Ab initio calculation of NMR shielding in phosphaalkenes X—PCY2

31P and 13C chemical shielding values, σiso, were calculated for a series of phosphaalkenes X—PCY2 with a variety of substituents such as XH, CH3, F, Cl, Br, OH, NH2, PH2, SiH3, CN and YH, CH3, NH2, SiH3, using the GIAO procedure at the RHF/6–311 + G(d,p) and RHF/6–311 + G(2d,2p) levels. The trends governing the variation of the geometric parameters and the natural charges and Wiberg indices are discussed. The contributions of the various molecular orbitals in 31P and 13C chemical shielding in the phosphaalkene H—PCH2 and ethylene, respectively, were compared. The 31P chemical shielding variation was determined by mixing of the ground and excited states. Good correlations were found between the experimentally measured δ C and δ P values in the series X′—PC(SiMe3)2 and those calculated for the series X—PC(SiH3)2, but on average the calculated 31P chemical shifts seem to be overestimated. A comparison with the calculated data (at the same level) for X—PC(SiMe3)2 indicates only a partial improvement. The 13C chemical shifts in phophaalkenes correlate within the subseries X—PCH2, X—PC(CH3)2 and X—PC(SiH3)2 with the charge variation at the corresponding carbon atom, reflecting the X—(PC) π-mesomeric interactions. The absence of the corresponding correlation with all phophaalkenes investigated manifestates limited validity of this generally used approach.