Ab initio calculation of NMR shielding in phosphaalkenes X—PCY2

Abstract

P-31 and C-13 chemical shielding values, sigma(iso), 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 P-31 and C-13 chemical shielding in the phosphaalkene H-P=CH2 and ethylene, respectively, were compared. The P-31 chemical shielding variation was determined by mixing of the ground and excited states. Good correlations were found between the experimentally measured SC and SP 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 P-31 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 C-13 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) pi-mesomeric interactions. The absence of the corresponding correlation with all phophaalkenes investigated manifestates limited validity of this generally used approach. Copyright (C) 1999 John Wiley & Sons, Ltd.