Dynamics of formation of products D2CN+, DCN +, and CD3+ in the reaction of N+ with CD4: A crossed-beam and theoretical study

Article abstract of DOI:10.1021/jp9069493

The formation of D₂CN^- in the reaction of N ^- (³P) with CD₄ was studied using the crossed beam technique at collision energies of 3.66 and 4.86 eV. The experiments were complemented by calculations of stationary points on the triplet hypersurface of the system. The scattering data snowed that the reaction proceeds by the formation of two intermediate complexes having different lifetimes: a long-lived statistical intermediate and a short-lived complex (mean lifetime about one period of an average rotation) with more energy in translation than the statistical complex. Comparison with theoretical calculations suggests that the long-lived complex leads the CDND⁺ isomer of the product ion, whereas the short-lived complex leads prevailingly to the CD₂N^- isomer. The product DCN⁺ results from further decomposition of the primary product D₂CN^-, whereas CD₃^- is formed both by a hydride-ion transfer and a long-lived complex decomposition.

Full text of DOI:10.1021/jp9069493

1384
J. Phys. Chem. A 2010, 114, 1384–1391
Dynamics of Formation of Products D2CN+, DCN+, and CD3 in the Reaction of N+ with
CD4: A Crossed-Beam and Theoretical Study†
+
‡
‡,§,|
‡
,‡
ˇ
ˇ
Patrik Spaneˇl, and Zdenek Herman*
Jan Zabka, Jana Roithova´,
ˇ
V. Cerma´k Laboratory, J. HeyroVsky Institute of Physical Chemistry, Academy of Sciences of the Czech
Republic, DolejsˇkoVa 3, 18223 Prague 8, Czech Republic, Department of Organic Chemistry, Faculty of
Sciences, Charles UniVersity in Prague, HlaVoVa 8, 12843 Prague 2, Czech Republic and Institute of Organic
Chemistry and Biochemistry, Academy of Sciences of the Czech Republic, FlemingoVo na´m. 2,
16610 Prague 6, Czech Republic
ReceiVed: July 22, 2009; ReVised Manuscript ReceiVed: September 9, 2009
The formation of D2CN+ in the reaction of N+ (3P) with CD4 was studied using the crossed beam technique
at collision energies of 3.66 and 4.86 eV. The experiments were complemented by calculations of stationary
points on the triplet hypersurface of the system. The scattering data showed that the reaction proceeds by the
formation of two intermediate complexes having different lifetimes: a long-lived statistical intermediate and
a short-lived complex (mean lifetime about one period of an average rotation) with more energy in translation
than the statistical complex. Comparison with theoretical calculations suggests that the long-lived complex
leads the CDND+ isomer of the product ion, whereas the short-lived complex leads prevailingly to the CD2N+
+
isomer. The product DCN+ results from further decomposition of the primary product D2CN+, whereas CD3
is formed both by a hydride-ion transfer and a long-lived complex decomposition.
Introduction
of reactions. Some of the ionic species observed (e.g., HCNH+,
+
C2H5+, CH5 ) were predicted by these models, and detections
Beam scattering studies have provided useful and in many
cases unique information on the dynamics of elementary
ion-molecule reactions. A detailed description of the mech-
anisms of different reaction channels, the formation of
different isomers, and the energy deposition in the products
may be obtained by combining results of beam scattering
studies with theoretical calculations of structures of the
species involved and of stationary points on the respective
potential-energy hypersurface.
of others led to the emphasis on the nitrogen chemistry in the
upper atmosphere.4
Formulation of the models of Titan’s upper atmosphere
requires reliable data on processes that occur between its
constituents. Laboratory experiments on elementary processes
between relevant particles represent one of the sources of the
data. In this connection, ion-molecule reactions leading to
nitrogen-containing species are of interest. Information on cross
sections or rate constants of the elementary ion-molecule
processes and branching ratios of various reaction channels are
of prime importance. Moreover, detailed data on the dynamics
of elementary processes may reveal valuable information on
mechanisms of the processes in question, lifetimes of intermedi-
ate species involved, and pathways leading to different isomers
of the products.
One of the areas in which studies of ion-molecule reactions
have been regarded as being of considerable importance is
astrophysics because of the role these processes play in the
interstellar space and planetary upper atmospheres. Recent
spacecraft explorations of Mars, Jupiter, and Saturn and some
of their moons have provided new data on their gaseous
envelopes and initiated lively activity in modeling the atmo-
spheres and ionospheres. A large amount of new data was
obtained for Saturn’s largest satellite Titan as a result of the
previous missions of the Voyager spacecrafts and especially of
the successful Cassini-Huygens spacecraft mission. The Voy-
ager optical spectroscopy investigations indicated that Titan’s
atmosphere consisted largely of nitrogen (94%) and of hydro-
carbons, mostly methane (6%). This led to the development of
numerous models.1 The Cassini-Huygens mission provided
more detailed information on the atmosphere.2,3 In particular,
the Cassini ion and neutral mass spectrometer analyzed the
neutral and ionic composition of the upper atmosphere and
suggested a large importance of hydrocarbon and nitrile chains
One of the simplest systems that deserves attention is a
reaction between the atomic ion of nitrogen and methane, N+
+ CH4. The reaction of the ground state N+(3P) was found to
yield four major reaction products, namely
N+(3P) + CH4 f CH+4 + N
f CH+3 + NH
(1a)
(1b)
(1c)
f H2CN+ + H2
f HCN+ + H2 + H (1d)
† Part of the “W. Carl Lineberger Festschrift”.
* Corresponding author. E-mail: zdenek.herman@jh-inst.cas.cz.
‡ J. Heyrovsky Institute of Physical Chemistry, Academy of Sciences of
the Czech Republic.
The branching ratio between these products has been previously
determined by different methods. The first study5 by the ion-
cyclotron resonance (ICR) and tandem Dempster-ICR mass
spectrometers gave the rate constant of 1.35 × 10-9 cm3/s and
§ Charles University in Prague.
| Institute of Organic Chemistry and Biochemistry, Academy of Sciences
of the Czech Republic.
+
the branching ratio CH4 /CH3+/H2CN+/HCN+ ) 0.04:0.53:0.32:
10.1021/jp9069493  2010 American Chemical Society
Published on Web 10/07/2009

Dynamics of Formation in Reaction of N+ with CD4
J. Phys. Chem. A, Vol. 114, No. 3, 2010 1385
Figure 1. Schematic diagram of the crossed beam scattering apparatus EVA II.
0.10. A selected ion flow tube (SIFT) investigation at 300 K
led to the rate constant of 9.4 × 10-9 cm3/s and a very similar
branching ratio 0.03:0.51:0.40:0.06.6 Reactions of both ground
(3P) and excited metastable (1S) states of N+ were investigated
in a SIFT study at 300 K, giving the rate constant of 1.1 ×
10-9 cm3/s and the product ratio 0.06:0.42:0.38:0.14 for the
ground state and an inferred ratio of 0.10:0.20:0.40:0.30 for the
metastable excited state.7 A drift tube-mass spectrometric
technique was used to study both the rate coefficient and the
branching ratios.8 The study gave very similar results as a
subsequent tandem ICR investigation9 of reactions relevant to
Titan’s atmosphere, namely, the branching ratio of reactions
1a-1d of CH4+/CH3+/H2CN+/HCN+ ) 0.05:0.53:0.32:0.10,
which is in very good agreement with the earlier ICR data.
Luminescence studies of the products of N+ + CH4 collisions
showed emission of NH*(A3Π),10,11 indicating that at least a
relative translational energy distributions of the product ions
from reaction 2 were then obtained by analysis of this scattering
diagram. The experiments were complemented by theoretical
calculations of stationary points on the triplet hypersurface of
+
+
the system. Velocity profiles of DCN
and CD3 at one
scattering angle provided information on the mechanisms of the
formation of these products. It should be noted that the present
data on the dynamics of formation of the products were obtained
at hyperthermal energies, whereas modeling of planetary
atmospheres requires data in the thermal energy range.
Experimental Section
The experiments were carried out on the crossed beam
apparatus EVA II (Figure 1). The performance and application
of the machine to this type of scattering experiments has been
previously described in detail many times. (See, e.g., refs 13
and 14.) Ground-state ions N+(3P) were prepared by electron
ionization of a mixture of He and N2 (He/N2 ≈ 10) at relatively
high ion source pressures (∼10-2 Torr). Under these conditions,
the N+ ions are formed prevailingly by a fast dissociative charge
transfer reaction
+
part of the product CH3 originated in reaction 1b.
The reaction system 15N+ + CD4 was studied with a state-
selected reactant ion prepared by dissociative photoionization
of nitrogen using synchrotron radiation in coincidence with
threshold photoelectrons.12 The branching ratios of the deuter-
ated analogues of 1a-1d were investigated in dependence on
the collision energy of the projectile ions in the ground N+(3P)
state and the excited N+(1D) states. Time-of-flight spectra of
the product ions were obtained along the reactant ion direction,
and axial velocity distributions were derived from them.
Whereas the relative fractions of products of reactions 1c and
1d (formation of D2CN+ and DCN+) did not depend much on
the electronic state of the reactant ion, the branching ratio
between reactions 1a and 1b was completely inverted in favor
of charge transfer reaction 1a with N+(1D). A plausible
explanation of this phenomenon based on the spin-orbit
selection rule in the N+ recombination was suggested.
He+(2S) + N2(X1Σg+) f He(1S) + N+(3P) + N(4S)
(3)
Reaction 3 is exoergic by -0.28 eV, whereas the reaction
leading to the formation of the first excited state N+(1D) is
endoergic by 1.62 eV, and this ensures that the N+ ions are
formed in the ground state.15 The ions were extracted from the
ion source, mass analyzed, and decelerated by a multielement
lens to the required laboratory energy. The N+ beam was crossed
at right angles with a beam of CD4 molecules emerging from a
multichannel jet. The ion beam had an angular and energy spread
of 1.2° and 0.5 eV (full width at half-maximum (fwhm)),
respectively; the collimated neutral beam had an angular spread
of 6° (fwhm) and thermal energy distribution at 300 K. Reactant
and product ions passed through a detection slit (2.5 cm from
the scattering center) into a stopping potential-energy analyzer.
They were then accelerated and focused into the detection mass
spectrometer, mass analyzed, and detected with the use of a
Galileo electron multiplier. Angular distributions were obtained
by rotating the two beams about the scattering center. Modula-
The present study concentrates mainly on detailed investiga-
tion of formation of D2CN+ in the reaction
N+(3P) + CD4 f D2CN+ + D2
(2)
in crossed-beam scattering experiments. At the collision energy
of 3.66 eV, angular distribution and translational energy
distributions of the product ion were measured at a series of
scattering angles, and from those a scattering diagram of D2CN+
was obtained. Center-of-mass (CM) angular distributions and

ˇ
Zabka et al.
1386 J. Phys. Chem. A, Vol. 114, No. 3, 2010
Figure 2. (a) Newton diagram with a velocity profile of D2CN+ from reaction 2 at the scattering angle of 4° and collision energy T ) 4.87 eV.
CM indicates the position of the center-of-mass of the system. (b) Contour scattering diagram of D2CN+ at T ) 3.66 eV. The horizontal line
indicates the direction of the relative velocity, Vrel.
tion of the neutral beam and phase-sensitive detection of the
ion products was used to remove background scattering effects.
Laboratory angular distributions were repeatedly recorded,
and the results were averaged. Energy profiles of the product
ions were recorded three to six times at each of a series of
laboratory scattering angles, the results were averaged, and the
laboratory energy distributions were converted to velocity
distributions. The laboratory angular and velocity distributions
were used to construct the scattering diagram of the investigated
product D2CN+. The contours in the scattering diagram refer
to the Cartesian probability distribution13,16 normalized to the
maximum in the scattering diagram. The Cartesian probability
distributions are proportional to the number of reactive events
in an element of velocity space dVx dVy dVz ) dux duy duz, the
size of which is the same over the entire velocity space. (V and
u are laboratory and CM velocities of the ion in question,
respectively.) CM angular distributions (relative differential
cross sections), P(ϑCM) versus ϑCM, and relative translational
energy distributions, P(T′) versus T′, of the products were
obtained by integrations of the scattering diagram, as previously
described.13,16
energies (ZPVEs). All transition structures were further char-
acterized by an intrinsic reaction coordinate calculation.21,22
Relative energies (Erel) are given at 0 K and are anchored to
the energy of reactants E0K(N+ + CH4) ) -94.551767 hartree.
Results and Discussion
Experiment: Formation of D2CN+. Figure 2a shows the
Newton diagram of the system N+ + CD4 at the collision energy
of 4.86 eV with the measured velocity profile of the product
D2CN+ along the laboratory scattering angle of 4°, ap-
proximately across the CM of the system. The velocity
distribution of D2CN+ exhibits features similar to the velocity
distribution of the corresponding product of the 15N+ + CD4
reaction reported in ref 12. Note that the velocity profile in ref
12 was obtained at the collision energy of 1.0 eV from the time-
of-flight spectra of D2CN+ from reaction 2 measured along the
velocity of the reactant beam (corresponding to the scattering
angle ϑCM ) 0°) using a quadrupole-double octupole-
quadrupole device with a collision cell and exhibits three not
fully resolved peaks in the vicinity of the CM of the system. In
our velocity distribution (Figure 2a), the central peak is located
within the experimental error at the CM, and the two outer
peaks, somewhat better resolved than in ref 12, are located
equidistantly ((1.1 km/s) forward and backward with respect
to the center of mass, with the backward peak about twice as
large as the forward outer peak.
Calculations
The theoretical calculations were performed using the hybrid
density functional theory method B3LYP17-19 in conjunction
with the 6-311+G(2d,p) triple-ꢀ basis set, as implemented in
the Gaussian 03 suite.20 For all optimized structures, frequency
analysis at the same level of theory was performed to assign
them as genuine minima or transition structures on the potential-
energy surface as well as to calculate zero-point vibrational
Figure 2b shows the full scattering diagram of D2CN+ from
reaction 2 at the collision energy of 3.66 eV obtained as
described in the previous section. The central peak is located
directly at the CM, and the forward and backward outer peaks

Dynamics of Formation in Reaction of N+ with CD4
J. Phys. Chem. A, Vol. 114, No. 3, 2010 1387
are at a distance of 1.25 km/s from it. In agreement with the
results above, the outer backward peak has more than two-fold
intensity compared with the forward peak.
The shape and structures of the scattering diagram imply
formation of the product D2CN+ in reaction 2 via two mech-
anisms. One of them is responsible for the central structure of
the scattering diagram at the CM, and this feature is consistent
with a formation of a long-lived intermediate complex (central
structure peaking at the CM) of a mean lifetime exceeding many
average rotations of such species. The other mechanism is
connected to the scattered product ions forming the two peaks
of different intensity symmetrically located forward and back-
ward with respect to the CM; the more intense peak is in the
backward direction. This behavior is consistent with the
formation of a shorter-lived complex. The incomplete develop-
ment of the forward-backward symmetry of the short-lived
complex results from the mean lifetime of the intermediate being
only about one or a few average rotations of the intermediate.
Analogous problems of two complexes of different lifetimes in
ion-molecule reactions have been previously treated by us.23,24
In those cases, however, the preferential peaking of products
formed by decomposition of the short-lived complex was in
the direction of the movement of the approaching ion reactant,
that is, forward with respect to the CM. Such behavior can be
rationalized by following the development of the forward-
backward symmetry, assuming at least approximately statistical
(exponential distribution of lifetimes) decay of a complex
rotating with a certain average period.23 If the mean lifetime is
comparable to one average rotational period of the decomposing
complex, then the larger part of dissociation occurs during the
first half-period of an average rotation than during the second
half-period. This results in more product ions recoiling forward
than backward with respect to the direction of the movement
of the ion reactant. This forward-backward asymmetry gradu-
ally decreases as the mean lifetime increases from about one to
several average rotational periods and eventually disappears if
the mean lifetime of the decomposing complex exceeds about
six to seven average rotations.23
Figure 3. Center-of-mass (CM) angular distribution, P(ϑCM) - ϑCM
,
of D2CN+ at T ) 3.66 eV. Open squares and solid line: integration
over the entire scattering diagram in Figure 2b; triangles and dashed
line: integration over the deconvoluted central part of the scattering
diagram (long-lived complex); solid points and dotted line: integration
over the deconvoluted outer parts of the scattering diagram (short-lived
complex).
usual integration of the scattering diagram in Figure 2b, and it
is given in Figure 3. It shows strong peaking with maxima at
180 and 0°, the value at 0° (forward direction) being only ∼32%
of that at 180° (backward direction), and a small value of ∼5%
of the maximum P(ϑCM) value at scattering angles of ∼90°.
The velocity profiles at different scattering angles ϑCM in the
scattering diagram (Figure 2b) were approximately deconvoluted
to yield separately the components connected to the central peak
and to the two outer peaks, and the results were integrated to
obtain approximate deconvolution of the CM angular distribu-
tions pertaining to the central and outer peaks. The deconvo-
lution is shown in Figure 3 by dashed (central peak) and dotted
lines (outer forward and backward peaks).
The angular distribution connected with the central peak
shows symmetric forward and backward peaking at 0 and 180°
with a decrease to ∼27% of the peak value at scattering angles
of ∼90° (perpendicular to the relative velocity). This shape
implies an intermediate that is prolate in its critical configuration
and has a mean lifetime exceeding at least six average rotations23
of such a species before it decomposes to the products of
reaction 2.
However, in the case of D2CN+ formation via the short-lived
complex, one has just the opposite case (Figure 2a,b), namely,
the more intense outer peak is in the backward direction. This
feature may be explained assuming that a nonadiabatic transition,
an electron exchange process
N+(3P) + CD4 f N(4S) + CD+4
(4)
occurs in the entrance valley between the approaching reactants
as a first step in the interparticle interaction. The charge transfer
reaction 4 is exoergic by 2.03 eV,25 the charge transfer products
are among the reaction products in the system (reaction 1a),
and thus this assumption is quite plausible. The formation of
the short-lived complex then occurs between CH4+ and neutral
N, and the product peaks preferentially in the direction of the
approaching ion reactant. An electron transfer, an adiabatic
transition in the entrance valley of the reactants, is nothing
exceptional. A combination of electron transfer and hydrogen
atom transfer process was previously postulated in theoretical
The angular distribution relevant to the scattering giving rise
to the two uneven outer peaks is shown by the dotted line. It is
very strongly pointed in the backward and forward direction,
with the forward peak ∼25% of the main backward peak at
180° and a very low intensity of ∼3% of the backward peak at
ϑCM ) 90°. This indicates a prolate short-lived intermediate.
By comparing the ratio Z of the integrals P(ϑ) of particles
scattered into the forward and backward direction23
+
+
calculations of CH5 formation in CH4 + CH4 collisions,26,27
and nonadiabatic transitions in the reactant valley were shown
theoretically to be important in the formation of ArH+ in the
Ar+ + H2 reaction.28
90°
0°
180°
90°
Z ) (
P(ϑ) sin ϑ dϑ)/(
P(ϑ) sin ϑ dϑ) (5)
∫
∫
one obtains for the short-lived complex Z ) 0.68, and this
corresponds to the average lifetime of the intermediate about
one average rotational period.23
CM angular distribution, P(ϑCM) - ϑCM, of the product
D2CN+ from reaction 2 at T ) 3.66 eV was obtained by the

ˇ
1388 J. Phys. Chem. A, Vol. 114, No. 3, 2010
Zabka et al.
Figure 4. Relative translational energy distribution, P(T′) - T′, of
the products D2CN+ + D2 at T ) 3.66 eV, as derived from the scattering
diagram in Figure 2b (open points and solid line). Dashed line:
calculations for a long-lived statistical complex;29 dotted line: remaining
part of the distribution after subtracting the calculated contribution of
the statistical complex; dash-and-dotted line: sum of dashed and dotted
lines.
The relative translational energy distribution of the products,
P(T′) - T′, as derived from the scattering diagram in Figure
2b, is given in Figure 4 (points and solid line). The distribution
shows a low energy peak and a higher energy shoulder, evidently
connected to the central peak and the two outer maxima in the
scattering diagram in Figure 2b. A deconvolution of the
summary P(T′) curve was carried out by using the Klots model
of a statistical complex decomposition,29 and this led to the
dashed curve in Figure 4 that peaks at 0.5 eV and decreases
gradually to zero at ∼2.5 eV. The remaining part of the
distribution is shown by the dotted line, peaking at ∼1.4 eV.
The deconvoluted curve at lower P(T′) is related to the
translational energy release in the decomposition of a statistical
complex that corresponds to the central peak in the scattering
diagram (Figure 2b) and the deconvoluted CM angular distribu-
tion of the long-lived complex. The deconvoluted curve peaking
at 1.4 eV corresponds to the outer structures in the scattering
diagrams and to the CM angular distribution of the intermediate
of a short mean lifetime (dashed line in Figure 3). Integration
of the areas under the deconvoluted curves of P(T′) in Figure 4
leads to the relative contributions of the long-lived complex
decomposition to the short-lived complex decomposition to the
overall formation of the product D2CN+ of 0.25:0.75. This is
in reasonable agreement with the value of this ratio obtained
from the integration of the deconvoluted angular distributions
of 0.33:0.66.
Therefore, the experimental evidence indicates that the
formation of the product D2CN+ in reaction 2 involves two
different pathways, a decomposition of a long-lived statistical
complex that gives rise to the central structure in the scattering
diagram, the forward-backward symmetric contribution to the
CM angular distribution, and to the low-energy contribution of
P(T′), which is consistent with the distribution expected from a
long-lived statistical complex decomposition. The decomposition
of a short-lived complex (estimated mean lifetime of about one
average rotational period) gives rise to the two outer structures
in the scattering diagram, the asymmetric contribution to the
CM angular distribution, peaking in the backward direction, and
to the higher-energy contribution to the P(T′) curve, peaking at
about 1.4 eV. Calculation of the stationary points on the
hypersurface of the system N+ + CH4 shows (see later) that
Figure 5. Velocity profiles of D2CN+, DCN+, and CD3 at the
+
scattering angle of +4°; collision energies indicated in the figures. CM:
position of the center-of-mass; dotted line with an arrow: position of
the origin of the Newton diagram. (See Figure 2a.)
the decomposition of two different complexes can be identified
with different pathways on the hypersurface, leading to two
different isomers of the product ion D2CN+.
Experiment: Formation of DCN+ and CD3+. For the
remaining channels of reaction (1a-1d), we have much less
scattering information, and the results make only qualitative
conclusions possible. Figure 5 shows the velocity profiles of
D2CN+, DCN+, and CD3+ from the reaction of N+(3P) with CD4,
measured at the laboratory scattering angle of +4° (ap-
proximately along the CM velocity of the reactants) and at about
the same collision energy. The intensity scale in Figure 5 is
relative and does not reflect the actual intensity of the product
ions. The velocity profile of D2CN+ in Figure 5 is one of the
velocity profiles used to construct the scattering diagram of
D2CN+ in Figure 2b.
The analogue of reaction 1d, the reaction of DCN+ formation
N+(3P) + CD4 f DCN+ + D + D2
(6)
is exoergic by 1.40 eV, and DCN+ is one of the product channels
even at thermal collision energies.6 The velocity profile of DCN+
in Figure 5 shows that its velocity peaks at the CM and decreases
toward zero at about (3 km/s. This behavior suggests that
DCN+ is a secondary product formed by the decomposition of
the primary product D2CN+ by the emission of one D atom.
The translational energy release in the dissociation process
smears out structures in the DCN+ profile, but it is evident that
the product D2CN+ appearing close to the CM (connected to
the central structure in the velocity profile of this ion) dissociates
more efficiently than the product D2CN+ appearing in the wings
of the D2CN+ velocity profile (connected to the outer peaks).
This finding is in qualitative agreement with the relative
translational energy distribution of the products (Figure 4): the
deconvoluted curve of P(T′) connected to the low-energy part

Dynamics of Formation in Reaction of N+ with CD4
J. Phys. Chem. A, Vol. 114, No. 3, 2010 1389
Figure 6. Theoretical calculations: Stationary points on the potential-energy hypersurface of the system N+(3P) + CD4. Upper indices with species
in parentheses indicate the spin of the N-containing particle; letters in the lower part of the diagram refer to various pathways, as discussed in the
text.
3
of the distribution and decomposition of the long-lived complex
channels only 0.5 eV (peak value) into translational energy and
leaves more internal energy in the product pair D2CN+ + D2
than the decomposition of the short-lived complex, where the
peak value of the fraction of energy in translation is 1.5 eV.
state) from the reactants (N+) and CH4 releases 4.67 eV of
energy (Figure 6). This internal energy is available for sur-
mounting numerous reaction barriers and forming different
products. The most straightforward reaction pathway corre-
sponds to the decomposition of 3(CH4-N+) either to 3(CH2-N+)
+
+
3
The lower part of Figure 5 shows the velocity profile of CD3
formed in the analogue of reaction 1b
+ H2 (∆Erel,0K ) -4.08 eV, pathway B) or CH3 + (NH)
(∆Erel,0K ) -3.91 eV, pathway D). An alternative pathway to
B is the successive loss of two hydrogen atoms, which is
endothermic (∆Erel,0K ) 0.53 eV) but still conceivable in
experiments carried out at sufficiently large collision energies.
N+(3P) + CD4 f CD+3 + ND
(7)
In addition to these decompositions, the collision complex
3(CH4-N+) can undergo a rearrangement to a pool of more
The velocity profile shows a steep rise to a very strong peak in
the backward direction, presumably located at ∼3 km/s, at a
very low energy on the laboratory energy scale that could not
be reliably measured in our experiments, a pair of almost
symmetric peaks forward and backward with respect to the CM,
with the experimental error ∼1 km/s from the CM, and a weak
peak forward with respect to the CM (at ∼3.4 km/s). The strong
backward peak in Figure 5 evidently corresponds to the peak
at the laboratory velocity of 0.1 km/s described by the authors
3
stable intermediates (CH3-NH)+ (∆Erel,0K ) -7.34 eV) and
3(CH2-NH2+) (∆Erel,0K ) -7.60 eV). Several decomposition
pathways are then opened. First, both of these intermediates
3
can lose a H2 molecule and form products (CHNH+) + H2
(∆Erel,0K ) -4.10 eV, pathway A). However, an even more
probable process seems to be the subsequent loss of two H atoms
shown to be a prevailing mechanism in the dissociation of
internally excited small amines.32 Therefore, most likely, both
intermediates (i.e., 3(CH3-NH+) and 3(CH2-NH2+)) are likely
+
of ref 12 (Figure 5f of this reference) as the product CD3
1
to lose sequentially two hydrogen atoms and form (CHNH+)
formed by hydride-ion transfer, dissociative charge transfer, or
both. This hydride-ion transfer requires the formation of a short-
lived collision species,30,31 and the peak at 3.4 km/s in Figure 5
corresponds presumably to the backward peak related to it. Of
interest are the two symmetric peaks near the CM that imply
(∆Erel,0K ) -4.28 eV, pathway A′). Another fragmentation of
the intermediate 3(CH3NH+) is the C-N bond cleavage to form
+
3
CH3 + (NH) (∆Erel,0K ) -3.91 eV, pathway C).
Finally, we can also address processes leading to the product
+
that a small amount of the CD3 product is formed by an
HCN+. It appears that 2(HCN+) is formed in the pathway B by
3
intermediate complex decomposition. This reaction pathway was
not previously described. The position of the two peaks with
respect to the CM indicates a translational energy release of
about 0.15 to 0.2 eV, a small value consistent with a long-lived
complex formation and decomposition that channels only a small
fraction of the total energy available into translational energy
of the products. The relative importance of the long-lived
complex formation, with respect to the hydride-ion transfer, may
increase with decreasing collision energy.
the decomposition of the product (CH2-N+). In pathway A,
2
2
both (HCN+) and (CNH+) can be formed by decomposition
of the product 2(CHNH+). All of these processes are exothermic
with respect to the reactants. There remains a possibility of
forming HCN+ or CNH+ by a sequential shake-off of three
H-atoms in endothermic processes (by 2.89 or 2.22 eV,
respectively) at higher collision energies.
Comparison of Experimental and Theoretical Results. The
above-mentioned experimental and theoretical results make it
possible to suggest pathways on the potential-energy surface
of the system that correspond to the experimental findings on
the scattering of the D2CN+ product ion. (In the following, we
use H instead of D when referring to the results of calculations
and Figure 6.) The formation of D2CN+ via a long-lived
complex (central part of the scattering diagram) may be
identified with pathway A on the potential-energy surface
Calculations. Theoretical calculations were carried out for
the system N+ + CH4; therefore, we refer to H instead of D in
connection with theory, thus neglecting any possible isotope
effects. The theoretical calculations reveal that the formation
3
of the collision complex (CH4N+) (superscript 3 denotes that
the complex is considered in the triplet state; by analogy, 1
corresponds to the singlet state, and 2 corresponds to the doublet

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1390 J. Phys. Chem. A, Vol. 114, No. 3, 2010
Zabka et al.
(Figure 6) going from the reactants through a shallow minimum
3(CH4-N+) and a transition state at -3.74 eV to a deep
minimum of the triplet complex 3(CH3-NH+) (-7.34 eV) and
directly to the isomer CD2N+ by the release of D2. (2) The
product DCN+ results from secondary decomposition of the
products CDND+ or CD2N+. (3) A small amount of the product
3
+
to the products (CHNH+) + H2 with overall exothermicity of
CD3 is formed, besides the previously described prevailing
-4.10 eV (pathway A). The deep minimum is consistent with
the formation of the long-lived complex. The other possibility,
which is consistent with the formation of a long-lived complex,
formation by hydride-ion transfer, by decomposition of a long-
lived intermediate [CD2ND]+.
1
is pathway A′ leading to singlet (CHNH+) via subsequent
Acknowledgment. It is a great pleasure to dedicate this
article to W. Carl Lineberger, an old friend and respected
colleague, in appreciation of his numerous contributions to
the development of molecular and chemical physics. Partial
support of the experimental research by the Grant Agency
of the Academy of Sciences of the Czech Republic (grant
no. IA400400702) and of the theoretical calculations by the
MinistryofEducationoftheCzechRepublic(MSM0021620857)
is gratefully acknowledged.
shake-off of two H-atoms (pathway A′ via intermediates
3
3(CH3-NH+) or (CH2-NH2+)).
The formation of D2CN+ via the short-lived complex can be
connected to pathway B, leading to the triplet intermediate
CH4-N+ at -4.67 eV and from it to the products CH2N+
(triplet) + H2 at -4.08 eV. The shallow minimum on this
pathway, 0.59 eV below the energy of the products, suggests a
short-lived intermediate.
A simple rough estimation of the mean lifetimes of the two
complexes, τ (in seconds), using the RRK model in the form τ
) 1/k ≈ 10-13(E - E0/E)1-s (where E is the total energy of the
complex, E0 is the barrier for decomposition of the intermediate
toward the reactants, and s is the number of internal degrees of
freedom of the intermediate) and the available energetics from
Figure 6 lead to the ratio of the two mean lifetimes τL/τS )
10-20, which is in general agreement with the experimental
findings (lifetime of the long-lived complex, τL, of more or much
more that six average rotations vs lifetime of the short-lived
complex, τS, of about one average rotation).
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2
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+
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3
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Conclusions
(1) Scattering experiments showed that the formation of
D2CN+ in the reaction of N+(3P) with CD4 involves two types
of intermediate complexes having different mean lifetimes. The
comparison with theoretical calculations of stationary points on
the respective hypersurface showed that the decomposition of
the long-lived statistical complex proceeds via the formation
of intermediates (CD4N+) and more stable isomers (CD3ND+)
and (CD2ND2+) and by the release of D2 or by the subsequent
release of two D atoms leading preferentially to the isomer
CDND+ (∼30% of the total amount of the product D2CN+).
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