The rate constant for the reaction of the cyanato radical, NCO(X2Pi), with the methyl radical, CH3(X2A2' '), has been measured to be (2.1 +/- 1.3(-0.80)) x 10(-10) cm3 molecule(-1) s(-1), where the uncertainty includes both random and systematic errors at the 68% confidence level. The measurements were conducted over a pressure range of 2.8-4.3 Torr of CH4 and at a temperature of 293 +/- 2 K. The radicals were generated by the 248-nm photolysis of ClNCO in a large excess of CH4. The subsequent rapid reaction, Cl + CH4, generated the CH3 radical. The rate constant for the Cl + CH4 reaction was measured to be (9.2 +/- 0.2) x 10(-14) cm3 molecule(-1) s(-1), where the uncertainty is the scatter of one standard deviation in the data. The progress of the reaction was followed by time-resolved infrared absorption spectroscopy on single rovibrational transitions from the ground vibrational level. Multiple species were detected in these experiments, including NCO, CH3, HCl, C2H6, HCN, HNC, NH, and HNCO. Temporal concentration profiles of the observed species were simulated using a kinetic model, and rate constants were determined by minimizing the sum of the squares of the residuals between experimental observations and model calculations. Both HCN and HNC seem to be minor products (<0.3% each) of the NCO + CH3 reaction. The peak concentrations of NH and HNCO were small, accounting for <1% of the initial NCO concentration; however, their temporal profiles could not be fit by the model kinetics. The observed C2H6 temporal profile always peaked at significantly higher concentrations than the model predictions, and several reaction models were constructed to help explain these observations. The most likely product channel seems to be the recombination channels, producing CH3NCO and CH3OCN.

The rate constant for the reaction of the cyano radical, CN(X(2)Sigma), with the hydroxyl radical, OH(X(2)Pi), has been measured to be (1.4 +/- 0.48) x 10(-10) cm(3) molecule(-1) s(-1) at a temperature of 292 +/- 2 K. The error estimate includes both systematic and random error at the level of one standard deviation. The measurements were carried out by simultaneously monitoring the temporal dependencies of the CN and OH radical concentrations on the same photolytic laser pulse that created the transient species. The rate constant was determined by two independent methods of data analysis: one based on the simulation of CN and OH concentration profiles using a detailed kinetic mechanism and the other based on a new integrated-profiles analysis.

The relative rates for the reaction of translationally energetic H atoms with XCN to yield HNC + X, HCN + X, and HX + CN, where X is Br or Cl, have been determined using time- and frequency-resolved absorption spectroscopy. The H atoms were created with an average translational energy of 92 kJ mol-1 by the laser photolysis of CH3SH at a wavelength of 248 nm. The measurements were carried out at a total pressure of several torr. The dominant channel for each reaction system was the production of hydrogen isocyanide, HNC. The relative branching ratios for H + BrCN and H + ClCN were determined to be 0.85/0.13/0.017 and 0.74/0.25/0.01 for the HNC/HCN/HX + CN channels, respectively. The ratio of the total reaction cross sections, σXCN, was found to be σBrCN/σClCN = 0.84.

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) x 10(-10) cm(3) 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.

The dynamics of the reactions of translationally energetic H atoms with BrCN, ClCN, and (CN)2 was studied by determining both the rotational state distribution and the translational energy disposition of the CN product ground vibrational level. The reaction was carried out using H atoms with a most probable translational energy of 92 kJ mol−1. The CN radical was monitored by time- and frequency-resolved absorption spectroscopy using the CN red system (A 2Π←X 2Σ) (2,0) band near 790 nm. Sub-Doppler resolution spectroscopy was used to determine the initial translational temperature of the CN(0,J) product. The fraction of the available reaction exothermicity that appeared as CN(0) rotational energy, fR, for H+XCN→HX+CN was 0.034±0.006, 0.061±0.02, and 0.13±0.007, for X=Br, Cl, and CN, respectively. Likewise, the fraction of the available reaction exothermicity that appeared as relative product translational energy, fT, was 0.52±0.25, 0.52±0.20, and 0.59±0.05, for X=Br, Cl, and CN, respectively. The absolute reaction cross sections for the H+XCN→HX+CN reactions were also measured to be 0.03, 0.02, and 0.3×10−16 cm2 for X=Br, Cl, and CN, respectively.

The integrated-absorption coefficients of several hyperfine lines of the magnetic dipole allowed transition of the bromine atom, Br, center at 3685.2 cm(-1) were measured, and a value for the square of the magnetic dipole transition moment of the Br atom was determined. A theoretical calculation for the magnetic dipole transition moment was also carried out using a relativistic ab initio atomic structure formulation. The theoretical value was in excellent agreement with the value predicted assuming pure LS coupling, and in reasonable agreement with experiment. The Br atom was generated in equal concentration with the cyano radical (CN) by the 193 nm photolysis of cyanogen bromine, BrCN. The CN radicals were titrated by the rapid reaction with C3H8 to generate HCN and a small amount of HNC. Both time-resolved and frequency-scanned infrared absorption spectroscopy were used to monitor the Br, HCN, and HNC species. The photolysis of BrCN at 193 nm produced both the ground state Br(P-2(3/2)) and the spin-orbit excited Br(P-2(1/2)) atoms, and the yield for the production of Br(P-2(1/2)) atoms was measured to be 0.31+/-0.01. The rate constants for the quenching of Br(P-2(1/2)) by BrCN and C3H8 at 293 K were also determined. [S0021-9606(99)01416-6].

The reaction of the cyano radical (CN) with hydrogen was studied by time-resolved infrared absorption spectroscopy of individual rovibrational states of HCN. The initial vibrational level distribution of HCN(v(1)0v(3)) was determined by plotting the time dependence of the fractional population of a vibrational level and extrapolating these curves to the origin of time. The experiments were carried out at two temperatures, 293 and 324 K, with similar results. It was estimated that about 50% of the available reaction exothermicity was deposited as vibrational excitation of the HCN product. Surprisingly, the HCN(101) vibrational level received a significant fraction of the observed vibrational population, implying that the CN vibration was not really a spectator bond in the reaction dynamics. Furthermore, the observed HCN(v(1)0v(3)) vibrations only account for about 27% of the initial HCN population produced in the title reaction. A significant fraction of the product HCN molecules must have been produced with the bending mode excited, likely in combination with the H-C stretch vibrations. (C) 1998 American Institute of Physics.

The reaction rate constant for the cyano (CN) radical with hydrogen and deuterium has been determined over the temperature range 293-380 K. The CN radical was detected by time-resolved near-infrared absorption using the CN red system (A2 Pi <-- X(2)Sigma) (2,0) band near 790 nm. These measurements were carried out at low pressures of Ar or He as carrier gas. The diffusion rate of CN in these mixtures was inferred from the diffusion rate of HCN(000) determined using time-resolved infrared absorption of HCN(000) around 3.0 mu m, simultaneously with the detection of CN. These measurements provide accurate thermal rate constant data that will enable a detailed comparison to be made between theoretical predictions and experimental measurements for this prototypical reaction system.

The title reaction has been studied in a crossed molecular beam apparatus. Both the product state distributions and the translational energy dependence of the reaction cross sections were measured under single collision conditions. Excellent agreement was found over a wide temperature range (26–3800 K) between rate constants deduced from the translational excitation function and recent thermal kinetic data. The rotational state distribution was found to be very cold compared to the reaction exothermicity, and could be described by a Boltzmann temperature of 110 K for all K-doublet levels. The vibronic state distribution was also found to be cold, with 70% of the products formed in the vibrational ground state. By comparing the molecular beam results for vibronic state distributions with those obtained from recent bulb experiments, it was conjectured that there appears to be a strong correlation between rotation in the reactants and bending excitation in the products.

The reaction of the cyano radical (CN) with ethane (C2H6) was studied using time resolved infrared absorption to monitor the product hydrogen cyanide (HCN) in individual ro-vibrational states. Pulse laser photolysis was used to provide an initial excess of the CN radical and the time dependence of individual ro-vibrational states of the high frequency antisymmetric stretching mode of HCN (0,0,v3) was followed. These experiments reveal that the initial product state distribution of HCN is not highly excited in the HCN(0,0,v3) vibrational manifold.

State-to-state collision dynamics of molecular radicals were investigated by the laser-induced fluorescence technique in a pulsed, crossed-beam apparatus. Dramatically different product state distributions were observed for two prototypical radicals, NCO(X2$PI) and CH(X2$PI). Based on a quantum scattering formalism and general considerations of the potential energy surfaces these observations were interpreted as generic features for the inelastic scattering of 2$PI radicals. The differences observed for NCO and CH are the results of well-known Hund's coupling classification of linear molecules.

State-resolved cross sections for the inelastic scattering of the linear triatomic radical NCO(X approximately 2II3/2) by He have been measured in a pulsed, cross-beam apparatus at a collision energy of 3.74 kcal mol-1. Dramatically different rotational state distributions were found depending on whether or not the spin-orbit state of NCO was preserved in the collision. It is proposed that the observed different distributions are generic to the inelastic scattering of a Hund's case (a) 2II radical, arising naturally as a dynamical consequence of the Renner-Teller effect.

State-resolved cross sections for the inelastic scattering of OH(X2-PI) with CO and N2 have been measured in a crossed molecular beam apparatus over the energy range from 0.5 to 5.5 kcal mol-1. The magnitude and shape of the excitation functions for the same OH final states were virtually independent of the two targets except near threshold. Both spin-orbit conserving and spin-orbit changing transitions have been investigated. Cross sections for spin-orbit changing transitions of large DELTA-N were approximately the same magnitude as those for spin-orbit conserving transitions. For small DELTA-N, however, the spin-orbit conserving transitions became more favorable. The LAMBDA-doublet ratio, PI(A')/PI(A"), in the PI-3/2 (F1) manifold differed from unity only for the N2 target for which the ratio became increasingly larger than unity as N' increased. The observed fine-structure effects were nearly the same as those from the corresponding (reverse) reactive collisions, and a close connection between the inelastic and reactive events has been conjectured.

The state-to-state integral cross sections for the inelastic scattering of CH(X 2Π) with He were measured in a newly constructed crossed molecular beam machine. Use of laser-induced fluorescence in an unconventional flux mode of detection provided single fine-structure state specific detection of the products. Two types of measurements were performed to further our understanding of the collision dynamics of open shell systems: (1) the product state distribution at a fixed and well-defined collision energy and (2) the dependence on collision energy of product state-resolved cross sections. A qualitative understanding of the collision dynamics can be obtained by properly factoring out features dependent on the fine-structure states, i.e., effects involving individual Λ-doublet states and features dependent on the rotational level alone, i.e., effects remaining after summing over all four fine-structure states associated with a given rotational quantum number. As for the fine-structure effects, a preferential population of product Λ-doublet states with reflection symmetry Π(A″) was observed. The physical origin of this observed electronic orbital alignment can be attributed to a quantum interference phenomenon, as detailed in the accompanying paper. At the rotational level, the dominance of rotational rainbow scattering is unambiguously identified from both the existence of dynamical thresholds and a strong correlation between rotational level distributions at fixed translational energy and level specific excitation functions. These effects combined with other experimental observations lead us to visualize the CH+He scattering dynamics in a novel fashion. The collision can be regarded as a series of approximately independent sequential events each mediated by different regions of the interaction potential during the course of the whole encounter.

The title reaction was studied in a new crossed molecular beam apparatus. Laser-induced-fluorescence was used to characterize both the reactant CH radical and the product CD radical in a state-resolved manner. Two types of dynamical information are reported here: (1) the excitation function for a specific reaction product state and (2) the product rotational state distribution at a well-defined collision energy.