NCO Radical Research Articles

ChemInform Abstract: Photochemistry of BrNCO and INCO

The absorption spectra of BrNCO and INCO in the region 160–400 nm are continuous in nature, and photolysis of these molecules with pulsed excimer lasers leads to dissociation. Photolysis of BrNCO and INCO at 308 and 351 nm, respectively, produces vibrationally cold ground state NCO radicals. Weak NCO A 2Σ+→X 2Π emission results from two‐photon absorption processes at these wavelengths. Photolysis of BrNCO and INCO at 193 and 222 nm, respectively, produces intense NCO A 2Σ+→X 2Π emission. The NCO(A) is a direct one‐photon photofragment in each case, and has considerable vibrational excitation. The yield of NCO(A) from BrNCO photolysis at 193 nm is approximately 16%; the yield of NCO(A) from INCO photolysis at 222 nm is near 13%. From the production of NCO(A) at these wavelengths and the short wavelength limit of the NCO A 2Σ+→X 2Π emission, limits on the heats of formation of these molecules are found to be ΔHf(BrNCO)≥−1.7 kcal mol−1 and ΔHf(INCO)≥10.9 kcal mol−1. These results demonstrate the preference o...

Computer controlled diode laser spectrometer with a helium evaporation cryostat and spectroscopy of thev 3 vibration of NCO

A computer-controlled diode laser spectrometer for the 1200 to 2500 cm−1 spectral region is described. The spectrometer has been applied to high resolution spectroscopy of the NCO radical at 5.2 μm. The lead-salt diode lasers are cooled to their operating temperature with a temperature adjustable helium evaporation cryostat. Computer-controlled tuning procedures for the frequency tuning of the diode lasers have been developed; they are independent of tables describing the tuning characteristics of the diode lasers. 41 lines of the antisymmetric stretching-vibrationv3 of the linear NCO radical have been observed. We were able to detect vibration-rotation transitions in both2π1/2 and2π3/2 fine structure sublevels. These measurements led to the precise determination of additional molecular constants.

Photochemistry of BrNCO and INCO

The absorption spectra of BrNCO and INCO in the region 160–400 nm are continuous in nature, and photolysis of these molecules with pulsed excimer lasers leads to dissociation. Photolysis of BrNCO and INCO at 308 and 351 nm, respectively, produces vibrationally cold ground state NCO radicals. Weak NCO A 2Σ+→X 2Π emission results from two-photon absorption processes at these wavelengths. Photolysis of BrNCO and INCO at 193 and 222 nm, respectively, produces intense NCO A 2Σ+→X 2Π emission. The NCO(A) is a direct one-photon photofragment in each case, and has considerable vibrational excitation. The yield of NCO(A) from BrNCO photolysis at 193 nm is approximately 16%; the yield of NCO(A) from INCO photolysis at 222 nm is near 13%. From the production of NCO(A) at these wavelengths and the short wavelength limit of the NCO A 2Σ+→X 2Π emission, limits on the heats of formation of these molecules are found to be ΔHf(BrNCO)≥−1.7 kcal mol−1 and ΔHf(INCO)≥10.9 kcal mol−1. These results demonstrate the preference of the halogen isocyanates for dissociation to doublet fragments by scission of the nitrogen–halogen bond, in contrast to the analogous halogen azides which photodissociate to singlet or triplet fragments.

Kinetics of the reaction of NCO with ethene and oxygen over the temperature range 295–662K *

Absolute rate constants for the reaction of NCO radicals with ethene were measured as a function of temperature (T=295-652K) and pressure (10–372 Torr). The high pressure rate of the reaction of NCO with ethene was observed to decrease with increasing temperature with the best fit to the data (T=295–447K) given by the following expression: k1=3.0×10−12exp⁡(230±300)/RT)cm3molecule−1s−1 At temperatures greater than 447K a rapid reduction in the observed rate was observed, which was interpreted as a rapid decomposition of the adduct formed. In addition, an upper limit on the rate of reaction with oxygen was measured at 295K (<1×10−16 cm3 molecule−1s−1) and 662K(<5×10−17 cm3 molecule−1s−1).

Dispersed fluorescence studies of the NCO radical

Rotational transitions to excited vibrational levels of the [Xtilde] 2Π state of the NCO free radical have been studied in the gas phase by dispersion of laser induced fluorescence from the (000) level of the first excited electronic state (A 2∑+). The vibrational levels involved are the 2Π vibronic components of the (100) and (020) states which perturb each other through a Fermi resonance. Transitions from the (000) level of the A state to the (100) level of the [Xtilde] state are much stronger than those to the k 2Π(020) levels which in turn are stronger than those to the μ2Π (020) levels. Measurements have been made of the wavenumbers of 199 such transitions and the whole data set has been fitted to a model which takes the perturbing effects of Renner-Teller coupling and Fermi resonance explicitly into account. The model is more than adequate, the quality of the fit being limited by the precision of the experimental data (about ±0·15 cm-1). The positions of the k 2Π and μ 2Π components of the (020) lev...

Hyperfine coupling constants of NCO in [formula omitted] by sub-Doppler spectroscopy

Molecular constants including 14N hyperfine coupling constants of the NCO radical in the A ̃ 2 Σ +(000) state have been determined precisely by using the microwave-optical double resonance (MODR) and intermodulated fluorescence (IMF) techniques, where the A ̃ 2 Σ +(000)- X ̃ 2 Π(000) band was pumped by a dye laser. The results are B = 12 056.559(41), D = 0.0045(16), γ = 22.06(17), b = 430.4(12), c = 80.3(28), and eQq = 2.6(36), all in MHz with one standard error in parentheses. It was found that, because the A ̃ 2 Σ + state closely approximates a coupling case ( b βS ), MODR did not provide any precise value for the Fermi contact term, but this constant was determined accurately by combining the MODR spectrum with the IMF spectrum.

An observation of thev1rotation-vibration band of the NCO radical by laser magnetic resonance spectroscopy

The v 1 (‘symmetric’ stretch) fundamental of the NCO radical in the [Xtilde] 2Π state has been studied by CO laser magnetic resonance at 7·8 μm. Only a single rotational transition, R 1(3/2), is close enough to a laser line to give a magnetic resonance spectrum. By using values for the spin-orbit coupling and rotational constants from recent optical work, a value for the band origin, v 1 - 1/2α A of 1272·9707(2) cm-1 has been obtained. The orbital contribution to the magnetic moment of NCO in the (1, 0, 0) level is found to be about 20 per cent smaller than expected. Such a result is quantitatively explicable in terms of the Fermi resonance between the (1, 0, 0) and the (0, 2, 0) levels of NCO.

Kinetics of the reactions of NCO radicals with H2 and NO using laser photolysis–laser induced fluorescence

Absolute rate constants for the reaction of NCO radicals with hydrogen (k6; T=592–913K) and nitric oxide (k7;T=294–538K) were measured using excimer laser photolysis-laser induced fluorescence at 416.8 nm. The following Arrhenius expressions summarize the data: k6=1.43×10−11e(−9000±1500)/RT k7=1.69×10−11e(390±320)/RT cm3 molecule−1 s−1. The experimental technique and results are discussed, and the importance of these reactions to combustion chemistry is addressed.

Microwave spectroscopy of the NCO radical in the ν2= 02Π, ν2= 12Δ, and ν2= 22Φ vibronic states

An extensive measurement of the microwave spectrum of the NCO radical has been made on the [Xtilde](0, 0, 0) 2Π i and [Xtilde](0, 1, 0) 2Δ i states, and the spectrum in the [Xtilde](0, 2, 0) 2Φ i state was also observed for the first time. From an analysis of the observed spectrum, the rotational, centrifugal distortion, spin-rotation coupling, and hyperfine coupling constants including eQq 2, the non-axially symmetric nuclear quadrupole coupling constant, were determined to good precision. The observed vibronic dependences of the hyperfine and spin-rotation coupling constants were discussed.

Reactions of CHF (X̃ 1A') and NCO (X̃ 2Π) Radicals

AbstractInfrared multiple photon dissociation has been used to produce ground state radicals CHF(X̃ 1A') and NCO (X̃ 2Π), and their kinetic behaviour in the presence of added gases studied by time resolved laser induced fluorescence. Both kinetics and the products of reactions of CHF with NO, NO2, O and N atoms, and of NCO with NO will be discussed.

The rate and mechanism of the reaction of HCN with oxygen atoms over the temperature range 540–900 K

Absolute rate constants for the reaction of O( 3 P) with hydrogen cyanide were measured over the temperature range 574–840 K by means of a laser photolysis-chemiluminescence technique. Using a laser photolysis-laser induced fluorescence technique, the NCO radical was observed to be a major product of the reaction. Rate constants derived from the rate of increase of NCO were obtained over the temperature range 540–900 K. The rate data were fitted with the following Arrhenius expression: k =9.8×10 −12 e −8000±1200(cal/mole)/RT cm 3 molecule −1 sec −1 Transition state calculations derived from Bond Additivity Corrections to Moller-Plesset fourth order perturbation theory (BAC-MP4), are used to extend the temperature range for the rate constant and provide branching ratios for products. The calculations suggest that reaction (1a), forming NCO+H, is the dominant channel at 840 K (≥66%), while reaction (1b), forming NH+CO, is believed to account for nearly all of the remaining (≤34%) of the products at this temperature. Based upon transition state calculations, extrapolation of these data to higher temperatures suggests that (1c), forming CN+OH, should begin to become important at temperatures greater than 2000 K.

Vacuum ultraviolet photoelectron spectroscopy of transient species

The He(I) photoelectron spectrum of the NCO(X 2Π) radical, produced from the gas phase reaction of atomic fluorine with isocyanic acid, has been recorded. Of the thirteen ionic states accessible from the neutral molecule with He(I) radiation, seven have been observed and assigned using the results of ab initio ΔSCF calculations. Evidence is presented to show that the X 3Σ- NCO+ potential energy surface is significantly different from those for the a 1Δ and b 1Σ+ ionic states. The first adiabatic ionization potential of NCO is measured as (11·76±0·01) eV and this leads to an estimate of D 0(N-CO+) of (4·40±0·16) eV. From the observed spectra predictions are made of possible electronic transition energies in NCO+.

Microwave spectroscopy of the NCO radical in thev2= 12Σ state

The microwave spectra of the NCO radical in the (0, 1, 0) μ 2Σ and (0, 1, 0) κ2Σ states have been observed by using a source-frequency modulation spectrometer. The radical was generated by the reaction of HNCO with microwave discharge products of CF4. The observed spectra were analysed using a hamiltonian that included the interactions of the (0, 1, 0) state with (1, 1, 0), (0, 1, 1) and (0, 3, 0), and the rotational, spin-rotation, centrifugal distortion, rovibronic interaction and hyperfine coupling constants were determined precisely for the (0, 1, 0) μ 2Σ and (0, 1, 0) κ2Σ states. From the rovibronic interaction constants two anharmonic potential constants W 1 and W 2 were calculated to be 29·5 (1) and 3·3 (1) cm-1, respectively, with three standard errors in parentheses. The effective B rotational constant in the (0, 1, 0) 2Σ state was found to differ by 1·99 (12) MHz from that in the (0, 1, 0) 2Δ state previously reported. This difference was accounted for by higher order rovibronic interactions; an algebraic expression was derived for the difference. This formula was applied also to BO2, CNC, CCC and CO2 +. For BO2 and CNC the calculated values agreed well with the observed values, whereas discrepancies remained for CCC and CO2 +.

Intracavity laser excitation of NCO fluorescence in an atmospheric pressure flame

Laser excited fluorescence of the NCO radical has been obtained using discrete prism selected lines of an argon ion laser pump source. To our knowledge this is the first time NCO fluorescence has been obtained in a flame environment. NCO was formed in a slightly rich atmospheric pressure CH4/N2O flame. This flame was placed inside the extended cavity of the argon laser to take advantage of the much higher light intensity levels. All of the available laser lines pump vibrational hot bands of the NCO A 2Σ+ ← X 2Π system. The 4658 Å line appears to be the most useful for probing NCO densities. This line pumps in the A 2Σ+(0,00,0) ← X 2Π (1,01,0) vibrational band. NCO is pumped to N′ = 31 by this line, probably via the Q231 transition although the R230 and P232 transitions could not be ruled out in the present analysis. The 4658 Å line was used to determine a relative NCO density profile through the reaction zone of a CH4/N2O flame. Profiles of C2, CN, and temperature were also obtained in this flame and are compared with the NCO profile. A lower limit of approximately 3×1014 cm−3 was placed on the peak NCO density in the flame. Attempts to find NCO or CN fluorescence in a CH4/air flame failed indicating probable differences in nitrogen chemistry for the two flames.

The laser magnetic resonance spectrum of the NCO radical at 5.2 μm

Lines in the ν 3 (“antisymmetric” stretch) fundamental of the NCO radical in the X ̃ 2 Π state were studied by CO laser magnetic resonance. The observations were assigned to P and R lines in the vibration-rotation band and lead to a precise determination of the vibrational interval and the anharmonic correction to the rotational constant: ν 3 = 1920.60645(19) cm −1, α 3 = 0.003338(21) cm −1. A single transition in the hot band (011)-(010), 2 Δ 5 2 - 2 Δ 5 2 was detected. This observation is used to determine the origin of the hot band as 1907.11892(20) cm −1, i.e., the anharmonicity parameter x 23 = −13.48753(28) cm −1.

Semiempirical all-valence-electron MO calculations on the electronic spectra of linear radicals with degenerate ground states

INDO-CI calculations are presented for CH, CF, NO, O2+, CCN, CNC, C2+, F2+, HF+, OH, CO2+, BO2, N3, N2O+, NCO, and diacetylene cation radicals. The results give a good account of transition energies for valence electronic states. The limits and merits of the method are briefly discussed.

Hyperfine Interactions of the Free NCO Radical in the Δ Vibronic State (v2 = 1)

Hyperfine Interactions of the Free NCO Radical in the Δ Vibronic State (<i>v</i>2 = 1)

Reactions of cyanogen radicals. Part 2.—Reactions with (CN)2 and O2

The gas phase reactions of CN radicals with (CN)2 and O2 have been studied as a function of temperature and vibrational excitation of the CN. The first reaction has Eact≃ 13.1 kJ mol–1 and only inefficient vibration → translational transfer occurred with the excited CN levels. The reaction with O2 has no activation energy, but the rate constants increase with CN vibrational level. NCO radicals were produced more rapidly than CN(X2∑; 0, 0 band) radicals were removed, indicating that vibrational excitation may contribute to the overall kinetics of this system.

Microwave spectrum of the NCO radical

The rotational spectrum of the linear triatomic radical NCO has been observed by means of a Stark modulated microwave spectrometer. The NCO radical was produced by mixing HNCO with the products of a microwave discharge in CF 4. The observed rotational transitions J = 5 2 ← 3 2 , and 7 2 ← 5 2 in the 2 Π 3 2 state and J = 5 2 ← 3 2 in the 2 Π 1 2 state have been analyzed by using Hougen's formulation for a linear triatomic molecule in a 2Π state. The rotational constant, the centrifugal distortion constant, the Λ-type doubling constant, the hyperfine constants and the dipole moment have been determined. Consideration of the Λ-type doubling constant leads to the conclusion that the upper electronic state perturbing the 2Π state is A ( 2 Σ +) .

THE PHOTOLYSIS OF ISOCYANIC ACID VAPOR

The photolysis of isocyanic acid vapor has been studied at temperatures ranging from −31 to 200 °C, and pressures from about 2 to 200 mm. Products identified were CO and N2 in a ratio which varied from 2.5 to 5, together with much smaller amounts of hydrogen. The mechanism[Formula: see text]accounts approximately for the products of the photolysis at low HNCO pressures and low temperatures. Additional reactions of NCO radicals with HNCO are postulated to account for increased production of CO and N2 as the temperature and HNCO pressures were increased.The effects of adding ethylene and hydrogen to the system were briefly investigated. The results are explained in terms of reactions of NH and NCO radicals with hydrogen and ethylene.