Reaction Product Identification from O(3P) + Benzene, Toluene, and 1

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Reaction Product Identification from O( P) + Benzene, Toluene, and 1,3,5-Trimethylbenzene Collisions THOMPSON M. SLOANE

Downloaded by UNIV LAVAL on July 12, 2016 | http://pubs.acs.org Publication Date: June 1, 1977 | doi: 10.1021/bk-1977-0056.ch006

Physical Chemistry Department, General Motors Research Laboratories, Warren, MI 48090

Reactions of oxygen atoms with aromatic hydrocarbons have received a great deal of attention in recent years because of their potential importance in combustion processes and in atmos pheric chemistry. (1) This paper presents the results of product analysis of single reactive c o l l i s i o n s at room temperature between O( P) atoms and benzene, toluene, and 1, 3, 5 - trimethyl benzene. The apparatus used in these experiments consisted of a quadrupole mass spectrometer housed inside a two-chamber, d i f f u sion-pumped, stainless steel vacuum system. The major features of the design were similar to the apparatus employed by Foner and Hudson. (2) The beam intersection angle was 60°. Products were observed at m/e = 94 and 66 for the reaction of O( P) with benzene. The product at m/e= 94 i s a long-lived C H O complex which, if stable, i s probably phenol. The product at m/e = 66 corresponds to the complex minus a molecule whose mass i s 28. a.m.u., undoubtedly carbon monoxide. The ionization efficiency measurement for this C H hydrocarbon product i s shown in Figure 1. There appears to be a small amount of product with an ionization threshold near 9.0 eV, and a much larger amount with a threshold near 9.6 eV. Cyclopentadiene has an ionization potential of about 9 eV. Molecules similar to 3-penten-1-yne have ionization potentials in the range 9.8 to 10.2 eV. (4) Assuming that 3-penten-1-yne has an ionization potential close to those compounds, the major product at m/e = 66 i s l i k e l y to be this open-chain C H hydrocarbon. The less intense product is probably cyclopentadiene. The hydrocarbon product must be in its ground electronic state for two reasons. F i r s t , the reaction would be too endothermic (ΔΗ = 20 kJ/mol) to take place at these low kinetic energies if the product were e l e c t r o n i c a l l y excited. Second, the measured ionization potential would be about 3 eV smaller for the excited product. Products were observed at m/e = 108, 106, and 80 for the reaction of oxygen atoms with toluene. These three product 3

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5

5

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Brooks and Hayes; State-to-State Chemistry ACS Symposium Series; American Chemical Society: Washington, DC, 1977.

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STATE-TO-STATE CHEMISTRY


• M A S S 66

Ο

Δ

Ο

Δ 9.0

8.0

Δ

10.0 E l e c t r o n Energy ( e V ) 3

Figure 1. Ionization efficiency measurement for the 0( P) + benzene reaction product at m/e = 66. Ο—measured intensity; Δ—intensity corrected for thefiniteenergy width of the ionizing electrons using the procedure in Ref. 3.

masses correspond t o the three products observed f o r 0 ( P ) + 1, 3, 5 - trimethylbenzene. Since more extensive data were obtained f o r t h i s l a t t e r r e a c t i o n , the r e s u l t s f o r 0( P) + toluene w i l l not be discussed f u r t h e r . The r e a c t i o n o f 0( P) with 1, 3 , 5 - trimethylbenzene y i e l d e d products a t m/e = 136, 134, and 108. Figure 2 shows the i o n i z a t i o n e f f i c i e n c y measurement f o r the product a t m/e = 136. The i o n i z a t i o n t h r e s h o l d i s about 8.2 eV. This product could be 2, 4 , 6 - t r i m e t h y l p h e n o l , 3 , 5 - d i m e t h y l a n i s o l e , or 3 , 5 dimethyl benzyl a l c o h o l . Since i t i s unclear how the i n t e r n a l energy o f the C H , 0 adduct (^400 kJ/mol i f i t i s the ground e l e c t r o n i c s t a t e ) might a f f e c t an i o n i z a t i o n p o t e n t i a l measure ment, no c o n c l u s i o n can be drawn about t h i s product's s t r u c t u r e . The i o n i z a t i o n e f f i c i e n c y measurement f o r the product a t m/e J

g

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6.

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= 134 i s shown i n Figure 3 . A l i k e l y product a t t h i s mass i s 3 , 5 - dimethylbenzaldehyde because the i o n i z a t i o n p o t e n t i a l o f methylbenzaldehyde i s 9.33 eV. (4j_ The i o n i z a t i o n e f f i c i e n c y measurement f o r the product a t m/e = 108 i s shown i n Figure 4. As i n the case of 0( P) + benzene, when CO leaves the adduct the r i n g may c l o s e to form trimethylcyclopentadiene ( I . P . = 7.96 eV ) or remain open to given an open-chain o l e f i n s i m i l a r to hydrocarbons having i o n i z a t i o n p o t e n t i a l s ranging from 8 . 6 to 9.1 eV. Although the measur ed data were too noisy f o r c o r r e c t i o n , they i n d i c a t e that a t l e a s t a major f r a c t i o n o f the products a t m/e = 108 i s an openchain o l e f i n r a t h e r than t r i m e t h y l c y c l o p e n t a d i e n e . A r e a c t i o n path which was not observed f o r these r e a c t i o n s i s the hydrogen atom a b s t r a c t i o n path to produce OH and an aromatic r a d i c a l . Even i f the aromatic r a d i c a l i s s c a t t e r e d "backward," i t should s t i l l be observed due t o poor beam collimation. -, 1 Tests f o r the p a r t i c i p a t i o n o f r e a c t i o n s o f 0( D) o r 0p( Δ ) i n the 0 atom beam and f o r r e a c t i o n s i n the i o n i z e r showed t h a t these p o s s i b l e i n t e r f e r i n g processes were n e g l i g i b l e . g The c o r r e l a t i o n diagram of Figure 5 shows f o r 0( P) + benzene that ground s t a t e r e a c t a n t s should c o r r e l a t e with e x c i t e d products. The observed products must then be produced by a s p i n -

ο ( P ) Io}j-*3

+ /

Ο

Measured

Δ

Corrected

Mass 1 3 6

. 1.0

-

.5

Ο

Δ Δ

Ο

ο Δ

Δ Δ 7.0

Δ

Δ 8.0

9.0

Electron Energy (eV)

10.0

Figure 2. Ionization efficiency measure ment for the 0( P) + 1,3,5-trimethylbenzene reaction product at m/e = 136 3


90


0 ( P) +J^)_^ I 3

/

30

r

^ ^

• Mass 1 3 4

\—

Δ

Δ


20 \ —

Δ 10

h

Δ

Δ Δ & 7.0

Δ

ι

Δ

Δ

Δ

Δ

Δ

I

Δβ.Ο

9.0 Electron Energy

10.0

11.0

12.0

(eV)

Figure 3. Ionization efficiency measurement for the 0( P) + 1,3,5-trimethylbenzene reaction product at m/e = 134. Only the corrected measurement is shown. 3

forbidden process. T u l l y has s u c c e s s f u l l y explained the r a p i d r a t e s of a number of s p i n - f o r b i d d e n r e a c t i o n s . (5) To determine whether h i s theory may e x p l a i n the s p i n - f o r b i d d e n nature of the 0 + benzene r e a c t i o n , the f o l l o w i n g two questions must be answered: (1) i s the lowest t r i p l e t s t a t e of phenol (T,) l o n g l i v e d enough? (2) Is the c r o s s i n g region between T, and S e a s i l y a c c e s s i b l e during the l i f e t i m e of the complex? To answer the f i r s t q u e s t i o n , the RRKM l i f e t i m e of T was e s t i m a t e d . For a r e l a t i v e k i n e t i c energy of 2.7 kJ/mol, the estimated l i f e t i m e was 10" seconds. A c r o s s i n g pointed l o c a t e d near the,-minimum of a 1300 cm v i b r a t i o n could be t r a v e r s e d 3.5 χ 10 times during t h i s l i f e t i m e . In order to answer the second q u e s t i o n , t h a t of the a c c e s s n

u

]

g


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Figure 4. Ionization efficiency measurement for the 0( P) + 1,3,5-trimethylbenzene reaction product at m/e = 108. Only the uncorrected measurement is shown. 3

8.0

9.0

E l e c t r o n Energy

10.0 (eV)

+50

0( D) + CgHg^A! 1

CO + H C = C C H = C H C H ( A ' 3

J

CO( l )+HC^CCH=CHCH ( A') 1

w

g

3

1

-50

-100 I CO + C H 5

6

OH

)po Figure 5.

c

6v

-OH C

C

O

+

C

5 6 H

s

3

Electronic state correlation diagram for the 0( P) + benzene system


92


-301.40


-301.50 h-

z -301.60

-301.70 T -

DISPLACEMENT

(Λ)

Figure 6. Energy of the T, and S states of phenol as a func tion of displacement from the T, minimum along a ring stretch ing vibrational mode 0

i b i l i t y of the c r o s s i n g r e g i o n , c a l c u l a t i o n s were c a r r i e d out with the ab i n i t i o SCF-M0 program Gaussian 70 using the ST0-3G b a s i s s e t . (7) The T-. and S p o t e n t i a l energy surfaces along e i g h t v i b r a t i o n a l modei of phenol were e x p l o r e d . For one mode, a c r o s s i n g was located about 160 kJ/mol above the minimum energy c o n f i g u r a t i o n of T,. The r e s u l t s f o r t h i s mode are shown in Figure 6. Due to the p r i m i t i v e nature of these l i m i t e d basis set s i n g l e - c o n f i g u r a t i o n c a l c u l a t i o n s , the r e l a t i v e p o s i t i o n s of the two surfaces are u n c e r t a i n . The two surfaces probably cross but where the c r o s s i n g i s l o c a t e d w i t h respect to the minimum i n the Τ , surface cannot be determined with chemical accuracy from these c a l c u l a t i o n s . These RRKM and molecular o r b i t a l c a l c u l a t i o n s i n d i c a t e t h a t T u l l y ' s theory of s p i n - f o r b i d d e n r e a c t i o n s i s a p l a u s i b l e explana t i o n f o r the s p i n - f o r b i d d e n nature of the 0( P) + benzene and Q


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methylbenzene r e a c t i o n s . More accurate c a l c u l a t i o n s would be h e l p f u l i n e x p l a i n i n g more clearly the details of these r e a c t i o n s .

Literature Cited 1.


J.

See Colussi, A. J., Singleton, D. L., Irwin, R. S., and Cvetanovic, R. J., J. Phys. Chem., 79, 1900-3 (1975), and references cited therein. 2. Foner, S. Ν . , and Hudson, R. L., J. Chem. Phys., 53, 4377-86 (1970). 3. Winters, R. E., Collins, J. H., and Courchene, W. L., J. Chem. Phys., 45, 1931-37 (1966). 4. Franklin, J. L., Dillard, J. C., Rosenstock, H. M., Herron, T., Draxl, K . , and F i e l d , F. H., "Ionization Potentials, Appearance Potentials, and Heats of Formation of Gaseous Positive Ions," NSRDS-NBS 26, U. S. Government Printing Office, Washington, D.C., 1969. 5. T u l l y , J. C., J. Chem. Phys., 62, 1893-8 (1975). 6. Hehre, W. J., Lathan, W. Α., D i t c h f i e l d , R., Newton, M. D., and Pople, J. Α., GAUSSIAN 70, Program #136, Quantum Chemistry Program Exchange, Indiana University, Bloomington, Indiana. 7. Hehre, W. J. and Pople, J. Α., J. Amer. Chem. Soc., 92, 2191-7 (1970).


Reaction Product Identification from O(3P) + Benzene, Toluene, and 1

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