Experimental and Theoretical Study of the Electronic Spectrum of the Methylene Amidogen Radical (H₂CN):  Verification of the ²A₁ ← ²B₂ Assignment

Abstract

A collaborative experimental and theoretical study of the electronic spectrum and excited-state photochemistry of H(2)CN has been carried out. The absorption spectrum, in the range of 287-278 nm, was measured through cavity ring-down spectroscopy. The radical was prepared by 193 nm photolysis of monomeric formaldoxime vapor. Two diffuse features were observed in the 34800-35800 cm(-1) spectral range, along with the A-X (1,0) band of the OH cofragment. The broad features were assigned through high-level ab initio calculations as vibronic transitions to the ground and 2b(1) (umbrella mode) levels of the second excited B (2)A(1) state from the ground X (2)B(2) state of H(2)CN. Rotational constants for the lower and upper levels of these transitions were computed from the expectation values of the moments of inertia tensor, using the appropriate vibrational wave functions. Experimental and simulated rotational profiles of these bands agree extremely well with each other for an assumed type-B electric dipole-allowed (2)A(1) <-- (2)B(2) transition appropriate to this transition. The former assignment to the dipole-forbidden (2)B(1) <-- (2)B(2) transition can be ruled out by these results. A theoretical investigation of the dissociation pathways for electronically excited H(2)CN is also presented. The upper states of the observed bands cannot dissociate directly but rather decay through internal conversion and subsequent dissociation to H + HCN fragments; higher b(1) levels are above the excited-state dissociation limit.

Photoinduced C–H bond fission in prototypical organic molecules and radicals

“…265 Ab initio theory confirms that the photoprepared B 2 A1 state of H2CN correlates with an excited state of HCN and that the observed fragmentation proceeds via non-adiabatic coupling to the ground state PES. 266…”

Neutral production of hydrogen isocyanide (HNC) and hydrogen cyanide (HCN) in Titan’s upper atmosphere

“…For the evaluation of chemical rate constants between 150 K and 200 K the presence of an energetic barrier is critical. When no information was available, the presence and the values of any energetic barriers to the entrance valley for the important reactions (in terms of production and/or loss rate) were calculated at the M06-2X/cc-pVTZ level using the Gaussian09 software package (Frisch et al 2009) Teslja et al (2006) using the Molpro software package (Werner et al 2010). For the other reactions, identified as being less important, the presence or the absence of a barrier was deduced from general considerations.…”

H2CN+ and H2CNH+: New insight into the structure and dynamics from mass-selected threshold photoelectron spectra

“…In the cation, on the other hand, there is only one stable singlet isomer, linear HCNH + , while four stable triplet cations with geometries similar to the neutral isomers have been computed. Various spectroscopic techniques have been applied to investigate the physical and chemical properties of methylene amidogen, [18][19][20][21][22][23] but the ionization energy (IE) is still not well established. The photoionization of methylene amidogen was investigated before by photoionization mass spectrometry.…”