Theoretical study of the photodetachment spectrum of the methylene amidogene anion (H₂CN⁻)

Abstract

The excited states of methylene amidogen (H2CN) have recently been studied extensively both by experiment and theory. However, the assignment of absorption spectra is in disagreement with the theoretical analysis. In the present study a theoretical photoelectron detachment spectrum is predicted which shows that a corresponding experiment could be suitable to decide whether the theoretical or the experimental assignment is correct. To this end, the electron affinity of H2CN was calculated very accurately. Vertical and adiabatic electron detachment energies were calculated by complete active space and multi-configuration self-consistent field (CASSCF and MCSCF) as well as by multireference configuration interaction (MRCI) methods. The MRCI wave functions were utilized to compute electronic detachment probabilities. Beyond the radical ground state, three significant detachments are found in the region around 4-5 eV and a further weak detachment at roughly 8.4 eV. Vibrational intensities were determined for the four lowest electronic states of the radical and vibrationally resolved photodetachment spectra were constructed. The predicted spectra show that the bands of the first 2B1 and 2A1 states are clearly distiguishable while the band for the nearby second 2B1 state is weak and strongly overlapping with the 2A1 manifold.

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

“…8 We also make a detailed comparison of the experimental spectra with rotational contours computed with rotational constants obtained by averaging over the b 1 umbrella motion on the ground and excited-state potential energy surfaces. 16 The experimental and simulated spectra agree extremely well with each other for an assumed type-B transition, appropriate to a 2 A 1 r 2 B 2 transition, and disagree significantly if the transition moment is assumed to lie along the a or c inertial axes. This provides strong confirmation for the 2 A 1 r 2 B 2 assignment made previously by Eisfeld.…”

“…Prior to the recent theoretical study by Eisfeld, 16 the consensus was that the observed electronic transition in H 2 CN was due to a dipole-forbidden but vibronically-allowed transition to the à 2 B 1 state. The experimental evidence for this assignment comes from the H atom photofragment study of Davis and co-workers.…”

“…This provides strong confirmation for the 2 A 1 r 2 B 2 assignment made previously by Eisfeld. 16 We also offer possible explanations for the value of the parameter measured by Davis and co-workers, 7 which is seemingly inconsistent with this assignment. The theoretical investigation of the dissociation pathways for electronically excited H 2 CN is also presented.…”

Electron collisions with the H2CN radical using theR-matrix method

“…Both Bernad et al [13] and Nizamov et al [14] assigned their observed absorption bands to a vibronically allowed 1 2 B 1 ← X 2 B 2 transition. Subsequently, Eisfeld and co-workers [15][16][17] carried out a series of theoretical and experimental studies of the electronic states of H 2 CN and assigned the observed electronic transition as the dipole-allowed 1 2 A 1 ←X 2 B 2 transition.…”

“…Cowles et al [18] measured the ultraviolet photoelectron spectra of H 2 CN − and * Corresponding author: wangkd@htu.cn deduced the adiabatic electron affinity of the H 2 CN molecule to be 0.511 ± 0.008 eV. Eisfeld [16] calculated the adiabatic electron affinity of 0.5152 eV calculated with zero-point energy correction. The rotational constants of H 2 CN were determined to be 9.4844, 1.3062, and 1.1423 cm −1 by the microwave spectroscopy [19].…”