Quartic force field predictions of the fundamental vibrational frequencies and spectroscopic constants of the cations HOCO+ and DOCO+

Only one fundamental vibrational frequency of protonated carbon dioxide (HOCO(+)) has been experimentally observed in the gas phase: the ν(1) O-H stretch. Utilizing quartic force fields defined from CCSD(T)/aug-cc-pVXZ (X = T,Q,5) complete basis set limit extrapolated energies modified to include corrections for core correlation and scalar relativistic effects coupled to vibrational perturbation theory and vibrational configuration interaction computations, we are predicting the full set of gas phase fundamental vibrational frequencies of HOCO(+). Our prediction of ν(1) is within less than 1 cm(-1) of the experimental value. Our computations also include predictions of the gas phase fundamental vibrational frequencies of the deuterated form of the cation, DOCO(+). Additionally, other spectroscopic constants for both systems are reported as part of this study, and a search for a cis-HOCO(+) minimum found no such stationary point on the potential surface indicating that only the trans isomer is stable.