Spatial and electronic structure, 29 Si chemical shifts, and the energetics of a wide series of the silylmethyl derivatives of phosphoric acid R 2 P(O)ZCH 2 SiMe 3-n Hal n (Z = O, NR 0 ,CH 2 , S; R = Me, NMe 2 , NEt 2 , OMe, 2R = -NMe(CH 2 ) 2 NMe-; Hal = Cl, F; n = 1-3) are studied at the MP2 and DFT (B3LYP and B3PW91) levels of theory with different basis sets. The compounds can exist in the noncoordinated (acyclic) form a and the P(O)fSi-coordinated (cyclic) form b. The relative stability of isomers b is determined not only by the degree of the orbital-or charge-controlled OfSi coordination (stabilizing factor) but also by theSi heterocycle strain energy, E strain (destabilizing factor). For this reason, only eight of 52 considered molecules exist exclusively in the chelate form b. The results of the NBO and AIM analysis suggest that the optimal combination of the stabilizing and destabilizing factors is achieved for the silylated phosphoramides R 2 P-(O)NR 0 CH 2 SiMe 3-n Hal n . According to the AIM and ELF analysis, relatively strong SiO interactions (1.9 A ˚< d SiO < 2.4 A ˚) in the coordinated forms b have a covalent component, whereas weak SiO interactions (d SiO > 2.4 A ˚) do not. The analysis of the relationship between the calculated and experimental values of the 29 Si chemical shifts for amides R 2 P(O)NR 0 CH 2 SiMe 3-n Hal n in CHCl 3 allows a realistic description of their structure in this medium.

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The plant pentacyclic alkaloid camptothecin and its structural analogs were extensively studied. These compounds are interesting due to the antitumor activity associated with K E Y W O R D Sbiological activity, camptothecin, properties, silatecans, synthesis

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