Geminal 2J(29Si‐O‐29Si) couplings in oligosiloxanes and their relation to direct 1J(29Si‐13C) couplings

Absolute values of (79) geminal ²J(²⁹Si‐O‐²⁹Si) couplings were measured in an extensive series of (55) unstrained siloxanes dissolved in chloroform‐d. Signs of ²J(²⁹Si‐O‐²⁹Si) in some (9) silicon hydrides were determined relative to ¹J(²⁹Si‐¹H) which are known to be negative. It is supposed that positive sign of the ²J(²⁹Si‐O‐²⁹Si) coupling found in all studied hydrides is common to all siloxanes. Theoretical calculations for simple model compounds failed to reproduce this sign and so their predictions of bond length and angle dependences cannot be taken as reliable. Useful empirical correlations were found between the ²J(²⁹Si‐O‐²⁹Si) couplings on one side and the total number m of oxygen atoms bonded to the silicon atoms, sum of ²⁹Si chemical shifts or product of ¹J(²⁹Si‐¹³C) couplings on the other side. The significance of these correlations is briefly discussed. Copyright © 2011 John Wiley & Sons, Ltd.