Employing gradient-corrected levels of density-functional theory Ž . DFT , medium-sized basis sets, and optimized geometries, chemical shifts are w xŽ . w Ž . x w Ž . x y calculated for VOCl F n s 0᎐3 , VF , VO OCH CH N , V CO , n 3yn 5 2 2 3 6 wŽ . Ž .x y w Ž . x Ž V CO N , as well as for the model compounds VO OMe Me n s 5 2 n 3yn . Ž 51 . 0᎐3 and their AlH adducts. Experimental trends in ␦ V are well reproduced 3 Ž 51 . Ž 51 . with DFT-based methods; for example, the slopes of the ␦ V vs. ␦ V calc expt linear regression lines are 0.92 and 1.03 at the GIAO-BP86 and GIAO-B3LYP levels, respectively. Ethylene polymerization observed with w Ž . Ž . X xŽ . V O иии AlX OR R X, R, RЈ s bulky alkyl, aryl, or silyl groups is 3 n 3yn
Ž . shown for model systems X s H, R s RЈ s Me to proceed by insertion of the olefin into a V-C bond via a transition state with approximate square-pyramidal Ž . coordination about vanadium. For the tri-and dialkyl derivatives n s 0, 1 , Ž similar activation barriers of ca. 19 kcalrmol are computed BP86 level including . Ž . zero-point energies , whereas that of the monoalkyl species n s 2 is predicted to be much higher, ca. 30 kcalrmol. The relevance of these results for the Ž 51 . apparent relationship between ␦ V and catalytic activities is discussed.