NCO Radical Research Articles

The 193-nm Photodissociation of NCO

The 193-nm photolysis of the NCO radical has been investigated. NCO was generated from the reaction of CN + O2, where the CN was produced by 193-nm photolysis of C2N2 close to the nozzle of a pulsed jet. A second 193-nm photon dissociated the NCO radical during the same laser pulse. At this photon energy both the N−CO and the NC−O bonds may break. N(2D,2P) and CO products have been detected using vacuum ultraviolet laser induced fluorescence. A direct measurement of the N(2D):N(2P) branching ratio yielded an upper limit of 72 ± 18. The CO vibrational distribution was modeled with prior distributions for each of the contributing channels with coproducts N(4S,2D,2P). Combination of the results from the prior model and the direct measurement yielded a branching ratio of N(4S):N(2D):N(2P) of (5.1 ± 1.8):(93.6 ± 4.8):(1.3 ± 0.3). For the N(2D) + CO product channel, the average energy disposal into product relative translation (8%) and CO vibration (24%) was determined, leaving 68% of the available energy to appear as CO rotation. This observation suggests that the geometry of the dissociating state of NCO is likely bent.

Removal of cyclohexanone in transition electric discharges atatmospheric pressure

Two new types of streamer-induced electric discharges operating in anon-uniform electric field in air at atmospheric pressure were applied to theremoval of volatile organic compounds (VOCs). The first type is a pulseless dc discharge with physicalproperties corresponding to the glow discharge. The second, also supplied by adc high voltage of both polarities, is a spontaneously pulsing dischargeoperating in the regime of the streamer-to-spark transition, the spark phasebeing too short to reach local thermodynamic equilibrium conditions. Bothdischarges are able to generate a non-thermal plasma, as resolved from theirrotational and vibrational temperatures.The influences of these discharges on the removal of cyclohexanone at variousgas flow rates and concentrations (600-6000 ppm) were compared. The removalefficiencies achieved were about 50-60%, and the energy costs were16-100 eV/molecule at various energy densities. Special conditions whereCO2 and other gaseous products are minor and dominant products appear inthe condensed phase can be obtained, especially in the spontaneously pulsingtransition discharge.We explain some plasmochemical processes induced by the discharges byconsidering heterogeneous effects of the copper electrode surface. The role ofactive nitrogen and the formation of the NCO radical are probably key factorsleading to the formation of the condensation product based on amino acids,here produced for the first time from VOCs, as well as in the overall energycycle resulting in low energy costs of the process.The small pilot-scale reactor based on the spontaneously pulsing transitiondischarge has been successfully applied to the removal of cyclohexanone in themixture with other VOCs with no noxious gas output. This validates thepossibility of the application of such a type of reactor for larger scales.

Kinetics of the NCO radical reacting with atoms and selected molecules

Rate constants for the reaction of isocyanate radicals (NCO) in its electronic ground state ( X̃ 2Π) with oxygen atoms were determined at 2.5 Torr total pressure in the temperature range 302–757 K. Excimer laser photolysis (ELP) of chlorine isocyanate (ClNCO) produced NCO radicals detected by laser-induced fluorescence (LIF). The reaction NCO + O exhibits a negative temperature dependence, described by the two-parameter equation: k NCO+O( T) = (4.3 −2.2 +3.2) × 10 −8 × T −1.14 −0.12 +0.08 cm 3 molecule −1 s −1. Measurements at 298 K and total pressures of 2.5 and 9.9 Torr, respectively, indicated a slight pressure dependence. For the reaction of NCO radicals with hydrogen atoms, the rate constant k NCO+H = (2.2 ± 1.5) × 10 −11 cm 3 molecule −1 s −1was obtained at 298 K and a total pressure of 2.6 Torr for the first time by a direct measurement. From a single measurement k = (3.8 ± 1.6) × 10 −11 cm 3 molecule −1 s −1 was determined at 548 K and 2.4 Torr total pressure. In addition, rate constants for the reactions of NCO radicals with molecular oxygen (O 2), carbon dioxide (CO 2), molecular hydrogen (H 2), and carbon monoxide (CO), which is a dissociation product of CO 2 in a microwave discharge, were measured at two different temperatures. At room temperature these reactions were slow and at the detection limit of the ELP/LIF technique. However, at elevated temperatures at least the rate constants of the reactions NCO + O 2 and NCO + H 2 become significantly larger and, therefore, should be taken into account, when modeling combustion processes under certain conditions.

Reaction kinetics of O(1D) with CF3CN

The rate constant for removal of O(1D) by reaction with CF3CN has been measured to be (1.3 ± 0.2) × 10−10 cm3 molecule−1 s−1 at room temperature by a time-resolved laser induced fluorescence (LIF) technique. The quantum yield for the electronic quenching O(1D) to O(3P) has been also measured to be 0.14 ± 0.02. The O(1D) and O(3P) atoms were detected by LIF using a vacuum ultraviolet laser as a probe laser. CN(X2Σ+) and NCO(X2Π) products were also detected by LIF using ultraviolet and visible lasers, respectively. The CN(X) radical was observed as a primary product, while the NCO(X 2Π) radical was not observed as a primary product in the present reaction.

Photochemistry of HNCO in Solid Xe: Channels of UV Photolysis and Creation of H2NCO Radicals

Photolysis of HNCO at wavelengths between 266 and 193 nm is studied in solid Xe with FTIR and laser-induced fluorescence methods. The channels HNCO → H + NCO (a) and HNCO → NH + CO (b) are operative in a Xe matrix. Channel b produces both isolated fragments and NH···CO complexes as characterized by the CO absorption. The MP2/6-311++G(3df,3pd) calculations are presented for the NH−CO complexes and compared with the experimental data. Photolysis of NCO produces mainly NO + C. A part of the carbon atoms form C2 after which C2- is created in a photoinduced charge transfer reaction. For comparison, in solid Kr, photolysis of HNCO produces additionally HOCN but this channel is absent in a Xe matrix. Upon annealing of the partially photolyzed matrix at 50 K, hydrogen atoms are mobilized and a radical H2NCO is formed by a reaction of a hydrogen atom with a HNCO molecule. Four IR absorptions of H2NCO are observed and they agree well with the MP2/6-311++G(3df,3pd) calculations. The assignment is supported by experiments with DNCO. The threshold for the photodecomposition of H2NCO is between 365 and 405 nm.

Kinetic investigation of NCO radicals reacting with selected hydrocarbons

Bimolecular rate coefficients for the reactions of isocyanate radicals (NCO) with ethane (C2H6), allene (C3H4), propene (C3H6), but-1-ene (C4H8) and but-1-yne (C4H6) were measured in argon as carrier gas. NCO radicals have been formed using excimer laser photolysis of chlorine isocyanate (ClNCO) and were detected in their electronic ground state, 2Π, by laser-induced fluorescence. The NCO+C2H6 reaction was investigated at 20 Torr total pressure in the temperature range 297–899 K. The reaction exhibits a positive temperature dependence which is well described by the following modified Arrhenius equation: In addition, measurements carried out at 651 K in the pressure range 5–528 Torr showed no pressure dependence, which supports the view that the NCO+C2H6 reaction proceeds as a simple H atom abstraction. For the reaction of NCO radicals with allene (C3H4), propene (C3H6), but-1-ene (C4H8) and but-1-yne (C4H6) bimolecular rate coefficients were determined at room temperature and at 20 Torr total pressure. Additionally, the reactions of NCO with C3H4 and C4H6 were studied at different total pressures. For both reactions, no pressure dependence was observed. The following rate coefficients were obtained at 298 K (in units of 10-11 cm3 s-1): kNCO+C3H4=(1.39±0.18), kNCO+C3H6=(4.29±0.20), kNCO+C4H8=(6.18±0.46) and kNCO+C4H6=(1.41±0.14). A correlation between the measured bimolecular rate coefficients of the NCO reactions with unsaturated hydrocarbons and their ionisation potentials was established.

A product study of the reaction of CH radicals with nitric oxide at 298 K

The product formation of the reaction of CH radicals with NO was studied in the gas phase at 100 Torr total pressure and 298 K. CH radicals were generated by excimer laser photolysis of CHClBr2/Ar mixtures. In the presence of nitric oxide, the formation of NH, CN and NCO radicals in their electronic ground states was monitored by laser-induced fluorescence. From the results of time resolved measurements of the radical concentrations, bimolecular rate coefficients for the reactions CH+X and X+NO [X=NH(X3Σ-), CN(X2Σ+) and NCO(X2Π)] were determined. From their kinetic behaviour, NH and CN radicals were unequivocally identified as direct reaction products of the reaction CH+NO, whereas the formation mechanism of NCO remains unclear.

Analytic UHF-CCSD(T) second derivatives: implementation and application to the calculation of the vibration-rotation interaction constants of NCO and NCS

An implementation of analytic open-shell UHF-CCSD(T) second derivatives is presented. To demonstrate applicability and test the accuracy of the UHF-CCSD(T) approach for the determination of spectroscopical parameters, vibration-rotation interaction constants were calculated for the ground (12Π) and first electronically excited (12Σ) states of the NCO and NCS radicals. In addition, harmonic vibrational frequencies for both states, the Renner-Teller parameter for the ground state, as well as the 12Π→12Σ excitation energy are reported. While the computed values are in good agreement with reliable experimental information for NCO, most of the data presented for NCS are predictions of quantities not well known from experiment.

On the à and B̃ electronic states of NCO and its clusters with nonpolar solvents

The B̃←X̃ and Ã←X̃ transitions of the NCO radical and its clusters with nonpolar solvents are studied in a supersonic jet expansion by employing laser-induced fluorescence techniques. Fluorescence excitation (FE) and hole burning spectra are recorded for the NCO radical and compared to previous work. NCO is clustered with Ar, N2, CH4, and CF4 nonpolar molecules to elucidate the effect of solvation on the radical energy levels and dynamics. FE spectra are detected for NCO 1:1 clusters showing blue shifts in their spectra with respect to that of the isolated NCO radical, while their 1:n counterparts show either red or blue shifts. Potential energy surface calculations are performed to evaluate the binding energies and geometries of 1:1 clusters in the X̃, Ã, and B̃ electronic states. The relatively long decay lifetime and red shifted fluorescence wavelength range observed for B̃ state clusters suggests that they decay first through internal conversion (IC) to à vibronic levels, and then experience rapid intracluster vibrational redistribution (IVR) and vibrational predissociation (VP), yielding ground state solvent molecules and NCO radicals at lower à vibronic levels. These à state NCO radicals subsequently emit, generating the Ã→X̃ band.

Ab initio coupled-cluster study of 2II radicals in ground and excited states: application to NCO and NCS

In this paper we report EOM-CC (equation-of-motion coupled-cluster) calculations on the X ̃ 2Π and B ̃ 2Π states of NCO and NCS radicals. Equilibrium bond length, rotational constant, harmonic vibrational frequencies, Renner—Teller parameter and spin—orbit splitting values are presented and compared to the available experimental data. The results show clearly that the calculated molecular parameters are accurate enough to be used in assigning experimental spectra.

Removal rates for the collisional quenching of various vibronic levels of ground state NCO by simple molecules (N2, O2, NO, CO2, N2O, and SO2)

The collisional behavior of NCO[X̃(0,n,0)] in specific vibronic states in the gas phase has been investigated in the time-domain by laser-induced fluorescence (LIF) on transitions within the system NCO(Ã 2Σ+–X̃ 2Π). The NCO radical was generated by the infrared multiphoton dissociation (IRMPD) of phenyl isocyanate (PhNCO) by means of a TEA-CO2 laser operating on the 9R24 line at λ=9.25 μm with subsequent monitoring of the vibronic levels of the X̃ state, characterized by Renner–Teller coupling, in the presence of N2, O2, NO, CO2, N2O, SO2, and PhNCO itself. The states probed were as follows: (0010)2Π3/2, (0010)2Π1/2, (0100)μ2Σ+, (0120)2Δ5/2, (0120)2Δ3/2, (0210)μ2Π3/2,1/2, (0230)2Φ7/2, and (0230)2Φ5/2. Various pairs of spin–orbit states were found to be tightly coupled kinetically. Thus, the time-evolution of the pairs of vibronic states (0010)2Π3/2 and (0010)2Π1/2; (0120)2Δ5/2 and (0120)2Δ3/2; (0230)2Φ7/2 and (0230)2Φ5/2 were found to be equal, yielding an effective local equilibrium within these spin–orbit components within experimental error. Further, states such as NCO(0100) and NCO(0120) were characterized by relatively long decay profiles in the presence of molecules such as CO2 and O2 where the contribution of rotational quenching to the overall decay process could be neglected. By contrast, NCO(0210) and NCO(0230) were removed on significantly faster time scales on collision with SO2. In the absence of extensive information required for solving the large set of coupled differential kinetic equations, albeit reduced in number of those states strongly coupled kinetically, such as a detailed knowledge of the nascent state distributions in NCO following IRMPD, not necessarily Boltzmann in character, the vibronic states were taken to behave independently as the most practical approach to this study. Absolute second-order rate data for the collisional quenching of NCO in the vibronic states (0010), (0100), (0120), (0210), and (0230) by the above molecular species are reported. No clear selection rules are apparent except for the low propensity rule ΔK=2 within the same vibronic state, i.e., μ 2Σ+(0100)–2Δ5/2(0120) and Π43/2,1/2(0210)–2Φ7/2(0230). This is presumed to reflect the role in the collisional interaction of the oscillating dipole in the vibronic state, facilitating ΔK=1, whereas ΔK=2 would involve the quadrupole which is smaller. It is found that the data for (V–V) and (V–T) energy transfer correlate best with the attractive part of the potential curves between the collision partners using the established Parmenter–Seaver plots, yielding well depths [(εMM/kB)1/2] for the vibronic states NCO[μ 2Σ+(0100), ∂25/2(0120), (0210), and Φ47/2(0230)], significantly larger than those of the closed shell collision partners and equal within experimental error. The data are also considered in terms of a multipolar attractive force model involving a collision complex where a sensible correlation is found between the computed and observed collision cross sections for O2, N2, CO2, N2O, and SO2 assuming no change in the multipoles with vibrational state.

Theoretical ab Initio Study of CN2O3Structures: Prediction of New High-Energy Molecules

Structures and energies of CN2O3 isomers have been investigated theoretically at the ab initio CCSD(T)/TZ2P//MBPT(2)/6-31G* level in search of new high-energy molecules. The most energetically favorable isomer, five-membered cyclic nitrous carbonate, has only a 16 kcal/mol dissociation barrier toward decomposition into CO2 and N2O. Nitroisocyanate, O2N−NCO, a collision complex in the gas-phase reaction of NCO and NO2 radicals, is 29 kcal/mol higher in energy than nitrous carbonate and has a 11 kcal/mol barrier in the exothermic decomposition into CO2 and N2O, which involves formation of an intermediate four-membered cyclic form. Nitrofulminate has a higher CN bond dissociation energy and is expected to be a more stable molecule than the earlier studied nitrosofulminate (Korkin, A. A.; et al. J. Phys. Chem. 1996, 100, 19840). Regarding its high exothermicity in decomposition (O2N−CNO → CO2 + N2 + 1/2O2; ΔE = −165 kcal/mol), nitrofulminate is suggested as a potential energetic oxidizer. Another candidate is its six-membered cyclic trimer, 2,4,6-trinitro-1,2,3-triazine 1,2,3-trioxide. The estimated gaseous heats of formations of nitrofulminate and trinitrotriazine trioxide are 71 and 108 kcal/mol, respectively. Trinitrotriazine trioxide and two other cyclic trimers, trinitroisocyanurate and 2,4,6-tri(nitrosooxy)-1,3,5-triazine, have been optimized at the HF/6-31G* and at the MBPT(2)/6-31G* levels and characterized by HF analytical harmonic frequencies. Another CN2O3 isomer, dinitrosocarbonyl, OC(NO)2, has a low stability toward decomposition into CO and NO radicals.

Kinetics of the Reactions of NCO Radicals with NO and NH3

AbstractThe rate constants for the reaction of NCO (X2Π) radicals with NO and NH3 were measured at 20 Torr total pressure in temperature ranges of 290–1098 K and 295–882 K, respectively, using ClNCO excimer laser photolysis for NCO radical formation in combination with laser‐induced fluorescence detection of NCO. In the temperature range investigated the NCO + NO reaction exhibits a negative temperature dependence which is described by the following three parameter Arrhenius equation: , with E0 in units of kJ/mol.For the NCO + NH3 reaction the measurements exhibit a positive temperature dependence over the temperature range investigated with a slight upwards curvature. A modified three parameters Arrhenius fit provides a good description of the experimental data: , with E0 in units of kJ/mol. In addition, the rate constant of the NCO + NH3 reaction was found to be pressure independent in the range 10–193 Torr at 295 K.

Theoretical prediction of the spin-orbit splitting in the NCO, NCS, HCCO and HCCS radicals

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Kinetics of the N+NCO reaction at 298 K

R. A. Brownsword, G. Hancock and D. E. Heard, J. Chem. Soc., Faraday Trans., 1997, 93, 2473 DOI: 10.1039/A702542D

The HNCO + F reaction system: VUV-photolysis of the cyanato radical via Rydberg states

The VUV-photolysis of the cyanato radical, NCO, formed in the reaction of isocyanic acid with fluorine atoms, leads to the formation of electronically excited CN fragments in the B-state. Excitation spectra in the range 105–200 nm and dispersed fluorescence spectra at selected wavelengths show that CN(B 2 Σ +) fragments are formed below λ rmex = 145 ± 1 nm. These spectra provide the first information on some properties of the excited states of the NCO radical above 6.5 eV, resulting in the tentative assignment of a ns-Rydberg series. It is proposed that the first member, 3s, gives rise to dissociation from a bent state, while the second member, 4s, and probably all higher members are governed by linear dissociative configurations. A vibrational progression in the ν 1-mode is superimposed on the 4s Rydberg transition.

DC corona discharge influence on chemical composition in mixtures of natural gas with air and its combustion exhaust with air

Experiments in dc supplied corona discharge in natural gas + air mixture and in combustion exhaust of natural gas + air mixture were realized. The influence of discharge on CO, CO2, NOx and other minority components was studied using IR absorption spectrometry. Production of NCO radicals in gas and consequent formation of NCO containing surface layers on a plate electrode was detected. In natural gas + air mixture after about 1 min oxygen poor combustion exhaust was produced due to slow combustion by corona discharge. Occurrence of various pulses in discharge current was typical.

The effect of solid CaO on the production of NO x and N 2O in fluidized bed combustors: Studies using pyridine as a prototypical nitrogenous fuel

Pyridine is typical of the nitrogenous species found in coal which decompose during devolatilization to yield HCN. Hence, pyridine was burned in an electrically heated bed of sand fluidized by O 2 + N 2. One product, NO, is found to have a concentration proportional to that of the pyridine in the fluidizing gases. A second, N 2O, has a concentration proportional to the square of the pyridine concentration. The mechanistic implications of this are discussed; it turns out that the production of NO and N 2O in this system is different from in a homogeneous gas-phase system. In particular, it appears that CN radicals adsorbed on sand are oxidised relatively rapidly to NO in: − CN → − CNO → CO + NO. The presence of 2 wt.% CaO in the bed increases the yield of NO by up to a factor of 20 and decreases that of N 2O slightly, without changing the dependences of [NO] and [N 2O] on [pyridine] noted above. The effects of oxygen concentration and temperature are investigated. It appears that CaO promotes a new heterogeneous reaction, whereby species in the pool of HCN and CN radicals are converted relatively rapidly to NO, resulting in a lower yield of N 2O. Certainly, NH 3 is found to react with CaO to produce NO. Addition of NO to the fluidizing air when burning pyridine establishes that N 2O is produced by reaction between NO and an intermediate which is most probably the NCO radical. Another effect of the presence of CaO is to reduce emissions of CO. It appears that CO burns catalytically on a CaO surface. Finally, CaO probably catalyses the reduction of NO and N 2O in: CO + NO → CO 2 + 1 2 N 2 and CO + N 2O → CO 2 + N 2.

The amounts of NO x and N 2O formed in a fluidized bed combustor during the burning of coal volatiles and also of char

Three quite different coals were burned batchwise in electrically heated beds of sand fluidized with O 2 and N 2 at either 800 or 900°C. The coal was injected into the bed either as a single piece (1 or 2 cm in size) or as 1.0 g batches of smaller particles (either 1.4–1.7 or 2.0–2.4 mm in diameter). The concentrations of NO x (i.e., [NO] + [NO 2]), N 2O, CO, and CO 2 in the off-gases were measured as functions of time whilst the coal was burning. It proved relatively easy to separate the emission of any one of these gases into that from the initial, devolatilization stage of combustion and that from the subsequent burning of the char. The first stage of coal combustion, when the volatile matter burns, turns out to be very important. For a low rank coal, 70%–90% of all the N 2O produced appears while the coal is undergoing devolatilization. The fraction drops to 40% for a coal with a low volatile content. The figures for NO x emissions range from 55% of the NO x arising from the volatiles in a low rank coal to 25% for a coal of high rank. In general, the effects of bed temperature and coal size were much less important than a coal's volatile content. Interestingly, changing the bed temperature altered the ratio [N 2O] [NO x] in the off-gases and not the total quantity of oxides of nitrogen emitted. Lower concentrations of O 2 resulted in slightly less N 2O and NO x being produced. The rates of production of NO x and N 2O during combustion of the volatiles were found to be proportional to one another. This seems to derive from a lack of mixing of the volatiles. The fact that large (> 1 cm) coal particles float on top of a fluidized bed during devolatilization is important and, e.g., can result in large particles producing less NO x and N 2O during devolatilization than tiny particles. The observations made during devolatilization conform to effectively all the fuel-nitrogen in the volatile matter being converted to HCN. Nitric oxide is then produced most probably heterogeneously on the sand from CN radicals, which alternatively can yield NCO radicals. N 2O is generated during devolatilization by the reaction NCO + NO → N 2O + CO occurring, probably in the gas phase. As for the burning of char in a fluidized bed, the particle size (> 1 mm) here is large enough for there to be control by mass transfer of oxygen to a particle. Oxidation is thus confined to the exterior of the char. It appears that CO, from burning char, is important, together with reaction between NO and solid carbon, in converting NO to N 2. This is why the yield of NO from char-N is less than that from volatile-N.

A shock tube study of H+HNCO→H 2+NCO and the thermal decomposition of NCO

The reaction of atomic hydrogen with isocyanic acid (HNCO) to produce molecular hydrogen and the NCO radical, H+HNCO→H 2 +NCO (3a) and the thermal dissociation reaction NCO+Ar→N+CO+Ar (4) have been studied in shock-heated mixtures of HNCO dilute in argon. Quantitative time histories of the NCO radical were measured behind the shock waves using cw, narrow-linewidth laser absorption at 440 nm. The second-order rate coefficients of reactions (3a) and (4) were determined to be k 3a =5.5×10 14 exp(−13.700/ T , K) ( f =0.50, F =1.5) T =2260−3250 K k 4 =2.2×10 14 exp(−27,200/ T , K) ( f =0.40, F =1.8) T =2370−3050 K cm 3 mol −1 s −1 , where f and F define the lower and upper uncertainty limits, respectively. The results for reaction (3a) were combined with the lower-temperature results from two previous studies for the reverse reaction (−3a), resulting in the expression: k 3a =9.0×10 7 T 1.66 exp(−7000/ T , K) cm 3 mol −1 s −1 , T =591−3250 K The experimental results for k 4 were used to calculate values of the collision efficiency ( β x ) and threshold energy ( E 0 ) for reaction (4) using unimolecular rate constant theory: β x =0.020 (−65%.+100%) E 0 =61.4±1.5 kcal/mol