Article pubs.acs.org/JPCA
Product Channels in the Reaction of the Hydroxymethyl Radical with Nitric Oxide Wenhui Feng, Erik Janssen, and John F. Hershberger* Department of Chemistry and Biochemistry, Dept. 2735, PO Box 6050, North Dakota State University, Fargo, North Dakota 58108-6050, United States ABSTRACT: The products of the reaction of CH2OH with NO were studied by infrared diode laser spectroscopy. Products were detected to determine the branching ratios of the CH2OH + NO reaction. HNCO was detected in 10.3 ± 2% yields. No other products were detected in significant quantities, indicating that adduct formation is the primary reaction pathway. Ab initio calculations of the potential energy surface show a low energy pathway to HNCO + H2O, but no other bimolecular channels, in agreement with the experiments.
1. INTRODUCTION The hydroxymethyl (CH2OH) radical is of interest to combustion chemists due to its importance in the combustion and atmospheric chemistry, as well as its capability to react with NOX species. Reactions of the CH2OH radical with NO2,1,2 O2,3 hydrogen halides,4−7 H2O,8 H2O2,9 several saturated and unsaturated hydrocarbons,10−13 OH,10 and HO2,10,14 have been studied by experiment or ab initio calculation. In the one previous experimental study of the kinetics of the title reaction, Pagsberg et al. suggested two possible product channels:1
CH 2OH + NO → CH 2O + HNO
0 ΔH298 = − 90.0 kJ/mol
(1b)
→ HCN + H 2O2
0 ΔH298
= −74.7 kJ/mol
(1c)
→ HNCO + H 2O
0 ΔH298
= −431 kJ/mol
(1d)
→ HCNO + H 2O
0 ΔH298
= −142 kJ/mol
(1e)
→ CO2 + NH3
0 ΔH298 = − 515 kJ/mol
(1f)
→ HCO + H 2NO
0 ΔH298
= 44.6 kJ/mol
(1g) (1h)
CH 2OH + NO + M → HOCH 2NO + M
(1a)
0 → NCO + OH + H 2 ΔH298 = 167 kJ/mol
CH 2OH + NO → CH 2O + HNO
(1b)
where the thermodynamic data are obtained from the NISTJANAF standard tables,17 and other published sources for HNCO, HCNO, and HNO,18 H2NO,19 and CH2OH.20 In this paper, we report infrared absorption measurements of the yield of product channels of the title reaction, as well as ab initio calculations of the potential energy surface. The experimental approach used in this study is to photolytically produce CN radicals, followed by the CN + CH3OH reaction:
They used pulse radiolysis to generate F atoms, followed by the F + CH3OH reaction to generate CH2OH radicals. Detection of CH2OH by UV absorption spectroscopy yielded a total rate constant of 2.5 × 10−11 cm3 molecule−1 s−1 at room temperature and 1 atm total pressure. On the basis of the UV absorption spectra obtained, they suggested that adduct formation may dominate the reaction but were unable to estimate the yield of any possible bimolecular product channels. There is one previous ab initio study of this reaction, in which Shin et al. performed calculations on the part of the potential energy surface involving adduct formation but did not consider subsequent rearrangement of the adducts into bimolecular product channels.15 In addition, an earlier ab initio study of nitromethane (CH3NO2) dissociation shares some critical points relevant to the title reaction.16 We note that in addition to (1b), there are several additional bimolecular channels that are thermodynamically possible. A more complete set of product channels is CH 2OH + NO + M → HOCH 2NO + M © 2016 American Chemical Society
ICN + hν(266 nm) → I + CN
(2)
CN + CH3OH → HCN + CH 2OH → HCN + CH3O
(3a) (3b)
The kinetics of reaction 3 have been reported in a previous publication in our laboratory.20 It is fast, with k3 = 2.5 × 10−11 cm3 molecule−1 s−1 at 298 K. A key point is that channel 3a was Received: December 15, 2015 Revised: January 19, 2016 Published: February 4, 2016
(1a) 1145
DOI: 10.1021/acs.jpca.5b12273 J. Phys. Chem. A 2016, 120, 1145−1152
Article
The Journal of Physical Chemistry A
3. RESULTS AND DISCUSSION 3.1. Product Yields. Of the list of probed species described above, three product molecules were detected by transient infrared absorption spectroscopy. Figure 1 shows transient
shown to be the dominant product channel, with an estimated yield of φ3a = 0.94, with only very small amounts (∼0.08) of channel 3b. This approach therefore represents a relatively clean route to hydroxymethyl radicals.
2. EXPERIMENTAL SECTION The experimental apparatus has been described previously.21 The photolysis laser was an Nd:YAG laser (Continuum Surelite-II) operating on the fourth harmonic. Typical reaction mixtures included ICN (0.10 Torr), CH3OH (1.0 Torr), NO (variable pressure), and SF6 buffer gas (1.0 Torr). Reaction products were detected by time-resolved infrared absorption spectroscopy using high resolution lead salt diode lasers (Laser Components). The UV and IR beams were copropagated through a 143 cm single pass absorption cell. The beams were then separated by 1/4 m monochromator and the infrared light was detected on a 1 mm InSb detector (Cincinnati Electronics, ∼1 μs response time). Transient signals were collected and signal averaged by a digital oscilloscope and stored on a computer. All measurements were taken at 298 K. ICN (Fluka, 97%) was purified by vacuum sublimation to remove dissolved gases. CH3OH (Sigma-Aldrich, 99.9%), CH3OD, and CD3OH (Cambridge Isotope Laboratories, 99% and 99.5% isotopic purity, respectively) were purified by several freeze−pump−thaw cycles at 77 K. NO (Matheson) was purified by freezing NO2 impurities at 230 K, then collecting the NO in a sealed liquid nitrogen trap, followed by several freeze−pump−thaw cycles. SF6 (Matheson) was purified by several freeze−pump−thaw cycles. HNCO samples (used for calibration purposes only) were synthesized as described by Fischer, et al, substituting sodium cyanate for potassium cyanate.22 HCNO samples were synthesized as described previously.23 The following molecules were probed using infrared diode laser absorption spectroscopy:
Figure 1. Typical transient signals for HNCO and HNO product molecules. Reaction conditions: P(ICN) = 0.1 Torr, P(CH3OH) = 1.0 Torr, P(NO) = 1.0 Torr, P(SF6) = 1.0 Torr.
signals for HNCO and HNO molecules; these molecules were detected only if all three reagents (ICN, CH3OH, and NO) were present. As shown, a large transient signal for HNCO formation is observed, as is an extremely small signal, on the edge of detectability, for HNO. Additionally, HCN was detected both with and without NO reagent, as shown in Figure 2. Transient signals for the other species listed above were negligible, but upper limits to detectable signals were estimated when possible. In general, the transient absorption signals displayed fast rises, with rise times on the order of a few microseconds to tens of microseconds, attributed to rapid formation of the detected product molecules, as well as relaxation of any rovibrational excited state populations to a
HCN (ν = 1 ← ν = 0) P(19) at 3251.823 cm−1
HNO (ν = 1 ← ν = 0) at 2669.891 cm−1 HNCO (ν = 1 ← ν = 0) R(15) at 2280.066 cm−1 and R(11) at 2277.39 cm−1
HCNO (ν = 1 ← ν = 0) R(10) at 2203.851 cm−1 CO (ν = 1 ← ν = 0) R(2) at 2154.59 cm−1 H 2CO (ν = 1 ← ν = 0) 515 nm at 2796.329 cm−1 and 330 nm at 2781.02 cm−1 CO2 (001 ← 000) P(6) at 2344.368 cm−1
Transient signals were collected both on and off resonance with the listed spectral lines. Very small off-resonance signals, which are attributed to thermal deflection effects, were subtracted from the on-resonance signals to obtain absorption profiles for the relevant molecules. The HITRAN database24 was used for locating and identifying HCN, CO, and H2CO lines. Other published sources were used to locate and identify HNO,25 HNCO,26 and HCNO27 lines.
Figure 2. HCN transient signals in the presence and absence of NO. Typical reaction conditions: P(ICN) = 0.1 Torr, P(CH3OH) = 1.0 Torr, P(NO) = 0 or 1.0 Torr, and P(SF6) = 1.0 Torr. 1146
DOI: 10.1021/acs.jpca.5b12273 J. Phys. Chem. A 2016, 120, 1145−1152
Article
The Journal of Physical Chemistry A Boltzmann distribution (the SF6 buffer gas is included to promote vibrational relaxation). This is typically followed by a slow decay on a ∼1 ms time scale, which for stable species is dominated by diffusional loss of the detected molecule out of the probed volume. The somewhat faster decay in the lower trace of Figure 2 is consistent with this interpretation, as diffusion is faster at the lower total pressure of 2.1 Torr, compared to the higher pressure of 3.1 Torr used in the upper trace. Peak−peak amplitudes were converted to absolute concentration using known line strengths from the HITRAN database for HCN, CO, and N2O. Calibration of HNCO signals was accomplished by filling the cell with varying pressures of HNCO and measuring static absorption signals. The absorption coefficient obtained for the R(11) line of HNCO at 2277.39 cm−1 was 2.4 ± 0.15 cm−1 Torr−1 (base e). A similar procedure was used to calibrate HCNO (resulting in 1.94 cm−1 Torr−1 for the R(10) line at 2203.851 cm−1), and CH2O (resulting in 0.196 cm−1 Torr−1 for the line at 2796.329 cm−1). HNO was calibrated by the following reaction sequence:
Figure 3. HNCO yield vs NO pressure. Reaction conditions: P(ICN) = 0.1 Torr, P(CH3OH) = 1.0 Torr, P(NO) = variable, P(SF6) = 1.0 Torr.
ICN + hν(266 nm) → I + CN
Table 1. Product Yields as a Fraction of CN Radicals Generateda
CN + CH 2O → HCN + HCO HCO + NO → HNO + CO
(Note that CN does not react quickly with NO at low pressures.) By measuring the photolysis laser pulse energy, the number density of CN radicals was estimated. In the limit of high CH2O and NO concentrations, this reaction sequence converts most CN radicals to HNO. Detection of the resulting transient signal for HNO then provides the calibration, leading to 0.069 cm−1 Torr−1 for the HNO line at 2669.891 cm−1. As shown in Figure 1, significant amounts of HNCO as well as small amounts of HNO are produced in this reaction. Figure 2 shows that significant amounts of HCN are produced, but with essentially identical yields with and without the NO reagent. This is attributed to the production of HCN in reaction 3 and indicates that production of HCN via channel 1c of the title reaction is small or insignificant. Figure 3 shows the number density of HNCO produced as a function of NO reagent pressure. At low NO pressures, reaction 1 competes with other CH2OH loss mechanisms such as reaction with trace O2, diffusional losses, etc. In the limit of high NO and CH3OH pressures, most CN radicals produced in the photolysis are converted to CH2OH in reaction 3a, and most CH2OH radicals react with NO. As shown, this high pressure limit is reached at pressures above approximately 0.5 Torr. The resulting product yields for detected product molecules as well as estimated upper limits of undetected molecules were compared to the initial CN radical number density, obtained from the laser pulse energy (typical values were ∼11 mJ/pulse), and the 266 nm absorption coefficient of ICN (0.009 cm−1 Torr−1, base e), and assuming a unity quantum yield for CN production. (Alternatively, one could compare product yields to the yield of HCN produced by reaction 3; this procedure yields similar results within the quoted experimental uncertainties.) This results in typical values of [CN]0 ∼ 4.37 × 1013 molecules cm−3. Table 1 shows the resulting yields. Channels 1g and 1h are not expected to be active, as they are endothermic. NCO is not directly detected, but any NCO produced in (1h) would be expected to produce N2O via the NCO + NO reaction; our lack of N2O detection implies that channel 1h is not active, with an
a
product
yield
HCN HNO HNCO HCNO CO CH2O CO2
