Hydrogen-carbon bond correlations

Figure 9.3 Shows the molecular orbitals for trans octatetraene ignoring, hydrogen-carbon bonds, and the filling of these orbitals with electrons. Both possible transitions that would result from a 1-electron model of the structure and the actual result including electron correlation effects are also shown, indicating that the lowest energy optical transitions are not from the HOMO to the LUMO states. Only a two-electron transition is allowed between these states for a photon-only process. Reprinted with permission from Bredas, Jean-Luc et al. Excited-state electronic structure of conjugated oligomers and polymers a quantum-chemical approach to optical phenomena . Accounts in Chemical Research 1999 32 267-276. [1] Copyright 1999 American Chemical Society.

Proton chemical shifts are very valuable for the determination of structures, but to use the shifts in this way we must know something about the correlations that exist between chemical shift and structural environment of protons in organic compounds. The most important effects arise from differences in electronegativity, types of carbon bonding, hydrogen bonding, and chemical exchange. 

---