The simple diatomic radical silicon phosphide (SiP) was recently detected by Koelemay and co-workers toward IRC +10216 (CW Leo), as reported in 2024. The work included measuring the pure rotational spectrum of the radical, which was not previously available. A number of transitions were identified in astronomical data collected at the 10m ARO telescope on Kitt Peak and the Submilimeter Telescope (SMT) on Mt. Graham. Some of the potential transitions were obscured by stronger features of other molecules, but SiP has characteristic doublet lines and resolvable hyperfine structure as well.

Silicon and phosphorus are below carbon and nitrogen in the periodic table, so one might expect SiP to behave like CN. The cyano radical is one of the most common astromolecules and a motif found in a large fraction of detected species. However, the ground state of SiP has ²Π_i symmetry rather the ²Σ⁺ symmetry found in CN. This means that the radical character is in the π system, not the σ system. It is more localized on P than on Si. When H is added to CN, the minimum structure is linear HCN; when H is added to SiP, the minimum structure is quasicyclic, with the H closer to P than to Si.