A Theoretical Study of ã⁴A₂ CH₂⁺

The potential energy surface and dipole moment surfaces of the  ã 4 A 2  electronic state of CH 2+  are calculated  ab initio  using an augmented correlation-consistent polarized valence quadruple-ζ (aug-cc-pVQZ) basis set, with the incorporation of dynamical correlation using the coupled cluster method with single and double excitations and perturbatively connected triple excitations [CCSD(T)]. We use these surfaces in the MORBID program system to calculate rotation and rotation–vibration term values for  ã -state CH 2+ , CD +2 , and CHD +  and to simulate the rotation and rotation–vibration absorption spectrum of CH 2+  in the  ã 4 A 2  electronic state. Our work is motivated by studies of CH 2+  that use the Coulomb explosion imaging technique and by the goal of predicting spectra that may be obtained from discharge sources. Although the  ã  state is the lowest-lying excited state above the  X̃ / Ã  ground state pair, it turns out to be relatively high-lying, and we determine that  T e ( ã )=30447.5 cm −1 . The equilibrium bond angle for  ã -state CH 2+  is only 77.1°; as a result the asymmetric top κ value is close to 0, and the molecule is equally far from the oblate and prolate symmetric top limits in this electronic state.