Relative ground and excited state energies of CH3(CH = CH)5CH = NC4H9, its hydrogen-bonded and proton-transferred species, and charge partitioning and distribution in the protonated Schiff base of retinal.

CH3(CH = CH)5CH = NC4H9 (compound 1) is structurally related to the Schiff base of retinal, the prosthetic group in visual pigments. Dilute solutions of a weak acid (phenol) and 1 in a hydrocarbon solvent, when subjected to decreasing temperature, show striking changes in electronic absorption spectra. Initially only the spectrum of compound 1 is present, but as the temperature is lowered, the absorbance of 1 decreases, and the spectrum of the H-bonded form of 1 appears and increases. Continued temperature lowering then causes a decrease in absorption of the H-bonded form and an appearance and rise in absorption of the proton-transferred form of 1. Concentrations of the various species are measured as a function of temperature, and by standard procedures, the thermodynamic constants for both reaction steps are computed. Values of delta H0 are taken as relative energies among the three ground states, and the lambda max value of each species yields relative energies among excited states. By employing data from electronic absorption spectroscopy, nuclear magnetic resonance (NMR) and theoretical calculations for retinal Schiff base, charge partitioning between nitrogen and the polyene chain and charge distribution among the carbon atoms of the polyene chain are calculated.