Chemistry Reference
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intermediate(s) is longer than its (their) rotational periods. At the higher
collision energies of 24.7 and 34.9 kJ/mol of the CN
CH
3
CCH reaction,
best fits of our data were achieved with slightly forward-biased values of
T(
þ
) with an intensity ratios of I(180
)/I(0
)
0.9). In all cases, the values
) show intensity in the whole angular range between 0
and 180
demonstrating again that the reactions follow indirect scattering dynamics
via C
4
H
4
N complex(es). The absence of polarization in the values of T(
of T(
)
might result from a poor coupling between the initial, L, and final orbital
angular momentum, L
0
, indicating that most of the total angular momentum
channels into rotational excitation of the heavy product(s).
Also for this system, as the first experimental results at the microscopic
level became available, ab initio calculations of the electronic structures of
the stationary points along the
2
A
0
C
4
H
4
N potential energy surface were
performed [83,84]. According to the ab initio calculations, the CN attacks
the
system of the methylacetylene molecule with the radical center located
on the
2
þ
orbital of the carbon atom. The consequent addition to the
b
carbon atom is a barrier-less pathway which leads to the deeply bound
(
250 kJ/mol) cis/trans-1-cyano-2-methyl-vinyl-2 radical intermediate,
INT1a/b(4). The INT1(4) trans and cis structures easily interconvert
through the transition state TS1i(4). The CN radical can also add to the
a
carbon of the methylacetylene, thus forming the trans/cis 1-cyano-1methyl-
vinyl-2 radical
intermediate, INT2a/b(4), which is slightly less stable
(
225 kJ/mol) than the previous couple. The destabilization is due to
the unfavorable repulsion of the cyano and methyl groups of INT2a/b(4).
The intermediates INT1b(4) and INT2b(4) can also isomerizes to each other
through a three-member cyclic intermediate, INT3(4). The addition to the
a
carbon atom of the methylacetylene is not favored. The electrophilic radical
attack on unsaturated hydrocarbon molecules [87] takes place towards the
carbon center which holds the highest electron density. Since the partial
delocalization of the methyl
-group orbital increases the spin density of the
b
carbon atom with respect to that of the
a
one, the cyano radical will
preferentially attack the terminal carbon atom. Also, the steric hindrance of
the bulky methyl group reduces the cone of acceptance of
a
-C and hence the
range of reactive impact parameters. Both effects direct the addition
towards
b
-C. In other words, while large impact parameters can only corres-
pond to the addition on the
b
-C, small impact parameters can bring also to
the
a
-addition. Small impact parameters, however, cannot account for the
rotational excitation of the reaction products, as experimentally evidenced
by the isotropy of the angular distributions. In conclusion, all considera-
tions suggest a predominant initial attack to the
b
-C of the methylacetylene
molecule and only minor contributions from small impact parameters con-
tributed to form INT2a/b(4).
The possible destinies of the initial addition intermediates INT1a/b(4)
and INT2a/b(4) are: 1. H atom elimination, 2. methyl group loss, and 3.
hydrogen atom migration. H atom elimination can occur in different ways.
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