Reactions of Azine Anions with Nitrogen and Oxygen Atoms: Implications for Titan's Upper Atmosphere and Interstellar Chemistry

Article abstract of DOI:10.1021/jacs.5b06089

Azines are important in many extraterrestrial environments, from the atmosphere of Titan to the interstellar medium. They have been implicated as possible carriers of the diffuse interstellar bands in astronomy, indicating their persistence in interstellar space. Most importantly, they constitute the basic building blocks of DNA and RNA, so their chemical reactivity in these environments has significant astrobiological implications. In addition, N and O atoms are widely observed in the ISM and in the ionospheres of planets and moons. However, the chemical reactions of molecular anions with abundant interstellar and atmospheric atomic species are largely unexplored. In this paper, gas-phase reactions of deprotonated anions of benzene, pyridine, pyridazine, pyrimidine, pyrazine, and s-triazine with N and O atoms are studied both experimentally and computationally. In all cases, the major reaction channel is associative electron detachment; these reactions are particularly important since they control the balance between negative ions and free electron densities. The reactions of the azine anions with N atoms exhibit larger rate constants than reactions of corresponding chain anions. The reactions of azine anions with O atoms are even more rapid, with complex product patterns for different reactants. The mechanisms are studied theoretically by employing density functional theory; spin conversion is found to be important in determining some product distributions. The rich gas-phase chemistry observed in this work provides a better understanding of ion-atom reactions and their contributions to ionospheric chemistry as well as the chemical processing that occurs in the boundary layers between diffuse and dense interstellar clouds.

Full text of DOI:10.1021/jacs.5b06089

Article
pubs.acs.org/JACS
Reactions of Azine Anions with Nitrogen and Oxygen Atoms:
Implications for Titan’s Upper Atmosphere and Interstellar Chemistry
Zhe-Chen Wang,*,† Callie A. Cole,† Nicholas J. Demarais,†,# Theodore P. Snow,‡,§
and Veronica M. Bierbaum*,†,§
†Department of Chemistry and Biochemistry, University of Colorado, Boulder, Colorado 80309, United States
‡Department of Astrophysical and Planetary Sciences, University of Colorado, Boulder, Colorado 80309, United States
§Center for Astrophysics and Space Astronomy, University of Colorado, Boulder, Colorado 80309, United States
S
* Supporting Information
ABSTRACT: Azines are important in many extraterrestrial environments,
from the atmosphere of Titan to the interstellar medium. They have been
implicated as possible carriers of the diffuse interstellar bands in astronomy,
indicating their persistence in interstellar space. Most importantly, they
constitute the basic building blocks of DNA and RNA, so their chemical
reactivity in these environments has significant astrobiological implications.
In addition, N and O atoms are widely observed in the ISM and in the
ionospheres of planets and moons. However, the chemical reactions of
molecular anions with abundant interstellar and atmospheric atomic species
are largely unexplored. In this paper, gas-phase reactions of deprotonated
anions of benzene, pyridine, pyridazine, pyrimidine, pyrazine, and s-triazine
with N and O atoms are studied both experimentally and computationally.
In all cases, the major reaction channel is associative electron detachment;
these reactions are particularly important since they control the balance
between negative ions and free electron densities. The reactions of the azine anions with N atoms exhibit larger rate constants
than reactions of corresponding chain anions. The reactions of azine anions with O atoms are even more rapid, with complex
product patterns for different reactants. The mechanisms are studied theoretically by employing density functional theory; spin
conversion is found to be important in determining some product distributions. The rich gas-phase chemistry observed in this
work provides a better understanding of ion-atom reactions and their contributions to ionospheric chemistry as well as the
chemical processing that occurs in the boundary layers between diffuse and dense interstellar clouds.
previously considered, were widely observed by CAPS in high
altitudes of Titan’s atmosphere.2,3,13,16−19 Although many
studies have examined cationic-neutral reactions, few have
focused on negatively charged reaction systems.3,20−24 To the
best of our knowledge, there are no kinetic studies or models
on the reactions of atomic species with aromatic anions in
Titan’s upper atmosphere.
Our knowledge of astrochemistry has grown immensely over
the last 80 years, albeit rather slowly in the beginning. The first
interstellar molecule (CH) was detected in 1937,25 and in the
25 subsequent years, only three additional species were
detected by optical telescopes. Since the 1960s, microwave
radio spectroscopy has led to many important discoveries
starting with the detection of OH.26 Today, nearly 200
interstellar and circumstellar species have been identified,
including a wide variety of neutrals and positive ions.27−29 The
first negative ion, OCN−, was identified in the icy mantles of
interstellar dust grains.30 However, gas phase negative ions were
1. INTRODUCTION
Titan, the only natural satellite in our solar system with a dense
atmosphere, has attracted increasing attention since the first
encounter with the Cassini spacecraft in 2004.1−11 The major
component in Titan’s upper atmosphere (∼1000 km) is N2
(>97%) with minor amounts of CH4 (2%) and N-bearing
organic species, as revealed by the ion neutral mass
spectrometer (INMS) onboard the Cassini spacecraft.11 The
main initial ion production mechanisms in Titan’s atmosphere
are dissociation and ionization of nitrogen by solar wind, UV
photons, and magnetospheric particles, generating very active
species, including atomic nitrogen.12 A large variety of neutral,
cationic, and anionic species have been observed by the INMS
together with the Cassini plasma spectrometer (CAPS),13
indicating that the complex chemistry in Titan’s upper
atmosphere may play an important role in the formation of
tholins.2,14 Similar chemistry may have taken place in the
atmosphere of early Earth before its buildup of oxygen 2.2 ×
109 years ago.15 The key precursors to heavy tholins are
considered to be aromatic.2,14 Furthermore, it was unexpected
that negatively charged molecules, whose existence was not
Received: June 11, 2015
© XXXX American Chemical Society
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not detected until 2006 and only six species, CN−, C3N−,
C5N−, C4H−, C6H−, and C8H−, have been confirmed to
date.31−37 The observation of interstellar negative ions is
exciting, because it indicates that anion reactions could be
important in the formation and distribution of species in the
interstellar medium (ISM). N and O atoms show high
interstellar atomic abundance.28 In the interstellar environment,
atomic species persist in more diffuse regions, while larger
cyclic molecules and anions reside in dense molecular clouds.
At the intersection of these regions, atoms chemically process
larger molecules and anions. Most previous ion-atom studies
have focused on reactions with cations.38−40 However, little is
known about the reactions of anions with N and O atoms,
despite their abundant interstellar and atmospheric pres-
ence.23,24,41−43
triazinide (C3H2N3 ) with ground state N(4S) and O(3P)
atoms are studied at room temperature (Scheme 1). The major
−
Scheme 1. Deprotonated Species Studied in This Work
Cyclic aromatic molecules are also ubiquitous throughout the
interstellar medium.2,44−47 Both neutral and ionic aromatic
hydrocarbons and N-containing aromatic hydrocarbons have
been suggested as the species whose visible absorption bands,
the diffuse interstellar bands (DIBs),48 have perplexed
astrophysicists and chemists alike for nearly a century.28,49−51
Additionally, N-containing aromatic hydrocarbons are impor-
tant prebiotic molecules. For example, pyrimidine constitutes a
key building block of nucleobases,52 and the simple pyrimidine
derivatives thymine, cytosine, and uracil are essential
components of nucleic acids. The nucleobases have been
formed in experimental simulations of Titan’s atmosphere,53
and the stability of their negative ions suggests their potential
importance in these environments.54 However, the specific
identity of high mass anions in Titan’s atmosphere must await
higher resolution observations. Studies of the reactions of azine
anions with interstellar and atmospheric species such as N and
O atoms are thus important for gaining insight into the
formation and persistence of complex molecules in these
environments.
reaction channel of these azine anions with N and O atoms is
associative electron detachment to produce electrons and
neutral products. The reactions of N atoms with azine anions
(reaction rate constants on the order of 10−10 cm3 s−1) are
generally faster than those with the carbon chain anions.23,24
Additionally, the reactions of azine anions with O atoms are
∼2−5 times faster than those with N atoms and show
complicated and intriguing ionic product patterns. Density
functional theory (DFT) calculations are used to study the
energies of the reactants, ion complexes, transition states, and
products to examine reaction mechanisms in detail as well as to
elucidate the spin conversion processes in the ion-atom
reactions.
2. EXPERIMENTAL METHODS
Measurements of the reaction rate constants and product distributions
were made using the tandem flowing afterglow-selected ion flow tube
(FA-SIFT) at the University of Colorado, Boulder. This instrument
has been described elsewhere,55 and only the salient details for these
experiments will be discussed here. Ions are generated using electron
and chemical ionization methods in the source flow tube. A small flow
of NH3 entrained in helium buffer gas is passed over a rhenium
Studies involving reactions with N and O atoms are relatively
rare, because the experiments are difficult to perform.38
Recently, our group has successfully investigated the intriguing
−
−
reactions of carbon chain anions (such as Cn and HCn ) as
well as N-containing hydrocarbon chain anions (such as
CH2CN−, CH3CHCN−, and (CH3)2CCN−) with atomic N(4S)
and O(3P).23,24 The reactions of Cn− with N atoms lead to the
formation of N-containing carbon chain anions such as CN−,
C3N−, and C5N−, which are among the observed interstellar
anions and are also observed in Titan’s upper atmos-
phere.3,31,35,36 Notably, the reaction rate constants of carbon
chain anions with O atoms are approximately 1 order of
magnitude larger than those with N atoms, and the neutral
product CO is generated rather than CnO−.24 The reactions of
N-containing hydrocarbon chain anions with N and O atoms
drew attention to spin-forbidden reactions and explored the
influence of spin conservation and conversion on the reactions
of interstellar species of high multiplicity such as N(4S) and
O(3P).23 N-containing hydrocarbon chain anions react rather
slowly with N atoms (reaction rate constants on the order of
10−11 cm3 s−1), because the relative energy of the doublet-
quartet crossing point is fairly close to the total energy of the
entrance channel of the reactants. The O atom reactions for N-
containing hydrocarbon chain anions are much faster (reaction
rate constants >7.0 × 10−10 cm3 s−1), and they generate similar
ionic products (for example, CN−) as the N atom reactions.23
−
filament to generate NH2 , which further reacts with azine molecules
to form the deprotonated anions.
−
−
NH2 + HA → NH3 + A
(1)
The anions are mass-selected with the SIFT quadrupole mass filter and
injected into the reaction flow tube. They are then entrained in helium
buffer gas (0.37 Torr, ∼ 200 std cm3 s−1) at 298 K and thermalized by
multiple collisions. The ion residence time in the flow tube is about 10
ms. The ion-neutral reaction is initiated by adding N or O atoms
through an inlet positioned 70 cm upstream of the sampling orifice.
Reactant and product ions are monitored with a quadrupole mass filter
coupled with an electron multiplier. Microwave discharge flow
techniques are used to generate N and O atoms in their ground
states, which are well-established methods for studying the reactions of
ions with atoms using the FA-SIFT.23,24,56−62
The N atoms are generated by flowing N2 through a microwave
discharge cavity operating at 30−60 W. The O(3P) atoms are formed
by the subsequent titration of the N atoms with NO (4% NO in He)
according to the reaction:63,64
N(4S) + NO → N2 + O(3P)
(2)
Because of difficulties in measuring the absolute number densities of
atoms, the total error in the rate constant measurements is estimated
to be 50%.
The main active species in the microwave discharge of N2 is N(4S).
Although appreciable concentrations of N(2D) and N(2P) metastable
excited atoms have been found near the discharge plasma,65,66 these
−
In this paper, the reactions of phenide (C6H5 −), pyridinide
(C5H4N−), 1,2-, 1,3-, and 1,4-diazinide (C4H3N2 ) and 1,3,5-
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+
excited atoms, as well as N2 (3Σu ), are destroyed within a few
performed using either the Berny algorithm73 or the synchronous
transit-guided quasi-Newton (STQN) method.74 For most cases, an
initial estimated structure of the transition state is obtained through
relaxed potential energy surface (PES) scans using an appropriate
internal coordinate. Vibrational frequencies are calculated to confirm
that the reaction intermediates have all positive frequencies and
transition state species have only one imaginary frequency. Intrinsic
reaction coordinate (IRC) calculations75,76 are also performed so that
a transition state connects two appropriate local minima in the
reaction paths. The hybrid B3LYP exchange−correlation functional77
is adopted. A Gaussian basis set 6-311++G(d,p) is used.78,79 Test
calculations indicate that basis set superposition error (BSSE) is
negligible, and therefore BSSE is not taken into consideration. The
zero-point vibration corrected energies (ΔEzpe) are reported in this
study. Cartesian coordinates, electronic energies, and vibrational
frequencies for all of the optimized structures are available in the
Supporting Information. The calculated energy for the reaction O2 →
milliseconds.67,68 The titration reaction shown in eq 2 is used to
generate O(3P), because this reaction is efficient.
Reaction 2 is one of the very few gas titration reactions with a visual
end point.69,70 Figure 1 shows the chemiluminescence of this titration
3
3O + O is 57.9 kcal mol−1 which agrees well with the experimental
value of 58.7 kcal mol−1. The calculated electron affinity for 3O is 37.1
kcal mol−1, which also agrees well with the experimental value of 33.7
kcal mol−1. On the basis of these test calculations, the estimated
uncertainty of the DFT calculations is 4 kcal mol−1.
4. RESULTS AND DISCUSSION
4.1. Generation of Atomic Species. The reactant ions
Figure 1. Titration processes involved in generating N and O atoms.
(a) N atom flow (yellow luminescence) with pure N2 flowing through
the discharge region; (b) coexistence of N and O atoms (blue
luminescence) due to the introduction of NO into the N atom flow;
(c) titration end point with the existence of only O atoms (no color
observed); (d) the coexistence of NO and O atoms after the titration
end point (yellowish-green luminescence).
phenide (C6H5 ), pyridinide (C5H4N−), 1,2-, 1,3-, and 1,4-
−
−
−
diazinide (C4H3N2 ) and 1,3,5-triazinide (C3H2N3 ) are
allowed to react with N and O atoms. Figure 2 shows a typical
process. The pure N2 flow is conducted through the microwave
discharge region and ∼1% of N2 dissociates to N(4S) atoms. The
visible yellow chemiluminescence is due to excited N2*, which is
formed by the recombination of N atoms (Figure 1a):
N + N → N2*
(3)
N2* → N2 + hυ
(4)
Addition of nitric oxide (NO) initiates the generation of O atom by
reaction 2. The coexistence of N and O atom in the flow tube causes
the blue chemiluminescence of NO* from the recombination of N +
O,71 shown in Figure 1b:
(5)
N + O → NO*
(6)
NO* → NO + hυ
The intensity of the chemiluminescence decreases as the end point is
approached until [NO]added = [N]0; when chemiluminescence is no
longer observed, [N] = 0 and [O] = [N]0 (Figure 1c). Further
addition of NO generates yellowish-green emission due to the excited
2B1 state of NO2* (Figure 1d):
−
Figure 2. Titration plot for the reaction of 1,3-diazinide (C4H3N2 )
with N and O atoms. Point A, microwave discharge off; point B,
microwave discharge on; region C, coexistence of N and O atoms;
point D, titration end point; region E, loss of O atom by reaction with
NO.
O + NO → NO2*
(7)
NO2* → NO2 + hυ
(8)
titration plot, where the logarithm of the intensity of 1,3-
diazinide (C4H3N2 ) is plotted versus the flow of NO. At point
A, molecular nitrogen is flowing into the system with the
microwave discharge off; therefore, no reaction is evident.
When the discharge is ignited, N atoms are formed, the ions
react, and their intensity decreases to the lower value at point B.
−
The generated NO2 can react rapidly with O to form O2 and NO.
Thus, after the titration end point, the O atoms will be converted to
O2, catalyzed by NO, causing the partial recovery of ionic reactant
signals. The removal of O atoms in this regime is very effective due to
the relatively long distance in our apparatus between the discharge
region and the flow tube.
As NO is added to the system, N is converted to O, and the ion
−
signal decreases due to the more rapid reaction of C4H3N2
+
3. CALCULATIONS
DFT calculations using the Gaussian 09 program72 are employed to
study reactions of azine anions with N and O atoms. These
calculations involve geometry optimization of various reaction
intermediates and transition states. Transition state optimizations are
O (region C). The intersection of the two lines at point D
represents the end point of the titration; the flow of NO at this
point is equal to both the N atom flow at the beginning of the
titration as well as the O atom flow at the end point. Further
C
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Table 1. Reactions of Phenide Anion and Deprotonated Azine Anions with N and O Atoms Studied with FA-SIFT
a
The branching fractions of associative electron detachment (AED) are obtained by comparing the decrease of the parent ion signal with the
b
c
d
intensity of total product ion signals. The total error is estimated to be 50%. Not detected. Branching fractions for ionic product distributions
are given in parentheses.
−
addition of NO beyond the end point, region E, causes the
recovery of the ion signal, because O atoms are rapidly
removed by reaction with NO to form NO2 (eq 7). The ion
loss, the atom flow rate, and other experimental parameters are
used to determine the pseudo-first order reaction rate
constants.
minor ionic product detected from the reaction of 1,4-C4H3N2
with N atoms is C4H3N−, corresponding to the formation of
neutral N2, reaction 9:
−
C4H3N2 + N → C4H3N− + N2
(9)
The rate constants for the reactions of azine anions with N
atoms are larger than those of N-containing hydrocarbon chain
anions with N atoms. For example, the reaction rate constant of
C5H4N− with N atom is 1.3 0.7 × 10−10 cm3 molecule−1 s−1,
which is much larger than that of CH2CN− with N atom (3.6
1.4 × 10−11 cm3 molecule−1 s−1).23
4.2. Reactivity of Azine Anions. 4.2.1. Reactions of N
Atoms. The rate constants and products for reactions of the
azine anions with N (4S) and O (3P) atoms are summarized in
Table 1. The dominant process for the reactions of azine anions
with N atoms is associative electron detachment (AED) to
form electrons and neutral products. There is no ionic product
A comparison of the reaction PESs of C5H4N− and CH2CN−
with N atoms is shown in Figure 3. The most favorable way for
the quartet 4N atom to approach CH2CN− is at the terminal C
atom of CH2CN− (Figure 3a), which shows a barrier of 1.1 kcal
mol−1 higher than the energy of the reaction entrance channel.
This barrier can be lowered to 0.5 kcal mol−1 (B3LYP/6-
31+G(d,p)) via doublet-quartet PES crossing.23 However, the
existence of either a transition state or crossing point near the
detected for the reactions of N atoms with C6H5 , C5H4N−,
−
−
−
1,2- and 1,3-C4H3N2 and 1,3,5-C3H2N3 . The fact that AED
dominates the chemistry between these species is relevant to
Titan’s atmospheric chemistry, as nitrogen incorporation has
been shown to greatly influence the chemistry of polycyclic
species in this environment.80,81 Moreover, AED converts the
identity of the negative charge carriers from ions to free
electrons, which impacts the balance of these species. The only
entrance channel of CH2CN− + N makes this reaction slow.
4
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N-containing hydrocarbon species, whose polymerization may
contribute to the formation of Titan’s tholins.2,82,83
4.2.2. Reaction of O Atoms with Phenide. The reactions of
deprotonated azine anions with O atoms are complex, because
various ionic products are observed experimentally in addition
to AED (Table 1). However, the AED process is still dominant.
The reaction rate constant of C6H5− with O atoms (9.0 4.5 ×
10−10 cm3 molecule−1 s−1) is roughly 5 times faster than that
with N atoms (1.8 0.9 × 10−10 cm3 molecule−1 s−1). The
−
−
observed ionic products for this reaction are C6H3 , C5H5 , and
−
HC2 , corresponding to the formation of neutral H2O, CO, and
C4H4O, respectively, shown in reaction 11:
−
3
C6H5 + O
Figure 3. Quartet PESs for the reactions of (a) CH2CN− and (b)
C5H4N− with N atoms. Note that the structure of TS2 is obtained by
fixing the N−C distance to 1.81 Å and fully relaxing all other
coordinates. The profiles are plotted for zero-point vibration corrected
energies (Ezpe) relative to the energy of the entrance channel.
→2C6H5O + e−
−98.7 kcal mol−1
−31.6 kcal mol−1
−58.5 kcal mol−1
−52.9 kcal mol−1
(11a)
(11b)
(11c)
(11d)
−
→3C6H3 + H2O
−
→3C5H5 + CO
−
→HC2 + C4H4O
Additionally, our current level of theory has a maximum
uncertainty of about 4 kcal mol−1, indicating that the small
barrier calculated in this reaction may be slightly lower than the
energy of the reactants, corresponding to the moderate reaction
According to our experimental observations, the generation of
C6H3− + H2O is the dominant ionic product channel (reaction
11b). The formation of carbon monoxide (reaction 11c) is not
surprising since CO is very stable and the generation of CO has
rate constant of 3.6
1.4 × 10−11 cm3 s−1. In contrast, the
4
−
−
approach of N atom to ground state C5H4N− is straightfor-
ward and barrierless with a significant energy release to form
NC5H4N− (Figure 3b). The subsequent electron detachment
leads to the formation of triplet neutral NC5H4N and an
electron, shown in eq 10. The zero-point vibration corrected
energies (Ezpe) are reported based on DFT calculations:
also been reported in the reactions of Cn and HCn with O
atoms.24 The ionic product HC2− is the smallest member of the
series HC2n−; the existence of the larger ions C4H−, C6H−, and
C8H− in this family has been confirmed in the ISM.32−34
4.2.3. Reaction of O Atoms with Pyridinide. Several ionic
products are observed from the reaction of C5H4N− + O
including predominantly CN−, C5H2N−, and C5H3N−,
corresponding to the formation of neutral C4H4O, H2O, and
OH, respectively (Figure 4). Other low-abundance products
C5H4N− + 4N → 3NC5H4N + e− −60.3 kcal mol−1
(10)
This comparison explains why the observed reaction rate
constants of N atoms with deprotonated azine anions are faster
than those with N-containing hydrocarbon chain anions. The
−
such as C3H3 , C3N−, and C4H4N− may arise from minor
−
reaction channels. Comparing the reactions of C6H5 and
C5H4N− with O atom, one can see that the reaction of N-
containing C5H4N− with O atom favors the formation of N-
containing ionic species as shown in Figure 4. The formation of
4
direct association of N atom to the deprotonated C atom site
of the six-membered ring is found to be a common mechanism
4
for the first attachment step of N atom with deprotonated
azine anions according to our DFT calculations.
It has been recently shown that the chemistry of negative
ions is unexpectedly rich and complex in the upper atmosphere
of Titan.2,3,12,20,25−27 The rate constants of reactions of N
atoms with azine anions reported here are thus crucial for the
proper modeling of anionic reactions in Titan’s atmosphere,
whose environment mimics the evolution of the atmosphere of
early Earth before its buildup of oxygen.15 The observed high
AED efficiency for the reactions of N atoms with both phenide
and all azine anions is surprising. This observation, coupled
with our former report of the high branching fractions of AED
in the reactions of N atoms with carbon chain anions,23,24
suggests a contribution of these processes to the electron-rich
nature of Titan’s upper atmosphere. Furthermore, subsequent
dissociation of neutral products is possible following the AED,
since these association reactions are highly exothermic (such as
reaction 10). The reformation of N2 molecules is feasible via
the interaction of free N atoms with the skeleton N atoms to
complete the nitrogen cycling in Titan’s ionosphere. This
argument is supported by the observation of the C4H3N− + N2
products from the reaction of 1,4-diazinide with N atoms
(Table 1). Furthermore, the cleavage of the stable six-
membered ring structure will enable the formation of small
Figure 4. Product distribution for the reaction of C5H4N− with O
atoms is shown in this mass spectrum. Some very low intensity signals
may arise from secondary reactions or system impurities.
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2
these species is also observed for the reactions of other azine
anions with O atoms (Table 1); these processes may be an
important source of various N-containing species observed in
Titan’s atmosphere and in the ISM. The Ezpe of this reaction is
studied by employing DFT calculations, shown in reaction 12:
exothermic than that to form P1. The loss of the OH radical
2
from 4 can generate the experimentally observed C5H3N− (4
→ P4). Unlike the formation of the 2OH radical, the
production of H2O requires a second hydrogen atom transfer
(4 → 4/5 → 5 → P3) with a high barrier of 16.6 kcal mol−1
(transition state 4/5) and thus is impossible on the triplet
potential energy surface. This calculated high barrier on the
3
C5H4N− + O
triplet PES for the formation of H2O + C5H2N− does not
3
→2C5H4NO + e−
−84.1 kcal mol−1
−31.1 kcal mol−1
−51.0 kcal mol−1
−11.1 kcal mol−1
−52.4 kcal mol−1
−50.7 kcal mol−1
−115 kcal mol−1
(12a)
(12b)
(12c)
(12d)
(12e)
(12f)
(12g)
agree with our experimental observations; therefore, we have
pursued this calculation in greater detail and discuss our
findings below.
The reaction is barrierless and more exothermic to form the
singlet products H2O + C5H2N− from the triplet reactants via a
crossing point as shown in Figures 6 and 7. This crossing point
3
→CN− + CH2COCHCH
→C5H2N− + H2O
2
→2C5H3N− + OH
−
→C3H3 + CO + HCN
→C3N− + H2O + C2H2
→C4H4N− + CO
The formation of three major products by the reaction of
C5H4N− with 3O atom on the triplet PES is shown in Figure 5.
1
Figure 6. PES for the reaction of C5H4N− with O on the singlet
surface. The profiles are plotted for zero-point vibration corrected
energies (Ezpe) relative to the energy of the entrance channel. The Ezpe
is given in kcal mol−1. The intermediates and transition states are
denoted in bold as n and n1/n2, respectively, and the products are
denoted in bold as Pn. CP designates the crossing point for the singlet
and triplet surfaces.
3
Figure 5. PES for the reaction of C5H4N− with O on the triplet
surface. The profiles are plotted for zero-point vibration corrected
energies (Ezpe) relative to the energy of the entrance channel. The Ezpe
is given in kcal mol−1. The intermediates and transition states are
denoted in bold as n and n1/n2, respectively, and the products are
denoted in bold as Pn.
can only occur between the reactants and the first encounter
complex (intermediate 6), because the singlet intermediate 6
possesses a very similar structure to the triplet intermediate 1,
The C5H4N− is deprotonated at the C4 site, which has been
verified by our former studies of its photoelectron spectrum.84
3
The interaction of O atom with the C4 site of the C5H4N− is
barrierless and associated with a large energy release (−79.3
kcal mol−1) to form intermediate 1. An electron can be lost
2
from 1 to generate the neutral doublet C5H4NO. This AED
process is highly exothermic (−84.1 kcal mol−1) and the
product branching fraction is experimentally estimated to be
∼0.9 (Table 1). From intermediate 1, hydrogen atom transfer
occurs either from the C3 atom to the oxygen or from the C2
3
to the C3 atom, leading to the formation of C5NH3−OH (4)
or c-3CH2COCHCHNC (2), respectively. From intermediate
2, the cleavage of the six-membered ring is feasible at the C2−
C3 bond and the formation of the CN− product can be
achieved (2 → 2/3 → 3 → P2). The generation of CN− +
3C4H4O (P2) is also a barrierless reaction channel, although the
reaction to form P2 (−31.1 kcal mol−1) is much less
Figure 7. Relaxed PES scan of the entrance channel of C5H4N− with
O for the singlet and triplet spin states. The singlet−triplet crossing
point is determined to possess the same structure and energy on both
singlet and triplet PESs.
F
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Article
but is 72.4 kcal mol−1 more stable than 1 (Figure 6). Starting
from 6, the process to generate P5 (H2O + C5H2N−) similarly
involves the consecutive hydrogen atom transfers (6 → 6/7 →
7 → 7/8 → 8 → P5) and the highest barrier (transition state
7/8) of this reaction pathway is −33.0 kcal mol−1 lower in
energy than the triplet reactants. In the reaction of C5H4N− +
O, the major reaction channel is AED, which has a branching
fraction of 90% (Table 1). The ionic product channel that
generates C5H2N− + H2O via the crossing between the singlet
and triplet potential energy surfaces contributes a branching
fraction of only 2.5%. This value corresponds to a rate constant
of 3.0 × 10−11 cm3 s−1, which is 40 times lower than the total
reaction rate constant for the C5H4N− + O reaction. The
products 2C5H3N− + 2OH (P4) and c-C4H4O + CN− (P6) can
also be formed from the singlet PES without barriers as shown
in Figure 6. The formation of H2O + C5H2N− is more
kinetically favorable than C5H3N− + OH on the singlet PES.
However, C5H3N− + OH can also be generated from the triplet
PES as shown in Figure 5, which gives a reasonable explanation
for the observed similar intensities of C5H2N− and C5H3N− in
Figure 4. The formation of CN− + C4H4O (triplet P2 in Figure
5 and singlet P6 in Figure 6) is the most favorable reaction
pathway on either the singlet or triplet PES, which agrees well
with the experimentally observed highest intensity signal from
−
3
Figure 8. PES for the reaction of 1,3-C4H3N2 with O on the triplet
surface. The profiles are plotted for zero-point vibration corrected
energies (Ezpe) relative to the energy of the entrance channel. The Ezpe
is given in kcal mol−1. The intermediates and transition states are
denoted in bold as n and n1/n2, respectively, and the products are
denoted in bold as Pn.
C4H3N2− with ground state O atoms is straightforward to form
the encounter complex (intermediate 12, − 77.2 kcal mol−1).
The binding energy of this process is sufficient for either the
AED channel to generate C4H3N2O + e− (P9, − 78.8 kcal
mol−1) or the following complicated hydrogen atom transfer
and ring-opening processes (12 → 12/13 → 13 → 13/14 →
14 → 14/15 → 15). From intermediate 15, the loss of the CN
CN−. The formation of C4H4N−, C3H3 , and C3N− is
−
thermodynamically favorable as shown in reactions 12e−12g,
and the low ion signals (Figure 4) indicate the probable
existence of relatively higher kinetic barriers on their reaction
pathways.
4.2.4. Reactions of O Atoms with Diazinides. Similar to the
reactions of N-containing hydrocarbon chain anions with O
atoms, the ionic CN− product is observed for all reactions of
deprotonated diazine anions with O atoms (see Table 1 for
details). However, in the reactions of N-containing hydro-
carbon chain anions with O atoms, CN− is the only major ionic
product,23 while there are complex product patterns from the
reactions of deprotonated diazine anions with O atoms. Four
3
moiety will lead to the products CN− + C3H3NO (15 → 15/
16 → 16 → P7), while rearrangement of the O atom inserted
into the carbon chain constructs a long-chain structure
containing an OCN group (intermediate 17). The breaking
of the O−C bond and the consecutive loss of the OCN moiety
from the terminal chain structure results in the formation of
OCN− + C3H3N (P8, − 34.5 kcal mol−1). The generation of
3
the singlet products, e.g., OCN− + C3H3N and CN−
+
C3H3NO may also be possible via a crossing point between the
triplet and the singlet PESs. Note that both OCN− and CN−
are important negatively charged interstellar species.
−
ionic products are observed for the reaction of 1,2-C4H3N2
with O atoms, corresponding to CN−, CCN−, C4H−, and
OCCN−, respectively. Among these anion products, C4H− and
OCCN− are dominant, and correspond to the formation of the
neutral species H2O + N2 and CH3CN, respectively. For the
4.2.5. Reaction of O Atoms with 1,3,5-Triazinide. The ionic
−
products observed for the reaction of 1,3,5-C3H2N3 with O
atoms are OCNN− and OCN−. The CN− product which has
been observed in reactions of other azine anions + O atoms is
not detected in this reaction. The reaction channels observed
are summarized in reaction 14:
reaction of 1,3-C4H3N2 + O, two ionic products, CN− and
−
OCN−, are detected with similar intensities. The reaction of
1,4-C4H3N2 + O leads to the generation of CN− and HC2N−
−
−
3
C3H2N3 + O
with the ionic branching fractions 0.55 and 0.45, respectively.
−
The reaction of 1,3-C4H3N2 with O atoms is studied in detail
→2C3H2N3O + e−
− 74.4 kcal mol−1
− 60.8 kcal mol−1
− 35.9 kcal mol−1
−
(14a)
(14b)
using DFT calculations. The parent neutral of 1,3-C4H3N2 is
1,3-diazine (pyrimidine), which is an important prebiotic
3
→OCN− + C2H2N2
molecule.52 The reaction rate constant for the reaction of
−
3
1,3-C4H3N2 with O atom is 1.2 0.6 × 10−9 cm3 s−1. With
AED as the major reaction channel, ionic products observed in
this reaction are OCN− and CN−, reaction 13:
−
2
→2OCNN + C2H2N
(14c)
−
As shown in Figure 9, the direct association of 1,3,5-C3H2N3
with an O atom forms the triplet intermediate 18 (−92.2 kcal
mol−1) that can undergo electron detachment, and the products
2C3H2N3O + e− (P12) are 74.4 kcal mol−1 below the total
energy of the reactants. From intermediate 18, the formation of
intermediate 20 with a CH2 moiety in the six-membered ring
can be realized via consecutive hydrogen atom transfers over
the ring skeleton (18 → 18/19 → 19 → 19/20 → 20). This
hydrogen atom transfer process allows the next ring-opening
step to form the OCNN moiety (20 → 20/21 → 21). The
direct elimination of OCNN from intermediate 21 leads to the
−
3
C4H3N2 + O
→2C4H3N2O + e−
− 78.8 kcal mol−1
− 41.4 kcal mol−1
(13a)
(13b)
(13c)
3
→CN− + C3H3NO
3
→OCN− + C3H3N
− 34.5 kcal mol−1
−
Figure 8 shows the reaction of 1,3-C4H3N2 with O atoms
on the triplet PES. The initial step for the combination of 1,3-
G
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reactions of H atoms in which reactivity correlates with
exothermicity.87 This result may reflect the fact that, for similar
processes, more exothermic reactions tend to have lower
energy barriers, and therefore a great number of states leading
to reaction. In addition, more complex product patterns are
observed for the O atom reactions. The ionic product CN− can
be observed for all studied reactions of azine anions with O
−
atoms except for the 1,3,5-C3H2N3 + O reaction; the latter
only generates OCNN− and OCN−. The mechanisms for the
formation of various ionic products via the reaction of azine
anions with O atoms are studied in detail using DFT
calculations. The spin conversion process is found to be
essential to the formation of C5H2N− + H2O from the reaction
of C5H4N− with O atoms, while many other ionic products of
azine anions + O reactions can be formed directly from triplet
PESs without barriers.
The temperatures in the ISM and in Titan’s atmosphere are
much lower than 298 K. Since the reaction rate constants of
azine anions with O atoms are close to the capture limit, no
strong temperature dependence is expected. However, in the
case of N atoms, strong temperature effects are possible;88 thus
temperature variable experimental studies are highly desired for
providing important benchmarks for theoretical assessments.
The ionic products observed herein (CN−, C3N−, C2H−,
C4H−, and OCN−) are important N-containing species that
exist in Titan’s atmosphere and the ISM. These reactions
illuminate the complex chemistry between N and O atoms and
azine anions, and synthesize other known interstellar and
atmospheric molecules in the process. This work uncovers
important mechanisms and, coupled with additional observa-
tions and modeling, may shed light on the identities,
abundances, and roles of negative ions in an array of
astrochemical environments.
Figure 9. PES for the reaction of 1,3,5-C3H2N3− with 3O on the triplet
surface. The profiles are plotted for zero-point vibration corrected
energies (Ezpe) relative to the energy of the entrance channel. The Ezpe
is given in kcal mol−1. The intermediates and transition states are
denoted in bold as n and n1/n2, respectively, and the products are
denoted in bold as Pn.
final products, 2OCNN− + 2C2H2N (P10). On the other hand,
the direct cleavage of the six-membered ring structure may
occur from 18, leading to the open ring intermediate 22 with
the formation of an OCN group. The generation of OCN− +
3C2H2N2 is straightforward from 22 with a barrier 48.7 kcal
mol−1 lower in energy than the initial reactants. According to
our experimental observations, AED is the dominant process
and the intensity of the OCN− signal is larger than that of
2OCNN− (Table 1), which agrees well with calculations on the
triplet PES. Although spin conversion might be possible during
the formation of the ionic products, the transfer between PESs
of different multiplicities is usually the rate-limiting step.
Therefore, it is reasonable to consider the reaction pathways
that occur within the PES of the same multiplicity to be the
major channels, given that the formation of products are
exothermic and barrierless. The computed results complement
the experimental observations well for the reaction of 1,3,5-
ASSOCIATED CONTENT
■
S
* Supporting Information
−
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/jacs.5b06089.
C3H2N3 with O atoms.
5. CONCLUSIONS
Cartesian coordinates and zero point corrected energies
for all of the optimized structures. (PDF)
Gas phase ion-atom reactions of deprotonated azines with N
and O atoms are studied both experimentally and computa-
tionally. From the boundary layers between diffuse and dense
interstellar clouds, to the atmospheres of early Earth and
satellites such as Titan, these reactions provide important
information regarding the chemical processing of atomic and
molecular species in these environments. In all cases, the major
reaction channel is AED, which transfers negative charge from
anions to free electrons. The reactions of the azine anions with
N atoms exhibit larger reaction rate constants when compared
to those of N-containing hydrocarbon chain anions, because
the association of N atom with the azine anions is
straightforward and barrierless on the quartet PES. There are
no ionic products observed for most of the reactions of the
azine anions with N atoms. The only ionic product observed is
AUTHOR INFORMATION
■
Corresponding Authors
*zhechen.wang@colorado.edu
*veronica.bierbaum@colorado.edu
Present Address
#Department of Physics, University of Auckland, Private Bag
92019, Auckland, New Zealand.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
C4H3N−, which arises from the reaction of 1,4-C4H3N2 + N
and corresponds to the formation of neutral N2; this process
provides a route for the regeneration of molecular nitrogen in
these environments.80
The reaction rate constants of azine anions with O atoms are
near the collisional limit;85,86 these reactions are more
exothermic and faster than the corresponding reactions with
N atoms. This behavior parallels our previous studies of the
−
■
This material is based upon work supported by the National
Science Foundation (NSF) Grant CHE-1300886 and by the
NSF Graduate Research Fellowship under Grant No. DGE-
1144083. We are grateful to the Extreme Science and
Engineering Discovery Environment (XSEDE), which is
supported by National Science Foundation grant number
ACI-1053575.
H
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