Potential-energy surfaces related to the thermal decomposition of ethyl azide: The role of intersystem crossings

The potential-energy surfaces of ethyl azide relevant to its thermal decomp osition have been studied theoretically. The geometries of minima and trans ition states on the S-0 surfaces, as well as the lowest energy points in th e seam of crossing of the triplet and singlet surfaces, have been optimized with the complete active space self-consistent field (CAS-SCF) method, and their energies, re-calculated with second-order multireference perturbatio n (CAS/MP2) theory and corrected by the zero-point energy (ZPE). The reacti on mechanism is described by the following steps: (1) CH3CH2N3--> CH3CH2N+N -2, (2a) CH3CH2N --> H-2+CH3CN; (2b) CH3CH2N --> CH3CHNH. The CN-N-2 fissio n of ethyl azide is the rate limiting step (1), leading to ethylnitrene eit her along a spin-allowed path (1a) or along an alternative spin-forbidden o ne (1b). Both 1a and 1b channels show barriers of similar heights for CN-N- 2 bond fission, Delta E=42 kcal/mol, Delta E being the energy difference be tween the minimum of the ground singlet state potential-energy surface of e thyl azide and either the singlet transition state (TS1) or the lowest ener gy point of the intersystem crossing (ISC1), respectively. The decompositio n of ethanimine formed in step (2b) has been studied as well and high energ etic transition states have been identified for its decomposition. (C) 2000 American Institute of Physics. [S0021-9606(00)30730-9].