This datum is in good agreement with that published recently
[Seetula and Slagle,10 (51.8 ^ 1.3) kJ mol~1].
A detailed expression of k
cross-combination rule:
/k
can be given via the
k1@2
18, Z 24, Z
k
k
/
/
k1@2k1@2
k1@2k1@2
3.5 On the role of addition/isomerisation/dissociation
reactions
18, Z
24, Z
18, Z 15 21, Z
24, Z 26 21, Z
15
\
\
(XXI)
k1@2
26
Addition/isomerisation/dissociation (AID) reactions may
introduce systematic errors in the determination of kinetic and
thermochemical data if they contribute to the formation of
any of the key products. For this reason, a detailed mecha-
nism including the most probable such processes was written,
and the formation of predicted products was checked.
where the approximation /
From eqn. (XX), on elimination of [R], k
\ /
was made.
18, Z
24, Z
/k
and
18, Z 24, Z
[CH ] via eqn. (VII), eqn. (XXI) and the equation r
\
3
C2H6
k
[CH ]2, respectively, we have
15
3
r
\ k
r
r1@2 /(k1@2r
)
(XXII)
(26)
~2
~2 TMHP2 C2H6 26 3MP2
The most probable AID reaction in the present system is:
where k is the rate constant of self-combination of R:
26
R ] 2MB2 H (CH ) CHC(CH ) CH(CH )C(CH )
2R ] R
3 2
3 2
3
3 2
2
H (CH ) CC(CH ) CH(CH )CH(CH )
The rate constant ratio, log k /k1@2 \ (10.7 ^ 0.3) [ (142.2
3 2 3 2 3 2
] 23DMB2 ] CH CHCH(CH )
3
~2 26
^ 2.8)/H, where H \ RT ln 10, was determined in an earlier
3
3 2
work.32
CH CHCH(CH ) ] CH ] 2ÈC H
The averages of the ratios &r /r
are listed in Table 2 for
3
3 2
3
4
8
i ~2
di†erent temperatures. These numbers suggest that the steady-
state concentrations of R are 89È94% of the equilibrium ones
and are independent of the temperature within experimental
error. We consider that these data justify application of the
23DMB2 is one of the expected products of the CH ] R dis-
3
proportionation [reaction (11)]:
r
(11) \ k [CH ][R]
(XVII)
23DMB2
11
3
method for the determination of * H¡(R).
f
On elimination of [R] via eqn. (VII) and [CH ] via r
\
3
C2H6
k
[CH ]2, we have
15
3
Acknowledgements
This work was supported by the Hungarian Research Foun-
dation (OTKA T016044) and the Foundation for Hungarian
Science.
k
r
r
11 TMHP2 C2H6 k115@2/
/
18, Z 16
r
(11) \
23DMB2
k
r
k1@2/
/
15 3MP2 26 24, Z 16
r
r
/
/
TMHP2 C2H6 18, Z 16
\ * (CH , R)
(XVIII)
2
3
r
/
3MP2 24, Z
References
There are no published data on * (CH , R). On the basis of
1
2
3
J. C. Jones, Combustion Science: Principles and Practice, Millen-
nium, Sydney, 1996.
2
3
published data on similar reactions M* (CH , (CH ) C
3 2
CHCH ) \ 0.06,29 * (CH , (CH ) CHCCH ) \ 0.04530 and
3 2
or 3C H ~) \ 0.0731N, we estimate
11
2
3
D. F. McMillen and D. M. Golden, Annu. Rev. Phys. Chem.,
1982, 33, 493.
W. Tsang, Heats of Formation of Organic Free Radicals by
Kinetic Methods, in Energetics of Organic Free Radicals, ed.
J. A. M. Simoes, A. Greenberg and J. F. Liebman, Chapman and
Hall, London, 1996.
A. Burkert and N. L. Allinger, Molecular Mechanics, ACS Mono-
graph, 1982.
S. W. Benson, T hermochemical Kinetics, J. Wiley & Sons, New
York, 2nd edn., 1976.
3
2
~
3
3
* (CH , 2C H
11
* (CH , R) \ 0.06.
2
2
3
3
5
5
On the assumption /
/
//
18, Z 16 24, Z
(11) is a factor of 3 less than the experimen-
\ 2, r
(11) was
23DMB2
estimated. r
tal r
23DMB2
at 575 K.
4
5
6
7
23DMB2
Since the other measured products expected from AID reac-
tions (C H and C H ) are also formed in small amounts, and
no contribution to TMHP2 and 3MP2 formation from AID
3
6
3 8
J. Berkowitz, G. B. Ellison and D. Gutman, J. Phys. Chem., 1994,
98, 2744.
reactions was predicted, the evaluation of K from the latter
p
products is warranted. Further, the small amounts of AID
W. Tsang, in Energetics of Organic Free Radicals, ed. J. A.
Martino Simoes, A. Greenberg and J. F. Liebmann, Blackie Aca-
demic & Professional, London, 1996, pp. 22È58.
J. C. Traeger and B. M. Kompe, in Energetics of Organic Free
Radicals, ed. J. A. Martino Simoes, A. Greenberg and J. F. Lieb-
mann, Blackie Academic & Professional, London, 1996, pp. 59È
107.
products suggest that higher oligomeric radicals do not yield
products in higher than negligible amounts.
8
3.6 On the approximation [R] ¿ [R]
eq
ss
A crucial point in the determination of * H¡ (R) is attain-
f
298
ment of a near-equilibrium concentration of R in the addition/
9
B. J. Smith and Leo Radom, J. Phys. Chem. A, 1998, 102, 10787.
10 J. A. Seetula and I. R. Slagle, J. Chem. Soc., Faraday. T rans.,
1997, 93, 1709.
dissociation process [reactions (2) and ([2)]. Near-
equilibrium concentrations of R were expected if &r @ r
,
i
~2
11 N. Cohen and S. W. Benson, Chem. Rev., 1993, 93, 2419.
12 L. Seres, M. Gorgenyi and J. Farkas, Int. J. Chem. Kinet., 1983,
15, 1133.
where subscript i refers to all reactions of R except disso-
ciation back to the reactants, and r is the rate of the latter
~2
reaction.
13 R. J. Cvetanovic and R. S. Irwin, J. Chem. Phys., 1967, 46, 1694.
14 K. R. Bull, R. M. Marshall and J. H. Purnell, Proc. R. Soc.
L ondon, Ser. A, 1975, 342, 259.
15 J. R. McNesby and A. S. Gordon, J. Am. Chem. Soc., 1957, 79,
5902.
&r was estimated from the products measured:
i
&r \ r
] r
] r
23DMB2
] r
223TMBa
i
23DMBa
] r
23DMB1
] r
(XIX)
16 T. Kortvelyesi, Z. Fekete and L. Seres, J. Chim. Phys., 1995, 93,
(DMA`R)
TMHP2
253.
where &r was somewhat overestimated: the combination
17 P. C. Durban and R. M. Marshall, Int. J. Chem. Kinet., 1980, 12,
1031.
18 P. L. Holt, K. E. McCurdy, J. S. Adams, K. A. Burton, R. B.
Weisman and P. S. Engel, J. Am. Chem. Soc., 1985, 107, 2180.
19 R. Renaud and L. C. Leitch, Can. J. Chem., 1954, 32, 545.
20 Z. Kiraly, M. Gorgenyi and L. Seres, J. Chim. Phys., 1999, 96,
591.
i
CH ] CH CHC(CH ) also yields small amounts of
3
3
3 3
223TMBa, and the non-terminal cross-combination CH
3
] 12DMA additionally yields 23DMB1 besides reactions (12)
and (14), respectively.
The rate of dissociation of R
21 J. C. Sternberg, W. S. Gallaway and D. T. L. Jones, in Gas Chro-
matography, ed. N. Brenner, Academic Press, New York, 1962,
ch. 18.
r
\ k [R]
(XX)
~2
~2
was estimated as follows.
Phys. Chem. Chem. Phys., 2000, 2, 349È354
353