An integrated interpretation of axomethane photodissociation dynamics
Although it has been known for more than 60 years the azoalkanes (R-N=N-R) dissociate under the influence of light or heat, few definitive conclusions have been reached on the mechanism of their photodissociation. The issue of whether the photodissociation is concerted or stepwise has been settled through kinetic resolution of the process into two steps, implying a methyldiazenyl radical intermediate. The author's ab initio CASSCF quantum chemical calculations confirm that stability of this methyldiazenyl radical, and the dissociative transition state on its ground state ([sup 2]A[prime]) surface has been located. A barrier height of 410 cm[sup [minus]1] has been found for this transition state, which leads to the dissociation into methyl radical plus N[sub 2]. The lowest energy path for this decomposition has been calculated using the CASSCF method with a 6-31G* basis set including nine electrons in nine active orbitals. Methyl-diazenyl's [nu][sub 5] mode (651 cm[sup [minus]1]) correspond closely to the dissociation coordinate. UHF calculations have revealed that the lowest-lying triplet state of azomethane has a perp configuration at its equilibrium geometry. Energies and methyldiazenyl vibration frequencies obtained from the quantum calculations have been combined with existing thermochemical data as input to an energy disposal analysis that includes both statistical and impulsive modelling of product state energy distributions measured with time-resolved coherent anti-Stokes Raman spectroscopy (CARS). The first dissociative step may have significant impulsive character. In the second step, the experimental nitrogen vibrational distribution is in excellent agreement with the prediction of the separate; statistical ensemble (SSE) model. Further experimental and theoretical research, particularly on the first step, will be required before an adequate understanding of azomethane photodissociation can be achieved.
- Research Organization:
- Rice Univ., Houston, TX (United States)
- OSTI ID:
- 7164685
- Country of Publication:
- United States
- Language:
- English
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Related Subjects
400500* -- Photochemistry
ALKANES
AZO COMPOUNDS
CHEMICAL REACTION KINETICS
CHEMICAL REACTIONS
CHEMISTRY
DECOMPOSITION
HYDROCARBONS
KINETICS
METHANE
ORGANIC COMPOUNDS
ORGANIC NITROGEN COMPOUNDS
PHOTOCHEMICAL REACTIONS
PHOTOLYSIS
PHYSICAL CHEMISTRY
RAMAN SPECTRA
REACTION KINETICS
SPECTRA