Halon Replacements - American Chemical Society

Chapter 27

Theoretical Prediction of Thermochemical and Kinetic Properties of Fluorocarbons 1

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M. R. Zachariah , P. R. Westmoreland , D. R. F. Burgess, Jr. , Wing Tsang , and C. F. Melius Downloaded by STANFORD UNIV GREEN LIBR on October 1, 2012 | http://pubs.acs.org Publication Date: May 5, 1997 | doi: 10.1021/bk-1995-0611.ch027

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Chemical Sciences and Technology Laboratory, Chemical Kinetics and Thermodynamics Division, National Institute of Standards and Technology, Gaithersburg, MD 20899-0001 Department of Chemical Engineering, University of Massachusetts, Amherst, MA 01003-3110 Sandia National Laboratories, P.O. Box 969, Livermore, CA 94551-0969 2

3

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

Downloaded by STANFORD UNIV GREEN LIBR on October 1, 2012 | http://pubs.acs.org Publication Date: May 5, 1997 | doi: 10.1021/bk-1995-0611.ch027

27.

Thermochemical and Kinetic Properties of FCs 359

ZACHARIAH ET AL.

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 MP4/631G(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. g^

(1)

where fy= Ay exp(-0Cjj ry)

(2)

EBAC

(Ai-Aj) = fij g

kij

and Aij and ay are calibration constants that depend on bond type and ry is the bond length at the Hartree-Fock level. and gkij = (1 - h hy)

(3)

ik

is the second-nearest neighbor correction where h

ik

= B exp(-a k

TABLE

ik

(r - 1.4 A)

(4)

ik

1: Bond Additivity Correction Parameters MP4/6-3 lG(d,p)//HF/6-3 lG(d)

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

CH

4

C2H6, C2H2

H 0 CH3OH, C H 0 HF CF H 2

2

4

2

Aii 38.61 1444.1 72.45 175.6 84.21 143.29 18.98

&jj 2.0 3.8 2.0 2.14 2.0 2.1 2.0

Atom Type H C O F

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

0 0.31 0.225 0.33

HALON REPLACEMENTS

360

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]: E pin = E(UMP3) - E(PUMP3) S

(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 Equilibrium 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 J A N A F number. Schnieder and Wallington [8] have recently completed a study of the thermochemistry of CF2O and related compounds using QCIbased 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 J A N A F assignment to be correct. In general, however, the agreement with experiment (where available and appropriate) was excellent.


27. ZACHARIAH ET AL.

Thermochemical and Kinetic Properties of FCs

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 B D E 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 B D E 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 B D E 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 B D E 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 B D E 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 to C F , 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 sp when bonded in ethane to sp » 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 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 B D E we can define an intrinsic bond energy which for ethane is in fact larger than the perfluoroethane, in keeping with one's expectations. 3

3

3

2

2

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 B D E shows the opposite to be true.


361

HALON REPLACEMENTS

362

Table 2. B A C - M P 4 Enthalpy of Formation kJ/mol SPECIES

BAC-MP4

EXPT

REF

4

-237.8 -452.9 -693.3 -933.1

A

CHF CF

-233.9 -451.0 -699.6 -934.3

•CHjF •CHF •CF

-31.4 -247.3 -471.5

-32.6 -247.7 -464.8

B B B

:CHF :CF 2

131.7 -203.3

125.5 -182.0

C C

236.4

255.2

C

CHF=0 CF =0 •CF=0

-395.0 -598.3 -182.8

-376.6 -638.9 -171.5

C C C

CHFjO CF 0*

-194.6 -405.8 -628.4

-655.6

D

CHjF CHJFJ


3

2

3

•CF

2

3

CHjFOH(JE)

CHFpH(G) CHF/)H(£) CF OH

-412.1 -420.9 -430.1 -672.0 -684.5 -918.8

CHjOF CHjFOF CHFjOF CFjOF

-91.9 -260.1 -520.9 -749.4

•CHFOH •CFpH(£) •CFjOHCG)

-239.4 -456.5 -463.2

•CHjOF •CHFOF

-42.3 -308.8

CF3OOH

-807.5

CHjFOO CHFjOO* CFjOO

-172.8 -401.2 -627.6

CF(0)OH

-615.0

CF^OH), CFj(0)OH FCOj

-903.0 -620.7 -336.5

CHjFOU(Z) CHjFOH(G)

3

A A A


27.

ZACHARIAH ET AL.

Table 2; continued


SPECIES

1243

CHF=CH«(E) CH =CF« CHF=CF«(Z)

123.0

2

CF =CF»

-2163

CjHF

REF

109.2 -42.7

a

EXPT

-41.0

CHF=CF-(E) CF =CH» 2


BAC-MP4

CHF=CH*(Z)

-67.8

118.0

125.5

c

31.8

20.9

c

E

454.0 CHF=C=0

-147.3

CF =C=0

-290.4

a

-CF=C=OCE)

69.0

CHjF-CHCKZ)

-322.6

CHjF-CHOCE)

-328.9

CHF -CHO(E)

-525.1

CHF -CHOCZ) 2

-538.9

CF,-CHO

-774.5

CHjF-CO(Z)

-169.9

CHjF-COKE)

-172.8

CHF -CO CHF/>

2

2

•CFPH(E)

2

CHrCHjF CHF CHF CH

-4.6

267.7 507.8

CHs-CHF,

-97.2

181.0

CH3-CHjF

129.5

72.7 |

+H + HF +H + HF +H + HF + HF

29.2 111.0 -189.3 -87.0 -431.0 -317.6 -545.2

45.1 |

+ HF

+ HF

-229.7 -314.7 -328.5 -718.9

+ HF + HF + HF +H

-714.4 -547.5 -473.5 -489.0 216.7 983

2

2

+ :CF

2

CHJF,

+H

CHF,

+H

---

+ :CHF

CF CF =0

+H

CF =0

+ HjO

2

CF 0 FCO(OH) FCO(OH) CH,-CHF« 2

163.0 147.9 161.0

-375.5

—»

2

177.6 84.6 175.1 157.0 76.6

+:CF

+H

4

624.0

-741.8 -109.6 -19.2 -248.6 -328.9 -300.0 -315.2 -301.9

+ HF

CH CHjF 4

-310.0

CHA

2

4

354.5 4163 329.2 465.8 797.3 311.1 751.9

CH =CH 2

r

*1

120.8 182.2 -121.8 -51.9 346.0 -3883 52.2

+H

•CHjF •CH, —» •CHF, -* •CHjF •CF, •CHF -* •CF, •CF=0 -* •CFjOH CHFjO* FC(0)OH

+ OH

2

CF^OH), FCO(OH) FCO, CO, CH^CH* CH^CHF

2

2

2

2

2

-

1 1 1 1 1 1

127.2 43.9 146.4 50.5 164.2 171.2 150.8 I 65.3 51.4 121.2J 125.8 1133 147.9 125.8 2923 W j

| | | |

1


369

370

HALON REPLACEMENTS

800 Substituted Methanes

-Bo E 3

600

HF Elimination H2 Elimination F2 Elimination F R b s t r a c t i o n by H H a b s t r a c t i o n by H Exp

1. 2. 3. 4. 5. 6.

Schug & UJagner, 1973 Schug et al., 1979 Kochuubei and Moin, 1971 Ulestenberg and Dehaas, 1975 Ridley, et al., 1972 Arthur & Bell, 1978


400

o CO 200

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 ( C H 2 , !CHF, 1

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.



27.

Thermochemical and Kinetic Properties of FCs 371

ZACHARIAH ET AL.

B.5

8.4

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

T

i

1

1

1

1

1

Fluorocarbene Insertion Reactions

1

1

$+ C H F 2

2

CH _ F + R 2

K

H

fl = CH F ; CH ; H ; HF; F 2

t_i

8

2

4

2

2

I

i

1

2

J

Number of Fluorine Rtoms (x) in methylene Carbon

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

1


372

HALON REPLACEMENTS

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 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)] t h


27.

ZACHARIAH ET AL.


373

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 CF O 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 CF H, CF HCL and CF O in H /O Flames", J. Chemie Physique, in press


2

3

2

2

2

2

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 Thermochemic 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


Halon Replacements - American Chemical Society

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