CHEMICAL-SHIFTS IN NMR SI-29 SPECTRA AND MECHANISMS OF INTRAMOLECULAR INTERACTIONS IN C,C-BIS(TRIMETHYLSILYL)PHOSPHAALKENES

The changes in the values of chemical shifts of P-31, C-13, Si-29 nuclei in the structure-similar phosphaalkenes of X(i)P = = C (SiMe3)2 series are considered. The specific character of electron effect of each of four groups of X(i) substituents on the delocalized pi-system -P = C (Si(E), Si(Z)) has been revealed. In the case of conjugation (hyperconjugation) effect intensification from X(i) the screening of Si(Z) atom increases as Si(E) atom loses its screening. In the case of X(i) with primary inductive effect (halogens, unconjugate groupings) the changes in screening of Si(Z) and Si(E) are symbate. The correlation of chemical shifts of delta(a) with Hammett sigma-constants and peculiarities of behaviour of ionization potentials of pi- and n(P)-energy levels in these compounds are under discussion. It is assumed the asymmetry in the degree of screening of Si(E) and Si(Z) atoms is caused by participation of unshared electron pair of P atom in the interaction with -C (SiMe3)2 grouping.