Chemistry Reference
In-Depth Information
FIGURE 14.4
Schematic potential energy surface for the CN/C
2
H
2
system (adapted
from
References 75
and
82
).
two, cyanoacetylene is the one favored by thermochemical considerations
(see Figure 14.4).
To establish whether the product thermodynamically favored is the one
really formed, we carried scattering experiments at two different collision
energies of 21.1 kJ/mol and 27.0 kJ/mol [75,82]. We were able to observe
scattered products at two different mass-to-charge ratios, 51 and 50, which
correspond to the ions C
2
HN
þ
and C
2
N
þ
, respectively. However, the LAB
distributions recorded at the two m/e values revealed identical patterns and
could be fit with the same CM functions. This unambiguously indicates that
the only product really formed under our experimental conditions has gross
formula C
2
HN and that it partly fragments to C
2
N
þ
in the electron impact
ionizer because of dissociative ionization. A signal at m/e
52 correspond-
ing to a possible stable reaction adduct of formula C
2
H
2
N was not observed.
¼
¼
The lack of signal observation at m/e
52 means that if a bound reaction
intermediate it fragments before reaching the detection region
because of its high energy content. The complete set of final measurements
were carried out at m/e
exists,
¼
50, since we had the best signal-to-noise ratio at
this mass. In
the LAB product angular distribution at
32
are
shown. The solid lines superimposed on the experimental results are the
calculated curves when using the CM best-fit functions reported in the
Another full set of data was recorded at a slightly higher
collision energy (27.0 kJ/mol) [82]. The laboratory angular distributions
recorded at both collision energies have a peak close to center-of-mass
position angle,
E
c
¼
21.1 kJ/mol together with the TOF spectrum recorded at
Y
¼
Y
CM
, and show about the same intensity in the backward
and forward directions (with respect to the CN radical beam direction).
Also, they are quite wide, with scattered products extending for about 35
.
The limiting circle shown in the Newton diagram of Figure 14.4 has been
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