Spectroscopic and Kinetic Investigation of Methylene Amidogen by Cavity Ring-Down Spectroscopy Boris Nizamov ² and Paul J. Dagdigian* Department of Chemistry, The Johns Hopkins UniVersity, Baltimore, Maryland 21218-2685 ReceiVed: October 7, 2002; In Final Form: January 24, 2003 Cavity ring-down spectroscopy (CRDS) has been used to study room-temperature chemical reactions of the methylene amidogen radical (H 2 CN). The radical was prepared by 193 nm photolysis of formaldoxime, H 2 - CNOH f H 2 CN + OH. CRDS signals from both H 2 CN and OH [A - X (1,0) band] were observed in the wavelength region 278-288 nm. By comparison of the strengths of the OH and H 2 CN signals, the oscillator strength of H 2 CN electronic transition in the 279-288 nm wavelength region was measured to be 4.5 × 10 -4 . To correct for the loss of the OH signal due to reactions of OH, the room-temperature rate constant for the reaction of OH with formaldoxime was measured, k(H 2 CNOH + OH) ) (1.5 ( 0.4) × 10 -12 cm 3 molecule -1 s -1 . Reaction of H 2 CN with a number of stable molecules [O 2 ,C 2 H 4 , CO, CH 4 ,H 2 ] could not be observed, and an upper limit to the reaction rate constants, < 1 × 10 -15 cm 3 molecule -1 s -1 , was derived. Self-recombination was the main removal process for the H 2 CN radical under the conditions of the experiment, with the rate constant k(H 2 CN + H 2 CN) ) (7.7 ( 2.5) × 10 -12 cm 3 molecule -1 s -1 . At high photolysis laser energies, for which the H 2 CNOH fractional dissociation was high, it was possible to study the reaction of H 2 CN with OH. A value of the rate constant for the OH + H 2 CN reaction, k(OH + H 2 CN) ) 6 × 10 -12 cm 3 molecule -1 s -1 , was derived. 1. Introduction The methylene amidogen radical (H 2 CN) is an important intermediate in a number of chemical processes. 1 It is an intermediate in the thermal decomposition of nitramine propel- lants, such as HMX and RDX. 2-4 This radical is also believed to play a role in the combustion chemistry of hydrocarbon flames containing nitrogen (through the N + CH 3 reaction 5,6 ), in the reburning of NO (through the CH 3 + NO reaction 7 ), and in the chemistry of the atmosphere of Titan and in interstellar clouds. 1,8 The H 2 CN radical has been investigated using a variety of experimental methods, including ESR spectroscopy, 1,9 flash photolysis, 10,11 IR and UV absorption in cryogenic matrixes, 12 photoionization studies, 13 and H-atom photofragment transla- tional energy spectroscopy. 14 The UV absorption bands of H 2 - CN near 280 nm were first observed some years ago in the flash photolysis of formaldazine (H 2 CN-NCH 2 ) and formal- doxime (H 2 CNOH) by Horne and Norrish 11,15 and Ogilvie and Horne. 10 The 193 nm photodissociation of formaldazine has been studied by molecular beam photofragment translational spec- troscopy with mass spectrometric detection of the H 2 CN fragments. 16 The UV absorption spectrum of H 2 CN trapped in solid argon was later reported by Jacox. 17 Dagdigian et al. 18 attempted to detect H 2 CN and its methylated analogues by laser fluorescence excitation of the bands near 280 nm with 193 nm photolytic preparation from various precursors (mainly the oximes). While the OH fragment from the photodissociation of the oximes was readily detected, they concluded that the fluorescence quantum yield for H 2 CN was negligible and that the excited state decays by predissociation. In a molecular beam study, Davis and co-workers 14 recently observed the bands of the H 2 CN radical near 280 nm through Rydberg atom detection of the H atom fragment from predis- sociation of electronically excited H 2 CN. The H 2 CN radical was prepared in a molecular beam by pyrolysis of formaldazine at the beam orifice. They also measured the kinetic energy distribution of the H atom fragments and thus estimated the internal excitation of the HCN cofragment, which was found to contain a wide distribution of internal excitation energies. This work showed that H 2 CN could be detected by laser-based methods; however, the particular scheme employed is not particularly amenable to kinetic and spectroscopic studies. In earlier work, Steif and co-workers 1,5,6 employed electron- impact ionization detection to study the kinetics in flow-tube experiments of some reactions involving the H 2 CN radical. They prepared H 2 CN by the reaction of N atoms with methane, and H 2 CN was detected by a mass spectrometer, with electron- impact ionization, through a sampling port. There were some problems associated with this method. In particular, vibrationally excited N 2 was also present in these experiments and interfered with the detection of H 2 CN since both species have the same mass-to-charge ratio (m/e ) 28). This interference could be alleviated by using very low electron energies (∼12 eV), but at the expense of a very poor signal-to-noise ratio. The use of deuterated reagent to generate D 2 CN allowed higher electron energies since the background at m/e ) 30 was much less. This group studied the reactions of N and H atoms with methylene amidogen, 19 as well as the formation of H 2 CN in the N + CH 3 reaction. 5,6 The determination of the rate constant for the N + H 2 CN reaction was complicated by the fact that N atoms are involved in both the formation and destruction of H 2 CN and that the rates of these processes are comparable. As an alternative ionization method to electron impact, photoionization can be applied to the detection 13 of H 2 - * Corresponding author. ² Present address: Department of Chemistry, The University of Cali- fornia, Berkeley, CA 94720. 2256 J. Phys. Chem. A 2003, 107, 2256-2263 10.1021/jp022197i CCC: $25.00 © 2003 American Chemical Society Published on Web 03/12/2003