A consistent model for the thermal decomposition of NCN3 and the singlet– triplet relaxation of NCN

Abstract

AbstractThe thermal decomposition of cyanogen azide (NCN3) and the subsequent collision‐induced intersystem crossing (CIISC) process of cyanonitrene (NCN) have been investigated by monitoring excited electronic state 1NCN and ground state 3NCN radicals. NCN was generated by the pyrolysis of NCN3 behind shock waves and by the photolysis of NCN3 at room temperature. Falloff rate constants of the thermal unimolecular decomposition of NCN3 in argon have been extracted from 1NCN concentration–time profiles in the temperature range 617 K <T< 927 K and at two different total densities: k(ρ ≈ 3 × 10−6 mol/cm3)/s−1=4.9 × 109 × exp (−71±14 kJ mol−1/RT) (± 30%); k(ρ ≈ 6 × 10−6 mol/cm3)/s−1=7.5 × 109 × exp (‐71±14 kJ mol−1/RT) (± 30%). In addition, high‐temperature 1NCN absorption cross sections have been determined in the temperature range 618 K <T< 1231 K and can be expressed by σ /(cm2/mol)= 1.0 × 108 −6.3 × 104 K−1 × T (± 50%). Rate constants for the CIISC process have been measured by monitoring 3NCN in the temperature range 701 K <T< 1256 K resulting in kCIISC (ρ ≈ 1.8 ×10−6 mol/cm3)/ s−1=2.6 × 106× exp (‐36±10 kJ mol−1/RT) (± 20%), kCIISC (ρ ≈ 3.5×10−6 mol/cm3)/ s−1 = 2.0 × 106 × exp (−31±10 kJ mol−1/RT) (± 20%), kCIISC (ρ ≈ 7.0×10−6 mol/cm3)/ s−1=1.4 × 106 × exp (−25±10 kJ mol−1/RT) (± 20%). These values are in good agreement with CIISC rate constants extracted from corresponding 1NCN measurements. The observed nonlinear pressure dependences reveal a pressure saturation effect of the CIISC process. © 2012 Wiley Periodicals, Inc. Int J Chem Kinet 45: 30–40, 2013