Thermal Decomposition of NCN3 as a High-Temperature NCN Radical Source: Singlet−Triplet Relaxation and Absorption Cross Section of NCN(3Σ)

Abstract

The potential of the thermal decomposition of cyanogen azide (NCN3) as a high-temperature cyanonitrene (NCN) source has been investigated in shock tube experiments. Electronic ground-state NCN(3Σ) radicals have been detected by narrow-bandwidth laser absorption at overlapping transitions belonging to the Q1 branch of the vibronic 3Σ+−3Π subband of the vibrationally hot 3Πu(010)−3Σg−(010) system at = 30383.11 cm(-1) (329.1302 nm). High-temperature absorption cross sections σ have been directly measured at total pressures of 0.2−2.5 bar, log[σ/(cm2 mol(-1))] = 8.9−8.3 × 10(-4) × T/K (±25%, 750 < T < 2250 K). At these high temperatures, NCN(3Σ) formation is limited by a slow electronic relaxation of the initially formed excited NCN(1Δ) radical rather than thermal decomposition of NCN3. Measured temperature-dependent collision-induced intersystem crossing (CIISC) rate constants are best represented by kCIISC/(cm3 mol(-1) s(-1)) = (1.3 ± 0.5) × 1011 exp[−(21 ± 4) kJ/mol/RT] (740 < T < 1260 K). Nevertheless, stable NCN concentration plateaus have been observed, showing that NCN3 is an ideal precursor for NCN kinetic experiments behind shock waves.