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Notes: Ab initio configuration interaction calculations with full geometric optimization along the Cl–CN dissociation coordinate have been carried out for several electronic states of ClCN. The calculations treat all low-lying singlet and triplet states and yield the vertical transition probabilities and oscillator strengths, as well as angular dependences near the Franck–Condon (FC) region and least energy paths for several electronic states as a function of rCCl. We find that the low-lying excited states derive from three electronic configurations: π3σ*, π3π*, and σπ4σ*. The lowest excited triplet and singlet states derive from the π3σ* configuration and give rise to bent 1,3A' and 1,3A‘ states. States arising from the π3π* configuration are linear (Σ and Δ states). There is evidence of surface crossings along the reaction coordinate between triplet states arising from the π3π* configuration and those arising from the π3σ* and σπ4σ* configurations. These crossings can be induced by slight bending of ClCN causing lowering of the symmetry. The calculated vertical excitation energies are in good agreement with features of the absorption spectrum, and suggest that the A continuum of ClCN involves transitions to the 2 1A' and 1 1A‘ states. The implications to the photodissociation dynamics of the cyanogen halides are discussed.

Notes: Large scale configuration interaction calculations have been carried out for the potential energy surfaces of the four lowest singlet states of nitric acid along the dissociation paths leading to OH (X 2Π) and NO2 in its ground X 2A1 and B 2B1 states. In order to gain a detailed picture of the dissociation dynamics, correlations are drawn between the forces experienced by these species upon photodissociation and observed oscillator strengths, vector coupling parameters, relative Λ doublet intensities and available energy distribution where this data is available. The calculations are in good agreement with experiment and support the conclusion that the transition to the lowest such excited state, 1 1A‘, is a vibronic transition whose electric dipole transition polarization is in plane, i.e., that the overall vibronic state observed is of A' symmetry. Dissociation from the ground state is shown to be associated with a tumbling motion of the OH due to the predominance of quadrupolar forces at intermediate distances.

Notes: The electronic absorption spectrum of NOCl in the region 620–180 nm is assigned by using vector properties of the NO photofragment and the results of ab initio calculations at the CI level. In assigning the electronic spectrum, we take into account the recoil anisotropy, rotational alignment, and Λ-doublet populations of NO, as well as the calculated vertical excitation energies, oscillator strengths, and the nature of the orbitals involved in the transitions. In the experiments, we use expansion-cooled samples and measure the recoil anisotropy parameters from the Doppler profiles of selected NO A 2Σ+←X 2Π rotational lines. The alignment parameters and Λ-doublet populations are derived from the rotational spectra using different laser polarizations and excitation–detection geometries. The theoretical calculations treat all low-lying singlet and triplet states. The calculations yield least energy paths for the excited states, with optimized rNO and ClNO angle as a function of rClN, as well as the angular dependences of the potentials and oscillator strengths of the singlet–singlet transitions. The following assignments are proposed for the main absorption bands: (1) E band—T1(1 3A‘)←S0(1 1A'); the transition borrows intensity by mixing with remote singlet states, predominantly the 4 1A' state; (2) D and C bands—S1(1 1A‘)←S0(1 1A'); the C band corresponds to excitation of ν'1 in S1; (3) B band—S3(2 1A')←S0(1 1A'); (4) A band—S5(4 1A')→S0(1 1A'). The assignments proposed here are in full agreement with all the experimental observations and the results of the calculations. Despite the shallow minima calculated for the T1 and S1 surfaces, dissociation on all the surfaces is fast, and the implications of the results to the dissociation dynamics are discussed.

Notes: Nascent CN(X 2Σ) rovibrational and kinetic energy distributions have been obtained for the title reaction using laser induced fluorescence, and regions of the HClCN potential energy surface appropriate to the observed HCl+CN channel, as well as the HCN+Cl and HNC+Cl channels, have been calculated ab initio at the MP4/3-21G* level. The CN spectator has low internal excitation; the average fractions of the available energy deposited in CN vibration and rotation are 〈 fV(CN)〉 ∼0 and 〈 fR(CN)〉 =0.06, respectively, with the rotational distribution corresponding to a temperature of ∼950 K. Sub-Doppler resolution spectroscopy on several CN B 2Σ←X 2Σ transitions provides an average value for the fraction of the available energy appearing as center-of-mass kinetic energy, 〈 fT〉 =0.33, as well as approximate kinetic energy distributions for specific CN V,R levels. Since the kinetic energy distributions are for specific CN levels, the corresponding HCl internal energy distributions are obtained by energy conservation, and the experimental results indicate a vibrational population inversion in HCl. Product energy disposal is similar to that of many "light+heavy–heavy'' systems which exhibit repulsive energy release and which prefer end-on rather than broadsideentrance channels. The dominant reaction pathways are probably HCN+Cl and HNC+Cl, with HCl+CN being minor. Because of the competitiveness between the chemically distinct product channels, the entrance channel associated with the observed products is more stereospecifically constrained than for a similar reaction which has a single set of products. The ab initio calculations suggest that end-on encounters can be reactive, thus accounting for the observed channel. A local minimum exists for a three-center structure involving hydrogen, chlorine, and carbon, and the barrier from here to HCN+Cl is small. Broadside approaches give rise to HCN+Cl, and we therefore surmise that the observed products derive predominantly from encounters in which the H atom approaches the chlorine over a modest range of angles and impact parameters. Linear approaches at the nitrogen end of the molecule are unreactive at the energies of the present experiments. However, there is a cis transition state leading to HNC+Cl which is low enough to participate in the overall chemistry. Thus, the ab initio calculations are consistent with the experimental observations, and suggest that the title reaction is a minor channel, and that CN+HCl is produced by direct reaction via attack at the chlorine.