31
P and 13 C chemical shielding values, iso , were calculated for a series of phosphaalkenes X-P CY 2 with a variety of substituents such as X H, CH 3 , F, Cl, Br, OH, NH 2 , PH 2 , SiH 3 , CN and Y H, CH 3 , NH 2 , SiH 3 , using the GIAO procedure at the RHF/6-311 C G(d,p) and RHF/6-311 C 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 31 P and 13 C chemical shielding in the phosphaalkene H-P CH 2 and ethylene, respectively, were compared. The 31 P chemical shielding variation was determined by mixing of the ground and excited states. Good correlations were found between the experimentally measured υC and υP values in the series X 0 -P C(SiMe 3 2 and those calculated for the series X-P C(SiH 3 2 , but on average the calculated 31 P chemical shifts seem to be overestimated. A comparison with the calculated data (at the same level) for X-P C(SiMe 3 2 indicates only a partial improvement. The 13 C chemical shifts in phophaalkenes correlate within the subseries X-P CH 2 , X-P C(CH 3 2 and X-P C(SiH 3 2 with the charge variation at the corresponding carbon atom, reflecting the X-(P C)mesomeric interactions. The absence of the corresponding correlation with all phophaalkenes investigated manifestates limited validity of this generally used approach.