Theoretical modeling of formic acid (HCOOH), formate (HCOO^-), and ammonium-nh₄⁺ vibrational spectra in astrophysical ICES

Ions embedded in icy grain mantles are thought to account for various observed infrared spectroscopic features, particularly in certain young stellar objects. The dissociation of formic acid (HCOOH) in astrophysical ices to form the formate ion (HCOO^-) was modeled with density functional theory cluster calculations. Like isocyanic acid (HOCN), HCOOH was found to spontaneously deprotonate when sufficient water is present to stabilize charge transfer complexes. Both ammonia and water can serve as proton acceptors, yielding ammonium (NH₄^-) and hydronium (H ₃O⁺) counterions. Computed frequencies of weak infrared features produced by stretching and bending modes in both HCOO^- and HCOOH were compared with experimental and astronomical data. Our results confirm laboratory assignments that a band at 1381 cm^-1 can be attributed to the CH bend in either HCOO^- or HCOOH, buta band at 1349 cm ^-1 corresponds to CO stretching in HCOO^-. Another feature at 1710 cm^-1 (5.85 μm) can possibly be assigned to a CO stretching mode in HCOOH, as suggested by experiment, but the agreement is less satisfactory. In addition, we examine and analyze spectroscopic features associated with NH₄⁺, both as a counterion to HCOO ^- or OCN^- and in isolation, in order to compare with experimental and astronomical data in the 7 μm region.