20
W. Feng, J.F. Hershberger / Chemical Physics 472 (2016) 18–23
Table 1
The product branching ratio of reaction has been estimated to
Raw product yields.a
be u9a = 0.21 0.1; u9b = 0.72 0.1 [20]. A subsequent secondary
reaction then produces 15N15NO:
Product
HCN
Yield (in units of 1012 molecule cmꢀ3
)
15NCO þ 15NO ꢀ! 15N15NO þ CO
ꢀ! 15N15N þ CO2
ð10aÞ
ð10bÞ
17.9 1.8
1.8 0.1
11.1 1.2
<10.3 2.0
1.0 0.2
OC18
DCN
CO
O
(Product
branching
ratios
at
296 K:
u10a = 0.44 0.07;
HNCO
15N2O
9.3 0.7
u10b = 0.56 0.07) [21,22].
We then detected the double labeled 15N15NO molecule, using
spectral line positions reported previously [19]. Although the
branching ratio into (9a) is modest, N2O is a very strong infrared
absorber, so this experiment has a reasonably low detection limit.
We found a small transient signal attributable to 15N15NO, shown
in Fig. 3.
a
Experimental conditions: P(HCNO) = 0.1 Torr, P(15N18O) = P(15N16O) = 1.0 Torr,
P(SF6) = 1.5 Torr (for HCN, NCO and DCN detection), P(CF4) = 1.5 Torr (for CO
detection only), 193 nm photolysis laser pulse energy ꢁ8.5 mJ. Uncertainties rep-
resent one standard deviation.
O þ HCNO ꢀ! H þ CO þ NO
CN þ HCNO ꢀ! NO þ HCCN
NCO þ HCNO ꢀ! HCN þ CO þ NO
ð2Þ
ð3Þ
ð4Þ
3.2. Product channel (1a), O + HCN
OH þ HCNO ꢀ! CO þ H2NO
ꢀ! HCO þ HNO
ð5aÞ
ð5bÞ
ð5cÞ
The procedure for quantification of channel (1a) is similar to
that described previously [8]. We directly detect the HCN photoly-
sis product by transient absorption, as shown in Fig. 2 (top panel).
The HCN yield from photolysis of HCNO could arise from channel
(1a), but also may have contributions from reaction (4) if channel
(1b) is significant. By including an excess of isotopically labeled
15N18O reagent, however, reaction (4) is suppressed:
ꢀ! NO þ H2CO
NH þ HCNO ꢀ! products
H þ HCNO ꢀ! products
ð6Þ
ð7Þ
NCO þ 15N18O ꢀ! N15N18O þ CO
ð10aÞ
ð10bÞ
Reactions (2)–(6) have been studied previously in our lab [2–6],
resulting in the following rate constants: k2 = (5.32 0.4) ꢂ 10ꢀ12
-
,
ꢀ! N15N þ OC18
O
cm3 moleculeꢀ1 sꢀ1
,
k3 = (1.04 0.1) ꢂ 10ꢀ10 cm3 moleculeꢀ1 sꢀ1
k4 = (1.58 0.2) ꢂ 10ꢀ11 cm3 moleculeꢀ1 sꢀ1
,
k5 = (3.4 0.3) ꢂ
where u10a = 0.44 and u10b = 0.56 [21,22], and k10 = (4.3 0.4) ꢂ
10ꢀ11 cm3 moleculeꢀ1 sꢀ1 [6]. Based on the rate constant of (4)
and (10), we estimate that a 10:1 ratio of 15N18O to HCNO is suffi-
cient to ensure that nearly all NCO radicals react by (10) rather than
(4), and therefore do not result in HCN production. Reaction (10)
will not suppress reaction (3), CN + HCNO, because the CN + NO
reaction is slow at the low pressures used in these experiments.
Reaction (3) produces HCCN radicals [4,7] which then react with
15N18O, producing labeled HC15N molecules:
10ꢀ11 cm3 moleculeꢀ1 sꢀ1 and k6 6 2.1 ꢂ 10ꢀ13 cm3 moleculeꢀ1 sꢀ1
at 298 K. While the rate of H + HCNO, reaction (7), is not
experimentally known, our previous study [8] showed that it was
too slow to affect the results of HCNO photolysis experiments.
Reactions (2)–(5) are fast and may produce some of the same
product molecules as those probed in these experiments,
potentially complicating the interpretation of product yields. The
approach used here is similar to that used previously [8], in which
we include additional reagents in the reaction mixture in order to
suppress or redirect these secondary reactions, as described in
detail below.
CN þ HCNO ꢀ! NO þ HCCN
ð3Þ
HCCN þ 15N18O ꢀ! HC15N þ NC18
In these experiments, however, we detect the non-labeled
HC14N isotope, which is not affected by this secondary chemistry.
The nitrogen-15 labeling is therefore important for this experiment
O
ð11Þ
Some additional secondary chemistry, potentially present in
this study but not relevant to our previous study, is due to the pos-
sible presence of channel (1f), forming CH + NO photoproducts.
This channel was not energetically accessible at 248 nm, but is at
193 nm. If this channel is active, the following reaction must be
considered:
CH þ HCNO ꢀ! products
ð8Þ
The rate constant of reaction (8) is unknown, but CH is a highly
reactive species, so it is likely that this reaction is fast. Products are
also unknown, but likely include many of the same molecules
probed in these experiments.
3.1. Product channel (1f), CH + NO
The most obvious approach to quantifying this channel would
be to directly detect and quantify the yield of NO products. This
approach is complicated, however, by secondary sources of NO,
including reaction (2)–(4), (5c), etc. We therefore used an alterna-
tive approach to detect this channel: we included an excess of
15N16
O
in the reaction mixture: HCNO (0.1 Torr)/15N16
O
(1.0 Torr)/SF6 (1.5 Torr). Under these conditions, any CH radicals
produced in (1f) will react with 15N16O as follows:
CH þ 15NO ꢀ! 15NCO þ H
ꢀ! HC15N þ O
ð9aÞ
ð9bÞ
Fig. 3. The 15N15NO transient IR absorption signal detected upon photolysis of
HCNO (0.1 Torr)/15N16O (1.0 Torr)/SF6 (1.5 Torr).