52
T. Okino and K. Yamanouchi
3.3 Three-Body Decomposition Pathways of Methylacetylene
and Methyl-d 3 -Acetylene
3.3.1 Three-Body Decomposition Pathways with CC Bond
Breaking
3.3.1.1 Methylacetylene
In methylacetylene, the following two types of three-body decomposition pathways
are identified in the CMI maps.
[Type I: C β C γ bond breaking]
I: C 3 H
3+
4 → H
+
+ C γ H
+
2 + C α C β H
+
(3.1)
[Type II: C α C β bond breaking]
II: C 3 H
3+
4 → H
+
+ C β C γ H
+
2 + C α H
+
(3.2)
where three carbon atoms are labeled using α, β, γ as H–C α ≡ C β –C γ H 3 .
The number of events in Pathway I (55 %) is as large as that of Pathway II (45 %).
This result indicates that the C β C γ bond breaking and the C α C β bond breaking
occur with almost the same probabilities. It is highly probable that a proton ejected
in these three-body Coulomb explosion processes is originated from a hydrogen
atom which is originally bonded to the C γ atom in the methyl group, but it may also
be possible that a proton is ejected from the C α atom in the methine group if the
hydrogen migration proceeds prior to the chemical bond breaking.
The proton distribution maps of Pathway I and Pathway II are shown in
Fig. 3.1(a) and 3.1(b), respectively. The proton distributions cover the entire θ range,
but there are clear two dense areas labeled by A 1 and B 1 in Pathway I and A 2 and
B 2 in Pathway II. The distribution A 1 shows that the protons are distributed around
the CH
+
2 moiety. The distribution B 1 shows that the protons are distributed around
the C 2 H + moiety. The probability of the distribution A 1 (78 %) is much larger than
that of the distribution B 1 (22 %). This result indicates that a proton is preferentially ejected from the methyl group accompanied by the C β C γ bond breaking that
is originally a single bond. The probability distribution A 2 (62 %) is much larger
than that of the distribution B 2 (38 %). This result also indicates that a proton is
preferentially ejected from the methyl group.
It is probable that the ejected protons are originated from the methyl group in
both Pathway I and Pathway II, however, in order to identify securely whether a
proton is ejected from methyl group or from methine group, we performed the CMI
measurements using partially deuterated species, methyl-d 3 -acetylene.
T. Okino and K. Yamanouchi
3.3 Three-Body Decomposition Pathways of Methylacetylene
and Methyl-d 3 -Acetylene
3.3.1 Three-Body Decomposition Pathways with CC Bond
Breaking
3.3.1.1 Methylacetylene
In methylacetylene, the following two types of three-body decomposition pathways
are identified in the CMI maps.
[Type I: C β C γ bond breaking]
I: C 3 H
3+
4 → H
+
+ C γ H
+
2 + C α C β H
+
(3.1)
[Type II: C α C β bond breaking]
II: C 3 H
3+
4 → H
+
+ C β C γ H
+
2 + C α H
+
(3.2)
where three carbon atoms are labeled using α, β, γ as H–C α ≡ C β –C γ H 3 .
The number of events in Pathway I (55 %) is as large as that of Pathway II (45 %).
This result indicates that the C β C γ bond breaking and the C α C β bond breaking
occur with almost the same probabilities. It is highly probable that a proton ejected
in these three-body Coulomb explosion processes is originated from a hydrogen
atom which is originally bonded to the C γ atom in the methyl group, but it may also
be possible that a proton is ejected from the C α atom in the methine group if the
hydrogen migration proceeds prior to the chemical bond breaking.
The proton distribution maps of Pathway I and Pathway II are shown in
Fig. 3.1(a) and 3.1(b), respectively. The proton distributions cover the entire θ range,
but there are clear two dense areas labeled by A 1 and B 1 in Pathway I and A 2 and
B 2 in Pathway II. The distribution A 1 shows that the protons are distributed around
the CH
+
2 moiety. The distribution B 1 shows that the protons are distributed around
the C 2 H + moiety. The probability of the distribution A 1 (78 %) is much larger than
that of the distribution B 1 (22 %). This result indicates that a proton is preferentially ejected from the methyl group accompanied by the C β C γ bond breaking that
is originally a single bond. The probability distribution A 2 (62 %) is much larger
than that of the distribution B 2 (38 %). This result also indicates that a proton is
preferentially ejected from the methyl group.
It is probable that the ejected protons are originated from the methyl group in
both Pathway I and Pathway II, however, in order to identify securely whether a
proton is ejected from methyl group or from methine group, we performed the CMI
measurements using partially deuterated species, methyl-d 3 -acetylene.