Ab initio structure and vibration-rotation dynamics of the formyl and isoformyl cations, HCO⁺/HOC

Accurate structure and potential energy surface of the formyl and isoformyl cation system, HCO⁺/HOC⁺, in its ground electronic state X̃ ¹Σ⁺ have been determined from ab initio calculations using the coupled-cluster approach in conjunction with the correlation-consistent basis sets up to septuple-zeta quality. Both the isomers are confirmed to be linear at equilibrium, with the total energy minimum of HOC⁺ lying 14 120 cm^-1 above that of HCO⁺ and the HCO⁺ → HOC⁺ isomerization energy barrier being 26 870 cm^-1 (in the Born-Oppenheimer approximation). The equilibrium structural parameters for HCO⁺ are estimated to be r_e(HC) = 1.0919 Å and r_e(CO) = 1.1058 Å, whereas those for HOC⁺ are estimated to be r_e(HO) = 0.9899 Å and r_e(CO) = 1.1544 Å. The vibration-rotation energy levels were predicted for various isotopologues using a variational approach and compared with the experimental data. For the spectroscopically well characterized formyl cation, the observed vibration-rotation energies and the effective rotational constants are reproduced to within about 2.3 cm^-1 and 1.7 MHz, respectively. The role of the core-electron correlation, higher-order valence-electron correlation, scalar relativistic, and adiabatic effects in determining the structure and vibration-rotation dynamics of both the isomers is discussed.