distances for single bonds between two sp3 carbons. However, an analysis of COR bond distances in >2000 ethers and carboxylic esters (all with sp3 carbon) shows that this distance increases with increasing electron withdrawal in the R group and as the C changes from primary to secondary to tertiary.63 For these compounds, mean bond lengths of the various types ranged from 1.418 to 1.475 A ˚. Certain substituents can also influence bond length. The presence of a silyl substituent b- to a CO (ester) linkage can lengthen the CO, thereby weakening it.64 This is believed to result from s-s* interactions in which the CSi s-bonding orbital acts as the donor and the CO s* orbitals acts as the receptor.