Reinterpretation of the electronic absorption spectrum of the methylene amidogen radical (H 2 CN)
The odd electronic absorption spectrum of H 2 CN has been of great interest to experiment. Herein, this system is reported on extensively by applying theoretical methods to the ground and low-lying excited electronic states. Employing a large breadth of high-level ab initio computations, including coupled cluster [CCSD(T) and CCSDT(Q)] and multireference configuration interaction [MRCISD+Q] methods, we comprehensively demonstrate that the most recent experimental and theoretical interpretations of the electronic spectrum of H 2 CN are in error. The previous assignments of the two broad features in the spectrum as the origin 0$^{0}_{0}$ (~35 050 cm –1 ) and 4$^{2}_{0}$ (~35 600 cm –1 )$\widetilde{B}$ 2 A 1 ←$\widetilde{X}$ 2 B 2 transitions are both found to be incorrect. The presently reported transition energies suggest that the higher energy band near 35 600 cm –1 is the true origin band. Moreover, from the computed anharmonic vibrational frequencies of the $\widetilde{X}$ 2 B 2 and $\widetilde{B}$ 2 A 1 states, we demonstrate that this ~550 cm –1 band spacing cannot be attributed to a simple vibronic transition, as claimed by the 4$^{2}_{0}$ assignment. Possible alternative explanations for the appearance of the lower intensity band near 35 050 cm –1 are discussed.