Directed gas-phase preparation of the elusive phosphinosilylidyne (SiPH2, X2A′′) and cis/trans phosphinidenesilyl (HSiPH; X2A′) radicals under single-collision conditions

The reaction of the D1-silylidyne radical (SiD; X-2 pi) with phosphine (PH3; X(1)A(1)) was conducted in a crossed molecular beams machine under single collision conditions. Merging of the experimental results with ab initio electronic structure and statistical Rice-Ramsperger-Kassel-Marcus (RRKM) calculations indicates that the reaction is initiated by the barrierless formation of a van der Waals complex (i0) as well as intermediate (i1) formed via the barrierless addition of the SiD radical with its silicon atom to the non-bonding electron pair of phosphorus of the phosphine. Hydrogen shifts from the phosphorous atom to the adjacent silicon atom yield intermediates i2a, i2b, i3; unimolecular decomposition of these intermediates leads eventually to the formation of trans/cis-phosphinidenesilyl (HSiPH, p2/p4) and phosphinosilylidyne (SiPH2, p3) via hydrogen deuteride (HD) loss (experiment: 80 +/- 11%, RRKM: 68.7%) and d-trans/cis-phosphinidenesilyl (DSiPH, p2 '/p4 ') plus molecular hydrogen (H-2) (experiment: 20 +/- 7%, RRKM: 31.3%) through indirect scattering dynamics via tight exit transition states. Overall, the study reveals branching ratios of p2/p4/p2 '/p4 ' (trans/cis HSiPH/DSiPH) to p3 (SiPH2) of close to 4 : 1. The present study sheds light on the complex reaction dynamics of the silicon and phosphorous systems involving multiple atomic hydrogen migrations and tight exit transition states, thus opening up a versatile path to access the previously elusive phosphinidenesilyl and phosphinosilylidyne doublet radicals, which represent potential targets of future astronomical searches toward cold molecular clouds (TMC-1), star forming regions (Sgr(B2)), and circumstellar envelopes of carbon rich stars (IRC + 10216).

Automated summary

The study investigates the reaction dynamics between the D1-silylidyne radical and phosphine, uncovering the formation of various intermediate products and pathways. Results show a branching ratio of close to 4:1 for the formation of trans/cis-phosphinidenesilyl/d-trans/cis-phosphinidenesilyl compared to phosphinosilylidyne.