Russian Journal of General Chemistry, Vol. 72, No. 8, 2002, pp. 1271 1275. Translated from Zhurnal Obshchei Khimii, Vol. 72, No. 8, 2002,
pp. 1354 1358.
Original Russian Text Copyright
2002 by Skorobogatov, Meilakhs, Pogosyan, Khripun.
New Mechanism of Photodissociation of Gaseous Acetone
G. A. Skorobogatov, A. G. Meilakhs, Yu. I. Pogosyan, and V. K. Khripun
St. Petersburg State University, St. Petersburg, Russia
Received February 28, 2001
Abstract It is found for the first time that photolysis of gaseous acetone under UV irradiation produces
ethane not only via recombination of methyl radicals, but also by the mechanism of induced predissociation.
Photolysis of acetone is a well-understood photo-
chemical process [1]. Gaseous acetone is even called
a classical actinometric system [2]. Actually, it is
As follows from Eq. (7), the average time of the
decomposition of CH3CO by scheme (6) at 333 K is
as short as 0.045 s.
firmly established [2] that at T
398 K in a wide
Clearly, if pathway (3) is realized, the quantum
range of near-UV wavelengths (250 320 nm) the
quantum yield of CO ( CO) formed by photolysis of
gaseous acetone is exactly one [schemes (1) and (2)].
yield
proves to be slightly lower than one, but
CO
the quantum yield of methyl radicals ( CH ) is always
3
no less than double
[Eq. (8)].
CO
h
2
.
CO
(8)
CH3COCH3
2CH3
At lower temperatures at
CO + 2CH3,
(1)
CH3
k2
Since gaseous acetone in photolytic kinetic studies
serves as a source of free methyl radicals, relation (8)
is a corner stone of gas-phase kinetics [3], allowing
one to estimate the initial concentration [CH3]0.
C2H6.
(2)
313 nm, along with
photodissociation (1), a less profound decomposi-
tion (3) becomes a significant pathway.
Whereas the above pattern seems quite clear, the
mechanism of photolysis of gaseous acetone cannot
be considered firmly established. The case in point is
that, according to our data, mild pyrolysis (500
600 K) of gaseous methyl iodide gives no ethane. We
also found that no ethane is also formed by photolysis
of methyl iodide. These findings led us to suggest that
the ethane resulting from photolysis of acetone is
mostly formed by induced predissociation (9) rather
than by recombination (2).
h
CH3COCH3
CH3 + (COCH3)*
CO + 2CH3 (3)
M
COCH3
Here M is the deactivating valence-saturated mo-
lecule, (COCH3)* is the excited radical with increased
vibrational temperature, which decomposes into CO
and CH3 if M has no time to deactivate it. The CH3
radicals formed by reaction (3) recombine by
scheme (2), as well as by schemes (4) and (5).
h
CH3COCH3
C2H6 + CO.
(9)
COCH3 + CH3
CH3COCH3,
(CH3CO)2.
(4)
(5)
Actually, some evidence for this mechanism could
be found in experiments with hard irradiation at
200 nm [3]. Recently, Scheld et al. [4] used the
molecular-beam technique to show that photodissoci-
ation of carbocyanide ( 193 nm) gives dicyan by the
intramolecular predissociation mechanism [scheme
(10)] with a probability of 6%.
COCH3 + COCH3
According to [3], even a vibrationally deactivated
radical CH3CO is quite unstable and decomposes by
scheme (6) similar by scheme (3) with a rate constant
defined by Eq. (7).
94 2%
CH3CO
CH3 + CO,
6740
(6)
(7)
COCN + CN,
h
CO(CN)2
(10)
6
2%
k6 =1.25 1010exp
, s .
1
CO +NCCN.
T/K
Thus, instead of photodissociation of gaseous
1070-3632/02/7208-1271$27.00 2002 MAIK Nauka/Interperiodica