An integrated interpretation of azomethane photodissociation dynamics

[摘要] Although it has been known for more than 60 years that 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. Recently, 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. Our ab initio CASSCF quantum chemical calculations confirm the stability of this methyldiazenyl radical, and the dissociative transition state on its ground state ($sp2$A$spprime$) surface has been located. A barrier height of 410 cm$sp{-1}$ has been found for this transition state, which leads to the dissociation into methyl radical plus N$sb2$. 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. Methyldiazenyl's $usb5$ mode (651 cm$sp{-1}$) was found to correspond closely to the dissociation coordinate. In addition, UHF calculations have revealed that the lowest-lying triplet state of azomethane has a perp configuration at its equilibrium geometry, suggesting that first-step photodissociation may occur from the T$sb1$ surface. Energies and methyldiazenyl vibrational 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). It appears that the first dissociative step may have significant impulsive character. In the second step, although some findings fall outside the range spanned by the impulsive and statistical models, 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.

[发布机构] Rice University

[效力级别] chemistry