On the spectroscopic constants, first electronic state, vibrational frequencies, and isomerization of hydroxymethylene (HCOH+)

The hydroxymethylene cation (HCOH${}^{+}$) is believed to be chemically independent of the more stable formaldehyde cation isomer in interstellar chemistry and may likely be a precursor to methanol in chemical reaction networks. Previous work is corroborated here showing that the trans conformer of HCOH${}^{+}$ is 3.48 kcal/mol lower than the cis on the potential energy surface. The small energy difference between the conformers and the much larger dipole moment of cis-HCOH${}^{+}$ (2.73 D) make this conformer more likely to be observed than trans-HCOH${}^{+}$ via telescopic rotational spectroscopy. A strong adiabatic shift is also predicted in the first electronic excitation into the 1 ²A′′/2 ²A state out of either conformer into a C₁ structure reducing the excitation wavelength from the near-ultraviolet all the way into the near-infrared. The full set of fundamental vibrational frequencies are also computed here at high-level. The 3306.0 cm${}^{-1}$ and 3225.3 cm${}^{-1}$ hydroxide stretches, respective of bare trans- and cis-HCOH${}^{+},$ are in agreement with previous theory but are significantly higher than the frequencies determined from previous experiment utilizing argon tagging techniques. This shift is likely because the proton-bound complex created with the argon tag reduces the experimental frequencies. Lower-level computations including the argon tag bring the hydroxide stretches much closer to the experimental frequencies indicating that the predicted frequencies for bare HCOH${}^{+}$ are likely well-described.