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The photodissociation spectroscopy and dynamics of the HNCN free radical have been investigated by fast beam photofragment translational spectroscopy. Predissociative transitions for both the B˜ [sup 2]A[sup ′]←X˜ [sup 2]A[sup ″] band and a higher-energy band system assigned to the C˜ [sup 2]A[sup ″]←X˜ [sup 2]A[sup ″] band were observed. Photofragment mass distributions indicate that N[sub 2] loss is the primary dissociation pathway. Translational energy distributions reveal a resolved vibrational structure of the N[sub 2] fragment, suggesting that the HNCN radical first isomerizes to a cyclic HCN[sub 2] intermediate. A dissociation mechanism is proposed in which electronically excited HNCN undergoes internal conversion to the ground state, followed by isomerization to cyclic HCN[sub 2] and dissociation through a tight three-center transition state. The HNCN bond dissociation energy D[sub 0] and heat of formation Δ[sub f]H[sub 0](HNCN) were determined to be 2.80±0.03 eV and 3.35±0.03 eV, respectively. © 2001 American Institute of Physics. [ABSTRACT FROM AUTHOR]

The spectroscopy and photodissociation dynamics of the A˜ [sup 3]Π and B˜ [sup 3]Σ[sup -] states of the CNN radical have been investigated by fast beam photofragment translational spectroscopy. Vibronic transitions located more than 1000 cm-1 above the A˜ [sup 3]Π←X˜ [sup 3]Σ[sup -] origin were found to predissociate. Photofragment yield spectra for the B˜ [sup 3]Σ[sup -]←X˜ [sup 3]Σ[sup -] band between 40 800 and 45 460 cm-1 display resolved vibrational progressions with peak spacing of approx. 1000 cm-1 corresponding to symmetric stretch 1[sub 0][sup n] and combination band 1[sub 0][sup n]3[sub 0][sup 1] progressions. Ground state products C([sup 3]P)+N[sub 2] were found to be the major photodissociation channel for both the A˜ [sup 3]Π and B˜ [sup 3]Σ[sup -] states. The translational energy distributions for the A˜ [sup 3]Π state are bimodal with high and low translational energy components. The distributions for the B˜ [sup 3]Σ[sup -] state reveal partially resolved vibrational structure for the N[sub 2] photofragment and indicate extensive vibrational and rotational excitation of this fragment. These results suggest that bent geometries are involved in the dissociation mechanism and provide more accurate values: Δ[sub f]H[sub 0](CNN)=6.16±0.05 eV and Δ[sub f]H[sub 298](CNN)=6.15±0.05 eV. These values, coupled with recent D[sub 0](RH) and D[sub 298](RH) values from Clifford et al. [J. Phys. Chem. 102, 7100 (1998)], yield Δ[sub f]H[sub 0](HCNN)=5.02±0.18 eV, Δ[sub f]H[sub 298](HCNN)=4.98±0.18 eV, Δ[sub f]H[sub 0](H[sub 2]CNN)=3.09±0.21 eV, and Δ[sub f]H[sub 0](H[sub 2]CNN)=3.09±0.21 eV. © 2000 American Institute of Physics. [ABSTRACT FROM AUTHOR]