The solution structure of (Me3Si)3CSiBr--an ab initio/NMR study

The first halosilylene stable in solution was investigated by ab initio/NMR calculations (IGLO SOS-DFPT PW91/B2//B3LYP/6-31+G(d)). The delta (29)Si(calc) of (Me(3)Si)(3)CSiBr (446 ppm) does not agree with the measured NMR signal at 106 ppm assigned to the free halosilylene. From the possible silylene complexes in the reaction solution, two structures agree with the observed NMR signal: the (Me(3)Si)(3)CSiBr(2) anion (delta (29)Si(calc)=124 ppm) and the unsolvated and solvated complex of the anion with two Li(+) (delta (29)Si(calc)=117 and estimated 134 ppm). Additionally the delta (29)Si(calc) of alkylsilylenes, R-Si-X, ranging from 200 to 900 ppm are presented to guide NMR identification in future silylene synthesis.