Ground, Excited, and Ionic States of the NCCNO Molecule: A HeI Photoelectron, Infrared, Ultraviolet, and ab Initio Investigation

Abstract

The cyanogen N-oxide molecule NCCNO was recently identified for the first time in the gas phase. In the present work, NCCNO, generated from its stable ring dimer dicyanofuroxan by low-pressure thermolysis, is characterized by HeI photoelectron, photoionization mass, ultraviolet, and infrared spectroscopies. From stop-flow and revaporization experiments the molecule may be categorized as semistable, having a lifetime of a few hours in the dilute gas phase. The ground state molecular geometry and vibrational frequencies are explored with ab initio calculations at the MP3, QCISD, and QCISD(T) levels for comparison with experiment. The standard correlated methods have some difficulty with the structure; density functional theory gives results closer to recent microwave data and suggests that bigger basis sets are necessary. The molecule is predicted to have a large amplitude deformation, suggesting possible quasi-linear behavior. For the ionic states, the semiempirical HAM/3 method gives very good agreement with the measured ionization energies. The low lying singlet excited states, optimized at the CIS level of theory, assist with the interpretation of the ultraviolet spectrum. Taken together, the ab initio calculations and the spectroscopic data suggest that both the ground state molecule and the ion have linear structures, but the lowest π* ← π electronic transition leads to a molecule strongly bent in the lowest excited singlet state.