. Trucks, J. B. Foresman, H. B. Schlegel, K.Raghavachari, M. A. Robb, J. S. Binkley, C. Gonzalez, D. J. DeFrees, D. J. Fox, R.A. Whiteside, R. Seeger, C. F. Melius, J. Baker, L. R. Martin, L. R. Kahn, J.Stewart, S. Topiol, J. A. Pople, Gaussian90, Gaussian Inc., Pittsburgh, Pa (1990).

7. Schlegel, H.B., J.Chem. Phys. 84, 4530 (1986) 8. Schnieder, W.F., and Wallington, T.J., J. Phys. Chem. 98, 7448 (1994) 9. Montgomery Jr., J.A., Michels, H.H., and Francisco, J.S., Chem. Phys. Lett.,

220 391 (1994)]

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10. Schug, K.P., Wagner, H. Gg., and Zabel, F., Ber. Bunsenges. Phys. Chem., 85, 167 (1979)

11. Hidaka, Y., Nakamura, T., and Kawano, H., Chem. Phys. Lett., 187, 40 (1991) 12. Tschuikow-Roux, E., J. Phys. Chem., 42, 3639 (1965) 13. Modica, A. P., and LeGraff, J.E., J. Chem. Phys. 44, 3375, (1966) 14. Schug, K.P., and Wagner, H. Gg., Z. Phys. Chem. 86, 59 (1973) 15. Zachariah, M.R., Tsang, W., Westmoreland, P.R., and Burgess Jr. D.R.F.,

"Theoretical Prediction of the Thermochemistry and Kinetics of Reactions of CF2O with Hydrogen atom and Water", submittted.

16. Biordi, J.C., Lazzara, C. P. and Papp, J.F., Fifteenth Symposium (International) on Combustion 917 (1974)

17. Richter, H., Vandooren, J., and Van Tiggelen, "Kinetics of the Consumption of CF3H, CF2HCL and CF2O in H2/O2 Flames", J. Chemie Physique, in press

References for Table 2

a. Rodgers, A.S; Chao, J.; Wilhoit, R.C.; Zwolinski, B.J.; J. Phys. Chem. Ref. Data 1974, 3, 117.

b. McMillan, D.F.; Golden, D.M.; Ann. Rev. Phys. Chem. 1982, 33, 493. c. Stull, D.R.; Prophet, H.; JANAF Thermochemical Tables, 1971, NSRDS-NBS

37. d. Batt, L.; Walsh, R.; Int. J. Chem. Kin. 1982, 14, 933-944. e. Chen, S.S.; Rodger, A.S.; Chao, J.; Wilhoit, R.C.; Zwolinski, B.J.; J. Phys.

Chem. Ref. Data 1975, 4, 441-456. f. Daubert, T.E.; Danner, R.P.; "DIPPR Data Compilation of Pure Compound

Properties," NIST Standard Reference Database 11 1985. g. Pedley, J.B.; Naylor, R.D.; Kirby, S.P.; Thermochemical Data of Organic

Compounds; Chapman and Hall: New York, NY, 1986. h. estimated using group additivity and/or bond dissociation energies. i. Rodgers, A.S.; ACS Symp. Ser. 1978, 66, 296. j. Martin, J.P.; Paraskevopoulos, G.; Can. J. Chem. 1983, 61, 861-865. k. Pedley, J.B.; Rylance, J.; Sussex-N.P.L. Computer Analysed Thermochemical

Data. Organic and Organometallic Compounds; University of Sussex: Brighton, England, 1977.

l. Stull, D.R.; Westrum, E.F., Jr.; Sinke, G.C.; The Chemical Thermodynamics of Organic Compounds; John Wiley: New York, NY, 1969.

References for Figure 5

Westenberg, A.A.; deHaas, N.; J. Chem. Phys. 62, 3321-3325 (1975). Ridley, B.A.; Davenport, J.A.; Stief, L.J.; Welge, K.H.; J. Chem. Phys. 57, 520 (1972). Arthur, N.L.; Bell, T.N.; Rev. Chem. Intermed. 2, 37-74 (1978). Kochubei, V.F.; Moin, F.B.; Kinet. Catal. 11, 712 (1971). Schug, K.P.; Wagner, H.Gg.; Z. Phys. Chem. 86, 59-66 (1973). Schug, K.P.; Wagner, H.Gg.; Zabel, F.; Ber. Bunsenges. Phys. Chem. 83, 167 (1979).

RECEIVED July 20, 1995

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