Photolysis of phosphine in the presence of acetylene and propyne, gas mixtures of planetary interest.

Phosphine (PH3) 1 has been observed in the atmospheres of Jupiter and Saturn. We have studied the photochemical reactions of this compound with acetylene (C2H2), an alkyne also detected in these atmospheres. The volatile products formed in these reactions were characterized by H, 31P and 13C NMR. The ethenylphosphine 2 is the first product formed in the photolysis of PH3 in the presence Of C2H2. Photolysis of PH3 in the presence of propyne (C3H4) led to the formation of the Z- and E-prop-1-enylphosphines and traces of 1-methylethenylphosphine. A reaction pathway is proposed. The initial step is the dissociation of PH3 to hydrogen and PH2 radicals. Addition of the phosphinyl radical on alkyne occurs as the next step. Vinylphosphines are then formed by radical combination. This proposed reaction pathway takes into account the nature of the products and studies devoted to the photolysis of germane (GeH4) or hydrogen sulfide (H2S) in the presence of alkyne. Attempts to detect the methylidynephosphine HC triple bond P (the isoelectronic compound of HC triple bond N), in the photolysis products of PH3-C2H2 mixtures were unsuccessful. The application of these findings to Jovian and Saturn atmospheric chemistry is discussed.