CASSCF and CEPA calculations for the photodissociation of hydroazoic acid. 2. Photodissociation into nitrogen and imidogen on the lowest 1A'' surface of hydroazoic acid

Abstract

6111 Figure 11. Pictorial view of the vs in-plane and v6 out-of-plane bending motions which are responsible for fragment rotation.be seen, the distribution is very sharply centered around the mean impact parameter listed in Table 11. Thus, probably only a small range of the PES is involved in the fragmentation process.S,o far, only information concerning scalar properties and the (8-J) correlation have been used to discuss the dissociation dynamics on the upper PES. The spatial distribution of the fragments is given by the bw parameter, which is a quantitative measure of the (ji-8) correlation. At low NH rotations the bw parameter is close to the limit of -0.5, which indicates a perpendicular alignment between I; and 8. Since for a A"-A' transition the dipole moment I; is perpendicular to the symmetry plane of the parent, one would expect this negative value for a planar dissociation geometry. The deviation of Ow from -0.5 is essentially caused by initial parent rotation about the axis that is in the plane of HN3 but perpendicular to the N" chain. Since the N2 partner products of those low rotationally excited NH fragments are generated at high JN2 (see Figure 2), the PES must be strongly dependent on the in-plane bending angle (Y"N. Therefore, the high N2 rotational motion should take place in the kitial plane of the parent and the alignment of the rotational vector JY2 should be parallel to the transition dipole moment E;. Indeed, this alignment at high JN2 was observed in the dissociation of HN3 at 283 nm.I8At high NH rotations, the HN3 system behaves diff%ently. The correlations between I; and 8 as well as between 8 and JNH become positive. Thus, a strong out-of-plane motion of the N-N-N chain must be responsible for fragment rotation, and the PES must also depend on the angle (Y'"". For a better visualization a cartoon of both types of motion is shown in Figure 11. NH products formed at high JNH correlate with N2 fragments at lower N2 rotations. (JN2 is still very high, but lower than the mean N2 rotation.) Therefore, one expects that the alignment of lower JN with respect to I; should be less positive than the alignment of higher JN2. Again, this behavior is observed in the experiment.18Roughly the same amount of NH product rotation is generated either via an @-plane or an out-of-plane motion as can be seen from the ( W ) correlation and Table 11. Thus, the torques d V /~3 a "~ and dV/da-should be similar. In order to generate the high N2 rotation, the potential energy difference between the configuration corresponding to the equilibrium geometry (of the ground state) and the minimum of the upper PES with respect to CY"N and CY'"" should be in the order of 1 eV. A "phase shift" in the induced cotational motion should occur for a generation of a positive ( G J ) correlation, i.e., a torsional motion of the NH rotor. Therefore, it seems to be more likely that the upper PES is constantly steep with respect to CY"N for all N2-NH separation distances, while the gradient with respect to CY'"" should be small in t...

The H+N2O→OH(2ΠΩ,υ′,N′)+N2 reaction: OH rotational angular momentum polarization

“…Torsional forces are still important in generating these higher OH angular momentum states, but NOH bending torques also appear to play some role. Gericke and co-workers 50 and Meier and Staemmler 52 have proposed, in the context of the molecular photodissociation of HN 3 , that a phase lag between the in-plane NNN and out-of-plane NNN bending motions is responsible for the torsional excitation in the dissociating HN 3 molecule, and a similar explanation would account for the behavior of the OH products of the HϩN 2 O bimolecular reaction. FIG.…”

“…Fortunately, such a comparison is made possible because of the detailed experiments carried out on the photodissociation dynamics of HN 3 by Gericke and co-workers, [49][50][51] and by the theoretical work of Meier and Staemmler. 52 In both systems, the OH/NH products are born rotationally and vibrationally cold, 4,49 with average excitation energies which depend little on available energy. Rotational excitation in the NH photofragment arises predominantly from torsional forces in the dissociating HN 3 species [49][50][51] ͑cf.…”

Nitrenes as intermediates in the thermal decomposition of aliphatic azides

“…The smallest active space to obtain correct results is composed of 10 electrons occupying 8 orbitals 11, 18. Thus, all the CASSCF calculations have been performed with such an active space.…”

UV Photodissociation Dynamics of HN3 in 190-248 nm

“…These absorption features are broad and nearly structureless. The weak feature at λ>220 nm has been assigned to AA ←XA [2,3,5]. At shorter wavelengths, the BA and CA electronic states are optically accessible, and the resulting UV absorption spectrum is strong, but no discrete structure is present.…”