Add time:07/25/2019         Source:sciencedirect.com

Carbon-13 NMR chemical shifts are reported for three series of Group IV aromatics p-XC6H4Y(CH3)3 where Y = C, Si and Ge. The shifts for the C(1), C(2), CH3 and C(α) carbons correlate with the substituent parameters σ, and σoR, and the slopes of the CH3 correlations suggest roughly equal transmitting abilities for carbon and silicon. For a given substituent X, the shift of the C(1) carbon varies in the order C>Ge>Si expected from electronegativity considerations. The other ring carbon atoms experience smaller changes corresponding to their respective positions ortho, para, and meta to Y. The shifts of the C(4) and C(2, 6) carbons vary Si>Ge>C, in agreement (for X = N(CH3)2 with atomic charge values from published CNDO/2 calculations for carbon and silicon which indicate that while silicon exerts an electron-releasing effect through the σ-framework, there is a net π-withdrawal registered at the C(4) and C(2, 6) carbon atoms relative to Y = C.

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