512

ISSN 1990-7931, Russian Journal of Physical Chemistry B, 2009, Vol. 3, No. 3, pp. 512–513. © Pleiades Publishing, Ltd., 2009.Original Russian Text © N.M. Kuznetsov, Yu.K. Karasevich, Yu.P. Petrov, S.V. Turetskii, 2009, published in Khimicheskaya Fizika, 2009, Vol. 28, No. 5, pp. 108–109.

One specific feature of the unimolecular decompo-sition of azomethane,

(I)

is that the preexponential factor of the rate con-

stant measured at high temperatures is low. Basedon our own and other direct spectroscopic measurementsof the kinetic characteristics of azomethane decomposi-tion in shock waves [1–9] at

T

= 850–1430 K and smallazomethane concentrations (40–2000 ppm), conditionsthat exclude the influence of secondary reactions at theinitial stage of the decomposition, we estimated it at

≈

10

9.9

–10

11.4

s

–1

. (1)

In this case, the expression for reads as [9]

[s

–1

] = 10

11.3

±

0.3

exp(– /

RT

), (2)

where the observed activation energy is

= (33.5

±

1.6)

kcal/mol. Expression (2) satisfactorily describesthe experimental data obtained by a tenfold variation ofboth the pressure and partial concentration ofazomethane [9]. The fact that no appreciable pressuredependence of the rate constant was observed upon awide variation of the pressure and temperature suggeststhat, under these conditions, azomethane decomposi-tion occurs in the high-pressure- limit regime.

Note that the RRKM theory [10] and the Slater the-ory [11], while based on different concepts of the acti-vated molecule, give, in their simplest versions,approximately the same values of the preexponentialfactor for the unimolecular decomposition in the high-pressure limit,

~10

13

s

–1

[12, 13]. Based on an analysisof a large number of reactions, the authors of the mono-

CH3( )2N2 2CH3• N2+

A1obs

k1obs

A1obs

k1obs

k1obs E1

obs

E1obs

graph [14] recommended a mean value of

A

∞

=

10

13.5

s

–1

.As can be seen, the preexponential factor of the rateconstant for azomethane decomposition at high temper-atures is 100 to 1000 times lower [10–14].

Along with the low value of the preexponential fac-tor, expression (2) features a low observed activation

energy . In [6–9], this fact was accounted for interms of the so-called concerted dissociation mecha-nism, according to which the abstraction of two groupsoccurs simultaneously. One distinctive aspect ofazomethane decomposition at high temperature is thatit cannot be interpreted as the sequential abstraction of

the two

C

radicals, since the

CH

3

radical (meth-yldiazenyl) is highly unstable. Using femtosecondtime-of-flight mass spectrometry, the authors of [15]estimated the lifetime of this radical at ~100 fs, a timeshorter than its rotation period and the characteristictime of collisions with bath-gas molecules. According

to the concerted mechanism, the

C

radials are con-secutively abstracted with a time lag of only severalhundred femtoseconds. The abstraction of the second

C

radical during the instantaneous dissociation of

the highly unstable methyldiazenyl radical (

CH

3

) ischaracterized by an exothermicity of ~16 kcal/mol [15].This results in a decrease in the observed activation

energy from ~ 50 kcal/mol, typical of C–N bonddissociation, to ~34 kcal/mol, an experimentally mea-sured value.

Thus, the rate constant for the high-temperaturedecomposition of azomethane differs markedly fromthe values predicted by the classical theory of unimo-lecular dissociation.

E1obs

H3• N2

•

H3•

H3•

N2•

E1obs

SHORTCOMMUNICATIONS

On the Mechanism of the High-Temperature Decompositionof Azomethane in Shock Waves

N. M. Kuznetsov, Yu. K. Karasevich, Yu. P. Petrov, and S. V. Turetskii

Semenov Institute of Chemical Physics, Russian Academy of Sciences, Moscow, 119991 Russiae-mail: stwill@mail.ru

Received August 4, 2008

Abstract

—The specifics of the high-temperature unimolecular decomposition of azomethane behind shockwaves is theoretically interpreted based on the concept of the anharmonicity of the symmetrical vibration of themolecule. The resonance detuning for the stretching of the two

CH

3

–N

bonds during decomposition is esti-mated. This estimate makes it possible to harmonize the measured and theoretically calculated parameters ofthe rate constant for azomethane decomposition.

DOI:

10.1134/S1990793109030270

RUSSIAN JOURNAL OF PHYSICAL CHEMISTRY B

Vol. 3

No. 3

2009

ON THE MECHANISM OF THE HIGH-TEMPERATURE DECOMPOSITION OF AZOMETHANE 513

The specifics of azomethane decomposition can beexplained by assuming that the stretching of one of the

CH

3

–N

bonds in the azomethane molecule makes theother

CH

3

–N

bond repulsive. Performing the imaginary

abstraction of one of the

C

groups at fixed lengths ofthe other bonds and then removing the fixation resultsin an increase in the kinetic energy of the products ofthe instantaneous dissociation of the methyldiazenylradical,

due to the release of the potential energy of repulsion.Of the entire set of vibrational and rotational states ofthe activated azomethane molecule, i.e., capable ofspontaneous dissociation, states that decay without theinvolvement of repulsive forces, will be characterizedby the minimum activation energy, equal to theenthalpy of dissociation (

D

= 34 kcal/mol [7–9]). It isnatural to assume that such states are symmetrical withrespect to the stretching of the

CH

3

–N

bonds.The coordinate of azomethane dissociation (reac-

tion

I

) corresponds to a synchronous expansion of the

CH

3

–N

bonds. In this case, the mechanism of unimolec-ular dissociation is a version of the concerted reactionmechanism. A brief description of this mechanism ispresented above. To ensure the complete symmetry of

the vibrations of the

C

groups, it is necessary thatthe phases of this motion along the reaction coordinatebe identical with respect to the middle of the N–N inter-atomic distance. The same requirement of synchronismis applicable to the other vibrations spatially close tothe reaction coordinate, which produce, due to theanharmonicity of molecular vibrations, the most pro-nounced effect on the reaction coordinate and motionalong it, more specifically, the two degenerate bendingvibrations of the methyl groups. The effect of the hin-

dered rotations of the

C

groups about the

CH

3

–N

and the C–H stretching vibrations in them is less impor-tant.

The probability of exact synchronization is vanish-ingly small, but slight shifts of the phases

δϕ

i

(

i

= 1, 2,and 3) produce only insignificant changes (increases) inthe activation energy with respect to its minimum valuecorresponding to synchronous motion. It stands to rea-son, that the maximum resonance detuning

δϕ

i

, whichdescribes the stretching of the two

CH

3

–N

bonds and

H3•

CH3N2• CH3

• N2+

H3•

H3•

produces only slight effect on the activation energy, ison the order of 1% of the total vibration phase,

2

π

, i.e.,

δϕ

i

/2

π

≤

0.01

. A significant resonance detuning isexpected to substantially alter the activation energy, afactor that makes azomethane decomposition impossi-ble. Correspondingly, the value of the preexponentialfactor decreases by about a factor of 100. A lower reso-nance detuning is associated with the disruption of thesynchronism in the bending vibrations and in the othertwo vibrations of the

CH

3

–N

groups, which, due toanharmonicity, influence the vibrations along the reac-tion coordinate.

REFERENCES

1. K. Glanzer, M. Quack, and J. Troe, in

Proc. 16th Int.Symp. on Combust.

(The Combust. Inst., Pittsburgh,1976), p. 949.

2. I. S. Zaslonko, A. G. Zborovskii, Yu. P. Petrov, et al., in

Proc. VIII All-Union Symp. on Combustion and Explo-sion

(Tashkent, 1986), p. 58.3. W. Moller, E. Mozzukhin, and H. G. Wagner, Ber. Bun-

seng. Phys. Chem.

90

, 854 (1986).4. A. G. Zborovskii, Yu. P. Petrov, V. N. Smirnov, et al.,

Khim. Fiz.

6

, 506 (1987).5. A. V. Bulgakov, Yu. P. Petrov, A. M. Tereza, et al., in

Proc. Russia-Japan Seminar on Combustion

(DICPRAS, Chernogolovka, 1992), p. 240.

6. H. Du, J. P. Hessler, and P. J. Ogren, J. Phys. Chem.

100

,974 (1996).

7. Yu. P. Petrov, S. V. Turetskii, and A. V. Bulgakov, Khim.Fiz.

26

(3), 26 (2007).8. Yu. P. Petrov, S. V. Turetskii, and A. V. Bulgakov, Khim.

Fiz.

27

(7), 39 (2008) [Russ. J. Phys. Chem. B

2

, 538(2008)].

9. Yu. P. Petrov, S. V. Turetskii, and A. V. Bulgakov, Kinet.Katal., No. 3 (2009, in press).

10. O. K. Rice and H. C. Ramsperger, J. Am. Chem. Soc.

50

,617 (1928).

11. N. B. Slater, Proc. Cambridge Phil. Soc.

35

, 36 (1939).12. N. M. Kuznetsov,

Unimolecular Reactions

(Nauka,Moscow, 1980) [in Russian].

13. C. Steel and K. J. Laidler, J. Chem. Phys.

34

, 1827(1961).

14. V. N. Kondrat’ev and E. E. Nikitin,

Kinetics and Mecha-nism of Gas-Phase Reactions

(Nauka, Moscow, 1974)[in Russian].

15. E. W. Diau, O. K. Abou-Zied, A. A. Scala, et al., J. Am.Chem. Soc.

120

, 3245 (1998).