THE ROTATIONAL SPECTRUM, STRUCTURE AND DIPOLE MOMENT OF ETHYNYLPHOSPHINE. $H_{2}PC\equiv CH$

The rotational spectra of two isotopic species of ethynylphosphine, $H_{2}PC\\equiv CH$ and $D_{2}PC\\equiv CH$, have been measured in selected regions between 8 and 124 GHz. The rotational constants as well as a complete set of quartic centrifugal distortion constants have been determined for both species. The substitution coordinates of the P hydrogens have been derived. If a reasonable structure is assumed for the $C\\equiv C-H$ group, the P--C length and $P-C\\equiv C$ angle may be determined from the moments of inertia. The derived structural parameters are $r(P-H)=1.414(5){\\AA}, r(P-C)=1.774(5){\\AA}, \\angle(P-C\\equiv C)=173(2)^{\\circ}, \\angle (H-P-H)=93.9(5)^{\\circ}$, and $\\angle (H-P-C)=96.6(5)^{\\circ}$, with the assumptions $r(C\\equiv C)=1.208{\\AA}, r(C-H)=1.058{\\AA}$, and $\\angle(C\\equiv C-H)=180.0^{\\circ}$. The $C\\equiv C-H$ group is bent away from the hydrogens. The dipole moment and its orientation have been determined from Stark effect measurements of both species. For $H_{2}PC\\equiv CH, \\mu_{s}=0.155(1)D, \\mu_{t}=0.555(1)D$ and $\\mu_{t}=0.576(1)D$. For $D_{2}PC\\equiv CH, \\mu_{a}=0.138(1)D, \\mu_{t}=0.564(3)D$ and $\\mu_{t}=0.580(3)D$. In the normal species the dipole moment makes angles of $53.4^{\\circ}$ and $70.5^{\\circ}$ with the P-H and P-C bonds respectively.