CONCERNING MECHANISM OF SINGLE-BOND SHORTENING . EVIDENCE FROM CRYSTAL STRUCTURES OF 1-BIAPOCAMPHANE 1-BINORBORNANE AND 1-BIADAMANTANE

The J13CH at the bridgehead carbon in norbornane is 139 Hz, indicating 28% s character. Using this number, the Dewar plot of bond length against s character (length = 1.692 - 0.0062 % s) predicts a central bond length in 1-binorbornane of 1.517 A. We find this bond length to be 1.515 ± 0.005 A. We therefore agree with Dewar that the concepts of conjugation or hyperconjugation are not necessary to explain single-bond lengths in compounds such as butadiene, styrene, or propylene. A correction for two H⋯H repulsions in 1-binorbornane reduces the central bond length to about 1.503 A and strengthens the case against conjugation in nonaromatic hydrocarbons. © 1968, American Chemical Society. All rights reserved.