Channeling of products in the hot atom reaction H + (CN)2 → HCN/HNC + CN and in the reaction of CN with CH3SH

Article abstract of DOI:10.1021/jp0108061

Infrared transient absorption spectroscopy was used to determine the total product branching fractions for the gas-phase hot atom reaction H + (CN)₂ → HCN/HNC + CN (a) and the reaction CN + CH₃SH → HCN/HNC + CH₃S/CH₂SH (b) at 293 K. The reactive H atoms had an initial mean translational energy of 92 kJ mol^-1, with a 38 kJ mol^-1 fwhm Gaussian energy distribution. The branching fractions determined for the product channels forming HCN and HNC, respectively, are 0.88 and 0.12 (±0.05) for reaction (a) and 0.81 and 0.19 (±0.08) for reaction (b). The bimolecular rate constant for reaction (b) was measured to be (2.7 ± 0.3) × 10^-10 cm³ molec^-1 s^-1 at 293 K. The observed product branching fractions for reaction (a) are consistent with the assumption that the average reactive cross sections for the two product channels are approximately equal above their respective energy thresholds. The results for reaction (a) are compared with the related H + XCN (X = Br, Cl) reactions. The large rate coefficient for reaction (b) suggests an interaction via a long-range intermolecular potential, which is facilitated by the small ionization energy of CH₃SH and large electron affinity of CN. The results for reaction (b) are compared with the related reactions of Cl and OH with CH₃SH.

Full text of DOI:10.1021/jp0108061

J. Phys. Chem. A 2001, 105, 6817-6825
6817
Channeling of Products in the Hot Atom Reaction H + (CN)2 f HCN/HNC + CN and in
the Reaction of CN with CH3SH
Brian K. Decker† and R. Glen Macdonald*
Argonne National Laboratory, Chemistry DiVision, 9700 South Cass AVenue, Argonne, Illinois 60439
ReceiVed: March 1, 2001; In Final Form: May 15, 2001
Infrared transient absorption spectroscopy was used to determine the total product branching fractions for the
gas-phase hot atom reaction H + (CN)2 f HCN/HNC + CN (a) and the reaction CN + CH3SH f HCN/
HNC + CH3S/CH2SH (b) at 293 K. The reactive H atoms had an initial mean translational energy of 92 kJ
mol-1, with a 38 kJ mol-1 fwhm Gaussian energy distribution. The branching fractions determined for the
product channels forming HCN and HNC, respectively, are 0.88 and 0.12 ((0.05) for reaction (a) and 0.81
and 0.19 ((0.08) for reaction (b). The bimolecular rate constant for reaction (b) was measured to be (2.7 (
0.3) × 10-10 cm3 molec-1 s-1 at 293 K. The observed product branching fractions for reaction (a) are consistent
with the assumption that the average reactive cross sections for the two product channels are approximately
equal above their respective energy thresholds. The results for reaction (a) are compared with the related H
+ XCN (X ) Br, Cl) reactions. The large rate coefficient for reaction (b) suggests an interaction via a long-
range intermolecular potential, which is facilitated by the small ionization energy of CH3SH and large electron
affinity of CN. The results for reaction (b) are compared with the related reactions of Cl and OH with CH3-
SH.
1. Introduction
The molecular weight of CN also happens to lie between the F
and Cl atomic weights. Consequently, in many respects the
polyatomic molecule cyanogen, N≡C-C≡N, resembles the
homonuclear diatomic halogens, X2 (X ) F, Cl, Br), whose
reactions with hydrogen atoms have been well studied.1 Thus,
the hot-atom reaction H + (CN)2 f HCN/HNC + CN is
perhaps the simplest five-atom system to compare with the
classical L + H-H triatomic systems, where L represents a
light-mass particle (usually hydrogen), and H a heavy-mass
particle (e.g., F, Cl, Br, I). The dynamics of this reaction has
been investigated in this laboratory, by measuring both the
disposal of energy into rotational and translational degrees of
Experimental and theoretical work on chemical reaction
dynamics traditionally has focused on binary atom-diatom
reactions in the gas phase,1 because these are the simplest and
most tractable systems involving the rupture and formation of
chemical bonds.2 The “classical” detailed atom-diatom reaction
may be written as A + BC f AB(V′, R′, T′) + C, where the
atoms A, B, and C usually are members of the hydrogen/halogen
group (H, F, Cl, Br, I), and V′, R′, and T′ represent the disposal
of reaction energy respectively into vibrational, rotational, and
translational degrees of freedom in the diatomic product.1 One
alternative macroscopic product channel is possible, viz., AC
+ B, if BC is heteronuclear. Binary reactions involving four
atoms can occur between either atom-triatom (A + BCD) or
diatom-diatom (AB + CD) pairs, with many more available
product channels (including different isomeric forms of the
triatomic species) and quantum states (with their corresponding
modes of molecular motion) than the three-atom case.2,3 Due
to the development of new experimental techniques in the past
10-15 years, the state-specific preparation and sensitive detec-
tion of triatomic and larger molecules has become possible.2
Likewise, due to advances in theoretical methods based on
quantum mechanics, accurate calculations of state-specific
dynamics now can be contemplated for reacting systems having
more than three atoms.4 Within the past decade, armed with
these new capabilities, workers have begun to extend the study
of detailed chemical reaction dynamics to include systems with
polyatomic reactants and products.2
2
freedom in the CN(X Σ) (V′ ) 0, R′, T′) product,7 and the
1
1
complete energy disposal for the minor HNC(0 ν2 0), ν2
)
00, 11, product.8 Global features of the dynamics, as dictated
by L + H-H kinematics, were found to be similar to the
analogous triatomic system: H + Cl2 f HCl + Cl,7-9 with
more than half of the available reaction energy, Eavail , appearing
as translational motion in the product.7,8 A major difference was
observed in the distribution of Eavail between rotational and
vibrational modes of excitation in the minor HNC product, for
which fR ) 0.32 and fV ) 0.13,8 an almost perfect reversal
of the situation for H + Cl2.8,9 This result can be explained in
terms of the potential energy surfaces (PESs) for the related H
+ NCCl10,11 and H + NCH12 systems. Collinear H-NC-X
reaction trajectories are highly repulsive on these PESs, and
thus HNC is more efficiently formed through bent configurations
which impart torque, and hence angular momentum, to the
nascent product. In contrast, atom-halogen reactions, H + X2
(X ) F, Cl, Br, I), have collinear or near-collinear transition
states,13 and the released reaction energy imparts little angular
momentum. The few data available for the HNCCN PES14 do
not consider the H-NC-CN interaction, but collinear trajec-
tories should be highly repulsive, as they are for the H-NC-X
(X ) H, Cl) interactions.8,10-12
The cyano radical, CN, is often called a pseudohalogen
because of its large electronegativity (between those of F and
Cl)5 and the great strength (∼179 kcal mol-1)6 of its triple bond.
* To whom all correspondence should be addressed. E-mail: macdonald@
anlchm.chm.anl.gov.
† Present address: Physical Sciences, Inc., 20 New England Business
Center, Andover, MA 01810.
10.1021/jp0108061 This article not subject to U.S. Copyright. Published 2001 by the American Chemical Society
Published on Web 06/19/2001

6818 J. Phys. Chem. A, Vol. 105, No. 28, 2001
Decker and Macdonald
The occurrence of different isomeric product channels
represents a fundamental feature of additional complexity in
reacting systems with four or more atoms. These product
channels represent different distributions of reaction trajectories,
and hence, their disposal of reaction energy provides comple-
mentary information about the dynamics and the PES of the
system. The title reaction is an extremely simple example of
this situation, for which only two isomers are possible. In the
present work, the branching fractions into the HCN- and HNC-
forming product channels of this reaction were determined under
bulk conditions at 293 K. A secondary reaction between CN, a
product of the title reaction, and CH3SH, the H-atom precursor,
contributed to the observed HCN and HNC. However, the
simplicity of the chemical mechanism employed for these
experiments permitted these contributions to be differentiated,
and the HCN/HNC branching fractions for the CN + CH3SH
reaction were determined as well. The bimolecular rate coef-
ficient for this reaction, which was required in the kinetic
analysis to extract the branching fractions, was measured at 293
K. The branching fractions of the title reaction, together with
our previous studies on the energy disposal in the CN and HNC
products,7,8 permitted an estimate to be made of the energy
disposal in the HCN product, and hence, in the entire system.
Figure 1. Typical time-resolved absorbance profile for HCN arising
from reactions 2a and 3a and monitored with the H12C14N (000) P(10)
absorption line. The circles represent experimental data. For clarity,
only every third data point for 0 < t < 0.2 ms, and every 30th data
point elsewhere, are shown in the plot. The solid curve is a nonlinear
least-squares fit of all data for t > 0.2 ms to the simple exponential
function, Aexp(-kt), with resulting fit parameters of A ) 296 and k )
308 s-1. The dominant loss process for HCN is diffusion from the
photolysis/probe zone. The fit extrapolation to t ) 0 was used to
estimate the nascent concentration of HCN from reactions 2a and 3a
(see text). The experimental conditions were PAr ) 2.45, P(CN)2 ) 0.061,
and PCH3SH ) 0.023 Torr at 293 K.
2. Experimental Section
Detailed descriptions of the experimental apparatus have been
given previously,15,16 and only a brief description follows. The
transverse flow reactor (TFR) consisted of a stainless steel
vacuum-tight vessel containing a Teflon box with interior
dimensions of 100 × 100 × 5 cm. The TFR was evacuated
through a liquid nitrogen trap by a 25 cfm mechanical pump to
a base pressure of a few mTorr, with a leak rate of <0.5 mTorr
min-1. The pump inlet was throttled to achieve various
experimental pressures in the range of 0.75-6.6 Torr. Gaseous
reagents were introduced from a separate vacuum system, their
partial pressures being determined from their known flow rates
and the total pressure in the TFR. The reagents were used
directly as supplied by the manufacturers, with the following
purities: Ar (AGA Gas, 99.9995%); CH3SH (Matheson, 99.5%);
(CN)2 (Matheson, 98.5%); C3H8 (AGA Gas, 99.5%); He (AGA
Gas, 99.9995%). The (CN)2 was further purified by pumping
on the frozen sample at -78 °C.
(infrared) laser pumped by a Coherent Innova model 400 Kr+
laser. The probe laser outputs were aligned and multipassed
collinearly through the photolysis region using a White cell
adjusted for 8 passes, for a total optical path length of 1136 (
5 cm. The diode laser was used to monitor the CN radical on
individual rovibronic transitions in the (2,0) vibronic bandhead
region of the red system (A2ΠrX2Σ) near 790 nm. Similarly,
individual rovibrational transitions in the HCN (X1Σ) ν3
(001)r(000) fundamental near 3.0 µm, and the HNC (X1Σ) ν1
(100)r(000) fundamental near 2.7 µm, were monitored with
the infrared laser. The absorption transients of CN and one of
the triatomic isomers were detected simultaneously after each
photolysis pulse. The transmitted intensity of the diode laser
probe beam was monitored with a fast Si PIN diode (New
Focus). Prior to introduction into the White cell, the infrared
probe beam was split into a reference beam and a signal beam,
whose intensities were monitored with separate liquid nitrogen-
cooled InSb infrared detectors (Cincinnati Electronics). The
reference detector output was used in the feedback circuit of
an optical modulator (ConOptics) that regulated the intensity
of the Kr+ pump laser; in this manner, the principal source of
noise in the infrared probe beam was suppressed. Further
suppression of low-frequency noise in both the diode and the
infrared probes was achieved by passing the transient absorption
signals through a high-pass electronic filter.15 The frequency
responses of the Si PIN diode and the InSb detectors with their
associated electronics were better than 20 and 7 MHz, respec-
tively.
A Lambda-Physik Compex 205 excimer laser operating at
248 nm was used to photolyze CH3SH at the S-H bond,17
thereby creating translationally hot H-atoms having a mean
center-of-mass (COM) kinetic energy of EH ) 92 kJ mol-1
,
with a Gaussian energy distribution of 38 kJ mol-1 full width
half-maximum (fwhm).7 The excimer laser was operated at a
nominal power density of 35-50 mJ cm-2 pulse-1, with a
repetition rate of 10 Hz; lower repetition rates of 2-5 Hz were
employed at the higher experimental pressures (>2 Torr) to
compensate for the slower removal of photolysis and reaction
products. Part of the transmitted excimer beam exiting the TFR
was monitored with a Molectron J-50 power meter; the
transmitted power was used to correct the data for pulse-to-
pulse fluctuations in photolysis power, as well as for its gradual
decline due to the build-up of UV-absorbing deposits on the
optical surfaces.7 These optics, especially the quartz entrance
window and the dichroic UV-IR mirror used to overlap the
photolysis and probe laser beams, were removed for cleaning
whenever the transmitted power had decreased by ∼25%.
The continuous-wave probe lasers were an Environmental
Optical Sensor model 2010-ECU tunable external cavity diode
laser, and a Burleigh Model 20 single-mode color center
Time-resolved and frequency-resolved transient absorption
profiles for CN, HCN, and HNC, under varied experimental
conditions, were collected at a temperature of 20.0 ( 0.5 °C
(293.2 ( 0.5 K). Time-resolved absorbance profiles (see Figures
1 and 2) were obtained by tuning the probe laser to the center
frequency of an absorption feature, ν0, and signal averaging
102-103 absorption transients with a LeCroy 9410 digital
oscilloscope triggered from a fast Si PIN photodiode monitoring
the photolysis laser pulses. The baseline probe laser intensity,
I0(ν0), was recorded with a Stanford SR-250 boxcar module

Product Channeling in the Hot Atom Reaction
J. Phys. Chem. A, Vol. 105, No. 28, 2001 6819
Figure 2. Typical time-resolved absorbance profile for CN arising
from reaction 2 and monitored with the 12C14N (A2ΠrX2Σ) (2,0) R1-
(11.5) absorption line. The circles represent experimental data. For
clarity, only every second data point for 0 < t < 0.5 µs, and every
20th data point elsewhere, are shown in the plot. The solid curve is a
nonlinear least-squares fit of all data for t > 1.4 µs to the simple
exponential function, Aexp(-kt), with resulting fit parameters of A )
832 and k ) 4.29 × 105 s-1. The dominant loss process for CN is
reaction 3, with a characteristic time of 2.33 µs. The first-order rate
constant, k, from this and other experiments were used to determine
the bimolecular rate constant, k3, for reaction 3 (see text and Figure 5).
The experimental conditions were PAr ) 2.40, P(CN)2 ) 0.084, and
PCH3SH ) 0.024 Torr at 293 K.
Figure 3. Typical frequency-resolved absorbance profiles for (a) HCN
arising from reactions 3a and 4a and monitored with the H12C14N (000)
P(10) absorption line, and (b) HNC arising from reactions 3b and 4b
and monitored with the H14N12C (000) R(9) absorption line. The filled
circles represent experimental data, while the solid curves are nonlinear
least-squares fits to the data assuming a Gaussian line shape, with the
residues indicated by the “X” symbols. The response of the 150-MHz
Etalon over the scan frequency range is shown above the respective
data plots in (a) and (b); the Etalon fringes from these traces were
used to calibrate the probe laser frequency. The experimental conditions
were PAr ) 0.359, PC3H8 ) 0.958, P(CN)2 ) 0.060, and PCH3SH ) 0.023
Torr at 293 K, with a boxcar width of 15 µs centered on delays of (a)
300 µs and (b) 100 µs. At these delay times, the translational energies
of the HCN and HNC products were equilibrated to the experimental
temperature of 293 K.
triggered a few hundred microseconds before the photolysis
laser. Triggering, collection of data, and other electronic control
functions were performed by a laboratory computer. Because
the bandwidths of both probe lasers were much narrower than
the 293 K Doppler-broadened line profiles of the absorption
features, frequency-resolved absorbance profiles providing more
accurate absorption measurements than the time-resolved pro-
files could be obtained18 (see Figures 3 and 4). Four Stanford
SR-250 boxcar modules were used simultaneously to monitor
I0(ν) and I(ν) for CN, and I0(ν′) and I(ν′) for one of the triatomic
isomers, where ν and ν′ are the frequencies of the diode and
infrared probe lasers, respectively. With appropriate time delays
determined from the time-resolved profiles, scans were made
by stepping the probe laser frequencies through the absorption
features in 10-20 MHz increments and accumulating the
absorption signal for a fixed number (typically 20-60) of laser
shots at each step.
and hence their optical densities, to be measured accurately,
which was required for the accurate determination of branching
fractions.
3. Results
The spectroscopic data needed to calculate line positions for
the CN red system19 and the HCN20 and HNC21,22 fundamental
transitions were obtained from the available literature. Because
HCN and HNC are both moderately light, linear hydrides, their
3.1. Reaction Mechanism. Only a few reactions were of
importance in the present system, which is described by the
following mechanism:
rovibrational transitions are widely separated (several cm-1
)
relative to their 293 K Doppler widths (∼0.008 cm-1),8 and
thus only one absorption feature was recorded per scan. Near-
infrared probe laser frequencies were measured to within (150
MHz accuracy by an evacuated Burleigh WA-20IR wavemeter.
Simultaneously with each frequency-resolved infrared absorption
scan, the output of a Burleigh CF-500 Etalon, with a free spectral
range (FSR) of 150 MHz, was recorded as a function of probe
laser frequency. The Etalon FSR was calibrated against the
known high-resolution absorption spectrum of acetylene near
3.0 µm,23 and the Etalon output in turn was used to calibrate
the frequency scale for each profile, thereby accounting for
variations in the relationship between the scan voltage and the
probe laser frequency as the laser was tuned to different
absorption features.8 This frequency-scale calibration allowed
the Doppler-broadened line profiles of the absorbing species,
CH3SH 248 nm8 CH3S + H*
(1)
H* + (CN)2 f HCN + CN f2a
f HNC + CN f2b
(2a)
(2b)
CN + CH3SH f HCN + CH3S/CH2SH f3a (3a)
f HNC + CH3S/CH2SH f3b (3b)
H* + M f H + M
(4)
where the fx (x ) 2a,b; 3a,b) are the branching fractions for the
indicated product channels. In several experiments, C3H8 was
added to the gas flow for the purpose of differentiating the

6820 J. Phys. Chem. A, Vol. 105, No. 28, 2001
Decker and Macdonald
TABLE 1: Summary of the Reaction Energetics for the H +
(CN)2 and CN + CH3SH Systems
reaction system
reactants products
a
∆H°r0
Eavail
(kJ mol-1
)
(kJ mol-1
)
Hb + (CN)2
HCNc + CNd
41 ( 8
101 ( 10
-157 ( 7
-129 ( 11
-95 ( 10
-69 ( 13
+55
-6
b
HNCe + CN
CN + CH3SHf
HCN + CH3Sf
HCN + CH2SHf
HNC + CH3S
HNC + CH2SH
a Ref 8. b ∆H°f0(H) ) 216.0 kJ mol-1 (uncertainty much smaller than
the precision shown); ∆H°f0((CN)2) ) 307.2 ( 4 kJ mol-1. Ref. NIST-
JANAF Thermochemical Tables; Chase Jr., M. W., Ed.; AIP: New
York, 1998. c ∆H°f0(HCN) ) 132.3 ( 4.2 kJ mol-1. Ref. Berkowitz,
J.; Ellison, G. B.; Gutman, D. J. Phys. Chem. 1994, 98, 2744.
d ∆H°f0(CN) ) 432 ( 5 kJ mol-1. Ref. Smith, I. W. M. In The Chemical
Dynamics and Kinetics of Small Radicals. Part 1.; Liu, K. P., Wagner,
A., Eds.; World Scientific: Singapore, 1995, p 214. e ∆H°f0(HNC) )
192.6 ( 8.2 kJ mol-1. Ref. Lee, T. J.; Rendell, A. P. Chem. Phys.
Figure 4. Typical frequency-resolved absorbance profiles for the
R1(11.5) and R1(7.5) pair of CN(0,J) transitions in the bandhead region
of the (2,0) band of the A2ΠrX2Σ “red” system of CN near 790 nm.
The origin of the wavenumber scale is at the center frequency of
R1(11.5). The filled circles represent experimental data, whereas the
solid curves are nonlinear least-squares fits to the data assuming a
Gaussian line shape, with the residues indicated by the “X” symbols.
The experimental conditions were PAr ) 2.40, P(CN)2 ) 0.084, and
PCH3SH ) 0.024 Torr at 293 K, with a boxcar width of 0.2 µs centered
on a delay of 0.8 µs. From these data, the translational temperatures of
the CN X2Σ (0,11.5) and (0,7.5) states, as measured from the fwhm of
the Gaussian population line shape, are 409 and 294 K, respectively.
Assuming a Boltzmann distribution of rotational states, the ratio of
peak absorbances, A0(R1(11.5))/A0(R1(7.5)), indicates a rotational tem-
perature of 380 K.
Lett. 1991, 177, 491. f ∆H°f0(CH3SH) ) -3.0 ( 0.1 kJ mol-1
;
∆H°f0(CH3S) ) 31.44 ( 0.54 kJ mol-1; ∆H°f0(CH2SH) ) 37.7 ( 2.0
kJ mol-1. Ref. Ruscic, B.; Berkowitz, J. J. Chem. Phys. 1993, 98, 2568.
1. Both channels of reaction 2 are substantially endothermic
with thermal H-atoms at 293 K, and thus only hot atoms with
sufficient COM translational kinetic energy, EH, are reactive.
The most probable available reaction energy, Eavail , shown in
Table 1 for channels 2a and 2b, was calculated from EH ) 92
kJ mol-1, ∆H°r0, and the thermal energy available in the rotation
and the low-frequency bending vibrations of (CN)2.8 Assuming
that channel (2b) has no additional potential energy barrier,
H-atoms with EH g 98 kJ mol-1 can contribute to this channel.
Yang et al.,14 using the ab initio BAC-MP4 method, have
calculated an activation barrier of ∼14 kJ mol-1 for channel 2a
relative to the products HCN + CN. Thus, H-atoms with EH g
51 kJ mol-1 can react.
products of reactions 2 and 3 (see section 3.4). All species were
lost by diffusion, which occurred on a time scale of tens of
milliseconds at the bath gas pressures employed,16 whereas
reactions (1-4) effectively were complete several microseconds
after the photolysis pulse. Relaxation of the HCN and HNC
rovibrational states to a Boltzmann distribution at 293 K
occurred within 100-300 µs, depending upon the conditions
of a particular experiment. Figure 1 illustrates a typical decay
profile for HCN, monitored in absorption on the P(10) line of
the ν3 fundamental transition. Diffusion is the dominant loss
process for HCN and for HNC.24 The analogous decay profile
for CN, obtained under similar experimental conditions and
monitored on the R1(11.5) line of the (2,0) vibronic band of
the red system, is shown in Figure 2.
The hot H-atoms created in reaction 1 were relaxed and
eventually thermalized by the bath gas (M) in reaction 4. For
comparison with future theoretical studies of the dynamics of
reaction 2, it is important that the observed product channels
arise from H atoms with their initial translational energy
distribution. Several observations indicate that this was indeed
the case. First, Park et al.28 have measured the thermalization
cross section for H atoms with ET ) 13 kJ mol-1 colliding with
Ar to be (9 ( 2) × 10-16 cm2. This is 30 times larger than the
estimate of the total reaction cross section, σR2, for reaction 2
by He et al.7 Furthermore, σR2 is expected to increase with
increasing collision energy as found in similar experimental and
theoretical studies on hot H-atom systems.29-31 Thus, reactive
collision events are strongly weighted toward the more energetic
H-atoms. Second, within the experimental scatter, there was no
difference in the measured branching fractions for a variety of
different conditions (see Tables 2 and 3). The concentration of
bath gas was always greater than the concentration of (CN)2,
>26:1 when only Ar or He was used and >9:1 when C3H8 was
used. Note that, due to its large manifold of rovibrational states,
the thermalization cross section should be much greater for C3H8
than for the noble gases.28 Thus, the collision frequency for
reaction 2 relative to that for thermalization was on the order
of 10-3 to 10-2, and thermalization effectively limited reaction
2 to those H atoms with the initial translational energy
distribution. A consequence of the dominance of thermalizing
collisions over reactive collisions is that the product concentra-
tion should be much smaller than the initial concentration of
H-atoms, [H]0, created in the photolysis pulse. Indeed, this was
verified experimentally by comparing the measured concentra-
The photolysis cross-section for CH3SH at 248 nm is 3.0 ×
10-19 cm2 molec-1 17
, implying <1.5% dissociation of CH3SH
under our experimental conditions. Thus, any reaction between
CN and CH3S/CH2SH can be neglected relative to reaction 3,
which is gas-kinetic (see section 3.3). The reaction between CN
and (CN)2 is slower by a factor of ∼105 than reaction 3 at 293
K.14 Although other product channels, e.g., the methyl abstrac-
tion forming CH3CN + HS, are exothermic for reaction 3, such
channels are not observed in the reactions of CH3SH with Cl25,26
and with OH,27 which proceed by rapid H-atom transfer (near
the gas-kinetic limit for Cl25). These alternative channels are
thermodynamically and sterically disfavored relative to simple
H-atom transfer.25 Furthermore, when C3H8, which reacts rapidly
with CN forming HCN exclusively, was introduced into the
reactant mixture in excess of CH3SH (see section 3.4), no
significant change was observed in the total yield of HCN +
HNC. Therefore the H-atom transfer product channels, (3a) and
(3b), represent the entire product of reaction 3, which is the
only significant process removing CN.
The Hess’s law reaction enthalpies at 0 K, ∆H°r0, for the
active product channels of reactions 2 and 3 are given in Table

Product Channeling in the Hot Atom Reaction
J. Phys. Chem. A, Vol. 105, No. 28, 2001 6821
TABLE 2: Summary of the Branching Fraction Measurements for the H + (CN)2 and CN + CH3SH Reactions (No Propane)
bath gas (M)
PM (Torr)
PC2N2 (Torr)
PCH3SH (Torr)
fa (HCN)
fb (HNC)
Ar
Ar
Ar
He
Ar
Ar
Ar
Ar
5.921
2.446
2.400
1.770
1.587
1.559
1.459
0.727
0.071
0.061
0.084
0.031
0.059
0.059
0.026
0.012
0.052
0.023
0.027
0.066
0.021
0.039
0.013
0.0064
0.645
0.678
0.619
0.663
0.766
0.636
0.732
0.774
0.355
0.322
0.381
0.337
0.234
0.364
0.268
0.226
average:
standard deviation:
0.689
0.060
0.311
0.060
TABLE 3: Summary of the Branching Fraction Measurements for the H + (CN)2 and CN + CH3SH Reactions (Propane
Added) and the Final Measured Branching Ratios for Reactions 2 and 3
PAr
PC3H8
PC2N2
PCH3SH
f ′
f ′
f2a
f2b
f3a
f3b
a
b
(Torr)
(Torr)
(Torr)
(Torr)
(HCN)
(HNC)
(HCN)
(HNC)
(HCN)
(HNC)
0
0
6.380
2.900
1.003
1.397
2.272
1.336
0.958
0.537
0.989
1.079
0.111
0.052
0.092
0.080
0.041
0.022
0.060
0.048
0.108
0.020
0.088
0.040
0.037
0.024
0.081
0.045
0.023
0.019
0.060
0.038
0.802
0.841
0.892
0.834
0.821
0.875
0.889
0.921
0.886
0.866
0.863
0.037
0.198
0.159
0.108
0.166
0.179
0.125
0.111
0.079
0.114
0.134
0.137
0.037
0.807
0.848
0.917
0.842
0.837
0.896
0.905
0.949
0.926
0.887
0.881
0.046
0.193
0.152
0.083
0.158
0.163
0.104
0.095
0.051
0.074
0.113
0.119
0.046
0.882
0.841
0.772
0.847
0.852
0.793
0.784
0.740
0.763
0.802
0.808
0.046
0.118
0.159
0.228
0.153
0.148
0.207
0.216
0.260
0.237
0.198
0.192
0.046
1.367
0.984
0
0
0.359
0.721
0
0
average:
standard deviation:
tion of the H-atoms that reacted, given by 1/2([HCN]0
+
to the optical density of the absorbing medium, xi, through the
absorption cross section:
[HNC]0] (see section 3.4), to an estimate of the initial
concentration of H-atoms, [H]0, created in the photolysis pulse.
The [H]0 was always at least 2 orders of magnitude larger.
3.2. Determination of the Relative Concentrations of the
Transient Species. Determinations of the transition moments,
j|µ|i , for the CN A2ΠrX2Σ (2,0) vibronic band32,33 and the
HCN X1Σ (001)r(000)34-36 and HNC X1Σ (100)r(000)37
fundamental vibrations have been reported in the literature. The
values of j|µ|i selected from the available data for the present
measurements have been tabulated in a previous publication.18
The integrated line strength, Sji, for these jri transitions can
be calculated from the expression38
I0(ν)
A(ν) ) Ln
) xiσ(ν)
(6)
(
)
I(ν)
In eq 6, I0(ν) is the probe laser intensity and I(ν) is the
transmitted intensity. Because Sji is independent of ν and g(ν)
is normalized, the optical densities of CN, HCN, and HNC in
the reaction vessel could be determined by integrating eq 6 and
rearranging to obtain
+∞
A(ν) dν
∫
-∞
xi )
(7)
8π3
3hc
Aji
Sji
2
Sji )
νjji| j|µ|i |
F
(5)
2J + 1
The integral in eq 7 was evaluated by fitting the experimentally
measured frequency-resolved absorbance profiles (see Figures
3 and 4) to a Gaussian line shape function using the commercial
software package PEAK FIT. Each fit yielded peak amplitude
and fwhm parameters that were used to calculate the area under
the absorbance profile.
where h and c have their usual meanings, νjji is the transition
wavenumber, J is the total angular momentum quantum number
of the lower (i) state, Aji is the appropriate Ho¨nl-London factor,
and F is the appropriate Herman-Wallis factor for the
vibrational transitions. For the linear triatomics HCN and HNC,
Aji is given by J + 1 for an R branch and J for a P branch
transition.39 The F factors were calculated using Herman-Wallis
coefficients measured for the HCN ν3 (1r0) fundamental
transition by Smith et al.,34 and for the HNC ν1 (1r0)
fundamental transition in this laboratory.22 For CN, whose
A2ΠrX2Σ transition is intermediate between Hund’s coupling
cases (a) and (b), the Aji were calculated from Earls’ formula40
and normalized by the method of Whiting et al.41,33
The absorption cross section at a frequency ν, σ(ν), is related
to the integrated line strength by σ(ν) ) g(ν)S, where g(ν) is
the normalized-line shape function.38 At 293 K and the pressures
of these experiments (see Tables 2 and 3), g(ν) has the well-
known Gaussian form of a Doppler-broadened absorption profile
for the species considered here. The Beer-Lambert absorption
law42 relates the experimentally determined absorbance, A(ν),
Because xi ) l[Ni] for a uniformly absorbing medium, where
l is the total optical path length and [Ni] the number density of
absorbers at ν, the absolute number densities of the absorbing
species could be determined in principle. However, because l
was identical for all of the absorbing species, the optical
densities sufficed for determining relative branching ratios, and
thus the additional uncertainty associated with measuring l was
avoided. Initial concentrations or optical densities, xi0, were
obtained by nonlinear least-squares fitting of the decay portions
of the time-resolved absorbance profiles to single-exponential
decay functions (see Figures 1 and 2). The fits provided an
extrapolation of the peak absorbance to the time origin (t ) 0)
for each species monitored under each set of experimental
conditions. The xi obtained from the frequency-resolved absor-
bance profiles recorded at various boxcar gate delays were then

6822 J. Phys. Chem. A, Vol. 105, No. 28, 2001
Decker and Macdonald
the CN already was substantially relaxed from its nascent energy
distribution. For the determination of k3, contributions from the
nonthermal population of CN were omitted by fitting only the
tail portion of the exponential decay in the time-resolved
absorbance profile, starting where the absorbance had decreased
to ∼80% of its peak value (see Figure 2 caption). Very little
difference was observed between these fits and those including
the entire decay; thus, the decay fits were overwhelmingly
weighted toward the thermalized CN.
Other potential sources of systematic error are the determi-
nation of CH3SH concentrations and variations in reactive
background impurities (e.g., reagent impurities and photofrag-
ments) under different experimental conditions. The flow meters
were calibrated by measuring the rate of pressure rise in the
known volume of the TFR chamber (see Experimental Section)
as a function of the scale flow. Accounting for uncertainties
associated with measuring the chamber volume and the pressure
change in a given time interval, the accuracy of these flow
calibrations is estimated to be within (3%. The variation in
reactive CH3SH-independent impurity levels from one experi-
mental run to another was responsible for the series of data
points having similar slopes (k3), but different y-intercepts (k0),
in Figure 5. While this affects the apparent scatter of the data,
clearly it should not influence the value for k3 derived from the
linear regression. Assuming a systematic uncertainty in the
exponential decay constants of (2% to account for the loss of
CH3SH by photolysis (which is <1.5%; see section 3.1) and
any possible CH3SH-dependent or pressure-dependent back-
ground reaction not considered here, the total systematic
uncertainty in k3 is estimated to be (5% in addition to the
precision of the linear regression. Thus, our reported value is
k3 ) (2.7 ( 0.3) × 10-10 cm3 molec-1 s-1 at 293 K.
Figure 5. First-order rates, k, for the removal of CN plotted as a
function of PCH3SH at 293 K. The linear least-squares fit to the data
gives k3 ) (2.7 ( 0.2) × 10-10 cm3 molecule-1 s-1, whereas the given
uncertainty reflects only the precision ((2σ) in the slope of the least-
squares fit line (r2 ) 0.991). The total uncertainty in k3 is estimated to
be ( 0.3 × 10-10 cm3 molecule-1 s-1 (see text).
extrapolated using the exponential fits to obtain xi0. For HCN
and HNC, which were removed only by diffusion, this correction
was small, ∼10% or less (see Figure 1). For CN, rotational and
translational relaxation occurred on the time scale of reaction
3, and the extrapolation to t ) 0 involved much greater
uncertainty (see Figure 2). Therefore, the branching fractions
for reactions 2 and 3 were determined without reference to the
initial concentration of CN (see section 3.4).
3.3. Measurement of k3. The binary rate coefficient for
reaction 3, k3, was determined at 293 K by measuring the first-
order decay of CN as a function of [CH3SH] in the reaction
vessel. Because the peak CN concentration was strongly
constrained by reactions 1 and 4 (see Section 3.1), reaction 3
was pseudo-first-order in [CN] for all values of [CH3SH]. The
first-order rate constant was obtained at various flows of CH3-
SH from a nonlinear least-squares fit of the decay portion of
the CN R1(11.5) temporal absorbance profile (cf. Figure 2) to
a two-parameter exponential decay function. The resulting first-
order rate constants, k, were plotted against [CH3SH], as shown
in Figure 5. The data were fit by linear regression to the equation
k ) k0 + k3[CH3SH], where k0 represents the [CH3SH]-
independent loss processes for CN (predominantly reactions with
background impurities). From the least-squares fit, k0 ) (1.36
The large value for k3 indicates that reaction 3 is gas-kinetic.
Large rate coefficients frequently have been observed in
reactions with CN, particularly with unsaturated hydrocarbons.43
In some cases, these reactions are faster than the hard-sphere
collision rate,44 especially at lower temperatures (i.e., down to
tens of kelvins45). Such behavior has been interpreted in terms
of “complex formation,” in which the particles interact through
a long-range intermolecular potential.44,45 The hard-sphere
collision rate coefficient for reaction 3, k3(hs), can be calculated
from estimates of the hard-sphere diameters of CN and
CH3SH.46 These were obtained from the van der Waals radii of
the atoms composing these molecules,47 together with the bond
lengths in CN48 and CH3SH,49 yielding an upper-limit value of
k3(hs) ) 2.5 × 10-10 cm3 molec-1 s-1 at 293 K. Thus, k3 is
marginally larger than k3(hs) at room temperature. The analo-
gous reaction of Cl with CH3SH is also rapid, with an
experimentally determined rate coefficient of (2.0 ( 0.3) ×
10-10 cm3 molec-1 s-1 at 298 ( 2 K,25 as compared to its upper-
( 0.52) × 105 s-1 and k3 ) (9.06 ( 0.29) × 106 Torr-1 s-1
,
where the given uncertainties represent the precisions ((1σ)
of the fit parameters. In molecular-cgs units, k3 ) (2.7 ( 0.2)
× 10-10 cm3 molec-1 s-1 at 293 K, where the error represents
the precision ((2σ) of the least-squares fit.
A possible source of systematic error in k3 is the nonthermal
energy distribution for CN at early times in the temporal profiles.
The nascent translational and rotational temperatures for CN
from reaction 2 have been determined to be Ttrans ) (1389 (
109) K and Trot ) (885 ( 45) K.7 Little or no vibrational
excitation is expected,7 and hence under our experimental
conditions, thermalization is very rapid. For example, the
frequency-resolved absorbance profiles of the neighboring CN
R1(11.5) and R1(7.5) lines shown in Figure 4 were collected at
a boxcar gate delay of 0.8 µs. The translational temperature of
the population in the J ) 11.5 state, as determined from the
fwhm of the Doppler-broadened R1(11.5) absorbance profile,
was 409 K, whereas that of the J ) 7.5 state population was
294 K. The R1(11.5)/R1(7.5) peak absorbance ratio suggests a
Boltzmann rotational temperature of 380 K. Thus, at 0.8 µs,
limit hard-sphere value of 1.9 × 10-10 cm3 molec-1 s-1
.
3.4. Determination of the Branching Fractions. Because
the HCN and HNC relaxed to their Boltzmann rovibrational
distributions on a much shorter time scale than diffusion, which
was the only significant process removing these species under
the present experimental conditions, their yields from reactions
2 and 3 were related to the extrapolated initial concentrations
(or optical densities), [HNC]0 and [HCN]0, in a simple way.
The sums of the HCN and HNC concentrations created in
reactions 2 and 3 are equal because all of the CN radicals created
in reaction 2 are assumed to be consumed by reaction 3. For
the HNC-forming product channels,

Product Channeling in the Hot Atom Reaction
2[HNC]0
J. Phys. Chem. A, Vol. 105, No. 28, 2001 6823
( 0.048). By adding in quadrature the mean precision of (0.046
from these 12 experiments, and the uncertainty of (0.067 in
the average value for f3b, the branching fractions obtained for
reaction 3 are f3a ) 0.808 and f3b ) 0.192 ((0.081).
fb ) f2b + f3b
)
(8)
([HNC]0 + [HCN]0)
where fb is the yield of HNC per (CN)2 molecule from reactions
2 and 3. A similar equation can be written for the yield of HCN.
However, the relative contributions from reactions 2 and 3
cannot be determined using the simple mechanism of reactions
(1-4) alone. To obtain the desired branching fractions, propane
(C3H8) was substituted for a portion of the inert bath gas in a
number of experiments, such that C3H8 was in excess of CH3SH.
Then reaction 9,
4. Discussion and Conclusions
Assuming that the channel forming HNC in reaction 2 has
no activation barrier and that the HCN-forming channel has a
small barrier14 of 14 kJ mol-1 (see section 3.1), the fractions of
H-atoms that are initially above the energy thresholds for
channels 2a and 2b are 1.00 and 0.30, respectively, as calculated
using the thermochemical data in Table 1 and the (92 ( 19) kJ
mol-1 fwhm Gaussian kinetic energy distribution from reaction
1.7 With the additional assumption that the average reactive cross
sections for channels 2a and 2b are approximately equal above
their respective energy thresholds, the predicted branching
fractions are f2a(pr) ) 0.85 and f2b(pr) ) 0.15, in good agreement
with the experimentally determined values of f2a ) 0.88 ( 0.05
and f2b ) 0.12 ( 0.05. The predicted branching fractions are
hardly sensitive to any uncertainty in the threshold for channel
CN + C3H8 f HCN + C3H7
(9)
which has a large binary rate coefficient, k9, of (8.3 ( 0.5) ×
10-11 cm3 molec-1 s-1 at 294 K,50 became the dominant process
removing CN. In a separate study in this laboratory, HNC was
searched for, but not detected, as a product of reaction 9 (with
CN generated by photolysis of (CN)2 at 193 nm). This
nonobservation establishes an upper-limit yield of ∼10-4 for
HNC from reaction 9, and thus the only significant product is
HCN. Neither HCN nor HNC reacts with propane. The altered
yield, f b′, of HNC is then given by the right-hand side of eq 8.
In the limit R3/R9 f 0, where Ri ) ki[CN][Mi] is the rate of
reaction i (with M3 ) CH3SH and M9 ) C3H8), reaction 3 is
“turned off”, and the desired branching fractions are simply f2b
2a. For example, a factor-of-two increase (i.e., to 28 kJ mol-1
in the barrier for this channel leads to predicted branching
)
(pr)
(pr)
fractions of f2a
) 0.84 and f2b
) 0.16. Conversely,
(pr)
(pr)
neglecting the barrier leaves f2a and f2b unchanged.
On the other hand, the possibility of an activation barrier and
the uncertainty in the reaction energetics for channel 2b
profoundly influence the predicted branching fractions, because
the energy threshold for this channel is close to the upper
inflection point in the Gaussian energy distribution for the hot
H-atoms. As a result, within the ( 10 kJ mol-1 uncertainty in
∆H°r0 for this channel (see Table 1), the predicted branching
) f ′ and f3b ) fb - f ′b. However, it is difficult to establish the
b
condition that R3 , R9, because k3 > k9 (see section 3.3 and
above) and [CH3SH] must be kept reasonably large for the
production of H-atoms. Thus, the branching fractions are given
more generally by
(pr)
fractions range widely, from f2a(pr):f2b ) 0.68:0.32 to 0.96:
f ′
R3
b
0.04. The assumption that only the initial kinetic energy
distribution of the H-atoms contributes significantly to the
observed product branching overestimates f2b, because channel
2a, with its lower threshold, is more accessible than channel
2b to a partially relaxed distribution of H-atoms. The additional
assumption, that the average reactive cross sections for channels
2a and 2b are equal above their respective reaction thresholds,
clearly oversimplifies the dynamics of reaction 2, yet leads to
predicted branching fractions that are in agreement with those
observed. Detailed calculations of the HNCCN PES are not
available. However, several stationary points for the CN + HCN
reaction have been computed by Yang et al.14 at the BAC-MP4
level of theory, and indicate that the adduct, NC(H)CN, is bound
by ∼130 kJ mol-1 relative to the H + (CN)2 exit barrier. While
it has been noted8 that this deep potential minimum could
facilitate secondary encounters, in which the HNC emerging
from reaction 2b reacts with the departing CN to form the more
exothermic HCN product, it is also possible that no such effect
occurs, and more detailed explorations of the HNCCN reactive
PES are needed to resolve this point. If secondary encounters
have an appreciable effect, it follows from the assumptions made
above that the average reactive cross section for channel 2b via
primary encounters is greater than for channel 2a.
f2b
)
-
fb
(10)
(11)
( )
R9
Rh9
(fb - f ′)
b
f3b
)
Rh9
where
R9
(R9 + R3)
k9
Rh9 )
)
(12)
[M3]
k9 + k
3[M9]
Table 2 lists the yields of HCN (fa) and HNC (fb) from the
experiments in which propane was not added. Despite the rather
large spread in the data, no discernible trend in these yields
with bath or reagent gas partial pressure is apparent. Table 3
lists the yields of HCN (f a′) and HNC (f ′) obtained from the
b
experiments in which C3H8 was added, as well as the branching
fractions for reactions 2 and 3 derived using eqs 10-12. From
eq 10, f2b was calculated for each value of f ′ using the average
b
value for fb shown in Table 2. The branching fraction for the
HCN-forming product channel, f2a, was calculated simply as 1
- f2b. The resulting branching fractions for reaction 2 are f2a
)
In previous work from this laboratory, the disposition of
reaction energy into the HNC product of channel (2b),8 and
into the CN coproduct of both channels of reaction 2,7 was
determined under near-single-collision conditions. Because
channel 2a forming HCN accounts for 88 ( 5% of the product
from reaction 2, the measured energy disposal in the CN
coproduct largely should reflect the dynamics of this channel.
By considering the energy disposal into CN as a linear
combination of contributions from channels (2a) and (2b), where
0.881 and f2b ) 0.119 (( 0.046), where the uncertainty
represents the (1σ precision from the 12 experiments shown
in Table 3. Here the uncertainty in fb is negligible, because the
term containing fb in eq 10 makes a small contribution relative
to the term containing f ′. Such is not the case with eq 11, and
b
for the (1σ range in fb, 0.251 e fb e 0.371, the range of average
calculated values (with (1σ precision) for f3b from the 12
experiments shown in Table 3 is (0.126 ( 0.044) e f3b e (0.259

6824 J. Phys. Chem. A, Vol. 105, No. 28, 2001
Decker and Macdonald
TABLE 4: Estimated Disposition of Reaction Energy in the
the HClCN surface. These calculations, in agreement with other
theoretical and experimental work,11,52 indicate that the channel
forming HCl (+ CN) in the H + ClCN reaction proceeds by
direct abstraction via a nearly collinear transition state, with a
H + (CN)2 and H + Cl2 Systems
product channel
fT
fR
fV
CN + HCN/HNCa
HCN + CNc
HNC + CNd
HCl + Cle
0.62
0.63
0.55
0.54
0.14
0.12
0.32
0.09
0.24b
0.25
0.13
0.37
large activation barrier of ∼91 kJ mol-1 10
.
Although no
theoretical data are available for the HBrCN PES, the compa-
rable small cross section and energy disposal for the abstraction
channel7 imply a large barrier and near-linear geometry for this
reaction as well. The cross section to form HNC is roughly the
same for BrCN and ClCN, and much smaller for (CN)2, which
reflects, at least in part, the difference in energetics; the HNC
channel is accessible to most of the initially hot H-atoms with
BrCN and ClCN,18 but not with (CN)2 (see Table 1). The cross
section to form HCN is smaller with BrCN than with ClCN,
which has been suggested to indicate geometrical effects due
to the larger Br atom.18 Indeed, it seems likely that the halogen
atom is largely responsible for the enhancement of the HNC
product relative to the more exothermic HCN product in the
XCN (X ) Br, Cl) reactions, by deflecting reactive trajectories
away from the C-atom.18
a Reference 7. b Refers to rovibrational excitation in the HCN/HNC
coproducts, assuming no vibrational excitation in CN (see ref 7). c This
work (see Discussion). d Reference 8. e Reference 51.
TABLE 5: Estimated Cross Sections (in units of 10-16 cm2)
to Form the Indicated Products in the Hot-Atom Reactions
H + XCN (X ) Br, Cl) and H + (CN)2
reactant
product
BrCN
ClCN
(CN)2
HCN
HNC
HX
0.22
1.5
0.026
0.5
1.5
0.02
0.26
0.036
the branching fractions determined in the present work are the
coefficients, the energy disposal in the HCN product can be
estimated from the data for HNC and CN, as shown in Table
4. As expected, the estimates for HCN are very close to the
results for CN. The disposition of reaction energy in the
analogous thermal triatomic reaction, H + Cl2 f HCl(V′, R′,
T′) + Cl,51 is included in Table 4 for comparison. Whereas
collinear and near-collinear trajectories contribute most of the
reactive flux in the H + Cl2 reaction,13 the experimental data
accumulated for the H + (CN)2 reaction (refs 7 and 8 and the
present work) support the expectation, based on theoretical
calculations for the related H + ClCN10,11 and H + HCN12
systems, that collinear and near-collinear H-NCCN geometries
are highly repulsive, and therefore trajectories that are transverse
relative to the NCCN molecular axis contribute most of the
reactive flux. The HNC product exhibits a large degree of
rotational excitation due to reactive trajectories having bent
H-NCCN geometries, which impart torque about the COM of
the system.8 The HCN product is created with much less
rotational excitation, because trajectories that approach the
C-atoms are closer to the system COM. Note that although the
energy disposition in the H + Cl2 reaction and the HCN-forming
channel of the H + (CN)2 reaction are quite similar (see Table
4), they arise from different sets of optimal reaction trajectories
relative to the molecular axis. The global features of the energy
disposition for the H + (CN)2 reaction are consistent with the
L + H-H kinematics of the H + Cl2 and other comparable
reaction systems.8
In another recent study,18 we reported the branching fractions
for the three accessible product channels of the related hot atom
reactions, H + XCN (X ) Br, Cl), where the H atoms were
generated in the same manner as in the present work. With these
cyanogen halides, HNC was the dominant product, with
branching fractions of 0.86 ( 0.03 and 0.74 ( 0.03 for X ) Br
and Cl, respectively. The total cross section for the H + XCN
reaction (in units of 10-16 cm2) was estimated to be 1.7, 2.0,
and 0.3 for X ) Br,18 Cl,18 and CN,7 respectively, within a
factor-of-two uncertainty. Table 5 presents the estimated cross
sections for the individual product channels in these reactions,
obtained from the measured branching fractions and the
estimated total reactive cross sections. It is clear by comparison
with the H + (CN)2 reaction that the halogen atom of XCN (X
) Br, Cl) strongly influences the dynamics. The most complete
theoretical study for any of these PESs to date is that of
Harding,10 who calculated 9 transition states and 14 minima on
Although k3 was measured only at 293 K, the kinetics of the
analogous reactions of Cl and of OH with CH3SH have been
determined in the 193-430 K25 and 244-430 K53 temperature
ranges, respectively. Both exhibit a weak negative dependence
on temperature, with Ea ) -1.3 ( 0.3 kJ mol-1 for Cl25 and
-3 ( 0.8 kJ mol-1 for OH.53 The absence of a significant kinetic
isotope effect (with CH3SD) in either case implies that both
proceed principally by an addition-elimination mechanism, in
which the radical forms an intermediate adduct with the
S-atom.25,26,54 Wilson and Hirst26 have located such an adduct
on the CH3S(Cl)H PES, which is bound by 57.1 kJ mol-1
relative to the Cl + CH3SH asymptote, at the MP2 level of
theory. Nicovich et al.25 have demonstrated that the reactivity
of Cl with the series of sulfides, RSR′ (R, R′ ) H, CH3),
correlates with IE(RSR′) - EA(Cl), where IE represents
ionization energy and EA electron affinity. As suggested by
these authors,25 the large electron affinity of Cl (EA(Cl) ) 3.61
eV55), combined with the relatively small IEs of the sulfides,55
is likely responsible for long-range electronic interactions that
lead to loose, polar transition states and hence to large rate
coefficients. The electron affinity of CN is comparable to that
of Cl (EA(CN) ≈ 3.8 eV55), and thus its kinetic behavior should
follow a similar mechanism. An abstraction mechanism also
has been predicted to be important in the Cl + CH3SH
reaction,26 although the lack of an experimentally observed
kinetic isotope effect25 does not support this prediction. With
CN, abstraction might be indicated by the product channel
forming HNC, because adduct formation via a CNsS interaction
is probably much less favorable than the NC-S interaction.
Acknowledgment. The authors thank Dr. G. He and Prof.
I. Tokue for their work in the initial stages of these experiments.
This work was supported by the U.S. Department of Energy,
Office of Basic Energy Sciences, Division of Chemical Sciences,
under Contract No. W-31-109-ENG-38.
References and Notes
(1) Polanyi, J. C. Science 1987, 236, 680.
(2) Schatz, G. C. J. Phys. Chem. 1995, 99, 516.
(3) Clary, D. C. J. Phys. Chem. 1994, 98, 10678.
(4) Clary, D. C. Science 1998, 279, 1879.
(5) Tyler, J. K.; Sheridan, J. Trans. Faraday Soc. 1963, 59, 2661.
(6) Costes, M.; Naulin, C.; Dorthe, G. Astron. Astrophys. 1990, 232,
270.
(7) He, G.; Tokue, I.; Macdonald, R. G. J. Chem. Phys. 2000, 112,
6689.

Product Channeling in the Hot Atom Reaction
J. Phys. Chem. A, Vol. 105, No. 28, 2001 6825
(8) Macdonald, R. G. J. Phys. Chem. A 2000, 104, 10202.
(9) Holmes, B. E.; Setser, D. W. In Physical Chemistry of Fast
Reactions. Vol. 2: Reaction Dynamics; Smith, I. W. M., Ed.; Plenum: New
York, 1980; p 83.
(10) Harding, L. B. J. Phys. Chem. 1996, 100, 10123.
(11) de Juan, J.; Callister, S.; Reisler, H.; Segal, G. A.; Wittig, C. J.
Chem. Phys. 1988, 89, 1977.
(12) ter Horst, M. A.; Schatz, G. C.; Harding, L. B. J. Chem. Phys.
1996, 105, 558.
(13) Levy, M. R. Prog. React. Kinet. 1979, 10, 1.
(14) Yang, D. L.; Yu, T.; Lin, M. C.; Melius, C. F. J. Chem. Phys.
1992, 97, 222.
(15) Bethardy, G. A.; Northrup, F. J.; Macdonald, R. G. J. Chem. Phys.
1995, 102, 7966.
(16) Bethardy, G. A.; Northrup, F. J.; He, G.; Tokue, I.; Macdonald, R.
G. J. Chem. Phys. 1998, 109, 4224.
(17) Barone, S. B.; Turnipseed, A. A.; Gierczak, T.; Ravishankara, A.
R. J. Phys. Chem. 1994, 98, 11969.
(18) Decker, B. K.; He, G.; Tokue, I.; Macdonald, R. G. J. Phys. Chem.
A (in press).
(19) Cerny, D.; Bacis, G.; Guelachvilli, G.; Roux, F. J. Mol. Spectrosc.
1978, 73, 154.
(20) Smith, A. M.; Coy, S. L.; Klemperer, W.; Lehmann, K. K. J. Mol.
Spectrosc. 1989, 134, 134.
(21) Burkholder, J. B.; Sinha, A.; Hammer, P. D.; Howard, C. J. Mol.
Spectrosc. 1987, 126, 72.
(22) Northrup, F. J.; Bethardy, G. A.; Macdonald, R. G. J. Mol.
Spectrosc. 1997, 186, 349.
(33) He, G.; Tokue, I.; Macdonald, R. G. J. Chem. Phys. 1998, 109,
6312.
(34) Smith, M. A. H.; Harvey, G. A.; Pellett, G. L.; Goldman, A.;
Richardson, D. J. J. Mol. Spectrosc. 1984, 105, 105.
(35) Varghese, P. L.; Hanson, R. K. J. Quant. Spectrosc. Radiat. Transfer
1984, 31, 545.
(36) Smith, I. W. M. J. Chem. Soc., Faraday Trans. 1981, 277, 2357.
(37) He, G.; Macdonald, R. G. Chem. Phys. Lett. 1999, 301, 175.
(38) Smith, M. A. H.; Rinsland, C. P.; Fridovich, B. In Molecular
Spectroscopy: Modern Research; Rao, K. N., Ed.; Academic: Orlando,
1985; p 112.
(39) Herzberg, G. Molecular Spectra and Molecular Structure III.
Electronic Spectra and Electronic Structure of Polyatomic Molecules;
Krieger: Malabar, Florida, 1991; p 226.
(40) Earls, L. T. Phys. ReV. 1935, 48, 423.
(41) Whiting, E. E.; Schadee, A.; Tatum, J. B.; Hougen, J. T.; Nicholls,
R. W. J. Mol. Spectrosc. 1980, 80, 249.
(42) Kroto, H. W. Molecular Rotation Spectra; Dover: New York, 1992;
p 68.
(43) Yang, D. L.; Lin, M. C. In The Chemical Dynamics and Kinetics
of Small Radicals. Part 1.; Liu, K. P., Wagner, A., Eds.; World Scientific:
Singapore, 1995; p 164.
(44) Copeland, L. R.; Mohammed, F.; Zahedi, M.; Volman, D. H.;
Jackson, W. M. J. Chem. Phys. 1992, 96, 5817.
(45) Sims, I. R.; Smith, I. W. M. Annu. ReV. Phys. Chem. 1995, 46,
109.
(46) Johnston, H. S. Gas-Phase Reaction Rate Theory; The Ronald Press
Company: New York, 1966; pp. 140-141.
(23) Auwera, J. V.; Hurtmans, D.; Carleer, M.; Herman, M. J. Mol.
Spectrosc. 1993, 157, 337.
(47) Emsley, J. The Elements; Clarendon Press: Oxford, 1990.
(48) Huber, K. P.; Herzberg, G. Molecular Spectra and Molecular
Structure IV. Constants of Diatomic Molecules; Van Nostrand Reinhold:
New York, 1979.
(49) Curtiss, L. A.; Nobes, R. H.; Pople, J. A.; Radom, L. J. Chem.
Phys. 1992, 97, 6766.
(50) Hess, W. P.; Durant, J. L., Jr.; Tully, F. P. J. Phys. Chem. 1989,
93, 6402.
(51) Anlauf, K. G.; Horne, D. S.; Macdonald, R. G.; Polanyi, J. C.;
Woodall, K. B. J. Chem. Phys. 1972, 57, 1561.
(52) Johnston, G. W.; Bersohn, R. J. Chem. Phys. 1989, 90, 7096.
(53) Tyndall, G. S.; Ravishankara, A. R. Int. J. Chem. Kinet. 1991, 23,
483.
(54) Butkovskaya, N. I.; Setser, D. W. J. Phys. Chem. A 1999, 103,
6921.
(55) Mallard, W. G.; Linstrom, P. J. NIST Webbook; NIST Standard
Reference Database Number 69; National Institute of Standards and
Technology: Gaithersburg, MD, 1998 (http://webbook.nist.gov).
(24) Diffusion actually occurs on a longer time scale than implied by
Figure 1, i.e., tens of milliseconds (see ref 16). The decay profile shown in
Figure 1 is distorted by use of the digital oscilloscope in its AC signal-
input mode, and by high-pass filtering. However, since the branching
fractions are determined from ratios of the extrapolated initial concentrations,
[HCN]0 and [HNC]0, this distortion has a negligible effect under similar
filtering conditions for the HCN and HNC decay profiles.
(25) Nicovich, J. M.; Wang, S.; Wine, P. H. Int. J. Chem. Kinet. 1995,
27, 359.
(26) Wilson, C.; Hirst, D. M. J. Chem. Soc., Faraday Trans. 1997, 93,
2831.
(27) Butkovskaya, N. I.; Setser, D. W. J. Phys. Chem. A 1999, 103,
6921.
(28) Park, J.; Shafer, N.; Bersohn, R. J. Chem. Phys. 1989, 91, 7861.
(29) Kessler, K.; Kleinermanns, K. J. J. Chem. Phys. 1992, 97, 374.
(30) Bradely, K. S.; Schatz, G. C. J. Phys. Chem. 1996, 100, 12154.
(31) Bradely, K. S.; Schatz, G. C. J. Chem. Phys. 1997, 106, 8464.
(32) Knowles, P. J.; Werner, H.-J.; Hay, P. J.; Cartwright, D. C. J. Chem.
Phys. 1988, 89, 7334.