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.

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

“…Meanwhile, reaction (1b) has been experimentally proven to be the main product channel of reaction (1) [6][7][8][9][10]. Simultaneously, experimental work on NCN reactions has been carried out [11][12][13][14][15][16][17]. Besides the photolysis of diazomethane (CH 2 N 2 ) in the presence of cyanogen (C 2 N 2 ), the photolysis or pyrolysis of NCN 3 served as a source of NCN radicals.…”

“…Since the NCN precursor molecules yield NCN in its electronically excited single state, 1 NCN, the formation of the combustion-relevant 3 NCN requires subsequent relaxation to the triplet ground state. In thermal systems, noticeable equilibrium concentrations of the singlet species can be ruled out due to a large singlettriplet splitting of about 121 kJ/mol [15]. Both from a theoretical and a practical point of view the question arises how fast the singlet triplet relaxation occurs.…”

“…To provide much needed high-temperature 3 NCN rate constant data, we have recently investigated the suitability of the thermal decomposition of NCN 3 as a 3 NCN radical source in shock tube experiments [15]. Measured 3 NCN concentration-time profiles at temperatures T < 1200 K clearly revealed induction times for 3 NCN formation on the order of 10-200 μs.…”

Shock Tube Measurements of the Rate Constant of the Reaction NCN + O2

“…Details on the employed difference UV absorption setup, which was operated with about 1 mW output power of a frequency-doubled continuous-wave ring dye laser, have been described elsewhere [19]. The observed absorption band of the triplet NCN ground state is a superposition of the [23].…”

“…The observed absorption band of the triplet NCN ground state is a superposition of the [23]. The corresponding strongly temperature dependent but nearly pressure independent absorption cross section, log (σ (base e)/(cm 2 mol −1 )) = 8.9 − 8.3 × 10 −4 × T/K, has been adopted from previous work [19]. Note that this value was recently put into question by Lamoureux et al [24] who reported a 2.6 higher value based on elaborated theoretical spectroscopic calculations referenced to the electronic transition moment obtained from zero pressure fluorescence lifetime measurements by Smith et al [25].…”