Molecular structure of acetyldimethylphosphine,MeC(O)PMe2: I. Gas phase electron diffraction study

An electron diffraction study of the MeC(O)PMe₂ molecule has been carried out which shows that the bond configuration at phosphorus in this compound is flattened from that observed in methylphosphines and dimethylcyanophosphine. This finding, as well as some lowering of barriers to inversion at phosphorus in acylphosphines, may be attributed to a contribution from conjugation of the amide type.Two models with different angles of rotation of the acetyl group about the PC_Ac bond, ψ, are consistent with the experimental data, those with ψ of 5.7 ± 6.6° (model I) and 78.8± 12.0° (model II) (ψ is measured relative to the C_s conformation with the C_MePC_Me bisector being cis with respect to the CO bond). Other parameters of the two models agree within uncertainties represented by three times standard deviation values 3σ (distances, r_a, in nm; angles in degrees): model I model II PC (average) 0.1863(2) 0.1862(2) CC 0.1536(9) 0.1540(9) CO 0.1219(4) 0.1218(3) CH 0.1094(6) 0.1094(6) C_MePC_Me 105.7 (0.9) 105.1 (3.3) C_MePC_Me 99.3 (2.0) 102.2 (7.5) PCO 123.0 (0.6) 121.9 (0.6) PCC 115.3 (0.9) 112.7 (0.9) The greater errors for phosphorus valence angles and the angle of rotation, ψ, in the case of model II are due to high correlation between these para A possible difference between the PC_Me and PC_Ac bond lengths has been estimated to be. ± 0.001nm(model I) or ± 0.003 nm(model II). Neither of the models can be preferred on the basis of the electron diffraction data; the results'obtained are also compatible with' the suggestion of large amplitude torsion motions.