Vibrational spectra of the radiation-induced cationic species resulting from phosphine monomers and dimers: A matrix isolation and CCSD(T) study.

Phosphine (PH3) is the simplest phosphorus compound detected in the extrasolar and planetary environments, where it can be subjected to ionizing radiation. In this study, we first report the vibrational spectra of the ionized molecules resulting from phosphine monomer and dimer in solid noble gas matrices upon X-ray irradiation. The assignment was based on the comparative studies using electron paramagnetic resonance and Fourier transform infrared spectroscopy, complemented by the quantum-chemical calculations at the valence-correlated spin unrestricted coupled cluster single-double and perturbative triple [UCCSD(T)] level of theory. We were able to observe three fundamentals of PH3+• and four fundamentals of P2H6+•. The spectroscopic data were also obtained for PD3+•, P2D6+•, and [PH3-PD3]+• radical cations. PH3+• demonstrates a large blue shift of the most intense infrared (IR) absorption related to the P-H stretching vibration (ν1) with respect to the corresponding vibration in the parent neutral. For the P2H6+• radical cation, the most intense IR absorption corresponds to a low-frequency deformational vibration. It was found that the P2H6+• radical cation reveals a photochromic behavior: it decays under the action of light with λ ≤ 400 nm and can be partially recovered after subsequent photolysis at λ = 445-525 nm. The observed transformations were attributed to the interconversion between P2H6+• and the PH4+…PH2• complex. The dynamics of these transformations was discussed using the data obtained for mixed P2H3D3+• species. The obtained results may be useful for future searches for the manifestations of such species in extraterrestrial environments and studies of the radiation-induced transformations of phosphine in icy media.